Pressure Systems

1. Pressure Systems

2. What is air pressure? • It is caused by the weight of all the air in the atmosphere pushing down on Earth. • It is also known as atmospheric pressure. • Air pressure changes with the height and also when air warms up or cools down. • Changes in air pressure cause changes in weather.

3. Differences in air pressure • Low Pressure • When air rises, it leaves behind an area of lower pressure, because the upward-moving air is not pressing down so hard on the surface.

4. Differences in air pressure • High Pressure • Areas of high pressure are formed where air is sinking back down, and so pushing down harder.

5. Differences in air pressure

6. Differences in air pressure • Since there are many areas of high and low pressure above the Earth’s surface due to uneven surface heating: • Air moves from high pressure to low pressure forming winds. • As a result, the greater the difference between high pressure and low pressure areas, the greater the wind speed is.

7. Highs and Lows • Pressure is different all over the world. • Lows are areas of low pressure with the lowest pressure at the center. • Lows usually bring wet, cloudy weather.

8. Highs are areas of high pressure with the highest pressure at the center. • Highs bring dry, sunnier weather. • The ways that these pressures change from day to day affects the weather changes.

9. Measuring Air Pressure • Air pressure is measured in millibars (mb) on a barometer. • The simplest kind of barometer is a mercury barometer and pressure is measured in (inches) of mercury

10. Pressure • Air exerts a pressure although we are not aware of it. • Pressure is a force, or weight, exerted on a surface per unit area and is measured in Pascals (Pa) • Atmospheric pressure is measured using a barometer • These usually measure in millibars (mb) • 1 mb= 100 Pa

11. Pressure • A mercury barometer measures the pressure by noting the length of mercury which is supported by the weight of the atmosphere. • 1 cm of mercury is equal to 13.33 mb • An aneroid barometer is a more compact way to measure atmospheric pressure.

12. Pressure • Air blows from areas of high pressure to regions of low atmospheric pressure. • A pressure system is a region of the Earth’s atmosphere where air pressure is unusually high or low. • High and low pressure systems form (and die) constantly due to thermodynamic interactions of the atmosphere and water

13. High Pressure Systems • In a high-pressure system, air pressure is greater than the surrounding areas. • This difference in air pressure results in wind or moving air. • In a high pressure area, air is denser than in areas of lower pressure. • The result is that air will move from the high-pressure area to an area of lower density, or lower pressure

14. High Pressure Systems • High pressure systems are associated with clear, cool weather • Around high pressure systems, winds flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. • Air moving from high pressure to low pressure follows a spiraling route due to the rotation of the Earth beneath the moving air.

15. Low Pressure Systems • Receives winds from high pressure systems. • As winds blow in, the air can be uplifted • The components of storms are attracted to regions of low pressure • For this reason, heavy precipitation and overcast conditions are often associated with low-pressure systems.

16. Pressure Systems on Maps • On weather maps, low-pressure systems are shown with a capital L, unless they are tropical storms or tropical cyclones • In these cases, the symbols for the weather conditions would be used. • High-pressure systems are shown with an H

17. Pressure Systems on Maps • Isobar Maps • Show where pressures are relatively high and low and show gradual or dramatic changes in pressure over a distance. • Isobar • A line that connects locations at equal pressure