Condensation: Dew, Fog, & Clouds Chapter 5
Condensation Condensation Water vapor in the air changes to a liquid and forms dew, fog, or clouds Water vapor requires a surface to condense on Possible condensation surfaces on the ground can be the grass, a car window, etc. Possible condensation surfaces in the air are tiny bits of particulate matter Called condensation nuclei (i.e. dust, smoke, Ocean salt crystals, etc.
The Formation of Dew & Frost Dew forms on objects near the ground surface when they cool below the dew point temperature and water vapor condenses on them More likely on clear nights due to increased radiative cooling
Figure 5.1 Dew forms on clear nights when objects on the surface cool to a temperature below the dew point. If these beads of water should freeze, they would become frozen dew.
Frost forms when temperature cools below the dew point and the dew point is below 0 C
Figure 5.2 These are the delicate ice-crystal patterns that frost exhibits on a window during a cold winter morning.
Condensation Nuclei Particles suspended in the air that, around which, water condenses or freezes. Figure 5.3 Hygroscopic nuclei are water-seeking, and water vapor rapidly condenses on their surfaces. Hydrophobic nuclei are water-repelling and resist condensation.
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 Valley fog created by cold air drainage High inversion fog
Figure 5.6 Visible satellite image of dense radiation fog in the southern half of California s Central Valley on the morning of November 20, 2002. The white region to the east (right) of the fog is the snowcapped Sierra Nevada range. During the late fall and winter, the fog, nestled between two mountain ranges, can last for many days without dissipating. The fog on this day was responsible for several auto accidents, including a 14-car pileup near Fresno.
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 forming fog as compared to lower elevation beaches.
Figure 5.9 (a) Radiation fog tends to form on clear, relatively calm nights when cool, moist surface air is overlain by drier air and rapid radiational cooling occurs. (b) Advection fog forms when the wind moves moist air over a cold surface and the moist air cools to its dew point.
Upslope Fog: moist air flows up an orographic barrier East side of the Rockies
Evaporation (mixing) Fog: Warm moist surface provides enough moisture to saturate a dry air parcel; short lived Steam fog Breath in winter
Figure 2 The mixing of two unsaturated air parcels can produce fog. Notice in the saturated mixed parcel that the actual mixing ratio (w) is too high. As the mixed parcel cools below its saturation point, water vapor will condense onto nuclei, producing liquid droplets. This would keep the actual mixing ratio close to the saturation mixing ratio, and the relative humidity of the mixed parcel would remain close to 100 percent.
Figure 3 The blue line is the saturation mixing ratio. The mixing of two unsaturated air parcels (A and B) can produce a saturated air parcel and fog.
Foggy Weather In general foggy conditions are not common for most locations in the US. However several areas do exist with a high frequency of fog. Two causes: Elevation Ocean currents
Figure 5.12 Average annual number of days with dense fog (visibility less than 0.25 miles) through the United States. (NOAA)
Fog dispersal Mix air with air craft or fans Introduce large particle into air to reduce total number of cloud droplets. Use dry ice to lower temperature below freezing.
Clouds Classification of clouds use Latin words to describe height and appearance. Factors described Height: low, mid, high, vertical Appearance: shape, density, color
Figure 5.13 Cirrus clouds.
Figure 5.14 Cirrocumulus clouds.
Figure 5.15 Cirrostratus clouds with a faint halo encircling the sun. The sun is the bright white area in the center of the circle.
Figure 5.16 Altocumulus clouds.
Figure 5.17 Altostratus clouds. The appearance of a dimly visible watery sun through a deck of gray clouds is usually a good indication that the clouds are altostratus.
Figure 5.18 The nimbostratus is the sheetlike cloud from which light rain is falling. The raggedappearing clouds beneath the nimbostratus is stratus fractus, or scud.
Figure 5.19 Stratocumulus clouds forming along the south coast of Florida. Notice that the rounded masses are larger than those of the altocumulus.
Figure 5.20 A layer of low-lying stratus clouds hides these mountains in Iceland.
Figure 5.21 Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis.
Figure 5.22 Cumulus congestus. This line of cumulus congestus clouds is building along Maryland s eastern shore.
Figure 5.23 A cumulonimbus cloud (thunderstorm). Strong upper-level winds blowing from right to left produce a well-defined anvil. Sunlight scattered by falling ice crystals produces the white (bright) area beneath the anvil. Notice the heavy rain shower falling from the base of the cloud.
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.
Figure 5.25 Lenticular clouds forming on the leeward side of the Sierra Nevada near Verdi, Nevada.
Figure 5.26 The cloud forming over and downwind of Mt. Rainier is called a banner cloud
Figure 5.27 A pileus cloud forming above a developing cumulus cloud.
Figure 5.28 Mammatus clouds forming beneath a thunderstorm.
Figure 5.29 A contrail forming behind a jet aircraft.
Figure 5.30 The clouds in this photograph are nacreous clouds. They form in the stratosphere and are most easily seen at high latitudes.
Figure 5.31 The wavy clouds in this photograph are noctilucent clouds. They are usually observed at high latitudes, at altitudes between 75 and 90 km above the earth s surface.
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 light or laser striking clouds and then amount and speed of reflected light recorded.
Figure 5 The laser-beam ceilometer sends pulses of infrared radiation up to the cloud. Part of this beam is reflected back to the ceilometer. The interval of time between pulse transmission and return is a measure of cloud height, as displayed on the indicator screen.
Cloud Observations Satellite Observations Geostationary satellites Polar orbiting satellites 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
Figure 5.32 The geostationary satellite moves through space at the same rate that the earth rotates, so it remains above a fixed spot on the equator and monitors one area constantly. 22,300 mi above
Figure 5.33 Polar-orbiting satellites scan from north to south, and on each successive orbit the satellite scans an area farther to the west about 850 km (510 mi)
Figure 5.34 Generally, the lower the cloud, the warmer its top. Warm objects emit more infrared energy than do cold objects. Thus, an infrared satellite picture can distinguish warm, low (gray) clouds from cold, high (white) clouds.
Figure 6 Sea-surface temperatures for February 19, 2004. Temperatures are derived mainly from satellites, but temperature information also comes from buoys and ships.
Figure 5.37 Infrared water-vapor image. The darker areas represent dry air aloft; the brighter the gray, the more moist the air in the middle or upper troposphere. Bright white areas represent dense cirrus clouds or the tops of thunderstorms. The area in color represents the coldest cloud tops.
Figure 5.38 A three-dimensional TRMM satellite image of Hurricane Ophelia along the North Carolina coast on September 14, 2005. The light green areas in the cut-away view represent the region of lightest rainfall, whereas dark red and orange indicate regions of heavy rainfall.