Water in the Air

1. Water in the Air Pgs. 36 - 43

2. Weather and Water • Weather is the condition of the atmosphere at a particular time and place. • The water cycle is the continuous movement of water from water sources to the air, ground and back again.

3. Water Terms • Condensation – occurs when water vapor cools and changes back into liquid droplets. • Evaporation – occurs when liquid water changes into water vapor. • Transpiration – the process by which plants release water vapor into the air through their leaves. • Precipitation – occurs when rain, snow, sleet, or hail falls from the clouds onto the Earth’s surface. • Runoff – water that flows across land and collects in rivers, streams and oceans

4. Humidity • The amount of water vapor or moisture in the air is called humidity. • As water evaporates, the humidity in the air increases. • The air’s ability to hold water vapor depends on air temperature. • As temperature increases so does the air’s ability to hold water. • What does that mean?

5. Relative Humidity • The amount of moisture the air contains compared to the maximum amount it can hold is relative humidity. • This amount is given as a percentage. • When air is holding 100% it is called saturated. • If air is not saturated you can still determine its relative humidity.

6. Calculating Relative Humidity • Pretend we know the classroom can hold 50g of water vapor. • Also, we know that there is 36 grams of water vapor in the air in this classroom. • If we take the amount of water vapor present and divide by the water vapor possible and multiply by 100, we get relative humidity. • 36g / 50g x 100= 72% relative humidity

7. Water Vapor vs. Temperature • If temperature stays the same, relative humidity changes as water vapor enters or leaves the air. • More water vapor at a temperature means a higher humidity. • If the water vapor in the air stays the same, relative humidity can change based upon the change in air temperature. • Warmer air can hold more water vapor than cooler air.

8. Measuring Relative Humidity • Psychrometer – an instrument used to measure relative humidity. • It is made of two thermometers. • One is a wet bulb thermometer, and one is a dry bulb thermometer. • The wet bulb thermometer has a damp cloth on it. • When air passes over it the water evaporates. • If humidity is high the water won’t evaporate. • The amount that evaporates determines relative humidity.

9. The difference in bulb readings will indicate the amount of water vapor in the air. • A large difference will mean there is little water vapor in the air.

10. The Process of Condensation • Before condensation can occur, air must be saturated, 100% relative humidity. • When air cools once saturated, condensation occurs. • Saturation can occur when water vapor is added to air or when air is cooled to its dew point.

11. Dew Point • Dew point – the temperature at which air must cool to be completely saturated. • Dew will form once air is saturated and has a surface to form on. • A glass of ice water is an example of air meeting its dew point.

12. Clouds • A collection of millions of tiny water droplets or ice crystals is called a cloud. • They form when warm rising air cools. • The air is saturated, and water vapor turns to liquid water. • Since condensation requires a surface, the dust and smoke in the atmosphere is the surface used to form clouds and rain.

13. Cumulus Clouds • Cumulus clouds look like big puffy cotton balls. • They tend to have flat bottoms and indicate fair weather. • Larger cumulus clouds can produce thunderstorms. • These are cumulonimbus clouds.

15. Stratus Clouds • Stratus clouds form in layers and cover large areas in the sky. • They are caused by gentle lifting air. • A nimbostratus cloud is one that produces light to heavy continuous rain. • A stratus cloud on the ground is called fog.

17. Cirrus Clouds • Cirrus clouds are thin feathery clouds found at high altitudes. • They form when wind is strong may indicate bad weather approaching.

19. Classifying Clouds • Clouds can be classified by both their type and their altitude. • High clouds: these are made at low temperature and made of ice crystals and are given the prefix cirro- • Middle Clouds: these are made of both ice crystals and water vapor and are given the prefix alto- • Low Clouds: these are made of water droplets and are given the prefix strato-

21. Precipitation • Precipitation is water, in solid or liquid form, that falls from the air to the Earth. • There are four major types of precipitation: rain, snow, sleet, and hail

22. Rain • Rain is the most common type of precipitation. • It is a liquid and raindrops fall when they are so large that they cannot be held up by the atmosphere. • Cloud droplets collide and join until they are large enough to be raindrops and fall to the Earth.

23. Snow and Sleet • Snow is the most common solid form of precipitation. • Snow forms when water vapor changes directly into a solid. • Sleet (freezing rain) forms when rain falls through a layer of freezing air. • The rain can fall as a solid or turn to a solid upon hitting a surface.

24. Hail • Hail is solid precipitation that falls as balls or lumps of ice. • Cumulonimbus clouds are typically the only producers of hail. • Sometimes large updrafts of air cause raindrops to go up to cooler temperatures and freeze. • They fall back down and then a larger updraft will force it back up. • The raindrop picks up moisture and particles and gets bigger and bigger until the updrafts can not support it.

25. Measuring precipitation • A rain gauge is an instrument used to measure rain fall. • It consists of a funnel and a cylinder for measuring. • Snow can be measured by both depth and water content. • A stick can show the depth and melting the snow will show water content.