Download Meteorology Chapter 6 – Air Pressure and Winds Air pressure – the

Meteorology Chapter 6 – Air Pressure and Winds Air pressure – the pressure exerted by the weight of air above. • Air pressure is exerted all around an object on Earth’s surface or in the atmosphere. That is, air pressure is exerted in all directions. Measuring Air Pressure • At sea level, the standard atmosphere exerts a force of 101,325 newtons per meter2. • The National Weather Service uses the unit “millibar’ to measure air pressure. ⇒ At sea level, atmospheric pressure equals 1013.25 millibars. • TV meteorologists often describe air pressure in inches. ⇒ This is because they use a mercury barometer to measure air pressure. ⇒ The mercury barometer uses a column of mercury in a glass tube as an indication of air pressure. ⇒ At sea level the mercury will rise to a height of 760 millimeters, which equals 29.92 inches. • An aneroid barometer measures air pressure without using a liquid. ⇒ The aneroid barometer uses a partially evacuated metal chamber that is very sensitive to variation in air pressure. It changes shape, compressing as the pressure increases and expanding as pressure decreases. Pressure Changes with Altitude ‐ The pressure at any given altitude in the atmosphere is equal to the weight of the air directly above that point. • As we ascend through the atmosphere, the air becomes less dense because of the lesser amount of air above. Consequently, there is a decrease in pressure with an increase in altitude. • The rate at which pressure decreases with altitude is not a constant. ⇒ The rate of decrease is much greater near Earth’s surface where pressure is high. ⇒ The standard atmosphere model depicts the idealized vertical distribution of atmospheric pressure. ⇒ According to the model, atmospheric pressure is reduced by approximately one‐half for each 5‐
kilometer increase in altitude. Influence of Temperature and Water Vapor ‐ In general, cold air is composed of comparatively slow‐
moving gas molecules that are packed closely together. • So, the density of this cold air increases and so will the pressure that it exerts. ⇒ This mass of cold air would be said to have high barometric pressure. • In contrast, in a mass of warm air the molecules would be farther apart, the air less dense, and the mass would produce low barometric pressure. • The amount of water vapor present in a volume of air influences the air’s density. ⇒ This is because a water molecule has less mass than a gas molecule of nitrogen or oxygen. • We can conclude that a cold, dry air mass will produce higher surface pressures than a warm, humid air mass. ⇒ Also, a warm, dry air mass produces higher pressure than an equally warm, but humid air mass. Airflow and Pressure • The movement of air can cause variations in air pressure: ⇒ Convergence – where there is a net flow of air into a region, as a result the air “piles” up. Results in a “taller” and heavier air column that exerts more pressure. ⇒ Divergence – a region where there is a net outflow of air Results in a drop in surface air pressure. Factors Affecting Wind • Wind – the horizontal movement of air which is the result of horizontal differences in air pressure. ⇒ Air flows from areas of high pressure to areas of low pressure. ⇒ Wind is nature’s attempt to equalize differences in air pressure. • If Earth did not rotate and if there were no friction, air would flow directly from areas of higher pressure to areas of lower pressure. ⇒ Because both factors exist, wind is controlled by a combination of force including: 1. The pressure‐gradient force 2. The Coriolis force 3. Friction Pressure‐Gradient Force ‐ When air is subjected to greater pressure on one side than on another, the imbalance produces a force that is directed from the region of higher pressure toward the area of lower pressure. • Pressure differences cause the wind to blow • The greater the differences in the pressure, the greater the wind speed • Pressure data are shown on surface weather maps by means of isobars. ⇒ Isobars are lines connecting places of equal air pressure. ⇒ The spacing of the isobars indicates the amount of pressure change occurring over a given distance and is expressed as the pressure gradient. ⇒ Closely spaced isobars indicate a steep pressure gradient and strong winds ⇒ Widely spaced isobars indicate a weak pressure gradient and light winds. The Coriolis Force ‐ If the only force involved in the horizontal movement of air, wind would cross the isobars at right angles due to the pressure‐gradient force. • That is not the case; winds cross the isobars at non‐right angles due to the Coriolis force. • All free‐moving objects, including wind, are deflected to the right of their path of motion in the Northern Hemisphere and the left in the Southern Hemisphere. • The magnitude of the Coriolis force is dependent on latitude. It is strongest at the poles, and weakens as you move toward the equator. • The amount of Coriolis deflection increases with wind speed. This is because faster winds cover a greater distance than do slower winds in the same time period. Geostrophic Winds – Are winds created when the Coriolis force is exactly equal and opposite to the pressure‐gradient force. • These winds flow in a straight path parallel to the isobars, with velocities proportional to the pressure‐gradient force. • A steep pressure gradient creates strong winds, and a weak pressure gradient produces light winds. • Buy Ballot’s Law: In the Northern Hemisphere if you stand with your back to the wind, low pressure will be found to your left and high pressure to your right. •
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Winds around cells of high or low pressure follow curved paths in order to parallel the isobars. ⇒ Winds of this nature, which blow at constant speed parallel to curved isobars, are called gradient winds. It is common practice to call all centers of low pressure cyclones and the flow around cyclonic. Cyclonic flow has the same direction of rotation as Earth: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Centers of high pressure are frequently called anticyclones and exhibit anticyclonic flow. Whenever isobars curve to form elongated regions of low and high pressure, these areas are called troughs and ridges. The flow about a trough is cyclonic; the flow around a ridge is anticyclonic. Friction as a factor affecting wind is important only within the first few kilometers of Earth’s surface. • Friction slows winds speed and as a result reduces the Coriolis force. So, the movement of air is at an angle across the isobars, toward the area of low pressure. • In whatever hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone. Vertical Airflow Associated with Cyclones and Anticyclones ‐ Vertical movement is usually very small when compared with horizontal airflow, but the vertical movement of air is important as a weather maker. • Rising air is associated with cloudy conditions and precipitation, whereas subsidence produces adiabatic heating and clearing conditions. ⇒ In a surface low pressure system, air is spiraling inward, and the net inward transport of air causes shrinking of the area occupied by the air mass in a process called horizontal convergence. ⇒ As the air converges horizontally it “piles up”, or increase in height. ⇒ The taller and heavier column of air above the should destroy the surface low pressure, but if the air diverges, or spreads out, at elevation at a rate equal to the horizontal inflow, then low pressure is maintained. • The opposite occurs at an area of high pressure. At the surface airflow is away from the center of high pressure (divergence), but at elevation, air is converging and the net result is the area of high pressure is maintained. • In summary: Air is rising in areas of low pressure promoting the growth clouds and chances of precipitation. The opposite is true of areas of high pressure; the air is sinking and warming so skies should be clear. Wind Measurement ‐ Two basic wind measurements—direction and speed—are important to the weather observer. • Winds are always labeled by the direction from which they blow; example: a north wind blows from the north to the south. • A wind vane is one instrument that is commonly used to determine wind direction. • A prevailing wind is a wind that blows more often from one direction than from any other. The prevailing westerlies in the United States consistently move the “weather” from west to east. • Wind speed is often measured using a cup anemometer, although an aerovane can be used to determine wind direction and speed.