Download Weather fronts transcript

Weather fronts transcript
You may have heard weather forecasters on TV talk about a ‘warm’ front or a ‘cold’ front approaching,
bringing with it a change in the weather. But what are they going on about? What is a weather front, and how does it effect our weather?
A weatherfront is the boundary between two bodies of air with different temperature and humidity. These differences depend on where the air has come from on the globe.
For example, air sitting over a tropical ocean becomes very hot and humid, while in contrast, air over the Arctic, becomes very cold and dry.
Thats logical enough, but what happens somewhere like Britain, where our climate is neither tropically
warm and humid or polar cold and dry? Over the Atlantic, these warm and cold air masses often meet. Just like oil and water, they don’t mix
easily and can become unstable as a result, giving us an active area of weather in which fronts form.
When air masses first collide, its impossible to know if we are looking at a warm or a cold front. It’s only
when one air mass dominates that we can distinguish between the two.
If the warm air is stronger than the cold air, then this is called a ‘warm front’. You may have seen this on
TV as a line with red semicircles along it. The semicircles point towards the cold air and the direction in which the front is moving.
If the cold air has the upper hand then it’s a ‘cold front’ which is shown by a line with blue triangles along it. The blue triangles point towards the warm air and again in the direction of movement. But what is happening when two opposing air masses meet and what kind of weather does each front produce? On a fine day, if you see a layer of very high cloud beginning to spread across the sky, usually from the
south west in Britain, it may mean that a warm front is approaching.
Warm air is less dense than cold air, so it tends to ride over the top of the cool air it is replacing. This creates a vertical slope to the frontal system with the front first being seen high in the atmosphere, miles ahead of the front at ground level.
As the warm, moist air is forced to rise, it cools and water vapour condenses into water droplets or ice crystals. These form the clouds that you see approaching, first as high cirrus clouds and then as thicker clouds. Its these interactions which form clouds.
If you measure the air pressure as the front approaches, you’d see that it’s falling.
This is because the column of dense, cold air above you is gradually being replaced with more and more warm and less dense air as the front moves closer. Layers of cloud also deepen and rain begins to fall. A warm front usually produces quite a long period of light and drizzly rain, with a shorter spell of heavier
rain at the beginning of the front. The rain or drizzle will eventually die out once the front has moved
through, but it will often stay pretty cloudy.
You might be thinking that a warm front would always mean nice warm weather, but unfortunately that’s not always the case.
All that rain and loss of sunshine may mean that temperatures are actually lower than they were before
the warm front arrived!
So why is it called a warm front? Because the cold, dry airmass has been replaced by the warmer, humid airmass - it just doesn’t feel much warmer.
Now, what about cold fronts? Warm fronts are easy to spot because you see the leading edge of high cloud approaching on the
horizon across clear skies.
However, warm fonts leave clouds behind, and the following cold front sneaks in under the cover of the
clouds.
This makes them hard to spot just using your eyes, as it’s difficult to see any changes that might be on
the way. But, remember that the pressure falls with a warm front because the air is less dense: well the
opposite is true with a cold front.
A cold front is a bit like a wedge, the air is colder and more dense than the warm air and scoops it up,
causing the pressure to rise, and we can spot that, using a barometer.
So, what kind of weather does a cold front bring?
Deep cloud layers form as the warm, moist air is forced to rise and cool.
These cloud layers produce lots of rain, often in bands and these bands tend to be quite narrow, with
heavy rain in them, sometimes with hail and even thunder.
Once the front passes there is often a dramatic clearance, with blue skies, bright sunshine as the cold air descends as the front passes. Temperatures fall at first, but they may rise again when the sunshine
appears.
However, this sunny weather doesn’t always last for very long, because if the cold air is warmed it will
rise, or ‘convect’ as weathermen say, and produce showery rain clouds such as cumulus or cumulonimbus.
As we’ve seen, weather fronts can be complicated things and bring some very varied weather which is
one of the reasons we have such an interesting time of things here in Britain. We’ve looked at how warm and cold fronts form, and what sort of weather they bring.
But if you want to find out more about weather fronts or anything else about weather and climate, then
visit the Met Office website.