Temperate (Extra-Tropical) Cyclones: Air Masses, Fronts, Origin and Development

At a Glance

• Region Middle and high latitudes, 35° to 65°
• Origin Wave on the polar front
• Direction West to east
• Structure Warm sector between cold and warm fronts
• Indian form Western disturbances
• Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows the topic on tropical cyclones and describes its counterpart of the middle latitudes. It uses the air masses and fronts described earlier, and explains the winter rain that reaches north India. Previous topic: tropopause Minimum speed Roughly 110 km per hour Polar front jet…">Jet Streams: Permanent and Temporary Types and Their Impact on Weather. Next topic: Polar Vortex and Its Relationship with Ozone Depletion.

How Do Air Masses and Fronts Give Rise to Temperate Cyclones?

In short: a temperate cyclone starts where warm and cold air masses meet at a front.

Temperate cyclones form in the middle latitudes, between 35 and 65 degrees, along the polar front, where cold polar air meets warm tropical air. The air masses move in opposite directions along the front. A small wave develops on the front, the warm air pushes into the cold and the cold air pushes into the warm, and the pressure falls at the point of the wave. The air begins to spiral anticlockwise into the low in the northern hemisphere. The properties of the air masses and fronts are described in Air Masses, Fronts and Extra-Tropical Cyclones.

What Are the Stages of a Temperate Cyclone?

1. Initial stage: the polar front is almost stationary, with cold air on one side and warm air on the other, and the winds blow parallel to it in opposite directions.
2. Incipient stage: a small wave forms on the front, often triggered by the jet stream aloft, as in Jet Streams: Permanent and Temporary Types and Their Impact on Weather. Warm air pushes into the cold air, and the cold air pushes into the warm; the pressure falls at the crest.
3. Mature stage: the wave deepens into a full cyclone with a clear warm sector of warm air, between the warm front in front and the cold front behind. The isobars are almost circular or V-shaped, and the winds blow anticlockwise and inward in the northern hemisphere.
4. Occlusion stage: the faster cold front overtakes the warm front, closing the warm sector and lifting the warm air from the surface, which forms an occluded front.
5. Dissipation stage: the warm air is cut off from the surface, the temperature contrast disappears, and the cyclone fills up and dies.

What Are the Features and Weather of a Temperate Cyclone?

The weather at a place as the cyclone passes follows a sequence. Ahead of the warm front the pressure falls, cirrus clouds thicken into nimbostratus, and steady rain begins. When the warm front passes, the rain stops and the temperature rises in the warm sector. When the cold front arrives, there are cumulonimbus clouds, squalls and heavy showers, followed by clearing skies, a fall in temperature and a rise in pressure. This is why weather in the middle latitudes changes so quickly. Behind the cyclone, a high pressure anticyclone with calm, clear and dry weather usually follows, as in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.

What Are Western Disturbances?

Western disturbances are the extratropical storms that form over the Mediterranean, the Black and Caspian seas and the Atlantic, and travel east in the subtropical westerly jet to reach northwest India and Pakistan between about October and May. They bring winter rain to Punjab, Haryana, Delhi and Uttar Pradesh, and snow to Jammu and Kashmir, Himachal Pradesh and Uttarakhand. The rain is a major benefit to the rabi crops, especially wheat, and the snow recharges the Himalayan glaciers and rivers. They also bring cold waves, fog, hailstorms and sometimes floods and landslides. When a western disturbance interacts with the monsoon, as in the extreme rain of June 2013 over Uttarakhand, the result can be disastrous.

As an analytical point, western disturbances show that India’s climate has a link with the mid-latitudes, and studies suggest that their pattern is changing, with fewer but more intense events in recent decades, which makes forecasting of winter rain and snow a growing concern for farmers and for the hill states. For a very fast-deepening mid-latitude cyclone, meteorologists use the term bomb cyclone, when the central pressure falls by at least 24 millibars in 24 hours.

How Do Tropical and Temperate Cyclones Compare?

In short: a tropical cyclone is a compact heat engine over warm seas, and a temperate cyclone is a large wave on a front.

Why Is This Topic Important?

• Concepts Stages and structure asked
• India Western disturbances and rabi crops
• Comparison Versus tropical cyclones

What Came Next?

The next topic explains the polar vortex and its relation to ozone depletion. Next topic: Polar Vortex and Its Relationship with Ozone Depletion.

Key People and Terms

• Wave cyclone Cyclone that starts as a wave on a front
• Warm sector Warm air between the two fronts
• Occlusion Closing of the warm sector
• Western disturbance Extratropical storm reaching India in winter
• Polar front theory Bjerknes theory of cyclone origin
• Bomb cyclone Rapidly deepening mid-latitude low

Exam Corner

Mains Angle

Questions ask for stages and the Indian link. These points add depth.

• Origin Temperature contrast on the polar front
• Impact Winter rain and snow in India
• Change Shifting western disturbances