Radiation and advection fog are two main types of fog that affect this area. Radiation fog occurs as the surface radiates heat into the atmosphere (radiation cooling) while advection fog occurs as warm moist air moves over a cool surface. In both cases, the temperature and dew point (moisture) will be the same.

For both radiation and advection fog to form, there needs to be ample low-level moisture present. However, they differ on how the temperature and dew point come together.

For radiation fog, perfect conditions would be clear skies, light winds and, of course, sufficient moisture. Clear skies allow heat near the surface to radiate back into "space" causing the temperature to fall. Light winds (0 to 5 MPH) limit the mixing of drier air down to the surface, thus keeping the dew point relatively constant. Stronger winds could allow for drier air to mix down (lowering the dew point). Ample low-level moisture might be a humid air mass already in place, but a good example of "ample moisture" would be moisture left over after a recent rainfall. So, if clear skies and light winds are expected after a rainy day, foggy conditions are a good bet. Areas near lakes, bayous, and rivers are more prone to fog development because of the proximity to a moisture source.

For advection fog, the best conditions occur typically in the winter and early spring months near a warm front as warm moist gulf air is advected northward over a colder continental air mass. So if warm moist gulf air is expected to spread northward on southerly winds after an outbreak of cold weather, foggy conditions are a possibility.

Now that we have discussed the conditions that help form radiation and advection fog, we turn to what causes them to dissipate. Advection fog will dissipate as the warm front moves northward and the surface has had time to heat up. Solar radiation (sunshine) will also help in this process.

Radiation fog will dissipate primarily due to solar radiation, but increase in winds or a cloud layer moving overhead will help in this process. The time of year also plays a big part in how long the fog will take to dissipate after sunrise. In the winter, fog will take longer to burn off because the longer nights allow the fog to thicken. In addition, the winter solar radiation is much weaker due to the angle of the sun. The opposite is true for summer, where shorter nights do not allow the fog to thicken as much, and the solar radiation is much stronger and lasts longer.

Meteorologists also use geostationary satellites to help determine when fog might dissipate. Visible satellite images are used to monitor areas of fog during the daytime. Areas of fog usually erode from the outer edges or where thin spots in the fog are located. However, the timing of fog dispersion is tricky at best, especially in the wintertime.

Bill Murrell
Meteorologist

The limiting factor of how high a particular weather phenomena reaches in the sky is the height of the "tropopause". This is the level or boundary where the troposphere (the layer of air next to the earth) ends and the stratosphere begins and is characterized by an inversion, i.e., an increase in temperature from the troposphere to the stratosphere. For classroom purposes, an average height of the troposphere is 30,000 to 50,000 feet.

The height of the tropopause varies depending on location (higher at the equator than the poles) and season (lower in the winter than in the summer).

Thunderstorms are the "tallest" weather phenomena that we experience here on the ground. An "average" height of thunderstorms is 30,000, however, depending on atmospheric conditions (i.e., the height of the tropopause) some thunderstorms can reach as high as 50,000 to 60,000 feet over the central U.S. and in the tropics near the equator. In some stronger thunderstorms, the upward vertical motion in them can "punch" through the tropopause and will actually penetrate into the stratosphere before stopping their upward growth.

Tornados develop inside thunderstorms so it's difficult to give an height with them except to note that the rotation associated very strong tornados has been observed on radar as high as 15,000 to 20,000 feet while the thunderstorm itself may be 40,000 to 60,000 feet tall.

The clouds, winds, and thunderstorms in a hurricane can reach as high as 40,000 ft, while the clouds and winds with a blizzard can reach 30,000.

Lee Harrison
Meteorologist-in-Charge