Water vapour, dew point and humidity

• Middelbare school
• vwo
• Leerjaar 2

Why do clothes get dry in a tumble dryer?

Why is it more likely to see dew in the morning after a warm day followed by a cold night?

Why can you dry your hair with a blow dryer

Dew point

Air can contain a certain amount of water.

The warmer the air the more water it can contain.

The dew point tells us how much water vapour 1 cubed meter of air can contain at a certain temperature.

When there is more water vapour than it can contain, the rest will condensate.

When temperature lowers, the amount of water vapour that the air can contain is less. So the water vapour will condensate.

This graph is figure 29 on page 113.
How much water can 1 cubed meter of air contain at a temperature of 20 C ?

1. 22 g
2. 20 g
3. 10 g
4. 17 g

This graph is figure 29 on page 113.
What is the temperature if the maximamum amount of water per m^3 is 40 g

1. 37 C
2. 32 C
3. 40 C
4. 48 C

At the end of a warm summerday it is 24  C. The air contains 16 g per m^3. Could this air hold even more water vapour?

During the course of the night and the evening the temperature decreases. At what temperature will the water in the air start to condense?

During the course of the night and the evening the temperature decreases. At what temperature will the water in the air start to condense?

1. 10 C
2. 18 C
3. 0 C
4. 24 C

Answer:

Look at 16 g. What temperature goes with it?

About 18 C

The temperature falls, finally reaching 8 C.

How many grams of water vapour will condense for each cubic meter of air?

The temperature falls, finally reaching 8 C.
How many grams of water vapour will condense for each cubic meter of air?

1. 7 g
2. 16 g
3. 9 g
4. 5 g

Answer:

Look at 8 C. You can see that the air can contain about 9g of water vapour per cubed meter. 

There was 16 g in the air and 9 gram will be left, so

16 - 9 = 7 g will condense.

During a warm day you may sweat.

Your sweat evaporates and that causes you to cool.

But some days you sweat a lot, but still don't cool.

We say then that the weather is sticky or muddy.

It has to do with the humidity of the air.

The humidity of air is a percentage.

It is calculated by the division of the content of water vapour in the air divided by the maximum content times 100%

Example:

The maximum amount of water vapour at 33 C is 34 g.

At a hot summer's day it is 33 C and the air contains 12 g of water vapour?

Calculate the humidity.

Data: maximum amount  = 34 g

           amount in air = 12 g

required: humidity

workings:

Answer h = 35%

h=​(maxamount)​​(amount)​​⋅100

h=​34​​12​​⋅100=35

Tekst

At a certain day the maximum amount of water vapour that the air can hold is 24 grams. The air contains 20 grams. Calculate the humidity.

1. 84%
2. 120%
3. 83%
4. 20%

Summary

The dew point is the point where water starts to condense. The aireither contains too much water vapour or the temperature drops.

humidity is the ratio between the amount of water vapour the air contains and the maximum amount.