LAYERS OF THE ATMOSPHERE

How many layers does the Earth's atmosphere contain?

The atmosphere, or "skin" of gas that surrounds the Earth, consists of six layers that are differentiated by temperature:

1. The troposphere is the lowest level. It averages about 7 miles (11 kilometers) in thickness, varying from 5 miles (8 kilometers) at the poles to 10 miles (16 kilometers) at the equator. Most clouds and weather form in this layer. Temperature decreases with altitude in the troposphere.

2. The stratosphere ranges between 7 and 30 miles (11 and 48 kilometers) above Earth's surface. The ozone layer, which is important because it absorbs most of the Sun's harmful ultraviolet radiation, is located in this band. Temperatures rise slightly with altitude to a maximum of about 32°F (0°C).

3. The mesosphere (above the stratosphere) extends from 30 to 55 miles (48 to 85 kilometers) above the Earth. Temperatures here decrease with altitude to -130°F (-90°C).

4. The thermosphere (also known as the hetereosphere) is between 55 to 435 miles (85 to 700 kilometers) above Earth's surface. Temperatures in this layer range to 2,696°F (1,475°C).

5. The ionosphere is a region of the atmosphere that overlaps the others, reaching from 40 to 250 miles (65 to 400 kilometers). In this region, the air becomes ionized (electrified) from the Sun's ultraviolet rays. It is divided into three subregions: 1) the D Region (40 to 55 miles [65 to 90 kilometers]); 2) the E Region (also called the Kennelly-Heaviside layer) at 56 to 93 miles (90 to 150 kilometers); and 3) the F Region (93 to 248 miles [150 to 400 kilometers]), which is further separated into the F1 layer and the F2 layer (also called the Appleton layer), with the dividing line being at about 150 miles (240 kilometers) above sea level.

6. The exosphere lies above the thermosphere and includes everything above 435 miles (700 kilometers) high. In this layer, temperature no longer has any meaning.

How is the troposphere defined?

The troposphere is considered as the layer closest to Earth and is also the region where temperatures reliably decrease with altitude. The troposphere is thickest at the Earth's equator, reaching heights of about 11 miles (17 to 18 kilometers), and this is, therefore, also where you will find the coldest tropospheric temperatures. It might seem rather ironic that, right above the world's steamiest tropical forests, temperatures can be as low as -110°F (-79°C).

Who figured out that temperatures rise in the stratosphere?

Pioneering French meteorologist Léon Philippe Teisserenc de Bort (1855-1913) conducted an experiment using helium balloons and temperature sensors. He learned that, after about seven miles (11 kilometers), the air stopped becoming cooler and leveled off for as high as the balloons could go. He concluded that the atmosphere was divided into two layers, which he named the troposphere and the stratosphere. Later, in the 1920s, meteorologists Gordon Miller Bourne Dobson (1889-1976) and F.A. Lindemann, First Viscount Cherwell (18861957), used studies of meteor trails to learn that temperatures warmed in the atmosphere as high as 30 miles (48 kilometers) up. Dobson concluded that ultraviolet radiation absorbed by the ozone in the stratosphere was the reason for the warmer air.

What mineral were scientists surprised to see in high quantities in the stratosphere?

Scientists discovered a higher-than-expected amount of salt in the mesosphere within the stratosphere. The current theory is that the salt has been left behind by meteor activity.

What is the tropopause?

The tropopause is the layer between the troposphere and the stratosphere that hovers around 10 miles (16 kilometers) above the ground. Within the tropopause is the tropopause break, a region through which water vapor and air can easily pass from the troposphere to the stratosphere.

How is the ionosphere important in the transmission of radio waves?

When ultraviolet light enters the atmosphere, it ionizes atoms in the ionosphere through a process called photoionization, which releases free electrons into this region of the atmosphere. It is these free electrons that make radio wave transmissions possible. Depending on the frequency of the radio waves, transmissions travel for shorter or longer distances. Lower-frequency waves bounce off the ionosphere at a lower elevation and thus travel a shorter distance than higher frequency waves. Very high frequency waves are used when communicating with satellites or anything out in space because they can completely escape the atmosphere.

British physicist Oliver Heviside (1850-1925) and American electrical engineer Arthur Edwin Kennelly (1861-1939) independently theorized the existence of an

A circa 1900 photograph of Teisserenc de Bort (left), standing with Blue Hill Meteorological Observatory founder and fellow atmosphere researcher Abbott Lawrence Rotch (1861-1912). (NOAA)

A circa 1900 photograph of Teisserenc de Bort (left), standing with Blue Hill Meteorological Observatory founder and fellow atmosphere researcher Abbott Lawrence Rotch (1861-1912). (NOAA)

Why are radio transmissions weaker at night than during the day?

At night, of course, much less light is entering the atmosphere, which means that less ionization is occurring and fewer electrons are available for radio waves to bounce off of. The result is that radio transmissions are weaker.

ionosphere and that certain wave frequencies would bounce off it and be reflected back to Earth. It was radio pioneer Guglielmo Marconi (1874-1937) who first took advantage of this theory to conduct the first transmission from Cornwall, England, to Newfoundland in 1901. The waves became known as radio waves, and Marconi is credited as the inventor of the radio. The E region of the ionosphere was named after Kennelly and Heviside to honor their work.

What is an ionospheric storm?

When a coronal mass ejection (solar flare) occurs on the Sun's surface, it can dramatically increase the amount of photoionization in the ionosphere. An overwhelming amount of free electrons in the upper atmosphere results, and this can cause problems with radio communications.

What is the mesosphere?

The mesosphere is the uppermost layer of the stratosphere. Below the mesosphere, at altitudes of 25 to 40 miles (40 to 65 kilometers), is a warm layer of the stratosphere that contains a high concentration of ozone molecules that block ultraviolet light.

Why is the thermosphere so hot?

While the temperature of the thermosphere can exceed 3,600°F (1,982°C), the atmosphere here is so thin that an ordinary thermometer would not register this temperature (a regular thermometer would indicate temperatures below freezing). Instead, special instruments are used to measure the speed of the few particles that are in the thermosphere, and these indicate the extraordinary highs. Atoms and molecules in the thermosphere are excited by radiation from outer space.

 
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