Weather

Water is so common that most of us rarely think about it. Nevertheless, it’s far from ordinary. No living things would exist without water in its liquid form. In addition, without water in its vapor, liquid, and frozen forms, the Earth’s weather as we know it wouldn’t exist.

Scientists looking for planets where life could possibly exist are interested in those in what they call the “circumstellar habitable zone,” also called the “Goldilocks zone,” where temperatures could be in the range that allows liquid water to exist on the surface.

Any such planets would have weather much like ours, with clouds, fog, falling rain, and possibly snow and sleet.

If humans ever go to Mars, which does not have bodies of water on the surface, they would be able to fly airplanes in the planet’s thin atmosphere. Weather briefings for Mars pilots would be much simpler than on Earth because clouds, fog, and precipitation wouldn’t cause poor visibility and low ceilings. Mars pilots wouldn’t have to worry about thunderstorms, hurricanes, blizzards, or airframe icing. Dust storms and tornado-like dust devils would be their only weather worries.

For meteorologists and other scientists, what most people call “water” is “liquid water—to distinguish it from water in its solid (ice) and gas (vapor) phases.

An ordinary glass of ice water illustrates how water can exist in its three phases at the same time and place. The water itself 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 often invisible gaseous phase, called water vapor. The air always contains some water vapor.

Water is the only natural substance that exists in all three phases at the temperatures and atmospheric pressures found at the Earth’s surface. Water’s existence in its three phases—and its changes among them—helps explain why it is the foundation of Earth’s weather.

As the National Weather Association says in its online Weather Theory for Pilots course, to learn about weather you should begin with the “The weather equation: Moisture � vertical motion � stability = weather.”

A CLOSE-UP LOOK AT WATER. A water molecule has two hydrogen atoms and one oxygen atom (at right), which is why its chemical abbreviation is H2O. The oxygen side of a water molecule has a slight negative charge, while the hydrogen sides have a slight positive charge.

The molecules’ electrical charges help account for water’s many unusual properties. For example, it becomes less dense as it cools to approximately 31 degrees Fahrenheit, which is why ice floats. The charges also help to account for water forming into six-sided ice crystals when it freezes.

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

HOW WATER POWERS STORMS. In addition to supplying the raw material for rain, snow, and ice, water’s phase changes contribute much of the energy that drives thunderstorms. Since hurricanes are made of thunderstorms, water is also the primary source of hurricane energy.

Phase changes add heat energy to or subtract it 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 its 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, thus adding to the thunderstorm’s power.

When water changes into a phase with more energy, such as raindrops evaporating into water vapor, it draws heat from its surroundings and cools them. For example, evaporation of perspiration on a hot day cools you by drawing heat from your body.

When ice crystals or water drops high in a cloud 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 a result, the temperature of the sinking air decreases. As the air cools, it becomes denser, which makes it fall faster.

In other words, the cooling of falling air adds more energy to a thunderstorm in the form of downdrafts.

If the sinking air cools enough, it can hit the ground as a blast of wind that quickly changes speed and direction. The strongest of such blasts are known as microbursts, a major hazard to aviation.

The total amount of water on Earth is 332.5 million cubic miles.