Water Vapor and Humidity D14-20

Where does water vapor come from?  When warm air touched cold glass, the air cools and droplets form  Water Vapor  the water in the air Water in the form of a gas Water in the form of a gas It’s invisible, colorless, odorless, and tasteless It’s invisible, colorless, odorless, and tasteless  Humidity  the amount of water vapor in the air This is NOT rain, fog, or clouds This is NOT rain, fog, or clouds

 Evaporation  the changing of a liquid into a gas This takes a large amount of energy This takes a large amount of energy The source of the energy is the Sun; each day the Sun turns trillions of tons of ocean water into water vaporThe source of the energy is the Sun; each day the Sun turns trillions of tons of ocean water into water vapor  When air is cooled, molecules in the air slow down; molecules of water vapor slow down too; if they slow down enough they collide and stick together and change to water droplets on cool surfaces

 Condensation  the changing of a gas into a liquid This is what you see on shower doors, on a cold drink glass, and on the grass in the morning This is what you see on shower doors, on a cold drink glass, and on the grass in the morning

What is Relative Humidity?  Humidity and Relative Humidity both describe the amount of water vapor in the air  2 factors determine the amount of humidity in the air There has to be water available to evaporate There has to be water available to evaporate The warmer the temperature, the faster the water evaporates The warmer the temperature, the faster the water evaporates This means if water is available, warm air will take on more water vapor than cold airThis means if water is available, warm air will take on more water vapor than cold air

 Relative Humidity  a comparison between how much water vapor is in the air and how much air could hold (at a given temp) if it were fully saturated It is written as a %; if the relative humidity is 50%, then the air is holding half of what it could if it were full It is written as a %; if the relative humidity is 50%, then the air is holding half of what it could if it were full  Relative humidity can change from day to day

 Can be used to predict how the air will feel The higher the relative humidity the less water can evaporate, the less sweat can evaporate, and the “stickier” the air feels The higher the relative humidity the less water can evaporate, the less sweat can evaporate, and the “stickier” the air feels  Relative humidity can also be used to predict when condensation will occur This is a hygrometer; they use it to find the relative humidity.

What happens when warm moist air cools? 1. Air can cool by being pushed upward over mountains by wind

2. Heating the air also causes it to rise.  When the Sun heats the ground, air rises above the ground warms and rises, as it rises, its expands and cools

3. Air can also pushed upward when cooler air and warmer air meet  The don’t mix when they meet  The lighter air is pushed over the heavier, cold air  The result is the warm air gets pushed higher into the atmosphere and it cools