8. Air Pressure and Winds

Photo 14: A barometer. 2011-06-29

Atmospheric Pressure

The Atmosphere is held against the earth by the force of gravity, which creates a pressure. Air pressure is generally measured in millibar (mb).  Standard sea-level air pressure is 1013.25 mb. Italian scientist Evangelista Torricelli (1608-1647) measured air pressure with an upside-down glass tube filled with mercury in a dish of mercury. The height of the mercury column in the tube measured air pressure on the mercury in the dish. The mercury was prevented from flowing out of the tube by a vacuum in the tube above the mercury.

Later scientists discovered that air pressure decreases with altitude gain.  However, air pressure also measured the flow air in the atmosphere; it could measure high and low pressure areas. And it could thus be used in the prediction of weather (photo 14), because air pressure often changes before the weather does.

Photo 15: Wind and rain meter, Lacombe, Alberta. 2011-06-29

Pressure-Gradient Force

Wind results from a horizontal difference in air pressure, air moves from high pressure areas to low pressure areas. This is called the pressure-gradient force. The Sun heats different parts of the earth at different times, and thus the Sun is often the driving force of winds.

Coriolis Force

The Coriolis force causes moving air to spin in a circular pattern, caused by the rotating of the Earth. Air is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force is strongest near the poles and weakest near the equator. The Coriolis force works strongest on large bodies of water and air. The popularized rotating water in the toilet bowl is not necessarily caused by the Coriolis force because shape and movement of the water in the toilet may be of greater affect to the water’s rotational direction than the Coriolis force.

Frictional Force

Another factor with air movement is the frictional force, which causes air close to the Earth’s surface to slow down due to friction.

Measuring Air Movement

Wind measurement is an important part of cloud movement, evaporation, and weather prediction. At our farm we measure air direction and force, as well as rain and temperature levels (photo 15). This data is analyzed by a computer, which regulates the opening and closing of the life stock barn walls. The ideal is as much ventilation as possible until temperature reaches below 0°C. The system also watches for rain blowing into one side of the barn, on which it will close that side of the wall.