Relative Humidity
Relative Humidity
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.
Air mass starts at 30 degrees Celsius, with 15 g H2O per cubic meter of air. It can hold a maximum of 30 g H2O. RH = 50%
With cooling, air still contains 15 grams H2O per cubic meter of air. But it can now only hold a maximum of 22 grams H2O. RH = 68%
With cooling, air still contains 15 grams H2O per cubic meter of air, equal to the maximum it can hold. This temp. is called the dewpoint. RH = 100%!
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.
The Orographic Effect
The Orographic Effect
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).
Click to expand for a long description
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.
Orographic Effect In Action
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.
Atmospheric Convection: Hadley Cells
Atmospheric Convection: Hadley Cells
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.
Video Review: Global Atmospheric Circulation (2:24)
Take a few minutes to review the video below to help you understand Global Circulation a little better.
In this animation, we're going to look at global wind patterns and talk about the reasons why the air circulates the way it does and also patterns of rising and sinking air and how that relates to precipitation. The engine that drives it all, I guess you could say, is the intense heating by the Sun that occurs only in the equator areas where the sun is shining is at a very high angle of incidence and this hot air near the equator being less dense Rises upward. It rises up, going to move toward the poles and then it gradually sinks at about 30 degrees north and south latitude. So we create these big spinning circles of air that we call the Hadley cells near the equator where the air is rising it loses its ability to hold moisture and you get a band of high rainfall and low pressure because there's air leaving the equator where the air sinks. In these, it belts at around 30 degrees north and south you get high pressure sinking air which creates areas of clear skies and desert climates now as this air circulates and tries to flow back toward the equator along the surface of the earth or as some of it heads toward the North Pole or toward the South Pole. The Coriolis effect, the spin of the earth, causes it to bend and turn and it's going to create the too big wind belts that prevail on our earth two out of three the trade winds north-northeast trade winds and southeast trade winds and then the prevailing westerlies. Now these winds curve the way they do because of the Coriolis effect the winds curve to the right of their path north of the equator, they curve to the left of their past south of the equator, and they end up flowing to the from east to west or from west to east. Now the other big factor is what's happening at the poles. At the poles the air is cold and the cold air wants to sink and as that cold polar air sinks it heads toward the equator and it bumps into this air heading toward the pole here and toward the South Pole here and it creates an area of rising air and again rising air produces high precipitation belts at about 60 degrees north and about 60 degrees south latitude. At the polls themselves, the precipitation is quite modest because the air is sinking and that creates low precipitation.
Energy Balance
Energy Balance
On average regions near the equator receive light at 90°. high latitudes receive light at low angles. Light energy is more concentrate near the equator. In other words, there is a greater flux per unit area (W/m2)
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.
Global Wind
Global Wind
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.
Video: The Coriolis Effect (03:05)
NARRATOR: If you've ever watched the news during a hurricane or wintertime nor'easter, you've probably noticed that big storms spin over time as they travel. In the Northern Hemisphere, they spin counterclockwise. But if you were watching a storm in the Southern Hemisphere, you'd see it spinning clockwise.
Why do storms spin in different directions depending on their location? And why do they spin in the first place? A storm's rotation is due to something called the Coriolis effect, which is a phenomenon that causes fluids like water and air to curve as they travel across or above the Earth's surface.
Here's the basic idea: Earth is constantly spinning around its axis from west to east. But because Earth is a sphere, and wider in the middle, points at the equator are actually spinning faster around the axis than points near the poles.
So imagine you were standing in Texas and had a magic paper airplane that could travel hundreds of miles. If you threw your airplane directly northward, you might think that it would land straight north, maybe somewhere in Nebraska.
But Texas is actually spinning around Earth's axis faster than Nebraska is because it's closer to the equator. That means that the paper airplane is spinning faster as well, and when you throw it, that spinning momentum is conserved. So, if you throw your paper airplane in a straight line toward the north, it would land somewhere to the right of Nebraska—maybe in Delaware. So, from your point of view in Texas, the plane would have taken a curved path to the right.
The opposite would happen in the Southern Hemisphere. An object traveling from the equator to the south would get deflected to the left.
So, what does this have to do with hurricanes spinning? Well, at the center of every hurricane is an area of very low pressure. As a result, the high-pressure air surrounding the center or "eye" of a storm is constantly rushing toward the low-pressure void in the middle.
But because of the Coriolis effect, the air rushing toward the center is deflected off course. In the Northern Hemisphere, the volumes of air on all sides of the eye keep getting tugged slightly to the right. The air keeps trying to make its way to the middle and keeps getting deflected, causing the entire system to spin in a counterclockwise direction.
In the Southern Hemisphere, where the Coriolis effect pulls air to the left, the opposite happens: storms spin around the eye in a clockwise manner.
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.
Global Wind Explained
Global Wind Explained
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:
- to the right in the Northern Hemisphere
- to the left in the Southern Hemisphere
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.
| Latitude | Barometric Pressure | Precipitation | Surface winds |
|---|---|---|---|
| 90° | High | Dry | Divergent |
| 60° | Low | Wet | Convergent |
| 30° | High | Dry | Divergent |
| 0° | Low | Wet | Convergent |