Temperate Cyclones

At a Glance

• Region About 35 to 65 degrees, both hemispheres
• Origin Waves on the polar front
• Size 1,000 to 2,000 km or more across
• Movement West to east with the westerlies
• Indian form Western disturbances in winter
• Exam link UPSC Prelims, Geography: Storms, Cyclones, Jet Streams and Air Masses

Where Does It Fit?

This is the eighth topic of the category on storms, cyclones, jet streams and air masses. The previous topic described air masses and fronts, which are the building blocks of the temperate cyclone. The next topic compares it with the tropical cyclone. Previous topic: Air Masses and Fronts. Next topic: Tropical Cyclones versus Temperate Cyclones.

What Is a Temperate Cyclone?

In short: a temperate cyclone is a travelling low pressure wave on the polar front, with a cold front and a warm front.

Temperate cyclones are the storms of the middle latitudes. They are known as extratropical cyclones because they form outside the tropics, and as wave cyclones because they begin as a wave on the front. The theory of their origin, the polar front theory, was put forward by the Norwegian school of Vilhelm Bjerknes and others around 1920, as in Air Masses and Fronts. Unlike tropical cyclones, which draw energy from warm seas, temperate cyclones get energy from the contrast of temperature between the cold and the warm air, and form over both land and sea. They are the dominant cause of rain and snow in Europe, North America and East Asia, and are most vigorous in winter, when the temperature contrast is highest.

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. 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 Pressure Belts and Pressure Cells.

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.

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 compares tropical and temperate cyclones. Next topic: Tropical Cyclones versus Temperate Cyclones.

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