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)

http://www.srh.noaa.gov/jetstream//atmos/heat.htm

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

http://www.srh.noaa.gov/jetstream//atmos/atmprofile.htm

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 placeThis ‘vertical mixing’ is whatcauses 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 pollutantsTemperature inversions, however, keep cool, dense air near the surface, reducing the vertical movementPollutants 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 afternoonin January after a cold night.

Wikimedia Commons

Other Favorable Conditions for Forming a Temperature

InversionLong nights ➯ greater duration of cooling

Inversions more common in winter due to longernightsInversions 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 sunin the sky (as in winter)➯ less daytime heating of airclose to the ground andsurface

Wikimedia Commons

Types of Temperature Inversions

Radiation Temperature Inversion

Subsidence Temperature Inversion

ShanghaiWikimedia Commons

What Causes a Radiation Temperature Inversion?

Mechanism1. Ground loses heat quickly after sunset 2. Air in contact with ground then cools quickly3. 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 afternoonPollutants 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 surfaceHigh pressure pushes the warm air mass down, effectively capping the cooler air below

Fog over San FranciscoWikimedia 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 inversionsMechanism1. 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, CAWarm, sunny climate and high level of pollutants (vehicle, shipping, port) causes a photochemical smogSmog 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, http://en.wikipedia.org/wiki/File:Los_Angeles_Pollution.jpg

The Great London Smog of 1952

The Great Smog (AKA “Big Smoke”) affected London in 1952

5 days in DecemberTemperature inversion that caused the worst air pollution event in UK history

Nelson’s Column, London, during the Great Smog, December 1952Wikimedia Commons

The Great Smog of 1952- 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 1952- 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 1952- 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 1952- 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 withyour care-givers as to what you need to do ifthere is a thermal inversion with a lot of pollution close to the ground

Practice the 3 Rs every day to limit overall pollutionfrom factories, power plants, transportation, andwaste disposal