bullet Katahatic (downslope) winds affect sea conditions off mountainous coastal areas,
bullet They develop at night when there are clear skies, a general slack pressure gradient, and rapid radiative cooling of the land.
bullet The air at X adjacent to the slope, becoming cooler and therefore denser than the air which is further away at the same level (Y), descends forming the katabatic wind.
bullet On reaching the foot of the slope the wind moves out to sea.
bullet Adiabatic warming of the air during descent is counteracted by conduction as it is in continuous contact with the colder mountain slope.
bullet When the slope is covered with ice or snow, which are effective insulators, very limited conduction takes place between the slope and the upper surface of the ice or snow cover. Thus during the cooling period overnight the upper surface experiences a more rapid decrease in temperature than that of a bare slope.

Under such conditions, the adjacent air, becoming very cold and dense, descends at a speed which can reach gale force conditions.


Occurs particularly in the Norwegian fjords and the ice covered regions of Greenland and Antarctica.


                                                Figure: Katabatic winds


bullet During the day an anabatic wind develops.
bullet With clear skies the slope absorbs solar radiation and heats the air (P) directly in contact.
bullet This air then flows up the slope, as it is at a higher temperature than the air (Q) further away at the same level.
bullet The air moving up the slope is subject to expansion and therefore cooling.
bullet The contact maintained with the warm slope ensures its continued ascent, with a maximum speed during mid-afternoon.
bullet Anabatic winds occur in the Alps in summer months when there is generally a slack pressure gradient.


                                        Figure: Anabatic winds



Source: Meteorology for seafarers

Updated: 21-Sep-2007