the atmosphere, pressure and forces
our thanks to http://www.ucar.edu and www.raa.asn.au (Copyright John Brandon)
 

The thin envelope of air that surrounds our planet is a mixture of gases, each with its own physical properties. The mixture is far from evenly divided. Two elements, nitrogen and oxygen, make up 99% of the volume of air. The other 1% is composed of "trace" gases, the most prevalent of which is the inert gaseous element argon. The rest of the trace gases, although present in only minute amounts, are very important to life on earth. Two in particular, carbon dioxide and ozone, can have a large impact on atmospheric processes.

Another gas, water vapour, also exists in small amounts. It varies in concentration from being almost non-existent over desert regions to about 4% over the oceans. Water vapour is important to weather production since it exists in gaseous, liquid, and solid phases and absorbs radiant energy from the earth.

Structure of the Atmosphere

The atmosphere is divided vertically into four layers based on temperature: the troposphere, stratosphere, mesosphere, and thermosphere. Throughout the Cycles unit, we'll focus primarily on the layer in which we live - the troposphere.

Troposphere

The word troposphere comes from tropein, meaning to turn or change. All of the earth's weather occurs in the troposphere.

The troposphere has the following characteristics.

  • It extends from the earth's surface to an average of 12 km (7 miles).

  • The pressure ranges from 1000 to 200 millibars (29.92 in. to 5.92 in.).

  • The temperature generally decreases with increasing height up to the tropopause (top of the troposphere); this is near 200 millibars or 36,000 ft.

    • The temperature averages 15C (59F) near the surface and -57C (-71F) at the tropopause.

    • The layer ends at the point where temperature no longer varies with height. This area, known as the tropopause, marks the transition to the stratosphere.

  • Winds increase with height up to the jet stream.

  • The moisture concentration decreases with height up to the tropopause.

    • The air is much drier above the tropopause, in the stratosphere.

    • The sun's heat that warms the earth's surface is transported upwards largely by convection and is mixed by updrafts and downdrafts.

  • The troposphere is 70% and 21% . The lower density of molecules higher up would not give us enough to survive.

Atmospheric Processes

Interactions - Atmosphere and Ocean

In the Cycles overview, we learned that water is an essential part of the earth's system. The oceans cover nearly three-quarters of the earth's surface and play an important role in exchanging and transporting heat and moisture in the atmosphere.

  • Most of the water vapour in the atmosphere comes from the oceans.

     
  • Most of the precipitation falling over land finds its way back to oceans.

     
  • About two-thirds returns to the atmosphere via the water cycle.

You may have figured out by now that the oceans and atmosphere interact extensively. Oceans not only act as an abundant moisture source for the atmosphere but also as a heat source and sink (storage).

The exchange of heat and moisture has profound effects on atmospheric processes near and over the oceans. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena. They also warm the climate of nearby locations. Conversely, cold southward flowing currents, such as the California current, cool the climate of nearby locations.

Energy Heat Transfer

Practically all of the energy that reaches the earth comes from the sun. Intercepted first by the atmosphere, a small part is directly absorbed, particularly by certain gases such as ozone and water vapor. Some energy is also reflected back to space by clouds and the earth's surface.


Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation.

Conduction is the process by which heat energy is transmitted through contact with neighbouring molecules.

Some solids, such as metals, are good conductors of heat while others, such as wood, are poor conductors. Air and water are relatively poor conductors.

Since air is a poor conductor, most energy transfer by conduction occurs right at the earth's surface. At night, the ground cools and the cold ground conducts heat away from the adjacent air. During the day, solar radiation heats the ground, which heats the air next to it by conduction.

Convection transmits heat by transporting groups of molecules from place to place within a substance. Convection occurs in fluids such as water and air, which move freely.

In the atmosphere, convection includes large- and small-scale rising and sinking of air masses and smaller air parcels. These vertical motions effectively distribute heat and moisture throughout the atmospheric column and contribute to cloud and storm development (where rising motion occurs) and dissipation (where sinking motion occurs).

To understand the convection cells that distribute heat over the whole earth, let's consider a simplified, smooth earth with no land/sea interactions and a slow rotation. Under these conditions, the equator is warmed by the sun more than the poles. The warm, light air at the equator rises and spreads northward and southward, and the cool dense air at the poles sinks and spreads toward the equator. As a result, two convection cells are formed.

Meanwhile, the slow rotation of the earth toward the east causes the air to be deflected toward the right in the northern hemisphere and toward the left in the southern hemisphere. This deflection of the wind by the earth's rotation is known as the Coriolis effect.

Radiation is the transfer of heat energy without the involvement of a physical substance in the transmission. Radiation can transmit heat through a vacuum.

Energy travels from the sun to the earth by means of electromagnetic waves. The shorter the wavelength, the higher the energy associated with it. This is demonstrated in the animation below. As the drill's revolutions per minute (RPMs) increase, the number of waves generated on the string increases, as does the oscillation rate. The same principle applies to electromagnetic waves from the sun, where shorter wavelength radiation has higher energy than longer wavelength radiation.


Most of the sun's radiant energy is concentrated in the visible and near-visible portions of the spectrum. Shorter-than-visible wavelengths account for a small percentage of the total but are extremely important because they have much higher energy. These are known as ultraviolet wavelengths.

Atmospheric oxygen

In the homosphere each gas exerts a partial pressure, the product of the total atmospheric pressure and the concentration of the gas. Thus as oxygen represents about 21% of the composite gases, the partial pressure of oxygen is about 21% of the atmospheric pressure at any altitude within the homosphere.

Interpolating from the pressure gradient graph above, oxygen partial pressure at selected altitudes is shown below. The decreasing partial pressure of oxygen as an aircraft climbs past 10 000 12 000 feet has critical effects on aircrew; the maximum exposure time for a fit person, without inspiring supplemental oxygen, is shown in the right hand column. Exposure beyond these times leads to unconsciousness.

Altitude O pressure Max. exposure
Sea level 210 hPa
7000 feet 165 hPa
10 000 feet 150 hPa
15 000 feet 120 hPa 30+ minutes
18 000 feet 105 hPa 2030 minutes
25 000 feet 80 hPa 35 minutes
30 000 feet 65 hPa 13 minutes
35 000 feet 50 hPa 3060 seconds
40 000 feet 30 hPa 1020 seconds

Atmospheric density

The average density of dry air in temperate climates is about 1.225 kg/m at mean sea level, decreasing with altitude.

There are several gas laws and equations which relate the temperature, pressure, density and volume of a gas. However the equation most pertinent to aeronautical needs is the equation of state:

   r = P/RT    where:
  r (the Greek letter rho) = density in kg/m
  P = the static air pressure in hectopascals
  R = the gas constant = 2.87
  T = the temperature in Kelvin units = C + 273

We can calculate the ISA standard sea level air density, knowing that standard sea level pressure = 1013 hPa and temperature = 15 C or 288 K

  i.e. Air density = 1013 / (2.87 288) = 1.225 kg/m

However if the air temperature happened to be 30 C or 303 K at the same pressure then density would = 1013 / (2.87 303) = 1.165 kg/m or a 5% reduction.


By restating the equation of state: P = RrT it can be seen that if density remains constant, pressure increases if temperature increases.

The ICAO International Standard Atmosphere

   The International Civil Aviation Organisation's International Standard Atmosphere [ ISA ] provides a fixed standard atmospheric model used for many purposes among which are the uniform assessment of aircraft performance and the calibration of some aircraft instruments. The model is akin to the average condition in mid-latitudes but contains the following assumptions:

  • dry air is assumed throughout the atmosphere
  • the mean sea level pressure = 1013.25 hPa
  • the msl temperature = 15 C [288 K]
  • the tropopause is at 36 090 feet [11 km] and the pressure at the tropopause = 226.3 hPa
  • the temperature lapse rate to 36 090 feet = 6.5 C per km or nearly 2 C per 1000 feet
  • the temperature between 36 090 and 65 600 feet [20 km] remains constant at 56.5 C.

The table below shows a few values derived from the ISA. Those pressure levels noted with a flight level designator are standard pressure levels used for aviation weather purposes, particularly thickness charts.

Pressure Flight level Temperature Air density Altitude
hPa   C kg/m feet
1013   15 1.225 msl
1000   14.3 1.212 364
950   11.5 1.163 1773
900   8.6 1.113 3243
850 A050 5.5 1.063 4781
800   2.3 1.012 6394
750   -1.0 0.960 8091
700 A100 -4.6 0.908 9882
650   -8.3 0.855 11 780
600 FL140 -12.3 0.802 13 801
550   -16.6 0.747 15 962
500 FL185 -21.2 0.692 18 289
450   -26.2 0.635 20 812
400 FL235 -31.7 0.577 23 574
350   -37.7 0.518 26 631
300 FL300 -44.5 0.457 30 065
250 FL340 -52.3 0.395 33 999
200 FL385 -56.5 0.322 38 662
150 FL445 -56.5 0.241 44 647
100   -56.5 0.161 53 083


Not immediately apparent from the ISA table is that the pressure lapse rate is about one hPa per 30 feet up to the 850 hPa level, then slowing to 40 feet per hPa at the 650 hPa level, 50 feet at the 450 hPa level, 75 feet at the 300 hPa level and so on, however, this provides a useful rule of thumb:

Rule of Thumb #1

    "An altitude change of 30 feet per hPa can be assumed for operations below 10 000 feet."

station pressure, sea level pressure and altimeter setting

Station pressure is the actual atmospheric pressure at the elevation of the observing station.

QFE: The pressure corrected to the official airfield elevation. An altimeter set to the particular airfield QFE reads zero when an aircraft is on the ground (strictly the height of the altimeter above the ground). In the circuit, the height indicated is the height above official airfield datum.

QNH: The pressure 'reduced' to mean sea level, assuming ISA temperature profile from the station/airfield to MSL. An altimeter set to the airfield QNH reads the elevation of the airfield when on the ground.

pressure systems

The pressure chart shows the distribution of atmospheric pressure. Pressure systems - depressions (LOW pressure regions) and anticyclones (HIGH pressure) are marked and Isobars are drawn on the chart to link areas with the same pressure. Isobar lines are drawn at 4mB interval (4 HPa) and weather frontal systems are marked using standard symbols.


Cold front

Warm front

Occluded front

Wind direction and some indication of strength can be deduced from the pressure chart. In the Northern Hemisphere, winds blow in an anti-clockwise direction around a depression (LOW) and in a clockwise direction around an anticyclone (HIGH). The closer the isobars are together, then the greater the pressure gradient and the higher will be the wind strength.

Pressure charts are a useful help in interpreting satellite images. The satellite image shows the pattern of cloud cover and with the help of the pressure chart, frontal systems can be identified and tracked over a period of time. Typically, rain will be associated with the passage of a front - identifying and tracking the fronts can allow the forecast of rain, changes of temperature, wind direction and speed etc.

low pressure areas also known as cyclones
ww2010.atmos.uiuc.edu

A low pressure centre is where the pressure has been measured to be the lowest relative to its surroundings. That means, moving in any horizontal direction away from the "Low" will result in an increase in pressure. Low pressure centres also represent the centres of cyclones.

A low pressure centre is indicated on a weather map by a red "L" and winds flow counter clockwise around a low in the northern hemisphere. The opposite is true in the southern hemisphere, where winds flow clockwise around an area of low pressure.

secondary low

The Secondary Low Centre within an occluded front is accompanied by rain, showers and possibly thunder.
 
Parameter Description
Precipitation Moderate to heavy precipitation and showers, some with thunder around the secondary low
Temperature  
Wind (incl. gusts) Cyclonically veering winds around the low
Other relevant information  

06 March 2002/06.00 UTC - Meteosat IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, black: no precipitation); position of Secondary Low indicated

  

through of low pressure

A trough of low pressure is an extended area of low pressure. On the weather chart, it has associated with it a trough line. The pressure there is lower than that at neighbouring points on either side of the trough line.

col

A col is a neutral region between two highs and two lows.  Weather conditions are apt to be unsettled.  In winter, the mixing of air of dissimilar air masses frequently produces fog.  In summer, showers or thunderstorms may occur.  While it is quite possible for weather conditions to be fair, generally speaking, cols may be regarded as regions of undependable weather.

High Pressure Centres also known as anticyclones

A high pressure centre is where the pressure has been measured to be the highest relative to its surroundings. That means, moving in any direction away from the "High" will result in a decrease in pressure. A high pressure centre also represents the centre of an anticyclone and is indicated on a weather map by a blue "H".

ridge of high pressure

An anti-cyclone ridge is a neck or ridge of high pressure with lower pressure lying on either side.  The weather in a ridge is generally fine to fair.

pressure changes

Pressure readings are taken at regular intervals (usually hourly) at weather stations.  Weather maps are prepared four times a day at six-hour intervals.  From these readings and maps, the changes in pressure can be observed and approaching weather forecast.  If a low, for example, is approaching a station, the pressure will steadily fall.  Once the centre of the low has passed by, the pressure will begin to rise.  This pattern of changing pressure is called pressure tendency.

forces encountered

coriolis  force

Since the globe is rotating, any movement on the Northern hemisphere is diverted to the right, if we look at it from our own position on the ground. (In the southern hemisphere it is bent to the left). This apparent bending force is known as the Coriolis force. (Named after the French mathematician Gustave Gaspard Coriolis 1792-1843). It may not be obvious to you that a particle moving on the northern hemisphere will be bending towards the right.

Consider this red cone moving southward in the direction of the tip of the cone. The earth is spinning, while we watch the spectacle from a camera fixed in outer space. The cone is moving straight towards the south.

Note that the red cone is veering in a curve towards the right as it moves. The reason why it is not following the direction in which the cone is pointing is, of course, that we as observers are rotating along with the globe.

Here we show the same image with the camera locked on to the globe.

Here we show the same image,with the camera fixed in outer space, while the earth rotates.

The Coriolis force is a visible phenomenon. Railroad tracks wear out faster on one side than the other. River beds are dug deeper on one side than the other. (Which side depends on which hemisphere we are in: In the Northern hemisphere moving particles are bent towards the right).
In the Northern hemisphere the wind tends to rotate counterclockwise (as seen from above) as it approaches a low pressure area. In the Southern hemisphere the wind rotates clockwise around low pressure areas.