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Secrets of the St. Lawrence

Sailors take warning - Fair weather and foul

The complexity of the atmosphere is due to the air's perpetual motion. The direction of the air is influenced by various phenomena related to the Earth's rotation, solar heating, and winds created by thermal exchange in each hemisphere from the equator to the poles.

In the Northern hemisphere, the Earth's rotation creates an enormous air circulation moving from West to East, becoming warmer in the South and cooler in the North, and picking up or shedding water vapour, all depending on the topography.

The wind is the perceptible manifestation of the air's perpetual motion. We tend to have the impression that wherever we happen to be the wind is blowing in a straight line. But this is only an illusion because we are so small compared with the gigantic dimensions of the atmosphere.

In fact, the wind, whose speed and direction we perceive through the objects it moves, is only a tiny part of the air masses travelling in a clockwise or counterclockwise direction. The term clockwise refers to spiral motion in the same direction as the hands of a clock.

HighWinds spinning in a clockwise motion.

When the air mass, in our hemisphere, is moving in this direction, it is turning around an invisible axis created by the area in which the pressure is highest. This area is called a high and is generally associated with blue skies and cooler temperatures.

LowWinds spinning in a counter clockwise motion.

Bad weather, with all the clouds, rain, thunderstorms, gales, abrupt changes in temperature and violent winds it brings, is associated with a low. The air moves in a counterclockwise direction, around the area in which the pressure is lowest.

Why does the air always follow a circular path? Gustave Coriolis, a Frenchman, explained its motion by saying that it is influenced by the rotation and spherical form of the Earth.

It is interesting to note that when you are out at sea, facing into the wind, the centre of a low is always on your right. In the St Lawrence valley, because of its channelling effect on the winds, it is more tricky for mariners to determine the exact location of a low.

Example of a synoptic scale represented with map of isobars with a high pressure system and low pressure system side by side. Air flows counter clockwise around a low and clockwise around a high.

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