The atmosphere, pressure and wind

The standard atmosphere

To compare airplane performance and calibrate instruments, the aviation world uses the International Standard Atmosphere: at sea level, 15°C (59°F) and 29.92 inches of mercury (1013.2 millibars). On average, temperature drops about 2°C per 1,000 feet and pressure drops about 1 inch of mercury per 1,000 feet in the lower atmosphere. Real days are almost never standard, which is why you correct for temperature and pressure.

Why wind blows

The sun heats the earth unevenly, making areas of high and low pressure. Air moves from high pressure toward low pressure. Because the earth rotates, the Coriolis force deflects moving air to the right in the Northern Hemisphere. The result:

• Around a high, air flows clockwise and outward. Highs usually bring descending air, fewer clouds and good weather (but sometimes haze).
• Around a low, air flows counterclockwise and inward. The rising air in lows often makes clouds and precipitation.

Near the surface, friction slows the wind and the Coriolis force weakens, so surface winds cross the isobars at an angle toward the low. That's why the wind direction at 3,000 feet is often different from the wind on the ground.

Local winds

• Sea breeze and land breeze: during the day, land heats faster than water, and cooler air flows from the sea toward land. At night it reverses.
• Mountain and valley winds: during the day, air flows up mountain slopes; at night, cool air drains down into valleys. Very common in places like Utah's Wasatch Front.