Air Pressure and Related Wind

By KD1LD

First let's consider pressure the result of air's weight. At the bottom of the earth's atmosphere sea level is the greatest with the standard value of twenty nine point nine two inches of mercury. It increases with a gain in altitude because of this correlation between altitude and pressure. The altitude my be determined by measuring the pressure. Pressure changes in the horizontal plain because the earth is unevenly heated. Since the earth's temperature is constantly changing, the density of the air above the earth is also constantly changing. This unceasing pressure is one of the basic factors of the weather. As a result of uneven heating there is a primary pressure set up in the earth's atmosphere. The intense heating and convergence near the equator causes the air to rise. Creating a low pressure belt that extends around the earth's surface in that region. Some air cools and descends near thirty degrees north latitude, some continues to the polar regions and descends. As a consequence, higher pressure is built up in both of those general areas. However, at sixty degrees north latitudes polar air moving southward over the earth and converges with the warm air moving north. The warmer air rises. The result is relativity lower pressure in the area. Pressure and pressure changes are measured with barometers the Macurail and Anaroid are the types commonly used. The value of the pressure is expressed simply as the height of the column of mercury in the tube. The same unit has bean adapted to anaroid type, thus standard pressure at mean sea level may be expressed as 29.92 inches of mercury. Another measuring unit is the millibar. 29.92 inches of mercury is equal to 1013.2 millibars. It would be well to remember that all pressure values on the surface pressure map are corrected to mean sea level. Lets make an example. The pressure at Ma solar Montana is 1032.9 millibars, Union South Dakota is 1026.1 millibars, Teahouse Indiana is 1011.2 millibars and Hatters North Carolina is 1023.0 millibars. In measuring the pressure at numerous points over a large area we can produce a picture of pressure system as they exist at any particular time. Now lets talk about iceobars. Simply stated iceobars are lines drawn through points of equal pressure. This data comes from weather stations and other sources. Iceobars then will show areas of relative high and low pressures. The high pressure symbol is a blue H for high and a red L for low pressure. The spacing of iceabars may vary depending on the scale of the map in use. One that is commonly used is four millibars. The change of pressure per unit of horizontal distance is called the pressure gradient. In this area the pressure has changed four millibars over a hundred miles distance on the horizontal plain. The pressure gradiant then in this area is four millibars per one hundred miles. The pressure gradient is the change in pressure with distance from high to low and perpendicular to the iceobars. Iceobars spaced closely indicate a strong pressure gradiant and conversely iceabars spaced widely on the map indicates a week pressure gradiant. It can be stated then that closely spaced iceabars or Strong pressure gradeant are closely associated with low pressure areas and relatively strong wind. While widely spaced iceabars are week pressure gradient and are more associated with high pressure areas and lighter wind. Pressure patterns are frequently elongated and produce a pressure ridge also known as a pressure trough. Frequently but not always cold fronts and warm fronts with there associated weather lie along these troughs. As fronts
and pressures systems move the pressure changes. The nature of the change and the degree of the change is reported by weather stations as the pressure tendency. This station shows a current pressure of 1017.4 millibars and the last three hours the pressure has bean falling steadily. The amount of change in the last three hours was a decrease of 1.2 millibars. This tenancy is important to the metrologist in forecasting the weather. We have bean talking about pressure changes on a horizontal plain, measured vertically pressure is much greater. It will be noted that with altitude the rate of pressure decreases rapidly. By the time the altitude of 1800 feet has bean reached the pressure has decreased almost fifty percent. Now lets see how the vertical pressure gradediant varies with the temperature within the column of air. In the standard atmosphere 850 millibar level is at five thousand feet, And the 700 millibar level is at an altitude of ten thousand feet. In cold air the density of the air has decreased and the iceobarric surface is lower thus the vertical pressure gradient has increased. In warm air the density of the air is decreased and the iceobaric surface is higher Thus the vertical pressure gradient has decreased. Because of this variation and the vertical pressure
gradient there is an error in aircraft altimeter called temperature error.

The horizontal pressure gradient force is the primary force that causes wind to flow. With this force alone the wind would flow quite predicably from the center of high pressure areas to low pressure centers. However, there are other forces acting on wind flow that modify the speed and direction. Among these are the corieolis force and friction force. The corieolis force is an apparent deflected force brought about by the rotation of the earth and does not change the wind speed. This apparent force couses a deflection of the wind to the right in the northern hemisphere. Wind produced by the pressure gradedint force and the collious force alone will be parallel to straight iceobars. The friction force decreases the wind speed and intern decreases the cillois force and changes the wind direction. The resulting wind flow in the lower two thousand feet flows at an angle of approximately thirty degrees across the iceabars from high pressure to low pressure. At and above two thousand feet the wind flow remains parallel to the iceabars at the respected levels. A parcel result of these forces near the surface has caursed the wind to flow across the iceabars out of highs. We call this divergence. As the wind diverges from a high pressure, the air near the center sinks. The subsiding air is heated by compression. This dries the air near the center of the high creating a sky clear of clouds. Because of this we generally associate good weather with higher pressures. In low pressure areas, the air flows inward and convergence. As the wind flows into a low pressure system the air rises to compensate for the convergence. As the air rises it expands and cools do to decreasing pressure. This cooling brings about saturation of the air and the formation of clouds. This frequency produces perspiration.

High pressure systems are called anticyclone. Low pressure systems are called cyclones. There are some pressure and penomena that requires special attention. The hurricane is a low pressure that develops over the water in low latitudes. As the air moves into the low and rises, clouds are form and release laten heat energy. The low intensifies resulting in more convergence, lift, condensation, and energy. It seems to feed on itself as it gains momentum. Wind speeds very from seventy five to one hundred and fifty miles an hour and sometimes even stronger. Tornadoes are the most violent type storm and are associated with intense cold fronts. And other times they are found in the southeast quadrant of the low pressure areas, or in the northeast quadrant of hurricanes

Tornadoes are extremely intense, but highly localized low pressure areas. They descend from
clouds as swelling funnels.

The jet stream is a meandering channel of high velocity --wind generally found between fifteen thousand and forty thousand feet. It's altitudes vari along it's course which is usually west to east but not always direct. Wind speeds of one hundred fifty to two hundred miles an hour sometimes even greater.