Valley and mountain breezes

The idea

By day sun-facing slopes heat faster than the valley floor, so warm air rises and cooler valley air moves up (valley breeze); after sunset slopes cool faster and cool, dense air flows down into the valley (mountain breeze).

A day in the hills

In hill towns such as Shimla and Dehradun, the wind often blows one way in the afternoon and the other way at night.

Why might the wind change direction between day and night?

• The slopes and valley floor heat and cool unequally
• The Sun changes colour
• The mountains move

Day and night

Open both tabs.

The wind turns round

Both breezes come from uneven heating. By day the slopes are the warmer place and air is pulled up them. At night the slopes are the cooler place and cold air slides down. Such daily changes in wind direction are commonly felt in hilly regions like Shimla, Dehradun and other Himalayan valleys.

By day, sun-facing slopes heat faster than the valley floor, so warm air rises and cooler valley air moves up: a valley breeze. After sunset the slopes cool faster and cool, dense air flows down into the valley: a mountain breeze.

Notes

By day sun-facing slopes heat faster, giving a valley breeze up the slopes. After sunset the slopes cool faster, giving a mountain breeze down into the valley.

Check yourself

What happens to the air over sun-facing slopes in the day?

In which direction does the valley breeze blow?

What flows down into the valley after sunset?

Why does the breeze change direction between day and night?

• It cools and sinks. That happens over the slopes after sunset, not in the day.
• It warms and rises — correct. Yes!
• It stays the same temperature as the valley. By day the slopes heat up more rapidly than the valley floor.
• Down the slopes into the valley. That is the mountain breeze, after sunset.
• Along the valley floor away from the hills. Cooler air from the valley moves up the slopes to replace the rising warm air.
• Up the slopes — correct. Yes!
• Cooler, denser air from the slopes — correct. Yes!
• Warm air from the valley floor. After sunset the valley floor is relatively warmer. The cool air from the slopes flows down.
• Rising air from the slopes. After sunset the air over the slopes becomes cooler and denser and flows down.
• The Sun changes its speed. The chapter explains the change by the slopes and the valley floor heating and cooling at different rates.
• The slopes and the valley floor heat up and cool down at different rates — correct. Yes!
• The valley changes its height. The change comes from different heating and cooling rates of the slopes and the valley floor.