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The Causes of Weather

The Causes of Weather. Air masses and pressure systems. Air Masses. An air mass is a very large mass of air that has the same properties, such as humidity and temperature, as the area over which the air mass forms. Most air masses form over tropical regions or polar regions.

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The Causes of Weather

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  1. The Causes of Weather Air masses and pressure systems

  2. Air Masses • An air mass is a very large mass of air that has the same properties, such as humidity and temperature, as the area over which the air mass forms. • Most air masses form over tropical regions or polar regions.

  3. Figure 1.14 and Table 1.2

  4. High Pressure Systems • When an air mass cools over an ocean or a cold region on land, a high pressure system forms. • Some high pressure systems are large enough to cover most of North America

  5. High Pressure Systems • As the air mass cools, the air becomes more dense. • The air mass contracts, and this contraction draws in surrounding air from the upper troposphere. The added weight of the extra air increases atmospheric pressure. • The dense, high pressure air moves outward toward areas of lower pressure. WIND!!

  6. High Pressure Systems • As the high pressure air sinks, it becomes warmer and drier. • As a result, high pressure systems often bring clear skies.

  7. Figure 1.15

  8. Low pressure Systems • Air masses that travel over warm land or oceans may develop into low pressure systems. • When an air mass warms, it expands and rises, making the layer of air thicker. • However, as the air rises, it cools. Water vapour in the air may condense, producing clouds or precipitation. • This is why low pressure systems often bring wet weather.

  9. Meanwhile, the expanding air mass pushes away air in the upper troposphere. Directly below, at Earth’s surface, the atmospheric pressure decreases.

  10. Figure 1.15

  11. High (A) and Low (B) pressure systems

  12. “When the pressure is low expect rain or snow. When the pressure is high expect blue skies”

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