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1_SevereWeather_I_Notes

# 1_SevereWeather_I_Notes - 1 Air pressure is the force that...

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Air pressure is the force that is exerted by the weight of air (our atmosphere) on the surface of the Earth. It is typically highest at the surface and decreases with increasing altitude. Air pressure may be classified as high or low pressure. A high pressure system is a region where the air pressure is higher than its surrounding area. Conversely, a low pressure system is a region where the air pressure is lower than its surrounding area. On a weather map, a high pressure system, or high, is denoted by the letter “H”, whereas a low pressure system, or low, is denoted with the letter “L”. Air always moves away from a high and air is always moving toward a low. 2
Regardless of your location on Earth, air pressure always flows from high pressure to low pressure. This is known as the pressure gradient force. The pressure gradient force, or PGF, is calculated as the change in pressure between two locations divided by the change in distance of those two locations. In the above figure, we can see both a high and low pressure system. The arrow in the figure represents the PGF and it is pointing from the high to the low. The blue lines on the figure illustrate isobars, or lines that connect points of equal air pressure. Note that the blue lines are closer together near the high. This is just a hypothetical situation. Isobars are not always necessarily closer near a high. In this example they are. Note that in this example, the distance from the 1008 mb isobar to the 1000 mb isobar is less than the distance from the 992 mb isobar to the 1000 mb isobar. Therefore, the PFG is higher from the 1008 mb isobar to the 1000 mb isobar than it is from the 1000 mb isobar to the 992 mb isobar. Why is this important? Wind speed is directly proportional to the PGF, so if the PGF is high, the wind speed is high and vice-versa. Therefore, the wind speed is stronger from the 1008 mb isobar to the 1000 mb isobar, and weaker from the 1000 mb isobar to the 992 mb isobar. 3

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The pressure gradient force is not the only force that acts to control the movement of air in our atmosphere. A second, important force is the coriolis force. The coriolis force results from the fact the Earth’s rotational velocity varies by latitude. As a result, objects that move through the atmosphere in the northern hemisphere are deflected to the right of their projected path, while objects that move through the atmosphere in the southern hemisphere are deflected to the left of their projected path. Thus, an object in the northern hemisphere that is moving from the south to the north will be deflected to the east. Conversely, if that same object is moving from the north to the south, it would be deflected to the west. In the southern hemisphere it is the opposite. In the above figure we see a similar drawing of the PGF. Note that in both hemispheres, the PGF moves from high pressure to low pressure. This never changes. The red lines in the above figure show the PGF as deflected by the coriolis force. Note that in the northern hemisphere, the PGF is deflected to the right of its projected path and in the southern hemisphere the PGF is deflected to the left of its projected path.
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1_SevereWeather_I_Notes - 1 Air pressure is the force that...

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