Predict The Weather (Part 3) - Understanding weather fronts

6/8/2012 11:24:02 AM

Understanding weather fronts

Check the charts for great shots.

Storm Depression

Description: Storm Depression

The letter ‘L’ marks two low-pressure systems, or depressions. The curved lines are isobars that link areas of equal atmospheric pressure. The big depression is a storm: pressures have fallen to 953hPa at the centre with tightly-packed isobars.

Types of front

Description: Types of front

Warm fronts are lines with red semi-circular flags. Cold fronts have sharp-edged blue flags. Purple lines with both flags are occluded fronts where the warm and cold fronts have merged and the cold air has lifted the warm air off the ground.

Warm front cross-section

Description: Warm front cross-section

Warm tropical air (left) collides with cold polar air (right) at the warm front (red line). The warm air rises, cools and clouds form along the front. Stratus clouds generally form featureless layers. Nimbostratus and stratus produce rain and snow.

Cold front cross-section

Description: Cold front cross-section

Cold air (left) spiraling around the storm depression catches up with the warm tropical air (right), and forces it off the ground at the cold front (blue line). The rising warm air often forms cumulonimbus. Visibility is poor beneath low cloud.

What’s that cloud?

Discover how to spot the most common types with our handy guide

Description: What’s that cloud?


Small round puffs of high cloud found in rows. Large numbers produce ‘markerel skies’ because they look like fish scales


Grey-white flat clouds often arranged in regular rolls, stripes or clumps. Bigger than cirrocumulus, they partially hide the sun


Rare clouds shaped like cows’ udders. The most impressive form at the rear of huge cumulonimbus clouds


Streamlined UFO-or disc-shaped clouds found on calm days downwind of hills or mountains. They rarely produce rain

Cirrus clouds

Thin and wispy high clouds found mainly in fair weather. Sheet-like cirrus can create ice halos around the sun


Dark thunderclouds, sometimes with a top shaped like a blacksmith’s anvil, which billow up to 10 miles into the atmosphere

Stratus clouds

Characterless layers associated with overcast skies. Stratus often produces drizzle. Nimbostratus brings rain or snow


Cotton ball-shaped clouds with sharp outlines. Fair-weather cumulus have flat tops. Those with cauliflower-shaped tops develop into cumulonimbus

Will it snow?

Snow presents great photo opportunities, which get better if you’ve prepared your kit for a wintery shoot. Weather charts showing ‘thickness’ can help you anticipate snow perhaps five days in advance. Thickness typically appears as dashed lines (red on the chart below), is measured in tens of metres, and gives a rough measure of the warmth of the atmosphere. Lower values mean the air is colder and denser, and snow is more likely. In northwest Europe, rain and snow are equally possible when thickness is 522 to 538. Snow is rare when thickness is more than 540, but likely to fall when thickness is less than 519.

In the weather chart below, snow may fall over mid-Sweden, north of Norway and off the southern tip of Greenland. Thicknesses are between 510 and 528, raising the possibility of snowfall. Clouds could form because air is warming and rising along the occluded fronts (thick lines with black flags) of passing low-pressure systems (‘L’ shapes).

Snow is more likely in a colder atmosphere because it can reach the ground without melting. Snowflakes form when ice freezes around dust and other particles in clouds, then the crystals collide and stick together. The flakes get too big and then fall towards the Earth’s surface. If they tumble through cold air, they reach the ground as snow. They can drop perhaps 300m (985ft) through warm air before the water turns into rain.

Snow expected

Description: Snow expected

You can predict snow using a map of mean sea level pressure (MSLP) and thickness.

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