Climatology

Atmosphere and insolation

Insolation is incoming solar radiation, and it is the ultimate energy source for the atmosphere, the oceans, and life. The amount received at any place depends on latitude, season, length of day, angle of the Sun's rays, and the transparency of the atmosphere. At low latitudes, the Sun stands high in the sky, so its rays are concentrated on a small area and pass through a short column of atmosphere, losing little energy. At high latitudes, the rays strike obliquely, spread over a larger area, and travel through more atmosphere, so less energy reaches the surface. This is the fundamental reason for the equator-to-pole temperature gradient, and it drives the whole atmospheric circulation, since the atmosphere must transport heat from the surplus region near the equator toward the deficit region near the poles. Seasons arise not from changing distance to the Sun but from the tilt of the Earth's axis, which changes the angle of rays and the length of day through the year, giving each hemisphere summer when it tilts toward the Sun and winter when it tilts away.

Temperature at the surface is modified by several factors beyond latitude. Altitude lowers temperature, because air expands and cools as it rises and because the thin air at height absorbs less outgoing radiation; the normal lapse rate, the average rate at which temperature falls with height in the troposphere, is a useful reference, though the actual rate varies with humidity and weather. Distance from the sea moderates temperature, because water heats and cools slowly, so coastal places have smaller annual ranges than continental interiors at the same latitude. Ocean currents carry warm or cold water along coasts and modify the air above them. Prevailing winds bring air from warmer or colder regions. Cloud cover reduces daytime heating by reflecting insolation and reduces night-time cooling by trapping outgoing radiation. Aspect matters on slopes: in the northern hemisphere, south-facing slopes receive more direct sun than north-facing slopes. The heat budget of the Earth as a whole balances: incoming shortwave radiation is matched, over the long term, by outgoing longwave radiation, with some energy reflected by clouds, ice, and bright surfaces, an effect measured as albedo. A surface with high albedo, such as fresh snow, reflects much energy and stays cold; a dark surface absorbs more and warms.

Atmospheric circulation and cyclones

Atmospheric circulation is the large-scale movement of air that redistributes heat and moisture. It begins with pressure belts. Warm air rises near the equator, creating a belt of low pressure, the equatorial low, where air is moist and rain is frequent. Air that rises there moves poleward aloft, cools, and sinks in the subtropics, creating belts of high pressure where air is dry and deserts are common. Further poleward, air rises again in the subpolar lows, and sinks over the polar highs. Between these belts blow the planetary winds: the trade winds, blowing toward the equator from the subtropical highs; the westerlies, blowing toward the poles from the subtropical highs in mid-latitudes; and the polar easterlies, blowing away from the polar highs. The Coriolis force, arising from the Earth's rotation, deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere, so winds do not blow straight from high to low pressure but curve, and the trade winds and westerlies take their characteristic diagonal paths. Friction near the surface weakens this deflection, so surface winds cross isobars at an angle, while winds aloft blow more nearly parallel to them.

Jet streams are narrow, fast-flowing ribbons of air in the upper troposphere, strongest near the tropopause, that steer weather systems and mark the boundaries between air masses. The subtropical jet and the polar-front jet are the most important, and their seasonal shifts and waviness — the formation of Rossby waves, ridges, and troughs — help explain why mid-latitude weather alternates between settled and stormy spells. Monsoons are the most dramatic seasonal reversal of circulation: in summer, land heats faster than sea, pressure falls over the heated continent, and moist air flows in from the ocean, bringing rain; in winter, the pattern reverses, and dry air flows out from the cooled continent. The monsoon is therefore not simply a rain, but a whole seasonal wind system, and its strength depends on the land-sea temperature contrast, the position of the jet streams, and the state of the oceans.

Cyclones are low-pressure systems around which air circulates inward and upward. Tropical cyclones form over warm oceans where sea-surface temperature, moisture, and the Coriolis force are sufficient; they have a calm eye at the centre, violent winds around it, and torrential rain, and they weaken rapidly over land or cold water where their energy supply is cut off. Their damage comes from wind, storm surge along coasts, and flooding rain. Temperate or mid-latitude cyclones form along the polar front where cold polar air meets warm tropical air; they are larger, less violent at the centre, and structured around warm and cold fronts, with a warm sector between them. As a temperate cyclone matures, the cold front catches up with the warm front, forming an occluded front, and the system gradually weakens. Anticyclones are high-pressure systems with air sinking and diverging at the surface; they bring settled, often clear and calm weather, and in winter they can trap cold air and fog near the ground. Local winds, such as sea and land breezes, mountain and valley breezes, and named regional winds, arise from local temperature differences and relief, and they matter for local climate even though they do not appear on a global circulation map.

Climatic classification

Climatic classification groups the endless variety of local climates into types, so that places with similar temperature and rainfall regimes can be compared. Any classification must choose its criteria, and different systems choose differently, which is why the same place can carry different labels in different systems. The Koppen system, the most widely used, is empirical: it classifies climates by observed temperature and precipitation, using thresholds that correspond roughly to vegetation boundaries, since vegetation is a visible integrator of climate. Its main groups are tropical rainy climates, dry climates, warm temperate climates, cold temperate or continental climates, and polar climates, with further divisions by seasonal rainfall pattern and by temperature. A place with rain in every month and high temperature falls in a different group from a place with the same annual rainfall concentrated in a single season, and that distinction matters greatly for agriculture. The strength of Koppen is its simplicity and its use of readily available data; its weakness is that it describes the result of climate rather than its causes, so two places in the same Koppen type may have arrived there by different circulation mechanisms.

The Thornthwaite system takes a different approach, based on precipitation effectiveness and thermal efficiency, and on the water balance: how much water is available for plant growth after evaporation is accounted for. It therefore distinguishes climates not only by how much rain falls but by how much of it remains useful, which depends on temperature and on seasonal timing. A place with moderate rainfall but low evaporation may be effectively moist, while a place with the same rainfall but high evaporation may be effectively dry. Thornthwaite's system is more demanding of data and more complex to apply, but it is closer to the needs of agriculture and water management, and it shows why annual rainfall totals alone are a poor guide to aridity. In examination answers, the useful comparison is not which system is correct in the abstract, but what each system is for: Koppen for broad, vegetation-linked mapping from simple data, Thornthwaite for water-balance and agricultural questions where evaporation and seasonality matter. Both systems remind the student that climate types are tools, not natural kinds, and that boundary places will always fit imperfectly.

ENSO events

ENSO, the El Nino-Southern Oscillation, is the most important single source of year-to-year climate variability on Earth. Under normal conditions in the tropical Pacific, trade winds blow from east to west, pushing warm surface water toward the western Pacific and allowing cold, nutrient-rich water to upwell along the South American coast. Air rises over the warm western Pacific, producing rain, and sinks over the cooler eastern Pacific, producing drier conditions; this east-west loop is the Walker circulation. During an El Nino event, the trade winds weaken, warm water spreads eastward across the Pacific, upwelling weakens, and the zone of rising air and heavy rain shifts eastward with it. During La Nina, the opposite phase, trade winds strengthen, the western Pacific becomes even warmer relative to the east, and the normal pattern intensifies.

The Southern Oscillation is the atmospheric side of the same phenomenon: a seesaw in surface pressure between the eastern and western tropical Pacific, measured by the pressure difference between stations on either side of the basin. When the pressure difference weakens, El Nino conditions tend to follow; when it strengthens, La Nina conditions tend to follow. Because the tropical Pacific is so large and its heat so great, a shift there disturbs the atmosphere worldwide through teleconnections — distant, statistically linked changes in rainfall, temperature, and storm tracks. Some regions become wetter and others drier, monsoons may weaken or strengthen, and the frequency of tropical cyclones in different basins may change. ENSO does not cause every drought or flood, and its effects vary from event to event and interact with other patterns, but no account of interannual climate variability is complete without it. For geographers, ENSO is also a lesson in scale: a change in ocean temperature in one basin is not a local event, because the atmosphere connects the whole planet.

Meteorological hazards and climate change

Meteorological hazards are weather and climate events that threaten life, property, and livelihoods. Tropical cyclones threaten coasts with wind, storm surge, and rain; their danger is greatest where low-lying coasts are densely settled and where warning and evacuation are weak. Thunderstorms bring lightning, hail, and intense short bursts of rain that can cause flash flooding, especially in urban areas where surfaces are sealed and drainage is overwhelmed. Drought is a slower hazard, defined not by a single dry day but by a prolonged deficit of rainfall relative to need, and its impact depends on water storage, irrigation, crop choice, and social vulnerability as much as on the rainfall deficit itself. Heatwaves, cold waves, and fog each carry their own risks, from heat stress to transport disruption. Floods, though driven by rain, become hazards through the interaction of rainfall intensity, catchment shape, soil saturation, river management, and settlement on floodplains.

Climate change is a long-term shift in the statistical properties of climate, distinct from year-to-year variability such as ENSO. Evidence comes from several independent lines: rising average surface temperatures recorded by instruments, retreat of glaciers and ice sheets, rising sea level from thermal expansion and melting ice, shifts in rainfall patterns, and changes in the frequency and intensity of some extremes. The physical cause is well understood in outline: greenhouse gases absorb outgoing longwave radiation and re-emit part of it toward the surface, warming the lower atmosphere. Human activity has increased the concentration of these gases, above all through burning fossil fuels and through land-use change, so the present warming is driven primarily by human causes superimposed on natural variability. Consequences already observed and projected include sea-level rise threatening low-lying coasts and islands, changes in rainfall that worsen both drought and flood risk in different regions, stress on agriculture where temperature thresholds are crossed, and shifts in the distribution of species and disease vectors. Responses divide into mitigation, reducing emissions and enhancing carbon sinks, and adaptation, adjusting settlements, water management, agriculture, and infrastructure to the changes that are already under way or unavoidable. Geography contributes to both: mapping who is exposed, who is vulnerable, and where interventions will help most, rather than treating climate change as a uniform global average.

Tropical and temperate cyclones compared

Koppen and Thornthwaite compared