The atmosphere is very thin The Atmosphere Troposphere barrier! Tropopause Earth radius 6,370 km (3,981 miles) The atmosphere extends upward to 500 km (321 miles), HOWEVER, 99% of all atmosphere gasses are below 32 km (20 miles) Therefore Although the entire atmosphere = 8% of earth’s solid radius 99% of gasses 0.005 = 0.5% (one half of one percent) of earth’s radius Cloud charts, radiosonde. instruments
For Wet Air add water vapor (up to 4% around here) Dry Air Two common gasses, N2 (78%) and O2 (21%), make up 99% of dry air. Other gasses, e.g. CO2 CH4 NO2 and water vapor H2O also play an important role by keeping the atmosphere warm, the “greenhouse effect”.
Heat vs. Temperature • Atoms in air are in constant motion, the energy of their motion is known as kinetic energy. Kinetic energy increases as the speed of atomic motion increases. Ek = 1/2mv2 (identify symbols) • Heat energy is the total kinetic energy of all the atoms in a substance. The more atoms present, the greater the heat. • Temperature represents the average kinetic energy of the atoms in a substance. A few atoms with rapid motion will have a higher temperature than many atoms with slow motion.
The atmosphere consists of four distinct layers thermosphere Ionized Gas Auroras in the Thermosphere top of mesosphere (Tropopause)
Ozone Atmosphere protects us from incoming comets, asteroids burn up ; blocks short wavelength radiation from the Sun.
Temperature changes in predictable ways with increasing altitude The thermosphere has very few atoms, but they are moving fast, so it has high Temperature Lapse Rates
Atmosphere Layers w/ Pauses 2. Pressure the weight/area of air above 3. Tropopause higher at equator Ozone layer equator poles 4. 75% of gases In Troposphere 5. lapse rate 6.5oC/km 6. Note change of sign of lapse rate at Tropopause (next slide)
When rising air hits the tropopause it cannot go much higher, so it spreads out. What does that remind you of? Mantle convection under lithosphere Lapse Rate in Troposphere
The apparent position of the sun at the NH Winter Solstice Tropic of Capricorn gets the most direct sunlight on about Dec 21st North Pole Dark 24hours/day, South Pole daylight 24 hours per day Earth’s Spin Axis is inclined 23½o to its orbit around Sun
Less than 10% of UV reaches the surface 7% is a good average Proportions of Solar Radiation Reaching Earth
(albedo) 100% in from sun – 30%reflected -19% absorbed = 51% reaches the earth Of that 51%, 23% used to evaporate water, and about 28% heats the Earth
Radiation Penetration 300 meters
Hydrologic Cycle The volume of water falling as precipitation is approximately 4.2 x1014 m3 (420 trillion m3) per year, many times greater than the moisture stored in the atmosphere. Water must be constantly cycled through the atmosphere to maintain such high precipitation volumes.
“water vapor” Water only compound in three states (liquid, gas, solid) on Earth’s surface. Heat energy is transferred through the atmosphere as water changes from one state to another. Heat from water is lost to the atmosphere during freezing, condensation, and “precipitation”. This heats the air, causing it to expand and, if possible, rise. The atmosphere’s heat is absorbed by water in processes such as melting, sublimation, and evaporation. Evaporation puts moisture (water vapor gas) into the atmosphere Condensation releases heat to atmosphere & forms cloud droplets “ “ These two transfer the most energy, are less common, don’t cause storms
Latent Heat • Latent heat is released to atmosphere as water changes from a less-ordered state to a more-ordered state “Latent heating of condensation” (gas to liquid). • Atmosphere’s Heat is absorbed by water as it changes to a less-ordered state “Latent cooling of evaporation” (liquid to gas) • The amount of heat lost or gained per gram of water is expressed in calories of latent heat.
Some useful units • One Gram is the mass of liquid water in a little cube, one centimeter on a side. • A centimeter is less than half an inch. It is 1/100 of a meter. • A meter is 39.37 inches, so a centimeter is about 0.394 inches • A mole of anything is 6.023 x 1023 • 1023 means 10x10x10x10x10x … x10 twenty three times
Latent heat amount the same either way • EXAMPLE: The latent heat of fusion, the heat released as water FREEZES, i.e. goes from liquid to solid, is 80 calories per gram of water. • The reverse reaction, the conversion of ice to water absorbs 80 calories of heat for each gram of water MELTED.
Changes of State and Weather • Changes in state where latent heat is released (freezing, condensation, “precipitation” [vapor to ice] ) • Changes in state where latent heat is absorbed (melting, evaporation, sublimation [ice to vapor] ). • Evaporation and condensation occur over large areas of Earth's surface contribute significantly to the generation of weather phenomena and the redistribution of heat on Earth’s surface. We will spend most of our time considering these two.
Note the LARGE number 585 calories Evaporation • Liquid water is converted to water vapor during evaporation. Heat is absorbed from the atmosphere to convert the liquid water to a less-ordered form, a gas, called “water vapor”. • “Latent Cooling of Evaporation”585 calories per gram are absorbed by water as it changes to gas. • Anything touching the water loses heat and cools. So do we as our water (“sweat”) evaporates We are mostly interested in the atmosphere losing heat. • Tusker Beer Anecdote
Condensation • Water vapor is converted to liquid water during condensation. Heat is released to the air as the vapor converts to the more-ordered liquid form. Nearby air heats up, expands, and usually rises. • “Latent Heating of Condensation” • Condensation starts at the cloud base. Cloud bases are made of tiny droplets of LIQUID water. (These may also freeze) • 585 calories per gram are released as water vapor is converted to liquid water . Objects nearby (e.g. atoms of N2 and O2 gas in the air) gain the heat that is released. • 585 calories/gram of water is really a lot of heat
Evaporation/Condensation transfers a lot of energy • Much more latent heat is lost/gained during changes between liquid and gas states than during changes between solid and liquid states. • Depends on the number of bonds that must be broken or modified between water molecules. • During freezing/melting these bonds are altered but generally do not break as the atomic structure changes slightly. • In contrast, during evaporation/condensation all the bonds between the molecules must be broken or formed, requiring much more energy.
Humidity • The presence of moisture (water vapor, an invisible gas) in the atmosphere is measured by the humidity of the air. • Humidity and condensation are closely related as condensation inevitably occurs when the air is saturated with moisture (100% humidity). “Latent Heat of Condensation” Gas to liquid droplet, heat is released to the atmosphere, air molecules move faster, move apart, less dense, rise
Relative Humidity and Dew Point • Absolute humidity measures the amount of water vapor in air. Grams H2O/m3 of air • Relative humidity measures the amount of water vapor in air relative to the maximum amount of water vapor the air could hold at that temperature. • Relative humidity increases with increasing water vapor or decreasing temperature. • Cold air can’t hold as much water vapor as warm air. • The DewPoint is the temperature at which air becomes saturated with moisture, i.e. it can’t hold any more.
Absolute Humidity • Absolute humidity measures the amount (mass) of water in a volume of air. Units are gramsH2O/meters3 • The absolute humidity of air varies with temperature; warm air can hold more moisture (water vapor, a gas) than cold air.
Heat flows from hot to cold: Why? • Warm air overlying cooler water? The air will warm the water. Example: near the equator • Cold air over warm water? Water warms air. Example: over the Gulf stream near Britain Demo: Collisions Heat: total kinetic (motion) energy of molecules in a packet of air of specified volume
Evaporation • When water is warmed, the bonds between the water molecules break as the velocity of the molecules increases and the liquid is converted to a gas phase. • This addition of water molecules to the air increases the vapor density, and thus the absolute humidity, gramsH2O/meters3of the air mass. • EVAPORATION INCREASES HUMIDITY
Moist Air vs. Dry Air 1 • Air with water vapor in it (Moist Air) is lighter than dry air Here’s Why: • When water vapor H2O is added to air, other gases are pushed aside. • Recall that dry air is mostly Nitrogen N2 and Oxygen O2 molecules.
Moist Air vs. Dry Air 2 OR “why moist air rises” • Water H2O “weighs” 18 grams per mole. Nitrogen N2 “weighs” 28 grams per mole Oxygen O2 “weighs” 32 grams per mole • The number of moles of molecules in air at constant T and P is constant. • Since light water molecules displace much heavier molecules, air with water vapor in it is “lighter”, less dense, more bouyant.
Relative Humidity • Relative humidity is expressed as a percentage. • Relative humidity measures the amount of moisture in air in comparison to the maximum mass/volume of moisture the air would contain when saturated. • Saturation is the point where increasing vapor density results in condensation (clouds)
25oC 72oF 12oC 53.6oF Various Temperature Scales
Warm air can hold more water vapor than cold air • For example, air with a temperature of 25oC and an absolute humidity of 11.5 g/m3 has a relative humidity of 50% because air at that temperature can hold up to 23 g/m3. • In contrast, the relative humidity of the same air would be 100% if the air was cooled to 12oC and the moisture content remains constant (11.5 g/m3), because 11.5 g/m3 is all the water vapor the cold 12oC air can hold.
Dew Point • Condensation occurs when the air becomes saturated with moisture (relative humidity = 100%). • As temperature falls the relative humidity of the air rises. • The temperature at which condensation begins is termed the dew point. • Condensed water forms clouds. A million cloud droplets may clump together to form a rain drop. wet lapse rate dry lapse rate TEMP
Air Pressure and Altitude • Air (atmospheric) pressure is the pressure exerted by the weight of the overlying column of air • 50% of all air lies below 5.5 km (3 miles) of altitude, therefore air pressure at this altitude (~500mb) is half of the air pressure at sea level (~1000 mb).
Coalescence - Making Rain • Condensation occurs on surfaces such as dust particles to form tiny cloud droplets. • The droplets are readily kept airborne by air turbulence. When they become so common that they collide and coalesce, the larger droplets fall, colliding with other droplets to eventually form a rain-drop. • Each rain drop contains approximately one million cloud droplets. • With decreasing temperatures (less than -10oC) ice crystals replace water droplets.
Lifting PV=nRT • When air is lifted, it expands, cools. • Density lifting (Buoyancy Lifting) occurs when a warm air mass, surrounded by cooler air, rises. The cold, denser air pushes under the warm, low density air. The warm low density air is forced up. • Frontal lifting occurs when warm air rises over cold air along a warm or cold front. • Orographic lifting takes place when air is forced to rise over a mountain range. Like oil & water http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm
Lapse Rates • For stable air, not rising or falling, the normal lapse rate is 4°C to 6.5°C per 1000m (2°F per 1000ft) • When air rises it cools at a relatively constant rate. If the air is unsaturated, this rate, called the dry adiabatic rate, is10°C per 1000m (5.5°F per 1000ft), • For saturated cold air the wet adiabatic rate ~ dry adiabatic rate. For warm air the wet adiabatic rate is less than the dry adiabatic rate. An average value of 6°C per 1000m (3.3°F per 1000ft) is commonly used. http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm
Adiabatic Processes • An adiabatic process takes place without a transfer of heat between the air parcel and its surroundings. In an adiabatic process compression always results in warming, and expansion results in cooling • A mass of warm dry air will rise through the stable air as long as the temperature of the warm air mass remains above that of the surrounding air. As it rises, it expands and cools. • This is because the density of warm air is less that the density of cool surrounding air http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm
Example: Assume that a warm air mass begins to rise with a temperature of 20oC and that the surrounding (stable) air has a temperature of 10oC. The temperature of the air mass and the stable air will be equalized at –5oC at an altitude of 2.5 km. NOTE: This is well below the tropopause
2. Above cloud base, water vapor condenses to liquid, releases heat, parcel rises faster! 1. If the air reaches the dewpoint . . . . . . before it stops, it will continue to rise at the wet adiabatic rate
Sometimes surface air is saturated with water, and a cloud forms at the surface FOG
Fronts • Frontal lifting occurs when two large air masses of contrasting density (temperature, moisture content) meet. • The boundary between the air masses is termed a front and may be 10 to 150 km (6-94 miles) across and hundreds of kilometers in length.
A warm front forms when a warm air mass displaces a cold air mass. The warm air rises above the colder air while pushing it. Condensation causes storms http://www.irkutsk.com/home/meteo/warmfront.jpg
A cold front forms when a cold air mass displaces a warm air mass. The cold air wedges under the warmer air while pushing up. The warm air holds more moisture, which condenses, resulting in storms Cold Front
Warm air is also forced upward when cold air approaches a warm air mass along a cold front. Cold fronts are steeper than warm fronts and cause cloud formation and precipitation to occur across a narrower area. http://www.irkutsk.com/home/meteo/warmfront.jpg
Sea Breeze and Land Breeze Land heats and cools faster than water
Convergence lifting occurs when two air masses collide, forcing some air upward as both air masses cannot occupy the same space. Thunderstorms result. Ha “ More thunderstorms pass over Miami than New York in a year” Florida Sea Breeze
Cloud names and meanings High (>6 km) Cirrus, cirrostratus, cirrocumulus Middle (2-6 km) Altostratus, altocumulus Low (< 2 km) Cumulus, stratocumulus, nimbostratus Rule of Thumb
Highs and Lows • High-pressure regions are dominated by cool or cold, descending air. • Low-pressure areas are associated with warm, rising air masses. • Good weather is associated with high-pressure, poor weather with low-pressure.
Surface winds in Northern Hemisphere converge CCW on low-pressure cyclones diverge CW from high-pressure anticyclones
Low (Cyclone) and High (Anticyclone) Divergence and Convergence These are not usually aligned vertically; a Low will follow its divergence aloft: good for storm forecasting