METEOROLOGY

GEL-1370

Chapter ThirteenChapter Thirteen

Global ClimateGlobal Climate

What we are going to learn?

We are going to learn the answers to the following questions– What are the factors that regular global climate?– How climates are classified?– What are the different types of climate?– What are micro-, macro- and meso-climates?

Climate

• Climate: Average of the day to day weather over a long duration• Microclimate: Small climatic region near or on the ground• Mesoclimate: Climate of a small area of the earth’s surface• Macroclimate: Climate of a much larger area (state or a country)• Global Climate: Climate extending over the entire earth• Two important weather elements: temperature & precipitation• Climate Controls are:

– Intensity of sunshine and its latitudinal variations– Ocean currents and thermohaline circulation systems– Relative distribution of land and water– Wind intensity and direction– Position of high- and low-pressure areas– Mountain barriers– Altitude

Global Temperatures

Global temperatures – contd.

• Isotherms are oriented East-West (same latitude locations receive same amount of solar energy)

• Bending of isotherms along the coastal margins is partly due to the unequal heating & cooling of land & water

• Variation in temperature between summer and winter will be far greater over continental interiors than along the west coastal margins of continents; climates of interior continental regions will be more extreme (higher summer temp and lower winter temp) than their counterparts in the west coast; west coast climates are typically quite mild for their latitude

• Highest mean temperatures occur in the tropics, but rather in the subtropical deserts of the Northern Hemisphere

Global Temperatures – contd.• Lowest mean temperatures occur over large land

masses at high latitudes; coldest areas of the world is the Antartic

• Snow and ice reflect ~80% of the sunlight that reaches the surface; much of the unreflected solar energy is used to transform the ice and snow into water vapor

• Southern hemisphere is cooler than Northern Hemisphere because:– Cold Antarctic

– Polar regions of the southern hemisphere reflect more incoming sublight

– Less land area found in the tropical and subtropical areas

Global Precipitation

• Global distribution of precipitation is closely tied to the general circulation of the atmosphere and to the distribution of mountain ranges & high plateaus

• Equatorial regions are typically wet, while the subtropics and the polar regions are relatively dry

• Precipitation is most abundant where the air rises; least abundant where it sinks

• In tropical regions, the trade winds converge along the ITCZ producing rising air --- heavy precipitation all year long; near 30 deg, the sinking air of the subtropical highs produces a ‘dry belt’ around the globe – presence of Sahara Desert of North Africa in this region

Annual global pattern of precipitation

Global Precipitation – contd.

• In the cold air of the polar regions there is little moisture --- winter storms drop light, powdery snow that remains on the ground for a long time because of the low evaporation rates; in summer a ridge of high pressure tends to block storm systems that would otherwise travel into the area --- precipitation is low in the arctic

• Heaviest precipitation is concentrated in the eastern states, as moisture from the Gulf of Mexico moves northward; cities on the plains receive more rainfall in summer ; those on the west coast have maximum precipitation in winter

• Cities in the midwest and East usually have abundant rainfall all year long

Annual global pattern of precipitation

Vertical cross section along a line running N-S illustrating mian global regions of rising and sinking air

Global climate – contd.

Hottest places on earth generally occurs in the subtropical deserts of the Northern Hemisphere, where clear skies and sinking air

The coldest places on earth tend to occur in the interior of high-latitude land masses; The coldest areas of the Northern Hemisphere are found in the interior of Siberia and Greenland; coldest area of the world is Antarctic

Wettest places in the world are located on the windward side of mountains where warm, humid air rises upslope

Variations in annual precipitation for three Northern Hemisphere cities

Effect of topography on average annual precipitation along a line from the Pacific Ocean through CA and western Nevada

Climatic Classification – The Köppen System• Similarity in climate (temperature and precipitation) in an

area forms the basis for classification• Köppen related distribution & types of vegetation to

climate; five major climatic types designated by A (tropical moist Climate), B (dry climate), C (moist mid-latitude climates with mild winters), D (moist mid-latitude climates with severe winters) & E (polar climates)

• Tropical Moist Climate (A): Monthly average temp above 18°C; no real winter season and year around warm temp.; abundant rainfall (>150 cm); extends from equator to ~15-25 ° N or S;

• Examples: Amazon lowland of South America, the Congo River Basin of Africa, and the East Indies from Sumatra to New Guinea;

Key to map of precipitation records thoughout the world

• Key

Some precipitation records.

Type – A Climatic zone – contd.

• High temperature and abundant rainfall --- dense, broadleaf, evergreen forest – tropical rain forest with vegetation; abundant sunlight allows for the growth of tangled shrubs forming impenetrable jungle

• Major types are tropical wet (Af) and tropical monsoon (Am) and tropical wet and dry (Aw);

• Tropical wet climate: seasonal temperature variations are < 3° C; greater variation in temperature between day & night (average high ~32°C & average low ~22°C); towering cumulus clouds form every day to produce heavy, localized showers in the afternoon; annual rainfall > 150 cm (windward side can exceed 400 cm/yr); high humidity & cloud cover tend to keep maximum temp from reaching very high values

Temperature and Precipitation data for Peru (latitude 4°S; tropical wet climate, Af)

Climate A Type – contd.

• In Peru, monthly rainfall variations are more than the temperatures --- due to migrating position of the intertropical convergence zone (ITCZ) & associated wind patterns

• Laterite: A soil formed under tropical conditions where heavy rainfall leaches soluble minerals from the soils leaving soil hard and poor for growing crops

• Tropical Monsoon Climate: When precipitation totals drop below 6 cm for perhaps one or two months; a tropical climate with a brief dry period of perhaps one or two months; examples coasts of southeast Asia, India, and in northeastern South America

Climate A Type – contd.

• Tropical wet and dry climate (Aw): Distinct dry season prevails; annual rainfall >100 cm, but monthly rainfall for more than 2 months is <6 cm --- rainforests cannot survive to this ‘drought’ giving rise to coarse savanna grass, scattered with low, drought-resistant deciduous trees

• Examples of Aw climate are: western Central America, north and south of the Amazon Basin (South America), southcentral Asia and northern Australia

• Dry Climate: Evaporation (E) + Transpiration (T) > Precipitation (P)

Climate data for Timbo, Guinea (11°N) – tropical wet-and-dry climate (Aw)

Dry Climates (Group B)• Deficient Precipitation [(E+T) > P] and irregular• Subtropical deserts extend from ~15-30° (often surrounded by

mountains)• ~26% of land area falls in this group• Major types: Arid (BW; 12% of the land) and Semi-arid (BS; 14%

of the land)• Arid region is divided as follows: a) BWh or BSh: Climate is hot &

dry with a mean annual temp of above 18°C; b) BWk or BSk: Climate is cold (in winter) and dry with mean annual temp < 18°C

• Examples for Arid region: West coast of South America, Africa and over much of the interior of Australia

• In North America, arid climate extends from northern Mexico into the southern interior of the US

• Most of the native plants are xerophytes (capable of surviving prolonged periods of drought)

Rain streamers (virga) in dry climates, as falling rain evaporates into the drier

Bushes and cactus found in the arid southwestern American deserts (BWh)

Dry climate – B – contd.

• In low-latitude deserts (BWh), maximum daytime during the summer can >50°C (40-45°C; (104-113°F) are more common); RH in the middle of the day 5-25%; minimum temp can drop below 25°C

• Mid-latitude deserts (BWk): Summer maximum temp reaches 40°C (104°F); winters are extremely cold with minimum temp below -35°C (-31°F); lie in the rain shadow of an extensive mountain chain (Sierra Nevada and Cascade mountains in North America), Himalayan mountains and Andes in South America

• Around the margins of the arid regions, where rainfall amounts are greater, the climate is gradually changes into semi-arid (BS); examples: Most of the Great Plains, southern coastal sections of California, & northern valleys of the Great Basin; annual precipitation 20-40 cm; examples are: Denver, Colorado

Arid (BWh) Climate – Phoenix, AZ (33.5°N)

Cumulus clouds in a semi-arid climate (BS) in Western North America

Climatic data for Denver, CO (40°N) – a semi-arid climate (BSk)

Moist subtropical mid-latitude climates (Group C)

• Humid with mild winters with the average temp of the coldest month -3°C to 18°C (27-64°F); commonly found on the eastern and western regions of most continents, 25-40° latitude

• Major types: Humid subtropical (Cfa), marine (Cfb) and dry-summer subtropical or Mediterranean (Cs)

• Humid subtropical climate (Cfa): Found principally along the east coasts of continents, ~25-40° latitude; summer is hot and muggy; summer dew-point temp and RH are high (exceeding 23°C or 73°F); Mobile, AL

• In winters, frost, snow, and ice storms are more common, but heavy snowfalls are rare

• Rainfall: 80-165 cm/yr, fairly well distributed in an yr

Climatic data for Mobile, Alabama (30°N) – a humid subtropical climate (Cfa)

Moist subtropical - Marine

• Marine (Cfb): Along western side of most continents from 40 to 60°; winds from the oceans moderate the climate; summers are quite cool

• The low annual temperature range in high latitude region – ocean’s influence keeps daily temp ranges low

• Coastal Mediterranean Climate: Precipitation: 30-90 cm/yr; summer afternoon temp climb above 34°C(93°F) and occasionally above 40°C (104°F);

• San Francisco, CA & Sacremento, CA is only 130 km apart (inland) and average July temp is 9°C (16°F) higher

Climatic data for Port Hardy, Canada (51°N) – marine climate

Comparison of a coastal Mediterranean climate (Csb; San Francisco) with an interior

Mediterranean climate (Csa) Sacromento

Moist Continental Climates (Group-D)

• Warm-to-cool summers and cold winters (warmest month average temp >10°C (50°F) & coldest monthly average temp drops below -3°C (27°F); D climates are controlled by large land masses; found only in the Northern hemisphere; D climates extend across North America and Eurasia (40° - ~70°)

• Major types: Humid continental with hot summers (Dfa; long and hot summer; mid-day temp exceed 32°C (90°F; nights are usually warm & humid)), humid continental with cool summers (Dfb; less humid; temp can exceed 35°C (95°F)) and subpolar (Dfe)

Comparison of a humid continental hot summer climate, Dfa (Des

Moines) with a humid continental cool summer climate (Winnepeg)

Subpolar climate (Dfc) – Fairbanks, AK (65°N); winters are severe, summers are short & cool, 1-3 months exceed mean

temp of 10°C (50°F)

Polar Climate – Group E

• Year-round low temp (average temp of the warmest month < 10°C (50 °F), but above freezing;

• Two major types: Polar tundra (ET) and Polar ice caps (EF)

• Polar tundra: ground is permanently frozen (Permafrost)• Annual precipitation < 20 cm (in lower latitudes, would

constitute desert, but in the cold polar regions, evaporation rates are very low and moisture is adequate

• Polar ice cap: Temp for every month is below freezing, plant growth is impossible & the region is covered with snow and ice – Greenland and Antartica; precipitation < 10 cm/yr

Climate data for Barrow, Alaska (71°N) – polar tundra climate (ET)

Highland Climates (Group H)

• Varying climate with altitude – 300 m elevation is equivalent to horizontal traveling 300 km northward (3°N)

• Ascending on a mountain, one can travel through many climatic regions in a relatively short distance

• Elevation in the central Sierra Nevada: Base of the mountain: semi-arid conditions; foothills: Mediterranean and the vegetation changes; higher elevations: subpolar and vegetation changes; near the summit, permanent patches of ice and snow, with some glaciers (in less than 4 km), the climate has changed from semi-arid to polar

Climatic data for Eismitte, Greenland (71°N); polar ice cap climate; ~ 3000 m above MSL

Vertical view of changing vegetation and climate due to elevation in the central Sierra Nevada

Chapter Summary• Micro-, meso-, and macro-climate with examples• Rainshadow desert and rainiest places• Rainforests exist in what climatic conditions?• What is a dry summer? Explain why it is a dry summer• Lowest temperature and precipitation in the world • Koppen classification system – type A, B• Monsoon• Characteristics of tropical wet climate, wet-and-dry climate• Most abundant climate type in the planet• Humid Subtropical climate; mediterranean climate; D-type

climate where it is found in US; • Which climate type normally has the larges and smallest annual

range in temp; subpolar temperature; polar ice cap climate; tundra climate;