1 Fog, dew and clouds Meteorology 10 Prof. Jeff Gawrych

Dew: air cooled to dewpoint

Frost: air cooled to dewpoint, but temp is at or below freezing point

4 How can clouds form? Air needs to be cooled to saturation. I.e. air cooled to dew point. I.e air reaches 100% RH 1)As air rises, it cools * How to force air to rise: –Topography –fronts –convection –Convergence ---> <---- 2) Surface cools off quickly by emitting radiation 3) Air moves over colder surface/ into cooler area

5 Three types of fog 1) Radiation fog –When surface cools off rapidly and reaches saturation. –A.k.a Valley fog/Tulle fog because it frequently occurs in the wintertime in the valleys (Sacramento/San Joaquin Valleys) –A low, extremely dense fog –Happens best on calm, clear days when radiation can most easily escape back to space. –Fog forms more easily on cold days because cold air cannot hold as much moisture as warm air –Why do valleys get colder than surrounding areas?

Radiation fog

7 Three types of fog 2) Advection fog –When warmer air moves over a colder surface, the air may be cooled to saturation Advection = movement –This is the SF/west coast fog that happens here, most commonly in late spring/summer –Why? Because sea surface temperatures (SST) close to the shore are much cooler than SSTs just a little bit offshore. –Upslope fog: As air moves up a slope, the gradually cooling air may cool to saturation and form fog

8 Advection fog

9 Three types of fog 3) Evaporation fog –A.k.a steam fog. –Explains why you can see your breath on cold days. –The warm, moist air from your mouth brings the outside air to saturation, creating a cloud. –Usually evaporates quickly.

Evaporation/steam fog

# of days with fog

12 Cloud characteristics Clouds are identified by their height (low, medium and high) and by their visual characteristics (flat or puffy). The four basic cloud groups are: –Cirrus: _________________________ –Cumulus: _______________________ –Stratus: ________________________ –Nimbus: ________________________

13 Where do these terms come from? The basic classification terminology has Latin roots: –Stratus (Latin for "layer") to describe sheet-like (stable) clouds. –Cumulus (Latin for "puffy") to denote a cloud with some puffy vertical (unstable) extent. –Cirrus (Latin for "curl of hair") denotes a wispy high level cloud. –Nimbus (Latin for "violent rain") means a rain cloud.

14 Cloud Classification As a first distinction, we classify clouds by the height of the cloud base. LOW CLOUDS Stratocumulus (Sc), Stratus (St) and are the primary low-level cloudsStratocumulus (Sc), Stratus (St) and are the primary low-level clouds

15 Cloud Classification As a first distinction, we classify clouds by the height of the cloud base. LOW CLOUDS Low clouds have a base below 5000ft.Low clouds have a base below 5000ft. Stratocumulus (Sc), Stratus (St) and are the primary low-level cloudsStratocumulus (Sc), Stratus (St) and are the primary low-level clouds

16 Cloud Classification MIDDLE CLOUDS Altostratus (As) and altocumulus (Ac) are the primary middle level clouds.Altostratus (As) and altocumulus (Ac) are the primary middle level clouds.

17 Cloud Classification MIDDLE CLOUDS Middle Clouds have a cloud base in the range of km ( ,000ft).Middle Clouds have a cloud base in the range of km ( ,000ft). Altostratus (As) and altocumulus (Ac) are the primary middle level clouds.Altostratus (As) and altocumulus (Ac) are the primary middle level clouds.

18 Cloud Classification HIGH CLOUDS Cirrus (Ci), Cirrostratus (Cs), and Cirrocumulus (Cc) are all high level clouds.Cirrus (Ci), Cirrostratus (Cs), and Cirrocumulus (Cc) are all high level clouds.

19 Cloud Classification HIGH CLOUDS High clouds typically refer to clouds with a base above 7 km (23,000ft) altitude.High clouds typically refer to clouds with a base above 7 km (23,000ft) altitude. Cirrus (Ci), Cirrostratus (Cs), and Cirrocumulus (Cc) are all high level clouds.Cirrus (Ci), Cirrostratus (Cs), and Cirrocumulus (Cc) are all high level clouds.

20 Cloud Classification High level clouds tend to be fuzzy in comparison to low level clouds.High level clouds tend to be fuzzy in comparison to low level clouds. This is primarily because high level clouds are made of ice crystals instead of droplets.This is primarily because high level clouds are made of ice crystals instead of droplets. Ice crystals usually evaporate slowly and thus have more time to diffuse.Ice crystals usually evaporate slowly and thus have more time to diffuse.

21 Cloud Classification Three primary cloud classes overlap the height bands: – Cumulus (Cu) – Cumulonimbus (Cb) –Nimbostratus (Ns) “nimb-” defines that the cloud is precipitating. “cumul-” defines that the clouds are a result of convection. “strat-” defines that the clouds are layered clouds. Various terms are commonly intermixed and aren’t limited to any given altitude.

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