The Earth's atmosphere exerts pressure on the surface. Pressure is measured in hectoPascals (hPa), also called millibars. Standard pressure at sea level is defined as 1013hPa, but we can see large areas of either high or low pressure. These areas are all relative to each other, so what defines a high will change depending on the area around it.
Weather chart
On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure) and depressions (areas of low pressure).
Ascending and descending air
Areas of high and low pressure are caused by ascending and descending air. As air warms it ascends, leading to low pressure at the surface. As air cools it descends, leading to high pressure at the surface.
In general, low pressure leads to unsettled weather conditions and high pressure leads to settled weather conditions.
Anticyclone (high pressure)
In an anticyclone (high pressure) the winds tend to be light and blow in a clockwise direction (in the northern hemisphere). Also, the air is descending, which reduces the formation of cloud and leads to light winds and settled weather conditions.
Depression (low pressure)
In a depression (low pressure), air is rising and blows in an anticlockwise direction around the low (in the northern hemisphere). As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. This is why the weather in a depression is often unsettled, there are usually weather fronts associated with depressions.
Atmospheric pressure and wind are both significant controlling factors of Earth’s weather and climate. Although these two physical variables may at first glance appear to be quite different, they are in fact closely related. Wind exists because of horizontal and vertical differences (gradients) in pressure, yielding a correspondence that often makes it possible to use the pressure distribution as an alternative representation of atmospheric motions. Pressure is the force exerted on a unit area, and atmospheric pressure is equivalent to the weight of air above a given area on Earth’s surface or within its atmosphere. This pressure is usually expressed in millibars (mb; 1 mb equals 1,000 dynes per square cm) or in kilopascals (kPa; 1 kPa equals 10,000 dynes per square cm). Distributions of pressure on a map are depicted by a series of curved lines called isobars, each of which connects points of equal pressure. At sea level the mean pressure is about 1,000 mb (100 kPa), varying by less than 5 percent from this value at any given location or time. Mean sea-level pressure values for the mid-winter months in the Northern Hemisphere are summarized in this first diagram, and mean sea-level pressure values for the mid-summer months are illustrated in the next diagram. Since charts of atmospheric pressure often represent average values over several days, pressure features that are relatively consistent day after day emerge, while more transient, short-lived features are removed. Those that remain are known as semipermanent pressure centres and are the source regions for major, relatively uniform bodies of air known as air masses. Warm, moist maritime tropical (mT) air forms over tropical and subtropical ocean waters in association with the high-pressure regions prominent there. Cool, moist maritime polar (mP) air, on the other hand, forms over the colder subpolar ocean waters just south and east of the large, winter oceanic low-pressure regions. Over the continents, cold dry continental polar (cP) air and extremely cold dry continental arctic (cA) air forms in the high-pressure regions that are especially pronounced in winter, while hot dry continental tropical (cT) air forms over hot desertlike continental domains in summer in association with low-pressure areas, which are sometimes called heat lows. A closer examination of the diagrams above reveals some interesting features. First, it is clear that sea-level pressure is dominated by closed high- and low-pressure centres, which are largely caused by differential surface heating between low and high latitudes and between continental and oceanic regions. High pressure tends to be amplified over the colder surface features. Second, because of seasonal changes in surface heating, the pressure centres exhibit seasonal changes in their characteristics. For example, the Siberian High, Aleutian Low, and Icelandic Low that are so prominent in the winter virtually disappear in summer as the continental regions warm relative to surrounding bodies of water. At the same time, the Pacific and Atlantic highs amplify and migrate northward. At altitudes well above Earth’s surface, the monthly average pressure distributions show much less tendency to form in closed centres but rather appear as quasi-concentric circles around the poles. This more symmetrical appearance reflects the dominant role of meridional (north-south) differences in radiative heating and cooling. Excess heating in tropical latitudes, in contrast to polar areas, produces higher pressure at upper levels in the tropics as thunderstorms transfer air to higher levels. In addition, the greater heating/cooling contrast in winter yields stronger pressure differences during this season. Perfect symmetry between the tropics and the poles is interrupted by wavelike atmospheric disturbances associated with migratory and semipermanent high- and low-pressure surface weather systems. These weather systems are most pronounced over the Northern Hemisphere, with its more prominent land-ocean contrasts and orographic (high-elevation) features.
A barometer is a scientific instrument used to measure atmospheric pressure, also called barometric pressure. The atmosphere is the layers of air wrapped around the Earth. That air has a weight and presses against everything it touches as gravity pulls it to Earth. Barometers measure this pressure. Atmospheric pressure is an indicator of weather. Changes in the atmosphere, including changes in air pressure, affect the weather. Meteorologists use barometers to predict short-term changes in the weather. A rapid drop in atmospheric pressure means that a low-pressure system is arriving. Low pressure means that there isn’t enough force, or pressure, to push clouds or storms away. Low-pressure systems are associated with cloudy, rainy, or windy weather. A rapid increase in atmospheric pressure pushes that cloudy and rainy weather out, clearing the skies and bringing in cool, dry air. A barometer measures atmospheric pressure in units of measurement called atmospheres or bars. An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). The number of atmospheres drops as altitude increases because the density of air is lower and exerts less pressure. As altitude decreases, the density of air increases, as does the number of atmospheres. Barometers have to be adjusted for changes in altitude in order to make accurate atmospheric pressure readings. Types of Barometers Mercury BarometerThe mercury barometer is the oldest type of barometer, invented by the Italian physicist Evangelista Torricelli in 1643. Torricelli conducted his first barometric experiments using a tube of water. Water is relatively light in weight, so a very tall tube with a large amount of water had to be used in order to compensate for the heavier weight of atmospheric pressure. Torricelli’s water barometer was more than 10 meters (35 feet) in height, which rose above the roof of his home! This odd device caused suspicion among Torricelli’s neighbors, who thought he was involved in witchcraft. In order to keep his experiments more secretive, Torricelli deduced that he could create a much smaller barometer using mercury, a silvery liquid that weighs 14 times as much as water. A mercury barometer has a glass tube that is closed at the top and open at the bottom. At the bottom of the tube is a pool of mercury. The mercury sits in a circular, shallow dish surrounding the tube. The mercury in the tube will adjust itself to match the atmospheric pressure above the dish. As the pressure increases, it forces the mercury up the tube. The tube is marked with a series of measurements that track the number of atmospheres or bars. Observers can tell what the air pressure is by looking at where the mercury stops in the barometer. Aneroid BarometerIn 1844, the French scientist Lucien Vidi invented the aneroid barometer. An aneroid barometer has a sealed metal chamber that expands and contracts, depending on the atmospheric pressure around it. Mechanical tools measure how much the chamber expands or contracts. These measurements are aligned with atmospheres or bars. The aneroid barometer has a circular display that indicates the present number of atmospheres, much like a clock. One hand moves clockwise or counterclockwise to point to the current number of atmospheres. The terms stormy, rain, change, fair, and dry are often written above the numbers on the dial face to make it easier for people to interpret the weather. Aneroid barometers slowly replaced mercury barometers because they were easier to use, cheaper to buy, and easier to transport since they had no liquid that could spill. Some aneroid barometers use a mechanical tool to track the changes in atmospheric pressure over a period of time. These aneroid barometers are called barographs. Barographs are barometers connected to needles that make marks on a roll of adjacent graph paper. The barograph records the number of atmospheres on the vertical axis and units of time on the horizontal. A barograph’s tracking tool will rotate, usually once every day, week, or month. The spikes in the graph show when air pressure was high or low, and how long those pressure systems lasted. A severe storm, for instance, would appear as a deep, wide dip on a barograph. Digital BarometersToday’s digital barometers measure and display complex atmospheric data more accurately and quickly than ever before. Many digital barometers display both current barometric readings and previous 1-, 3-, 6-, and 12-hour readings in a bar chart format, much like a barograph. They also account for other atmospheric readings such as wind and humidity to make accurate weather forecasts. This data is archived and stored on the barometer and can also be downloaded onto a computer for further analysis. Digital barometers are used by meteorologists and other scientists who want up-to-date atmospheric readings when conducting experiments in the lab or out in the field. The digital barometer is now an important tool in many of today’s smartphones. This type of digital barometer uses atmospheric pressure data to make accurate elevation readings. These readings help the smartphone’s GPS receiver pinpoint a location more accurately, greatly improving navigation. Developers and researchers are also using the smartphone’s crowdsourcing capabilities to make more accurate weather forecasts. Apps like PressureNet automatically collect barometric measurements from each of its users, creating a vast network of atmospheric data. This data network makes it easier and faster to map out storms as they develop, especially in areas with few weather stations.
Fast Fact
Storm Glass
A storm glass is a type of barometer used centuries ago. A storm glass is a sealed glass container with an open spout, partly filled with colored water. If the water level in the spout rises above the water level in the container, observers expect low pressure and stormy weather.