Determining Atmospheric Winds using Satellite Observations
Magnitude and Direction of Forces
Weather results from the interaction of many forces. When discussing these interactions we are concerned with the direction and magnitude, or strength, of the force exerted on an object. These two factors, represented in diagrams by arrows, determine what effect the force has on the object. The arrow points in the same direction as the force. The length of the shaft represents the magnitude.
Two or more forces can act to pull or push at the same point on a body. These forces can be replaced by a single force, the net force or resultant, which reproduces the same effect on the body as the separate forces. Suppose we apply two forces to a body that will only act to move the object. If the two forces act in the opposite direction and with different magnitudes, the object will move in the direction of the stronger force (as in a tug-of-war). These two forces can be represented by a single force whose direction is the same as the stronger force and whose magnitude is the difference between the two forces.
Forces often act at an angle to each other. In this situation we need an approach to determine the magnitude and direction of the net force. One way is to graphically construct a parallelogram using two forces to represent two of the sides. The diagonal of the parallelogram represents the net force of the two given forces. The length of the diagonal represents the magnitude of the combined forces. The direction of the net force is along the diagonal and away from where the two forces are applied.
Laws of Motion
Applying a force often results in movement. When an object is changing its position with reference to another body, we say it is in motion. As with force, motion has a magnitude and a direction. The speed of the object, the distance traveled in a given amount of time, is the magnitude of the motion.
Wind is air in motion relative to the ground. Weather reports include wind speed and direction. The average wind speed and average wind direction are usually reported. Wind speed is reported on weather maps in knots--one nautical mile per hour (equivalent to 1.1508 statute miles per hour or 0.5144 meters per second). If the wind speed is strong (greater than 15 knots) and highly variable, the weather report will include wind gusts, the maximum observed wind speed.
Wind direction is reported as the direction from which the wind is blowing. It is reported with respect to compass directions or the number of degrees east of north. A north wind blows from the north to the south . Windward refers to the direction the wind is coming from, while leeward denotes the direction the wind is blowing towards. The prevailing wind direction of a region is the most frequently observed wind direction during a given period of time.
Wind that changes speed or direction has undergone an acceleration. If a body increases its speed at a constant rate, it undergoes a uniform acceleration. A free falling body is a good example of an object that undergoes uniform acceleration. If we neglect air resistance, a falling body undergoes an acceleration of 9.8 meters per second for each second it falls. The object's velocity increases 9.8 meters per second after each passing second. This downward acceleration results from the earth's gravitational pull and is called the acceleration of gravity. Galileo discovered that the acceleration of two falling object is the same, even if their weights are very different. Astronaut David Scott demonstrated this on August 2, 1971 by dropping a hammer and a feather at the same moment while on the moon. The moon has no atmsophere so there was no significant friction from air which would affect the fall of the feather more than the hammer in our atmosphere. The objects hit the ground at the same time, though the acceleration was less than 9.8 meters per second per second.In the seventeenth century Sir Isaac Newton established the three fundamental laws which describe the motion of bodies: the law of inertia, the law of momentum, and the law of reaction.
Newton's first law of motion states that a body at rest tends to stay at rest while a body in motion tends to stay in motion traveling at a constant speed and in a straight line until acted upon by an outside force. If a bus suddenly starts the passengers lurch backward. The passengers are initially at rest and want to remain at rest when the bus first starts. The moving bus is exerting a force that eventually gets the passengers moving at the same speed. If the bus suddenly stops, the passengers surge forward. They wantend to remain in motion. If the bus makes a quick, sudden right turn the passengers, who want to continue traveling in a straight line, are crammed towards the left side of the bus. The resistance of an object to changing its velocity is called inertia.
Air at rest will tend to stay at rest until a force puts it in motion.
The momentum of an object is its mass multiplied by its velocity. Two objects can have the same speed, but the one with more mass has greater momentum. Newton's second law states: When a force acts on a body, the body's momentum is changed by an amount that is proportional to the applied force and the amount of time the force acts on the body. Hitting a baseball with a bat is good example of this law. If you want to hit the ball far, you have to swing the bat hard to increase the force applied to the ball on contact. You also want to follow through with your swing to keep the bat in contact with the ball as long as possible.
Applying a force to an object changes its momentum by changing the speed at which it travels. A light breeze will not move a sailboat as fast as a strong steady wind. Back to our bus for a final example, as the bus accelerates, its momentum, and the momentum of the passengers inside, increases.
Firing a gun moves a bullet forward, but there is an equal force in the backward direction referred to as the 'kick.' Newton's third law states that for every action (force) there is an equal and opposite reaction (force). When a cup of coffee is placed on a table, a downward force is exerted on the table because of gravity. The table exerts an equal and opposite force in order to support the cup.
Forces that Move the Air