Category Science

HOW IS AIR PRESSURE MEASURED?

          An instrument called a barometer is used to measure air pressure. A mercury barometer consists of a glass tube standing in an open dish of mercury. The air pressure pushes against the mercury and forces it up the tube. The level of the mercury is recorded against a scale. Mercury barometers are clumsy, and mercury is poisonous, so aneroid barometers are more commonly used. A sealed metal box inside the barometer is connected to the pointer on the clock-like face. The vacuum inside the metal box means that an increase in pressure will squash it; a drop in pressure will make it expand. These changes make the pointer move around the dial.

          Even though we can’t see air, it is real and has pressure. The pressure of the atmosphere changes. It is higher at sea level, and lessens as you go higher up in the atmosphere. Some weather systems have slightly higher pressure than others – you may have heard of High pressure and Low Pressure weather system.

          Let’s look at how atmospheric pressure is measured. For a long time, atmospheric pressure has been measured by a mercury barometer. The first was invented in 1643 by one of Galileo’s assistants. A mercurial barometer has a section of mercury exposed to the atmosphere. The atmosphere pushes downward on the mercury (see image). If there is an increase in pressure, it forces the mercury to rise inside the glass tube and a higher measurement is shown. If atmospheric pressure lessens, downward force on the mercury lessens and the height of the mercury inside the tube lowers. A lower measurement would be shown. This type of instrument can be used in a lab or a weather station, but is not easy to move! Measurements from a mercury barometer are usually made in inches of Mercury (in Hg).

          An aneroid barometer can be used in place of a mercury barometer. It is easier to move and is often easier to read. This instrument contains sealed wafers that shrink or spread out depending on changes of atmospheric pressure. If atmospheric pressure is higher, the wafers will be squished together. If atmospheric pressure lessens, it allows the wafers to grow bigger. The changes in the wafers move a mechanical arm that shows higher or lower air pressure (see image).

          Either a mercury barometer or an aneroid barometer can be set up to make constant measurements of atmospheric pressure. Then it is called a barograph (see image). The barograph may constantly record pressure on paper or foil wrapped around a drum that makes one turn per day, per week, or per month. Nowadays, many mechanical weather instruments have been replaced by electronic instruments that record atmospheric pressure onto a computer.

          Atmospheric pressure can be recorded and reported in many different units. This can get a little confusing! As mentioned, a mercury barometer makes measurements in inches of Mercury (in Hg). Pounds per square inch (abbreviated as p.s.i.) is common in the English system of units, and the pascal (abbreviated Pa) is the standard in the Metric (SI) system. Since the pressure exerted by Earth’s atmosphere is of great importance, pressure is sometimes expressed in terms of “atmospheres” (abbreviated atm). In weather, the bar and millibar (mb) describe pressure. You’ll often hear millibar used by meteorologists when describing low or high pressure weather systems.

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HOW DOES A HIGH-PRESSURE AREA FORM?

 

 

          An area of high pressure is created where the air is cold. The cold air sinks, pushing down and creating high pressure. This causes the air molecules to be squashed together, creating heat. As the air warms up, it tends to bring warm and pleasant weather.

Since surface air pressure is a measure of the weight of the atmosphere above any location, a high pressure area represents a region where there is somewhat more atmosphere overlying it.

High pressure areas are usually caused by air masses being cooled, either from below (for instance, the subtropical high pressure zones that form over relatively cool ocean waters to the west of Califormia, Africa, and South America), or from above as infrared cooling of winter air masses over land exceeds the warming of those airmasses by sunlight.

As the airmass cools, it shrinks, allowing air from the surroundings to fill in above it, thus increasing thte total mass of atmosphere above the surface, which then results in higher surface barometric pressures.

The pressure difference between the high pressure area and its lower-pressure surroundings causes a wind to develop flowing from higher to lower pressure. But because of the rotation of the Earth, the wind is deflected to the right (in the Northern Hemisphere) which then causes the wind to flow in a clockwise direction around the high pressure zone.

 

          In an anticyclone, air masses drop extensively. At the same time, the air warms itself up, so that no condensation and consequently no cloud formation can take place. Near to the ground, the air flows out of the anticyclone in the direction of depression – it diverges. Hence, there is no formation of fronts in altitude. During the subsidence of the air masses, an inversion forms. That is where the clouds are dissolved.
An anticyclone is builded quiet slowly. The forces of circulation in the subtropic areas lead to stable anticyclones.

          Because of the differences in the origin or development, the anticyclones are divided into three categories:

          A cold anticyclone originates if air cools off, for example, in winter above a cool land mass (e.g., Central Asian high). Then the air has a bigger density and exerts a higher pressure on the base. In the middle latitudes, it can also originate in the form of flat wedges in the back of cyclones as a ridge of high pressure.

          A dynamic anticyclone is generated by the Rossby-waves (Polar front, Jet Stream). The dynamic Azores anticyclone exerts, on this occasion, a big influence on the weather of Central Europe.

         A high anticyclone is an anticyclone which appears at big heights and is thus shown in high weather maps. It is always connected with a ground low-pressure area, because with the warming of the surfaces, the vertical pressure gradient is lowered and reflects itself the relative atmospheric pressure reduction on the ground with increasing height in a pressure relatively higher to the horizontal surroundings. Hence, one can derive the other way around a height low-pressure area also from a ground anticyclone (also thermal anticyclone).

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HOW DOES A LOW-PRESSURE AREA FORM?

 

 

 

         AN AREA of warm air can create low pressure because warm air rises, reducing the level of air pressure. If the warm air evaporates water on the surface, clouds may form, producing the rain and bad weather associated with low pressure.

Since surface air pressure is a measure of the weight of the atmosphere above any location, a low pressure area represents a region where there is somewhat less atmosphere overlying it.

Low pressure areas form when atmospheric circulations of air up and down remove a small amount of atmosphere from a region. This usually happens along the boundary between warm and cold air masses by air flows “trying” to reduce that temperature contrast. The air flows that develop around the low pressure system then help to accomplish that reduction of contrast in temperature, with the colder air flowing under the warmer air mass, and the warmer air flowing over the colder air mass.

“Thermal lows” occur when an air mass warms, either from being over a warm land or ocean surface. For instance, a very weak thermal low forms over islands heated by the sun, which then causes a sea breeze to form with oceanic air flowing toward the island. Similarly, very cold winter air flowing over the Great Lakes produces localized low pressure over the relatively warmer lake waters.

Low pressure can be enhanced by the air column over it being warmed by condensation of water vapor in large rain or snow systems. The warming causes the air layer to expand upward and outward, removing some of the air from the column, and thus reducing the surface air pressure. The most extreme example of this is the intense low pressure that forms in the eye of a hurricane, where latent heat release from rain formation causes warming of the air column within the eye. It the most intense hurricanes and typhoons, over 10% of the atmosphere can be removed from the eye of the storm through this process.
But outside of the tropics, as mentioned above, low pressure centers are usually associated with extratropical cyclone systems, along with their fronts and precipitation systems.

 

Interesting facts:

The lowest air pressure in the world occurs in intense tropical cyclones where condensation of water vapor to form clouds and rain releases heat that warms the air column in the eye of the storm. The lowest pressure ever recorded was in the eye of Typhoon Tip, in the tropical western Pacific Ocean, on October 12, 1979: 25.69 inches of mercury (870 millibars). Since average sea level pressure is 29.92 inches (1013.23 millibars), this record pressure was about 14% lower than normal, indicating that 14% of the atmosphere’s mass had been removed from the column of air.

 

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WHAT CAUSES AIR PRESSURE?

          Air pressure is created by the effect of gravity pulling the atmosphere towards the Earth. It can vary according to temperature, causing different amounts of pressure in different parts of the world. It also changes according to altitude —pressure is greater at sea level because there is more air pushing down than there is at higher altitudes.

          The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

          Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

          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). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.

          As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

          Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness.

          Decompression sickness, also called “the bends”, is also a problem for scuba divers who come to the surface too quickly.

          Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.

          Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.

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WHAT IS ABOVE THE TROPOSPHERE?

          The layer directly above the troposphere is called the stratosphere. The stratosphere is warmer than the upper part of the troposphere and this warm, relatively heavy air acts like a lid, trapping clouds in the troposphere. Going up through the layers, the air gets thinner and thinner — only in the lower parts of the troposphere is there enough air to breathe normally.

          The stratosphere is a layer of Earth’s atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.

          The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause.

          Ozone, an unusual type of oxygen molecule that is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun. Temperatures rise as one moves upward through the stratosphere. This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.

          The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds (PSCs) are the exception. PSCs appear in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. They appear to help cause the formation of the infamous holes in the ozone layer by “encouraging” certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds.

          Air is roughly a thousand times thinner at the top of the stratosphere than it is at sea level. Because of this, jet aircraft and weather balloons reach their maximum operational altitudes within the stratosphere.

          Due to the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long times. Such is the case for the ozone-destroying chemicals called CFCs (chlorofluorocarbons). Large volcanic eruptions and major meteorite impacts can fling aerosol particles up into the stratosphere where they may linger for months or years, sometimes altering Earth’s global climate. Rocket launches inject exhaust gases into the stratosphere, producing uncertain consequences.

          Various types of waves and tides in the atmosphere influence the stratosphere. Some of these waves and tides carry energy from the troposphere upward into the stratosphere; others convey energy from the stratosphere up into the mesosphere. The waves and tides influence the flows of air in the stratosphere and can also cause regional heating of this layer of the atmosphere.

          A rare type of electrical discharge, somewhat akin to lightning, occurs in the stratosphere. These “blue jets” appear above thunderstorms, and extend from the bottom of the stratosphere up to altitudes of 40 or 50 km (25 to 31 miles).

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WHAT HAPPENS IN THE TROPOSPHERE? HOW FAR UP DOES THE TROPOSPHERE REACH?

          The Troposphere is sometimes called the weather layer. Here, the air is constantly moving as it is heated and cooled in a process known as convection. Clouds form as water in the atmosphere evaporates and then condenses. This movement of air, heat and water creates the world’s weather systems.

          The height of the troposphere varies between different areas of the Earth. At the Equator, for example, it stretches to about 20km (12 miles) above the surface. At the poles, the layer reaches a height of about 10km (6 miles).

          The troposphere is the lowest layer of Earth’s atmosphere. The troposphere extends from Earth’s surface up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level. Most of the mass (about 75-80%) of the atmosphere is in the troposphere, and almost all weather occurs within this layer. Air is warmest at the bottom of the troposphere near ground level. As one rises through the troposphere the temperature decreases. Air pressure and the density of the air also decrease with altitude. The layer immediately above the troposphere is called the stratosphere.

         Nearly all of the water vapor and aerosol particles in the atmosphere are in the troposphere. Because of this, most clouds are found in this lowest layer as well.

          The troposphere is heated from below; sunlight warms the ground or ocean, which in turn radiates the heat into the air immediately above it. Temperature drops off at a rate of about 6.5° C per km (about 3.6° F per thousand feet) of increased altitude within the troposphere. This is why mountaintops are much cooler than lower elevations nearby. Since warm ‘parcels’ of air are less dense than colder air, warmer air is buoyant and tends to rise up from Earth’s surface towards the top of the troposphere. If you’ve ever watched cumulonimbus thunderstorm clouds form and grow on a hot summer day, you’ve seen this rising air in action. The air in the troposphere is ‘well mixed’ because it is constantly churning and ‘turning over’ as warm air at the surface rises and colder, denser air at altitude descends to take its place. This is not the case for all layers in the atmosphere. At the top of the troposphere the temperature drops to a chilly -55° C (-64° F)!

          The boundary between the top of the troposphere and the layer above it, the stratosphere, is called the tropopause. The height of the tropopause (and thus the top of the troposphere) varies with latitude, season, and between day and night. The troposphere is thickest in the tropics, where the top of the layer can be as high as about 20 km (12 miles or 65,000 feet) above sea level. At mid-latitudes, the typical height of the tropopause is around 11 km (7 miles or 36,000 feet), while near the poles it can dip down to as low as 7 km (4 miles or 23,000 feet). The jet stream, a fast-moving “river of air” that can zip along at speeds up to 400 km/hr (250 mph), is located just below the tropopause.

         Air gets ‘thinner’ with increasing altitude. That’s why mountain climbers sometimes need bottled oxygen to breathe, and why it is so easy to get ‘winded’ while hiking in high mountains or even visiting someplace at elevation.

          The lowest part of the troposphere, right next to the surface of Earth, is called the “boundary layer”. Differences in the surface texture (mountains, forests, flat water or ice) affect winds in the boundary layer.

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