Flight Training - July 2012 - 41

forces to some degree. A liquid takes on the shape of the container it’s in, but the attractive forces are strong enough to hold the molecules together in the container as they vibrate. In a gas, the molecules are moving fast enough to almost completely overcome the attractive forces, allowing them to fly away from each other unless they’re in a closed container. Gravity keeps all of them from escaping the bonds of Earth.
HOW WATER SUPPLIES STORM ENERGY.

form when warm, humid air rises from the surface. As it rises it grows cooler, eventually becoming cold enough for the water vapor to begin condensing into cloud drops, which warms up the air. This causes the air to rise even faster

WATER’S ENERGY POWERS THUNDERSTORMS. Thunderstorms

Water supplies storm energy because it either gives off or takes up energy when it changes among its phases. Each phase change involves either taking up or giving off heat. For example, to evaporate and become vapor, water has to gain heat, which means evaporation carries heat away from the liquid or anything the water is on. The most familiar example of this is the perspiration that carries heat away from our bodies, cooling us when it’s hot. In order to condense back into liquid, water vapor has to lose heat. All of water’s phase changes involve either a gain or a loss of heat. In brief, here are the phase changes that water goes through and whether the change adds or takes away energy from the surroundings, including the atmosphere when the water is in the air: • Condensation of vapor into liquid warms the air. • Freezing of liquid into ice warms the air. • Deposition of vapor directly into ice warms the air. • Melting of ice into liquid cools the air. • Evaporation of liquid into vapor cools the air. • Sublimation of ice directly into vapor cools the air.

and farther. As the air continues to rise, growing colder all of the way, vapor in the air begins depositing directly into ice, and water drops in the rising air freeze to form more ice crystals. The energy both processes create causes the air to rise even farther and faster, which adds to the thunderstorm’s violence. Eventually ice crystals or water drops grow large enough to begin falling and dragging air down with them. Compression heating warms this sinking air. But, phase changes to higher-energy forms, such as raindrops evaporating into vapor and ice melting into water and sublimating into vapor, draws energy from the sinking air, which offsets the compressional heating. As the air cools it becomes denser, which makes it fall faster. Water’s phase changes increase the speeds of both the updrafts and downdrafts in a thunderstorm. Wind shear between updrafts and downdrafts increases incredible turbulence in thunderstorms. If the sinking air cools enough it can hit the ground as a blast of wind that quickly changes the ambient wind speeds and directions. The strongest of such blasts are known as microbursts, a major hazard to aviation.

involved in determining whether water is vapor, liquid, or ice. Imagine we can see water vapor in the air in a closed container that keeps the water from interacting with the surroundings. If we could see them, the water molecules in the air as vapor would be moving at a wide range of speeds. The same is true of the water molecules making up the liquid in the container; they are moving at a range of speeds. But, the average speed of the vapor molecules is higher than the average speed of the liquid molecules. We would also see some of the molecules in the liquid going fast enough to escape, flying into the air as water vapor. At the same time, some of the vapor molecules are going slowly enough to become liquid in the container. If the temperatures of the water and the air above it have been constant for a while, the numbers of molecules leaving and entering the water would be equal. If we heated the water and the air above it, the number of water molecules moving fast enough to become vapor would increase because the average speeds of both the water and vapor molecules increases. As the temperature stabilizes at the MOISTURE'S MANY PHASES higher value, the number of vapor molecules also stabiEva po r lizes at a higher value. Vapor ra po ti o va Co n to n d en When this happens, ice sa to ti o or meteorologists say the air is n ap “saturated” with water vapor, because no more vapor will enter the air unless the temperature is increased. When something cools the air and Ice Liquid water, some vapor molecules would condense back into liquid and the air becomes saturated with fewer vapor Fre ezing molecules at the lower - liquid to ice temperature. Meltin g - ice to liquid The amount of water vapor needed to saturate the air at Phase changes that cool the surroundings Phase changes that warm the surroundings any temperature is known as the saturated mixing ratio.
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CLOUDS, FOG, AND SUPERCOOLED WATER. More than temperature is

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Flight Training - July 2012

Table of Contents for the Digital Edition of Flight Training - July 2012