Earth Science 1111

Lecture 1: Overview and Atmospheric Composition and

Structure

The Atmosphere

The atmosphere is the envelope of gases that surround our planet

It is a very thin layer compared to the radius of the Earth

We refer to this layer informally as “air”

The majority of the energy that drives the atmosphere is from the Sun.

The atmosphere receives most of this energy from the surface.

Imbalances of energy drive the winds. Eventually, all energy degrades down to friction.

State of the Atmosphere

We commonly express the state of the atmosphere by measuring it using the following variables:

• Pressure

• Temperature

• Wind

• Humidity

• Cloud cover

• Precipitation type and amount

• Visibility (distance one can see horizontally)

Types of Measurements

Contact Measurements – Also called in-situ measurements; A measurement that is conducted by sticking a probe into the medium being measured (thermometer).

Remote Sensing – A measurement conducted by remote means and not from directly within the medium being measured (satellites).

Weather

Weather is the state of the atmosphere at a particular time and a particular place.

An example of weather information would be the temperature at Glen Ellyn today at 7 p.m.

Climate

Climate is the long-term state of the atmosphere at a particular location.

Can be expressed as averages and extremes.

“Normal” conditions is simply a 30-year average of that particular variable.

Examples of climate information would be:• The normal high at O’Hare on August 1 is 83

degrees.

• The record amount of precipitation at O’Hare on August 1 is 2.55 inches back in 1961.

More Definitions

Meteorology is the study of the Earth’s atmosphere, its motions and behavior

Climatology is the study of the Earth’s climate, its past and future

Atmospheric Composition

Dry air (neglecting water vapor) is composed of the following gases:

Nitrogen (N2) = 78%

Oxygen (O2) = 21%

Argon (Ar) = 1%

Trace Gases

Dust, bacteria, and other particulates

Atmospheric Composition

Trace Gases

• Gases that are only found by examining a million or billion air molecules

• Examples include:

Carbon Dioxide (CO2) = 340 ppmv

Neon (Ne) = 18 ppmv

Helium (He) = 5 ppmv

Methane (CH4) = 2 ppmv

Hydrogen (H2) = 0.5 ppmv

Atmospheric Molecules

Note that the Oxygen, Nitrogen, and Hydrogen that we breathe are not single atoms of that gas, but instead are two atoms of that gas bonded together in what is called a diatomic molecule.

Ozone is an example of a triatomic molecule with three oxygen atoms.

Atmospheric Composition

The dry air components in the air are well-mixed, meaning they do not vary widely with location or time at the surface.

Water vapor (H2O) is a variable gas. You will see it depends on temperature and the availability of liquid water. A volume of air can have between 0% and 4% water vapor, and this varies both in time and space.

Vertical Layers of the Atmosphere

The Earth’s atmosphere has 4 distinct layers that are identified by the way temperature changes with height. These layers are:

- Troposphere

- Stratosphere

- Mesosphere

- Thermosphere

Troposphere

This is the layer closest to the Earth’s surface

Almost all the weather phenomenon we know of occurs in this layer.

The depth of the troposphere varies by the season (deeper in summer)

Temperature on average decreases with height.

Stratosphere

The next layer above the troposphere

The boundary between the stratosphere and troposphere is called the tropopause.

The ozone layer is found here

Some clouds can form here near the poles

Temperature increases with height

Very little mixing occurs between the troposphere and stratosphere.

Mesosphere/Thermosphere

The mesosphere is the next layer above the stratosphere.

Clouds (noctilucent) are rare in the mesosphere.

The thermosphere rests above the mesosphere, and no clouds exist.

Temperature decreases with height in the mesosphere and increases with height in the thermosphere.

The highest temperatures for the atmosphere occur in the thermosphere, but it doesn’t feel hot because the atmosphere is so thin up there.

Atmospheric Density

Density and pressure in the atmosphere decrease exponentially with height

Approximately 75% of the total atmosphere is in the troposphere

Approximately 99% of the atmosphere is the troposphere and stratosphere