Chapter 5. Observing the Atmosphere

Atmospheric scientists collect and analyze weather data to reveal insights as to how the atmosphere behaves. We can neither choose nor change the conditions that we measure. Our understanding of how the atmosphere behaves, and thus our ability to forecast changes, has improved over the last two decades. Much of this improvement has come from technological advances.

New technologies of observing the atmosphere allow scientists to gather a steady stream of weather data for weather analysis. These technologies include new satellites and radars, as well as fully automated surface observations. Computers continue to increase in speed and storage capacity. These result in improved analysis of weather data and more sophisticated weather models used for prediction.

The complexity of the atmosphere requires that we make a number of different measurements. We need to observe and analyze air temperature, atmospheric pressure, atmospheric humidity, visibility, wind direction and wind speed, cloud distribution, cloud type, and precipitation amount and type. To obtain the most useful information, observations of the state of the atmosphere should be made at many places and as continuously as possible.

Our first means of observing the atmosphere were based on the impressions weather makes on our senses. These observations are still very important, although instruments are quickly replacing human observations. Instruments make more objective measurements than the human senses, which vary from person to person. In this chapter, we will discuss some of the instruments used to make surface weather observations. We will then interpret some common optical features of the atmosphere. Satellite and radar observations are explained at the end of this Chapter.

Surface Weather Observations

Across the world, weather stations routinely monitor surface weather conditions. Weather observations are taken at the same time everywhere to accurately represent the state of the atmosphere at a particular time. In the United States, staff of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and private citizens and businesses typically make these measurements.

Chapter 1 listed some of the common weather parameters graphically represented on the surface station model. The station model is one way to make lots of information accessible to the weather forecaster. The meteorogram is another method. The meteorogram is a chart of one or more weather variables at a station over a given period of time (Box 5.1).

The Automated Surface Observing System (ASOS) is a surface weather observing system implemented by the NWS, the FAA, and the Department of Defense (DoD). The ASOS is the United States' primary surface weather observing network. ASOS is designed to support weather forecast activities and aviation operations by providing more information on the atmosphere, more frequently and from more locations (Figure 5.1). The location of the instruments is an important factor in making accurate measurements. The weather instruments must be located to be representative of the entire area. This section discusses some of the instruments used to make these measurements.

Temperature

The thermometer is the most recognized instrument for measuring temperature. Liquid filled thermometers and metallic thermometers measure temperature by measuring the expansion and contraction of objects when they are heated or cooled. The mercurial thermometer is a type commonly used to measure temperatures in a laboratory; however, most modern thermometers are electronic..

ASOS (Automated Surface Observing System) makes use of a resistance thermometer to measure outside temperature automatically. The electrical resistance of substances varies with temperature. We can determine the temperature of the wire, and thus the surrounding air, by measuring the resistance of a platinum or nickel wire. The resistance of these wires increases with increasingvaries with temperature in a known fashion.

To measure the air temperature accurately, thermometers are shielded from the sun to minimize solar energy gains. The thermometer must also be ventilated to reduce the effects of the shielding!

 

Humidity Measurements

Dew point temperature is measured by ASOS using a dew-point hygrometer. In a dew-point hygrometer, a laser light is shined on a mirror. The light reflects off the mirror onto an instrument that measures the intensity of the reflected light (Figure 5.2). The mirror surface is then chilled. When the mirror surface has been cooled to the dew point temperature, dew forms on the mirror. The water droplets (or ice crystals for the frost point) scatter the light from the laser, reducing the light received by the detector (Figure 5.2b). The mirror is then warmed above the dew point to evaporate the dew, and then cooled again to make a new measurement.

A psychrometer is often used to measure relative humidity. A psychrometer consists of two ventilated mercurial thermometers, one of which has a wet wick around its bulb and is called the wet bulb. Evaporation of water off the wick removes heat from the thermometer. The temperature drops according to the rate of evaporation. To operate correctly, the thermometers have to be ventilated by either whirling the instrument (sling psychrometer) or by drawing air by using a fan (aspirated psychrometer). After a few minutes, the wet bulb temperature will stabilize at a particular temperature, referred to as the wet-bulb temperature. A table is used to convert the temperature difference between the wet bulb and the dry bulb thermometers to relative humidity.

Pressure

Pressure is measured with a barometer. Evangelista Torricelli invented the mercury barometer, discussed in Chapter 1, in 1643. This consists of a long tube that is open at one end. Air is removed from the tube and the open end immersed in a dish of mercury. The weight of the air above will then balance a column of mercury in the tube. The height of the mercury is a measure of the atmospheric pressure.

The aneroid barometer is a flexible metal box, called a cell, which is tightly sealed after air is partially removed. Changes in external air pressure cause the cell to contract or expand. The size of the measured cell represents the atmospheric pressure.

Wind Measurements

A wind vane measures wind direction (Figure 5.3). A typical wind vane has a pointer in front and fins in back. When the wind is blowing, the wind vane points into the wind. For example, when the wind vane points northward, that is a north wind.

A windsock is often used at airports to enable pilots to quickly determine the wind direction along the runway. A windsock is a cone-shaped bag with an opening at both ends.

A cup anemometer is used to measure wind speed. The cups catch the wind and produce pressure difference inside and outside the cup. The pressure difference causes the cups to rotate. Electric switches are used to measure the speed of the rotation, which is proportional to the wind speed.

Precipitation Measurements

The key to precipitation measurements is to accurately intercept the falling precipitation and record amount and intensity. Precipitation amount is determined by a rain gauge. This consists of a funnel-like receiver above a bucket.

ASOS uses a freestanding rain gauge. Precipitation falls into the upper portion of the rain gauge, which is called the collector. The collector is heated to melt any frozen precipitation, such as snow or hail, for collection in the bucket. The collected water is funneled into a tipping bucket. The tipping bucket measures water depth in increments of 0.01 inch. It is called a tipping bucket because as water is collected, the tipping bucket fills to the point where it tips over and empties out, indicating 0.01 inches of water has been measured. A wind-shield is placed around the collector to reduce wind updrafts and wind streamlines that alter rain trajectories.

Cloud Ceiling

Cloud ceiling is defined as the height of the lowest layer of clouds. Pilots can determine cloud height when they note the altitude of the plane as they fly through a cloud layer. Trained observers on the ground can also estimate cloud altitude.

ASOS uses a laser-beam ceilometer (pronounced se-lom'-i-ter) to measure cloud height. The ceilometer uses invisible laser radiation to detect cloud levels. This ceilometer sends pulses of infrared radiation upward (Figure 5.4). If a cloud is present, part of this beam is reflected off the cloud and sent back to the ceilometer. The time interval between when the pulse is transmitted and when it is received is a measure of the cloud height. Unfortunately, the current ASOS ceilometer cannot measure clouds above 12,000 feet.

Visibility

Visibility is a measure of the opacity of the atmosphere and is expressed in terms of the horizontal distance at which a person can see and identify specified objects. Humans can estimate visibility if they know the distances between objects and themselves. If an object 1/4 of a mile away cannot be seen, than the visibility must be less than 1/4 of a mile.

ASOS uses a light scattering technique to measure visibility. The visibility instrument projects a flash of light over a very short distance. The atmosphere scatters the light (see below). The amount of scattered light is related to the visibility. The light scattered by the intervening atmosphere is measured by a receiver and converted into a visibility value.

Upper-air Weather Observations

Meteorologists monitor the upper atmosphere weather by using a radio-equipped meteorological instrument package carried into the atmosphere by a balloon. These packages are called radiosondes. Radiosondes measure the vertical profiles of air temperature, relative humidity, and pressure between the ground and about 30 km (19 miles or 4 mb). Wind speed and direction can be determined by tracking the balloon position with time. When winds are also measured, the observation is called a rawindsonde (Figure 5.5). Measurements using rawinsondes are made worldwide twice each day, 0000 UTC and 1200 UTC.

A sounding is a plot of the vertical distribution of temperature, dew point, humidity and winds. Soundings are plotted on special graphs called thermodynamic diagrams. The Stuve diagram is the simplest form of the thermodynamic diagram (Figure 5.6). In the Stuve thermodynamic diagram, temperature is represented on the x-axis and pressure on the y-axis. The straight lines that slope upward from right to left are the dry adiabats. The dry adiabatic lines represent a decrease in temperature of 10C for each kilometer, the dry adiabatic lapse rate. The dashed slightly curved lines represent the wet adiabats. These lines represent the moist adiabatic lapse rate experienced by a rising saturated air parcel.

Sensual Observations

Major changes in the way weather data are gathered, processed and disseminated have occurred over the last two decades. The focus has been on providing automated weather observations of high quality in a nearly real time mode. Getting more information on the atmosphere, more frequently and from more locations is important for improving weather forecasts and warnings. However, we should not lose sight of the importance of making our own observations of the weather. This section focuses on visual observations of the weather.

Light and Color

Human vision is sensitive to light with wavelengths between approximately 0.39 and 0.78 microns (Table 5.1). Each color of the rainbow corresponds to a particular range of wavelengths. White light is composed of all visible wavelengths.

Much of what we observe about the atmosphere deals with how light interacts with molecules or objects suspended in the atmosphere, such as water drops. It is important to briefly describe some rules that govern how light interacts with objects. This is important for explaining visual observations as well as observations from satellites and radars.

Laws of Reflection and Refraction

Suppose light traveling through air impinges on a pool of water. When the light ray strikes the boundary between the air and water, some of the light is transmitted into the water and some of the radiant energy is reflected (Figure 5.7). The light that penetrates the water also changes directions; it is refracted. Light that is incident on the water and turned back into the direction is referred to as reflected light.

Reflection

To determine the law of reflection we shine a ray of light on a mirror and measure the angle at which it is reflected. To measure this angle an imaginary line is drawn perpendicular to the surface where the light ray strikes the surface. This imaginary line is called the normal. Angles are measured with respect to this normal (Figure 5.7). The angle at which the light strikes the mirror, the angle of incidence, always equals the angle of reflection (Figure 5.7). This simple law of physics describes how a single ray reflects off a surface and holds whenever light is reflected.

There are two types of reflection: specular reflection and diffuse reflection. A mirror is a specular reflector and your clothes are diffuse reflectors. Specular reflection occurs on smooth objects. When a bundle of parallel light rays undergo specular reflection, they all reflect at the same angle and so remain parallel (Figure 5.8). The individual light rays that make up the beam of light are reflected in the same relative position as they struck the surface. Specular reflection results in images. Reflections of images by water are examples of specular reflections. You cannot see such an image in a piece of cloth. This separates reflection off a smooth surface, specular reflection, from reflection off a rough surface or diffuse reflection.

A rough surface is one in which height variations on the surface have a size that is similar to the wavelength of the incident light. When a bundle of parallel rays strikes a rough surface, the reflected rays are no longer parallel (Figure 5.8). Images cannot be formed by diffuse reflection.

Reflection effects produced in the atmosphere

Reflection of sun light off a calm lake generates a perfect image of the sun. When sunlight reflects off slightly ruffled water, the reflected image appears as a bright patch known as sun glint (Figure 5.9). The ripples of water act as separate slanting mirrors reflecting partial images of the sun. Sun glint is an example of specular reflection.

Tiny ice crystals can also act as tiny mirrors, reflecting the light that shine on them. If you live in a region where snow is common, you may have noticed that freshly fallen snow can sparkle. The new fallen crystals lie in random positions on the surface. As the sunlight strikes them, some of the rays are reflected, forming a partial image of the sun.

Refraction

If you shine a beam of light on a container of water or a block of glass, and make careful observations, you will note the following:

These three observations summarize the laws of refraction.

When light is traveling from water into air at an oblique angle, it bends away from the normal, since the light ray is traveling from a medium of high optical density to a medium of low optical density. As the angle of incident of this water ray increases (Figure 5.10), a position occurs for which the ray exiting the water travels along the air-water interface. This angle is referred to as the critical angle. If the incident angle is greater than the critical angle, the ray cannot pass through the interface, as it reflects back into the water. This condition, called total internal reflection, occurs only when light travels into a medium with a lower index of refraction.

Refraction effects produced in the atmosphere

As light rays from the sun stream through the atmosphere, they bend as they pass from the rarefied air of high altitudes to the more dense air near the surface. Because of the gradual change in the number of molecules with altitude, refraction causes the path of the light rays to be curved rather than the sharp change exhibited when entering water. Because of refraction, when we view a star at night it appears to be higher in the sky than it truly is (Figure 5.11). The difference between the starís true position and its apparent position depends on how close to the horizon the star is. In fact, stars that are near the horizon are actually much lower in the sky than they appear. Stars that we can see very near the horizon are really below the horizon! Refraction also explains why stars twinkle (Box 5.3).

Mirages

You probably have observed mirages while driving along a highway on a hot summer day. The mirages may appear as inverted images of cars on the road or they make the pavement appear to be wet (Figure 5.12). These mirages are not optical illusions any more than reflections in mirrors are illusions. Mirages are refracted images.

There are two types of mirages, inferior and superior. In the inferior mirage, the refracted image appears below the true object, while in a superior mirage it lies above the object. Inferior mirages result when temperature decreases with distance from the surface while superior images form in conditions of a temperature inversion.

The appearance of a puddle of water on the road that disappears when approached is a common sight on many highways. Images of distant cars often appear with these watery looking surfaces. This "highway mirage" occurs because of the temperature gradient that exists near the road. While it is very common in summer, it also occurs in winter. The inferior mirage forms as light rays originating from above the horizon refract through warmer air near the ground (Figure 5.13). The light rays continually change direction until they no longer travel towards the surface, but are propagating away from the ground! The original light rays appear to be originating from a reflecting surface lying on the ground. The watery mirage that appears on the road results from rays of light coming from the clear sky.

Temperature inversions can also generate mirages. Rays passing upward through the inversion refract downward toward the surface and a mirage image appears above the true position of the object (Figure 5.14). Superior mirages can bring objects into view that are below the horizon!

The appearance of a mirage is a function of the temperature gradient. Strong, non-uniform temperature gradients above relatively coldwarm water may cause what appear to be cliffs, or castles, on the horizon. This type of mirage often appears near the Strait of Messina near southern Italy and was named by Italian poets as Fata Morgana (Italian for Morgan the fairy). According to Celtic legend, King Arthur had a fairy half-sister, Morgana, who lived in a crystal palace beneath the water. The Italian poets attributed this complex mirage as her dwelling. The Fata Morgana is an example of a superior mirage.

Halos

A halo, a whitish ring that encircles the sun (or moon), is another optical phenomenon that owes its existence to refraction of light by ice crystals. Different crystal habits, crystal orientations, and solar zenith angles can produce a wide variety of halos. The most commonly observed halo is the 22-degree halo. With a fully extended arm, the 22° halo encircles the sun at about a hand's width from the center of the sun (Figure 5.15). The inner portion of a halo may have a dull, reddish tint, owing to the fact that red light is refracted least.

Small columnar ice crystals will form the 22° halo. Light rays enter a crystal side and refract. Light refracts as it exits the crystal (Figure 5.16). Because the crystals are randomly oriented in space, there are many different directions for which light rays can enter the crystals. More light rays are refracted at this 22° angle than any other, producing the concentration of light we call the halo.

Dispersion

When sunlight passes through a triangular glass prism, it separates into all the colors of the rainbow (Figure 5.17). This distribution of colors results because different colors, defined by their wavelength, refract by different amounts. Red light refracts the least and violet light the most. The longer the wavelength of the light, the smaller the amount of refraction. The separation of colors is referred to as dispersion. You can buy lead crystals to hang in your windows that will cast "rainbows" when a beam of sunlight strikes them and is dispersed by refraction. Cloud particles suspended in air also generate beautiful optical phenomena because of dispersion. An example of this is the rainbow.

The Rainbow

The single, bright, colored arc sometimes observed following a rain shower is a primary rainbow (Figure 5.18). Red is the outermost color of the arc and violet is always the innermost color. On occasion, you may have seen two rainbows at once, the lower rainbow is the primary rainbow and the higher, fainter, colored arc the secondary rainbow. The color sequence of the secondary rainbow is opposite to the primary, red is on the inside of the arc and violet on the outside. To explain the occurrence of rainbows we need to trace rays of light as they enter and leave large drops of water.

Sun's rays that are concentrated by refraction and reflection produce the rainbow. Since the amount of refraction depends on wavelength, each color of light is reflected and refracted at different angles. Figure 5.19 displays the dispersion from the primary and secondary rainbow for red and violet light. Red appears above violet because refraction decreases with increasing wavelength. This explains why the outside of the primary rainbow arc is red and the inner portions are violet. Red light appears below violet in the secondary rainbows, because of the second reflection, explaining why the color sequence of the secondary rainbow is a mirror image of the colors in the primary rainbow. But why is the rainbow circular in shape?

The rainbow is an assembly of different rays of light leaving millions of falling raindrops. A raindrop shape is three-dimensional and light rays that form the rainbow originate from different segments of the myriad of these falling spheres. This gives the rainbow its circular shape. Each portion of the rainbow you view is light scattering by a particular group of raindrops at a particular angle of the sky! Most of the light leaving a particular drop does not reach your eyes. Because red light is scattered at an angle of approximately 138° and violet at approximately 139.7° , the red colors of the primary rainbow come from falling drops that are higher in the sky, and the violet appears from the drops closer to the ground.

Scattering

Halos occur in thin cirrus clouds that cover the sky. Portions of halos can be seen when the sky is partially covered with thin clouds. Halos are not observed in thick cirro-stratus because of multiple scattering. When we say energy is scattered, we mean that it has changed its direction of propagation.

Small particles interact with light differently than larger ones. In discussing halos and rainbows, we followed a ray of light through an ice or water particle. We can do this because the ice particle is much larger than the wavelength of the radiation, 20 m m versus 0.6 m m. For much smaller particles, such as gas molecules, this ray tracing method does not work.

Scattering by particles that are small with respect to the wavelength of the incident radiation is referred to as Rayleigh scattering. Until Lord Rayleigh described scattering by small particles in 1869, the reason for the blueness of the sky was a mystery. While all colors are scattered by air molecules, as demonstrated by Lord Rayleigh, violet and blue scatter most. The sky looks blue, not violet, because our eyes are more sensitive to blue light.

Sunlight passes through more air at sunset and sunrise than during the day when the sun is higher in the sky (Figure 5.20). More atmosphere means more molecules to scatter the violet and blue light. This is why sunsets are often yellow, orange and red (Figure 5.21). If the path is long enough, all of the blue and violet light scatters out of your line of sight, while much of the yellow, orange and red colors continue along the undeviated path between your eyes and the sun.

The sky on a clear day in the afternoon is brightest blue overhead, while near the horizon the sky appears a milky blue. The whitish tint on the horizon is due to the scattering by many more molecules close to the ground, in addition to scattering by haze particles. A single molecule scatters violet and blue light better than red or orange. Air molecules scatter red light, just not as effectively as blue light. Multiple scattering occurs when light interacts with more than one particle. In the presence of many single scattering events, enough light of all colors scatters to combine into white light. The density of molecules is greatest near the ground, thus light traveling across a line of sight near the horizon is scattered by many more molecules than light traveling from directly overhead. With a large number of scattering events, enough red, yellow and orange light is scattered so that when combined with the blues and greens, makes the sky appear whitish. Multiple scattering explains why piles of salt, sugar, and snow crystals appear white, although the individual crystals are translucent. Multiple scattering also explains why haze near the horizon causes the sky to appear whitish and why clouds often look white. Scattered light by clouds is also important in climate change studies (Box 5.3).

The bottoms of clouds appear grayish, sometimes ominously, not because of absorption, but because of multiple scattering. Multiple scattering, the cumulative effect of many single scattering events, re-directs the light out the tops and the sides of the cloud allowing very little light to be transmitted out the cloud base. The base therefore appears dark (Figure 5.22). Halos are diffused in thick cirrostratus clouds through scattering by the multitude of ice crystals.

Satellite Observations

Observations from instruments flown on satellites are used extensively in weather analysis. Weather satellites fly around the Earth in two basic orbits: a Geostationary Earth Orbit (abbreviated as GEO) and a Low Earth Orbit (also called LEO) (Figure 5.23).

Geostationary satellites orbit the Earth as fast as Earth spins and therefore appear to hang over a single point over the Earth. A geostationary satellite must be located over the equator. An instrument in a geostationary satellite does not view the entire earth and has a poor view of the polar regions. The advantage of a satellite in the geostationary Earth orbit is that an instrument has a continuous view of the mid-latitude and tropical regions. Satellites in this orbit are good for weather studies as you can track the movement of the storms. The satellite loops displayed on your favorite weather channel are observations from a satellite in a geostationary satellite. The United States typically operates two geostationary satellites called GOES (Geostationary Operational Environment Satellite). One has a good view of the east coast and the other GOES is focused on the west coast. Other satellites in geostationary orbit include the European METEOSAT (METEOrological SATellite) which views the eastern Atlantic Ocean, Africa and Europe and the Japanese GMS (Geostationary Meteorological Satellite) which has a good view of Asia, Australia and the west Pacific Ocean.

Low Earth Orbiting (LEO) satellites because they orbit the Earth at a much lower altitude than the GEO satellites. LEO satellites that are in a sunsynchronous orbit circle the earth from pole-to-pole. A satellite in this orbit views all regions of the earth in a single day and is good for global weather studies. Sunsynchronous orbits are also referred to as polar orbits as the polar regions are frequently viewed, for example 14 times a day.

In the tropical regions, a LEO satellite flies by a particular region of the earth twice a day¾ for example once in the afternoon and once in the evening. NOAA (National Oceanic and Atmospheric Administration) typically maintains two polar orbiting satellites. One views the United States at approximately 2 p.m. and 2 a.m. local time and the second views regions of the US around 10 a.m. and 10 p.m. local time.

Flown onboard satellites are instruments that measure electromagnetic energy that is either reflected or emitted by our planet. These instruments are called radiometers. Two common types of radiometers are used in satellite meteorology. One type measures the amount of visible light from the Sun reflected back to space by the Earthís surface or by clouds. The second measures the amount of radiation emitted by the surface or clouds.

Interpreting Satellite Images

The radiometers flown on satellites are not cameras although they do produce images. The radiometer uses moving mirrors to view different regions of Earth. The instrument begins at a starting point in one direction and then scans across a scene line-by-line making observations as it proceeds. The observational data is recorded in a two-dimensional format which, when displayed on a computer monitor or television screen, makes up an image. The smallest part of this image is the pixel, which stands for "picture element." The data about the observed radiation represented by each pixel is presented in terms of a grayscale. Each pixel could be one of 256 possible gray shades, ranging from 0 for pure black to 255 for pure white.

Analysis of a satellite image allows meteorologists to locate thunderstorms, hurricanes, fronts, and fog. These weather events can be tracked using time sequences of satellite images, allowing weather forecasters to predict their movement over short time periods. The horizontal wind speed and direction can also be determined by tracking cloud features in a time sequence of satellite images.

Visible Imagery

A visible satellite image represents sunlight scattered by objects on Earth. Differences in the albedo of clouds, water, land, and vegetation allow us to distinguish these features in the imagery. Dark areas in a visible satellite image represent geographic regions where only small amounts of visible light from the Sun are reflected back to space. The oceans are usually dark while snow and thick clouds are bright (Figure 5.24).

The number of water drops or ice crystals the cloud is composed of primarily determines the brightness of a cloud in the visible image. Stratus have lots of particles, thus scatter lots of solar radiation, and appear white in a visible image. Fog is also very easy to see on visible satellite images. On the other hand, thin cirrus are difficult to see because of the few ice particles that scatter the solar radiation.

Infrared Imagery

The infrared radiometers on satellites measure radiation with wavelengths of 10 to 12 m m. For easy interpretation, the radiant energy measured by infrared radiometers is converted to a temperature. So, in infrared radiometric images, cold objects are white and hot surfaces appear black (Figure 5.25). An advantage of the infrared, or IR, satellite image over the visible image is that it is available day and night. Time sequences of infrared images are animated and shown on television news programs.

All objects emit radiation. The emissivity of an object is a measure of how close an object is to a blackbody. The amount of radiation an object emits depends on the object's temperature and its emissivity. With adjustments for differences in emissivity, an IR instrument measures the temperature of land, water, and clouds. IR imagery can be used to distinguish low clouds from high clouds. Low clouds are relatively warm and appear gray in satellite IR images. Thick cold clouds, like the tops of thunderstorms, appear bright white.

The good way to interpret satellite images is to view visible and infrared imagery together. Difference in the solar and infrared properties of different clouds allows us to distinguish different cloud types (Figure 5.26). For example, stratus clouds can be easily separated from thick cirrus. Stratus appear gray in the IR image and bright white in the visible image, while thick cirrus are white in both images. Adding a water vapor channel further enhances our analysis of atmospheric conditions.

Water Vapor Imagery

The infrared radiometers on satellites also measure radiation with wavelengths between 6.5 and 6.9 microns. Images made from these observations are referred to as water vapor imagery. This imagery (Figure 5.27) is a valuable tool for weather analysis and forecasting as it represents flow patterns of the upper troposphere.

Water vapor is transparent to radiation at visible and 10-12 micron wavelengths. This is why visible and IR satellite imagery are used to observe surface features and clouds. However, water vapor is a very efficient absorber and emitter of radiation with wavelengths between 6.5 and 6.9 microns. So, satellite radiometers measuring the amount of radiation emitted by the atmosphere at these wavelengths can be used to detect water vapor in the atmosphere. The water vapor satellite image displays the water vapor concentration in the atmospheric layer between 200 and 500 mb. Black indicates low amounts of water vapor and milky white shows high concentrations. Bright white regions correspond to cirrus clouds. In the middle latitude regions, zones with strong contrast in water vapor amount often indicate the presence of a jet stream.

Radar Observations

Precipitation is an important weather and climate variable. Global, annual average precipitation patterns are an important piece of the atmospheric circulation picture. Precipitation is also very important on smaller time and space scales. A large amount of precipitation over a short time interval poses hazards to transportation and can cause dangerous flooding. One method of measuring precipitation is with a rain gauge. Radar is another means of measuring precipitation.

Radar technology was rapidly developed during World War II to track flying aircraft and the movement of ships. The word radar is the acronym for RAdio Detection And Ranging. Data collected from weather radar are now extensively used to track the development, direction and speed of storms. Radar data are routinely shown on television weather reports as images.

A radar transmitter sends out narrow pulses of radio waves. Precipitation sized particles are large enough to scatter radio waves so they can be detected by radar (Figure 5.28). Radio waves are one type of electromagnetic energy. Some of the scattered radar waves are scattered back to the transmitting point and can be detected. The received signal is called the radar echo. The radar echo indicates how far away the rain is from the radar and the intensity of the precipitation.

The time the radar signal takes to reach the precipitation and travel back to the radar station determines how far away the rain is. The direction of the precipitation is simply determined from the direction the radar is pointing.

The intensity of the radar echo indicates the intensity of the precipitation, which is the amount falling (e.g., millimeters of rain per hour). Relatively high amounts of returned energy indicate high rainfall rates. The amount of energy scattered is proportional to the size of the particle and the concentration of particles. Large particles and high concentration of particles both imply high rainfall rates.

The radar echo is displayed in a color image (Figure 5.29). Colors represent the amount of energy scattered back to the radar site, or the reflectivity. High reflectivities are colored red and indicate high precipitation rates. Radar reflectivity is measured in decibels (abbreviated dBZ). Radar reflectivity images usually represent radar reflectivity measured in dBZ and not rainfall amount.

A map summarizing all the national weather radar data (Figure 5.30) allows meteorologists to quickly determine regions of precipitation. When overlaid with satellite imagery, meteorologists can determine the type of weather system producing the precipitation.

Doppler Radar

Doppler radar is used to detect precipitation and rotation of a thunderstorm. Radar can also be used to measure how fast the raindrops are moving relative to one another. Just as important as the relative speed, Doppler radar determines the direction of the precipitation, which is away from or towards the location of the radar. Before addressing why this is important, let's see how this works.

Sound travels as a wave. As with all waves, a particular sound is characterized by its frequency¾ the number of waves passing a given location in a given amount of time. As you listen to a police or ambulance siren approach and then pass by, the sound you hear changes; the pitch is higher (the frequency increases) as the sound approaches you and then lowers as it moves away (Figure 5.31). This is known as the Doppler effect, named after Johann Christian Doppler who explained this phenomenon.

The Doppler effect is a shift in the frequency of electromagnetic waves (or sound waves) that arise from a moving source. A Doppler radar measures the relative direction of particles that move towards or away from the radar as the radar waves are scattered by the particles (Figure 5.32). If a radio wave is scattered by two particles, one moving towards you and one moving away, the scattered energy from the two particles has a shift in the frequency. This shift is an indication of the relative speed of the two particles. Doppler radar can only measure the relative speed in terms of moving towards or away from the antenna.

Relative motion of cloud particles measured by a Doppler radar is color coded and displayed in an image for quick analysis by a forecaster (Figure 5.33). Typically the cool colors (greens and blues) represent motion towards the radar and the warm colors (reds and yellows) indicate motion away from the radar. Warm colors next to cool colors indicate rotation within the storm. Figure 5.33 is a radar display in Doppler mode (also known as velocity mode) of a thunderstorm that spawned a tornado and caused destruction in Oklahoma City, OK in May 1999. An arrow marks the region likely to have a tornado, where over a small distance the winds are traveling towards the radar and then away from the radar.

Wind Profiles

A recent application of Doppler technology is the wind profiler. Wind profilers are a modern way to routinely measure the wind aloft using Doppler radar. Dust, molecules, insects, and turbulent eddies that move with the wind scatter the radar beams transmitted by the wind profiler. As these targets move toward or away from the wind profiler, the returning radar pulse changes frequency. By measuring the change in the frequency of the returned radar beam, we can determine the wind speed of the air. This provides continuous measurement of vertical wind at a given location.

Summary

Observations of the atmosphere are an important part of understanding weather and climate. Each day thousands of observations of the weather are made across the world. The instruments used to make these measurements have become standard tools in meteorology. To provide more observations of the atmosphere, the United States weather service uses automated weather observations, such as ASOS. ASOS is designed to support aviation operations and weather forecasting by replacing manual surface observation techniques.

Light is electromagnetic energy with wavelengths between 0.4 and .7 microns. Laws of reflection, refraction, and scattering describe the occurrence of many observed wonders. Reflection occurs at boundaries. When radiation is reflected, it is turned back into the medium through which it originated. In reflection, the angle of the reflected ray equals the incident angle. Sun glint and sparkling snow result because of reflection.

Refraction is the process in which the direction of energy propagation is changed due to changes in the index of refraction. Mirages are examples of refraction that arise because of a strong gradient in the index of refraction that results from a steep temperature gradient near the earthís surface. Refraction is wavelength dependent. In traveling between air and water, red light refracts less than blue. Halos result from the refraction of sunlight.

A rainbow is an example of refraction and reflection of light through drops of water. As a ray of light enters and leaves the drop, some of the energy is refracted and some is reflected. The primary rainbow is produced by light that undergoes two refractions and one reflection. The secondary rainbow requires two refractions and two reflections inside the drop. The colors appear because of dispersion of the sunlight. The secondary rainbow is fainter and wider than the primary rainbow. Because of the two reflections, the colors of the secondary rainbow are reversed from the primary, red being on the inside of the arc.

Satellites are extensively used to observe the atmosphere. The three most common satellite images are the visible, the infrared and the water vapor images. The visible images provide detail on cloud structure but are not available during the night. The infrared satellite images provide daily images and are useful for determining cloud top temperature and thus cloud altitude. The water vapor image provides an analyst with information on the atmospheric structure of the upper atmosphere.

Radars (for RAdio Detection And Ranging) are used to locate regions of precipitation and the intensity of the precipitation. Radars must be capable of transmitting and receiving electromagnetic radiation. Weather radars measure the amount of electromagnetic radiation that is scattered by large water and ice particles, and convert this energy into information about the precipitation. Doppler radars also can provide information on the wind patterns within a storm by tracking the movement of precipitation particles. Chapter 11 will describe how radars can be used to identify regions of a thunderstorm that might develop into a tornado.

Terminology

You should understand all of the following terms. Use the glossary and this Chapter to improve your understanding of these terms.

Anemometer

Barometer

Ceilometer

Cloud ceiling

Dispersion

Doppler effect

Doppler radar

Halo

Hygrometer

Index of refraction

Inferior mirage

Law of reflection

Light

Mirage

Meteorogram

Psychrometer

Radar

Radiosonde

Rain gauge

Reflectance

Refraction

Rainbow

Rawinsonde

Satellite IR image

Satellite visible image

Satellite water vapor image

Scattering

Sounding

Superior mirage

Thermometer

Wind profiler

Wind vane

 

Review Question

  1. Why should temperature measurements be made in a shady and ventilated location?
  2. Describe reflection and refraction.
  3. Explain how refraction lengthens our day.
  4. Explain the similarity and differences between mirages and images in a mirror.
  5. Explain the difference between an inferior and superior mirage.
  6. Explain refraction, reflection and scattering.
  7. Why would continental cumulus appear brighter than a maritime cumulus of the same water content?
  8. Can moon-light produce a rainbow?
  9. Explain why clouds are white, though they are composed of water drops which are transparent.
  10. Visit an art gallery to determine if rainbows, halos, or other optical phenomena are correctly drawn. For example, are the colors in the correct order?
  11. What are the similarities and differences between an infrared satellite image and a water vapor image?
  12. Describe how a radar works.
  13. How is dew point measured?
  14. How is wind speed and direction measured?
  15. What is a sounding?
  16. Why is it important to have automated weather observations?

Web Activities

Tracing rays through a raindrop

Photogallery of optical phenomena

Practice analyzing Meteorograms

Identify clouds on satellite images

Identify precipitation on radar images

Practice multiple choice exam

Practice true/false exam

 

Box 5.1 The Meteorogram

The station model is used by meteorologists to analyze the spatial distribution of weather at a given time. The meteorogram allows a meteorologist to analyze how meteorological variables vary in time at a given location. The accompanying figure is an example of a meteorogram for Madison, WI on August 5 and 6, 2000. Time, in UTC, runs along the x-axis. There are different weather parameters plotted as a function of time. The top portion of the figure lists visibility in miles. Below that are precipitation in inches and current weather conditions, using the key discussed in Chapter 1. Wind speed (in knots) and direction are also shown, along with peak wind gusts. Cloud base altitude and cloud coverage is plotted in the second panel from the top, and pressure below. The bottom panel plots temperature and dew point temperature.

Clouds were present during most of this time period. Precipitation occurred through most of the evening on Aug 5, with a thunderstorm at 2100 UTC. At 0000 UTC on August 6, 1.5 inches of precipitation were measured. The rain gave way to drizzle at 0300 UTC on August 6, and fog sets in by 0500 UTC. Notice that the winds were very light during periods of fog and visibility was reduced. The dew point and temperature were equal during rainy and foggy periods. Near sunrise on August 6 at 1200 UTC, the temperature increased as the sun warmed the Earth and lower atmosphere. The dew point temperature also increased, probably because dew was evaporating into the air raising the dewpoint. The fog turned to haze at 1600 UTC and lifted by 1700 UTC on August 6, 2000.

Box 5.2 Twinkle, twinkle little star.....

While we often assume that the atmosphere is uniform in structure, it is in reality always varying. As star light, considered to be a point source of light, traverses through the atmosphere it continually undergoes small deviations in its direction of travel due to differences in the temperature, density and moisture content in the air it travels through. These departures cause rapid changes in the apparent position or brightness of the star making it appear to twinkle.

As small perturbations in atmospheric structure are often caused by turbulence, we can consider how a parcel of air, whose index of refraction is different from its environment, that moves into our line of sight would affect a starís appearance. Because of refraction, when the ray encounters the air parcel, the star light will appear to come from position B instead of its true position, A. As the parcel moves out of the line of site, the next ray of starlight may encounter another parcel with different index of refraction, changing the apparent position of the star. If the refraction is strong, the star may fade or even disappear for a moment!

The twinkling of stars is referred to as astronomical scintillation. It is most apparent on clear, cold, and windy nights. The effect is greatest for stars near the horizon as they pass through more atmosphere and are therefore likely to encounter a larger number of air parcels with different indices of refraction.

Each ray of starlight travels through the atmosphere along a slightly different path than the preceding and following ray, causing the star to twinkle. The circle represents a pocket of air moving through the atmosphere, with an index of refraction different then its surroundings.

Box 5.3 Multiple Scattering and Climate Change

Energy gains and losses are important in climate. Clouds modify the energy budget of the atmosphere and the earthís surface. The effect of clouds on climate not only depends on how much cloud there is but is also dependent on the size of the particles composing the cloud. Take a piece of glass and smash it. Group the large particles separately from the small ones. The pile of small pieces, because of multiple scattering, appears brighter and whiter than the larger ones. This also happens in clouds. If the amount of water in two clouds is the same, but one cloud contains very large drops and the other very small drops, the cloud containing the small particles will appear brighter, just like the two masses of glass pieces. Multiple scattering therefore has implications for climate and climate change.

If the average particle size of a cloud were to suddenly become smaller, the cloud would become brighter and more solar energy would leave the top of the cloud and then the atmosphere. That energy would not be available to warm the ground or ocean. The average size particle in clouds is reduced if the number of cloud condensation nuclei (CCN) increases. By increasing the CCN, the water content of the cloud is distributed over more particles so the average size of the particles decreases.

In theory, changing the number of particles existing in the atmosphere could modify the global energy budget and therefore climate.