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Secrets of the St. Lawrence

Sailors take warning - Keeping a weather eye

You have probably heard about depressions or lows, without knowing exactly what the words mean, other than simply bad weather. There are a number of signs that a low is approaching.

In the following example, the centre of the low is approaching from the Southwest and moving North of Baie-Comeau. The typical weather sequence for someone on the Trajectory A-B will be as follows:

1. high streaky white clouds move rapidly toward the East

  • a light Easterly wind begins to blow
  • the barometer starts fallinga veil of cloud follows, along with a halo around the sun

2. a veil of cloud follows, along with a halo around the sun

3. the cloud cover thickens

  • the sun appears to be behind smoky glass
  • greyish streaks hang from the clouds
  • the barometer continues to fall
  • the wind becomes Southeasterly and freshens

4. it starts raining

  • haze forms and is replaced by fog
  • the air becomes warmer
  • the swell is now from the Southwest

Warm front

5. the wind is now blowing from the Southwest

  • the barometer falls more slowly
  • the sun begins to pierce the clouds
  • the air becomes warmer
  • visibility improves

The weather is pleasant. The warm front has just passed. This is the warm part of the low.

6. off in the distance you can see large anvil clouds

  • thunder rolls
  • the barometer continues to fall slowly
  • the clouds have dark streaks hanging down
  • turbulent roll clouds threaten
  • a West-Northwest swell produces cross seas

Cold front

7. the wind suddenly shifts to West-Northwest and doubles in speed

  • showers reduce visibility
  • the barometer starts to climb quickly
  • the air becomes cooler

A squall has just occurred, associated with the passing of a strong cold front.

8. the sky clears

  • the wind drops, but there is still a good breeze
  • the humidity level drops
  • visibility improves
  • the barometer rises more slowly
  • a high has moved in
  • the good weather continues until the next low appears.

The low has moved North of the St Lawrence, as is generally the case during the summer. (If you find yourself under the north part of the low, the scenario will be different because you will be spared the passing of the fronts.)

Low pressure with counter clockwise winds, a warm front connected to the low brings rain and fog ahead of it.  A cold front trails the warm front and at the center of the low and along the cold front are clouds that can produce thunderstorms. A cross section of low pressure system demonstrates that the warm front at the front of the depression brings gradually darkening skies, starting with cirrus ending with nimbostratus clouds. Both nimbocumulus and nimbostratus clouds can precipitate. The cold front brings altocumulus and nimbocumulus clouds.

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