Winds are moving currents of air, part of a convective process. We usually limit the term "wind" as a name for currents that are mostly horizontal, but in fact it is not useful to consider the horizontal movements without also including the updrafts and downdrafts that complete the convection current.

When light, low pressure air begins to rise because it is less dense than the air above it, cooler, higher pressure air will flow across the earth's surface to replace it. We can think of the process as involving two zones of air, one of sinking high pressure air and the other of rising low pressure air. Surface winds always move from the high pressure zone towards the low, but because the movement is affected by the rotation of the earth, winds always follow a somewhat circular path to their destination. Consider the rising current in the low pressure zone. The upper atmosphere rotates faster than the lower. (It has to because it has farther to travel in 24 hours.) Because this effect is stronger above the equator that it is above the poles, a rising (or a falling) current in the northern hemisphere feels this effect more strongly along its southern margin.

Drag or the action of friction between the rising air and the air next to it causes the current to circulate in a counter clockwise fashion. Winds around northern hemisphere lows always follow a counter clockwise path. The opposite occurs in the sinking current of a high. The southern edge of the high is slowed more rapidly than the northern edge and so it drags the winds into a clockwise rotation around the high.

These circulation patterns can be used to predict weather. Because our weather comes from the west, a northwest wind means that a high pressure with cool, clear air is approaching. A southwest wind heralds a low pressure system with storms. It also allows weather forecasters to predict the severity of a winter storm by tracking the likely path of a low. A low passing to the south of Philadelphia can pick up lots of moisture from the Atlantic while northeast winds keep the temperature in Philadelphia below freezing. These are the storms that can bring a lot of snow. Storms passing to the north of Philadelphia bring mostly rain because the moisture is carried to the city by south and southeast winds.

Winds, because they are a convective process, are controlled by pressure gradients resulting from differences in air density. Warm, moist air is low density air and so creates zones of low pressure. These typically form over tropical or subtropical oceans. High pressure air is cold dry air that develops over arctic and sub-arctic continents. The velocity of the wind that moves from the high to the low is controlled by the pressure difference that develops and the distance between the two air masses. The greater the difference and the closer the two masses the stronger the winds. This difference can be measured as a pressure gradient, a measure of the pressure difference divided by the distance. Locally wind speed can also be affected by the upward movement of air in the center of the low. This is an important factor in powerful storms like hurricanes, tornadoes and thunderstorms. The faster the air moves up, the faster the winds must blow into the center of the low to replace it. The effect of a rapid updraft is to create extremely low pressure in the center of the low causing a large pressure gradient. Moisture in the air enhances upward movement in the low. As the rising air cools, water vapor condenses releasing potential energy as heat. This slows the adiabatic cooling of the air. The rising air remains warmer and so rises faster than it would if it had not been warmed by condensation.

Thinking in systems terms, it is possible to describe the process controlling wind speed as relating to the variables wind speed, pressure gradient, humidity, temperature gradient and velocity of updrafts in the low. Important processes would be convection, evaporation, condensation, wind and mixing of air masses, and the heating/cooling of air. Some of these variables and the processes that control them are included in this flow chart. A second flow chart shows the effect of updraft velocity on wind speed as an indirect effect through pressure gradient. Both flow charts are accurate. The first one was included to remind the reader that the updraft velocity is more of a local effect within the low while temperature and humidity differences between air masses is a contrast between high and low pressure systems.

A key point in the flow chart is the potential for winds to either decrease or increase the water vapor content (humidity) of the low pressure air. One would expect winds, because they are mixing the high and low pressure air, to reduce the water content of air and that is true when the winds blow over land or over very cold oceans. However, if the wind is blowing over a warm ocean, it will warm and draw water into the air by evaporation. When this happens, the wind can continually replace moisture that the low pressure air is losing to condensation and rain. In the graphical analysis of the system we will have to consider that winds can effect the pressure gradient in a variety of ways from simply decreasing the gradient by mixing air to increasing the gradient by adding lots of water vapor.

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