Humidity and dew point
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HUMIDITY AND DEW POINT. Humidity. amount of water vapor in the air. Specific Humidity. ACTUAL amount of water vapor in the air at a given time and place Units - g of water vapor / Kg of air. Saturated Air At a steady temperature the amount of water vapor is at it’s peak (maximum)

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HUMIDITY AND DEW POINT

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Humidity and dew point

HUMIDITY AND DEW POINT


Humidity

Humidity

  • amount of water vapor in the air


Specific humidity

Specific Humidity

  • ACTUAL amount of water vapor in the air at a given time and place

    • Units - g of water vapor / Kg of air

  • Saturated Air

    • At a steady temperature the amount of water vapor is at it’s peak (maximum)

      Rate of condensation = Rate of evaporation

    • Dependent on air temperature

      • Higher the air temp. - greater the amount of water vapor in the air

      • Water vapor in air doubles every 10°C


Dew point

Dew Point

  • The temperature at which air is cooled to become saturated and condensation begins

  • More water vapor in the air

  • Less the air has to cool

  • Higher the Dew Point

weather.stives-town.info


Relative humidity

Relative Humidity

  • Comparison of the specific humidity and the maximum amount of water vapor that the air can hold at a given temperature and pressure

    • UNIT - %

Relative humidity = specific humidity X 100%

maximum capacity

100% Rel. Hum. – saturated air

0% Rel. Hum. – dry air


Psychrometer

Psychrometer

  • Instrument to measure humidity

  • Dry bulb – measures air temp.

  • Wet bulb – has a water soak wick around the bulb – water evaporates – cools the air


Evaporation is a cooling process

Evaporation is a cooling process


Psychrometer1

Psychrometer

Dry air - greater evaporation

- greater cooling

- greater the difference in the 2 temperatures

High humidity – no evaporation (saturated air)

- temperatures of the 2 bulbs will be the same or very close


Evaporation question

Evaporation question

Why do you feel so hot and sticky on hot, humid days in the summertime?


Answer

Answer

High humidity means air has a lot of water vapor in it. Your body sweats but the water has no where to go so it stays on your body and you feel sticky and aren’t cooled by the evaporation of the sweat.


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