The explanation for spatial variations in precipitation centers on the concept of relative humidity. The relative humidity is the water vapor pressure (numerator) divided by the equilibrium vapor pressure (denomator) times 100%. The equilibrium vapor pressure occurs when there is an equal (thus the word equilibrium) flow of water molecules arriving and leaving the condensed phase (the liquid or ice). Thus there is no net condensation or evaporation (Alistair Fraser, PSU).
Now, if the water vapor pressure is greater than the equilibrium value (numerator is greater), there is a net condensation (and a cloud could form, say). And that is not because the air cannot hold the water, but merely because there is a greater flow into the condensed phase than out of it.
Relative humidity describes the amount of water vapor actually in the air (numerator), relative to the maximum amount of water the air can possibly hold for a given temperature (denominator). It is expressed as a percentage:
If the relative humidity (RH) is 100%, this means that condensation would occur. On a typical hot muggy summer day, RH might be around 60-80%. In a desert, RH is commonly around 15-25%.
One important consequence is that when air masses change in temperature, the relative humidity can change, even if the actual amount of water vapor in the air does not (the numerator in our equation, which is defined by the saturation curve, stays the same, but the denominator changes with temperature). Figures 11-13 below show an example of this process. As the air cools, the relative humidity increases. If the air mass were cooled enough to become saturated (hit the solid black curved line), condensation would occur. This temperature is called the dew point.
In the same way, changes in relative humidity occur when warm moist air is forced to rise or, conversely, when cool dry air descends. For example, when an air mass moves over mountains, it cools as it rises, and when it reaches the dewpoint, water will condense. This forms clouds, and if the air mass cools enough, the condensation becomes rapid enough to form precipitation.
To take the concept of relative humidity outdoors, let's consider why it rains in some areas and we have deserts in others. There are two primary reasons for this. Both are related to the transport, rise, and fall of air masses that lead to temperature changes, and ultimately in the amount of water vapor that the air can hold. These are the orographic effect, and atmospheric convection.
In both cases, cooling and warming of air masses occurs because they are forced upward or downward in the atmosphere. The decrease in air temperature with elevation is known as the atmospheric (or adiabatic) lapse rate, as shown below, and is related to decreasing air density and pressure with increasing altitude (as air rises, it expands due to decreased pressure, leading to lower temperature). A typical average lapse rate is around 7° C per km of altitude change. If an air mass begins rising and has not reached the dewpoint temperature, it follows a dry adiabatic lapse rate, with the rate of cooling due nearly entirely to decreasing pressure, as shown in Figure 14. Once the airmass temperature reaches the dewpoint during continued rise, water droplets begin to condense (forming clouds) and the airmass follows a moist adiabatic lapse rate (Figure 14), for which the rate of cooling with elevation decreases because of the addition of some offsetting heat to the airmass from the process of condensation (termed latent heat).
The orographic effect occurs when air masses are forced to flow over high topography. As air rises over mountains, it cools and water vapor condenses. As a result, it is common for rain to be concentrated on the windward side of mountains, and for rainfall to increase with elevation in the direction of storm tracks. With continued cooling past the dewpoint, the amount of water vapor in the air cannot exceed saturation, so water is lost from the air via condensation and precipitation.
On the leeward side of mountain ranges, the opposite occurs: the air descends and warms. As it does so, it is capable of holding more water vapor (recall the saturation line in the relative humidity plot above). However, there is no source of additional water, so the descending air mass increases in temperature but the amount of water vapor remains constant. Because the air has lost much of its original water content, as it descends and warms its relative humidity decreases. These areas are called rain shadows and are commonly deserts. You’ve probably noticed this same process in action when you heat your house or apartment in the winter – warming the cold air leads to dry conditions – one of the reasons people often put water pots or kettles on their wood stoves.
The animation below shows an airmass trajectory superimposed on a Google Earth image of western North America. The point of this animation is to provide an explanation of the orographic effect and the changes in temperature and water content of an airmass passing over several mountain ranges. The animation shows the "rain shadow" effect that results in desert regions behind large mountain ranges. An inset graph at bottom right illustrates combinations of temperature (x-axis) and moisture content (y-axis) in grams per cubic meter of the air mass as it passes over various topographic features on the land surface.
There is a second, larger-scale effect that also plays a key role in the global distribution of precipitation and evaporation. Fundamentally, these patterns are also explained by the rise and fall, and cooling and warming of air masses – as is the case with the orographic effect – but in this case, their movement is a result of atmospheric convection rather than transport over topographic features.
As you have seen, there are regular climate and precipitation bands on the Earth – latitudes where most of the Earth’s tropical and temperature rainforests, deserts, polar deserts (also known as tundra) tend to occur. This global pattern – along with prevailing global wind patterns and storm tracks, are driven by atmospheric convection.
It all starts with solar radiation. Because of the Earth’s curvature, the tropics (between 23.5° N and S latitude) receive a larger flux of solar radiation per unit area on average than higher latitudes. Because the Earth’s axis is tilted, during Northern hemisphere summer, the peak influx of solar radiation occurs at 23.5° N latitude. During the Southern hemisphere summer, the maximum occurs at 23.5° S. (Incidentally, these latitudes define the tropics of Cancer and Capricorn.) Annually, the highest flux of solar energy per unit area occurs at the equator, as shown below.
As a result, the air around the equator becomes warmest. It holds quite a bit of water, too – based on the fact that, as you’ve seen above, warm air has a higher capacity to carry moisture.
Take a few minutes to review the video below to help you understand Global Circulation a little better.
The differential heat input from solar radiation input and loss by infrared radiation is a critical part of maintaining equability (relatively low gradients in temperature from low to high latitudes) on the Earth. The energy balance figures indicate that above about 40 degrees North and South (e.g., the latitude of New York City) of the equator the loss of heat by radiation (infrared), on average, exceeds the input of heat from the sun (visible). What does that imply for our climate? One might think that this should result in permanent snow or ice above this latitude. Right? Indeed, during the last glacial epoch, about 21 thousand years ago, thick continental ice sheets extended to nearly 40 degrees North in North America (just north of I-80). But normally, because of the heat gradient created by the imbalance between solar input and infrared radiation, the atmosphere (and ocean) is set in motion to redistribute heat from low to high latitudes. Otherwise, the tropics would be excessively hot and the high latitudes excessively cold—at all times. Next, we will see how this circulation works.
As this warm air rises due to its lower density, it cools. Once it cools past the dewpoint, condensation occurs and clouds form. With continued rise and cooling, the air cannot hold the moisture and precipitation falls.
In response to that rising air, surface air must flow in to fill the vacated space. The rising air results in a low-pressure center. This is why when you hear about low pressure in the forecast, is typically associated with rising air masses and therefore with crummy weather. The air rushing in toward the equator defines the trade winds. These winds converge on the equator but blow to the West because of Earth’s rotation. This rotational effect is known as the Coriolis effect. We won’t get into that in detail here, but if you are interested, check out the video below.
These flows drive convection cells, with dimensions that are controlled by the viscosity and density of air, and by the thickness of the atmosphere. The air that rose from the equator flows North and South at the top of the cell and eventually descends at around 30° N or S latitude. As the cool, now dry air descends it warms. Sound familiar?
Just as occurs when air descends on the leeward side of mountain ranges and causes rain shadows, the amount of water that the descending and warming air could hold increases. But there is no additional moisture to be found, so the actual amount of water vapor in the air mass remains more-or-less fixed. These descending limbs of the Hadley cells form high-pressure centers and would be regions where persistent dry conditions should prevail – leading to the Earth’s desert belts that include the Gobi, Sahara, Arabian, and the Australian Outback (not just a steakhouse!).
The equatorial convection cells are known as Hadley Cells. There are two more in each hemisphere, also driven by the uneven distribution of incoming solar radiation density; these are Farrell and Polar cells. Check out the diagram of this process below.
The illustration below portrays the global wind belts, three in each hemisphere. Note that the U.S. lies primarily in the Westerly Wind Belt with prevailing winds from the west. Each of these wind belts represents a "cell" that circulates air through the atmosphere from the surface to high altitudes and back again. The cells on either side of the Equator are called Hadley cells and give rise to the Trade Winds at Earth's surface. How do we explain this pattern of global winds and how does it influence precipitation?
We'll start at Earth's equator, where solar radiation is the highest year around. Air near the equator is warmed and rises because it is less dense (mass/unit volume) than the air around it as shown in Figure 21 below.
The rising air creates a circulation cell, called a Hadley Cell, in which the air rises and cools at high altitudes moves outward (towards the poles) and, eventually, descends back to the surface. The continual heating and rise of air at the equator create low pressure there, which causes air to move (wind) towards the equator to take the place of the air that rises. On the other hand, sinking air creates high pressure at the surface where it descends. A gradient of pressure (high to low) is formed that causes air to flow away from the high and towards the low pressure at the surface.
The Earth would have two large Hadley cells if it did not rotate. But, because it does rotate, the rotation of the Earth leads to the Coriolis effect. You should view the short video on this so-called "effect" or "force." (The Coriolis Effect [2]). Without going into detail as to why rotation creates this apparent force, the Coriolis effect causes winds (and all moving objects) to be deflected:
The Coriolis effect causes winds to deflect as they travel within circulation cells and results in the two large hypothetical Hadley cells breaking into six smaller cells, which looks something like the diagram below (and the first figure in this series).
Ok, so, we now have some idea about the origin of global wind systems that result from pressure gradients at Earth's surface. How does this produce precipitation, and where? Precipitation occurs where moisture-laden air rises, either by heating at the equator or by running up and over a more dense air mass. As the rising air cools its capacity to hold water decreases (relative humidity increases) and, at some point, saturation with respect to water vapor is reached. Then, condensation--clouds and rain!
The diagrams above and below portray just the Hadley cell circulation, that is driven by heating in the equatorial region. On the surface, wind moves away from high pressure (High) and toward low pressure (Low). Convergence occurs near the equator (winds blow in towards one another) and Divergence occurs under the descending air that forms high-pressure belts. The final figure (Figure 26) shows all six cells diagrammatically, along with the pressure variations at the surface of the Earth and zones of typical wet and dry belts. Note particularly the dry belts near 30 degrees North and South.