# Weather_instruments by chenmeixiu

VIEWS: 23 PAGES: 18

• pg 1
```									 Weather

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
matter.
 The rate of motion of these particles
results in their kinetic energy and is
measured as temperature.
http://physics.csustan.edu/java/tutorials/temperature/temperature.htm
1. Thermometer
 A thermometer is a device used to measure the outside air
temperature.
 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
 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
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.
(km/h).
3. Psychrometer
 A psychrometer (sike ro
meeter) is a device that
measures the amount of
relative humidity in the
air.
 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
analogy:
 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
hold!
 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
air.
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.
Activity
 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
pressure?

```
To top