Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance  becomes cooler than the air above it. Frontal inversions occur when warm air spreads over a layer of  cooler air, or cooler air is forced under a layer of warmer air.  The relationship between dew point and temperature defines the concept of relative humidity. The  dew point, given in degrees, is the temperature at which the air can hold no more moisture. When the  temperature of the air is reduced to the dew point, the air is completely saturated and moisture begins  to condense out of the air in the form of fog, dew, frost, clouds, rain, or snow.  If air reaches the saturation point while temperature and dew point are close together, it is highly  likely that fog, low clouds, and precipitation will form. There are four methods by which air can  reach the saturation point. First, when warm air moves over a cold surface, the air temperature  drops and reaches the saturation point. Second, the saturation point may be reached when cold air  and warm air mix. Third, when air cools at night through contact with the cooler ground, air reaches  its saturation point. The fourth method occurs when air is lifted or is forced upward in the  atmosphere.  On cool, clear, calm nights, the temperature of the ground and objects on the surface can cause  temperatures of the surrounding air to drop below the dew point. When this occurs, the moisture in  the air condenses and deposits itself on the ground, buildings, and other objects like cars and  aircraft. This moisture is known as dew and sometimes can be seen on grass and other objects in the  morning. If the temperature is below freezing, the moisture is deposited in the form of frost. While  dew poses no threat to a small UA, frost poses a definite flight safety hazard. Frost disrupts the flow  of air over the wing and can drastically reduce the production of lift. It also increases drag, which  when combined with lowered lift production, can adversely affect the ability to take off. A small UA  must be thoroughly cleaned and free of frost prior to beginning a flight. To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. It appears individually or  in groups and is known as either an air mass or orographic thunderstorm. Heating of the air near the  Earth’s surface creates an air mass thunderstorm; the upslope motion of air in the mountainous  regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are  nonfrontal bands of thunderstorms or squall lines. Since rising air currents cause cumulonimbus clouds, they are extremely turbulent and pose a  significant hazard to flight safety. For example, if a small UA enters a thunderstorm, the small UA could  experience updrafts and downdrafts that exceed 3,000 fpm. In addition, thunderstorms can produce  large hailstones, damaging lightning, tornadoes, and large quantities of water, all of which are  potentially hazardous to an aircraft.  Standing lenticular altocumulus clouds are formed on the crests of waves created by barriers in the  wind flow. The clouds show little movement, hence the name  standing . Wind, however, can be  quite strong blowing through such clouds. They are characterized by their smooth, polished edges.  The presence of these clouds is a good indication of very strong turbulence and should be avoided.  Standing Lenticular Altocumulus Clouds. Clouds  Dew and Frost Methods by Which Air Reaches the Saturation Point Temperature/Dew Point Relationship Remote Pilot – Small Unmanned Aircraft Systems Study Guide  25 