1. Dew, Frost, and Fog

2. Humidity Humidity -Amount of water vapor in a given volume of air
Absolute Humidity – The actual amount of water vapor in a given volume of air –in grams per cubic meter
Relative Humidity – The percentage of water vapor in a given volume of air
100% Saturated (Full), it cannot hold any more water vapor - condenses

3. The warmer the air, the more water vapor it can hold
The colder the air, the less water vapor it can hold
So when air is cooled, it will be able to hold less water and can quickly become saturated

4. Relative Humidity

5. Dew and Frost Temperature quickly drops at night
Cools air directly above surface
Air molecule shrinks (Humidity increases) until air is saturated
The dew point is the point at which the air becomes 100% saturated and vapour becomes liquid
Water droplets form on grass = Dew
If temperature is below freezing when this happens = Frost

7. Fog
Air that is close to the Earth that reaches its dew point and begins to condense
“Low lying cloud”

8. Radiation Fog Forms during clear and cool nights
Ground cools air near surface
When dew point is reached, condensation happens and fog will form.
Does not form when there is wind, as the wind would cause the cool air to mix with warmer air

9. Radiation Fog

10. Radiation Fog

11. Advective Fog
Occurs when moist air passes over a cool surface by advection (wind) and is cooled.
Common when tropical air encounters cooler waters. Eg. California coast
When warm air passes over a cold current, it can create advective fog

12. Advective Fog

13. Advective Fog

14. Advective Fog

16. What do you need to know about clouds?
High Clouds – meters
Mostly ice crystals, not precipitation-forming
Cirrus clouds are high, wispy clouds
Cirrostratus and cirrocumulus take shapes like the types they are named after
Stratus= wide, flat layers
Cumulus= puffy

17. Low clouds –less than 3000 meters
Stratus clouds – layers of grey cloud, usually cover most of the sky – can cause light precipitation
Nimbostratus – dark, thick layers of cloud
-produce lots of precipitation
Stratocumulus –layers of puffy clouds, not much precipitation

18. Middle clouds -2000 to 7000 meters
Altostratus – gray and layered, lots of moisture
Altocumulus – white and grey, puffy

19. Vertical Clouds – generated by rising air currents (convection)
-can reach great heights
Cumulus- dark base, tall white clouds
Cumulonimbus –air circulates quickly in this cloud, can produce lighting and hail
-anvil shaped vertical cloud
-associated with extreme weather

20. Orographic precipitation
Warm, moist air is forced over a mountain
As the air rises, it cools and condenses
There is heavy rain on the windward side of the mountain
On the leeward side, the air lacks moisture and often creates a rain shadow (area with little rain)
Decreasing pressure creates adiabatic cooling
Increasing pressure creates adiabatic heating

21. Frontal precipitation
When 2 air masses meet, the warmer mass will be forced to rise
As the air rises it cools and the moisture condenses, causing precipitation
Warm fronts are when a warm mass moves over a colder mass and rises with a gradual slope
Warm fronts bring precipitation over a large area

22. Cold Front
A cold front is when a cold mass moves into a warm mass
It forces the warm air up at a steep angle
This creates vertical clouds (cumulonimbus) that can cause heavy, short periods of precipitation
Cold fronts move faster than warm fronts and create more severe precipitation, such as hail

23. Convectional Precipitation
Occurs in tropical areas or in the middle of large land masses
The sun heats the ground, causing the warm air to rise
As the air rises it cools and condenses, usually creating a cumulonimbus cloud
These clouds are associated with short, heavy rains and sometimes hail and thunderstorms

24. Thunderstorms and Hailstorms
Thunderstorms are formed when warm, moist air is forced upwards
The rapid rising and cooling of air creates extreme precipitation and winds
As water droplets are circulated in this rising and falling air, they freeze as they go up and are coated with more water as they go down
They may continue to rise and fall, growing in size until they are so large that they fall to the ground

25. Intense storms can cause lightning and thunder
Lightning is the discharge of electricity from the clouds to the surface of the earth or other clouds
It is caused when the ice particles in the cloud rub against each other, forming an electric charge
The lightning bolt is caused when the electricity moves from one polarity to the other

26. Lightning reaches immense heat, as high as 28000C
This heat causes the surrounding air to expand rapidly, just like an explosive
This explosion creates the sound waves that we hear as thunder

27. Assignment
Explain the difference between relative humidity and aboslute humidity.
Explain what dew point is and how it relates to the creation of clouds
Explain how the two types of fog are formed.
Explain how convection causes the creation of cumulonimbus clouds
In your own words, explain the 4 types of precipitation we discussed.
How does convection lead to the creation of hail and lightning?