Pressure Systems

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.

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.

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

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.

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.

 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.

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

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

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 mb  An aneroid barometer is a more compact way to measure atmospheric pressure.

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

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

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.

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.

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

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