5th Lecture - Humidity

HUMIDITY

5th Lectureby

Muhammad Shahbaz Gulzar

Water sources

Oceans

Ice sheets & glaciers

Groundwater

Lakes

Atmosphere

Streams

Water moving from earth to atmosphere

Evaporation - water changes from liquid to vapor

Transpiration - water taken up through plant roots

Sublimation - water changes from solid to vapor

Evapotranspiration - sum total of evaporation & transpiration together

Water moves from atmosphere to earth

Condensation - water changes from vapor to liquid

Deposition - water changes from vapor to solid

Humidity

Amount of water vapor in atmosphere (air)

Expressions of humidity: Specific humidity

Relative humidity

Dew Point

Specific humidity

Also known as the mixing ratio

Described as the mass (weight) of water vapor per mass of dry air

Grams (water) / kilogram of air g/kg

Higher temperature, higher maximum specific humidity

Warm air contains (can hold) more water vapor than cold air

Maximum water vapor per temperature

TEMPERATURE GRAMS/KG OF AIR

90 40

80 30

70 20

60 10

50 5

40 2

Relative humidity

Relative Humidity is a measure of the amount of moisture in the air compared with the amount of moisture the air can hold.

Relative humidity is expressed as a percentage of how much moisture the air could possibly hold at the temperature it happens when you measure it.

Can change when either the temperature or the amount of water vapor changes

Indicates how near the air is to saturation

With no change in water vapor content

As the temperature increases the relative humidity will go down

As the temperature decreases the relative humidity will go up

Saturation

When the relative humidity is 100% for a given temperature

If the air is less than 100% relative humidity, it is unsaturated air

If the air is above 100% relative humidity, it is supersaturated

At warmer temperatures, the molecules of water vapor in air move quickly and dont easily come together.

At cooler temperatures, molecules in air move more slowly.

The slower movement allows water vapor molecules to stick together and form droplets of liquid water.

Dew point

The dewpoint is the temperature of air which is needed for condensation or dew (at that particular temperature).

If you take a glass of ice water and it develops condensation on the glass surface, the air on the glass has condensed to its dewpoint and created dew.

Dewpoint actually measures how much water vapor is in the air.

These temperatures are given in Fahrenheit.

Relative humidity measuring instruments

Sling psychrometer

Hair hygrometer

Hygrograph

Psychrometer

Two mercury thermometers side by side

One is dry bulb, the other wet bulb

Dry bulb tells actual temperatureWet bulb shows how much water can be evaporated temperature lowers as water is evaporated.

The difference in temperature on the 2 thermometers is an indication of the amount of water vapor in the air.

Larger the difference on the reading the lower the relative humidity

If air is saturated the thermometers will have the same reading

When the Wet bulb temperature = the dry bulb temperature

100% HUMIDITY!!!

Wet- and Dry Bulb Psychrometer

Dry air: the water will evaporate quickly and cause a large drop in the wet-bulb temperature.

This makes the difference in readings on the 2 thermometers greater.

Moist air: little water will evaporate from the wet-bulb and the temperature decrease will be small.

The difference between the wet bulb and dry bulb will be small.

Humidity stretches the hair (human /animal / synthetic) while dryness shortens it.

The hygrometer has the job of stretching a hair between a fixed and a movable point to measure humidity over time.

Other kinds of hygrometers use materials with electrical resistance that varies with the amount of moisture absorbed.

With such hygrometers, a measurement of electrical resistance can be calibrated as a humidity measurement.

Hair Hygrometer