4.2.3 - The Atmosphere (KQ2): Weather Patterns and Events

Recall back to 4.2.1 when we discussed wind.  It is air movement from areas of high pressure (cold) to areal of low pressure (warm).  This happens because the warm air rises, creating the low pressure below.  The cool air rushes in, causing wind.  This is the basis for most weather events and forecasting which we will discussed here.

The three major types of weather phenomena we will discuss will be anticyclones (high pressure systems), cyclones (low pressure systems...aka hurricanes), and temperate frontal depressions.

ANTICYCLONES and CYCLONES

Anticyclones and cyclones are best shown in the following video.  You may need to watch it a few times to get it,but he does a really good job of explaining them.

TEMPERATE FRONTAL DEPRESSIONS

Before we get into what temperate frontal depressions are, we will need to know what those symbols we always see on weather maps mean.  You would have seen some in the video above.  Look at and study the graphic below.

When two air masses meet, they do not mix readily due to differences in temperature and density.  A front is an imaginary line separating two contrasting air masses.  Fronts are areas where rainfall takes place.

A warm front is found when warm air is advancing and rises over cold air, which is denser and heavier.  As the warm air rises it is cooled and condensation takes place.  Continued condensation leads to the growth of water droplets and when heavy enough these fall as rain.

A cold front occurs when heavier cold air is advancing and pushes beneath a body of lighter warm air.

The name of the front is specified by the temperature of the moving air.  

Depressions are areas of low pressure, bringing unsettled weather of cloud, rain and wind.  

Example: For the UK, they form over the Atlantic Ocean where warm tropical air meets cold polar air.  Fronts occur where these two different air masses meet and are unable to mix.  The depression becomes more extensive as it moves eastwards towards the UK.  By the time a depression reaches the UK it has a warm and cold front.  The cold front travels faster than the warm front squeezing the warm sector air upwards between the fronts.  Eventually the cold front will catch up with the warm front and, as the warm sector disappears from the ground, form an occluded front.  The depression dies out when the warm air has completely risen and cooled. It now has underlying cold air and the temperature differences have equalled out.

Forecasting Depressions

Differences in pressure are shown on a weather map by isobars, which are lines joining places of equal pressure.  Pressure is measured in millibars (mb) and it is usual for isobars to be drawn at 4mb intervals.  The closer together the isobars, the greater the difference in pressure (the pressure gradient) and the stronger the wind.  Winds follow, or bend slightly inwards from the isobars.  You can see an example of isobars on a couple of weather maps below.  We will discuss them in class.  They work very similar to elevation on a topographic map.

This next one shows many features that were given in the legend at the beginning of the section.

Looking at the isobars can tell forecasters how strong winds may be, where fronts may occur, and then combined, what weather conditions are most probable to come.
In addition to the weather charts, forecasters will use satellite data and infra-red photography to look at the weather and make their predictions.  View the fun cartoon below to get some more info on satellite use in forecasting.