"United States Current Weather And Travel Forecast For Today"
US Current Conditions US Current Radar Oklahoma Radar US Travel Forecast Today
Search How do I find it? Subscribe to paper
HomeNewsTravelMoneySportsLifeTechWeatherWeather Inside Weather
Sections Ask the Weather Guys Beach conditions Blog: The Weather Guys Daily highs and lows Hurricanes and storm coverage International forecasts Ski conditions Travel resources U.S. forecasts Essentials Alerts Current weather Precipitation Radar Satellite Temperature Thunderstorm
Cars Event tickets Jobs Real estate Shop Online degrees Find a forecast:
Ask the Experts: Air pressure
Q: How should a sports team traveling to a high-altitude location prepare for the thinner air?
A: Any athlete planning to perform well at high altitude must be in good respiratory health. Air pressure decreases with increasing altitude, meaning that an athlete gets fewer oxygen molecules with each breath. Less oxygen taken in means less oxygen carried by hemoglobin to exercising muscles and thus reduced performance. While performance in sports that involve running or swimming will be reduced compared to sea-level performance, any training that focuses on stamina will help athletes adjust to thinner air in high altitudes.
There's plenty more about air pressure on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, September 19, 2007)
--------------------------------------------------------------------------------
Q: What is a "pressure gradient" and what does it have to do with weather?
A: A “gradient” is the change in a quantity over distance. With atmospheric pressure, air tends to move clockwise away from high pressure and counterclockwise toward low pressure in the Northern Hemisphere. A large change in pressure over a relatively small distance, a large “pressure gradient,” can result in strong winds. On weather maps, locations of equal pressure are connected by isobars, typically drawn as white lines. When the isobars are tightly packed, locations within that large pressure gradient can expect windy conditions.
There's plenty more about the pressure gradient force on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, August 27, 2007)
--------------------------------------------------------------------------------
Q: What separates high- and low-pressure systems?
A: When it comes to air pressure, everything is relative. There is no threshold value that indicates high or low pressure. The only thing that separates centers of high and low pressure are regions that have lower pressure readings than the high -- and higher pressure readings than the low. Since air pressure is essentially the weight of the atmosphere above a location, weather maps are drawn to compare air pressure readings at different locations.
High-pressure systems usually form where the air converges aloft. As the air converges in the upper levels of the atmosphere, its own weight forces it to sink. The sinking air spirals outward, clockwise in the Northern Hemisphere, counterclockwise south of the Equator. Low-pressure systems often form where the air diverges aloft, allowing for air from lower levels to rise. As air rises, it cools and often condenses into clouds and precipitation. . There's plenty more about highs and lows on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, August 12, 2007)
--------------------------------------------------------------------------------
Q: What do meteorologists mean by "500 millibar heights?"
A: Since air pressure decreases with altitude in the atmosphere, the 500 millibar height is just the altitude above sea level at which a barometer would read 500 millibars. Typically this is around 18,400 feet. However, the average temperature of the column of air can affect that height. If the air in the column is warm, the 500 millibar level will be higher. If the column is cool, the 500 millibar level will be lower. When a meteorologist says that the 500 millibar heights are dropping over a particular location, that means that colder air is moving into the area.
Learn more about air pressure on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, July 18, 2007)
--------------------------------------------------------------------------------
Q: What causes the winds to blow as they do around high- and low-pressure areas?
A: Winds near the Earth’s surface rotate counterclockwise toward the center of areas of low pressure and clockwise outward from the center of areas of high pressure in the Northern Hemisphere, with an opposite flow (clockwise around areas of low pressure and counterclockwise around areas of high pressure) occurring in the Southern Hemisphere. The main reason for this pattern is the Coriolis force, which results from the Earth’s rotation on its axis and deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Without the Coriolis force, air in the Northern Hemisphere would flow directly toward the center of areas of low pressure from all directions and directly out from areas of high pressure, with the opposite occurring in the Southern Hemisphere.
For more, see this USA TODAY resource page on understanding the Coriolis force.
(Answered by Sean Potter, a certified consulting meteorologist and science writer in New York City, June 18, 2007)
--------------------------------------------------------------------------------
Q: What is a Bermuda high?
A: A Bermuda high is a semi-permanent area of high pressure that forms over the Atlantic Ocean during the summer. It’s usually centered near the island of Bermuda and can cover as much as 2,000 square miles of the Atlantic Ocean. It’s often the key summer weather player for most of the eastern USA, as the clockwise circulation around the high brings hot, humid winds to the East, especially the Southeast.
The location and strength of the Bermuda high can also affect the tracks of Atlantic hurricanes. In 2004, the high was in the far southwest Atlantic Ocean and some scientists believe this position helped steer hurricanes toward Florida that year, when four hurricanes battered the state.
(Answered by Elizabeth Caldwell of USA TODAY's weather staff, June 13, 2007)
--------------------------------------------------------------------------------
Q: What are the highest and lowest barometric pressures ever recorded?
A: The highest barometric pressure ever recorded on Earth was 32.01 inches, measured in Agata, U.S.S.R., on December 31, 1968. Agata is located in northern Siberia. The weather was clear and very cold at the time, with temperatures between -40° and -58°.
The lowest pressure ever measured was 25.69 inches, set on Oct. 12, 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.
An excellent reference for extreme weather records like this is Paul F. Krause's technical report Weather and Climate Extremes, the contents of which I haven't found anywhere online. The report can be ordered through the National Technical Information Service, which is part of the U.S. Department of Commerce.
(Answered by Doyle Rice, USA TODAY’s weather editor, January 24, 2007)
--------------------------------------------------------------------------------
Q: What does the term 1045 "millibars" mean?
A: This is a metric measurement of atmospheric pressure, which is in turn a measure of the weight of the atmosphere at a particular location. At sea level, standard air pressure is 1013.2 millibars, so a reading of 1045 millibars would indicate relatively high atmosphere pressure. Other units commonly used to measure atmospheric pressure include inches of mercury and hectopascals. 1045 millibars equals about 30.86 inches of mercury.
Learn more about air pressure on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, January 21, 2007)
--------------------------------------------------------------------------------
Q: What’s the relationship between barometric pressure and weather?
A: Air has weight, and a barometer measures the changes in air pressure above. When a high-pressure area is in control, the air sinks. Sinking air inhibits the development of clouds. When the air sinks, more force pushes down toward the ground, so the barometric pressure increases. Conversely, when a low-pressure area moves in, the air rises, cools and condenses out moisture, which forms clouds and precipitation. Since a rising column of air above weighs less, the barometric pressure falls.
When the pressure is high or rising, the next 12 to 24 hours should stay dry, although it might also be very cold. In fact, colder air is denser than warm air so the highest barometric pressure readings are in winter. Pilots love to fly in winter because it is easier to take off, and is often smoother when in flight. When the pressure is dropping or very low, a storm is approaching or occurring. The lower the pressure the stronger and more intense the storm.
It's hard to believe that we still use the barometer today to help us forecast the weather even though we now have computer models, satellite images and Doppler radar. Italian physicist and mathematician Evangelista Torricelli invented the mercurial barometer in 1644! Can you imagine walking around back then before electricity, and understanding that the atmosphere has weight and inventing an instrument to measure it.
Although Galileo is sometimes given the credit, Torricelli, a friend of Galileo's, was the first to use a denser liquid and correctly explain how the weight of the atmosphere affected the water in Galileo's barometer. (If water is used, the instrument would have to be much taller. Mercury was used because the liquid is so dense the changes are rather small, only a matter of inches.) On average, sea level pressure supports 30 inches of mercury.
These USA TODAY resource pages explain high- and low-pressure areas and how a barometer works.
(Answered by meteorologist Topper Shutt of WUSA-TV in Washington, D.C., January 10, 2007)
--------------------------------------------------------------------------------
Q: What is a "dirty ridge?"
A: Typically ridges -- high-pressure areas -- are associated with sinking air in the atmosphere and relatively cloud-free skies. However, sometimes clouds and even precipitation can result within an area of high pressure. This can occur when cold air moves in over wet ground, becomes saturated and forms low clouds, fog and/or drizzle. If the air is cooler near the surface and warmer aloft within the core of high pressure, an inversion will form that may trap the clouds and moisture for days. Such a high-pressure system is referred to as a "dirty ridge" or a "dirty high."
Learn more about dirty ridges on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, January 9, 2007)
--------------------------------------------------------------------------------
Q: Do troughs tend to bring stormy weather?
A: Yes. A trough is an elongated region of low atmospheric pressure, usually depicted by a dashed orange line on a weather map. Low surface pressure often indicates upward motion in the atmosphere, which can be associated with cloudy or stormy weather occurring along and east of the trough axis. Pressures are higher west of the trough, indicating sinking motion in the atmosphere, which usually prevents clouds and precipitation from forming.
Learn more about how troughs affect weather at this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, January 3, 2007)
--------------------------------------------------------------------------------
Q: What are you measuring when you measure air pressure?
A: When you dive to the bottom of a swimming pool, you feel pressure on your ears due to the weight of the column of water on top of you. Likewise, when you are standing on the surface of the Earth, you are at the bottom of a vast ocean of air. Though it may not seem like it, air does indeed have mass and weight. If you don't believe me, move your hand back and forth or, better yet, step outside into a stiff wind. What you are sensing are air molecules crashing into you.
In detecting atmospheric pressure, a barometer measures the weight of the column of air stretching directly above it from the surface of the Earth to the top edge of the atmosphere. This column of atmosphere exerts, on average, about 14.6 pounds of pressure on every square inch of your body at sea level. Why aren't we then crushed by the atmosphere? The pressure inside the body is the same as the pressure outside the body, resulting in no net pressure.
Learn more about atmospheric pressure from this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, December 27, 2006)
--------------------------------------------------------------------------------
Q: How do high- and low-pressure systems form?
A: When it comes to weather, everything is relative. That is, the center of high pressure or the center or low pressure simply means that the atmospheric pressure reading at that point is higher or lower than readings from all other surrounding locations.
High-pressure systems usually form where air converges high above in the atmosphere. As the air converges aloft, its own weight forces it to sink. The sinking air spirals outward, clockwise in the Northern Hemisphere, counterclockwise south of the Equator. Low-pressure systems often form where the air diverges aloft, allowing for air from lower levels to rise. As air rises, it cools and often condenses into clouds and precipitation. . There's plenty more about highs and lows on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, December 17, 2006)
--------------------------------------------------------------------------------
Q: If air pressure is necessary to keep water from boiling, how can there be water in space?
A: By definition, open space is essentially a vacuum and virtually devoid of all molecules, including water. However, planets that have an atmosphere could have liquid water, given the right combination of temperature and atmospheric pressure. Mars, for example, has a very thin atmosphere (100 times thinner than Earth's). Unfortunately, the extremely low atmospheric pressure of this thin atmosphere does not allow liquid water to exist. A cup of water transported to the Martian surface would instantly boil or freeze, depending on the temperature. There is a very small amount of water vapor in the Martian atmosphere with larger water reserves locked up as ice at the poles and beneath the Martian surface.
Air pressure does affect the boiling of water on Earth, as water boils when its saturation vapor pressure reaches the atmospheric pressure. At Earth's surface, the boiling point of water is 212°F. At higher altitudes above sea-level, the boiling point decreases because atmospheric pressure decreases with altitude; therefore, the required saturation vapor pressure of the water decreases.
Learn more about the water in the atmosphere on this USA TODAY resource page. Also of interest is this recent report about liquid water on the Martian surface.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, December 10, 2006)
--------------------------------------------------------------------------------
Q: Is there any known correlation between air pressure and headaches?
A: While there have been numerous studies of the link between air pressure and other weather conditions and headaches, a conclusive correlation has yet to be determined. Congestion headaches due to air trapped in the sinuses could be magnified by decreases in atmospheric pressure, as the trapped air expands in response to lower environmental pressure. Migraine headaches can be linked to weather sensitivity, but the weather conditions that bring them on can vary from individual to individual.
Find out how weather conditions can impact health on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, October 26, 2006)
--------------------------------------------------------------------------------
Q: How does the weather change as air pressure at Earth's surface rises and falls?
A: High pressure is the result of sinking air in the atmosphere, which usually corresponds with clear skies. Low pressure indicates rising air, which cools and condenses, forming clouds and precipitation. So, in general terms, if the pressure is rising at your location, that means that high pressure is building in and skies should gradually clear. If your barometer reading is dropping, that means that low pressure is on the way and stormy weather can be expected.
Barometers measure atmospheric pressure. Find out how barometers work on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, October 17, 2006)
--------------------------------------------------------------------------------
Q: When the barometric pressure is given for Denver, is that the actual pressure or is it corrected to sea level by some formula?
A: The pressure reported for Denver, or any official observation station for that matter, is not the actual pressure on the surface, but rather is the pressure corrected to sea level. The reason this is done is so that meaningful maps of constant pressure lines, called isobars, can be drawn for stations across the USA. These maps are useful for picking out areas of relative high and low pressure. If pressure readings were not corrected, places like Denver would almost always have lower pressure than spots at lower elevations. Essentially, the map would reflect topography, rather than weather systems in the atmosphere.
Learn more about air pressure on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, July 24, 2006)
--------------------------------------------------------------------------------
Q: I've read that when the pressure goes up, the air goes down. Can you explain this to me?
A: Though it may not seem like it, air does indeed have mass and weight. If you don't believe me, move your hand back and forth or, better yet, step outside into a stiff wind. What you're sensing are air molecules crashing into you.
In detecting atmospheric pressure, a barometer measures the weight of the column of air that stretches directly above it from the Earth’s surface to the top of the atmosphere. If your barometer’s reading goes up, this means that more air molecules are moving into that column of the atmosphere than are leaving it. Essentially, this "piling on" of air molecules increases air density and causes it to sink.
Learn more about the ups and downs of the atmosphere from this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, June 20, 2006)
--------------------------------------------------------------------------------
Q: How does cloud cover relate to barometric pressure?
A: The vertical motion of the atmosphere is often reflected in the barometric pressure that's measured at the Earth's surface. Low pressure is associated with rising air: when air rises, it cools and water vapor condenses, forming clouds. Conversely, high pressure typically means sinking air. When air sinks, it warms and its relative humidity decreases, making it difficult for clouds to form.
Learn more about how high- and low-pressure areas impact cloud cover and precipitation on this USA TODAY resource page.
(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, December 21, 2005)
--------------------------------------------------------------------------------
Q: What are all the all-time records for high and low barometric pressure in the USA?
A: Using average sea level pressure of 29.92 inches of mercury as a point for comparison, the highest barometric pressure ever recorded in the USA was 31.85 inches in Northway, Alaska, in January 1989. The lowest barometric pressure ever recorded was associated with the landfall of the Labor Day hurricane in Key West, Florida in 1935, which registered a minimum pressure of 26.35 inches of mercury. Both are also records for North America.
It is likely that tornadoes have had lower barometric pressures, but they have not become part of the official record.
Learn more about atmospheric pressure on this USATODAY.com resource page.
(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 24, 2005)
--------------------------------------------------------------------------------
Q: Where in the U.S. does the barometric pressure fluctuate the least? The most?
A: According to Christopher C. Burt's book, Extreme Weather: A Guide & Record Book, Honolulu, Hawaii, is home to the smallest difference in pressure extremes, with a record high of 30.32 inches of mercury and a record low of 29.34 inches. For the continental U.S., San Diego boasts the most steady pressure, with a record high of 30.53 inches and a record low of 29.37 inches.
As for largest fluctuations between record highs and record lows, St. Paul Island, Alaska, has a record high pressure of 30.86 inches of mercury and a record low of 27.35 inches, a whopping 3.51 inches difference. In the continental U.S., Charleston, S.C. has a record high of 30.85 inches and a record low of 27.64, with the difference being 3.21 inches.
Learn more about barometric pressure on this USA TODAY resource page.
(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 17, 2005)
--------------------------------------------------------------------------------
Q: Is atmospheric pressure present underground?
A: Yes, and it increases with depth the further underground you go. A good way to think about atmospheric pressure is to realize that, here on the Earth's surface, we are at the bottom of an "ocean" of air. If you go underground, perhaps into a cavern, you add an additional depth of air, just like you would by diving deeper into a pool of water.
There's plenty more about "understanding air pressure" on this USA TODAY resource page.
(Answered by Bob Swanson, USA TODAY's assistant weather editor, October 30, 2005)
--------------------------------------------------------------------------------
Q: What is barometric pressure?
A: Barometric pressure is the pressure of the air that's pressing in all directions. In regular weather forecasts, the meteorologists are almost always talking about the air's pressure at the ground. The air's pressure is caused by the weight of all the air above the ground pressing down - gravity pulls air toward the earth just as it pulls everything else. It's called "barometric" pressure because an instrument called a barometer is used to measure air pressure.
--------------------------------------------------------------------------------
Q: What does a barometer measure?
A barometer measures the pressure of the air pushing on it. On the average, at sea level, the air has a pressure of 14.7 pounds per square inch. This means a square one inch on each side has 14.7 pounds of air pressure on it. We don't feel it because the pressure is pushing with equal force in all directions. Instead of using pounds per square inch, barometers in the U.S. measure the pressure in inches of mercury. This is how high the pressure would push mercury into a tube that has the top sealed off from the air. A reading of 29.92 inches of mercury is the same as 14.7 pounds per square inch.
--------------------------------------------------------------------------------
Q: How does air pressure affect the weather?
A: In general, falling air pressure means that clouds and precipitation are likely. Rising air pressure signals that clear weather is likely. A USA TODAY Online graphic on air pressure explains why this is so. Our storms and fronts index has links to a lot more information on the relation between air pressure and weather. You'll also find good information about air pressure and the weather in our how the weather works index.
--------------------------------------------------------------------------------
Q: What are millibars?
A: Millibars are a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a convenient measure of Earth's air pressure. Inches of mercury, the number used for surface air pressure in the U.S., is not a direct measure of pressure. Instead, inches of mercury tell you how high the pressure pushes the mercury in a barometer. The use of direct pressure measurements goes back to the late 19th century when the great Norwegian meteorologist Vilhelm Bjerknes, the leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather.
--------------------------------------------------------------------------------
Q: What are hectoPascals?
A: In the International System (SI) of measurements, the unit of pressure is the Pascal, named after Blaise Pascal, the 17th century scientist who made important discoveries about air pressure. The standard atmospheric pressure at the Earth's surface of 1013.25 millibars is equal to 101,325 Pascals. To avoid large numbers, air pressure is reported in hectoPascals, which are the same as millibars. In many nations, you are now likely to hear reports such as, "air pressure, 1020.0 hectoPascals." This is the same as 1020.0 millibars.
--------------------------------------------------------------------------------
Q: What is the formula for converting pressure in millibars of pressure to inches of mercury?
A: You don't really need a formula. The "standard" atmospheric pressure at sea level is 29.92 inches of mercury, or 1013.2 millibars. In other words, these numbers are the same, but in different measurement systems. Anyway, if you see a pressure on a weather map of, say, 1016 millibars, you can convert to inches of mercury by multiplying by 29.92 and dividing by 1013.2 to come up with 30.00 inches of mercury.
Why? Think of the rules for cancellation. When you multiply by inches of mercury and divide by millibars, the millibars cancel out and you're left with inches of mercury. And, it's OK to do the multiplication and division because the numbers represent the same air pressure. A number divided by itself is 1 and when you multiply a number by 1 you get the original number. To go the other way from inches of mercury to millibars, you just divide by inches of mercury and multiply by millibars. This method is a good way to do all sorts of conversions without memorizing a bunch of formulas, as long as you know one equivalent set of numbers in the two systems.
--------------------------------------------------------------------------------
Q: How can I find the barometric correction for elevation ?
A: The El Paso, Texas, National Weather Service Office has a weather calculator posted on the World Wide Web. One of the calculations you can perform on it is correcting station pressure for the altimeter setting. You can also use the rule of thumb that pressure decreases about 1 inch of mercury for each 1,000 foot altitude gain. For more information on this, go to USA TODAY's online barometric pressure corrections text. Detailed information is also found in Federal Meteorological Handbook Number 1, which is the official guide to taking weather observations. Chapter 11 covers pressure measurements and corrections.
--------------------------------------------------------------------------------
Q: How does air pressure affect the temperature at which water boils?
A: Water boils at lower temperatures at lower air pressures. This is why water boils at lower temperatues at high elevations, such as in the mountains, where the air pressure is lower. As water boils, bubbles of water vapor rise to the surface of the water and escape. In order for these bubbles to make it to the surface, the saturation vapor pressure must equal the atmospheric pressure, otherwise the bubbles would collapse. The saturation vapor pressure is directly proportional to the temperature of the water. In other words, the higher the temperature, the higher the saturation vapor pressure. At higher elevations, the atmospheric pressure is less than at lower elevations. This means that the saturation vapor pressure needed to allow bubbles to escape into the air is less, which means that the temperature water must be heated to in order to bring it to a boil is also less. Meteorology Today by Donald Ahrens has an excellent discussion about this concept. Cooking things in boiling water at higher elevations takes longer than at lower elevation because of the lower boiling temperaturer. Once water begins to boil, its temperature remains constant as the energy from the heating is used to convert the liquid water to water vapor. For example, water boils at 95 degrees Celsius, 203 degrees Fahrenheit, in Denver, which is about 5,000 feet above sea level. Our water in the atmosphere homepage has links to more information about how water behaves in the atmosphere.
Click Here To Return To Link Page.
Updated January 02, 2012