A broad spiral of white clouds over the Pacific northeast of Japan.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Conceptual pressure centre. surface flow and vertical motion enlarged.

Northern hemisphere · low pressure, surface convergence and ascent. Spin is read from above.

How to read these results

Blue and coral traces mark example cold and warm fronts. This is an extratropical system, not a tropical eyewall. Follow the parcels to connect surface convergence with rising air.

Cyclone and anticyclone

Compare pressure and airflow around a cyclone and an anticyclone, then relate each pattern to the weather it can bring.

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Why it happens

Around a low-pressure system, air tends to converge near the surface; around a high it tends to diverge. Rotation and friction determine the circulation direction and cross-isobar flow.

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MODIS observed this extratropical cyclone northeast of Japan on 22 February 2012. Cloud bands trace the low-pressure circulation.

Try a comparison

Switch to Anticyclone and distinguish divergence from rotation. Name the exaggerated vertical motion.

Reading the result

These idealised pressure systems show circulation patterns; cloud and rainfall depend on moisture and stability as well as pressure.

A common misunderstanding

High pressure always brings hot, sunny weather.

High pressure is commonly associated with descending air, but surface weather also depends on season, moisture and temperature. An anticyclone can bring clear skies or persistent fog and low cloud. In winter, weak winds and clear nights can allow strong cooling near the ground.

Cases

Exam language

Cyclone

A circulation around a low-pressure centre: anticlockwise in the Northern Hemisphere and clockwise in the Southern. Tropical and extratropical cyclones form through different processes.

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MODIS observed this extratropical cyclone northeast of Japan on 22 February 2012. Cloud bands trace the low-pressure circulation.

Anticyclone

An atmospheric circulation around a region of high pressure, commonly associated with sinking air. Surface winds turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Clear skies are common, but fog and low cloud can persist.

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NOAA surface analyses during a late-February 2022 storm. Isobars join equal pressure; front symbols identify analysed boundaries between air masses.

Depression

In meteorology, a region of low atmospheric pressure; in UK usage, commonly an extratropical cyclone.

Convergence

The movement of air towards an area, bringing neighbouring flows together. Near the surface it can encourage rising air, cloud and rain if enough moisture is present. Convergence describes a flow pattern; it is not itself a type of cloud.

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Aqua MODIS observed Cyclone Evan near Fiji on 16 December 2012. Its cloud bands spiral around the centre; a mathematical logarithmic spiral is an approximation, not a measured exact fit.

Divergence

The spreading of air away from an area. Surface divergence commonly accompanies sinking air in high-pressure systems, while divergence aloft can help sustain rising air below. Always state the height at which the flow is diverging.

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GOES-East cloud observations over the Americas, 22 April 2014. Tropical cloud bands and midlatitude systems provide evidence of atmospheric circulation. The three-cell pattern is a long-term zonal average, not three fixed cloud rings in this single image.

Comma cloud

A curved, comma-like cloud pattern often seen in satellite views of developing or mature extratropical cyclones.

Extra-tropical

Outside the tropics. An extratropical cyclone is a storm system usually associated with fronts and horizontal temperature contrasts.

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