–      Forecasting weather can be difficult because there are many factors involved in weather such as the motion of air and water masses.

–      Weather forecasters (meteorologists) use complex computer models, weather instruments, and observations of weather patterns to predict the weather.

–      However, through careful observation, weather can be predicted by any individual.

–      When older people complain of nerve and joint pain, they usually predict a low pressure system (and therefore an upcoming storm).  This is because under low air pressure, dissolved gases in the blood form bubbles causing pain.

–      Birds are also indicators of air pressure.  In a low pressure system, birds fly lower than normal.

–      Let’s look at some weather instruments that are used:


–      A thermometer measures the temperature of air.

–      The temperature of the air is determined by how fast air molecules are moving.  It feels warm when air molecules are moving faster (because a lot of collisions are occurring) and colder when molecules are moving slower (therefore less collisions).

–      Why would you want to measure temperature in the shade and not place the thermometer in direct sunlight?

–      Direct sunlight will cause the air molecules to move faster and therefore increase the measured temperature.


–      A hygrometer measures the humidity of the air immediately around it

–      Humidity is a measure of how much water vapour is in the air.

–      In general, humidity increases before it rains and decreases after the rain.  Whey does this make sense?  Because humidity measures how much water vapour is in the air and you would expect water vapour to increase just before it rains.

–      When a warm front (a warm mass of air) approaches, humidity will increase.  During the warm front humidity will be very high, and will decrease slightly as it leaves.

–      When a cold front (a cold mass of air) approaches, humidity will stay steady.  As soon as the cold front hits, humidity will be very high and decrease quickly when it leaves.

Wind Vane

–      A wind vane shows the direction of the wind.

–      Wind direction can tell us about what type of weather to expect.

–      Winds from the north or northwest usually bring cooler weather.

–      Winds from the south or southwest usually bring fair and warmer weather.

–      Winds from the southeast, east, and northeast usually bring rain.


–      An anemometer is used to measure wind speed.

–      Wind speed ranges from “calm” (0-1 km/h) to a “fresh breeze” (30-38 km/h) to “storm” (88-101 km/h) to “hurricane” (greater than 119 km/h). (See table on page 205)


–      A barometer measures air pressure.  Air pressure measures the force of the air pressing down on the Earth.

–      Cold air exerts more pressure than warm air.

–      If the pressure is increasing, the approaching air is colder and heavier.

–      If the pressure is decreasing, the approaching air is warmer and lighter.


  • HumidityHumidity measures the percent of water in the air.  It ranges from 0 to 100%.  Toronto usually has an above average humidity because it is near a large lake.
  • Air PressureAir pressure is measured in “kilopascals” (kPa). Average air pressure is 101.3kPa. The highest recorded air pressure is 108.6kPa and occurred in December 2001 in Mongolia. The lowest recorded air pressure is 87.0kPa and occurred in October 1979 in the center of a typhoon.
  • TemperatureTemperature is a measurement of how fast molecules move and is measured in degrees Celsius (oC). Room temperature is approximately ­20oC to 25oC.

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