the causes of weather

The Causes of The Causes of WeatherWeather

Air masses and pressure Air masses and pressure systemssystems

Air MassesAir Masses

►An air mass is a very large mass of air An air mass is a very large mass of air that has the same properties, such as that has the same properties, such as humidity and temperature, as the area humidity and temperature, as the area over which the air mass forms. over which the air mass forms.

►Most air masses form over tropical Most air masses form over tropical regions or polar regions. regions or polar regions.

Figure 1.14 and Table 1.2Figure 1.14 and Table 1.2

High Pressure SystemsHigh Pressure Systems

►When an air mass cools over an ocean When an air mass cools over an ocean or a cold region on land, a high or a cold region on land, a high pressure system forms. pressure system forms.

►Some high pressure systems are large Some high pressure systems are large enough to cover most of North enough to cover most of North AmericaAmerica

High Pressure SystemsHigh Pressure Systems

►As the air mass cools, the air As the air mass cools, the air becomes more dense. becomes more dense.

►The air mass contracts, and this The air mass contracts, and this contraction draws in surrounding air contraction draws in surrounding air from the upper troposphere. The from the upper troposphere. The added weight of the extra air added weight of the extra air increases atmospheric pressure. increases atmospheric pressure.

►The dense, high pressure air moves The dense, high pressure air moves outward toward areas of lower outward toward areas of lower pressure. WIND!!pressure. WIND!!

High Pressure SystemsHigh Pressure Systems

►As the high pressure air sinks, it As the high pressure air sinks, it becomes warmer and drier. becomes warmer and drier.

►As a result, high pressure systems As a result, high pressure systems often bring clear skies.often bring clear skies.

Figure 1.15Figure 1.15

Low pressure SystemsLow pressure Systems

► Air masses that travel over warm land or Air masses that travel over warm land or oceans may develop into low pressure oceans may develop into low pressure systems. systems.

►When an air mass warms, it expands and When an air mass warms, it expands and rises, making the layer of air thicker. rises, making the layer of air thicker.

►However, as the air rises, it cools. Water However, as the air rises, it cools. Water vapour in the air may condense, producing vapour in the air may condense, producing clouds or precipitation. clouds or precipitation.

► This is why low pressure systems often bring This is why low pressure systems often bring wet weather.wet weather.

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

Figure 1.15Figure 1.15

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

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