Aviation professionals such as pilots and air traffic controllers are required to pass examinations covering the theory of several subjects including meteorology. Such knowledge is crucial and is applied daily during actual operations.1
Weather, consciously or unconsciously, affects the daily choices we all make. Do we dress to stay warm or cool? Do we stay indoors? Our health, food production, general well-being and ability to travel safely whether by land, sea or air are all affected.
The actual weather we experience is primarily the product of the Earth’s life-sustaining atmosphere and of heating from the Sun. The weather is perpetually changing, shaped by air currents within the atmosphere which is kept in place by its own weight under gravity.
Atmosphere
The atmosphere is essentially a mixture of gases extending upwards from the Earth’s surface for some 6,000 miles.2 It blends gradually into space. The bulk of the atmosphere is close to the Earth’s surface - within about 10 miles - and it is in that band where most aviation takes place.
The EarthHow website notes the composition of the atmosphere: nitrogen (c.78 %) and oxygen (c.21 %) and other gases including argon and carbon dioxide. Any given volume of air is capable of holding water vapour.
Even though the percentage taken up by carbon dioxide is small (<1 %) it is vitally important because, by absorbing infra red radiation, it helps earth to retain heat.3
Image: Composition of atmosphere: EarthHow
Layers
Five atmospheric “layers” are recognised - Atmosphere (nationalgeographic.org).
The Troposphere extending from the surface to around 10 miles at the equator and 7 miles at the Poles. Most of the water vapour in the atmosphere, along with dust and ash particles, are found in the troposphere—explaining why most of Earth’s clouds are located in this layer. Temperatures in the troposphere decrease with altitude. The rate of temperature change with altitude is known as the lapse rate.
Immediately above the troposphere is the Stratosphere extending upwards to about 30 miles. Temperatures in the stratosphere increase with altitude. A high concentration of ozone (O3) is found at this level. Ozone absorbs some of the incoming solar radiation, shielding life on Earth from potentially harmful ultraviolet (UV) light, and is responsible for the temperature increase in altitude. The top of the stratosphere is known as the stratopause.
Above the stratosphere is the Mesosphere extending to over 50 miles above the earth surface. Temperatures decrease in the mesosphere with altitude - e.g. to about -90°C (-130°F). The upper boundary of the mesosphere is called the mesopause.
The fourth layer is the thermosphere extending upwards to around 400 miles. Temperatures increase with altitude and solar radiation makes the upper regions of the thermosphere very hot, reaching temperatures as high as 2,000°C (3,600°F).
The uppermost layer, that blends with what is considered to be outer space, is the exosphere.
Global circulation
Incoming heat from the Sun is known as INSOLATION and it causes the air to be in continual circulation. As the earth rotates, the surface facing the Sun receives heat. The fact that the earth is tilted4 by 23.5o as it orbits the Sun also plays a part.
A considerable temperature difference arises between the two poles and the equator but air circulation prevents the equator become hotter and hotter and the poles becoming increasingly cold.
Three circulation “cells” are recognised: the HADLEY5 cell is from the equator to about latitude 30o/40o (North or South), the FERREL6 cell is from there to about 60o/70o (North or South), and the POLAR cells are from there to the poles.
Image: Air circulation cells. ITCZ (Inter-tropical convergence zone), STJ (sub-tropical jets),PFJ - Polar front jets.
The Global Atmospheric Circulation - Royal Meteorological Society
Global circulation patterns - Met Office
Global atmospheric circulation - Atmosphere and climate - BBC Bitesize
The cells have a major impact on regional climates - Earth's Atmospheric Circulation System - Geosciences.
Atmospheric pressure
In the International System of Units7, the base units of time, length and mass are, respectively, the second, metre, kilogram.
Force is an influence causing an object to change its velocity - that is, to change either its speed, direction of movement or both. Forces can be applied by contact (e.g. a push or a pull, air resistance etc) or by action at a distance (e.g. gravity, magnetism etc).
Force is measured in NEWTONS8 (N) where 1 N is the force required to accelerate 1 Kilogram (Kg) at a rate of 1 metre per second per second.
Pressure is force per unit of area. This is measured in PASCALS9 (Pa). A Pascal is 1 Newton per square metre.
Air pressure is the FORCE exerted on a surface by the air above it and it is measured in HECTOPASCALS (hPa) where 1 hPa = 100 Pa. The hectopascal is equal to the millibar which was for many years used in meteorology and aviation.
Pressure gradient expresses the change in air pressure from one place to another. Air will flow from high pressure toward low. The greater the difference in pressure, the greater the pressure gradient and the stronger the resultant wind.
Standard atmosphere
The International Organization for Standardisation (ISO) was founded on 23 February 1947 and (as of January 2024) it has published over 25,000 international standards covering almost all aspects of technology and manufacturing. One such standard is an International Standard Atmosphere (ISA) which is a model of the Earth’s atmosphere used as a reference for aeronautical and aerospace purposes.10
The International Civil Aviation Organization (ICAO) publishes their "ICAO Standard Atmosphere" as ICAO Doc 7488.11
The basics are - Temperature at mean sea level +15C (288.15K) and air pressure 1013.25 hPa. Temperature falls at a lapse rate of 1.98C per 1000 feet up to 36,090 feet (11km). Above that the temperature is assumed to be constant at -56.5C (216.65K). That applies up to 65,000 (20K).12
Image: International Standard Atmosphere
Charts
ISOBARS are lines drawn on a chart joining places of equal atmospheric pressure. By convention, isobars are shown at intervals of 4hPa. Here is a chart for 16 May 2024 showing isobars (thin black lines).
The chart shows low pressure (999) to the west of the Iberian peninsula. Air pressure over Germany is shown as 1005 and to the north of Scotland 1007. Air circulation is anti-clockwise around low pressure and so the result was a a gentle easterly wind over Denmark flowing toward Scotland.
The isobars are well spaced on this chart. The closer the isobars, the greater the pressure gradient and the stronger the wind speed.
Here is a chart from October 2023 when Storm Babet hit the UK
Buys Ballot
Christophorus Henricus Diedericus Buys Ballot13 was a Dutch chemist and meteorologist whose name is given to the principle (“law”) that, in the Northern Hemisphere, if a person stands with their back to the wind, the atmospheric pressure is low to the left and high to the right. This is because wind flows counter-clockwise around low pressure zones in the Northern Hemisphere. In the southern hemisphere the reverse applies.14
The Bus Ballot law was useful to masters of both sail and steam vessels, assisting them in steering their vessels away from the centre of hurricanes and other severe disturbances at sea. Prior to radio, satellite observation and the ability to transmit timely weather information over long distances, the only method a ship's master had to forecast the weather was observation of meteorological conditions (visible cloud formations, wind direction and atmospheric pressure) at his location.
In modern times, we are familiar with satellite images of hurricanes with the wind rotating about a centre - (see image at the top of this Viewpoint).
The movement of air around a low pressure system is brought about by the CORIOLIS15 effect but that and other material is in Weather (2).
Links of interest
Earth's Atmosphere: A Multi-layered Cake - NASA Science
Atmosphere (nationalgeographic.org)
Layers of Earth's Atmosphere - (ucar.edu)
MetLink - Royal Meteorological Society In Depth - The Global Atmospheric Circulation
Intertropical Convergence Zone (ITCZ) - Met Office
World Meteorological Organization (wmo.int)
Global Observing System (GOS) | World Meteorological Organization (wmo.int)
Convert Kelvin to Celsius (unitconverters.net)
Convert Pressure, Hectopascal (convertworld.com)
Theoretical knowledge examinations | Civil Aviation Authority (caa.co.uk)
There appears to be no general consensus on an upper limit.
If CO2 Is Only 0.04% of the Atmosphere, How Does it Drive Global Warming? – State of the Planet (columbia.edu)
Earth's spin, tilt and orbit - Understanding Global Change (berkeley.edu)
George Hadley (1685 - 1768) - Wikipedia
William Ferrel (1817 - 1891) - Wikipedia
The International System of Units (SI), 2019 Edition (nist.gov). Units of measurement is a complicated topic.
Newton (unit) - Wikipedia
Pascal (unit) - Wikipedia
Details are set out in ISO 2533:1975 - Standard Atmosphere and see ISO 2533:1975 Standard Atmosphere | Pacific Certifications
Manual of the ICAO Standard Atmosphere - extended to 80 kilometres / 262,500 feet (Doc 7488) | ICAO Store
Fuller details at International Standard Atmosphere Theory Guide.pdf
C. H. D. Buys Ballot (1817 - 1890) - Wikipedia
Buys Ballot's law - Wikipedia
Gaspard-Gustave de Coriolis (1792 - 1843) - Wikipedia