Lecture 1.3 – Structure of the Atmosphere

Today – we answer the following:

• How big is that atmosphere?

• Why is it like a cake?

• Why is cold in Denver?

• Why do mountaineers need oxygen?

Thickness

• The atmosphere is a very thin (relatively) layer of gas over the surface of the Earth

• Earth’s radius ~ 6400km

• Atmospheric thickness ~ 100km

• (If you travel 100km horizontally you don’t even get to St. Louis. If you do it vertically you’d be in space!)

The Relationship Between Air Pressure and Altitude

Pressure decreases as yougo up in height.

The change is pressure isnot constant. The pressuredecreases exponentiallywith increasing height.

Air Density and height

Pressure & DensityPressure & Density

Figure 1.7Figure 1.7

Gravity pulls gases Gravity pulls gases toward earth's toward earth's surface, and the surface, and the whole column of whole column of gases exerts a gases exerts a pressure of 1000 pressure of 1000 hPa at sea level, hPa at sea level, 1013.25 mb or 29.92 1013.25 mb or 29.92 in.Hg.in.Hg.

Pressure and Density Decrease with Height

© 1998 Wadsworth Publishing Co. -- From Ahrens, Essentials of Meteorology

Vertical Pressure ProfileVertical Pressure Profile

Pressure increases at a Pressure increases at a curved rate curved rate proportional to altitude proportional to altitude squared, but near the squared, but near the surface a linear surface a linear estimate of 10 mb per estimate of 10 mb per 100 meters works well.100 meters works well.

Figure 1.8Figure 1.8

Layers by temperature

• The atmosphere can be divided into layers based on temperature characteristics.

• This layering of the atmosphere also represents real physical barriers in that within the layers there is lots of vertical motion and mixing of air.

• This does not happen between layers.

Layers of the atmosphere

• Troposphere

• Stratosphere

• Mesosphere

• Thermosphere

Atmospheric LayersAtmospheric Layers

Figure 1.9Figure 1.9

8 layers are defined by constant 8 layers are defined by constant trends in average air trends in average air temperature (which changes temperature (which changes with pressure and radiation), with pressure and radiation), where the outer exosphere is not where the outer exosphere is not shown.shown.

The Troposphere

• Where we live (all the time)

• Contains 80% of the mass of the atmosphere

• Is between 8-16km (5-10 mi) deep

• Deeper at the equator than the poles

• WHERE WEATHER HAPPENS

Temperature Structure of the Atmosphere

Warming in thestratosphere

© 1998 Wadsworth Publishing -- From Essentials of Meteorology, 2nd Ed., by C.D. Ahrens, p. 9.

The Stratosphere

• Contains the ozone layer

• Where ultra-violet radiation is absorbed– This means that we are protected from

harmful high-energy radiation from the sun– This also means that the stratosphere is

warmer than the top of the troposphere because it has absorbed that energy

Ozone

• Is a variable gas

• At the surface– Is caused by chemical reactions between a

variety of pollutant gases (such as nitrogen oxides)

– Mostly caused by vehicle emissions– Is an irritant

Ozone

• In the stratosphere– Is a beneficial gas that absorbs ultra-violet

radiation– Protects us from this harmful radiation– Is broken down by chemical reactions with

chlorine containing gases (chlorofluorocarbons – CFCs): Man-made compounds used in aerosol sprays, refrigerators and air-conditioners

Summary of Chapter 1

• To answer our questions– The atmosphere is pretty big, but on the other

hand…– It’s like a cake because it has layers– It’s cold in Denver because the troposphere

gets colder as you go up– Mountaineers need oxygen because the air is

less dense at altitude and this means there are less oxygen molecules

Chapter 1 - Homework

• Read Chapter 1 of Ahrens

• From Ahrens, Page 24: Questions for review numbers: 2, 3, 11, 12

• Due: Friday September 5th (at the start of class)