1. Pressure Systems

2. Importance of Pressure Systems
High and low air pressure is important to predicting the weather because differences in air pressure causes wind to form.

3. High Pressure Systems
A high pressure system occurs when dry, cool air spirals in a clockwise direction, bringing mild weather and sunny, blue skies.
A high pressure system generally means good weather because it attracts cool and dry air. On a weather map a high pressure system is shown by a blue H.

4. High Pressure System
Air sinks slowly to warmer, lower altitudes. As the air nears the ground, it spreads outwards, toward areas of lower pressure.
Most high-pressure systems are large and change slowly. When it stays in the same location for a long time, an air mass may form.
High-pressure systems bring clear and calm weather.
Air moves down, out, and around!

5. Low Pressure System
A low pressure system is moist, warm air swirling in a counter-clockwise direction. Low pressure systems usually mean stormy, wet weather.
This is because low pressure systems generally attract warm and moist air. On a weather map a low pressure system is shown by a red L.

6. Low Pressure System
A small area of low pressure can develop into a larger system.
A low-pressure system forms around a low pressure center.
Air moves quickly around and inwards, toward the lowest pressure center. Then, up to higher altitudes.
Often formed along the boundary of warm and cold air masses.
Part of the boundary between the masses moves south, forms a cold front.
Part of the boundary moves north and forms a warm front.
A center of low-pressure forms where the ends of the two fronts meet.
Can cause very stormy weather.
Air moves up, inward, and around.

7. Air Pressure on a Weather Map
Areas of High and Low pressure on shown on a weather map with an H or an L.

8. Pressure Systems
Pressure differences can cause air to move in ways that make a high or low pressure area the center of a whole system of weather.

9. Air Pressure Affects the Weather
Low air pressure usually results in Bad weather: stormy, cloudy, overcast.
High air pressure usually results in Good weather: clear skies, no precipitation
Air pressure in a weather system reflects the amount of water in the air, which affects the weather.