Seasonal and local winds: breezes and mountain winds

Detail

1. Seasonal winds: when the whole wind pattern shifts

• Seasonal winds are winds whose direction changes with the season. They change because the belts of heating, pressure and wind move north and south as the overhead sun moves.
• In the northern summer, the sun is overhead near the Tropic of Cancer (23.5° N). The belts move north.

• In the northern winter, the belts move south.

• The monsoon is the strongest example. It is seen most clearly over South and South-East Asia.

• Summer: land heats faster → a low-pressure area forms over north-west India → the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet) moves north → moist south-west monsoon winds are pulled in from the Indian Ocean → rain.

• Winter: land cools faster → high pressure over the land → dry north-east winds blow from land to sea. The Coromandel coast (Tamil Nadu) gets rain from these winds after they cross the Bay of Bengal.

• Link to El Niño and La Niña:

• El Niño (unusual warming of the central and eastern tropical Pacific) disturbs this seasonal pattern. It is often linked with a weaker Indian monsoon.

• La Niña (the cool phase) is often linked with a normal or strong monsoon.

• Local winds are small-scale winds. They come from differences in how surfaces heat and cool, either over a day (diurnal cycle) or over a year (annual cycle). Examples are land and sea breezes and mountain and valley winds.

2. Sea breeze (by day)

• Sea breeze: a cool wind that blows from the sea to the land during the day.
• How it forms:
• Cause: land has a lower specific heat than water, so it heats faster in the morning sun.
• Process: the air over the land warms and rises → a low-pressure area forms over the land → the sea stays cooler, so its pressure is relatively higher → cool air moves from sea to land.

• Result: coastal towns get cooler afternoons. The breeze is strongest in the afternoon and can reach tens of kilometres inland.

• Sea-breeze front: the leading edge of the cool sea air as it moves inland.

• When the front passes a place, the air temperature can fall by 8–11 °C (15–20 °F). Humidity rises and the wind changes direction [2].

• Thunderstorms often form along this front. In Florida (USA), sea-breeze storms can be pushed up to halfway across the peninsula when the wind blows from the east [2].

• Indian example: Chennai (Tamil Nadu, Bay of Bengal coast)

• An IMD (India Meteorological Department) study published in the Mausam journal in 2002 used March–October data from 1969–1983. It found that the Chennai sea breeze is shallow, under 1 km deep [3].
• The study showed that the sea breeze controls the daily rise and fall of temperature and humidity in the city [3].

3. Land breeze (at night)

• Land breeze: a wind that blows from the land to the sea at night.
• How it forms:
• Cause: after sunset, land cools faster than water.
• Process: the air over the land becomes colder and denser → pressure becomes higher over land → the heavy air flows out over the sea. There it pushes under the lighter, warmer sea air and forces it upward [6].

• Result: a gentle offshore wind, strongest late at night and early in the morning [6].

• Why it is weaker than the sea breeze:

• The temperature difference between land and sea is smaller at night than by day.
• Cool ground slows the rising and sinking of air.

• Hills and buildings block air flowing out to sea [2][6].

• Use by fishermen: traditional fishermen sail out with the early-morning land breeze and come back with the afternoon sea breeze. This is still seen along the Kerala, Tamil Nadu and Konkan coasts.

• Effect on climate: land and sea breezes together give coasts an equable climate (a climate with only a small gap between day and night temperatures, and between summer and winter). Compare Mumbai (coastal, mild) with Nagpur (inland, extreme).

Feature	Sea breeze	Land breeze
Time	Day; strongest in the afternoon	Night; strongest late night to early morning
Direction	Sea → land	Land → sea
Low pressure over	Land	Sea
Strength	Stronger	Weaker, because the night-time contrast is smaller
Effect	Cools coastal afternoons; storms can form at its front	Mild; carries fishing boats out to sea

4. Valley breeze (by day)

• Valley breeze: a daytime wind that blows up the valley and up the mountain slopes. A wind that moves uphill like this is called an anabatic wind.
• How it forms:
• Cause: the sun heats the slopes. The air touching the slopes becomes warmer than air at the same height over the middle of the valley.
• Process: the warm air rises along the slopes and pulls air up the valley behind it.

• Result: the rising air cools, so clouds form over the peaks in the afternoon. They often bring afternoon showers.

• Where seen: Himalayan valleys such as Kullu (Himachal Pradesh) and Kashmir, and the valleys of the Alps. Afternoon clouds over the peaks are common there in summer.

5. Mountain breeze (at night)

• Mountain breeze: a night-time wind that blows down the slopes into the valley.
• How it forms:
• Cause: at night the slopes lose heat by radiation (heat escaping to the sky) and cool fast.
• Process: the air touching the slopes becomes cold and dense. Gravity pulls it down into the valley floor. This sinking of cold air is called air drainage.

• Result: cold air collects at the bottom with warmer air above it. This is a temperature inversion (a layer where temperature rises with height instead of falling). The valley floor gets frost and fog.

• Human use: frost-sensitive crops are planted on the middle slopes, away from the frosty valley floor. Examples are tea in Darjeeling and Assam and orchards in Himachal. Settlements also avoid cold valley bottoms.

Feature	Valley breeze	Mountain breeze
Time	Day	Night
Direction	Up the valley and slopes (anabatic)	Down the slopes into the valley
Driving force	Heated slopes; warm air rises	Cooled slopes; cold, dense air sinks
Weather result	Afternoon clouds and showers over peaks	Valley inversion, frost, fog

6. Katabatic winds

• Katabatic wind: cold, heavy air from high plateaus and ice fields that flows downhill under gravity. It is the mountain breeze on a very large scale.
• How it forms:
• Cause: the surface of a high plateau or ice sheet loses heat to space by radiation.
• Process: the air touching it becomes very cold and dense, then flows down the slope.

• Result: strong, cold winds blow towards the coast.

• Strongest examples: the Antarctic and Greenland ice sheets.

• Terra Nova Bay, Victoria Land, Antarctica (Ross Sea): fierce katabatic winds blow down from the Trans-Antarctic Mountains. They push sea ice away from the coast and keep a polynya (an area of open water surrounded by sea ice) open through the southern winter. The Drygalski Ice Tongue stops new ice from drifting back into the bay. A MODIS image from NASA's Aqua satellite captured this on 16 October 2007 [4].

• Bora: a cold katabatic wind of the Adriatic

• Direction and region: a very strong, cold wind from the north-east onto the Adriatic coasts of Italy, Slovenia and Croatia [5].
• Mechanism: cold air crosses the mountains from the east and drops down to the coast. This is why it is classed as a gravity (katabatic) wind [5].
• Strength: it often blows at more than 100 km per hour. It has knocked people down and overturned vehicles [5].

Prelims Hooks

• Sea breeze = by day, from sea to land, with low pressure over the land. Land breeze = by night, from land to sea, with high pressure over the land.
• The land breeze is weaker than the sea breeze because the land–sea temperature contrast is smaller at night.
• Valley breeze = daytime, upslope (anabatic). Mountain breeze = night-time, downslope. Trap: "mountain breeze blows by day" is wrong.
• Mountain breeze → air drainage → valley temperature inversion → frost on the valley floor. This is why tea and orchards are planted on slopes.
• The bora is a cold katabatic wind from the north-east onto the Adriatic coast (Italy, Slovenia, Croatia). Do not mix it up with the warm, dry downslope winds foehn (Alps) and chinook (Rockies).
• The strongest katabatic winds blow off the Antarctic and Greenland ice sheets. In Terra Nova Bay (Ross Sea) they keep a polynya open.
• After a sea-breeze front passes, the temperature can fall by 8–11 °C, and storms often form along the front [2].
• The sea breeze at Chennai is shallow, under 1 km deep (IMD study, 1969–1983 data) [3].
• The monsoon is the best example of a seasonal wind. It is caused by the seasonal shift of pressure belts and the ITCZ, not by a daily heating cycle.

Mains Points

• Local winds shape coastal and mountain climates (GS-I):
• Sea and land breezes give coastal cities such as Mumbai and Chennai an equable climate. Inland Delhi and Nagpur have extreme climates.

• Valley and mountain winds control the daily pattern of clouds, rain, frost and fog in Himalayan valleys.

• Effects on livelihoods and land use:

• Traditional fishing trips follow the daily cycle of land and sea breezes.
• Hill farmers use air drainage: tea, coffee and fruit go on the slopes, away from frosty valley floors.

• Valley inversions trap smoke and pollution in hill towns in winter.

• Disasters and risk (GS-III):

• Storms along a sea-breeze front can bring sudden heavy rain to coastal cities.
• Katabatic winds such as the bora are dangerous to transport and shipping.

• Antarctic katabatic winds make polynyas, which affect how sea ice forms and moves [4][5].

• Seasonal winds and the ENSO link:

• The monsoon comes from the seasonal shift of pressure and wind belts. Its strength changes from year to year with El Niño and La Niña.
• This makes ENSO monitoring central to India's farm planning and drought preparedness.