Condensation: Dew, Fog, & Clouds Chapter 5
The Formation of Dew & Frost Dew forms on objects near the ground surface when they cool below the dew point temperature. More likely on clear nights due to increased radiative cooling White frost forms when temperature cools below the dew point and the dew point is below 0 C
Condensation Nuclei Particles suspended in the air that around which water condenses or freezes. Hygroscopic: water attracted Hydrophobic: water repelling
Haze Dry condensation nuclei (above dew point) reflect and scatter sunlight creating blueish haze Wet condensation nuclei (75% relative humidity) reflect and scatter sunlight creating grayish or white haze
Fog Saturation reached condensation forms a cloud near the ground Radiation fog: ground cools through conduction and radiation; ground fog Vll Valley fog created tdby cold air drainage High inversion fog
Fog Advection Fog: warm moist fog moves horizontally (advects) over a cool surface. Summer fog on the Pacific coast Observation: Headlands Air converges and rises over headlands d forming fog as compared to lower elevation beaches.
Advection Fog in San Francisco
Fog Upslope Fog: moist air flows up an orographic barrier East side of the Rockies Evaporation Fog: Warm moist surface provides enough moisture to saturate a dry air parcel; short lived Steam fog Breath in winter
Upslope Fog
Example of Evaporation Fog
Foggy Weather In general fog not common for most location in the US. However several areas do exist with a high frequency of fog. Two causes: Elevation Ocean currents
Average Annual Days of Dense Fog
Clouds Classification of clouds: use Latin words to describe height and appearance Factors described Height: low, mid, high, vertical Appearance: shape, density, color
Major Cloud Types
Cloud Base Heights
Basic Cloud Types and Heights
Cirrus above 20,000 ft, thin, ice crystals
Cirrocumulus
Cirrostratus sometimes get halos- often in advance of a storm
Altocumulus thunderstorms often develop
Altostratus
Nimbostratus usually have light to moderate precipitation- darker than altostratus
Stratocumulus
Stratus
Cumulus Also called air weather cumulus or cumulus humilis
Cumulus Congestus (well defined top- not fuzzy)-usually summertime
Cumulonimbus This has an anvil top from wind shear Usually fuzzy top- thunderstorm clouds From continued growth of cumulus congestus
Some Unusual Clouds Not all clouds can be placed into the ten basic cloud forms Unique atmospheric processes and environmental conditions create dramatic and exotic clouds Unusual clouds and weather balloons often cause of UFO reports
Atmospheric Rotors Rotors and Mountain Waves in the lee of the Sierra Nevada Improved understanding and prediction important for aviation safety in complex terrain Forming a strongly coupled system with overlying mountain waves and underlying boundary layer August 7, 2007 Grubisic, AMS 12th Mesoscale Conference, Waterville Valley, NH 32
Lenticular
More Lenticular Clouds
Mammatus Associated with sinking air often with severe weather
Cloud Observations Sky conditions: cloud coverage divided into eighths and each amount associated with term such as scattered clouds Observations: cloud ceilings Ceilometer used at airports to determine height from clouds by laser striking clouds and then amount of reflected light recorded Lidar = Light Detection And Ranging
Cloud Observations Satellite Observations Geostationary, polar orbiting Visible light provides a black and white picture of clouds Infrared approximates cloud temperature which infers height Satellites measure many other variables: sea surface temperatures, ozone, upper level features, snow cover, land cover
Geostationary Satellite
Polar Orbiting Satellites
High clouds cold Lower clouds warmer Just cold ground