19.1 Understanding Air Pressure & Wind

What is air pressure? It is the force of the weight or air on a surface. It is exerted in all directions. The air pressure pushing down on an object exactly balances the air pressure pushing up on the object.

How is air pressure measured? A barometer is a device used for measuring air pressure. When air pressure increases, the mercury in the tube rises. When air pressure decreases, so does the height of the mercury column. Unit for measuring pressure is millibar. (Standard sea-level pressure = 1013.2 millibars).

Mercury Aneroid YouTube: Weather Barometer - Understanding Barometer Readings

What causes wind? The sun is the ultimate energy source for most wind. Creating areas of high pressure and low pressure.

Mr. Fetch's Earth Science Class Air flows from areas of higher pressure to areas of lower pressure. This causes air to move creating WIND! H L YouTube: Indoor Skydiving Mr. Fetch's Earth Science Class Mr. Fetch’s Earth Science Classroom

What factors affect wind? Three factors combine to control wind: pressure differences (pressure gradient), the Coriolis effect, friction.

How do pressure differences affect wind? Wind is created from differences in pressure. Pressure differences make wind stronger or weaker. The greater the differences in pressure, the greater the wind speed.

What are isobars? Barometers in weather stations collect pressure data. This data is shown on weather maps using isobars. Isobars are lines on a map that connect places of equal air pressure.

What is a pressure gradient? The spacing between isobars shows the pressure gradient. Remember that wind moves from higher pressure to lower pressure. Closely spaced isobars indicate a steep pressure gradient and strong winds. Widely spaced isobars indicate a weak pressure gradient and light winds.

Light Winds (SLOW) Strong Winds (FAST)

What is the Coriolis effect? The Coriolis effect describes how Earth’s rotation deflects all free-moving objects or fluids, including wind & water. Air does not move in a straight line because of Earth’s rotation. Coriolis Effect

How does Coriolis Effect change wind? Changes only wind direction and NOT wind speed. Is affected by wind speed – the stronger the wind, the greater the deflection. Is strongest at the poles and weakens toward the equator. The change in direction happens because the Earth rotates 15° to the east every hour.

YouTube: Coriolis

Coriolis Effect Northern Hemisphere Southern Hemisphere

How does friction affect wind? The friction layer is the layer of air that is affected by friction. Surfaces such as hills and mountains act to change speed & wind direction. Generally speaking: If Friction then Wind Speed

What are jet streams? Above the "friction layer", winds tend to blow parallel to isobars. These are jet streams. Jet streams are fast-moving rivers of air that travel between 320 and 480 kilometers per hour (200-300 mph) in a west-to-east direction.

Air pressure & Jet streams