Co-op Week Six {High and Low Pressure; Fronts}

A) We remembered how wind is created when air moves from a high pressure to a low pressure area.  (Just like when the air rushes out of a balloon)  This lesson focuses on what high and low pressure systems are, how weather fronts are formed, and we can forecast weather by knowing a little bit about these things!

B) Most weather is determined by the location and movement of large air masses.  The weather inside an air mass is fairly stable, and it's on the boundaries where it is contact with other air masses of different temperature, humidity, and pressure that unstable weather occurs. 

• cold air masses form in polar regions


• warm air masses form in the tropics


• continental air masses from over land, and maritime air masses form over water


• air masses can be described as a combination - such as a warm continental air mass which may be low humidity; or a warm maritime air mass which is probably humid

C) Eventually the air masses move away from where they formed, moved by global winds.  When two air masses meet, they don't easily mix! The cooler (and more dense) mass will push against the warmer air mass and the place where they meet is called a front.  Whether it's a warm front or a cold front depends on which air mass is the one moving.

• warm front = when warmer air takes the place of cooler on the ground.  The warmer air slides up against the cooler air in a shallow slope.  Clouds form along the front line, and the first ones we'd see as a warm front approaches are the high-altitude clouds, followed by middle and low altitude clouds, and usually precipitation.


• cold front - when cold, heavy air moves towards warm, light air.  The cold air pushes under the warm air, making a slight bulge or a steep slope.  Mid-altitude clouds and usually precipitation are on the leading edge of the cold front.


• occluded front - when two cooler air masses meet and push a warm air mass up on top of both

What are cold fronts and warm fronts?

Most fronts produce clouds and precipitation because the cooler air cannot hold much humidity and as the moisture condenses, clouds are formed.  Warm air masses usually have lower air pressure.  Since most weather changes occur on the "frontlines", watching air pressure helps meteorologists track front movement and predict weather.

D) Air pressure is measured with a barometer.  We made a very simple barometer in class - flexible plastic tubing bent into a U shape and one end sealed with duct tape.  At home, the kids will pour colored water into the tube so that it is about one inch on either side of the "U"  They will need to mark the height of the water on the plugged side of the tube, and then mount it somewhere where it won't be disturbed.  If the air pressure increases, it will push down on the open end of the tube, forcing the water farther up the plugged side.  If the air pressure decreases, the water should drop below the marking on the plugged side.  I have another type of barometer that we will make in a later class, and I showed that to the kids and explained how it worked.

E) We looked at weather maps showing High and Low Pressure systems, millibars, and fronts and talked about how they are shown on weather maps.  I had printed out the weather map with fronts and pressure systems from that day and we worked on interpreting the information a bit.  As I was driving to class yesterday, the weather report on the radio said we could expect an Alberta Clipper this weekend.  So we should be seeing some changes in air pressure as it comes closer.