NATS 101

Lecture 2

Vertical Structure of the Atmosphere

Vertical Structure of the atmosphere

1. Pressure

2. Density

3. Temperature

30

Height(km)

20

10

0

Can be thought of as weight of air above you.

(Note that pressure acts in all directions!)

So as elevation increases, pressure decreases.

Higher elevation Less air aboveLower pressure

Lower elevation More air aboveHigher pressure

Pressure:

Pressure Decreases Exponentially with Height

Logarithmic Decrease

• For each 16 km increase in altitude, pressure drops by factor of 10.

48 km - 1 mb 32 km - 10 mb

16 km - 100 mb 0 km

- 1000 mb

100 mb

10 mb

1 mb

16 km

32 km

48 km

Equation for Pressure Variation

We can Quantify Pressure Change with Height

What is Pressure at 2.8 km?(Summit of Mt. Lemmon)

Use Equation for Pressure Change:

p(at elevation Z in km) = pMSL x 10 -Z/(16 km)

Set Z = 2.8 km, pMSL = 1013 mb

p(2.8 km) = (1013 mb) x 10 –(2.8 km)/(16 km)

p(2.8 km) = (1013 mb) x 10 –(0.175)

p(2.8 km) = (1013 mb) x 0.668 = 677 mb

What is Pressure at Tucson?

Let’s get cocky…

How about Denver? Z=1,600 m

How about Mt. Everest? Z=8,700 m

You try these examples at home for practice

Use Equation for Pressure Change:

p(at elevation Z in km) = pMSL x 10 -Z/(16 km)

Set Z = 800 m, pMSL = 1013 mb

______Density: = (kg/m3 or g/cm3)

massvolume

Initial State

Incompressible fluid

Initial State

Compressible fluid

The density of the gases that make up the atmosphere is constantly changing. In addition, the atmosphere is compressible.

Sea-level

Near sea level, air density ~ 1.2 kg m-3.

Denver, CO

At Denver CO, (~1.6 km altitude – or 1 mile), air density is approximately 85% of that at sea level, or 1.01 kg m–3.

Density and Pressure Variation

Key Points:

1. Both decrease rapidly with height

2. Air is compressible, i.e. its density varies

Ahrens, Fig. 1.5

“Standard atmosphere” is calculated based on profiles at 30 latitude.

Temperature Stratification

Divide into several vertical layers based on electrical, temperature, and chemical (homogeneous/heterogeneous), characteristics.

Together with the change in density with height, this gives the atmosphere its structure.

Troposphere

Majority of the weather occurs

here

The lapse rate is theaverage decrease in temperature withheight ~ 6.5°C/km

Contains 80% of the atmospheric

massTropopause

Depth ranges from ~8 km at the poles to ~16 km in

the tropics

Rapid decrease in temperature with

height

Layer of most interest to this course!!!

Little weather occurs here

Isothermal inlowest 10 km

Lapse rate is 0

Contains ~19.9% of the atmospheric

mass

Stratopause

Temperature increases withheight from 20-~50 km

(Temperature inversion)

Ozone layer

The ozone layer absorbsmuch of the incoming

solar radiation, warmingthe stratosphere, and

protecting us from harmfulUV radiation

Layer of some interest to this course!!!

Stratosphere

Temperature onceagain decreases

with height

Mesosphere

Temperature onceagain increases

with height

Thermosphere

Neither of these layers have much interest for the Meteorologist

Ionosphere

- extends from the upper mesosphere into the thermosphere.

Contains large numbers of charged particles called ions. Ions are atoms or molecules that have gained an electron or lost an electron so that they carry a charge. This occurs in the upper atmosphere because the molecules are being constantly bombarded by solar radiation.

Important for reflecting AM radio waves back to Earth.

Also responsible for the aurora borealis (northern lights) and aurora australis (southern lights).

Divide the atmosphere into several vertical layers based on temperature characteristics. Together with the change in density with height, this gives the atmosphere its structure.

Troposphere

Stratosphere

Mesosphere

Thermosphere

Summary:

Temperature Inversion

Stable layer

Temperature inversion

tropopause

T-profile