This day-time up valley winds and nightly down-valley winds are commonly found in the mountainous regions. In other words, these diurnal wind systems develop frequently over areas with large differences in relief.
These winds are chiefly caused by temperature differences between two points at the same elevation, but with a different distance from the earth’s surface. During daytime the slopes of mountains heat up rapidly because of intense insolation, but the free atmosphere at the same elevation over the lowlands is not heated to the same extent.
This results in warm air moving up along the slope. This upslope breeze is called the valley breeze. Anabatic wind is another name given to valley breeze. The valley breeze is sometimes accompanied by the formation of cumulus cloud near mountain peaks or over escarpments and slopes.
Occasional afternoon thundershowers on warm and humid days may also be produced by this kind of upslope winds in the mountainous regions. However, at night the temperature difference between mountain slopes and free atmosphere at the same elevation is reversed.
Nocturnal radiation brings about a more rapid cooling of the mountain slopes as a result of which the cool air drains into the valley below. This down-slope wind is called the mountain breeze. That is why the cold and dense air pockets are found in the lowest spots.
Mountain breeze is also called the Katabatic wind. These down-slope breezes produce radiational fogs in the valleys, particularly during the cold season. Valley breezes are more common in summer, while mountain breezes have greater preference for cold season.
There are large regional variations in the pattern of these wind systems. These breezes may occur over a single mountain or valley, along mountain ranges or escarpments or they may occur between extensive mountains and lowlands.
Valley breezes are generally more intense and more persistent. In summer when the insolation is very strong, these winds continue even after sunset. Sometimes they persist throughout the night. In the Himalayan ranges this is the common feature.
On occasions these breezes reinforce the prevailing winds on the windward slopes of mountains and cause heavy orographic rainfall. Such a type of rainfall is maximum in the afternoon which coincides with the period of maximum insolation. On the leeward slopes of mountains, the valley breezes are usually suppressed by the winds of general circulation.