How do we measure relative humidity?

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Lesson 21 How do we measure relative humidity?
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Have you ever tried to air dry clothes on a humid
day?
It took a long time, didnt it?
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The air already contained many water molecules,
so the water from the clothes evaporated very
slowly.
Evaporation means to change to a gas.
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How fast evaporation takes place depends on the
humidity.
When the water does evaporate something happens.
The surface that had been wet becomes cooler.
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For example, when the sweat on your body
evaporates, your skin cools down.
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Now you know two important facts about
evaporation.
-Water evaporates faster when the air is dry.
-When water evaporates, the temperature goes down.
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These facts make it possible to measure relative
humidity.
Relative humidity is measured with a hygrometer.
One kind of hygrometer is the wet and dry bulb
thermometer.
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The wet and dry bulb thermometer has two
thermometers.
One of the thermometers has a wet cloth around
the bulb. This is the wet bulb.
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The thermometer without the wet cloth is the dry
bulb.
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-the dry bulb measures the normal air temperature.
-the temperature of the web bulb thermometer
depends on how fast the water evaporates.
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When the air is saturated, no evaporation takes
place.
The temperature of the wet bulb thermometer is
the same as the dry bulb thermometer
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When air is not saturated, evaporation does take
place.
The wet bulb cools down, and it has a lower
temperature than the dry bulb.
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If the wet bulb has a much lower temperature than
the dry bulb, it means the air is not very humid.
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To figure out the relative humidity of the air,
you need to know two things
-The dry bulb temperature
-The difference between the wet and dry bulb
temperatures.
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Once you know these two temperatures, you use a
chart to find the relative humidity.
Relative humidity is stated in a percent.
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Lets try one.
The dry bulb temperature is 28, and the wet bulb
is 24.
What is the difference between the two
temperatures?
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So, on the left side of the chart, we find the
dry bulb temperature of 28.
Along the top, we find the difference of 4.
Like a multiplication chart, we follow the two
lines until they meet.
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What is the relative humidity?
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Please remember that scientists always measure in
the metric system, and American weather people
measure in the English system.
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When you hear the relative humidity on TV, it was
always found using degrees Fahrenheit.
Scientists will always measure in degrees
Celsius.