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 K and boils at K, T(K) = T(C)

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 mb Inches of Mercury (“Hg): Sea level pressure is “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 = V g /V air 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 (O 2 ): 21% Trace Constituents: Argon (Ar), about 0.9% Water vapor (H 2 O), up to ppmv Carbon dioxide (CO 2 ), 350 ppmv Ozone (O 3 ), near zero at the surface, up to 10 ppmv in the stratosphere Methane (CH 4 ), 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 (e s ) represents the maximum vapor pressure of water in air. Vapor pressure is determined for equilibrium over liquid water or over ice e s 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 / e s (%) 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 (O 3 ) 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 (O 2 ). 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 Circulation The 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 = meters) Concentrations from 1 to 1000 per cm 3 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 pressure 2) 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 Formation Why 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 continued Low 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 (Extra slides follow)

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)