Local Winds, Air Masses & Fronts – NDA Geography Notes

The planetary winds govern the broad patterns of global circulation. But at the local and regional scale, a different set of winds operates, shaped by local temperature differences, terrain, and seasonal pressure changes. These are local winds, predictable, named, and often having dramatic effects on the weather and people of the regions where they blow.

This chapter also introduces air masses: large bodies of air with uniform temperature and humidity: and fronts: the boundaries between contrasting air masses where most of the world’s significant weather events occur.

NDA tests this chapter primarily through local wind identification and matching questions. The names of local winds, their regions, and their characteristics, hot or cold, dry or moist, are the most reliably tested facts.

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Air Masses

An air mass is a large body of air that has roughly uniform temperature and humidity throughout its horizontal extent. Air masses form when air stays over a region long enough to take on the temperature and moisture characteristics of that surface.

Source regions: the areas where air masses form: must be large, flat, have uniform surface characteristics, and have calm or gentle winds so the air stays in place long enough to be modified.

The four main types of air mass based on source region:

Type	Code	Source Region	Character
Maritime Polar	mP	Cold ocean at high latitudes	Cold and moist
Continental Polar	cP	Cold land at high latitudes (Siberia, Canada)	Cold and dry
Maritime Tropical	mT	Warm ocean in the tropics	Warm and moist
Continental Tropical	cT	Warm, dry land in tropics (Sahara, Arabia)	Hot and dry

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Fronts

A front is the boundary between two air masses of different temperature and humidity. When a cold air mass meets a warm air mass, they do not mix easily. Instead they form a sharp boundary. Most precipitation and severe weather occurs along fronts.

The science of fronts was developed largely by Norwegian meteorologists in the early 20th century, sometimes called the Bergen School.

Front Type	What Happens	Weather Produced
Cold Front	Cold air pushes under warm air (wedge); steep, rapid uplift	Cumulonimbus, heavy rain in narrow belt, thunderstorms, sharp temperature drop; moves fast
Warm Front	Warm air slides gently over cold air; gradual uplift	Widespread layered clouds (stratus), steady light rain over large area; gradual warming
Occluded Front	Cold front catches warm front; warm air lifted completely off surface	Complex mixed weather
Stationary Front	Neither air mass advances	Persistent cloud and light rain for days

Cold fronts move faster than warm fronts.

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Local Winds: Overview

Local winds are winds specific to a particular region, caused by local temperature differences, terrain effects, or seasonal pressure patterns rather than global pressure belts.

For each local wind, know: the name, the region where it blows, and whether it is hot or cold, dry or moist. NDA tests these facts through match-list questions.

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Hot and Dry Local Winds

Foehn (Föhn)

A warm, dry wind that blows down the northern slopes of the Alps in Europe. Moist air from the Mediterranean rises on the southern slopes, cooling and losing moisture as rain. When it descends on the northern side, it warms up rapidly (dry adiabatic rate is faster than moist adiabatic rate). The result is a warm, dry wind on the lee side. This process, warming and drying on the downwind side of a mountain, is called the Foehn effect and applies to all similar winds worldwide.

Chinook

A warm, dry wind that blows down the eastern slopes of the Rocky Mountains in the USA and Canada, the North American equivalent of the Foehn. Its name in Arapaho means “snow eater”: it can melt large amounts of snow very quickly, raising temperature by 20–30°C in just a few hours.

Sirocco

A hot wind that originates in the Sahara Desert and blows northward across the Mediterranean Sea to southern Europe (Italy, Spain). In North Africa it is hot and dry, but as it crosses the Mediterranean it picks up moisture and arrives as a warm, humid, oppressive wind. Sirocco is the hot wind blowing from Africa toward the Mediterranean coast. [NDA 2007-II]

Khamsin

A hot, dry, dusty wind blowing from the Sahara Desert across Egypt and the eastern Mediterranean. It typically blows for about 50 days between March and May (khamsin means fifty in Arabic). Similar to the Sirocco in origin.

Harmattan

A dry, dusty wind blowing from the Sahara Desert westward and southwestward across West Africa toward the Gulf of Guinea coast. Blows during the winter months (November to March). In the coastal areas of West Africa it is called the “doctor”: it brings some relief from the humid, oppressive conditions. Harmattan is the dry, dusty wind blowing from the Sahara toward West Africa. [NDA 2011-II]

Loo

A hot, dry, and oppressive wind blowing from the west and northwest across the Indo-Gangetic Plain (northern India and Pakistan) during the hot season: May and June. It can cause heatstroke and death if a person is exposed for extended periods.

Other Hot Winds

Santa Ana: hot, dry; from high desert interior toward the Pacific coast of southern California, USA. Notorious for spreading wildfires, occurring in autumn.

Simoom (Samoom): hot, dry, suffocating wind across the Arabian Peninsula and Sahara, carrying large amounts of dust and sand.

Brickfielder: hot, dry wind from the interior of Australia toward the southeastern coast.

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Cold Local Winds

Mistral

A cold, dry, strong wind blowing from the north or northwest down the Rhône Valley in southern France toward the Mediterranean. Blows particularly in winter and spring: can be violent enough to overturn vehicles and damage crops. Mistral is a cold wind blowing in France. [NDA 2007-II]

Bora

A cold, dry, powerful wind blowing from the northeast down into the Adriatic Sea: affecting the coastal areas of Croatia, Slovenia, and northeastern Italy (around Trieste). It blows from the cold continental interior outward to the relatively warm Adriatic.

Blizzard

A severe winter storm with very cold temperatures, strong winds, and blowing snow, reducing visibility to nearly zero. Associated with polar regions, particularly Antarctica and the northern interior of North America and Russia.

Buran (Purga)

A cold, violent blizzard wind blowing across Central Asia and Siberia (Russia and surrounding regions). In Siberia it is called Purga.

Pampero

A cold, violent wind blowing from the southwest across the Pampas grasslands of Argentina: originating from cold southern latitudes.

Lavante (Levante)

A cold, moist wind blowing from the east across the Strait of Gibraltar: affecting southern Spain.

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Sea Breeze and Land Breeze

Sea breezes and land breezes are examples of diurnal (daily) local winds: they change direction twice a day, driven by the different heating and cooling rates of land and water.

Sea Breeze (Day)

During the day, land heats up faster than the adjacent sea. Air over land becomes warm, expands, and rises, creating low pressure. Cooler, denser air from the sea flows in to replace it. Wind blows from sea to land: sea breeze. Sea breezes are strongest in the afternoon, when the temperature difference between land and sea is greatest.

Land Breeze (Night)

At night, land cools faster than the sea. Air over land becomes cooler and denser, creating high pressure. Wind blows from land to sea: land breeze. Land breezes are strongest in the early morning hours.

Why does land heat and cool faster than water? Land has a lower specific heat capacity than water. Water absorbs large amounts of heat with little temperature change. Land heats up quickly but also cools quickly, which is why coastal areas have smaller temperature ranges than continental interiors.

Breeze	Time	Direction	Reason
Sea Breeze	Day	Sea → Land	Land heats faster; low pressure over land
Land Breeze	Night	Land → Sea	Land cools faster; high pressure over land

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Mountain Breeze and Valley Breeze

Similar to sea and land breezes but driven by terrain height differences.

Valley Breeze (Day)

During the day, mountain slopes heat up faster than the valley floor at the same elevation. Air on slopes warms, rises, and flows up the slope. Wind blows from the valley upward along the slope: valley breeze (anabatic wind).

Mountain Breeze (Night)

At night, mountain slopes cool rapidly by radiation. Cold, dense air flows downward along the slopes into the valley. This is the mountain breeze (katabatic wind).

Katabatic wind = cold air flowing down a slope under gravity. The term katabatic is also applied to larger cold outflow winds, like the cold outflow from the Antarctic ice sheet that creates strong winds around Antarctica.

Breeze	Time	Direction	Type
Valley Breeze (Anabatic)	Day	Valley → Slope	Warm; air rises up heated slope
Mountain Breeze (Katabatic)	Night	Slope → Valley	Cold; cold air drains downslope under gravity

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