Foehn wind

Geography glossary

Also called: Fohn, Foehn · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

The Foehn (also spelt föhn or fohn) is a warm, dry wind that blows down the leeward side of a mountain range. The leeward side is the side facing away from the wind. The air has already lost its moisture as rain or snow on the windward side (the side facing the wind). As it sinks on the far side, it gets squeezed, and this warms it up.

The name comes from the wind in the north-facing valleys of the Alps. Scientists also use "foehn" as the general name for every wind of this type. That is why UPSC often asks about the foehn together with its look-alikes: the Chinook, Zonda and Norwester.

Explanation

Key terms you need first

Adiabatic change: air heats or cools only because it is squeezed or allowed to expand. No heat is added or taken away.
Dry adiabatic lapse rate (DALR): air that is not saturated cools by about 10 °C per km as it rises. It warms by the same amount as it sinks.
Saturated (wet) adiabatic lapse rate: saturated air cools more slowly, by about 5–6 °C per km. This is because condensing water vapour gives out latent heat (heat stored in water vapour and released when it turns into liquid).

How a foehn forms (cause → process → result)

Windward side: moist air rises
Moist wind hits the mountain and is pushed up.
It cools and becomes saturated. Clouds form, and rain or snow falls.
Above the cloud base, the air cools only at the slow wet rate.
Summit: the air is now dry
The air has dropped most of its moisture on the windward slope.
Leeward side: dry air sinks
The dry air sinks down the slope and is squeezed.
It warms at the fast dry rate of about 10 °C/km.
Result: warm, dry wind at the foot of the slope
The air arrives warmer and much drier than it was at the same height on the windward side.
It cuts relative humidity and melts snow quickly.
NCERT angle: Class 11, Ch 9 describes this process but does not name the wind.

Conditions it needs

A high mountain barrier that lies across the path of the wind.
Moist air on the windward side, so that rain or snow can fall there.
A pressure gradient (a difference in pressure from one side to the other) that pushes the air over the ridge and down the far side.

Effects of the Alpine foehn

Melts snow early in spring.
Helps grapes ripen in the Alpine valleys.
Raises avalanche risk, because snow melts suddenly.
Raises fire risk, because the air and vegetation become very dry.
The leeward side lies in a rain shadow (a dry area behind a mountain).

Where it occurs

Foehn proper: the north-facing valleys of the Alps (Switzerland, Austria), at about 46–47° N. It blows mainly in spring and autumn.
The same kind of wind elsewhere:

Wind	Region	Season	Main effect
Chinook	Eastern slopes of the Rockies: Alberta (Canada), Montana and Wyoming (USA)	Winter and spring	The "snow eater". It clears pastures so cattle can graze, and can raise temperature by 20 °C or more within hours
Zonda	Eastern (leeward) slopes of the Andes, Argentina (Mendoza–San Juan)	Mainly May–November	Hot, dry and dusty. It harms crops and health
Norwester	Canterbury Plains, east of the Southern Alps, South Island, New Zealand	Spring and summer	Hot, dry and gusty. It dries the soil

Rain-shadow regions shaped by the same physics:
In India: the Deccan, east of the Western Ghats.
In the world: Patagonia, east of the Andes.

Don't confuse with

Chinook: this is the same type of wind, not a different process. The foehn belongs to the Alps. The Chinook is the foehn of North America, on the eastern slopes of the Rockies.
Mistral / Bora: both are cold, dry winds. The Mistral blows down the Rhône valley in France. The Bora blows on the Adriatic coast. They get their speed because a narrow valley squeezes the air (the funnel effect). They are not warmed by sinking.
Loo: a hot, dry westerly over the north-western plains of India in May–June. It gets its heat from advection (heat carried sideways by moving air from the Thar Desert). It is not a wind that warms by sinking down a mountain.
Sirocco: a Saharan wind that starts dry but picks up moisture over the Mediterranean. It reaches Sicily and southern Italy hot and humid. A foehn stays dry.

Prelims Hooks

Foehn → Alps → helps grapes ripen. Chinook → Rockies → "snow eater".
The foehn warms at the dry adiabatic rate (about 10 °C/km) as it sinks. On the windward side, rising air cools at the slower wet rate (about 5–6 °C/km) once it is saturated.
Zonda is the foehn of Argentina (Andes). Norwester is the foehn of New Zealand's Canterbury Plains. Trap: neither is a cold wind.
Trap: the foehn is warm and dry. The Mistral and Bora are cold, even though the Mistral also starts near the Alps.
The Chinook can raise temperature by 20 °C or more within hours. It shortens the time cattle must be fed indoors in winter.
"Foehn" is also the general name for all warm, dry winds that blow down the lee side of a mountain.

Mains Points

How relief shapes climate (GS-I):
Moist air is forced up a mountain, drops rain on the windward side, and warms as it sinks on the leeward side.
This one chain explains the foehn, Chinook, Zonda and Norwester.
It also explains rain-shadow regions such as the Deccan behind the Western Ghats and Patagonia behind the Andes.
Effects on economy and livelihoods:
Helpful: the foehn ripens grapes in the Alps. The Chinook clears snow from pastures so cattle can graze.
Harmful: sudden snowmelt raises avalanche risk. Dry, warm air raises fire risk in the valleys.
Disaster and climate-change angle (GS-III):
Air that sinks and warms makes vegetation very dry, so fires spread fast.
Hotter, drier conditions make downslope winds more dangerous: more wildfires and more heat stress.
Avalanche warnings and fire-weather alerts are needed in foehn-prone valleys.

Related concepts

Named regional winds of the world

Sources

1Class 11, Ch 9 "Atmospheric Circulation and Weather Systems" (primary)