Composition and structure of Earth’s Atmosphere

the atmosphere

composition of the atmosphere

400.0 0.040

0.00018 1.8

atmospheric CO2 concentrations at

Mauna Loa, Hawaii

Earth-Sun Relationships

•  Energy received from the Sun drives weather and climate, so it is obvious to start with changes associated with the Sun and our orbit around the Sun

•  The regular changes of Earth’s orbit around the Sun, and Earth’s rotation about its tilted axis, control seasonal and diurnal cycles and dominate the climatology of Earth

indirect rays

Earth’s surface, and lower atmosphere,

are unevenly heated

low latitudes receive more energy per unit

area than higher latitudes

distribution of incoming solar radiation

heat budget

caused by Earth’s orientation-inclination of the axis seasons

Length of day

length of day

daily variations in air temperature

heat (energy, W/m2) temperature (average energy, oC)

humidity (water vapor, g/kg) pressure (force/area, mb = 100 N/m2)

winds (circulation, m/s) cloud cover (liquid/solid water)

precipitation rate (mm/day)

Environmental Variables

Figure 1.1

temperature

Figure 1.2

seasonal temperature differences

Figure 1.3

structure of the atmosphere

temperature vs. height

in the troposphere temperature decreases with height

the average lapse rate is about 5-9 degrees C per kilometer depending on the humidity

in the stratosphere temperature increases with height

in the stratosphere ozone absorbs incoming ultraviolet radiation

80% of the mass of Earth’s atmosphere is in the troposphere,

we all live in the troposphere

ozone

ozone layer maximum at 20-25 km

tropopause height varies with latitude

Figure 1.6

atmospheric pressure pressure is a force per unit area exerted by the weight of air above

-- about 1 kg/cm2 or 14.7 lb/in2 at the surface of Earth (equivalent to a 10 meter column of water)

units of pressure are N/m2 where N = Newton = force required to accelerate a 1 kg mass 1 m/s2

The SI unit for pressure is the Pascal (Pa) 1 Pa = 1 N/m2

a standard atmosphere (and average typical of a mid latitude location) exerts a pressure of 101,325 Pa at sea level

the unit adopted by the National Weather Service is the millibar (mb) 1 mb = 100 Pa

--> standard sea level pressure = 1013.25 mb

factors affecting atmospheric pressure

1.  gas molecules are able to fill space available

2.  gas molecules bounce off one another when they collide (and off the wall if in a container)

3.  the atmosphere is bounded above (gravity) and below (ground)

Air pressure is the force per unit area exerted against a surface by continuous collision of gas molecules

pressure is partly determined by temperature consider a change in temperature while holding density (volume) constant

temperature increase --> speed of molecules increases (force increases, higher pressure)

pressure is partly determined by density

Density (# molecules/volume) increase --> increase in # of collisions (higher pressure)

consider a change in density (volume) while holding temperature constant

ideal gas law Boyle's Law - at constant temperature, the volume of gas varies inversely with pressure

p1V1 = p2V2

Charles' Law - at constant pressure, the volume of a given mass is directly proportional to absolute temperature--> increase in temperature results in increase in volume

V1 / V2 = T1 / T2

Combine these two laws to obtain ideal gas law, or equation of state

p = ρ R T

where p = pressure, ρ = density, R = gas constant, and T = temperature

pressure changes with altitude

Figure 1.9

pressure and density vs. height

pressure at sea level is around 1000 millibars

pressure decreases with height exponentially

50% of the mass of Earth’s atmosphere is below an

altitude of 5-6 km (the 500 millibar height)

pressure is a force per unit area

Figure 1.7

reducing pressure to sea level

sea level pressure on Earth

seasonal pressure and wind patterns

Figure 1.15

pressure changes with temperature

pressure changes with temperature

hydrostatic equilibrium

concept: –  the vertical pressure gradient force is equal and opposite to the gravitational force

hydrostatic equation

Where: Δp represents change in pressure, Δz, change in altitude, ρ, air density, and g, acceleration of gravity

(Δp / Δz) = - ρg

the rate at which pressure decreases with height equals the product of air density times the acceleration of gravity

hydrostatic equation examples

cold column: ρ = 1.3 kg/m3

(Δp / Δz) = - 1.3 (9.8) = -12.8 Pa/m

thus:

pressure declines more rapidly in a cold, dense air column than in a warm air column

water’s changes of state

Water’s Changes of State

amount of water vapor in air

1.  absolute humidity = mass of water vapor per volume of air (g/m3)

2.  mixing ratio = mass of water vapor in a unit mass of dry air (g/kg)

3.  relative humidity = ratio of air's water vapor content to its capacity

4.  dewpoint temperature is the temperature at which air is saturated

(100% relative humidity)

humidity

relative humidity

changes with added moisture

changes with temperature

relative humidity

saturation mixing-ratio

Figure 1.11

Which has more Moisture?

Which has higher Relative Humidity?

Higher Temperature Lower Relative Humidity with MORE Moisture!

Lower Temperature Higher Relative Humidity

with LESS moisture!

diurnal changes in relative humidity

cloud cover