Weather and climate
Elements :- Precipitation, cloudness, temperature, humidty, wind, air pressure, wind speed and
direction.
Air is a mixture of discreet gases. Nitrogen 83%, oxygen 21%, other gases, CO2 is 0.036% and it absorbs
heat energy from earth.
Components of air:
Water vapour- about 4% of air volume, forms clouds and precipitation, absorbs heat energy from earth
Aerosols- tiny solid and liquid particles, water vapour can condense on solids, reflect sunlight and help
color sunrise and sunset.
Ozone- O3, absorbs harmful UV radiation, concentrated about 10 to 15 km above the surface and
human activity is depleting it by adding CFCs(chlorofluorocarbons)
Structure of atmosphere:
Pressure is the weight of air above. Average sea level pressure is more than 1000 millibars. Pressure
decreases with altitude.
Atmospheric layers:- Troposphere- Bottom layer. Temperature decreases with altitude. Thickness varies
and outer boundary known as tropopause. Stratosphere- temperature increases at the top,
stratopause. Mesosphere – temperature decreases, mesopause. Thermosphere – fraction of
atmosphere’s mass, gases moving at a high speed.
Earth-sun
Seasons – caused by earth’s changing orientation to the sun. Special days are summer solstice june 2122, rays located at tropic of cancer, longest day 73.5. Winter solstice dec 21-22, rays at tropic of
Capricorn, shortest day 26.5. Autumnal equinox sep 22- 23, rays at equator. Spring equinox march 2122,rays at the equator.
Atmospheric heating
Heat is always transferred from warmer to cooler objects. Conduction – through molecular
activity(handle). Convection – Mass movement within a substance, usually vertical motion(boiling
water).
Radiation- through vacuum(fire), consists of different wavelengths. Laws- all objects emit radiation,
hotter objects emit more than cooler ones, the hot ones have short wavelengths and good absorbers
are good emitters.
Atmosphere is largely transparent to incoming solar radiation. Effects are reflection-albedo, scattering
and absorption. Visible radiation reaches the surface, where 50% is absorbed.
Temperature measurement: Daily max and min. Human perception- anything that influences the rate
of heat loss in the body influences the sensation of temperature.
Important factors- Air temperature, Relative humidity, Wind speed and sunshine.
Land &water – Land heats and get hotter than water, lands cools and gets cooler than water.
Temperature maps: Isotherm – line connecting places of equal temperature. Temperature adjusted at
sea level, January and July represent extremes. In southern hemisphere, Isotherms are straighter and
more stable. They show ocean currents.
Moisture, cloud and precipitation
Calorie- heat needed to raise the temperature of one gram of water one degree celcius
Latent heat – stored or hidden heat, not derived from pressure change. To change state heat must be
absorbed or released. Latent heat of vaporization – 600 calories per gram of water are added.
Condensation- vapour changes to liquid, heat energy released –latent heat of condensation. Melting,
Freezing latent heat of fusion –heat energy is released. Sublimation – solid to gas, 680 calories per gram
of water added. Deposition- Vapour changed to solid heat is released.
Humidity- amount of water vapour in air. Saturated air is air that is filled with vapour to capacity. Warm
air has grater capacity, and vapour adds pressure to the air. Measure humidity- Mass of water vapor in
a unit of air compared to the remaining mass of dry air. Relative humidity -Ratio of the air’s actual water
vapor content compared with the amount of water vapor required for saturation at that temperature in
%. Saturated air – 100% relative humidity, content equals capacity.
Relative humidity changed by two ways: Add moisture raises relative humidity, removing moisture
lowers relative humidity, lower temperature raises relative humidity.
Dew point temperature – temp where parcel of air would need to be cooled to reach saturation.
Cooling the air below dew point causes condensation. WATER VAPOUR = 2G/KG * RH(%)=G/KG
Two types of hygrometer used to measure humidty: Pshychrometer- compares temp of wet buld and
dry bulb. Air saturated the both thermometers equal. Greater the difference btn thermometers, lower
humidity. Hair hygrometer – reads humidity directly.
Adiabatic temp changes occur when: Air is compressed- motion of air molecules increases, air will
warm. Air expands- air will cool, rising air will expand due to decreasing air pressure. Adiabatic rates:
dry- unsaturated air, rising air expands and cools, descending air is compressed and warms. Wetcommences at condensation level, air reaches dew point, condensation occurs and heat is released.
Process that lift air:
Orographic lifting – elevated terrains act as barriers, result in rainshadow dessert.
Frontal wedging- cool air act as barriers to warm air, fronts are part of the system called middle latitude
cyclones.
Localized convective lifting- occurs where unequal surface heating causes pockets of air to rise because
of their buoyancy. Convergence – where the air is flowing together and rising(low pressure).
Stability of air:
Stable air- resists vertical displacement(cooler & denser than surrounding air, wants to sink), No
adiabatic cooling, adiabatic stability occurs when the environmental lapse rate is less than the wet
adiabatic rate. Widespread clouds with little vertical thickness, precipitation is light to moderate.
Absolute instability- Acts like a hot air balloon, rising air(warmer & less dense than surrounding, rise till
raches altitude with equal temp), adiabatic cooling, environmental lapse rate is greater than the dry
adiabatic rate, conditional instability when atmosphere is stable for an unsaturated parcel of air but
unstable for a saturated parce.
Condensation:- vapor to liquid forming dew, fog. Vapor requires surface to condense on like grass or
window, condensation nuclei in the atmosphere like dust and smoke.
Clouds:- made of millions water droplets or tiny crystals of ice. Classification : Form: Cirrus- high, white,
thin. Cumulus- associated with fair weather. Stratus- sheets or layers that cover much of the sky.
Height: high clouds 6000(cirrus, cirrostratus, cirrocumulus), middle clouds(altostratus and altocumulus),
low clouds 2000(stratus, stratocumulus, and nimbostratus).
Fog- atmospheric hazard. Cloud with base near the ground. Form because of radiation cooling or
movement of air over cold surface. Types of Fog: Cooling: Advection fog- warm moist air moves over
cool surface. Radiation fog- earths surface cools rapidly, forms during cool, clear, calm nights. Upslope
fog- humid air moves up slope, adiabatic cooling occurs. Evaporation: Steam fog- Cool air moves over
warm water and moisture is added to the air, steaming appearance. Frontal fog- Forms during frontal
wedging when warm air is lifted over colder air, rain evaporates to form fog.
Precipitation:
Formation: Bergeron process- temperature in cloud is below freezing, ice crystals collect vapor, large
snowflakes form and fall to the ground or melt during descent and fall as rain. Collision-coalescence
process- warm clouds, large hydroscopic condensation nuclei, large droplets form, droplets collide with
other droplets during decent. Forms: Rain and drizzle, snow, sleet(occurs when warmer air overlies
colder, rain freezes as it falls), glaze, hail(hard rounded pellets, occurs in large cumulonimbus clouds,
pellets fall to the ground when they become too heavy), rime(freezing of supercooled fog). Measuring
precipitation: standard rain gauge for rain, depth or water equivalent for snow.
Air pressure and wind
Atmospheric pressure- force exerted by the weight of the air above. Decreases with increasing altitude.
Units in millibars(mb). Measured by barometer.
Wind- Horizontal movement of air from high pressure to low pressure.
Controls of wind: Pressure gradient force- Isobars( lines of equal air pressure), pressure
gradient(pressure change over distance). Coriolis effect- deflection in the wind due to earths rotation,
right in northern hem and left in the southern hem. Friction- surface slows air movement.
Upper air winds: generally blow parallel to isobars(geostrophic winds), jet streams
Cyclones and anti-cyclones:
Cyclone- center of low pressure, pressure decreases towards center. Winds associated with cyclones:
northern hem- inward(convergence), counterclockwise. Southern hem- inward(convergence),
clockwise. Often bring clouds and precipitation. Anti-cyclone – center of high pressure, increases
towards the center. Winds associated with anti-cyclone: Northern hem- outward(divergence),
clockwise. Southern hem- outward(divergence), counterclockwise. Associated with subsidizing air.
General atmospheric circulation:
Idealized global circulation: equatorial low pressure zone- rising air, abundant precip. Subtropical high
pressure zone- subsidizing, stable air, trade winds(air from subtropical high to equatorward), westerly
winds(air from subtropical high to poleward). Subpolar low pressure zone- warm and cool winds
interact. Polar high pressure zone- cold subsidizing air, easterly winds(air spread equatorward), polar
front(polar easterlies collide with the westerlies). Influence of Continents: Obvious in the northern
hem- monsoon(occur over continents, warm)
Circulation in mid-latitudes: Air flow is interrupted by cyclones.
Types of wind: Land breeze(night)- land cools faster, so does air above it, high pressure cause wind to
blow to sea. Sea breeze(day)- land heats up faster, air above it expands, low pressure cause wind to
blow from the sea. Mountain breeze(night- cool air) & valley breeze(day-warm air).
Wind measurement: direction(wind vane), speed(cup anemometer).
El nino: A countercurrent that flows southward along the coasts of Ecuador and Peru. Warm and
appears during the Christmas season. Pressure changed between the eastern and western Pacific called
the Southern Oscillation. Effects are highly variable depending in part on the temperatures and size of
the warm water pools.
La nina: Opposite of el nino. Triggered by colder than average surface temperatures in the eastern
Pacific. Typical la nina winter: Blows colder than normal air over the Pacific Northwest and northern
Great Plains while warming much of the rest of the United States. Greater precipitation is expected in
the Northwest. Influence on the state of weather and climate almost everywhere.
High pressure regions: Subsidising air, divergent winds, dry conditions. Low pressure regions: Ascending
air, converging winds, ample precipitation.
Large landmasses in the middle latitudes often have less precipitation toward their centers. Mountain
barriers also alter precipitation patterns- Windward slopes receive abundant rainfall from orographic
lifting, Leeward slopes are usually deficient in moisture.
Weather patterns and severe storms
Air masses:- Large body of air, similar temperature and moisture at any given altitude. Source region –
the area where an air mass acquires its properties. Classification of air mass:- By latitude: Polar(high
latitudes, cold), Tropical(Low latitudes,warm). By natue of surface: Continental(form over land, dry),
maritime(form over water, humid air). Four basic types of air masses: Continental Polar, Continental
tropical, Maritime polar, Maritime tropical. cP(north America)- winter(cold dry air), summer(cool
relief), lake-effect snow(air picks up moisture from lakes). mT(North America)- warm, moist, unstable
air. cT(southwest and mexico)- hot,dry. mP- brings precipitation to western mountains.
Fronts:- Boundary that separates air masses of different masses. Types of fronts: Warm Fronts- Warm
air replaces cool air, line with semicircles, clouds become lower as the front nears, light precipitation.
Cold Fronts – Cold air replaces warm air, line with triangles, faster than warm front, high precipitation
intensity. Stationary fronts- Flow of air on both sides of the front is almost parallel to the line of the
front, position doesn’t move. Occluded front- Cold front overtakes warm one, cold air wedges the warm
air, precipitation occur due to warm air rise.
Middle-latitude cyclone: Life cycle- form along a front where air masses are moving parallel to the front
in opposite directions, flow of air is counterclockwise cyclonic circulation, warm and cold front forms,
cold front catches up to warm front, pressure gradient weakens and fronts discontinue. Middle-latitude
cyclones move eastward across the United States. Role of air aloft: Cyclones and anticyclonesgenerated and maintained by upper-level air flow. Cyclone- low pressure system. Anticyclone- high
pressure system.
Thunderstorms: Features- Cumulonimbus clouds, heavy rainfall, lightning. Occurrence-Florida. Stages of
dev:- require- warm moist air, instability, surge causes air to rise higher, precipitation forms and cooling
after.
Tornadoes: Features- violent windstorm, rotating column of air that extends down from a
cumulonimbus cloud, low pressures inside causes the air to rush into the tornado. Occurrence and devMost frequent from April through June, associated with severe thunderstorms, conditions for
formation(occur most often along a cold front, associated with huge thunderstorms called supercells).
Forecasting- Tornado watch and warning when sighted.
Hurricanes: Violent storms, speed excess of 119 km/h. Typhoons(western pacific), cyclones(indian
ocean). Parts- Eyewall(near the center, rising air, heaviest rainfall), Eye(very center, precipitation, wind
subsides, air gradually descends, warmest part of the storm). Formation and decay- all tropical waters
except south atlantic and eastern sout h pacific, energy from condensing water vapor, diminish in
intensity(move over cooler clean water, move onto land), factors that affect amount of hurricane
damage(strength of storm, size and population density, shape of the ocean bottom), inland damage
from torrential rains.
World Climates and Global Climate Change
Climate is an aggregate of weather. Koppen classification of climates: Uses mean monthly and annual
values of temperature and precipitation. Five principle climate groups: Humid tropical A, Dry B, Humid
middle-latitude with mild winters C, Humid middle-latitude with severe winters D, Polar E. Precipitation
is B. Humid tropical A- Tropical wet and dry(Poleward of wet tropics and equatorward of the tropical
deserts, tropical grassland, rainfall). Dry B- Evaporation exceeds precipitation and there is a constant
water deficiency, boundary determined by annual precipitation and temperature, causes of desserts and
steppes. Humid middle-latitude with mild winters C- humid subtropics, marine west coast, dry summer
subtropics. Humid middle-latitude with severe winters D- Land-controlled climates. Polar E- Enduring
cold, meager precipitation, ice cap climate EF.
Climate change: Paleoclimate- Direct temperature measurement from boreholes.
Ice cores- provide oxygen from gas bubbles, oxygen has two stable isotopes 18 and 16
Human impact on global climate: clearing land for agriculture(urbanization), using fire, overgrazing for
marginal land. Global warming: Water vapor and carbon dioxide absorb heat and are largely responsible
for the greenhouse effect of the atmosphere, burning fossil fuels has added great quantities of carbon
dioxide to the atmosphere. Global temperature have increased.
Feedback mechanisms- possible outcomes of altering the climate systems. Positive feedback
mechanisms- reinforce the initial change. Negative - produce results that are just the opposite of the
initial change and tend to offset it.
Consequence of global warming: Altered distribution of the world’s water resources and the affect on
the productivity of agricultural regions. Rise in global mean sea level. Changing weather patternshigher frequency and intensity of hurricanes, shifts in the paths of large-scale cyclonic storms, changes
in frequency and intensity of heat waves and droughts.