Download What are air masses?

What are air masses?
James Chubb
If you want to understand the weather you really need to know about things called air masses. But air is a
pretty tricky thing to look at, being see through and all that. So, first of all, what exactly is an air mass?
Well, an air mass is a name given to a large chunk of air that develops certain characteristics because of
where it is on the planet. For example, air that sits over North African deserts is very dry and very hot.
We call this type of air a ‘tropical continental’ air mass because it is in the tropics and sits over
continental land, rather than over the sea. With the right weather patterns, this air can come all the way to
Britain. As it moves north, it has to cross the Mediterranean Sea, Spain and France, but it will still keep
some of the dry and hot characteristics it originally had. So a tropical continental air mass brings us dry,
warm and mostly clear skies. It might bring more than just good weather, as this sort of air mass can give
spectacular sunsets and deposits of fine red dust on the ground if its picked up desert dust from the
Sahara!
Another easy one to spot is a ‘polar continental’ air mass and the name describes exactly what it is. This
air mass is called ‘polar’ because it comes from around the North Pole and ‘continental’ because it
travels over the land rather than the sea. If this air makes it to the UK, it could have come from two
different directions. Either from the East, moving south from the Pole, across Siberia and Eastern Europe.
Or it can come from the north east, traveling over Scandinavia before it gets to us. This air mass brings
some of our coldest weather in the winter. At the end of 2010 the UK was in the grip of a polar
continental air mass and temperatures plummeted to a very chilly minus 18C in Wales, the lowest on
record for November. During their journey to us, often across the North Sea, these air masses may pick up
enough moisture to create heavy snow showers along the east coast. In Northumberland on 30 November
2010, over 40cm of snow fell this sort of air mass. 40 cm!
As we’ve seen so far, any air mass that travels over land tends to bring us dry weather. Depending on
where the air started from, this can be warm, dry weather or cold, dry weather. But what about weather
that travels over water? Air masses which have a long journey across the sea will change from its original
type. Looking at its starting point will give us a clue about what weather it will bring. ‘Polar maritime’ air
arrives in Britain from the north west. It starts over northern Canada or Greenland, where it is cold and
usually pretty dry. However its long journey over the Atlantic ocean means the character of the air mass
changes dramatically! As the maritime air mass travels across the warm sea, the air is heated and
moistened from below. Warming causes the air to rise, and as the air rises it cools, condensing moisture in
the air forming clouds. When this air mass arrives in the UK we tend to get a mixture of sunshine, rain,
hail, sleet and even sometimes snow. As we are so familiar with this sort of mixed weather, its not a
surprise to find out this is one of the most common air masses over the UK. When we hear forecasters
predict sunshine and showers, it’s probably because polar maritime air is heading our way.
We’ve looked at just 3 of the 6 main types of air masses that we can experience in the UK. When an air
mass originates far from the UK, it changes on its journey to us. A continental air mass traveling over
land changes only a very little, while a maritime air mass is altered massively as it interacts with the sea.
Some air masses don’t take a direct route and can change as they move over warm or cold seas. The most
severe weather we experience occurs when dry and cold air masses from the north clash with warm and
humid air masses from the tropical oceans to the south.
We don’t have enough time to cover every possible air mass in this video, but we’ve had a look at the
basic types and how they change. If you want to find out more, there’s loads more information on the Met
Office website.