The Atmosphere

1. The Atmosphere B5: Weather Factors Part 3 – Precipitation

2. Water in the Atmosphere • Water Cycle – The movement of water between the atmosphere and the Earth surface • Water vapor enters the atmosphere through evaporation and transpiration • It then condenses into clouds • Eventually it forms precipitation and falls back to the Earth • On Earth it becomes surface run off to begin the cycle again

3. Water in the Atmosphere • Evaporation and Transpiration – Process in which surface water enters the atmosphere • Evaporation – process in which liquid molecules escape into the air as water vapor • Transpiration – the evaporation of water from the leaves of plants • Humidity – the amount of water vapor in the air • The air’s ability to hold water vapor depends on its temperature • Warm air can hold more water vapor than cool air

4. Water in the Atmosphere • Relative Humidity – Percentage of water vapor that is actually in the air compared to the maximum amount the air can hold at a particular temperature • Example – At 10o C, 1 cubic meter of air can hold at most 8 grams of water vapor. • If there were actually 8 grams in the air, the relatively humidity would be 100% • Saturated – Air that has 100% relative humidity • If there were actually 4 grams in the air, the relatively humidity would be 50%

5. How Clouds Form • Clouds form when water vapor in the air condenses to form liquid water or ice crystals • Condensation – process in which water vapor in the air becomes liquid water • Condensation requires 2 conditions; a cooling of the air and the presences of particles • Role of Cooling – Cold air holds less water vapor • As air cools, it can nolonger hold all the water vapor • The water vapor condenses into tiny droplets of liquid water or ice

6. How Clouds Form • Dew Point – the temperature at which condensation begins • If the dew point is above freezing, water droplets form. If it is below freezing, ice crystals called frost form • Role of Particles – In addition to a change in temp, particles must also be present • For water vapor to condense, it needs a surface for the droplets or crystals to form on • In clouds formation, the most common particles are salt crystals, dust from soil, and smoke

7. How Clouds Form • On Earth, water vapor condenses on any solid surface like grass or window panes • Frost – on Earth, if the dew point is below freezing, the ice that forms is called frost

8. Types of Clouds • Scientists classify clouds into 3 main types based on their shape: cirrus, cumulus, and stratus • Cirrus Clouds – Wispy, feathery clouds that form at high altitudes • Made of icecrystals because they form very high up where the temperatures are low • Cirrocumulus – rows of cirrus clouds often indicate a storm is on its way • Cirrus Clouds

9. Types of Clouds • Cumulus Clouds – Fluffy, rounded clouds that form lower in the atmosphere • Not very tall and usually indicate good weather • Cumulonimbus – very tall cumulus clouds that often produce thunderstorms • Cumulus Clouds

10. Types of Clouds • Stratus Clouds – Flat, gray clouds that form in layers • Usually produce drizzle, rain, or snow • Stratus Clouds • Fog – a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface • Usually forms at night after a warm, humid day • When the ground cools, the air just above it cools to it’s dewpoint

11. Precipitation • Precipitation – any form of water that falls from clouds and reaches Earth’s surface • For precipitation to occur, water droplets or ice crystals must grow heavy enough to fall through the air • One way for droplets to grow is by colliding and combining with other droplets

12. Types of Precipitation • Rain – Most common liquid precipitation • Most common types of precipitation include rain, sleet, freezing rain, snow, and hail • Water droplets must be atleast 0.5 mm in diameter • Droplets that are smaller than 0.5 mm are called drizzle • Precipitation that is even smaller is called mist

13. Types of Precipitation • Sleet – Rain that has frozen in the air into solid drops of ice • Raindrops fall through a layer of air that is below freezing • Freeze and form ice smaller than 0.5 mm in diameter • Freezing Rain – Rain that freezes when it touches the ground instead of in the air • Rain falls through a layer of cold air near the ground but does not freeze in the air

14. Types of Precipitation • It freezes into smooth, thick layers on surfaces • Often referred to as an ‘icestorm’ • Can cause a lot of damage when the weight of the ice breaks tree • branches or takes down power lines • Snow – Water vapor that is converted into ice crystals called snowflakes while in clouds • All have unique shapes and can join to form larger crystals

15. Types of Precipitation • Hail – Round pellets of ice larger than 5mm in diameter • Only forms inside cumulonimbus clouds during thunderstorms • Ice pellets start to fall until strong updrafts carry it up through the cloud again • Each time it goes through a cold layer, a new layer of ice is formed and it begins to fall again • The more cycles it makes, the larger the hailstone that is formed • Eventually it becomes heavy enough to fall out of the cloud to the ground

16. Types of Precipitation • Hail stones can grow very large depending on how many times they cycle through the cloud • They can also cause tremendous damage to crops, • buildings, • and cars

17. In Closing • Describe the water cycle