Temperature Inversions

Why is the Earth’s Surface Warm? Some of the sun’s energy is converted to heat as it penetrates the earth’s atmosphere The radiant heat of the sun also warms the surface of the earth NASA

Heating of Air The warmed surface of the earth heats the air just above it in the troposphere by convection Troposphere = the lower atmosphere that we live in Some of the sun’s energy also heats the air through conduction and the rising of warmed air (convection)

Atmospheric Layers and Temperature As it rises, this warmed air in the troposphere expands and cools Density of the air decreases with the expansion of air, making it ‘lighter’ in weight m

Air Temperature in the Lower Atmosphere Temperature at earth’s surface= about 62°F/17°C Temperature at about 12 miles altitude = about 60°F/-51°C Height of troposphere is approx. 12 mi. at equator Air temperature in the troposphere therefore gets cooler as altitude increases

Air Movement As the warm air rises, cooler air (more dense, therefore heavier), rushes in to take its place This ‘vertical mixing’ is what causes wind Thermal column. The cloud (A) is above the ground. The sun increases the temperature of the ground which will then warm the air above it (1). The bubble of hot air starts to rise (2) and begins to cool as it expands and moves upward into cooler air. Due to its lower temperature, the air becomes cold enough to contract and then move downward (3) to repeat the cycle. Wikimedia Commons

What is a Temperature Inversion? A ‘Temperature Inversion’ occurs when the normal profile of warm at the surface and cooler as altitude increases is ‘inverted’ or turned upside down Cool air will be found at lower altitudes, with warmer air above during an inversion Height (y-axis) versus Temperature (x- axis) under normal atmospheric conditions (black line). The path D-C illustrates an inversion aloft. Wikimedia Commons

Temperature Inversions and Pollution The normal movement and mixing of cold and warm air in our atmosphere creates turbulence, which helps to lift and disperse pollutants Temperature inversions, however, keep cool, dense air near the surface, reducing the vertical movement Pollutants stay near the surface Pollutants become concentrated as more are added by everyday activities Pollutant concentrations may become hazardous

Favorable Conditions to Form a Temperature Inversion Calm winds ➯ reduce vertical mixing Clear skies ➯ increase rate of surface cooling Less ‘greenhouse effect’ Rising smoke in Lochcarron, Scotland forms a ceiling over the valley due to a temperature inversion. The picture was taken on an afternoon in January after a cold night. Wikimedia Commons

Other Favorable Conditions for Forming a Temperature Inversion Long nights ➯ greater duration of cooling Inversions more common in winter due to longer nights Inversions are stronger in winter because of the greater difference in temperature between the cool surface air and the warmer air aloft A very low angle of the sun in the sky (as in winter) ➯ less daytime heating of air close to the ground and surface Wikimedia Commons

Types of Temperature Inversions Radiation Temperature Inversion Subsidence Temperature Inversion Shanghai Wikimedia Commons

What Causes a Radiation Temperature Inversion? Mechanism 1. Ground loses heat quickly after sunset 2. Air in contact with ground then cools quickly 3. Air above surface stays warm since air is a poor conductor of heat Cooling of the air at night near the ground

Radiation Temperature Inversions Morning warming by the sun will often reverse a temperature inversion by afternoon Pollutants built up during the night also disperse Wikimedia Commons

Radiation Temperature Inversions and Topography Cooler, denser air cannot rise up over mountains to laterally disperse pollutants Mountains also block the low angle of the winter sun, reducing warming of the air Cloudy weather will block the sun, further decreasing warming Mexico City Wikimedia Commons Areas in a mountain basin may experience prolonged radiation temperature inversions

What Causes a Subsidence Temperature Inversion? A high altitude mass of warm air moves into an area over cooler air near the surface High pressure pushes the warm air mass down, effectively capping the cooler air below Fog over San Francisco Wikimedia Commons Fog may be seen under the inversion cap Subsidence temperature inversions are usually short- lived

Subsidence Temperature Inversions and Topography Areas with mountains on three sides and the ocean on the fourth may experience frequent and prolonged subsidence temperature inversions Mechanism 1. Ocean breezes cool the surface air 2. Mountains surrounding prevent the cooler, heavier air from dispersing over the mountains and removing pollutants

Subsidence Temperature Inversions and Smog These conditions are especially problematic in areas like Los Angeles, CA Warm, sunny climate and high level of pollutants (vehicle, shipping, port) causes a photochemical smog Smog persists due to frequent subsidence temperature inversions for at least half the year (summer and fall) Low rainfall (15” per year) limits clearance of smog David Iliff,

The Great London Smog of 1952 The Great Smog (AKA “Big Smoke”) affected London in days in December Temperature inversion that caused the worst air pollution event in UK history Nelson’s Column, London, during the Great Smog, December 1952 Wikimedia Commons

The Great Smog of Meteorological Causes Weather had been very cold, so more low-grade, high sulfur coal (the least expensive) was being burned than usual This increased sulfur dioxide in the air significantly There was very little wind, thus little dispersal of pollutants Cold, stagnant air was trapped under a layer of warmer air above A high pressure area settled over London, decreasing air movement further (a subsidence temperature inversion) Prevailing winds blew heavily polluted air across the English Channel from industrial areas of Europe

The Great Smog of Human Causes London had recently changed from an electric tram system to diesel buses; this diesel exhaust added pollution and particulates to the mix Tarry particles of soot were also in the mix from chimney smoke, commercial, and industrial sources

Great Smog of Health Impact As Londoners were used to fog, there was no panic initially The very young and very old, however, began to have respiratory problems, as did those who already had chronic respiratory problems Public transportation and even ambulances had to stop running, as the fog was so thick that visibility was only a few yards

Great Smog of Health Impact 4,000 people died from respiratory tract infections, lung infections, etc. during the days of the smog 8,000 more people died from respiratory problems in the months just after the Great Smog This episode made governments aware of the need for legislation to clean up the air to prevent a lethal situation in the future

Thermal Inversions in Georgia These sometimes happen in the metro Atlanta area after a cold fall or winter night when many people have been using their wood-burning fireplaces, or have burned leaves during the day Try to view the area from a higher vantage point to see the full effect of the inversion Want to see a temperature inversion?

How to Help Limit Temperature Inversions and Protect Yourself When the conditions are conducive to forming an inversion, reduce activities that might contribute, such as extra driving, burning wood, leaves, or refuse, etc. Wood smoke contains much more particulate pollution than oil- or gas-fired furnaces Follow the Air Quality Index reports online, in the newspaper, or on radio and follow recommendations for any change in activity If you have respiratory problems, make a plan with your care-givers as to what you need to do if there is a thermal inversion with a lot of pollution close to the ground Practice the 3 Rs every day to limit overall pollution from factories, power plants, transportation, and waste disposal