Wind Essays - Atmospheric Dynamics, Vortices, Wind

Wind Wind can tell us many things about weather conditions. A person knowledgeable about local weather can take the temperature and dew point, wind data and altimeter setting and make a pretty good estimate of what the present weather is. He can throw away all but the wind and still give you an approximate weather outlook. There is a simple key to understanding wind: Variations in pressure from place to place create wind, because air tends to flow from high pressure to low, as water flows from high ground to low. This phenomenon has led to the creation of a model to explain worldwide air circulation. Because cooler air is denser than warmer air, its pressure is higher. As air at the equator warms and rises, the pressure in the area lowers. Heavier, cooler air tends to flow toward the low, and as it does, it causes the warmer air to flow upward and poleward, where it cools and develops higher pressure. This air, in turn, will then tend to flow back toward the equator. The same principle of circulation--from areas of high pressure to areas of low--also governs the circulation around the high- and low-pressure areas that move across our country. Isobars are lines drawn on a map to link points of equal atmospheric pressure. The textbook rule says that wind flows parallel to the isobars, but this appears to conflict with the basic principle of flow. If air flows from high pressure to low, shouldn't its movement be perpendicular to the isobars? The answer is that the rotation of the earth deflects the wind flow about 90 degrees, causing the air to circulate around the pressure systems instead of directly from one system to another. In the Northern Hemisphere, the flow is clockwise around high-pressure areas and counterclockwise around lows. This fact gives us a start on wind-based weather information. If you stand with your back to the wind, low pressure will be to your left. If there's a strong crosswind component aloft, worse weather is ahead if the wind is from your left and better weather if the wind is from your right. Knowing these basics of wind flow will never take the place of a good weather map and the information we get from the National Weather Service, but it can be helpful when the forecasts are all going sour. The strength of wind flow is governed by how much the barometric pressure changes within a given distance. As they tell you in basic meteorology, the closer the isobars are to each other, the stronger the wind. Strong winds mean the highs are very high and the lows are very low. Air assumes the properties of the area over which it flows: Warm air comes form the south, cold air from the north. Moist air comes from over water, dry air from over large land masses. The vigor with which the wind blows is an indication of how much moisture, or how much dry air, is overspreading the area. Winds in the middle and high levels tell the meteorologist a lot, too. Wind speeds around highs differ from those around lows. A high is usually large in area and the pressure change near its center tends to be gradual. Also, because of friction effects, the surface winds tend to flow clockwise and outward (about 30 degrees) around a high. The wind is thus light and the weather usually good near its center. The flow around a low is counterclockwise and inward, so the wind tends to accelerate nearer the center, which is usually small compared to the center of a high. The strength and direction of wind flow also provide excellent clues to approaching weather. If a cold front passes your location, a good northwesterly flow of cold, dry air settling in behind it is an indication that the high behind the front is of respectable size and strength. If the flow behind the front is weak, the high probably doesn't amount to much, and the odds of having a prolonged period of good weather following the front are slim. If the weatherman mentions an approaching low-pressure system but the winds outside are light, the low is probably far away or weak. If the wind is freshening rapidly, the low is probably strong. The stronger the wind circulating around the low, the more severe will be any storm associated with it. Furthermore, as lows deepen the circulation around them increases while the movement of the low across the country slows. Severe blows