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Weather Maps: Dew Point

Dew Point

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A calculation combining air temperature and relative humidity, given in degrees Celsius. It represents the heating effect felt due to a lack of body moisture evaporation, and gives the average person a "feeling" for how hot and stuffy the air is. Humidex is reported only during the summer months.

In Canada the area most affected by this combination of very high temperature and high humidity is southern Ontario and Quebec as moist tropical air pushes up from the Gulf of Mexico in the summer.

Use the following formula to calculate Humidex:
Humidex = T + 5/9 x (e - 10)
where:
e = vapour pressure(6.112 x 10^(7.5 x T/(237.7 + T)) x H/100)
T= air temperature (degrees Celsius)
H= humidity (%)


Wind chill is the cooling our body feels when the impact of temperature and wind are combined. Normally, on a relatively calm day our body is able to provide some protection from the outside temperature by heating up a thin layer of air that lies close to the skin. This added insulation is known as a boundary layer. On windy days, however, this insulating layer gets taken away, leaving our skin more exposed to the outside temperature. It takes time and energy for our body to warm up another layer of air, and if this layer continually gets blown away, eventually our skin's temperature will fall and our body will feel colder.

The wind can also make our body feel colder through the evaporation of any existing moisture from our skin. Through this process more heat is drawn from our body.


A sudden significant increase in, or rapid variation of wind speed. Usually a gust lasts less than twenty seconds.


A squall is the rapid onset of strong winds with speeds increasing to at least 16 knots and sustained at 22 knots or more for a minimum of one minute.


Indicates an area of high pressure. In a high, air will slowly descend and flow out in a clockwise direction at the ground. Normally a high will bring mainly sunny skies to an area.


Indicates an area of low atmospheric pressure. In a low, air is flowing counterclockwise into the centre of the Low. The air will rise and cool often resulting in clouds and precipitation.


A cold front is the leading edge of colder air. In front of it, you usually have warmer, more humid air. Behind the front lies much cooler or colder and drier air. The cause for cold fronts is colder air masses migrating southward from the polar regions. It is part of the world's natural energy circulation or cycle, this is how the earth "balances" out the warm and cold air masses around the earth.


A front can best be described as the border between two different air masses. A warm front is the leading edge of a milder or warmer air mass. A warm front travels in such a way that it results in warm air replacing colder air. This happens as the warm air rises up and over the cold air below. As the warm air travels upwards, it begins to mix with the cold air aloft and condenses to form clouds. You can usually expect periods of rain or drizzle for many hours as the front approaches.


An elongated area of low pressure.


The jet stream is like a current or river of air in the upper atmosphere, 7,000 to 13,000 metres up. It's created when cold and warm air masses come together. In the mid-latitudes of the Northern Hemisphere winds will tend to move from the west to the east. The jet stream, quite simply put, is the apex of these winds in the upper atmosphere. The minimum criterion for jet stream speed is 93 kilometres per hour. The location and orientation of the jet stream changes from day to day. Weather patterns are influenced by the position, strength and orientation of the jet stream.




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