Weather

If Earth had no water, clouds, fog, and precipitation wouldn’t cause poor visibility and low ceilings. No thunderstorms, hurricanes, or blizzards would ground pilots, and no one would have to worry about airframe icing.

On the other hand, without Earth’s water, no pilots or aircrews would have the exciting job of flying into hurricanes.

It’s common but far from ordinary

Put a few ice cubes in a glass, pour in some water, and then contemplate what you’re looking at. Water is the only natural substance that exists as a liquid, a solid, and a gas at temperatures and atmospheric pressures found on the Earth’s surface.

The water itself (“liquid water” in science speak) is the liquid phase. The ice is water’s solid phase. The millions of water molecules zipping around in the air above the water and ice in the glass are water’s invisible gaseous phase, called water vapor.

Water’s existence in all three phases—and its changes among them—helps explain why it is weather’s foundation. In a Web weather course it’s developing, the National Weather Association’s aviation committee sums this up: “Moisture � Vertical Motion � Stability = Weather.” (For more on stability, see “Atmospheric Stability is Key,” March 2010 Flight Training.)

If you want to sound like a meteorologist, start referring to water in any of its phases as “moisture.”

An extreme close-up

This image is a representation of a water molecule with two (white) hydrogen atoms and one (red) oxygen atom; thus its chemical abbreviation “H2O.” The oxygen side of a water molecule has a slight negative charge, while the hydrogen sides have a slight positive charge.

The pattern of water molecules’ electrical charges help account for water’s many unusual properties, such as its becoming less dense as it cools to approximately 31 degrees Fahrenheit—which is why ice floats. It also helps account for water forming into six-sided ice crystals when it freezes.

The interactions of water in its three phases in the atmosphere are so complex that an entire branch of atmospheric science, called cloud physics, studies them. Improved forecasts of clouds and precipitation are one of the practical results of such studies.

If you want a $50 word for water, call it dihydrogen monoxide, which refers to each water molecule’s two hydrogen atoms and one oxygen atom.

How water powers storms

In addition to supplying the raw material for rain, snow, and ice, water also supplies much of the energy that drives thunderstorms. Since hurricanes are made of thunderstorms, water is the primary source of hurricane energy.

Phase changes add or subtract heat energy to or from the surroundings because water vapor contains more heat energy than the same amount of liquid water, and liquid water contains more heat energy than ice. When water changes to a phase with less energy, such as vapor condensing into water drops, the water gives up the extra energy—called latent heat—to the surroundings. When this energy is added to the rising air that is building a thunderstorm, it causes the air to rise faster and farther than it otherwise would have; this adds to the thunderstorm’s power.

When water changes into a phase with more energy, such as water drops evaporating into water vapor, it draws heat from the surroundings, cooling them. (This is why the evaporation of perspiration cools you.) When ice crystals or water drops begin falling they drag air down with them. Compression heating warms this sinking air. But phase changes to higher-energy forms, such as raindrops evaporating into vapor, draw energy from the sinking air, which offsets the compressional heating. As the air cools it becomes denser, which makes it fall faster.

If the sinking air cools enough, it can hit the ground as a blast of wind that quickly changes wind speeds and directions. The strongest of such blasts are known as microbursts, a major hazard to aviation. (For more on microbursts see “Menacing Microbursts,” July 2007 Flight Training.)

The diagram below sums up which phase changes add heat to the air, and which draw heat from the surrounding air. Note that the label “deposition” is on the phase change from water vapor directly to ice. Aviation texts often use “sublimation” here, although U.S. meteorologists stopped using sublimation for the vapor-to-ice change more than 30 years ago.

Water does not necessarily freeze at 32 degrees F

Despite what they might have told you in school, water doesn’t necessarily freeze at 32 degrees F (0 degrees Celsius). While relatively large amounts of water will usually begin freezing at 32 degrees F, water drops, such as those that make up a cloud, can cool to minus 40 degrees and still be liquid.

By the way, minus 40 degrees is the same temperature on the Fahrenheit and Celsius scales, but that’s just a coincidence. Both scales were developed long before scientists discovered that water drops can remain liquid until they are approximately this cold.

Water drops that are liquid even though they are colder than 32 degrees F are said to be supercooled. They are dangerous because they will freeze as soon as they hit something, such as an aircraft. Supercooled cloud, drizzle, or rain drops can coat an airplane that runs into them with ice, which changes the shapes of wings to degrade lift and increase drag. Ice on an airplane’s horizontal stabilizer can affect pitch control, and different patterns of ice on the two wings can cause unwanted rolling motions.

Since air aloft is usually colder than the air at the surface, you don’t have to take off in frigid temperatures to fly into icing.

Above: View from the pilot’s seat of a NOAA WP-3 inside the eye of Hurricane Katrina on August 28, 2005, when it was a Category 5 storm. The P-3 is heading toward the eyewall where a hurricane’s strongest winds are concentrated. NOAA Photo by Lt. Mike Silah, a NOAA Corps pilot