CHAPTER 19.1Understanding Air Pressure

Air pressure is very important for making changes in the weather.

Air pressure is the pressure exerted by the weight of air above.

Average air pressure at sea level is 1 kg/cm2

Air pressure is exerted in all directions. The air pressure pushing down on an object balances the air pressure pushing up on the object.

MEASURING AIR PRESSURE

Air pressure is measured using the unit millibar. Standard pressure is 1013.2 millibars.

A barometer is used to measure air pressure. It is a glass tube, closed at one end, filled with mercury. The tube was then put upside down in a dish of mercury. As pressure changed the amount of mercury in the tube changed.

An aneroid barometer is smaller and more portable.

FACTORS AFFECTING WIND

Wind is horizontal air movement.

Wind is made for the horizontal differences in air pressure. Air always flows from an area of higher pressure to lower pressure.

Because the Earth is unequally heated to makes pressure differences. Solar radiation is the energy source for most wind.

Three things control wind: pressure differences, Coriolis effect, and friction.

PRESSURE DIFFERENCES

The greater the pressure differences the greater the wind speed. Pressure changes are called a pressure gradient.

If there is a large pressure gradient, then it will cause high speed winds.

On a map closely spaced isobars indicate a steep pressure gradient and high winds. Widely spaced isobars will show areas with light winds.

CORIOLIS EFFECT

Remember this is how the Earth’s rotation affects moving objects. All free-moving objects or fluids (including wind) are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Because of this effect the wind is always directed at right angles to the direction of the airflow. It only affects wind directions and not wind speed. It is affected by wind speed – the stronger the wind, the greater the deflection. It is strongest at the poles and weaker toward the equator where it becomes nonexistent.

FRICTION

Friction on the Earth’s surface causes air movement to slow down. That changes the winds direction.

Jet streams are fast-moving rivers of air that travel between 120 and 240 kilometers per hour from west to east direction. (this is high wind)

For air close to Earth’s surface, the roughness of the terrain determines the angle of airflow. Over the smooth ocean surface friction is very low so the angle of airflow is small. Over rugged terrain there is a lot of friction so wind moves more slowly and at greater angles.