Meteorological Factors Affecting Visibility Dr Chip West Astronomical

Meteorological Factors Affecting Visibility Dr. Chip West

Astronomical Tools

Deployment of the Hubble Space Telescope in Earth orbit, April 24, 1990

Structure of the atmosphere • Pressure is the weight of the air above • Average sea level pressure • 1013. 2 millibars • About 14. 7 pounds per square inch • Pressure decreases with altitude • One half of the atmosphere is below 18, 300 Ft. • One quarter of the atmosphere is below 8000 ft. • Ninety percent of the atmosphere is below 10 miles

Thermal structure of the atmosphere Troposphere • Temperature decreases with altitude (environmental lapse rate) • 6. 5˚C per kilometer (average) • 3. 5˚F per 1000 feet (average) Stratosphere • About 12 km to 50 km • Temperature increases at top • Outer boundary is named the stratopause Mesosphere • About 50 km to 80 km • Temperature decreases • Outer boundary is named the mesopause Thermosphere • No well-defined upper limit • Fraction of atmosphere's mass • Gases moving at high speeds

Classification of clouds according to height and form

Classification of clouds according to height and form (continued)

Cumulus clouds

Stratus clouds

Cirrus clouds

Aurora

Fog v Most fogs form because of • Radiation cooling, or • Movement of air over a cold surface v Types of fog • Advection fog – warm, moist air moves over a cool surface • Radiation fog • Earth's surface cools rapidly • Forms during cool, clear, calm nights • Upslope fog • Humid air moves up a slope • Evaporation fogs • Steam fog • Cool air moves over warm water and moisture is added to the air • Frontal fog, or precipitation fog • Forms during frontal wedging when warm air is lifted over colder air • Rain evaporates to form fog

ANNUAL FOG FREQUENCY 8 7 DAYS 6 5 4 3 2 1 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Advection Fog • Usually in the morning • Last usually few hours • May take visibilities to less than 1 mile • Forms first and dissipates last in low lying areas

Haze

A weather map showing isobars

Cyclonic and anticyclonic winds in the Northern Hemisphere

Airflow associated with surface cyclones and anticyclones