AIR PRESSURE & WIND

#1 Air pressure is the weight of the atmosphere as it pushes down on the Earth’s surface. Normal,standard air pressure comes out to be 14.7 pounds per square inch at the Earth’s surface.

More #1 We do not notice the air pressure unless it changes significantly. Air pressure decreases with an increase in altitude. Also, moving air produces lower air pressure than air that is not moving. Air that has high humidity levels has less air pressure than dry air does, all other things being equal.

#2 The instrument used to measure air pressure is called a barometer. When the barometric pressure is given in a weather forecast, it is usually given as a number followed by a unit of inches or sometimes millibars (mb). A mercury barometer is actually a long tube filled with liquid mercury.

More #2 The reason the pressure is measured in inches is that the tube of mercury will rise/fall with changes in air pressure. Standard air pressure is about inches of mercury.

Even More #2 What really matters to forecast weather is the pressure trend (if the pressure has been rising or falling over the past few hours). Generally, falling pressure means the weather will change, probably getting cloudy, rainy, etc. Increasing pressure means the weather will clear (improve). Steady pressure means the weather will not change very much.

Map showing isobars

#3 On a weather map, areas of equal pressure are connected by lines called isobars. (Iso means “same” and bar refers to barometric pressure.) If the isobars on a weather map are very close together, then the pressure is changing a lot (steep pressure gradient) and it is very windy. If the isobars are widely spaced, then there is a low pressure gradient and the winds are fairly calm.

#4 The reason the wind blows is because the atmosphere is always trying to reach equilibrium in air pressure everywhere. In order for that to happen, the air has to move constantly from high to low pressure areas. Since the Earth is a sphere and also has many different surfaces (like 70% water) the Earth does not heat evenly everywhere.

#5 Generally speaking, wind always flows from areas of high pressure to areas of low pressure.

#6 Wind speed is measured with an instrument called an anemometer. Wind speed is usually given in miles per hour or km/hr. Wind direction is just as important as wind velocity. Wind direction is always given as where the wind is coming from. So a north wind is coming from the north, a southeast wind is coming from the southeast, and so on. Wind direction is determined with a wind vane or wind sock.

#7 To say the wind always blows from a high to low pressure system is not the whole story. There are many other factors affecting how winds blow. For example, the fact that the Earth rotates on it’s axis causes something called the Coriolis effect.

More #7 Winds in the northern hemisphere are curved to the right Winds in the southern hemisphere are curved to the left, we won’t worry about that right now.

#8 Friction is another force that affects the wind. Friction caused by object on the Earth’s surface (trees, buildings, mountains) caused wind to slow down.

#9 Near the upper level of the troposphere, winds are very fast ( km/hr) The location of the jet stream has a major role in determining our weather here in MN.

THE END!!!!