Unit 2: Atmosphere // Section 5: Vertical Motion in the Atmosphere
To see how atmospheric motion generates climate and weather systems, it is useful to start by picturing an air parcel—a block of air with uniform temperature and pressure throughout. The air parcel can change over time by rising, falling, or emitting or absorbing heat. If the pressure of the surrounding environment changes, the pressure of the parcel changes, but it does not exchange heat or chemicals with the surroundings and therefore may behave differently from its surroundings.
Many weather patterns start with rising air. According to the Perfect Gas Law, if an air parcel stays at a constant pressure, increasing its temperature will cause it to expand and become less dense. Contrary to popular belief, however, air does not rise simply because it is warm. Rather, air that becomes warmer than its surroundings—for example, if it is warmed by heat radiating from the Earth's surface—becomes buoyant and floats on top of cooler, denser air in the same way that oil floats on top of water. This buoyant motion is called convection.
Water is also a key factor in weather and climate. As shown in Table 1, two to three percent of the atmosphere typically consists of water vapor. Weather forecasts often report the relative humidity, which compares the amount of water vapor in the air to the maximum amount that air can hold at that temperature. When relative humidity reaches 100 percent, the air has reached saturation pressure and cannot absorb any more water vapor. (High humidity levels make people feel uncomfortable because little or none of their sweat can evaporate and draw heat from their skin.) As air warms, the amount of water vapor that it can hold rises exponentially. Consequently, atmospheric water vapor concentrations are highest in warm regions and decrease toward the poles (Fig. 5).
Figure 5. Mean distribution of atmospheric water vapor above Earth's surface, 1988–1999
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Source: Courtesy Cooperative Institute for Research in the Atmosphere, Colorado State University.
Atmospheric water vapor contributes to weather patterns in several ways. First, adding water vapor to the air reduces its density, so adding moisture to dry air may make it become buoyant and rise. Secondly, moist air carries latent energy, the potential for condensation of water vapor to heat the air. Liquid water absorbs energy when it evaporates, so when this water vapor condenses, energy is released and warms the surrounding environment. (As we will see in the next section, thunderstorms and hurricanes draw energy from the release of latent heat.) The dew point, another key weather variable, denotes the temperature to which air would have to cool to reach 100 percent relative humidity. When an air parcel cools to its dew point, water vapor begins condensing and forming cloud droplets or ice crystals, which may ultimately grow large enough to fall as rain or snow.
When a rising air parcel expands it pushes away the surrounding atmosphere, and in doing this work it expends energy. If heat is not added or removed as this hypothetical parcel moves—a scenario called an adiabatic process—the only source of energy is the motion of molecules in the air parcel, and therefore the parcel will cool as it rises. (Recall from Figure 1 that that in the troposphere, temperature falls 6.5°C on average with each kilometer of altitude. The actual decrease under real-world conditions, which may vary from region to region, is called the atmospheric lapse rate.)
A dry air parcel (one whose relative humidity is less than 100 percent) cools by 9.8°C for each thousand meters that it rises, a constant decrease called the dry adiabatic lapse rate. However, if the parcel cools enough that its relative humidity reaches 100 percent, water starts to condense and form cloud droplets. This condensation process releases latent heat into the parcel, so the parcel cools at a lower rate as it moves upward, called the moist adiabatic lapse rate.
Atmospheric conditions can be stable or unstable, depending on how quickly the temperature of the environment declines with altitude. An unstable atmosphere is more likely to produce clouds and storms than a stable atmosphere. If atmospheric temperature decreases with altitude faster than the dry adiabatic lapse rate (i.e., by more than 9.8°C per kilometer), the atmosphere is unstable: rising air masses will be warmer and less dense than the surrounding air, so they experience buoyancy and will continue to rise and form clouds that can generate storms. If temperature falls more gradually with altitude than the dry adiabatic lapse rate but more steeply than the wet adiabatic lapse rate, the atmosphere is conditionally unstable. In this case, air masses may rise and form clouds if they contain enough water vapor to warm them as they expand (Fig. 6), but they have to get a fairly strong push upwards to start the condensation process (up to 4000 meters in the figure). If temperature falls with altitude more slowly than the moist adiabatic lapse rate, the atmosphere is stable: rising air masses will become cooler and denser than the surrounding atmosphere and sink back down to where they started.
Figure 6. Conditional instability
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Convection is not the only process that lifts air from lower to higher altitudes. When winds run into mountains and are forced upward the air cools, often forming clouds over windward slopes and the crests of hills. Convergence occurs when air masses run together, pushing air upward, as happens often in the tropics and in warm summer conditions in midlatitudes, generating thunderstorms. And when warm and cold air fronts collide, the denser cold air slides underneath the warm air layer and lifts it. In each case, if warm air is lifted high enough to reach its dew point, clouds will form. If lifting forces are strong, the system will produce tall, towering clouds that can generate intense rain or snow storms.
As discussed in section 9, "Feedbacks in the Atmosphere," clouds are important factors in Earth's energy balance. Their net impact is hard to measure and model because different types of clouds have different impacts on climate. Low-altitude clouds emit and absorb infrared radiation much as the ground does, so they are roughly the same temperature as Earth's surface and thus do not increase atmospheric temperatures. However, they have a cooling effect because they reflect a portion of incoming solar radiation back into space, increasing Earth's albedo and reducing the total input of solar energy to the planet's surface.
In contrast, high-altitude clouds tend to be thinner, so they do not reflect significant levels of incoming solar radiation. However, since they reside in a higher, cooler area of the atmosphere, they efficiently absorb outgoing thermal radiation and warm the atmosphere, and they radiate heat back to the surface from a part of the atmosphere that would otherwise not contribute to the greenhouse effect.