What are air masses?

Tom Bendall

Tom Bendall

My main interests are weather prediction and climate modelling, in particular the physics that is used within them, describing the motion of the atmosphere and ocean. I'm currently studying for an MRes at Imperial College London. My current work focuses on how we can describe oceanic and atmospheric processes whose scale is smaller than the scales in our model, but have a significant effect on the fluid.
Tom Bendall

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One of the most admirable (or most irritating) aspects of children can be their thirst for understanding. This often manifests itself through a seemingly never-ending chain of questions beginning with “why…”?

Trying to explain the weather on a particular day can feel a bit like this. Every answer leads to another question, which can turn the process into a fruitless task! However, inspired by the name of this blog, this is the quest that we would now like to set upon.

This post is meant to launch a series of blog entries, with the eventual goal of untangling the causes of weather on a particular day. For the moment, we’ll just focus on the weather over London.

However, in order to reach our goal, we need to first provide answers for several of the most common `why` questions. We’ll then be able to refer to these when trying to justify the weather each day!

The first concept in this series concerns air masses. This is possibly the most important aspect in determining what the weather is like.

What is an air mass? Simply, it is a large body of air – thousands of kilometres in extent. The crucial point for the weather in the United Kingdom is that there are different types of air mass that can move over the country.

Typically these air masses can be categorised by the direction from which they’ve come. Each of these types of air mass will have different characteristics, and are related to different types of weather.

The UK Met Office website lists six types of air mass that affect the UK. These are labelled by the direction from which they come and whether it was from land (continental) or sea (maritime).

Each of these air masses has characteristic temperatures and humidities – i.e the amount of moisture they contain (for instance maritime masses bring more water with them). They also tend to occur at different times of the year. 2018 has been a year of extreme weather in the UK: the cold and snowy February was associated with polar continental and arctic maritime air masses, while the very hot summer featured tropical continental air.

Air MassDirectionAssociated Weather
Arctic MaritimeComes from the north, from the Arctic.Cold, snowy weather in winter, particularly to Scotland.
Polar MaritimeComes from the north-west, from Greenland and the Arctic sea.Generally brings frequent showers. In winter these are often over the western and northern sides of the British Isles, but in summer the showers are heaviest of the east. This is the most common air mass to affect the British Isles.
Returning Polar MaritimeOriginates over Greenland and Arctic sea, but comes from the west via the Atlantic ocean.This air has travelled further over the Atlantic ocean than Polar Maritime. It is usually dry but can bring a lot of clouds.
Tropical MaritimeFrom the south-west, from over the Atlantic sea.Warm but moist air, bringing low cloud.
Tropical ContinentalFrom the south-east, with air originating over North Africa.Hot dry air. Most common in summer.
Polar ContinentalFrom the east or north-east, with air originating over central or north-eastern Europe.The air is very cold but dry. Can bring clear skies and severe frosts, or if it travelled over the North Sea brings rain or snow showers.

You can read more about these air masses on the Met Office website:

https://www.metoffice.gov.uk/learning/atmosphere/air-masses/types

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