Fog, dew and clouds Meteorology 10 Prof. Jeff Gawrych

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

2. Dew: air cooled to dewpoint

3. 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 • 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?

6. 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.

10. Evaporation/steam fog

11. # 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 clouds

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

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

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

18. 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.