Hydrometeors are any water or ice particles that have formed in the atmosphere or at the Earth’s surface as a result of condensation or sublimation. Water or ice particles blown from the ground into the atmosphere are also classed as hydrometeors. Some well-known hydrometeors are rain, fog, snow, clouds, hail, and dew, glaze, blowing snow, and blowing spray. Scattering by hydrometeors has an important effect on signal propagation.
Fog is a cloud of small water droplets near ground level and sufficiently dense to reduce horizontal visibility to less than 1,000 m. Fog is formed by the condensation of water vapor on condensation nuclei that are always present in natural air. This can result as soon as the relative humidity of the air exceeds saturation by a fraction of 1%. In highly polluted air, the nuclei may grow sufficiently to cause fog at humidities of 95% or less. Three processes can increase the relative humidity of the air:
- 1. Cooling of the air by adiabatic expansion
- 2. Mixing two humid airstreams having different temperatures
- 3. Direct cooling of the air by radiation (namely, cosmic ray radiation)
According to physical processes of fog creation, there are different kinds of fogs usually observed: advection, radiation, inversion, and frontal. We do not discuss their creation, but refer the reader to literature [5—7].
When the air becomes nearly saturated with water vapor (relative humidity RH ^ 100%), fog can form assuming sufficient condensation nuclei are present. The air can become saturated in two ways, either by mixing of air masses with different temperatures and/or humidities (advection fogs), or by the air cooling until the air temperature approaches the dew point temperature (radiation fogs).
Fog models, which describe the range of different types of fog, have been widely presented based on measured size distributions . The modified gamma size distribution (1.9) was used to fit the data. The models represent heavy and moderate fog conditions. The developing fog can be characterized by droplet concentrations of 100—200 particles per cm3 in the 1—10 pm radius range with mean radius of 2—4 pm. As the fog thickens, the droplet concentration decreases to less than 10 particles per cm3 and the mean radius increases from 6 to 12 pm. Droplets less than 3 pm in radius are observed in fully developed fog. It is usually assumed that the refractive index of the fog corresponds to that of pure water. Natural fogs and low-level clouds are composed of spherical water droplets, the refractive properties of which have been fairly well documented in the spectral region of interest.