1. Weather Notes

2. Weather
The atmospheric conditions, along with short-term changes, of a certain place at a certain time
Weather conditions can change within a few hours or last over several days in a row
Weather depends on several variables that affect atmospheric conditions

3. Weather Variables
Air Temperature – the measure of the average kinetic energy of molecules in the air
Air Pressure – the force that a column of air applies on the air or a surface below it
Wind speed and direction (wind is the movement of air from areas of high pressure to areas of low pressure)

4. (Variables Continued)
Humidity – the amount of water vapor in the air
Cloud coverage
Precipitation – water, in liquid or solid form, that falls from the atmosphere

5. Relative Humidity vs. Dew Point
Relative humidity tells us how close air is to saturation.
Dew Point tells us the temperature at which water vapor will become saturated enough to condense in the atmosphere to either form clouds, fog, or dew

6. Cloud Formation
Water heated by sunlight evaporates and turns into water vapor that rises in the atmosphere and cools
When the cooling air reaches its dew point, water vapor condenses on small particles (like dust, ash, and pollen) to form droplets
The droplets combine to form clouds

7. Types of Clouds
Stratus – low altitude clouds that form in flat layers and are gray
Cumulus – fluffy white clouds that look like cotton balls and form at medium altitudes
Cirrus – high altitude clouds that are wispy and made of ice crystals

8. Clouds That Form Precipitation
Nimbostratus – very dark gray low altitude clouds that produce light steady precipitation
Cumulonimbus – clouds that extend from low to high altitude and produce thunderstorms, heavy rain, and often hail

9. Types of Precipitation
Rain – precipitation that reaches Earth’s surface as droplets of water
Snow – precipitation that reaches Earth’s surface as solid, frozen crystals of water.

10. (precipitation continued)
Sleet – snow melts as it falls through a layer of warm air and refreezes when it passes through a layer of below-freezing air to form ice pellets
Hail – a small piece of ice that is repeatedly lifted and dropped by an updraft within cumulonimbus clouds and adds a layer of ice with each lifting

11. The Water Cycle
A series of natural processes by which water continually moves among oceans, land, and the atmosphere
Evaporation – water on Earth’s surface turns to water vapor when heated and rises in the atmosphere

12. (water cycle continued)
Condensation – water vapor in the atmosphere condenses back into a liquid to form clouds
Precipitation – liquid or solid water that falls from the atmosphere