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9.25 Winds - General

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Diurnal Wind Variation

Wind develops as a result of spatial differences in atmospheric pressure. Generally, these differences occur because of uneven absorption of solar radiation at the Earth's surface. Wind speed tends to be at its greatest during the daytime when the greatest spatial extremes in atmospheric temperature and pressure exist.

Just as the sun heats different surface locations differently, different heights above the ground are also warmed differently. This is because the sun doesn't warm air nearly as much as the ground. The ground is the biggest heater of the air, and so the air closest to it is usually the warmest.

How do different temperatures at different heights cause winds at 5000 feet to affect surface winds? When the sun heats the ground during summer, it starts a process called mixing. To understand mixing, imagine a container of salad dressing. If it sits unused for long enough, the oil component floats to the top and forms a layer. When a spoon stirs it back together, the dressing once again looks like a single liquid, even though before mixing, it looked like two different substances. Now imagine that the atmosphere is like a bottle of salad dressing, with a layer of faster winds on top and a layer of gentle winds on the bottom. Instead of a spoon, temperature differences start the stirring process. When the winds from 5000 feet mix with the slower winds at the surface, the two speeds blend together like salad dressing with its oil, leaving a single wind speed throughout the heights where mixing occurred. Now that all the factors for the process of mixing are in place, it's time to go into more detailed explanations of how and why they all occur.

As mentioned above, the ground gets much more warmth from the sun than air does or the ocean. This makes the ground warmer than the air above it, which can be observed by touching a sidewalk on a sunny summer day. When the sun first warms the ground in the morning, two processes called conduction and convection occur. As the ground becomes warmer, the layer of air immediately above it also becomes warmer through conduction. Conduction alone doesn't cause the mixing, though. Convection is more important in starting that process. Once conduction heats the air near the surface, it rises through convection, which leads to the next step in the explanation of why winds from 5000 feet mixed with those at the surface: eddies.

What exactly is an eddy? Eddies form because of the convection of surface air. As air warmed by the ground rises, cooler air from above sinks down to replace it. The cycle continues as the ground continues to warm surface air. Eddies gradually become taller because as the sun heats the ground more through the morning, convection carries the air higher and higher. This vertical circulation causes mixing of the faster moving winds from 5000 feet with the calmer air at the surface.

As eddies mix the faster winds from 5000 feet with calmer winds at the surface, a property called momentum comes into play, which will explain why the two wind speeds change. Momentum is essentially an object's tendency to continue moving or to stay at rest. While the salad dressing example from above illustrates how the two wind speeds mix together to become one, it doesn't explain why the speeds change. Thus, the tendency of the faster winds to keep moving fights with the tendency of the surface air to stay still as they are mixed together, and the result is a mixture of wind that is slower than the winds from 5000 feet and faster than the winds from the surface.

The last factor involved with what happens are eddies. A warm and sunny day makes it much more likely for eddies to form than if it were cool and cloudy because of the sunlight needed to start air at the surface rising. So, faster winds at 5000 feet are mixed to the surface by eddies that form because of the ground being warmed by the sun. The momentum from faster winds mixed down to the surface start moving surface air along at speeds faster than could be expected.

Forecasting computers simply don't take everything into account when calculating area forecasts around the country. The air higher up can usually be safely ignored when forecasting surface conditions, but under extreme conditions, more attention needs to be given to all aspects of the atmosphere.