HUMIDITY AND

DEW POINT

CONDENSATION� H2O can exist in solid, liquid, or gaseous states. Change from liquid to gas is evaporation. Change from gas to liquid is condensation. Change to/from the solid to gas is sublimation.� Evaporation occurs where water is plentiful (over oceans, water and transpiring vegetation)� Condensation occurs where the relative humidity 100% (saturation) which occurs when evaporation is rapid or where the air is cooled.� Rapid evaporation occurs over water or forests.

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TE - EvaporationT - TranspirationC - CondensationPpt - Precipitation

When air is saturated, the rate of evaporation equals condensation

DEW POINTThe 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.

Humans tend to react with discomfort to a high dew point (i.e., greater than

15 °C (59 °F)).

Thermoregulation: body sweats and as sweat evaporates, the body is cooled.

At higher dew points (higher humidity sweat does not evaporate and the

body does not cool

Discomfort also exists when dealing with low dew points (i.e.,

below −30 °C (−22 °F)).

The drier air can cause skin to crack, become irritated more easily

and will dry out the respiratory paths.

RELATIVE HUMIDITYRelative humidity is a measure of how much water vapor the air actually could "hold" at a certain temperature.

The relative humidity represents how close the air is to saturation.

Saturated air will have an RH of 100 %. You need the RH of 100% to have rain form in clouds.

To measure relative humidity use a sling psychrometer (hygrometer)

Dry bulb thermometer and a wet bulb thermometer mounted together

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.

How relative humidity (RH) is measured?

Wet- and Dry Bulb Psychrometer

Wet- and Dry Bulb Psychrometer Psychrometric Chart.

If the difference between wet and dry bulb is 6º F and the temperature is 72º F (dry bulb), then the RH is 54%.

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.

When the Wet bulb temperature = the dry bulb temperature………

100% HUMIDITY!!!

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.

The "wetter or damper" you feel,, the higher is the

relative humidity.

If you feel the air is dry around you, the relative

humidity is low.