Humidity Water Vapour in the Air

The more water vapour in the atmosphere A) the air becomes heavier. B) the air becomes lighter. C) the air stays the same

What are the components of air?

Air components Dry air = 99% Nitrogen & Oxygen gas These gases are heavier than water vapour SO if water vapour replaces dry air the atmospheric pressure will go down. Therefore, humid or ‘wet’ air exerts less atmospheric pressure than dry air.

What does a decrease in atmospheric pressure suggest about the weather?

Warm, humid air is approaching and the temperature will increase. So an increase in atmospheric pressure suggests that cool, dry weather is on its way.

Humidity is: A measure of the amount of water vapour in the air (page 20) veFrizzbutkeepcurl.aspx

What factor primarily determines how much water vapour the air can hold?

Temperature Which temperature can hold more water vapor?

The factor that primarily determines how much water vapour the air can hold is: Temperature Warmer air has a greater capacity for holding water vapour than cooler air

The amount of water vapour actually present in the air is called specific humidity The air becomes saturated when the specific humidity equals the capacity of air to hold water at a specific temperature What happens when the air cools below this temperature?

Condensation DEW POINT is the temperature at which condensation will occur. The Dew point is the temperature the air must be in order to be saturated. Remember, cooling the air makes it unable to hold as much water vapor, so water will condense out at a certain temperature. The ice makes the air near the glass cooler and it reaches its’ dew point, cool air can’t hold as much moisture and condensation occurs

Most often we here another term when referring to humidity

Relative Humidity RELATIVE HUMIDITY is the amount of water in the air compared to the amount of water the air could possibly hold. Humidity values are usually given as Relative Humidity. –Examples: –if the air is holding half the water it could hold, it’s Relative Humidity is 50%. –If the air is holding ALL the water it can hold it is saturated and the relative humidity is 100% –If the air is holding no water, relative humidity is 0% –If it holding a quarter of the water it could hold, 25%

Saturation If the air is SATURATED, it is holding ALL of the water that it can hold. –WARM air expands and can hold more water vapor than COLD air, so it takes more water to saturate warm air. If the air is saturated, its Relative Humidity is 100%, and if the temperature drops PRECIPITATION will occur.

Water capacity of air at different temperatures

Measuring Humidity Relative Humidity is measured using a PSYCHROMETER. –A psychrometer is made of two thermometers. One is covered with a wet cloth. When air moves over the wet cloth, evaporation occurs and lowers the temperature on that thermometer. If you compare the temperature on the two thermometers you can get the relative humidity.

Ex. 1 Dry Bulb = 14 degrees C Wet Bulb = 10 degrees C Difference is = 4 1 st – look at dry bulb reading (14) 2 nd – find difference (4) 3 rd – RH is where they meet = 60%

Difference is 4-3 = 1 RH =85 % Dry Bulb= 4 degrees Wet Bulb= 3 degrees

Ex. 3 Dry Bulb = 6 degrees C Wet Bulb = 6 degrees C Difference is 6-6 = 0 RH =100%

Activity 2-1B Using the Humidex Scale Complete this activity on page 51 pass in when completed.