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July-August 2009

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Weather Queries

I have received confusing information on atmospheric pressure and what changes in pressure might imply about the weather. Increasing pressure is associated with improving weather, and high pressure with benign weather. Decreasing pressure is associated with deteriorating weather, and low pressure with bad weather. Very low pressure is associated with severe storms. A friend “explained” that this is so because water weighs less than the other two major components of air— oxygen and nitrogen—and so when there’s more water vapor in the air (leading to wet weather), the overall pressure goes down. When people sense a storm coming, complaining of “pain in the joints,” is it because of a pressure change?

Britt Bunyard
Chagrin Falls, Ohio


You received confusing information, indeed. Air consists of many gases. Nitrogen is the most plentiful component of dry air by far, occupying 78.1 percent of the total volume. Next is oxygen at 21.0 percent. The concentration of these two gases does not vary in time. The many other gases in dry air make up the remaining 0.9 percent. Dry air and water vapor together make up the air we breathe, but the concentration of water vapor can vary from close to 0 to nearly 4 percent by volume. The molecular weight of water (H2O) is 18 grams per mole, while nitrogen (N2) is 28 grams per mole, and oxygen (O2) is 32 grams per mole. (A mole of a substance is its weight in grams of 6.02 x 1023 atoms or molecules.) Each gas in the air exerts a partial pressure, and the sum of all partial pressures is the total pressure that a barometer measures and our bodies experience. It’s true that a molecule of water vapor “weighs” less than a molecule of oxygen or nitrogen, but this does not imply that the total pressure will decrease when the concentration of water vapor goes up, just because there’s more of the lighter gas in the mixture. Most of the time, the pressure at the ground is the result of the weight of all the overlying air. If a passing weather disturbance in the jet stream temporarily removes some of the overlying air in a column above us, the surface pressure falls.

Most of us are familiar with the maxims mentioned in your question about high and low pressure, and rising and falling pressure. These are generally true in mid-latitudes where the terrain is flat. In areas with hilly or mountainous terrain, the wind direction is often more important than the atmospheric pressure. For example, I live on the eastern edge of the foothills of the Rocky Mountains in Colorado. Bad weather with rain or snow often begins when the pressure rises following passage of a cold front. The wind begins to blow from the east or northeast, uphill from the plains to the foothills. This leads to cloud formation and precipitation. In the Midwest, precipitation often occurs during a time of minimum pressure when the cold front passes, and then the weather improves.

Low pressure often brings foul weather because fronts are associated with low pressure systems, and both warm and cold fronts bring precipitation. When the pressure drops to very low values, the wind is usually strong as well because of strong pressure gradients (big changes in pressure over short distances). The most extreme cases of this occur in tornadoes and hurricanes.

Finally, you asked whether arthritis pain might be aggravated by falling pressure. This is plausible, in that arthritic joints are inflamed and under pressure because of increased joint fluid. If the outside pressure drops, the inflamed joint could stretch and swell further, increasing the pain. Many people claim that the pain brought on by arthritis is weather-sensitive, but, to my knowledge, no clear scientific connections between weather and arthritic pain have been established. Almost none of the experiments reported so far have been conducted under carefully controlled conditions, in which the patients under study were not aware of the environmental conditions affecting their pain. I would appreciate hearing from readers who know otherwise.

How do forecasters predict the size of hailstones? That is, how can they state that golfball-sized hail might fall on a certain day?
Cynthia Hilliard
Pueblo, Colorado


Forecasters often attempt to predict hail size on days when thunderstorms are expected. To estimate hail size, they need to estimate the strength of the thunderstorm updraft. To estimate the strength of the updraft, they need a measure of potential instability in the atmosphere that supports thunderstorm development. And finally, to estimate atmospheric instability, they must refer to data from balloon soundings that measure the vertical variation of temperature and moisture.The three steps in this progression all deserve a more complete explanation.

The size of hail depends upon the strength of the updraft. Hail must be suspended in the thunderstorm updraft in order for it to have time to grow. In general, the larger the hailstone, the faster it falls; therefore, stronger updrafts are required to suspend larger hailstones. Liquid water in the thundercloud coats the hailstone at sub-freezing temperatures. The water freezes to the hailstone, adding to its size. Eventually, either the hailstone becomes too large for the updraft to suspend it in the growth region of the cloud, or the updraft weakens. Either way, the hailstone falls out. Laboratory and theoretical calculations relate the fall speed of hail (equivalently, the updraft speed) with hail size.
The strength of the updraft depends upon how much potential instability is available in the atmosphere. First, a few words about stability. Imagine a volume of air hypothetically isolated from its surroundings. If such a volume of air is forced upward, it will cool at a known rate, based upon thermodyamical considerations. If the forced lifting ceases and the volume of air spontaneously returns to its starting position, the temperature stratification (how the temperature varies with altitude) of its surroundings is said to be stable. This happens if, upon lifting, it becomes cooler (denser) than its surroundings. But if it accelerates upward, away from its starting position, the temperature stratification of its surroundings is said to be unstable. This happens if, upon lifting, the volume of air becomes warmer (less dense) than its surroundings. Most of the time atmospheric instability, if it exists at all, is only potential.

To realize this instability, air close to the surface has to be lifted until it becomes buoyant and can begin rising on its own. There are a number of ways in which low-level air can be forced aloft: passage of a cold front, a particularly large thermal, air flowing up sloping terrain, or, as is often the case, converging air streams close to the ground. Air forcibly lifted becomes buoyant only after some of its water vapor begins to condense, usually at some distance above cloud base. Once a cloud forms, the heat of condensation is released internally within the cloud updraft air. If enough heat is released, the updraft temperature can exceed that of air outside the cloud at the same level. At this point, the cloud updraft air becomes buoyant and will begin rising on its own: the instability is realized! When this happens, cumulus clouds take on a cauliflower shape; they bubble and boil upward.

The amount of potential instability depends upon the vertical variation of temperature and moisture. To assess potential instability, meteorologists examine measurements of temperature and dewpoint (a moisture parameter) obtained by a sounding balloon as it rises through the atmosphere. They plot this information on a thermodynamic chart. Using features available on this chart, they can visualize what would happen to a volume of air, lifted from the surface, that has a starting temperature and dewpoint matching the values expected during the heat of the day. The dewpoint tells the meteorologist how much water vapor is available to be condensed. The meteorologist compares the temperature of the rising air, which cools at a known rate as it is lifted, with the air temperature of its surroundings. Usually at some distance above the cloud base, the heat of condensation released within the rising volume of air makes it warmer than the air outside the cloud. Henceforth, it rises like a hot air balloon, accelerating upward. A layer of stable air at the top of the troposphere invariably slows and stops the updraft. The amount of buoyant energy accumulated by the rising volume depends upon two things: how much warmer it is than its surroundings at each level, and over what altitude range it remains buoyant. The accumulated buoyant energy is a measure of potential instability. It has a name: convective available potential energy, or CAPE. It is measured in terms of energy per unit mass, or joules per kilogram. A large value is 4000. CAPE is used to calculate the maximum updraft
speed in a thunderstorm. For more details about tracking a rising volume of air on a thermodynamic chart and assessing CAPE, see the Weatherwise web site at www.weatherwise.org.

To summarize: 1) Data from a balloon sounding is necessary for the calculation of potential instability. 2) A measure of potential instability is necessary for calculation of the updraft speed. 3) An estimate of updraft speed is necessary for calculating hail size. A number of assumptions are buried in the calculations of potential instability, maximum updraft speed, and hail size. Even so, forecasters are usually successful in distinguishing between days when hail will be small, medium, or large.          

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA’s Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author in care of Weatherwise; 1319 18th St. NW; Washington, D.C. 20036; or by email to ww@heldref.org.


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**Web Extra**

More information about how to assess potential instability from atmospheric sounding data:

To assess potential instability, meteorologists look on a thermodynamic diagram for the path a volume of air would take if it were lifted from near the surface. I’ve sketched a simple thermodynamic diagram with temperature along the horizontal axis and altitude along the vertical axis. A sounding balloon ascending through the atmosphere measures the temperature and dewpoint (a measure of moisture) at each altitude. The red and blue curves depict temperature and dewpoint measurements, respectively, from the surface to 16 kilometers (over 52,000 feet). Sounding balloons are released twice a day around the world at 0000 and 1200 Greenwich Mean Time (GMT), which is the local time in Greenwich, England. In the United States, forecasters examine the 1200 GMT sounding (morning, local time) for signs of instability that could favor thunderstorm development and hail formation later in the day. Here, I’m showing a sounding typical of a summer afternoon.

Some background information will aid in understanding of the rest of the information on the diagram. The decrease of temperature with height is called the lapse rate. The atmospheric lapse rate seldom exceeds 1°C cooling per 100 meters of altitude or 5.4°C per 1000 feet (this is called the dry adiabatic lapse rate). The stability in this case is considered neutral, because, when this lapse rate occurs, vertical motions occur freely. Rising or sinking volumes of air maintain the same temperature as their surroundings. Dry adiabatic lapse rates are common in the lowest few kilometers of the atmosphere on sunny summer afternoons. Thermals are common, and this is when soaring pilots like to fly. A dry adiabat represents the dry adiabatic lapse rate on a sounding diagram. The straight gold line on the diagram is a particular dry adiabat─one that passes through the measured surface temperature.


The lapse rate is often less than 1°C of cooling per 100 meters altitude. In fact, the temperature sometimes increases with height. This is called an inversion. Inversions represent stable stratification of the air because they inhibit vertical motion. A volume of air rising through an inversion quickly finds itself cooler than its surroundings and sinks back toward its origin. The tropopause is a semi-permanent stable layer, either an inversion or a layer of nearly constant temperature, at altitudes of 25,000 to 40,000 feet. This stable layer invariably stops the vertical development of clouds. It is at the upper limit of the troposphere that most weather occurs.

To find out whether the measured temperature and moisture profiles favor buoyancy within a cloud, we can follow a volume of air lifted from the surface. In this example, the starting temperature and dewpoint are the same as those measured by the balloon when it was released. The temperature trajectory taken by this volume of air is shown by the green curve. It is determined by thermodynamics. From 1-3 km altitude, the air cools at the dry adiabatic lapse rate, 1oC for each 100 meters of lift. That is why the green curve follows the gold line at first. The dewpoint of the air is decreasing, too, but not as fast as the temperature. Eventually, the two converge at cloud base, near 3-km altitude. Above this altitude, the temperature and dewpoint within the rising volume are identical.

Above 3-km altitude, condensation occurs continuously, and the addition of heat within the rising volume causes its rate of cooling with altitude to change markedly, as can be seen by the abrupt change in trajectory at cloud base. The trajectory now follows a moist adiabat, representing a lapse rate initially about half that of dry adiabatic, but eventually approaching dry adiabatic at greater altitudes because the rate of condensation (and condensation heating) decreases with decreasing temperature.

Note that the rising air is cooler than its surroundings between about 2- and 4-km altitude. Forced lifting must continue in this altitude range. If not, the volume of rising air will never become buoyant. The altitude where the green curve, representing the temperature changes experienced by the rising air, crosses the red curve, representing the measured temperature in air surrounding the cloud, is called the level of free convection, near 4 km. Above this level, in-cloud air is buoyant, and the updraft accelerates. The cumulus cloud bubbles and boils upward until finally, near the tropopause, the updraft loses its buoyancy (near 13 km), and the air it has lofted from the lower atmosphere spreads laterally into an anvil-shaped cloud.

On an appropriate thermodynamic diagram (a skew-T / log-p diagram, not the one shown here), the area between the red and green curves (but only where the green curve lies to the right of the red curve) is proportional to the buoyant energy imparted to the rising air. A computer can calculate the value, which is called the convective available potential energy (CAPE). The CAPE is a quantitative measure of potential instability.

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