Presentation on theme: "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."— Presentation transcript:
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. C E Ppt T E - Evaporation T - Transpiration C - Condensation Ppt - Precipitation When air is saturated, the rate of evaporation equals condensation
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.
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 HUMIDITY Relative 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 temperature Wet 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.