What is a warm front?

A warm front occurs at the leading edge of  a warm air mass that is moving to replace a cooler air mass.  We already know that warm air moves over the top of cold air.  This is because the air masses have different temperatures and densities.  Warmer tropical air is forced over the cooler polar air.  This forms a bulge on the front called a warm front.

Diagram showing a warm front. The warm air mass is moving to replace the cooler air mass and at the boundary a warm front forms.

Fronts can be several hundred kilometres in width.  The air behind a warm front is warmer than the air ahead of it.  If a warm front passed overhead when you were standing outside, then you would feel the air warming up.  The air would also be more moist (humid) than before.

Let's now look at a weather forecast map showing a warm front. Study the map carefully and then answer the quiz questions to help you understand how to recognise a warm front.

The weather forecast map below shows a warm front over the UK at 12pm. The front is the orange line with the oval lumps on it.  It travels in the direction of the wind, which is blowing from the south-west to the north-east.  You can see that the weather conditions ahead of and behind the front are different.

We said before that the temperature ahead of the warm front is usually colder than the temperature behind the warm front.  We can see the difference on this map.

Weather forecast for 12pm

Symbols on the map

• Thin grey-blue lines: These are isobars. They link areas with equal air pressure. the values marked on the lines, e.g. "1016" are air pressures in millibars
• Numbers in blue arrowed circles: These show the wind speed (in miles per hour) and wind direction
• Numbers in black circles: These are air temperature in degrees Celsius
• Icons: These symbols show the main weather features such as light cloud, rain or sunshine
• Thick blue and orange lines: These show weather fronts, i.e. where differing air masses meet

Questions