Climate for a wet planet: Water in all its guises

Water has profound effects on climate. The most obvious thing about the Earth, viewed from afar, is that 2/3 of its surface is covered by oceans. These are in contact with the air at the water's surface, as visible to the eye. What is not visible to the eye is that oceans and other bodies of water evaporate unceasingly, putting water into the air in molecular form. The water can condense, with the results visible as clouds, haze, and fog. Less often, it precipitates and falls back to earth as rain or snow. Most of the effect that water has on climate is due to its much higher density and its far larger capacity to absorb and retain heat, for a given increase in temperature, compared to dry air.

The essential effect of water is that the Earth's climate is - compared to what it would be otherwise - warmer, less prone to large temperature variations, and more stable.

1. Water vapor makes the air warmer overall than it would be otherwise. The coastal tropics experience the most intense heating (from the Sun more or less directly overhead), the most evaporation, and higher temperatures overall (considered over the entire day-night cycle) than, say, deserts at the same latitudes (where the air heats up rapidly during the day, but cools off quickly at night). Water vapor acts like a blanket in the atmosphere. But even more importantly, the condensation of water vapor in the air releases the vapor's latent heat and acts like a steam bath.

An ideal blanket is one that absorbs and retains heat well, but doesn't transmit the heat through from one side to another. Although heat is conducted through water better than it is in air, it is water vapor's strong absorption, retention, and release of heat that create the steam bath- and blanket-like effects in humid air.

2. The presence of liquid and evaporated water moderates the change of temperature in time and space. Water is harder than air to heat up, but if hot, acts as a heat reservoir. It's harder to raise the temperature of a system of (dry air + water) than dry air alone. But, once warmed up, the (dry air + water) system cools off more slowly. We're all familiar with such effects, which are spelled out in more detail below.

If the Earth's atmosphere were dry, its thinness (low density) would make it prone to more violent winds and larger temperature variations. A good comparison case is Mars, which has a thin, cold atmosphere routinely subject to extreme wind storms, more extreme than anything here on Earth.

An opposite extreme is Venus. Its atmosphere, although very different from Earth's in some ways, illustrates how a very dense gas, as an excellent heat reservoir, tends to exhibit little variability of temperature - and thus little "weather."

Let's break these assertions down further to the basic physics.*

Mass density and mechanical inertia. Both air and water are "fluids" in physics lingo. But air is a gas and much less dense than water, about 770 times less dense.

The amount of mass in a given volume of fluid is a measure of that fluid's mechanical inertia - that is, the tendency for it to stay still if already still and to keep moving if already moving. A given force will accelerate a parcel of air much more quickly than the same volume of water, but it's also easier to stop that parcel of air than already-moving water. That's why winds are much more variable than convective streams (like the Gulf Stream) in the ocean.

A few years ago, some fairly imprecise new measurements of the Gulf Stream were compared to fairly imprecise old measurements from the 1950s. The two measurements were actually consistent with each other, within both their large measurement uncertainties. But their central values were quite different, leading to an absurd speculation that "global warming" was going to stop the Gulf Stream - and thus, paradoxically, lead to a cooling in Europe, which is warmed an extra amount by the relatively warmer Gulf Stream streaming through an otherwise cold north Atlantic. The law of inertia means that large ocean current changes take much longer (decades or centuries) than changes in wind patterns. The Gulf Stream is safe, for now, at least.

Heat capacity, thermal conduction, and thermal inertia. Heat capacity is the amount of heat energy it takes to warm something up by one degree. The heat capacity of water, per volume, is about 3000 times that of dry air, at constant density. The amount of heat energy that causes a one degree rise of air temperature causes a temperature rise of about 1/3000th of a degree in the same volume of water. It's a lot harder to heat water up than air. By the same token, once it is heated up by that one degree, the water retains about 3000 times as much heat energy as does the same volume of air heated up by the same one degree. You can think of heat capacity as a thermal version of inertia: the more heat capacity, the harder to heat up, but also the longer to cool off.

Land has a smaller heat capacity than water. That is why temperatures rise and fall in more extreme ways in continental interiors than on coasts. A nearby large body of water moderates extreme temperature changes. The land-water contrast also gives rise to strong winds near the shore. During the day, the more rapid heating just over the land surface causes the air to rise there, sucking in air from over the water - an onshore wind. At night, the air cools more rapidly over the land and sinks, pushing the air out over the water - an offshore wind.

It's also interesting to compare how easily air and water conduct heat. The essential material property is the thermal conductivity, a measure of how fast heat travels through a substance. Water conducts heat about 25 times faster than air. It's not a huge ratio, but it's enough to make heat conduction important in the ocean and at the air-ocean interface, while still being unimportant in the air itself (compared to convection and radiation: see the earlier discussion on heat transport).

Evaporation and latent heat. The major effect of water on the atmosphere, apart from the contact of air with liquid water surfaces, results from the water in the air itself, either "dissolved" at the molecular level as an invisible vapor or "condensed" as clouds of suspended liquid droplets.

When liquid water evaporates by absorbing heat, it absorbs more than just the heat implied by the heat capacity figure. The heat capacity of liquid water assumes that the water, in fact, remains liquid. When water evaporates to a gas, it's undergone a change of phase, and that requires an additional heat input called the heat of vaporization. Once in the air, the water vapor carries around all that extra heat that it needed to get there starting from liquid form. This extra is the latent heat.

Saturation, relative humidity, and dew point. The amount of water vapor air can absorb is limited; at any given temperature, this definite limit marks the saturation point. The saturation amount of water vapor rises as temperature rises. The absolute humidity is the ratio (by weight) of water vapor to air; it's rarely ever more than about 1% by number. The important number for weather is the relative humidity, the ratio of the actual absolute humidity to the maximum absolute humidity possible at a fixed temperature (the saturation value of absolute humidity).

Imagine an atmosphere with a fixed pressure, absolute humidity, and a relative humidity less than 100%. It has some temperature, but imagine lowering the temperature - and thus the saturation humidity - until the saturation humidity falls to the actual humidity. The temperature for which that is true is the dew point, and it's less than the actual temperature. If the relative humidity were exactly 100%, the actual and dew point temperatures would be equal. If the temperature were to fall below the dew point, the humid air would start getting rid of vapor by turning it back into liquid water droplets.

Water vapor and infrared absorption. Water vapor - dissolved water molecules in the air - continues to enjoy its extra heat absorption and retention powers (in comparison to dry air). It can absorb incoming solar radiation as heat, but its main effect in the air is to absorb and retain extra heat re-radiated from the Earth's surface (infrared radiation). This creates the "blanket effect" and makes the air warmer than it would be if it were dry.

Of course, the heated-up water vapor itself re-radiates heat, until the heat reaches the upper atmosphere and makes it back to space. But that takes time, a longer time than if the Earth's re-radiated heat simply faced a dry atmosphere on its way out.

Condensation, clouds, and radiation. If a parcel of humid air moves from a warmer to a colder region, cold enough that the dew point now rises above the parcel temperature, some of the water vapor will condense into tiny liquid droplets - steam, essentially. A big blob of floating steam in the air is otherwise known as a cloud, and that is in fact how clouds actually form.

The two most important such cloud-forming situations result from vertical and horizontal motion, respectively. A parcel of humid air, having acquired some water vapor at the surface, rises in altitude. At higher altitudes the temperature falls, eventually enough for the cloud to condense. That's why clouds usually form at some altitude above the surface. Another common situation is when a colder, drier air mass slams horizontally into a warmer, wetter air mass. At the surface of contact, vapor condenses rapidly and extensively. That is the origin of weather fronts. A less common but still familiar siutation is when warm, humid air cools down enough at night to set off condensation near the ground - fog, which is nothing more than a cloud at the surface.

Clouds have an additional effect on the movement of radiation in the atmosphere, because their surfaces are generally fairly white. That means they reflect visible and ultraviolet radiation away from themselves, while they also absorb essentially all incident heat radiation. The cloud bellies absorb and re-emit heat both up and down, while their tops reflect incoming solar radiation back into space, significantly altering the distribution of heat. Heat emitted back down makes the air warmer than if the cloud were not there. OTOH, radiation reflected back into space makes the air colder than it otherwise would have been. The two effects compete with each other, and the net effect depends on cloud details and other conditions. You might call the reflectivity of clouds a "tin-foil effect."

Condensation and release of latent heat. When water condenses from vapor to liquid, it releases into the dry air all that latent heat it was carrying around - the reverse of vaporization - appropriately called the heat of condensation. Since this is how a steam bath works, it's fitting to call this the "sauna effect."

Evaporation-condensation is the most important way that water vapor warms the atmosphere beyond what it would be dry - in addition to the smaller effects of the water vapor heat-absorbing "blanket". The net warming effect of water vapor can be dramatic. For example, although temperature falls with altitude in the lower atmosphere, the rate of decrease is quite a bit less for humid air (a quarter to a third less) than for dry.

Closing the cycle: Condensation and precipitation. In an air free of dust and other tiny particulates, those liquid water droplets would stay suspended in the atmosphere. But sometimes they don't. The droplets start out as so tiny that their weight is not enough to overcome the other forces (like updrafts) that they're buffeted with.

But if a droplet latches on to a little solid particle, it can accumulate more liquid from other droplets. If the growing droplet gets big enough, its weight can become large enough to overcome the forces that keep it up in the air. Enough such mega-droplets, and you get rain. If the air temperature near the ground is low enough, it comes down as ice crystals (snow). Even if it isn't cold enough near the ground, the temperatures aloft might be so cold that large ice clumps form and stay solid all the way down, where they land as hail.

Some of this fallen water gets absorbed by the ground. But most of it ends up in bodies of water and eventually back into the ocean, to repeat the cycle.
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* The basic principles and numbers are taken from the Physics Vademecum of the American Institute of Physics (out of print) and the books of Byers and Fleagle & Businger.