Temperature, precipitation, and runoff are different expressions of the same coupled climate system. Radiation supplies energy, circulation redistributes heat and moisture, and water moves among ocean, atmosphere, ice, soil, groundwater, rivers, and living things.

Circulation patterns vary naturally

Large-scale circulation is not fixed. Several recurring patterns redistribute heat and moisture without creating or destroying them.

ENSO

The El Niño–Southern Oscillation is a coupled ocean–atmosphere pattern in the tropical Pacific.

Under neutral conditions, easterly trade winds push warm surface water toward the western Pacific. The thermocline is deep in the west and shallow in the east, allowing cold nutrient-rich water to upwell near South America. Warm western-Pacific water supports rising air and heavy rain, while air returns eastward aloft as part of the Walker circulation.

During El Niño, the trade winds weaken, warm water and tropical convection shift eastward, the eastern thermocline deepens, and normal upwelling weakens. During La Niña, the trades and east–west contrast strengthen, bringing enhanced eastern-Pacific upwelling and cooler central/eastern tropical water.

ENSO changes the probabilities of rainfall, drought, storm tracks, fisheries conditions, and global temperature, but it does not determine one location’s weather. Events differ in strength and spatial pattern, and the oscillation is irregular rather than clock-like.

PDO, NAO, and AO

The Pacific Decadal Oscillation is a broad North Pacific sea-surface-temperature and wind pattern that can persist for years. In its positive phase, the central North Pacific is typically cooler while the eastern and coastal rim is warmer; the negative phase approximately reverses that pattern. These phases do not mean a uniformly warm or cold Pacific. ENSO, atmospheric variability, ocean memory, and long-term change can all contribute to the index.

The North Atlantic Oscillation describes variation in the pressure contrast between the Icelandic Low and Azores High. A positive phase usually means a stronger pressure gradient and stronger Atlantic westerlies; a negative phase usually means weaker westerlies and a shifted storm track.

The Arctic Oscillation is a broader hemispheric contrast between Arctic and midlatitude pressure. A positive phase tends to confine cold air more strongly toward the Arctic; a negative phase allows a weaker or more distorted circumpolar flow. NAO and AO overlap, but the NAO emphasizes the Atlantic sector. Neither index proves that a particular local event had one cause.

Temperature is not heat

Temperature is an intensive property that describes thermal state and determines the direction of spontaneous heat transfer. It is related to microscopic molecular energy, but it is not the total energy stored in an object.

Heat is energy transferred because of a temperature difference. It flows spontaneously from higher to lower temperature. Two objects can have the same temperature while containing very different internal energy because their mass, material, or phase differs.

Sensible heat changes temperature without changing phase. Latent heat is absorbed or released during melting, freezing, evaporation, condensation, sublimation, or deposition.

Why the warmest hour comes after noon

Insolation usually peaks near local solar noon, but surface and air temperature commonly peak later. The surface keeps warming as long as total energy gains exceed total losses. In the early afternoon, absorbed solar radiation can still be greater than emitted longwave, sensible, latent, and ground-heat losses even though sunlight has started to decline.

Temperature reaches its maximum when gains and losses become equal. It then falls as losses exceed gains. The daily minimum commonly occurs near sunrise after a full night of net cooling. The often-quoted 3 p.m. maximum is only typical: clouds, fronts, sea breezes, wind, soil moisture, snow, and urban materials can shift the timing.

Six controls on temperature

Latitude

Solar altitude generally decreases poleward, spreading sunlight over a larger area and increasing its atmospheric path. Seasonal changes in day length also grow with latitude. Latitude creates the broad equator-to-pole gradient, but elevation, oceans, currents, winds, clouds, and land cover produce large departures.

Elevation

Tropospheric temperature decreases with height by an average environmental lapse rate of about

$$6.5\,^{\circ}\mathrm{C\,km^{-1}}.$$

If downtown Montreal is $24\,^{\circ}\mathrm{C}$ at 50 m and Mont Tremblant’s summit is at 875 m, the height difference is $0.825\ \mathrm{km}$. An average estimate is

$$24-(6.5)(0.825)\approx18.6\,^{\circ}\mathrm{C}.$$

The real lapse rate varies and can reverse in an inversion. Mountains are not literally always cooler than adjacent lowlands at every moment.

Land–water contrast

Water changes temperature more slowly than land because:

1. evaporation spends energy as latent heat;
2. sunlight penetrates below the water surface;
3. water has a high specific heat, about $4.18\ \mathrm{J\,g^{-1}\,K^{-1}}$;
4. waves, convection, and currents mix heat through depth and move it horizontally.

A representative dry soil or rock may have a specific heat near $0.8\ \mathrm{J\,g^{-1}\,K^{-1}}$, though real soil values vary with composition and moisture. The energy that warms 1 kg of water by $1\,^{\circ}\mathrm{C}$ could warm 1 kg of such dry material by roughly

$$\frac{4.18}{0.8}\approx5.2\,^{\circ}\mathrm{C}.$$

A maritime climate therefore has milder winters, cooler summers, and a smaller annual range. A continental climate has larger seasonal extremes. Vancouver and Winnipeg sit near the same latitude, but ocean influence makes Vancouver much milder.

Ocean currents

Warm currents transfer heat to the atmosphere and can raise coastal temperatures; cold currents cool nearby air and often encourage stable marine layers and fog. Currents also influence humidity and precipitation. Western Europe’s mild climate reflects ocean heat storage, North Atlantic currents, prevailing westerlies, and atmospheric circulation together—not the Gulf Stream alone.

Prevailing winds

At midlatitudes, westerlies often give west coasts air that recently crossed an ocean and east coasts air that crossed a continent. This strengthens maritime conditions in places such as Vancouver and continental conditions farther inland. Mountains, monsoons, and seasonal pressure shifts modify the pattern.

Clouds

Clouds reflect shortwave radiation, often making cloudy days cooler. They also absorb and emit longwave radiation, increasing downward infrared and often making cloudy nights warmer. Low bright clouds usually have a strong shortwave cooling effect; high cold clouds can have a stronger greenhouse effect. Clouds generally reduce the daily temperature range.

Reading global temperature maps

An isotherm connects points with equal temperature for a stated averaging period. Isotherms shift poleward in summer and equatorward in winter, but they bend because land warms and cools faster than ocean, currents move heat, and elevation lowers temperature. The thermal equator traces the warmest mean temperature at successive longitudes for a specified period. It migrates seasonally and does not have to follow one fixed-temperature isotherm or the geographic equator.

Annual temperature range is largest in high-latitude continental interiors, where seasonal insolation changes strongly and the ocean provides little moderation. It is smallest in the tropics, where solar altitude and day length change little through the year. The Northern Hemisphere has larger continental extremes because it contains more land at middle and high latitudes.

Where precipitation is greatest and least

Precipitation requires moisture, cooling to saturation, condensation or deposition, particle growth, and enough fall speed for hydrometeors to reach the surface. Global patterns follow vertical motion:

• The ITCZ is wet because trade winds converge and warm humid air rises.
• The subtropical highs near $20^\circ$–$30^\circ$ N/S are dry because air sinks, compresses, warms, and lowers relative humidity.
• Midlatitude precipitation increases along the polar front, where cyclones and fronts lift air.
• Polar regions are dry in absolute terms because cold air contains little water vapour.

Terrain modifies these belts. Moist air rises and cools on windward slopes, while descending leeward air produces a rain shadow. Cold currents can stabilize coastal air and suppress rainfall; continental interiors lie far from ocean moisture sources.

Representative annual totals range from more than $2000\ \mathrm{mm}$ in tropical rain forests to less than $250\ \mathrm{mm}$ in hot deserts. Some windward tropical mountains exceed $10{,}000\ \mathrm{mm}$, while high-latitude tundra can receive less than $300\ \mathrm{mm}$. These are examples, not universal boundaries.

Rain gauges provide direct measurements but can miss precipitation because of wind undercatch, evaporation, splash, poor siting, and sparse mountain coverage. Snow is especially difficult to measure. Radar and satellites improve spatial coverage but still require calibration.

Amount, phase, timing, and intensity matter

The same annual precipitation can produce different runoff regimes.

• Snow stores winter precipitation until spring melt releases it, often producing a concentrated freshet.
• High-intensity precipitation can deliver water to the surface faster than the basin can absorb and store it, increasing rapid runoff.

Runoff is therefore not precipitation alone. Antecedent storage and the pathways through the basin matter.

Most water is salty; most freshwater is not in rivers

About 97% of Earth’s water is saline, almost all in the oceans. Only about 2.5%–3% is fresh. Of that freshwater, roughly two-thirds is frozen in glaciers and ice sheets, while most of the remainder is groundwater. Freshwater lakes and rivers together hold less than 0.01% of all Earth’s water; soil moisture, atmosphere, and living organisms hold other tiny fractions.

A small store can still carry a large or important flux. Rivers contain a tiny fraction of global water but continually move water, sediment, nutrients, and carbon from land toward the ocean. The amount in storage and the rate of turnover are different properties.

The global hydrologic cycle

Water moves through linked processes:

• evaporation from ocean, lakes, soils, and wet surfaces;
• transpiration through plants;
• sublimation from snow and ice;
• condensation and cloud formation;
• atmospheric transport;
• precipitation;
• interception by vegetation;
• infiltration into soil and percolation toward groundwater;
• surface runoff and channel flow;
• shallow throughflow and deeper groundwater flow;
• storage in ocean, ice, lakes, wetlands, soil, aquifers, atmosphere, and organisms.

Globally, over a sufficiently long period,

$$P\approx E,$$

because total precipitation must balance total evaporation. The regional pattern is not closed locally. Oceans evaporate more water than they receive as precipitation; the atmosphere exports vapour to land. Land receives more precipitation than it returns by evapotranspiration; rivers and groundwater carry the surplus back to the ocean.

For a land area or drainage basin over one stated interval, write all terms as compatible volumes or area-equivalent water depths:

$$P=ET+Q+\Delta S,$$

where $P$ is precipitation, $ET$ evapotranspiration, $Q$ runoff exported during the interval, and $\Delta S$ the storage change over that same interval. A rate budget instead uses $dS/dt$ and fluxes per unit time. Over a long averaging period, the average storage-change rate may become small, but storage change remains central during wet seasons, droughts, snow accumulation, groundwater depletion, and reservoir filling.

Why runoff patterns differ from rainfall maps

Annual runoff depth in $\mathrm{mm\,yr^{-1}}$ is annual discharge volume divided by basin area, which allows differently sized basins to be compared. Runoff is usually high in humid tropics, wet monsoon regions, and cool windward mountains. It is low in deserts, continental interiors, rain shadows, and places where evapotranspiration consumes most precipitation.

A cool humid basin may turn a large fraction of precipitation into runoff because available energy limits evapotranspiration. A hot basin can receive substantial rain yet produce little annual runoff because evapotranspiration returns much of the water to the atmosphere. Infiltration is an internal transfer: infiltrated water may be stored, exported across a basin boundary, or later sustain streamflow. Storage reduces runoff over an interval only when that storage increases. Climate supplies the broad pattern; basin properties determine the route and timing.