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

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.

We are now going to look at some weather maps. They're taken from the Telegraph website and although they are quite old they're useful because they show us air temperatures and the position of fronts.  The maps will help us understand how to recognise a warm front.


The chart below shows a warm front over the UK on the 21st January 1998.  Although it is old, it shows a front very clearly. The front is the black line with the oval lumps on it.  It travels in the direction of the wind - in this case, 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.

British Isles weather chart 21 Jan 1998 - Telegraph

[Source: Daily Telegraph]

Symbols on the map

  • Thin black lines: These are isobars. They link areas with equal air pressure. the values are pressures in millibars, with inches in brackets
  • Arrows: Direction of the wind (wind speed is shown in miles per hour)
  • Numbers in black circles: Air temperature in degree Celsius (degree Fahrenheit is shown in brackets)
  • Icons: Show main weather features such as light cloud, rain, sunshine, etc.
  • Thick lines: Fronts, either cold (marked with triangles), warm (semi-circles) or occluded (both marks)


  1. What is the change in temperature ahead of and behind the warm front?
  2. What other weather conditions change ahead of and behind the front?