** LESSON: How do we trace air masses? **
An airmass is characterised by its temperature (cold or mild) and its humidity (dry or moist).
A tool called potential equivalent temperature is used to trace air masses. That's a mouthful of a name!
Passing the maths behind it, it basically combines temperature and humidity into one chart.
As an exemple, the chart below shows a Polar Maritime air mass (blue colours represent cold and dry air).
Milder air is coming in from the south-west and, indeed, a few days later, the air mass is replaced by a Tropical Maritime air mass (red colours represent mild and moist air). See below:
As a result, the weather to be expected will be a lot different.
And this until a new airmass with its own properties replaces it.
Simple isn't it?