Smog - The first definition of smog was a mixture of smoke and fog in the atmosphere which reduces visibility. Over time smog has now changed its meaning to restricted visibility due to pollution emitted from human activities. A type of smog known as photochemical smog, common to heavily populated areas, forms when UV light from the sun reacts with low level pollutants such as nitrogen oxides. On hot days this reaction can create ozone gas at the surface which is dangerous to humans.

Exosphere - The Exosphere is the outermost layer of the atmosphere. It extends from the top of the thermosphere and is 6,200 miles thick. In this layer, atoms and molecules escape into space and satellites orbit the earth. Some scientists place the outer reaches of the exosphere about 120,000 miles from the earths surface, making it near the half way point to the moon.

Insolation - Insolation is the amount of solar radiation that reaches the earth and the atmosphere. The amount of insolation at a specific location varies due to the length of day and the angle of the sun. During the winter in the northern hemisphere, the shorter days compiled with the lower angle of the sun results in much lower insolation compared to what is received during the summer months.

Computer Model - A system of computers which run complex physics equations on meteorological data to produce mathematical forecasts. Forecast models are first initialized with the current conditions in the atmosphere, and then thru a series of complex algorithms, they model what will be happening anywhere from 12 to 180 hours from the current time. There are several computer models that meteorologists utilize in predicting the weather.

Altostratus Clouds - Are gray or blue-gray middle level clouds composed of ice crystals and water droplets. These clouds usually cover the entire sky. In the thinner areas of the cloud, the sun may be dimly visible as a round disk.

Windward - The side of an object facing the wind. For example, if the wind is blowing in from the west, an object's west facing side would be the windward side. Conversely, the east side of the object would be the leeward side.

Crepuscular Rays - The meteorological term used to describe rays of sunshine that poke through holes in the cloud cover. They oftentimes have a celestial quality to them.