Understanding Mid-Latitude Cyclones & Anticyclones: Highs, Lows, Fronts & Jet Streams, Study notes of Earth, Atmospheric, and Planetary Sciences

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Cyclones and Anticyclones

in the Mid-Latitudes

Val Bennington

November, 2008

Anticyclones

• High pressure systems
• Just air masses with temperature and

moisture varying slightly over large area

• Clear, calm, pretty dry
• Blob-like, with small pressure gradients

and slower winds

Anticyclone (High)

• Which way does the

wind blow?

• Does air diverge or

converge at the

surface?

• Does air converge

or diverge above the

high?

Anticyclone (High)

• Which way does the wind blow? - -> anti-cyclonic = clockwise!
• Does air diverge or converge at the surface? - ->Diverges!
• Does air converge or diverge above the high? -->Converges!

Anticyclones (Highs)

• Generally boring weather - clear, calm
• Linger for a while, but can be nice
• Trap air near surface (sinking motion)
• Blob-like air masses
• Air mass stays long can take on

characteristics of land it is over

Fronts and Cyclones!

What is a Cyclone?

• A cyclone is simply an area of low

pressure around which the winds flow

counterclockwise in the Northern

Hemisphere and clockwise in the

Southern Hemisphere

• Cyclones form and grow near the front
• Cyclones (lows) are cloudy, wet, stormy

Cyclones have converging air

at surface that rises!

Cold Fronts

Cold Front

• Cold air is more dense than warm air!
• As the dense, cold air moves into the warm air region, it forces the warm air to rapidly rise just ahead of the cold front.
• This results in deep convective clouds, occasionally producing strong to severe thunderstorms (depending on how unstable the atmosphere ahead of the cold front is).
• Often, the precipitation along a cold front is a very narrow line of thunderstorms

Warm Fronts

Warm Front

• A transition zone where a warm air mass replaces a cold air mass
• Drawn as a red line with red half-circles pointing in the direction of the front’s movement
• TEMPERATURE CONTRAST ALONG WARM FRONTS IS GENERALLY LESS DISTINCT (SMALLER GRADIENT)

Occluded Fronts

Occluded Front

• A region where a faster moving cold front has caught up to a slower moving warm front.
• Generally occurs near the end of the life of a cyclone
• Drawn with a purple line with alternating semicircles and triangles

Stationary Fronts

• Front is stalled
• No movement of the temperature gradient
• But, there is still convergence of winds, and forcing for ascent (and often precipitation) in the vicinity of a stationary front.
• Drawn as alternating segments of red semicircles and blue triangles, pointing in opposite directions

Locating Fronts

Here, the winds are rapidly changing counterclockwise across this temperature gradient.

The winds are blowing warm air from the south.

This is a warm front.

Locating Fronts

In this case, the winds are also rapidly changing counterclockwise across this temperature gradient, indicating positive vorticity.

The winds are blowing cold air from the northwest.

This is a cold front.