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Chapter: 11th 12th std standard Geography earth space Higher secondary school College Notes

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Weather Maps : Winds - Low, High

The direction and speed of the wind is represented by symbols. A circle represents the station, and is usually filled with a symbol representing the cloud cover. The wind feather points in the direction from which the wind comes. Each long barb represents 10 knots (about 15 m/s or 1.1 mph), and each half-barb 5 knots.

Winds: The direction and speed of the wind is represented by symbols. A circle represents the station, and is usually filled with a symbol representing the cloud cover. The wind feather points in the direction from which the wind comes. Each long barb represents 10 knots (about 15 m/s or 1.1 mph), and each half-barb 5 knots. If there is only a half-barb, it is not drawn at the end of the feather, but a short distance from the end, so it is not mistaken for a full barb. The feather shaft alone signifies a wind of 1-2 knots.

A 'west wind' blows from the west, as in common usage. Study the upper-atmosphere charts and observe the wind speed decrease at lower altitudes. A calm wind is represented by an outer circle concentric with the cloud cover circle; clear skies and no wind is represented by concentric circles.

 

A monsoon wind is a seasonal wind blowing from the ocean onto land in summer, normally bringing moisture with it. The wind reverses in the winter, becoming a cold, dry wind. The name is from the Arabic for 'season.' Monsoon winds are famous in India, where they bring life-giving rains. Monsoons also occur in the United States. Monsoon winds are probably driven by a continental high in the winter and a continental low in the summer, which explains the seasonal reversal in direction. The southern hemisphere does not have monsoons, since there are no continents in high mid-latitudes to heat and cool strongly with the seasons, only the constant ocean.

 

Highs and Lows: The next thing to notice on the weather maps are the highs and lows, marked by large L's and H's, with their centres marked by circles containing a cross, and their central pressures given. There are local highs and lows; the pressure of a low in one part of the chart may be higher than the pressure of a high in another part. The winds will circle anticlockwise around a low, and clockwise around a high. The winds around lows are usually much more intense than those around highs. In some cases, the winds will seem to disregard the isobars, but these will be only light winds. The lows, or cyclones, will be accompanied by considerable cloud (look for black station circles), while the highs, or anticyclones, will generally be associated with clear skies.

 

Air does not take the short path and simply flow on the surface from a high to a low. There are two reasons why this happens: pressure gradient and Coriolis force. If we start at the North Pole, no matter which way we head the earth will be moving anticlockwise (eastwardly) beneath us, and again our path will deviate to the right. If, in mid-latitudes, we move directly eastward, the parallel of latitude will curve away to the north, and we will appear to be deflected to the right. The Coriolis force in every case gives the right answer. A special kind of low is the tropical depression, tropical storm, or hurricane, which derives its energy from the moisture provided by warm sea surfaces.


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