plume, the smoke from a forest fire, or the debris falling from a nuclear mushroom cloud. Sand Sand particles may be picked up from dry surfaces by the wind at wind speeds as low as 21 knots and carried to moderate heights. Stronger winds may carry sand to extreme heights. The term sand storm refers to blowing sand that reduces visibility from 5/16 to 5/8 mile, while the term heavy or severe sand storm means that visibility is less than 5/16 mile. The only hazard to aviation caused by haze and smoke is reduced visibility. But dust, ash, and sand can also clog engine intakes and be very abrasive to moving components.  Aircraft  flying  through  these  conditions may experience fatal engine failure. HYDROMETEORS Hydrometeors  are liquid or solid water particles falling  through,  suspended  in,  or  condensing/subliming from the atmosphere, as well as solid or liquid water blown from the surface by wind. The term refers to all forms of condensation, such as clouds, fog, dew, and frost; all forms of precipitation, such as rain, drizzle, snow and hail; and all forms of moisture blown about by the wind. Condensation/Sublimation  Forms Many  of  the  weather  elements  identified  as hydrometeors  are  formed  by  the  condensation  or sublimation of water vapor in the air or on surfaces. Clouds and fog are hydrometeors of suspended liquid or solid moisture suspended in the air. Dew and frost are hydrometeors  of  moisture  that  condense  or  sublime directly on surfaces or on the ground. CLOUDS.—Clouds are the visible form of water vapor, and consist of minute suspended droplets of liquid water or ice particles. Fog is a cloud on the earth’s surface. Liquid water droplets develop from gaseous water vapor by the process of condensation.  Solid  water particles or ice crystals develop by the process of sublimation. During sublimation, gaseous water vapor bypasses the liquid state and goes directly from a gas to a solid, thereby releasing heat into the atmosphere. Three  factors  are  necessary  for  cloud  formation: sufficient moisture, hygroscopic nuclei, and a cooling process. Moisture  is  supplied  by  evaporation  and  is distributed  vertically  by  convection  currents  and horizontally  by  winds. Hygroscopic nuclei are small particles on which water  vapor  can  condense  or  sublime.  Hygroscopic nuclei actually attract water vapor. The most effective hygroscopic nuclei are the by-products of combustion, sulfuric acid and nitric acid particles, and salts (such as sodium chloride raised from the sea surface). Dust particles may contain sufficient salts or acids to become hygroscopic nuclei, but dust particles in general are not effective  hygroscopic  nuclei.  The  presence  of hygroscopic  nuclei  is  a  must  for  water  vapor  to condense. Air has been super-saturated in laboratories to over 400% before condensation began in the absence of  hygroscopic  nuclei. In actual conditions, in the presence of abundant hygroscopic nuclei, condensation may begin at relative humidities near 70%. Saturation of the air is reached when the relative humidity reaches 100%. At this point, the evaporation rate from liquid water droplets to water vapor equals the condensation rate from water vapor to liquid water, or theoretically, the sublimation rates from gas-to-solid and from solid- to-gas are exactly equal. Hygroscopic nuclei are also called condensation nuclei  and  sublimation  nuclei  when  referring  to  the specific process of condensation or sublimation. A  cooling process  aids in condensation, since it increases the humidity of the air without increasing the amount  of  water  vapor  present. The  higher  the humidity,  the  easier  condensation  proceeds. The cooling  process  most  frequently  associated  with condensation is adiabatic expansion. When a parcel of air is lifted higher in the atmosphere (where the pressure is lower), it expands and its temperature decreases. Another  important  cooling  process  is  radiational cooling. Simply put, as the sun goes down, the air cools because the heat source, the sun, is no longer available to maintain the heating. FOG.—Fog is a suspension of small visible water droplets  (or  ice  crystals)  in  the  air  that  reduces horizontal  and/or  vertical  visibility  at  the  earth’s surface. Fog is a stratus cloud on the surface of the earth. It is distinguished from smoke, haze, or dust by its dampness and gray appearance. Fog usually does not form or exist when the difference between the air temperature and the dew-point temperature is greater than   4   Fahrenheit   degrees   (2   Celsius   degrees). However,  at  temperatures  below  -2O°F  (-29°C), freezing fog, or ice fog, may form when the dew-point temperature is as much as 8°F (4 Celsius degrees) lower than the air temperature. Freezing fog is composed entirely of ice crystals that sparkle brilliantly in light. When the air temperature is between 32°F and -20°F, 1-31