1. Moisture in the air

2. Heavy and Light air Heavy Air: (high pressure) – cold, because the molecules are closer together. – Dry, because nitrogen and oxygen are heavier than water vapor. – Think of beautiful blue skies and light breezes. Light Air: (low pressure) – warm; the molecules have more energy and are farther apart. – Humid, because water vapor is a very light molecule. – Think of days that are ‘muggy’ and warm.

3. Water vapor and energy – Water vapor enters the atmosphere due to evaporation from surface water and ice. – Energy is required for evaporation, which takes heat away from the surface. – Warm air has more energy, and can therefore cause more evaporation. – Cold air cannot cause as much evaporation, so the water vapor tends to condense or sublimate. – Condensation releases heat back into the air.

4. Dewpoint temperature – The dewpoint temperature is the temperature at which water vapor will CONDENSE to become drops of water or SUBLIMATE to become ice crystals. – The air must be cooled for this to happen. – The dewpoint depends on the amount of water vapor in the air. It DOES NOT depend on the air temperature. It indicates how humid air is. – A sling psychrometer, compares a ‘wet’ bulb to a ‘dry’ bulb. – A high dewpoint indicates a high concentration of water vapor. Dry air has a low dewpoint. novalynx.com Flickr.com

5. Relative Humidity – Relative humidity is a percent. – It compares the amount of water vapor in the air, (humidity), to the amount of water vapor the air can hold at the present air temperature. – WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD AIR DOES. – The relative humidity indicates the likelihood of dew or frost or fog forming. – 100% RH at the surface indicates that fog is probably forming. This means that the air is saturated with water vapor. – Clouds are always at 100% RH.

6. WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD AIR DOES. Places with low RH%: hot deserts. Cold Antarctica. Places with high RH%: cold, coastal regions that get lots of snow or rain. Hot coastal places (like New Orleans) and rainforests. ultimatehandyman.co.uk

7. Using a sling psychrometer Two thermometers: – The ‘dry bulb’ just measures air temperature. – The ‘wet bulb’ measures the temperature after evaporation from the wet ‘sock’ cools the thermometer. – On a dry day, there will be a lot of evaporation and the wet bulb temperature will get much cooler. – On a humid day, there may be no evaporation, and the wet bulb temperature will be the same as the dry bulb temperature.

8. Determining dewpoint temperature: page 12 ESRT regentsearth.com http://regentsearth.com/Illustrated%20ESRT/P age%2012%20(DP%20&%20RH)/RelHumDewP tMC.html

9. Determining relative humidity: page 12 ESRT http://regentsearth.com/Powerpoints/Powerp oint%20Index.htm

10. What is ‘humid’ air? Can two air masses with the same relative humidity have different temperatures? Different dew points? What does dew point really tell us? When the air temperature and dew point temperature are almost the same, then _________________???? Review book pages 174- 177 https://castlelearning.com/review/login/l ogin.aspx https://castlelearning.com/review/login/l ogin.aspx