Weather_instruments by chenmeixiu



Measuring Change
            Lesson Objectives
In this lesson you will learn:
 identify the function of the following weather
  instruments used in a weather station:
  thermometer, psychrometer, aneroid
  barometer, anemometer, and rain gauge
               Measuring Change
Measuring Temperature
 Temperature is one of the weather
  conditions measured on a regular basis.
 Temperature is defined ad the measure of
  the average kinetic energy of a sample of
 The rate of motion of these particles
  results in their kinetic energy and is
  measured as temperature.
               1. Thermometer
 A thermometer is a device used to measure the outside air
 How does a thermometer work?
 The thermometer is based on the idea that matter expands
  when heated and contracts (shrinks) when cooled. The
  liquid inside the thermometer behaves this way.
 As the thermometer is heated the liquid inside warms up
  and expands (takes up more space). Since there is no
  where to go but up, the liquid rises up the thin tube.
 When the thermometer is cooled, the liquid then contracts
  (shrinks) and moves back down into the bulb.
 Does it matter if the thermometer is placed
  in the sun or shade?
 The thermometer must be placed in the
  shade. If it is placed in the sun you will not
  measure the outside air temperature
  accurately. The direct sunlight will cause
  the liquid in the thermometer to warm up
  giving you a false reading.
       Measuring Wind Speed
 In winter a high wind speed will make us feel
  much colder than on a day when the wind is
  very calm even though the temperature is
  the same in both cases. This is known as
  the "wind chill factor."
                2. Anemometer
 An anemometer (annie mom meeter) is
  used to measure the speed of the wind.
Weather forecasters'
anemometers have the
propeller connected to a small
generator and computer that
can accurately count and
calculate the revolutions per
minute into kilometers per hour.
            3. Psychrometer
 A psychrometer (sike ro
  meeter) is a device that
  measures the amount of
  relative humidity in the
 Humidity is the measure
  of the amount of moisture
  (water vapour) in the air.
   Measuring Relative Humidity
 Consider that our sample of air is saturated. This
  means that it is holding its maximum amount of
  water. Under this condition, the air sample would
  have a relative humidity of 100%. If we could
  remove half of the water from our air sample, it
  would no longer be saturated. The relative
  humidity would now be 50% since it now holds half
  the maximum amount of water that it could hold
  when saturated.
   Measuring Relative Humidity

 To make this a little simpler we can use an
 Alex has big hands. He can hold 100 jelly beans
  when his hands are cupped together. If he has
  83 jelly beans in his hands, he is holding 83% of
  the maximum number that he could possibly
 This is just like the air. The air can hold a
  maximum amount of water. If we can measure
  how much water is actually in the air we can then
  figure out its percentage or relative humidity.
   Measuring Relative Humidity
 Consider the following problem: If your
  sample of air when saturated (relative
  humidity of 100%) held 5 grams of water (in
  the form of vapour), how many grams would
  be in your air sample, if the relative humidity
  was reduced to 20%?
 If the relative humidity is reduced to 20%,
  there would be only 1 gram of water
  vapour present in the air sample.
What does humidity have to do with
          the weather?
 Warm air can hold more water than cold air. The
  warm air is more spread out than cold air and
  because of this there is more space for water
  vapor between the warm air molecules. The more
  water that is in the air the greater the chance for
  precipitation (rain or snow). So warm, wet air is
  usually associated with wet, stormy weather. The
  hygrometer can detect a change in humidity and
  thus predict a change in the weather.
         4. Aneroid barometer
An aneroid (an er oyd) barometer, commonly called a
barometer is a device used to measure surrounding air
pressure. Air pressure is the force that air pushes down
on the earth.
Cold air is denser (thicker and heavier) than warm air.
The colder the air the more we can squeeze into the same
size air mass. As a result cold air is heavier than warm
So a high barometric pressure is usually associated with
cool, clear, dry air – fine weather.
 If the air pressure
  begins to drop that
  means a low pressure
  system is moving in. A
  low pressure system is
  usually warmer, moist
  air - poor weather on
  the way.
However, if we experience a sudden change in
elevation (altitude) such as when we travel in an
airplane, we may notice a pain against our ear
drums, or even pain in our sinuses. This pain is
due to the imbalance of the air pressure. The
pain usually goes away as soon as the pressure
balance is once again equalized. Sometimes
our ear pressure is balanced suddenly as our
ears "pop."
             5. Wind vane
 A wind vane (weather vane) is a tool that
  measures the direction of the wind. A
  typical wind vane would have pointer that
  can spin with one end larger than the other
  and compass bearing.
   Weather Balloons

 Weather vanes can only measure wind
  direction a few meters off the ground. Large,
  helium-filled weather balloons are used to
  measure winds high above the earth's
  surface. The balloons move with the same
  speed and direction of the wind. Using
  computer tracking devices the speed and
  direction of the balloon and thus the
  wind can be measured.
 Place a small quantity of water into a can and
  place on a hot plate until the water begins to boil.
  The water vapour (steam) will drive out the air
  from within the can. Place a lid on the can to
  prevent air from entering. Remove the can from
  the hot plate and allow the can to cool. As the can
  cools the water vapour (steam) will condense to
  liquid water. Observe what happens to the can as
  it cools. Write a paragraph in which you attempt to
  explain why the can behaved in the way it did.
  How is this lab experience related to atmospheric

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