7.3 Moisture, Fog and Frost - Practice Questions

Moisture is added to air by

• sublimation and condensation.

• evaporation and condensation.

• evaporation and sublimation.

Answer explanation

As water changes from one state to another, an exchange of heat takes place. These changes occur through the processes of evaporation, sublimation, condensation, deposition, melting, and freezing. However, evaporation and sublimation are the only ways water vapor is added into the atmosphere.

Why is frost considered hazardous to sUAS operations?

• Frost decreases lift capacity by changing the aerodynamic shape of the propellers.

• Frost decreases lift capacity by spoiling the smooth flow of air over the propellers.

• Frost increases control effectiveness by slowing the airflow.

Answer explanation

Frost is considered hazardous to sUAS operations because it spoils the smooth flow of air over the propellers or wings and makes it harder for the aircraft to generate lift. Frost decreases the lift capacity of the aircraft.

Frost does not change the basic aerodynamic shape of the wing, but the roughness that even a very thin layer of frost creates on an aircraft’s propellers or wings will drastically impact your aircraft’s performance.

If the temperature is 64°F (and if the temperature/dewpoint spread is too small and decreasing), what type of weather is most likely to develop?

• Fog or low clouds

• Freezing precipitation

• Thunderstorms

Answer explanation

Fog typically occurs when the temperature of air near the ground is cooled to the air’s dew point. Remember, the dew point is the temperature at which the air will have 100% humidity -- it’s fully saturated with water vapor. At this point, the water vapor in the air condenses and becomes visible in the form of fog.

What is "dew point"?

• The temperature at which condensation and evaporation are equal.

• The temperature at which dew will always form.

• The temperature to which air must be cooled to become saturated.

Answer explanation

The dew point, given in degrees, is the temperature at which the air can hold no more moisture, so it’s at 100% humidity. As moist, unstable air rises, clouds often form at the altitude where temperature and dew point reach the same value. At this point, the air is completely saturated, and moisture begins to condense out of the air in the form of fog, dew, frost, clouds, rain, hail, or snow.

The dew point is the temperature air needs to be cooled to become saturated or achieve a relative humidity of 100%. Once the air is saturated, it cannot hold any more water in the gas form. Further lowering of the temp increases saturation and the water vapor starts to condense and appears typically in the form of fog, rain, or frost

The amount of water vapor which air can hold depends on the

• air temperature.

• dewpoint.

• stability of the air.

Answer explanation

The dew point, given in degrees, is the temperature at which the air can hold no more moisture, so it’s at 100% humidity.

But the actual variable that affects the dew point is air temperature!

Clouds, fog, or dew will always form when

• relative humidity reaches 100 percent.

• water vapor condenses.

• water vapor is present.

Answer explanation

Fog typically occurs when the temperature of air near the ground is cooled to the air’s dew point. Remember, the dew point is the temperature at which the air will have 100% humidity -- it’s fully saturated with water vapor. At this point, the water vapor in the air condenses and becomes visible in the form of fog.

Low-level turbulence can occur (and icing can become hazardous) in which type of fog?

• Rain-induced fog

• Steam fog

• Upslope fog

Answer explanation

Steam fog is common over bodies of water during the coldest times of the year, and because of that, it’s also known as sea smoke. Steam fog forms when cold, dry air moves over warm water. As the water evaporates, it rises and resembles smoke. Low-level turbulence can occur (and icing can become hazardous) with the presence of steam fog.

Advection fog will most likely form when

• an air mass moves inland from the coast.

• a light breeze blows colder air out to sea.

• there's a warm, moist air mass on the windward side of mountains.

Answer explanation

Advection fog is common in coastal areas 1) where sea breezes can blow moist air over cooler land masses, and 2) when air masses move inland from the coast. Think about the beautiful and foggy winter beaches of northern California, Oregon, or Washington.

Wind is required for advection fog to occur. Winds of up to 15 knots allow the fog to form and intensify; above a speed of 15 knots, the fog usually lifts and forms low stratus clouds. This type of fog can persist for days.

Radiation fog will most likely form when

• there is warm, moist air over low, flatland areas on clear, calm nights.

• moist, tropical air moves over cold, offshore water.

• cold air moves over much warmer water.

Answer explanation

Radiation fog typically forms in low-lying areas like a mountain valley. Radiation fog happens when the surrounding air reaches its dew point, and the ground cools rapidly due to terrestrial radiation. It’ll form on clear nights, with relatively little to no wind present.

Frost is most likely to form on a UAV's wings or propellers:

• when the temperature of the wing or propeller's surface is below the dew point of the surrounding air

• when the dewpoint is lower than 0° C.

• when the temperature of the wing or propeller's surface is below the dew point of the surrounding air AND the dew point is lower than 0° C.

Answer explanation

When it comes to your unmanned aircraft, frost is most likely to form when the dew point is lower than 0° C, and when the temperature of your propellers’ or wings’ surface is below the dew point.