Forestry & Forest Sciences · Foundations · also Agrometeorology, Forest Microclimate, Climatology
Weather and climate as they shape forest distribution, growth and hazards: radiation, temperature, moisture and wind, inside and above the canopy.
In the exams: Background to IFoS silviculture (site factors) and fire topics; ICAR AICE-JRF 59 Unit 2 (climatic site factors) and Unit 12
Forest meteorology and climatology study the weather and climate that shape where forests grow, how fast they grow and what threatens them, and also how forests in turn modify the air around and above them. Climate sets the broad limits of forest distribution: the boreal forest ends where summers become too short and cool, tropical rainforest gives way to savanna where the dry season lengthens, and treelines on mountains follow growing-season temperature. Within these limits, weather controls the year-to-year growth recorded in tree rings, the timing of flowering and frost damage, the water available to trees, the storms that blow them down and the conditions under which fires start and spread. Forests are not passive: their dark, rough, transpiring canopies absorb radiation, return water vapour to the atmosphere, slow the wind and create a cooler, moister, calmer microclimate beneath them.
This manual covers the atmosphere and its radiation balance and energy budget; temperature regimes, frost and heat; precipitation, rainfall interception and fog drip; humidity and evapotranspiration; wind and windthrow; forest microclimate and edge effects; the climate classifications of Köppen and Holdridge; fire weather and fire danger rating systems; and the instruments and weather stations used to measure all of these in and around forests. The physics is kept to what a forester needs to interpret observations and indices; climate change science and forest carbon are developed in the forests and climate subject, and catchment water balance in the forest hydrology subject.
The subject supports silviculture, because site factors such as frost, drought and exposure determine species choice and regeneration methods; forest protection, because fire danger and windthrow risk depend on weather; and forest hydrology, because interception and evapotranspiration control water yield. Examiners test it through definitions (albedo, Bowen ratio, lapse rate, inversion, frost pocket, interception loss, occult precipitation, vapour pressure deficit, katabatic and foehn winds), the classification systems, the structure of fire danger rating systems, the instruments and their siting, and short calculations: net radiation, partitioning of energy, temperature change with altitude, interception loss, mean areal rainfall, vapour pressure deficit, evapotranspiration from a water balance or pan, tree slenderness, shelterbelt protection distance, Holdridge potential evapotranspiration ratio and simple fire danger indices. Postgraduate and doctoral forestry examinations, civil-service forest examinations in many countries, forestry engineer licensing and wildland-fire qualifications draw on this material.
The atmosphere is a thin envelope of gases: by volume, dry air is about 78 percent nitrogen, 21 percent oxygen and just under 1 percent argon, with carbon dioxide at more than 0.04 percent (above 420 parts per million in the mid-2020s) and variable water vapour, which ranges from almost nothing to about 4 percent and is the most important greenhouse gas. The lowest layer, the troposphere, contains most of the mass and nearly all weather; it extends to about 8 km over the poles and 16 to 18 km over the equator, and temperature in it falls with height, by about 6.5 °C per kilometre on average (the environmental lapse rate). Above lies the stratosphere, where ozone absorbs ultraviolet radiation and temperature rises with height, then the mesosphere and thermosphere. Rising unsaturated air cools at the dry adiabatic lapse rate of about 9.8 °C per kilometre; once saturated, condensation releases latent heat and the rate falls to a saturated adiabatic rate that is commonly about 4 to 7 °C per kilometre. Whether a parcel of air keeps rising depends on how the environmental lapse rate compares with these rates: an atmosphere that cools faster with height than rising air is unstable, favouring convection, thunderstorms and tall smoke columns, while one that cools more slowly, or warms with height in an inversion, is stable. The Sun radiates mostly shortwave radiation (wavelengths roughly 0.3 to 3 µm, peaking in the visible); the solar constant, the irradiance at the top of the atmosphere perpendicular to the beam at the mean Earth–Sun distance, is about 1361 W m⁻². At the surface, solar radiation arrives as direct beam and diffuse radiation scattered by air, aerosols and clouds; together they are called global radiation. About 45 to 50 percent of solar energy at the surface is photosynthetically active radiation (PAR, 0.4 to 0.7 µm). The Earth emits longwave (infrared) radiation, and greenhouse gases and clouds absorb and re-emit part of it, keeping the mean surface temperature near 15 °C rather than the roughly −18 °C it would have without them.
The net radiation (Rn) of a surface is the balance of incoming and outgoing shortwave and longwave radiation: Rn = (1 − α)K↓ + L↓ − L↑, where K↓ is incoming shortwave radiation, α the albedo (the fraction of shortwave reflected), L↓ incoming longwave from the sky and L↑ longwave emitted (and reflected) by the surface, which follows the Stefan–Boltzmann law. Forest canopies are dark and deep, and light reflected within the canopy is often absorbed before escaping, so forests have low albedos: roughly 0.08 to 0.15 for conifers and 0.15 to 0.20 for broadleaves in leaf, compared with about 0.20 to 0.25 for grass and crops and 0.8 or more for fresh snow. With K↓ = 800 W m⁻², α = 0.10, L↓ = 350 and L↑ = 420 W m⁻², net radiation is 720 + 350 − 420 = 650 W m⁻². Net radiation is partitioned by the surface energy balance: Rn = H + LE + G + S, where H is sensible heat flux (heating the air), LE latent heat flux (energy used in evaporation and transpiration), G soil heat flux and S heat stored in biomass and canopy air, with a very small share used in photosynthesis. Over forests G and S are usually small compared with H and LE. The Bowen ratio, β = H/LE, describes the partition: well-watered forests in humid climates have Bowen ratios of about 0.2 to 0.5, using most energy for evapotranspiration, while dry forests and drought-stressed canopies have ratios of 1 to more than 2. If Rn = 500 W m⁻², G = 20 W m⁻² and β = 0.5, then H + LE = 480, LE = 480 / 1.5 = 320 W m⁻² and H = 160 W m⁻². Because forests are aerodynamically rough, they exchange heat and water vapour with the atmosphere efficiently, so their canopy temperatures stay close to air temperature when transpiring. Land-cover change alters this balance: tropical deforestation usually warms the local surface by reducing evapotranspiration, whereas in snowy boreal regions forests mask bright snow, so their low albedo has a warming effect that partly offsets the cooling from carbon storage.
Air temperature follows daily and annual cycles. The daily minimum usually occurs around sunrise, after a night of radiative cooling, and the maximum in early to mid-afternoon, after peak radiation, because the surface continues to gain heat for some hours after noon. The daily range is largest under clear skies, in dry air, in continental interiors and on open land, and smallest under cloud, near oceans and beneath forest canopies. The annual range increases with latitude and continentality. Temperature controls forest distribution and growth through several measures. Growing-season length and warmth, often expressed as degree-days above a base temperature, limit growth in cold regions. Worldwide, alpine and polar treelines occur where the mean temperature of the growing season falls to roughly 6 to 7 °C, a relation established by Körner and colleagues; the older rule that treelines approximate the 10 °C isotherm of the warmest month is also widely cited. Minimum temperatures limit species ranges: the hardiness zones used by horticulturists and foresters are based on average annual extreme minimum temperatures, and species that rely on supercooling are excluded where minima regularly fall below about −40 °C. In mountains temperature falls with altitude at roughly the environmental lapse rate, so a site at 2500 m is about 13 °C cooler than one at 500 m in the same region (2 km × 6.5 °C per km), which, together with rainfall, produces vegetation zones on mountain slopes. Aspect modifies temperature: in mid-latitudes, slopes facing the equator receive more radiation and are warmer and drier than slopes facing the poles, which carry different species. Soil temperature, damped and delayed with depth, controls root growth, decomposition and the thawing of frozen ground.
Frost occurs when temperatures fall to 0 °C or below at the surface or in the air. Radiation frost forms on clear, calm nights when the ground and vegetation lose longwave radiation to the sky, cool below the air above them and chill the air near the surface; the result is a temperature inversion, with colder air near the ground and warmer air above. Because cold air is dense, it drains downslope like water and collects in valleys, basins and hollows, forming frost pockets (frost hollows) that may be several degrees colder than slopes above them. On hillsides a zone of relatively mild nights, the thermal belt, often lies above the pooled cold air. Advection frost occurs when a mass of cold air moves into a region, often with wind and cloud, affecting all topographic positions. Ground frost is recorded by a thermometer just above the grass surface, which reads lower than a screen thermometer on radiation nights. Frost damage to forests includes killing of new shoots and flowers by late spring frosts, injury to shoots that have not hardened before early autumn frosts, frost cracks and frost rings in stems, frost heaving of seedlings in wet, fine-textured soils, and winter desiccation of evergreen foliage when the soil is frozen. Frost risk is reduced by keeping a shelterwood or nurse canopy over young regeneration, which reduces radiative heat loss, by avoiding planting of frost-sensitive species in hollows, by maintaining gaps in barriers that would dam cold air, by controlling tall weeds that trap cold air near seedlings, and by matching provenance phenology to the site. High temperatures also damage trees. Heat waves raise leaf temperatures above photosynthetic optima and can cause leaf scorch, especially when drought closes stomata and removes transpirational cooling; thin-barked species suffer sunscald and bark necrosis when suddenly exposed by thinning or edge creation; and seedlings can be killed by soil surface temperatures above about 50 to 60 °C on bare, dark soils in strong sun. Heat waves combined with drought have caused large-scale forest dieback on several continents.
Precipitation forms when moist air cools, usually by rising, until it is saturated and water vapour condenses on aerosols to form cloud droplets, which must then grow large enough to fall. Growth occurs by collision and coalescence of droplets, dominant in warm tropical clouds, and by the ice-crystal (Bergeron–Findeisen) process in cold clouds, where ice crystals grow at the expense of supercooled droplets because the saturation vapour pressure over ice is lower than over water. Three lifting mechanisms produce three types of precipitation. Convective precipitation results from surface heating that drives air upward in showers and thunderstorms, typical of the tropics and of summer afternoons. Orographic precipitation results from air forced over mountains, so windward slopes are wet and leeward areas lie in a rain shadow; this explains many forest boundaries, such as the wet forests on windward slopes and the dry woodlands or grasslands in the lee. Frontal or cyclonic precipitation results from air rising along fronts in mid-latitude depressions or in the converging air of tropical cyclones. Monsoon systems, driven by seasonal reversal of winds between land and ocean, bring strongly seasonal rainfall to South and Southeast Asia, West Africa and northern Australia. Rainfall is measured with standard gauges, collectors of known orifice area whose height above ground follows national practice, and with recording gauges such as tipping-bucket and weighing gauges that give intensity. Gauges should be sited in the open, away from obstacles; a common rule is that obstacles should be at a distance of at least two to four times their height, so in forests gauges are placed in large clearings or above the canopy on towers. Snow is measured as depth and as water equivalent. Mean areal rainfall over a catchment can be estimated by the arithmetic mean of gauges, by Thiessen polygons that weight each gauge by the area nearest to it, or by isohyets (lines of equal rainfall). For example, three gauges recording 800, 1000 and 1200 mm with Thiessen weights of 0.40, 0.35 and 0.25 give a mean of 970 mm.
When rain falls on a forest, part is caught by leaves, branches and stems and evaporates back to the atmosphere without reaching the ground: this is interception loss. The rest reaches the floor as throughfall, which passes through gaps or drips from the canopy, and stemflow, which runs down branches and trunks. Gross rainfall P = throughfall + stemflow + interception loss; throughfall plus stemflow is net precipitation. If a forest receives 1000 mm of rain in a year, with 750 mm of throughfall and 30 mm of stemflow, interception loss is 220 mm, or 22 percent. Interception loss depends on canopy storage capacity (the water the canopy can hold, commonly around 1 to 2 mm for many forests), on the number, size and intensity of storms, and on the evaporation rate during and after rain. Many small showers lose a larger proportion than a few large storms. Evergreen conifers with dense, rough foliage typically intercept more than deciduous broadleaves, and annual interception losses of about 10 to 35 percent of rainfall are common, with higher values in some conifer forests and in humid climates with frequent light rain. Physically based models, such as the Gash model, estimate interception from canopy and climate parameters. Stemflow is usually a small share of rainfall but concentrates water and nutrients at the base of trees, and is larger in smooth-barked species with upward-angled branches, such as beech. In snowy forests, conifer canopies intercept snow, part of which sublimates directly to vapour, reducing the snowpack under the canopy compared with open areas. Where clouds or fog move through forests, leaves and needles capture droplets that drip to the ground, a process called fog drip, cloud water interception or occult (horizontal) precipitation. It is important in tropical montane cloud forests and in coastal fog belts, such as those supporting coast redwood forests in California and fog-fed forests in the Canary Islands and parts of the Andes and Africa, where it can add a substantial share to measured rainfall and is especially valuable in dry seasons; clearing such forests can reduce water yield, unlike most forests.
Humidity describes the water vapour in air. The most physically useful measure is vapour pressure, e, the partial pressure exerted by water vapour, in kilopascals. The saturation vapour pressure, es, the maximum vapour pressure air can hold at a given temperature, rises steeply and nonlinearly with temperature: a common approximation, the Tetens equation, gives es (kPa) = 0.6108 × exp(17.27 T / (T + 237.3)), about 1.23 kPa at 10 °C, 2.34 kPa at 20 °C, 3.17 kPa at 25 °C and 4.24 kPa at 30 °C. Relative humidity (RH) is e/es × 100; because es changes with temperature, RH changes through the day even when the amount of vapour does not, typically being highest near dawn and lowest in mid-afternoon. The vapour pressure deficit (VPD), es − e, is the drying power of the air and drives transpiration; at 25 °C and 60 percent RH, e = 1.90 kPa and VPD = 1.27 kPa. Because es rises with temperature, warming at constant relative humidity increases VPD, one reason warmer droughts stress trees more. The dew point is the temperature to which air must be cooled at constant pressure to become saturated; dew and frost form when surfaces cool below it. Other measures include absolute humidity (mass of vapour per volume of air), specific humidity and mixing ratio (mass of vapour per mass of moist or dry air). Humidity is measured with psychrometers, which compare a dry-bulb thermometer with a wet-bulb thermometer cooled by evaporation, the wet-bulb depression being larger in drier air (aspirated psychrometers ventilate both bulbs at a standard speed); hair hygrometers and hygrographs, which use the change in length of hair with humidity; and electronic capacitive or resistive sensors, standard in automatic weather stations. Forest canopies raise humidity beneath them by transpiration and reduced mixing, which helps seedlings, mosses and epiphytes and lowers the flammability of fine fuels.
Evapotranspiration (ET) is the combined loss of water by evaporation from soil, water and wet surfaces (including intercepted rain) and by transpiration through stomata. Potential evapotranspiration is the rate from an extensive surface amply supplied with water; reference evapotranspiration (ET₀), defined by FAO, is the rate from a hypothetical well-watered grass surface of specified height and resistance, calculated by the FAO-56 Penman–Monteith equation from radiation, temperature, humidity and wind. Simpler methods use temperature alone (Thornthwaite, Hargreaves) and are used where data are limited, at the cost of accuracy. Pan evaporation from a standard open pan, such as the Class A pan, multiplied by a pan coefficient that is often about 0.7 but depends on humidity, wind and the pan's surroundings, provides another estimate; 6 mm per day of pan evaporation with a coefficient of 0.7 indicates about 4.2 mm per day of reference ET. Actual evapotranspiration from forests depends on available energy, VPD, canopy conductance (which stomata control), aerodynamic coupling and soil water. Forests are tall and rough, so they are well coupled to the atmosphere: their transpiration responds strongly to stomatal opening and VPD, whereas short, smooth crops are more decoupled and their transpiration follows available energy more closely, a contrast described by the decoupling coefficient of Jarvis and McNaughton. Forests generally evaporate more water annually than grasslands or crops in the same climate, because of interception loss from large canopies, deep roots that maintain transpiration in dry periods, low albedo and rough canopies; global analyses of catchments, such as those by Zhang and colleagues, summarise how annual ET increases with rainfall and is higher for forest than grass. Annual ET of a catchment can be estimated from the water balance P = ET + Q + ΔS: with 1200 mm of rainfall, 450 mm of streamflow and negligible storage change over a year, ET is about 750 mm. ET is measured directly by eddy covariance on flux towers, which record turbulent fluctuations of vertical wind and water vapour above the canopy, by sap flow sensors in stems for transpiration, and by lysimeters for small vegetation.
Wind is air moving from high to low pressure, deflected by the Earth's rotation (the Coriolis effect) and slowed by friction near the surface. Above the friction layer, wind blows nearly parallel to isobars (the geostrophic wind); near the ground it slows and turns towards low pressure. Over a vegetated surface, mean wind speed increases roughly logarithmically with height: u(z) = (u*/k) ln((z − d)/z₀), where u* is the friction velocity, k the von Kármán constant (about 0.4), d the zero-plane displacement and z₀ the roughness length. For forests, rough rules of thumb put d at about two-thirds of canopy height and z₀ at about one-tenth, so a 20 m stand has d ≈ 13 m and z₀ ≈ 2 m; the forest effectively raises the ground surface and greatly increases turbulence. Within the canopy, wind speed falls sharply, often roughly exponentially with depth, and the forest interior is calm except in gusts, which penetrate as intermittent turbulent eddies. Local wind systems matter for forests. Valley and mountain winds blow upslope and up-valley by day as slopes heat, and downslope and down-valley at night as cold air drains (katabatic flow), carrying cold air into frost pockets and smoke into valleys. Sea and land breezes reverse daily on coasts. Foehn winds, called chinook in North America and by other local names elsewhere, are warm, dry winds descending the lee of mountains after losing moisture on the windward side; they raise temperature, lower humidity sharply and are associated with extreme fire danger in many regions. Wind is measured with cup or sonic anemometers and wind vanes, by WMO convention at 10 m above open, level ground, and described by the Beaufort scale in field observations. Tropical cyclones (hurricanes, typhoons), mid-latitude windstorms, thunderstorm downbursts and tornadoes cause the most severe forest damage.
Wind damage to trees takes two forms: uprooting (windthrow), when the root–soil plate fails, and stem breakage (windsnap), when the stem fails. Which occurs depends on the relative strength of anchorage and stem. The wind force on a tree increases with the square of wind speed and with crown area and height, and turning moments at the base are greatest for tall trees with large crowns. Susceptibility increases with tree height; with slenderness, the ratio of height to diameter at breast height (both in the same units), for which values above roughly 80 to 100 are often cited as indicating instability, although thresholds vary with species and site; with shallow rooting caused by waterlogging, compacted or shallow soils over rock; and with wet, soft soils during storms. Stand history matters: trees grown in dense stands are slender and depend on mutual support, so heavy or late thinning and the creation of new edges suddenly expose them to wind they have not acclimated to, and damage is common in the first years after thinning or clear-cutting adjacent stands. Trees acclimate to wind by thigmomorphogenesis, growing thicker stems and stronger roots where they are exposed, which is why trees at old edges are more wind-firm than those newly exposed. Species differ: shallow-rooted species on wet soils, such as many spruces, are vulnerable, while deep-rooted species on well-drained soils are more stable. Snow and ice loading add weight to crowns and cause breakage in dense young stands. Management reduces risk by early and moderate thinning that allows trees to develop taper and roots, avoiding heavy thinning late in the rotation in exposed stands, shorter rotations where tall stands are at high risk, designing harvest boundaries along natural wind-firm features, feathering or creating wind-firm edges, improving drainage, mixing species and choosing stable species on exposed sites. Process-based risk models combine tree dimensions, soil and wind climate to estimate critical wind speeds for stands. Shelterbelts and windbreaks protect farms and settlements: wind speed is reduced for a distance commonly of about 10 to 20 times the belt height downwind, and to a lesser extent upwind, with moderately porous belts (around 40 to 50 percent porosity) giving longer protection and less turbulence than dense ones.
A forest canopy creates a distinctive microclimate beneath it. Radiation is the primary control. The canopy intercepts most incoming solar radiation, and the fraction transmitted decreases exponentially with leaf area according to Beer's law, I = I₀ e^(−k × LAI), where k is the extinction coefficient (often about 0.5 for broadleaves, varying with leaf angle and clumping); a canopy with LAI 4 and k = 0.5 transmits about 14 percent of light at the floor, and dense tropical and conifer canopies often transmit only 1 to 5 percent. Light under canopies is not only reduced but changed: leaves absorb red and blue light and transmit far-red, so the red-to-far-red ratio falls, a signal that seedlings detect through phytochrome and that triggers shade-avoidance growth; brief, bright sunflecks provide a large share of the light available to understorey plants. Because the canopy absorbs radiation by day and slows longwave loss at night, temperatures near the forest floor are buffered: daytime maxima are lower and night minima higher than in the open, so the daily range is reduced. Syntheses of measurements from many forests have found that understorey maximum temperatures in warm periods are, on average, several degrees cooler than outside, with greater buffering under dense, tall and multi-layered canopies. Humidity is higher and VPD lower beneath the canopy because of transpiration and reduced mixing. Wind speed is much lower. Carbon dioxide concentrations near the floor can rise at night from soil and plant respiration. These conditions favour shade-tolerant seedlings, mosses, ferns and amphibians, slow the drying of fuels and litter and influence decomposition. The forest microclimate also buffers understorey species against regional warming, so that species in closed forests may experience less warming than open-air records suggest; loss of canopy through disturbance or harvest removes this buffer abruptly.
Canopy gaps and forest edges create gradients between the forest interior and the open. In small gaps, radiation at the floor increases with gap size, and in mid-latitudes the brightest part of a gap is displaced towards the side away from the equator because the sun shines in at an angle; soil temperature, daily temperature range and evaporation increase, while humidity drops. Larger gaps approach open conditions and, on clear nights, can be colder than both the open field and the forest because cold air pools in them. Edges between forest and cleared land are exposed to more radiation, higher temperature, lower humidity and stronger wind than the interior, and these changes penetrate some distance into the forest. Studies of tropical forest fragments, notably the long-term Biological Dynamics of Forest Fragments experiment in the Amazon, found edge effects on microclimate, tree mortality and species composition reaching tens of metres to a few hundred metres into the forest, depending on the variable; edges facing the equator and the prevailing wind are most affected, and new edges are harsher than old edges that have developed a dense side canopy. Fragmentation therefore reduces the effective interior habitat by more than the area cleared, and small or narrow fragments may have no true interior. Elevated tree mortality at edges, drier fuels and greater wind penetration also make fragmented forests more vulnerable to fire and windthrow. Silviculture uses microclimate deliberately. Shelterwood systems retain a partial canopy that moderates temperature extremes and frost for regenerating seedlings of species that need protection; gap sizes in group selection are chosen to favour shade-tolerant or light-demanding species; nurse crops shelter sensitive species; and retaining buffer strips along streams keeps water and riparian microclimate cool. Understanding edge orientation and gap size helps foresters predict where regeneration will succeed, where frost will occur and where trees may be damaged by sunscald or wind.
Climate classification groups places with similar climates, usually using monthly temperature and precipitation, and often reflects vegetation boundaries, which inspired the first systems. The most widely used is that of Wladimir Köppen, first published around 1900 and revised by him and later by Rudolf Geiger, hence often called the Köppen–Geiger system. It has five main groups. A (tropical) climates have every month with a mean temperature of 18 °C or more; they are subdivided into Af (tropical rainforest, every month with at least 60 mm of rain), Am (tropical monsoon, a short dry season compensated by heavy rain) and Aw (tropical savanna, a pronounced dry season). B (dry) climates are those where evaporation exceeds precipitation, defined by a threshold of annual precipitation that depends on mean annual temperature and the season of rain; they include BW (desert) and BS (steppe), with h (hot) and k (cold) variants. C (temperate) climates have the coldest month mean between −3 °C (or 0 °C in some versions) and 18 °C, and D (continental or cold) climates have the coldest month below that threshold with at least one month above 10 °C. Their second letters describe seasonality of rain: f (no dry season), s (dry summer, the Mediterranean climates) and w (dry winter), and third letters summer warmth: a (hot summer, warmest month at least 22 °C), b (warm summer), c (cool summer) and d (very cold winter). E (polar) climates have every month below 10 °C, divided into ET (tundra, warmest month between 0 and 10 °C) and EF (ice cap, all months below 0 °C). The 10 °C warmest-month boundary between D and E approximates the polar treeline. Forests correspond broadly to Af and Am (rainforests), Aw (dry forests and savanna woodland), Cf (temperate deciduous and evergreen forests), Cs (sclerophyll woodlands and forests), Df (boreal and mixed forests, Dfc being the classic boreal climate) and Dw (East Asian continental forests). Walter–Lieth climate diagrams, which plot monthly temperature and precipitation with 10 °C corresponding to 20 mm, show arid months where the precipitation curve falls below the temperature curve.
Leslie Holdridge proposed his life-zone system in 1947 and expanded it in 1967, mainly from work in tropical America. It classifies the climate that determines natural vegetation using three variables, arranged on logarithmic axes as a triangular grid of hexagons. Mean annual biotemperature is the mean of temperatures in which values below 0 °C (and, in later versions, above 30 °C) are set to zero, because plant growth is assumed to stop outside that range. Mean annual precipitation is the second variable. The potential evapotranspiration ratio, the third, is potential evapotranspiration divided by precipitation, with potential evapotranspiration estimated as 58.93 × biotemperature in millimetres. A site with biotemperature 20 °C and annual precipitation 2000 mm thus has potential evapotranspiration of about 1179 mm and a ratio of about 0.59, a humid climate. The diagram defines latitudinal regions and altitudinal belts (from tropical through subtropical, warm temperate, cool temperate, boreal, subpolar and polar, or from basal through premontane, lower montane, montane, subalpine, alpine and nival) and humidity provinces from superarid to superhumid, giving named life zones such as tropical dry forest, tropical moist forest, tropical wet forest, lower montane rain forest and boreal moist forest. The system is widely used in Latin America for ecological mapping and has been applied globally to project shifts of life zones under climate change; its limitation is that it ignores seasonality, so sites with the same totals but different dry seasons fall in the same zone. Other systems include Thornthwaite's classification based on a moisture index from precipitation and potential evapotranspiration, Whittaker's biome diagram plotting biomes against mean annual temperature and precipitation, and national or regional forest type classifications that combine climate with vegetation. For foresters, classification helps match species and provenances to sites, transfer silvicultural knowledge between regions of similar climate, and anticipate how climate change will shift the suitable range of species.
Fire needs fuel, oxygen and heat, the fire triangle; how a fire behaves once burning depends on fuel, weather and topography, the fire behaviour triangle. Weather is the most variable element. Temperature heats fuels and dries them; relative humidity controls the moisture content of fine dead fuels, which respond within hours; wind supplies oxygen, tilts flames towards unburnt fuel, carries embers ahead of the fire and is the main driver of rate of spread; precipitation wets fuels; and atmospheric stability determines whether smoke columns can grow into towering convective plumes that make fires erratic and generate their own winds. Drought, the cumulative lack of rain over weeks to months, dries heavy fuels, duff and live vegetation, increasing the fuel available to burn and fire intensity. Dead fuels are classified by timelag, the time they take to lose about 63 percent of the difference between their moisture content and the equilibrium moisture content set by temperature and humidity: 1-hour fuels are less than about 0.6 cm in diameter (grass, needles, fine twigs), 10-hour fuels about 0.6 to 2.5 cm, 100-hour fuels about 2.5 to 7.6 cm and 1000-hour fuels about 7.6 to 20 cm. Fine fuels therefore track the daily cycle of humidity, so fire danger peaks in mid-afternoon, when temperature is highest and humidity lowest, and falls at night when humidity recovers; fires that keep burning actively through the night signal very dry conditions. Topography interacts with weather: fire spreads faster upslope, slopes facing the equator are drier, and foehn and other downslope winds produce many of the most destructive fires. Large fire events are often linked to synoptic patterns such as blocking highs, heat waves, dry cold fronts that bring wind shifts, and to climate variability such as El Niño, which brings drought and severe fire seasons to Indonesia, Australia and the Amazon. Climate change has lengthened fire weather seasons across much of the world.
Fire danger rating combines weather and fuel information into indices that guide preparedness, resource deployment, restrictions on burning and public warnings. The Canadian Forest Fire Weather Index (FWI) System, developed in the 1970s and finalised in 1987, is the most widely used internationally, including in Europe and Southeast Asia. It uses daily noon measurements of temperature, relative humidity, wind speed and 24-hour rainfall to compute three fuel moisture codes, the Fine Fuel Moisture Code (FFMC, for litter and fine fuels), the Duff Moisture Code (DMC, for loosely compacted organic layers of moderate depth) and the Drought Code (DC, for deep, compact organic layers and heavy fuels), and three fire behaviour indices: the Initial Spread Index (ISI, from wind and FFMC), the Buildup Index (BUI, from DMC and DC) and finally the Fire Weather Index (FWI, from ISI and BUI), a general index of fire intensity. The United States National Fire Danger Rating System, introduced in 1972 and revised several times, computes components such as the Spread Component, Energy Release Component and Burning Index from weather, fuel models and topography, with observations at fire weather stations in the early afternoon. Australia long used the McArthur Forest Fire Danger Index, which combines a drought factor, temperature, relative humidity and wind speed, and introduced the Australian Fire Danger Rating System in 2022 with simplified public rating levels. Simpler indices are used in many countries: the Angström index, developed in Sweden, I = R/20 + (27 − T)/10, with R relative humidity in percent and T temperature in °C, where values below 2.5 indicate high fire risk; the Nesterov index, developed in Russia, sums daily values of T × (T − Td), using temperature and dew point, over the days since significant rain; and the Keetch–Byram Drought Index tracks cumulative soil moisture deficit. For example, at 25 °C and 30 percent relative humidity, the Angström index is 1.5 + 0.2 = 1.7, indicating high risk. Ratings are expressed in classes (such as low to extreme or catastrophic), and must be calibrated locally against fire occurrence and behaviour, since the same index value means different things in different fuels and climates.
Standard weather observations follow guidance of the World Meteorological Organization (WMO) so that records are comparable. Air temperature and humidity are measured in a ventilated radiation shield, traditionally a white louvered wooden thermometer screen (the Stevenson screen) and now often a smaller multi-plate shield, with sensors at about 1.25 to 2 m above level, short grass. The traditional set comprised dry- and wet-bulb thermometers for humidity, a maximum thermometer (mercury, with a constriction that keeps the highest reading) and a minimum thermometer (alcohol, with an index that marks the lowest reading), and recording thermographs and hygrographs; the Six's thermometer combines maximum and minimum in one instrument. A grass minimum thermometer just above the grass tips records ground frost. Soil temperatures are read at standard depths such as 5, 10, 20, 50 and 100 cm. Precipitation is measured with standard and recording rain gauges (tipping-bucket, weighing or siphon types) and snow gauges or snow boards. Wind speed is measured with cup or sonic anemometers and direction with a vane, at 10 m above open ground. Sunshine duration has traditionally been recorded with the Campbell–Stokes recorder, a glass sphere that focuses sunlight to burn a trace on a card, and now with electronic sensors, sunshine being defined as direct beam irradiance above 120 W m⁻². Radiation is measured with pyranometers (global shortwave), pyrheliometers (direct beam), net radiometers and quantum sensors for PAR. Atmospheric pressure is measured with barometers (mercury or aneroid) and barographs. Evaporation is measured with pans such as the Class A pan, 121 cm in diameter and 25.4 cm deep, mounted on a wooden platform, and evapotranspiration with lysimeters. Today most stations are automatic weather stations, with electronic sensors, a data logger, a power supply such as a solar panel and telemetry, recording at intervals of minutes; they require regular maintenance and calibration, and quality control of data to detect sensor drift, gaps and errors.
Stations serving forestry have particular purposes and siting problems. Standard climate stations are sited on open, level ground with short grass, away from trees and buildings, so they represent regional conditions; forest clearings large enough to avoid obstruction, or towers above the canopy, are used to obtain comparable data in forested regions. Fire weather stations, such as the remote automatic weather stations operated by fire agencies, are sited to represent fire environments: open sites with standard exposure, measuring temperature, humidity, wind at a standard height, rainfall and often fuel moisture with sensors such as fuel-moisture sticks, at standard observation times (noon local standard time for the Canadian FWI System, early afternoon for the US system), feeding fire danger calculations. Research stations measure microclimate within stands, in gaps and along edges, with sensors at several heights to describe profiles of temperature, humidity, wind and light, and soil sensors for temperature and moisture. Flux towers, extending above the canopy, carry fast-response sonic anemometers and gas analysers for eddy covariance measurement of carbon dioxide, water vapour and heat exchange, as in the global network of flux sites, plus radiation sensors and profiles. Tree-based sensors such as dendrometers, which record stem diameter changes, and sap-flow probes link weather to tree responses. Siting rules matter: sensors must be shielded from direct sun, ventilated, mounted at standard heights, and placed away from obstacles, roads, and heat sources; changes in site, sensor or exposure must be documented in metadata, because they create inhomogeneities in long records. Networks of low-cost sensors and remote sensing now extend coverage, but they need calibration against standard instruments. Long, homogeneous records from forest stations are valuable for detecting climate trends, calibrating tree-ring reconstructions and models, and evaluating the forest microclimate buffering discussed earlier.
A student uses this subject to explain site factors in silviculture and to read weather data. Core skills are computing net radiation and the Bowen-ratio partition, temperature change with altitude, interception loss from throughfall and stemflow, Thiessen mean rainfall, VPD from temperature and humidity, ET from a water balance or pan, slenderness, shelterbelt protection distance, the Holdridge PET ratio and the Angström index, and naming Köppen climates from monthly data. Common errors are confusing relative humidity with VPD and radiation frost with advection frost.
A forester five years into practice uses weather knowledge daily: avoiding frost hollows for sensitive species, keeping shelterwood cover over regeneration, scheduling thinning to limit windthrow, siting and maintaining fire weather stations, reading daily fire danger indices to set burning restrictions and crew readiness, planning prescribed burns within weather prescriptions, and estimating water yield changes from interception and ET when planning plantations in catchments.
A senior professional with twenty-five years of experience sets policy on fire preparedness and wind risk across large estates or agencies, decides when a fire danger system needs local recalibration, plans species and provenance choices for future climates using climate classification and projections, and weighs forest effects on local water and climate in land-use decisions, such as afforestation in water-scarce catchments or protection of cloud forests.
A researcher with a PhD and long experience studies forest–atmosphere exchange with flux towers, microclimate buffering, interception and fog capture processes, wind damage mechanics and fire weather under climate change. Such researchers develop and test fire danger and wind risk models, contribute to assessments of forest–climate interactions, supervise doctoral students and examine candidates, so they must link classical micrometeorology with current climate science.
Values given for albedo, interception loss, canopy storage, slenderness thresholds, shelterbelt protection distance and fire index thresholds are typical ranges that vary with species, stand structure and climate and must be calibrated locally. The manual does not derive the equations of boundary-layer physics or explain fire behaviour modelling, weather forecasting, or climate model projections; national variants of instruments and observing practice are not described. Fire danger systems are described in outline only; operational use requires each system's official documentation.
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