Define Col, Ridge & Trough ?

A Col is an area between two highs and two lows situated alternately. Light variable winds are experienced but not for long. Sudden change of weather is likely. Relative humidity is fairly high and lightning may be seen. No definite pattern of weather is associated with a Col.

A ridge or wedge is an area of high pressure jutting into areas of low pressure. The isobars are

curved, with the high pressure inside, and are generally far apart. They do not necessarily form

closed shapes. A ridge may form by itself or it may be the outer fringes of an anticyclone far away.

The weather associated with a ridge is like that of an anticyclone – no precipitation, light winds, no

clouds.

A trough is an area of low pressure jutting into areas of high pressure. The isobars are curved, with the low pressure inside, but they do not form closed shapes. The pressure gradient is fairly high resulting in strong winds. The winds blow from the high pressure areas towards the areas of low pressure, being deflected to the right in the NH, and left in the SH, by Coriolis force. Bad weather is associated with a trough. Before the trough, pressure falls and weather deteriorates. After the passage of a trough, pressure rises and weather improves.

There are two forms of trough:

Non-frontal trough: In this case, the isobars curve gently. When a non-frontal trough passes over an observer, the wind veers gradually in the NH and backs gradually in the SH. The “U” of the non-frontal trough always points towards the equator.

Frontal trough: A frontal trough exists at the boundary between two different air-masses. The ‘V’ formed by the isobars always points towards the equator. On crossing a frontal trough, the isobars change direction suddenly by about 90° – veers in the NH and backs in SH. Squalls may be experienced, accompanied by lightning and heavy precipitation. Since one air-mass is replaced by another, a sudden change of temperature also is experienced on its passage.