1. Air Pressure and Winds Dr. R. B. Schultz

2. Air Pressure Air pressure is the pressure exerted by the weight of air above. Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch. Another way to define air pressure is that it is the force exerted against a surface by the continuous collision of gas molecules.

3. Units and Instruments of Pressure A millibar (mb) equals 100 newtons per square meter. Standard sea-level pressure is 1013.25 millibars. Two instruments used to measure atmospheric pressure are: –the mercury barometer, where the height of a mercury column provides a measure of air pressure (standard atmospheric pressure at sea level equals 29.92 inches or 760 millimeters), and –the aneroid barometer, which uses a partially evacuated metal chamber that changes shape as air pressure changes.

4. mercury barometer

5. aneroid barometer

7. The Ideal Gas Law In still air, the two factors that largely determine the amount of pressure that a particular gas brings to bear are: temperature and density. The relationship that exists among these variables, called the ideal gas law, can be shown by the expression: pressure=temperature x density x constant. A change in any one of the variables will cause a predictable change in the others. The pressure at any given altitude is equal to the weight of the air above that point. Furthermore, the rate at which pressure decreases with altitude is much greater near Earth's surface. The "normal" decrease in pressure experienced with increased altitude is provided by the standard atmosphere, which depicts the idealized vertical distribution of atmospheric pressure.

10. Wind Wind is the result of horizontal differences in air pressure. If Earth did not rotate and there were no friction, air would flow directly from areas of higher pressure to areas of lower pressure. However, because both factors exist, wind is controlled by a combination of: –(1) the pressure-gradient force, –(2) the Coriolis force, and –(3) friction.

11. Pressure-Gradient Force and Pressure Maps The pressure-gradient force is the primary driving force of wind that results from pressure changes that occur over a given distance, as depicted by the spacing of isobars, lines drawn on maps that connect places of equal air pressure. The spacing of isobars indicates the amount of pressure change occurring over a given distance, expressed as the pressure gradient.

13. Hydrostatic Equilibrium Closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate a weak pressure gradient and light winds. The sea breeze is a common example of wind generated by a horizontal pressure gradient. There is also an upward directed, vertical pressure gradient which is usually balanced by gravity in what is referred to as hydrostatic equilibrium. On those occasions when the gravitational force slightly exceeds the vertical pressure gradient force, slow downward airflow results.

14. Coriolis Effect The Coriolis force produces a deviation in the path of wind due to Earth's rotation (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). The amount of deflection is greater at 60° latitude than at 40° latitude, which is greater than at 20°. Furthermore there is no deflection observed for the airflow along the equator. The amount of Coriolis deflection also increases with wind speed. Friction, which significantly influences airflow near Earth's surface, is negligible above a height of a few kilometers. In airflow above a few kilometers, where the effect of friction is small enough to disregard, as the speed of the wind increases the deflection caused by the Coriolis effect also increases.

18. Geostrophic Winds Winds in which the Coriolis force is exactly equal and opposite to the pressure gradient force are called geostrophic winds. Geostrophic winds flow in a straight path, parallel to the isobars, with velocities proportional to the pressure-gradient force. Winds that blow at a constant speed parallel to curved isobars are termed gradient winds. In centers of low pressure, called cyclones, the circulation of air, referred to as cyclonic flow, is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Centers of high pressure, frequently called anticyclones, exhibit anticyclonic flow which is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Whenever isobars curve to form elongated regions of low and high pressure, these areas are called troughs and ridges, respectively.

20. Cyclones and Anticyclones Near the surface, friction plays a major role in redistributing air within the atmosphere by changing the direction of airflow. The result is a movement of air at an angle across the isobars, toward the area of low pressure. Therefore, the resultant winds blow into and counterclockwise about a Northern Hemisphere surface cyclone. In a Northern Hemisphere surface anticyclone, winds blow outward and clockwise. Regardless of the hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.

26. Fair and Stormy Weather A surface low-pressure system with its associated horizontal convergence is maintained or intensified by divergence (spreading out) aloft. Inadequate divergence aloft will weaken the accompanying cyclone. Because surface convergence about a cyclone accompanied by divergence aloft causes a net upward movement of air, the passage of a low pressure center is generally related to unstable conditions and stormy weather. On the other hand, fair weather can usually be expected with the approach of a high-pressure system. As the result of these general weather patterns usually associated with cyclones and anticyclones, the pressure tendency or barometric tendency (the nature of the change of the barometer over the past several hours) is useful in short- range weather prediction.

28. Wind Measurements Two basic wind measurements, direction and speed, are important to the weather observer. Wind direction is commonly determined using a wind vane. When the wind consistently blows more often from one direction than from any another, it is called a prevailing wind. Wind speed is often measured with a cup anemometer.

29. Key Terminology Air pressureMercury barometerAneroid barometer Ideal Gas LawStandard atmospherePressure-Gradient Force Coriolis ForceIsobarsPressure gradient Hydrostatic equilibriumGeostrophic WindsCyclones AnticyclonesGradient windsTroughs RidgesConvergenceDivergence Pressure tendencyWind vanePrevailing wind Cup anemometer