Aim : What is relative humidity? Do Now : In your notes, describe what the term “humid” means.

Aim: What is relative humidity?

Do Now: In your notes, describe what the term “humid” means.

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

- Warmer air can hold more water vapor than cooler air.

Cold air

Warm airWHY? - Because it expands!

II. Relative Humidity (RH) describes “how full” the air is with water vapor, expressed as a percent (%).- compare the size of the air to the amount of water.

- Changing the temperature changes the size of the air and changes the Relative humidity.

- Saturation occurs when the air contains the maximum amount of water vapor that it can hold at any given temp. and pressure.

**When saturated, warm air contains more water vapor than cold saturated air.

Cold air

Warm air

Both parcels are at 100% RH

- When the Relative humidity = 100%, the air is Saturated and Condensation may occur.

III. Changing the Relative Humidity

a. Water vapor enters or leaves the air. (evaporation or

condensation)

b. The temperature changes. (changes the size of the air)

How can you change the Relative Humidity? (Think what is the definition of RH)

IV. Dew Point Temperature - the temp. at which the air will be

saturated with water vapor (RH = 100%)

=

- The closer the air temp. is to the dew point temperature:

– the air is more saturated, RH is closer to 100% and there is better chance for condensation. 1) At what time

is the RH the highest?

2) At what time is RH the lowest?

3) When is the greatest chance of rain?

V. Measuring Dew Point and RH

- using a Sling Psychrometer, you can measure the temperature of the air by “slinging” the thermometers in the air.

- There is a dry bulb temperature and wet bulb temperature.

- The wet bulb temperature is always lower.

Example:

A student measure the temperature of the air using a sling psychrometer and found that the Dry Bulb Temp. = 240C and the Wet Bulb Temp. = 200C. What is the wet bulb depression temperature?

- calculate difference between dry and wet bulb temps. This is called the wet bulb depression.

Example:Dry bulb = 240C Wet Bulb = 200C

Wet bulb depression = 40C- Now use the ESRT page

12 charts to look up the Dew Point Temperature and the Relative Humidity.

Dew point = 18 C. RH = 69%