The Earth’s Atmosphere

The Earth’s Atmosphere

Atmospheric Variables

We use a variety of variables to describe the atmosphere,

For example:

Temperature

Pressure

Mixing ratio

Discussing the atmosphere requires an understanding of some important atmospheric variables

Temperature

Temperature is a measure of the average speed of the molecules, faster motion = higher temperature.

Temperature is a fundamental quantity for understanding the weather, radiation, and chemistry of the atmosphere.

Temperature scales:

Fahrenheit (F): water freezes at 32°F and boils at 212°F

Celsius (C): water freezes at 0°C and boils at 100°C, T(F) = (9/5) T(C) + 32

Kelvin (K): water freezes at 273.15 K and boils at 373.15 K, T(K) = T(C) + 273.15

Pressure

Atmospheric pressure can be thought of as the weight per unit area of the column of atmosphere above a given height.

Pressure scales:

Millibars (mb): Sea level pressure is 1013.25 mb

Inches of Mercury (“Hg): Sea level pressure is 29.92 “Hg

Barometer

Pressure continued

Since the number of air molecules above some altitude decreases with height, pressure likewise decreases with height.

Pressure decreases exponentially with altitude

Density, Mixing Ratio, Partial Pressure

Density

Air density is determined by pressure and temperature, d = P / RT

R is the universal gas constant

Warmer temperatures or lower pressures correspond to lower air density

Mixing Ratio

The abundance of a gas in the atmosphere can be described by the mixing ratio.

Volume mixing ratio is the volume of gas per unit volume of air, Q = Vg/Vair

Partial Pressure

The partial pressure for a given gas is the pressure exerted by that gas alone.

For example we may want to know the partial pressure of just water vapor

Composition of the Atmosphere

•The atmosphere is comprised of a variety of gases:

Major Constituents (99%):Nitrogen (N): 78%Oxygen (O2): 21%

Trace Constituents:Argon (Ar), about 0.9%Water vapor (H2O), up to 10000 ppmvCarbon dioxide (CO2), 350 ppmvOzone (O3), near zero at the surface, up to 10 ppmv in the stratosphereMethane (CH4), 1.7 ppmv and others…..

ppmv = “parts per million by volume”

Water in the Atmosphere•Water exists in 3 states: solid (ice) – liquid – gas (water vapor)

•The saturation water vapor pressure (es) represents the maximum vapor pressure of water in air.

Vapor pressure is determined for equilibrium over liquid water or over ice

•es is a function of temperature alone, and decreases at colder temperatures.

•Relative humidity is the ratio of the water vapor content to the water vapor capacity:

RH = 100 x e / es (%)

•Dew point is the temperature to which the air would have to be cooled to achieve 100% relative humidity.

Vertical Structure of the Atmosphere

•Layers in the atmosphere are defined by temperature

•Earth's atmosphere thins out to near nothingness several hundred kilometers above the surface

•99% of the total mass of the atmosphere exists below 30 km altitude

Troposphere and Stratosphere

Troposphere•0 to 15 km altitude

•The lowest region of the atmosphere, where life & weather exist.

•Temperature decreases with altitude.

•Long-wave radiation emitted from Earth is absorbed by the atmosphere, the atmosphere becomes less dense with increasing altitude, less air to absorb

•Top of the troposphere is known as the tropopause

Stratosphere •15 to 50 km altitude

•Temperature increases with altitude.

•Heating occurs because ozone (O3) absorbs ultraviolet radiation from the Sun.

•Top of the stratosphere is known as the stratopause

Mesosphere and Thermosphere

Mesosphere •50 to 90 km altitude

•Temperature decreases with altitude

•The lowest temperatures in the entire atmosphere are found at the mesopause during summer at high latitudes, 130 K (-226°F) can occur

•Top of the mesosphere is known as the mesopause

Thermosphere •90 to 500 km altitude

•Temperature increases with altitude above 90 km, and is constant above 200 km.

•This heating is due to absorption of solar radiation (wavelengths less than 0.2 microns) by molecular oxygen (O2).

•The highest temperatures in the atmosphere can be found in the thermosphere, 2000 K can occur

Atmospheric Circulation

Atmospheric motion, or wind, exhibits a range of horizontal scales.

Planetary scale:

broadest features of the global circulation, features with horizontal dimensions comparable to the size of continents or oceans, for example the persistent west to east winds.

Synoptic scale:

waves with horizontal dimensions on the order of several hundreds of kilometers, for example high and low pressure systems.

Mesoscale:

waves with horizontal dimensions on the order of tens to hundreds of kilometers, for example mountain lee waves.

Global CirculationThe broadest features of the global circulation are driven by the overall temperature distribution:

•Warm air at the equator rises and flows towards the poles

•Cold air at the poles sinks and flows towards the equator

The coriolis force turns these winds resulting in the “three cell” circulation

Weather Patterns

Weather patterns are more complex than the global circulation

Areas of high and low pressure change the weather frequently

Driving Forces Behind Wind

•Pressure Gradient

Air flows from high to low pressure (“downhill”)

•Coriolis

Caused by the rotation of the earth, wind deflects to the right in the northern hemisphere

•Centripital

Present when winds are in rotation

•Friction

Air moving along the Earth’s surface is slowed by friction

Pressure Gradient Force

Air flows from areas of high pressure (density) to areas of low pressure (density)

Pressure on weather maps is indicated by “isobars,” or lines of equal pressure

The pressure gradient force is in the direction from high to low pressure

weak pressure gradient force: light winds

strong pressure gradient force: strong winds

Coriolis Force

High pressure in N hemisphere

Add Coriolis

bend to the right

Low pressure in N hemisphere

Add Coriolis

bend to the right

The Coriolis force is an apparent force that explains the deflection of a body moving across a rotating surface.

The rotation of the Earth causes the wind to

•deflect to the right of its path in the northern hemisphere

•deflect to the left of its path in the southern hemisphere

The coriolis force:

Increases with increasing wind speed

Is zero at the equator and strongest at the poles

Condensation

•Condensation occurs when the relative humidity exceeds 100%

•Water only condenses on a surface

•Dew and frost condense on surfaces such as plants or windshields

•In the atmosphere water condenses on condensation nuclei (CN).

Condensation Nuclei (CN)

CN are tiny particles suspended in the atmosphere

CN stay aloft in the air for many days. They are so small that their weight is less than their air resistance.

Radius typically from 0.1 to 1 microns (micron = 10-6 meters)

Concentrations from 1 to 1000 per cm3 of air

Not all particles are good CN, effective CN are:

•Soluble (for example salt)

•Or wettable (for example clay or minerals)

But not hydrophobic (for example oils)

Ice Nuclei

•Water does not always freeze at 32° F

•Water existing at temperatures below freezing is called “supercooled”

•Some particles cause supercooled water to freeze, these particles are known as ice nuclei

•Without ice nuclei, pure water would need to be –40° F to freeze

•Some CN are also good ice nuclei, others are not

Clouds in the Atmosphere

Most clouds occur in the troposphere

There are exceptions:

Noctilucent clouds (NLCs) occur in the mesosphere

Polar stratospheric clouds (PSCs) occur in the stratosphere

•Clouds are a collection of water drops and/or ice crystals•Clouds form when water vapor in the atmosphere condenses•Condensation only occurs on CN•Water vapor condenses when the relative humidity exceeds 100%

This can happen if one or both of the following occurs:1) The air is cooled, reducing the saturation vapor pressure2) Water vapor is added to the air

Rising air expands, expanding air cools, so rising air can cause clouds

Cloud Formation

Imagine an air parcel, rising upward through the atmosphere.

The air parcel expands as it rises and this expansion causes the temperature of the air parcel to decrease.

As the parcel rises, it cools, and the humidity increases until it reaches 100%.

When this occurs, cloud droplets begin forming as the excess water vapor condenses on CN particles.

Above this point the cloud droplets grow by condensation in the rising air.

If the rising motion is sufficiently intense and enough water vapor is present, precipitation will develop.

Cloud FormationWhy does air rise?

An air parcel will rise naturally if the air within the parcel is warmer than the surrounding air (like a hot air balloon).

As the earth is heated by the sun, bubbles of hot air form (called thermals) and rise upward from the warm surface.

Convergence is an atmospheric condition that exists when there is a horizontal net inflow of air into a region.

When air converges along the earth's surface, it is forced to rise since it cannot go downward.

Cloud Types

Clouds are classified into broad categories

High level:

•cirrus clouds

•Altitudes above 20,000 feet

•Composed primarily of ice crystals

•Typically thin and white in appearance

Mid level:

•altocumulus, altostratus

•Altitudes between 6,500 to 20,000 feet.

•composed primarily of water drops, sometimes ice crystals

Cloud Types continuedLow level: Stratus,

•nimbostratus

•Altitudes below 6,500 feet

•Usually composed of water drops

•Uniform, covers entire sky

Vertically Developed:

•cumulus and cumulonimbus (thunderstorms)

•Cloud top heights in excess of 39,000 feet

•Composed of water and ice together, often producing hail

Cloud Types continued

Other cloud types that are uncommon:

Noctilucent clouds (NLCs)

occur in the mesosphere at polar latitudes

Polar stratospheric clouds (PSCs)

occur in the stratosphere at polar latitudes

Summary

What is the atmosphere composed of?

What are the layers of the atmosphere, how are they defined?

What are the forces that govern wind?

What is required to form cloud particles?

The End

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Atmospheric Observations

The most common parameters measured:

Temperature: Thermometer

Pressure: Barometer

Humidity: Hygrometer

Wind speed and direction: Anemometer

Precipitation: rain gage (rain), ruler (snow depth)

Other parameters measured:

Cloud coverage & movement: Radar, satellite, human observer

Precipitation: using Radar

Atmospheric Observations

In Situ Measurements, instrument is in contact with the subject•Surface: weather stations in most towns, observations every hour, temperature, pressure, humidity precipitation, wind, clouds•Balloon: one or two sites per state, observations twice a day, temperature, pressure, humidity, winds, from the surface to the tropopause

Remote Measurements, instrument is far from the subject•RADAR (radio detection and ranging): 1 or 2 per state, clouds & precipitation, storm movement•Satellites: cloud images, water vapor measurements

Geostrophic Wind

Winds aloft (above ~1000 m) flowing in a straight line, a balance between 2 forces:

•Pressure gradient force (PGF)

•Coriolis ‘force’ (CF)

A wind that begins to blow across the isobars is turned by the Coriolis ‘force’ until Coriolis ‘force’ and PGF balance

Gradient Wind

Winds aloft in rotation, a balance of 3 forces

•Pressure gradient force (PGF)

•Coriolis ‘force’ (CF)

•Centripital force (Ce)