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Fog, dew and clouds

Meteorology 10Prof. Jeff Gawrych

Dew: air cooled to dewpoint

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

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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

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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

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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

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Advection fog

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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# of days with fog

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Cloud characteristicsCloud 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: ________________________

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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.

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Cloud ClassificationCloud ClassificationAs a first distinction, we classify As a first distinction, we classify

clouds by the height of the clouds by the height of the cloud basecloud base..

LOW CLOUDSLOW CLOUDS

• Low clouds have a base Low clouds have a base below 5000ft.below 5000ft.

• Stratocumulus (Sc), Stratocumulus (Sc), Stratus (St) and are the Stratus (St) and are the primary low-level cloudsprimary low-level clouds

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Cloud ClassificationCloud ClassificationMIDDLE CLOUDSMIDDLE CLOUDS

• Middle Clouds have a cloud Middle Clouds have a cloud base in the range of 2 - 7 km base in the range of 2 - 7 km (6000-23,000ft). (6000-23,000ft).

• Altostratus (As) and Altostratus (As) and altocumulus (Ac) are the altocumulus (Ac) are the primary middle level clouds.primary middle level clouds.

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Cloud ClassificationCloud ClassificationHIGH CLOUDSHIGH CLOUDS

• High clouds typically refer High clouds typically refer to clouds with a base to clouds with a base above 7 km (23,000ft) above 7 km (23,000ft) altitude.altitude.

• Cirrus (Ci), Cirrostratus Cirrus (Ci), Cirrostratus (Cs), and Cirrocumulus (Cs), and Cirrocumulus (Cc) are all high level (Cc) are all high level clouds.clouds.

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Cloud ClassificationCloud Classification

• High level clouds tend to be High level clouds tend to be fuzzy in comparison to low fuzzy in comparison to low level clouds.level clouds.

• This is primarily because This is primarily because high level clouds are made high level clouds are made of ice crystals instead of of ice crystals instead of droplets.droplets.

• Ice crystals usually Ice crystals usually evaporate slowly and thus evaporate slowly and thus have more time to diffuse.have more time to diffuse.

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Cloud ClassificationCloud ClassificationThree primary cloud classes overlap the height bands:Three primary cloud classes overlap the height bands:

– Cumulus (Cu)Cumulus (Cu)

– Cumulonimbus (Cb)Cumulonimbus (Cb)

– Nimbostratus (Ns)Nimbostratus (Ns)

““nimb-”nimb-” defines that the cloud is precipitating. defines that the cloud is precipitating.

““cumul-”cumul-” defines that the clouds are a result of convection. defines that the clouds are a result of convection.

““strat-”strat-” defines that the clouds are layered clouds. defines that the clouds are layered clouds.

Various terms are commonly intermixed and aren’t limited Various terms are commonly intermixed and aren’t limited to any given altitude. to any given altitude.

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