AMS Weather Studies Introduction to Atmospheric Science, 4th Edition Chapter 5 Air Pressure

Case-in-Point Mount Everest World’s tallest mountain – 8848 m (29,029 ft) Same latitude as Tampa, FL Due to declining temperature with altitude, the summit is always cold January mean temperature is -36 °C (-33 °F) July mean temperature is -19° C (-2 °F) Shrouded in clouds from June through September Due to monsoon winds November through February – Hurricane-force winds Due to jet stream moving down from the north Harsh conditions make survival at the summit difficult Very thin air Wind-chill factor Most ascents take place in May © AMS

Driving Question What is the significance of horizontal and vertical variations in air pressure? Air pressure is an element of weather we do not physically sense as readily as temperature and humidity changes This chapter examines: The properties of air pressure How air pressure is measured The reasons for spatial and temporal variations in air pressure

Important changes in weather can accompany relatively small variations in air pressure. In this chapter we examine the properties of air pressure, how air pressure is measured, and the reasons for spatial and temporal variations in air pressure.   We also consider how air responds to changes in pressure when moving vertically through the atmosphere. Defining Air Pressure next.

Defining Air Pressure Air exerts a force on the surface of all objects it contacts The air is a gas, so the molecules are in constant motion The air molecules collide with a surface area in contact with air The force of these collisions per unit area is pressure Dalton’s Law – total pressure exerted by mixture of gases is sum of pressures produced by each constituent gas Air pressure depends on: Mass of the molecules and kinetic molecular energy Air pressure can be thought of as the weight of overlying air acting on a unit area Weight is the force of gravity exerted on a mass Weight = (mass) x (acceleration of gravity) Average sea-level air pressure 1.0 kg/cm2 (14.7 lb/in.2) Air pressure acts in all directions That is why structures do not collapse under all the weight Air Pressure within the house next.

• Air Pressure Measurement The barometer is the instrument used to measure air pressure and monitor its change.   Mercurial barometer next.

There are two basic types of barometers There are two basic types of barometers. The first is the mercury barometer. The second type is the aneroid barometer. The mercurial (mercury) barometer is the more accurate of the two but also more cumbersome to work with. Picture of a mercurial barometer next.

The height of the mercury column changes as air pressure changes. The average air pressure at sea level will support the mercury column in the tube to a height of 29.92 inches. The height of the mercury column changes as air pressure changes. Falling pressure will allow the mercury to drop; whereas increasing pressures forces the column of mercury to rise in the tube. Rising and falling of the mercury means something next.

The second type is the aneroid barometer An aneroid barometer is less precise, but more portable than a mercury barometer It has a chamber with a partial vacuum Changes in air pressure collapse or expand the chamber This moves a pointer on a scale calibrated equivalent to mm or in. of mercury New ones are piezoelectric – depend on the effect of air pressure on a crystalline substance

Air Pressure Measurement Forecasting uses air pressure and tendency values changes over time Barometers may keep a record of air pressure These are called barographs provides a continuous trace of air pressure variations with time making it easier to determine pressure tendency. © AMS

Air Pressure Units Units of length Units of pressure Millimeters or inches Inches typical for TV Units of pressure Pascal – worldwide standard Metric scale Sea-level pressure = 101,325 pascals (Pa) 1013.25 hectopascals (hPa) 101.325 kilopascals (kPa) Bars – U.S. A bar is 29.53 inches of mercury A millibar (mb) is the standard used on weather maps, meaning 1/1000 of a bar Usual worldwide range is 970 – 1040 mb Lowest ever recorded - 870 mb (Typhoon Tip in 1979) Highest ever recorded – 1083.8 mb (Agata, Siberia) © AMS

Variations in Air Pressure w/Altitude Overlying air compresses the atmosphere the greatest pressure is at the lowest elevations Gas molecules are closely spaced at the surface Spacing increases with altitude At 18 km (11 mi), air density is only 10% of that at sea level Because air is compressible, the drop in pressure with altitude is greater in the lower troposphere Then it becomes more gradual aloft Vertical profiles of average air pressure and temperature are based on the standard atmosphere – state of atmosphere averaged for all latitudes and seasons Even though density and pressure drop with altitude, it is not possible to pinpoint a specific altitude at which the atmosphere ends ½ the atmosphere’s mass is below 5500 m (18,000 ft) 99% of the mass is below 32 km (20 mi) Denver, CO average air pressure is 83% of Boston, MA © AMS

Average Air Pressure Variation with Altitude Expressed in mb

Air Pressure changes more with elevation than latitude next.

Because air pressure drops with increasing altitude at rates that can be readily determined, an aneroid barometer can be calibrated to monitor altitude. Such an instrument is called an altimeter. Picture of a plane on 870 mb traveling from New Orleans to New York next.

Changes in surface air pressure en route are one cause of differences between indicated and true altitude. Aircraft altimeters are equipped with a moveable scale than enables the pilot to adjust altimeter readings based on reports from weather stations along the route. The pilot is following the 870 mb level from New Orleans to New York. The pilot thinks he is a 1220 m but as he goes north his actual height gets lower so that over New York his actual height is really 1190 m. Note the pilot is following the 870 mb level from New Orleans to New York. The pilot thinks he is a 1220 m but as he goes north his actual height gets lower so that over New York his actual height is really 1190 m. STOP

Horizontal Variations in Air Pressure Horizontal variations are much more important to weather forecasters than vertical differences Local pressures at reporting locations are adjusted to equivalent sea-level values to make up for changes in elevation This shows variations of pressure in the horizontal plane This is mapped by connecting points of equal equivalent sea-level pressure, producing isobars

Horizontal Variations in Air Pressure Horizontal changes in pressure can be accompanied by significant changes in weather In middle latitudes, a continuous procession of different air masses brings changes in pressure and weather Temperature has a much more pronounced affect on air pressure than humidity In general, the weather becomes stormy when air pressure falls but clears or remains fair when air pressure rises Air pressure varies continuously © AMS

Horizontal Variations in Air Pressure Influence of temperature and humidity Rising air temperature = rise in the average kinetic energy of the individual molecules In a closed container, heated air exerts more pressure on the sides Density in a closed container does not change No air has been added or removed The atmosphere is not like a closed container Heating the atmosphere causes the molecules to space themselves farther apart This is due to increased kinetic energy Molecules placed farther apart have a lower mass per unit volume, or density The heated air is less dense, and lighter. © AMS

Horizontal Variations in Air Pressure Influence of temperature and humidity, continued Air pressure drops more rapidly with altitude in a column of cold air Cold air is denser, has less kinetic energy, so the molecules are closer together 500 mb surfaces represent where half of the atmosphere is above and half below by mass This surface is at a lower altitude in cold air vs. in warm air Increasing humidity decreases air density The greater the concentration of water vapor, the less dense is the air due to Avogadro’s Law We often refer to muggy air as heavy air, but the opposite is true Muggy air only weighs heavily on our personal comfort factor

Influence of temperature and humidity, continued Cold, dry air masses are the densest These generally produce higher surface pressures Warm, dry air masses generally exert higher pressure than warm, humid air masses These pressure differences create horizontal pressure gradients Pressure gradients cause cold or warm air advection Air mass modifications can also produce changes in surface pressures Conclusion: local conditions and air mass advection can influence air pressure

Horizontal Variations in Air Pressure Influence of diverging and converging winds Diverging = winds blowing away from a column of air Converging = winds blowing towards a column of air Diverging/converging caused by : Horizontal winds blowing toward or away from some location (this chapter) Wind speed changes in a downstream direction (Chapter 8) © AMS

Influence of Diverging and Converging winds If more air diverges at the surface than converges aloft, the air density and surface air pressure decrease If more air converges aloft than diverges at the surface, density and surface pressure increase

Highs and Lows Isobars are drawn on a map as previously discussed U.S. convention – these are drawn at 4-mb intervals (e.g., 996 mb, 1000 mb, 1004 mb) A High is an area where pressure is relatively high compared to the surrounding air A Low is an area where pressure is relatively low compared to the surrounding air Highs are usually fair weather systems Lows are usually stormy weather systems Rising air is necessary for precipitation formation Lows are rising columns of air. Highs are sinking columns of air. © AMS

Winds are strong where isobars are closely spaced because of greater horizontal pressure differences, but are weak when isobars are widely spaced as the horizontal differences are less.   Isobars generally are more closely spaced near the center of a Low than near the center of a High. The Gas Law next.

• The Gas Law We have discussed variability of temperature, pressure, and density → these properties are known as variables of state; their magnitudes change from one place to another across Earth’s surface, with altitude above Earth’s surface, and with time The three variables of state are related through the ideal gas law, which is a combination of Charles’ law and Boyle’s law The ideal gas law states that pressure exerted by air is directly proportional to the product of its density and temperature, i.e. pressure = (gas constant) x (density) x (temperature) Air density drops as temperature increases next.

Conclusions from the ideal gas law Density of air within a rigid, closed container remains constant. Increasing the temperature leads to increased pressure Within an air parcel, with a fixed number of molecules: Volume can change, mass remains constant Compressing the air increases density because its volume decreases Within the same air parcel: With a constant pressure, a rise in temperature is accompanied by a decrease in density. Expansion due to increased kinetic energy increases volume Hence, at a fixed pressure, temperature is inversely proportional to density

Conservation of energy Expansional cooling – when an air parcel expands, the temperature of the gas drops Compressional warming – when the pressure on an air parcel increases, the parcel is compressed and its temperature rises Conservation of energy Law of energy conservation/1st law of thermodynamics → heat energy gained by an air parcel either increases the parcel’s internal energy or is used to do work on the parcel A change in internal energy is directly proportional to a change in temperature More on expansional cooling and compressional cooling next.

Conservation of Energy The law of energy conservation states that energy is neither created nor destroyed but can change from one form to another. When the air is compressed, work is done on it but when air expands, the air does work on its surroundings. Energy is either supplied (during compression) or released (during expansion). Adiabatic process next.

Conservation of Energy If the air is compressed, energy is used to do work on the air If we allow the air to expand, the air does work on the surroundings

Adiabatic Processes During an adiabatic process, no heat is exchanged between an air parcel and its surroundings The temperature of an ascending or descending unsaturated parcel changes in response to expansion or compression only Dry adiabatic lapse rate = 9.8 C°/1000 m (5.5 °F/1000 ft) Once a rising parcel becomes saturated, latent heat released to the environment during condensation or deposition partially counters expansional cooling Moist adiabatic lapse rate = 6 C°/1000 m (3.3 °F/1000 ft) → this is an average rate © AMS

Dry adiabatic lapse rate describes the expansional cooling of ascending unsaturated air parcels

Should a rising air parcel cool to the point that the relative humidity nears 100% (saturation) and condensation or deposition (vapor to solid) takes place, the ascending air parcel no longer cools at the dry adiabatic rate. It now cools more slowly at the moist adiabatic lapse rate of 60C per 1000 m. Picture of dry and adiabatic lapse rate next.

More on moist adiabatic lapse rate next.

Warm saturated air has more water vapor than cool saturated air Warm saturated air has more water vapor than cool saturated air. For the same temperature change, the greater condensation or deposition in warm saturated air releases more latent heat than condensation or deposition in cool saturated air.   The moist adiabatic lapse rate ranges from about 4 Celsius degrees per 1000 m for very warm saturated air to very near 9.8 Celsius degrees per 1000 m for very cool saturated air. For convenience, we use an average value of 6 Celsius degrees per 1000 m for the moist adiabatic lapse rate. Conclusions next.

Atmospheric Stability To view this animation, click “View” and then “Slide Show” on the top navigation bar.

• Conclusions Air pressure drops rapidly with altitude in the lower atmosphere and then more gradually aloft.   Adjusting a station’s barometer readings to sea level eliminates the influence of weather station elevation on air pressure. More conclusions next.

Surface air pressure depends on air density, which in turn, is governed by air temperature and, to a lesser extent, by the concentration of water vapor in air.   Diverging or converging winds may also affect air density and surface air pressure. Changes in air temperature trigger the phase changes of water that cause clouds to form or to dissipate. Basic Understandings next.

• Basic Understandings Air pressure is the weight of a column of air acting on a unit area of Earth’s surface. The pressure at any specified altitude is equal to the weight per unit area of the air column above that altitude.   A barometer is a weather instrument that monitors changes in air pressure. Air pressure and air density decrease rapidly with increasing altitude in the lower atmosphere and then more gradually aloft. More Basic Understandings next.

About 50% of the mass of the atmosphere occurs below and altitude of 5 About 50% of the mass of the atmosphere occurs below and altitude of 5.5 km, and 99% is below an altitude of 32 km.   Cold air masses are denser and exert higher pressure at Earth’s surface than do warm air masses. As a rule, temperature has a much more significant influence on air density and air pressure than does humidity. Air pressure may fluctuate in response to divergence and convergence of air, which is produced by changes in wind speed and direction. More Basic Understandings next.

Important changes in the weather often accompany relatively small changes in air pressure at Earth’s surface. High or rising pressure signals clearing or continued fair weather, whereas low or falling pressure means stormy weather.   During an adiabatic process, no heat is exchanged between an air parcel and its environment. Last slide next.

AIR ASCENDS, EXPANDS AND COOLS. AIR DECENDS, COMPRESSES AND WARMS LAST