Climatology is the study of weather patterns over long times and the delineation of distinct climate regions.
To classify climate regions consideration must be given to the principal elements of climate:
insolation
temperature
pressure
air masses
precipitation
as well as humidity, seasonality of rainfall, winds, cloud cover, and types of weather disturbances.
Climate classification could be based on
1) causative factors such as latitude, elevation, interaction of specific air masses.
Such a climate classification system would be termed a genetic classification.2) observed effects such as temperature and precipitation data.
This type of climate classification system would be termed an empirical classification.
One of the most widely used climate classification systems is the one developed by Wladimir Köppen and his student Rudolph Geiger, introduced in 1928 and revised thereafter.
The Modified Köppen-Geiger system is based on:
average montly temperatures
average monthly precipitation
total annual precipitation
It doesn't consider winds, temperature extremes, precipitation intensity, amount of sunshine, cloud cover, net radiation, or any causative factors such as latitude or elevation. Yet it is simple, based on readily available date and few rules, and presents a reasonable description world climates.
Modified Köppen-Geiger Climate Categories
First the world is subdivided into these major categories.
A - Tropical (equatorial - between the Tropic of Cancer and Tropic of Capricorn)
B - Dry (arid desert and semiarid grassland [steppe] climates)
C - Mesothermal (humid subtropical, Mediterranean, and marine west coast climates)
D - Microthermal (humid continental and subarctic climates)
E - Polar
H - Highland
All but B, the dry climates, are based only on average temperatures. Categories A through E generally follow a latitudinal pattern, A at the equator through E at the poles. However the pattern is more complicated in many regions because of effects such as rain shaddows and the influence of cool ocean currents offshore.
Each category is broken down into specific climate types:
A - Tropical Climates (average temperature of coolest month > 18 °C (64 °F))
Af - tropical rain forest, wet all year; all months have more than 6 cm / 2.4 in rain
Am - tropical monsoon, short dry season and a very wet rainy season
Aw - tropical savanna, mixed grassland/woodland climates, wet summer, dry winter
B - Dry Arid and Semiarid Climates (precipitation < potential evaporation & transpiration from vegetation)
BWh - hot, low-latitude deserts, (mean annual temp > 18 °C (64 °F))
BWk - cold, mid-latitude deserts, (mean annual temp < 18 °C)
BSh - hot, low-latitude steppe, (mean annual temp > 18 °C)
BSk - cold, mid-latitude steppe, (mean annual temp < 18 °C)
C - Mesothermal Climates (average temperature of coldest month between 0 °C and 18°C, warmest month > 10 °C, seasonal climates)
Cfa, Cwa - humid subtropical, hot summer (f: moist all year; w: dry winter)
Cfb, Cfc - marine west coast, mild winters, cool summers
Csa, Csb - Mediterranean dry-summer (s=summer drought, a=hot summer, b = mild summer)
D - Microthermal (cool temperate to cold: average temp of warmest month > 10 °C, coldest month < 0 °C)
Dfa, Dwa - humid continental, hot summer (warmest month > 22°C (71.6 °F); f: moist all year; w: dry winters)
DFb, Dwb - humid continental, mild summer (warmest month < 22°C)
Dfc, Dwc, Dwd - subarctic, cool summers, cold winters (c: 1-4 months > 10 °C; d: coldest month < -38 °C)
E- Polar Climates (always cold, warmest month below 10 °C)
ET - tundra (warmest month 0-10 °C, snow covered 8-10 months)
EF - ice cap (warmest month < 0 °C, polar regions)
EM - polar marine (all months > -7°C, warmest month above 0 °C)
H- Highland Climates
Because of the decrease in temperature with increasing elevation, the slopes of mountains may repeat the climate regions poleward of the location of the mountain. For example, a large mountain range whose base is in a humid subtropical climate region may have the equivalents of humid continental, subarctic, tundra, and polar ice cap climates at progressively higher elevations.
Climate Differences: Comparing the East and West Coasts of Continents
East coasts are more humid and receive more rainfall because of warm ocean currents along the coast. Midlatitude cyclones pull in warm, humid maritime tropical air (mT) from over the warm currents. East coasts are colder in winter because the prevailing winds bring cold/dry continental polar (cP) air from the interior.
West coasts are drier because of cold ocean currents offshore. Midlatitude cyclones and the prevailing westerlies carry cool air, which can't hold as much moisture, from off the cold ocean. West coasts are cooler in the summer than east coasts because of cool air coming from over the cold currents. West coasts are milder in winter because of cool, somewhat humid, maritime polar (mP) air coming off the ocean (which is not as cold as the continental polar air from the interior).
Idealized West Coast Climate Pattern
ET tundra
Dfc subarctic
Cfb marine west coast - cool summer
Csa Mediterranean - hot summer
BSh steppe
23.5 °N BWh desert
BSh steppe
Aw savanna
equator - - - Af - - - - rainforest - - - - - - - - - - - - - - - -
Aw savanna
BSh steppe
23.5 °S BWh desert
BSh steppe
Csa Mediterranean - hot summer
Cfb marine west coast - cool summer
Dfc subarctic
ET tundra
Idealized East Coast Climate Pattern
ET, EF tundra, polar ice cap
Dfc, Dfd subarctic
Dfb humid continental - mild summer
Dfa humid continental - hot summer
Cfa humid subtropical - moist all year
Aw tropical savanna
equator - - - Af - - - - - rainforest - - - - - - - - - - - - - - - - -
Aw tropical savanna
Cfa humid subtropical - moist all year
Dfa humid continental - hot summer
Dfb humid continental - mild summer
Dfc, Dfd subarctic
ET, EF tundra, polar ice cap
The climates described above are only averages over many years. There are variations in climate from year to year. Some years are wetter than average, some years are drier, some winters are colder, some summers are hotter. There are also cyclic variations in climate, some short term, on the order of a decade or less per cycle (El Nino), some very long term, on the order of tens of thousands of years (glacial cycles). Ongoing human-induced climate change has been in the works for over a hundred years and will likely affect climate for decades or centuries to come (global warming).
El Niño - Southern Oscillation (ENSO)
Every few years ocean circulation and atmospheric pressure patterns in the tropical Pacific Ocean are disrupted. The eastern tropical Pacific warms, preventing the upwhelling of cold, nutrient rich waters. Fish catches plumet. This frequently occurs around Christmas (the celebration of the birth of the baby Jesus, El Niño). Strong low pressure that normally lies over the western Pacific migrates to the central and eastern Pacific causing rains to diminish in the western Pacific region. This has caused drought and raging wildfires in Australia during past El Niños. Abnormally high rainfall and flooding may occur in California because of warming of the eastern Pacific.
Global Warming
The Earth's temperature has risen by a significant amount during the last 120 years. Part of that warming is probably caused by natural variations in the Earth's energy budget, for example, slight variations in the sun's output as inferred from the sunspot cycle. But there is a concensus among climate scientists that a significant portion of that warming is the result of increasing greenhouse gases in the atmosphere resulting from human activities.
Greenhouse Gases (absorb infrared radiation, thereby keeping the atmosphere and the Earth's surface warm)
Carbon dioxide (CO2) content in the atmosphere has increased from 280 parts per million (ppm) in 1888 to more than 370 ppm today largely as a result of industrialization and burning of coal, oil, and natural gas.
Methane (CH4) content in the atmosphere is rising 1% per year largely because of agriculture: dairy cattle and flooded rice fields.
Chorofluorocarbons (CFCs), in addition to damaging the ozone layer are probably also responsible for a signifanct share of human-caused global warming.
Warming, Present and Future
Global average temperature rose approximately 0.8 °C (~ 1.4 °F) between 1880 and 1995. The average global temperature is warmer than any time during the past 500 years and perhaps much longer.
To predict the amount of warming that may occur during the next century requires a large and complex computer program called a general circulation model (GCM). These require enormous computer power. Not even today's supercomputers are capable of doing the work without some short-cuts. Present GCMs are imperfect tools but improving.
The current best GCM projections for the amount of warming in the next century indicate that global temperatures will be 2 to 3.5 °C (3.6 to 6.3 °F) by the year 2100. But warming will be less than the average in the tropics and much more than the average at high latitude. So, high latitude ecosystems and the polar ice caps will be hard hit.
Consequences of Global Warming
Warming causes sea level rise by melting of glaciers, returning the meltwater to the ocean, and by thermal expansion of the oceans as they warm. Sea level rose about a half foot or so during the 20th century and is predicted to rise a 2 to 3 feet during the 21st century, perhaps much more if ice sheets in Antarctica and Greenland become unstable. Rising sea level is and will continue to cause increasing erosion and flood damage to the coastline during storms. Eventually, low-lying cities, and even entire countries like the Netherlands, Bangladesh, and the Pacific Islands will be in danger of inundation.
Other negative consequences include:
Ecosystems losses as climate shifts faster than ecosystems can migrate.
Changing rainfall patterns will make some areas drier, and crops less reliable, while other areas will become wetter, turning marginal farmland into productive grain belts.
Tropical diseases like malaria could expand their range.
Tropical cyclones (hurricanes) will increase in intensity as ocean water warms.