PRESSURETEMPERATURE

MOISTURE

We remember that pressure is the weight of a column of air above a surface (and only the air, even if an airplane, rooftop, bird, etc. happens to be in that particular column – but why does pressure not include those items? b/c they are effectively weightless, either supported by a wall (for a rooftop) or buoyant on their own (for a plane/bird/insect etc.)

In meteo., we have a very convenient way to measure atmospheric pressure: Millibars (abbreviated “mb”)

Standard sea-level pressure is 1013.2 mb.

500 mb is considered the “halfway” point of the atmosphere (half of the atmosphere lies beneath 500mb, the other ½ lies above it)

100 mb is often around the level of the tropopause (so 90% of air lies in the troposphere– no wonder most weather occurs there!)

Because air pressure decreases naturally as we rise in the atmosphere, or up a mountain, we must make corrections to the air pressure owing to elevation above sea level. 

These corrections are easily made by adding the air pressure that would be exerted by the air column at that elevation. 

Here we have a great example of the “700 mb surface” – and also the strong relationship between pressure and temperature

-- Notice (lower-panel) that the 700 mb “surface” (shaded) slopes downward from the equator to the poles. According to this graphic, at the equator, you’d have to rise 3,200 meters (2 miles) to reach 700 mb; at the pole, you only need to rise 2,800m to reach 700 mb.

This difference in height is directly related to the temperature of the air: warm air is less-dense & thus occupies greater volume. In panel A, temp is uniform, & 700 mb surface is flat.

Example of a constant-pressure surface weather map: 500 hPa. Can identify regions of high height and low height. Data for this map come from radiosonde (balloon) releases at 00Z and 12Z daily. Visit http://weather.unisys.com/gfsx/init/gfsx_500p_init_eur.html

How to interpret this chart: a warm column of air will normally expand. Thus the 500 millibar (mb, aka hPa) level will be found higher above the earth’s surface. For instance: the 500 hPa heights over Austria are ~ 5800 m, but over the north Atantic are only ~ 5300 m. This 500 m difference is one of the factors that generate wind (& hence weather)

On this chart: colors represent 500 hPa height, contoured every 60 m; dark lines represent sea level pressure, contoured every 4 mb

Equation of state & relationships

P = ρ R TPressure = density x gas constant x temperature• Thus, if we hold P constant (like on a “constant-pressure” weather

map, 500 mb etc.), and T increases, what must density do?• Relationship becomes more complex when all 3 variables change at

the same time – and when we add water vapor (which affects density) to the air

• Adding water vapor makes air less dense. Why? – Avagadro’s rule: for constant P, volume, & T, n (# of molecules)

must remain constant. Therefore, if we displace O2 (atomic weight 32) and N2 (atomic weight 28) with H2O (atomic weight 18), we have lowered the density of that parcel of air

The U.S. Geological Survey estimates that the Earth has about 326 million cubic miles of water. This includes all of the water in the oceans, underground and frozen in glaciers.

Water and the Water Cycle

The Hydrologic Cycle involves the continuous circulation of water in the Earth-atmosphere system. Of the many processes involved in the hydrologic cycle, the most important are

• evaporation, • transpiration, • condensation, • precipitation, and • runoff.

Evaporation the process by which water changes from a liquid to a gas or vapor Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in the atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration.

Evaporation is a cooling process

Transpiration is the evaporation of water from plants through stomata. Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues. In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Of the transpired water passing through a plant only 1% is used in the growth process of the plant. The remaining 99% is passed into the atmosphere.

How much water do plants transpire?During a growing season, a leaf will transpire many times more water than its own weight. an acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, a large oak tree can transpire 40,000 gallons (151,000 liters) per year.

Condensation is the process by which water vapor in the air is changed into liquid water. In the atmosphere, condensation may appear as clouds, fog, mist, dew or frost, depending upon the physical conditions of the atmosphere.

Condensation is crucial to the water cycle because clouds may produce precipitation, which is the primary route for water to return to the Earth's surface.

Condensation is the opposite of evaporation. Condensation releases heat to the atmosphere

At any given temperature, there will eventually be an equilibrium between the number of molecules evaporating, and the number of molecules condensing.  When the number of molecules evaporating balances the number of molecules condensing, we say that the air above the liquid water is saturated. 

Latent heat is critical to energetics of the atmosphere

Key to understanding latent heat is to separate the “air parcel” from the “environment”

Latent heat is exchanged from the parcel to the environment

Clausius-Clapeyron derivation

where es is saturation water vapor pressure over a plane water surface, T is a temperature, Lv is latent heat of evaporation, and Rv is water vapor gas constant.

Integrate the equation, note that at 273 K, es is 611.2 Pa and that Lv is slightly a function of temperature. We get:

Start with the differential relationship between vapor pressure and temperature:

Tells us the saturation vapor pressure of water increases with increasing temperature

Warmer air has greater saturation vapor pressure and thus more energy (Gibbs free energy) is available for phase change to/from gas

Amount of moisture in the atmosphere is a function of temperature

Thus July vapor pressures are typically greater than January vapor pressures

We can describe the actual amount of water vapor in the atmosphere at any given time in a number of different ways:

Absolute humidity = density of water vapor (gm of water vapor/m3 of air)

Specific humidity = portion of atmospheric mass accounted for by water vapor = mass of water vapor/(mass of dry air + mass of H2O vapor)

Mixing ratio = mass of water vapor/mass of dry air

Vapor pressure = portion of total atmospheric pressure (P) due to water vapor (i.e. the partial pressure of water vapor) = e

Relative humidity = actual water vapor in air/maximum water vapor possible

Relative humidity depends on two factors: the actual amount of moisture in the atmosphere the temperature

remember that temperature determines how much water vapor can be in the air at saturation

RH=[(vapor pressure)/(saturation vapor pressure)] X 100%

RH=[(mixing ratio)/(saturation mixing ratio)] X 100%

We can express/calculate Relative Humidity in a variety of ways:

Remember, RH = vapor pressure / saturation vapor pressure

VP remains relatively constant during the day (varies with evaporation & precipitation)

SVP, though, varies directly with temp

Thus as temp warms during the day, RH decreases

Because RH varies greatly each day, it is not a good measure of moisture in the atmosphere

So you can have a change in relative humidity in one of two ways:1) Change the amount of water vapor available; if there is liquid water present, for instance, a lake, you can have an increase in relative humidity by evaporation from the surface of the lake. You’re adding water vapor, so the humidity increases.

Steam fog over a lake.

2) The other way is to change the temperature of the air, while holding the water vapor constant. Even though there is no water source, and no water vapor is added, a lowering of air temperature results in a rise of relative humidity.

The amount of water vapor that could be present at saturation is less at the lower temperature, so the existing amount of water vapor represents a higher percentage of the saturation level of the air.

Similarly, a rise in temperature results in a decrease in relative humidity, even though no water vapor has been taken away.

Key point to remember: Given that the amount of water vapor is held constant, then if you-- reduce the temperature, the relative humidity goes up-- increase the temperature, the relative humidity goes down.

If the amount of water vapor in the air remains constant, the relative humidity will change only as a result of the change in temperature; therefore, the relative humidity will be higher in the morning (coolest part of the day) than in the afternoon (warming part of the day).

Only about 3,100 cubic miles of this water is in the air, mostly as water vapor, but also as clouds or precipitation, at any one time, the Geological Survey estimates.

While this is a small share of Earth's water, our planet would be very different without it. If Earth's air didn't contain as much humidity as it does, our weather would be like that of Mars: And, without all of this water in all of its forms, life on Earth would probably not exist.

The storehouses for the vast majority of all water on Earth are the oceans: of the 332,500,000 cubic miles (mi3) (1,386,000,000 km3) of the world's water supply, about 321,000,000 mi3 (1,338,000,000 km3) is stored in oceans (~ 96.5%) it is also estimated that the oceans supply about 90% of the evaporated water that goes into the water cycle.

The vast majority, almost 90 percent, of Earth's ice mass is in Antarctica, while the Greenland ice cap contains 10 percent of the total global ice mass.

Photos from: www.uscg.mil/.../polarsea/ Antarctica_Photos.htm

Water sourceWater volume, in cubic miles

Water volume, in cubic kilometers

Percent offreshwater

Percent oftotal water

Oceans, Seas, & Bays 321,000,000 1,338,000,000 -- 96.5

Ice caps, Glaciers, & Permanent Snow

5,773,000 24,064,000 68.7 1.74

Ground water 5,614,000 23,400,000 -- 1.7

    Fresh 2,526,000 10,530,000 30.1 0.76

    Saline 3,088,000 12,870,000 -- 0.94

Soil Moisture 3,959 16,500 0.05 0.001

Ground Ice & Permafrost 71,970 300,000 0.86 0.022

Lakes 42,320 176,400 -- 0.013

    Fresh 21,830 91,000 0.26 0.007

    Saline 20,490 85,400 -- 0.006

Atmosphere 3,095 12,900 0.04 0.001

Swamp Water 2,752 11,470 0.03 0.0008

Rivers 509 2,120 0.006 0.0002

Biological Water 269 1,120 0.003 0.0001

Estimate of global water distribution

Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp.817-823.

• Water freezes at 32o F (0oC) and boils at 212oF (100oC) at sea level, but 186.4°F at 14,000 feet• Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot• The high specific heat index of water helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans

According to columnist Cecil Adams, "a modest-size cloud, one kilometer in diameter and 100 meters thick, has a mass equivalent to one B-747 jumbo jet." But, with all that mass being spread over such a large volume of space, the density, or weight (mass) for any chosen volume, is very small.

http://www.straightdope.com/classics/a980313a.html,

Even though a cloud weighs tons, it doesn't fall on you because the rising air responsible for its formation keeps the cloud floating in the air.

Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. Evaporation and transpiration change liquid water into vapor, which ascends into the atmosphere due to rising air currents. Cooler temperatures aloft allow the vapor to condense into clouds and strong winds move the clouds around the world until the water falls as precipitation to replenish the earthbound parts of the water cycle.

There is always water in the atmosphere (i.e., RH never equals 0%). If all of the water in the atmosphere rained down at once, it would only cover the ground to a depth of 5 to 50 mm.

By examining precipitable water content, you get an idea of not only the quantity of moisture in the depth of the troposphere, but also the temperature for the layer

PrecipitationPrecipitation is the result when the tiny condensation particles grow too large for the rising air to support, and thus fall to the earth.

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail.

It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth.

Most precipitation falls as rain.

Ice caps influence the weatherIn some regions, precipitation collects throughout the year as ice or snow.

Just because water in an ice cap or glacier is not moving does not mean that it does not have a direct effect on other aspects of the water cycle and the weather.

Ice is very white, and since white reflects sunlight (and thus, heat), large ice fields can determine weather patterns. Air temperatures can be higher a mile above ice caps than at the surface, and wind patterns, which affect weather systems, can be dramatic around ice-covered landscapes.

Some glacier and ice cap facts Glacial ice covers 10-11 percent of all land. According to the National Snow and Ice Data Center (NSIDC), if all glaciers melted today the seas would rise about 230 feet (70 meters). During the last ice age (when glaciers covered more land area than today) the sea level was about 400 feet (122 meters) lower than it is today. At that time, glaciers covered almost one-third of the land. During the last warm spell, 125,000 years ago, the seas were about 18 feet (5.5 meters) higher than they are today. About three million years ago the seas could have been up to 165 feet (50.3 meters) higher. Largest surface area of any glacier in the contiguous United States: Emmons Glacier, Washington (4.3 square miles or 11 square kilometers)

Extent of global ice during the last ice age (~20,000 years ago)