The CLIMATE section of MatchClover provides information on the important aspects of weather and climate that affect clovers and other forage species selection and management.

Climate refers to the long-term history of temperature, precipitation, and radiation for a given region. Climate is the principal factor affecting suitability of forage species or cultivars for a given location. In contrast, weather includes day-to-day and short-term extremes in temperature, precipitation as rain, snow or hail, relative humidity, wind, and solar radiation at a given site. Forage producers must be aware of the climate, i.e. the average year, when selecting adapted species and cultivars. They must be aware of weather events that alter year-to-year productivity and influence day-to-day management decisions.

Climate Classification Systems

Aristotle made one of the earliest attempts to classify the climates on earth with his discussion of Temperate, Torrid, and Frigid Zones. There have been other systems, but the one in almost universal use today is the Köppen system, developed in 1928 by German climatologist and amateur botanist Wladimir Köppen.

The Köppen System

The Köppen system uses letters to denote the six major climate regions and their 24 sub-classifications. These regions are based on average monthly temperature and precipitation values. The Köppen system does not fully account for such factors as cloudiness, solar radiation, wind, or even extremes in temperature, but it remains a useful guide to average climate trends. The individual zones are shown (Figure 1) with clearly delineated boundaries, but it is important to note that the areas between zones represent a gradual transition between climates.

Figure 1.  Köppen-Geiger 1976-200 climate classification map.

Köppen updated and modified his system of climate classification until his until his death in 1940. Since then it has been modified by a number of geographers, most notably the late Glen Trewartha of University of Wisconsin, whose version is probably in the widest use today.

The modified Köppen classification uses six letters to divide the world into six major climate regions, based on average annual precipitation, average monthly precipitation, and average monthly temperature:

Each category is further divided into sub-categories based on temperature and precipitation (Table 1).

For example, the U.S. states located along the Gulf of Mexico are designated as "Cfa." The "C" represents the "mild mid-latitude" category, the second letter "f" stands for the German word feucht or "moist," and the third letter "a" indicates that the average temperature of the warmest month is above 72°F (22°C). Thus, "Cfa" gives us a good indication of the climate of this region, a mild mid-latitude climate with no dry season and a hot summer.

Köppen also used vegetation to aid in climate classification, including tropical rainforest, tropical wet and dry season vegetation, low-latitude steppe, low-latitude desert, Sclerophyll forest, mid-latitude deciduous forest, Boreal forest, and tundra vegetation.


Arnfield, John A. 2017. Köppen climate classification. https://www.britannica.com/science/Koppen-climate-classification

Chen, D. and H. W. Chen, 2013: Using the Köppen classification to quantify climate variation and change: An example for 1901–2010. Environmental Development, 6, 69-79, 10.1016/j.envdev.2013.03.007. http://hanschen.org/koppen/

Köppen climate classification: https://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification.

Rosenberg, Matt. "Koppen Climate Classification System." ThoughtCo, Mar. 30, 2017, http://thoughtco.com/koppen-climate-classification-system-1435336.

Spatial Modeling of Climate

More recently, climate station data has been used to create spatial models of climate elements including precipitation, minimum and maximum temperature, relative humidity, and solar radiation. These models have typically used distance weighting approaches that work well for flat conditions. Mountainous areas with detailed topography and coastal regions, however, have been poorly modeled by these purely statistical approaches. PRISM was developed to address these issues and allow for detailed, accurate modeling of real-world conditions (Daly et al., 2002). The climate spatial layers used in this project were developed with PRISM.

Table 1.  Köppen climate classification chart.

A Tropical humid Af Tropical wet No dry season
    Am Tropical monsoonal Short dry season; heavy monsoonal rains in other months
    Aw Tropical savanna Winter dry season
B Dry BWh Subtropical desert Low-latitude desert
    BSh Subtropical steppe Low-latitude dry
    BWk Mid-latitude desert Mid-latitude desert
    BSk Mid-latitude steppe Mid-latitude dry
C Mild Mid-Latitude Csa Mediterranean Mild with dry, hot summer
    Csb Mediterranean Mild with dry, warm summer
    Cfa Humid subtropical Mild with no dry season, hot summer
    Cwa Humid subtropical Mild with dry winter, hot summer
    Cfb Marine west coast Mild with no dry season, warm summer
    Cfc Marine west coast Mild with no dry season, cool summer
D Severe Mid-Latitude Dfa Humid continental Humid with severe winter, no dry season, hot summer
    Dfb Humid continental Humid with severe winter, no dry season, warm summer
    Dwa Humid continental Humid with severe, dry winter, hot summer
    Dwb Humid continental Humid with severe, dry winter, warm summer
    Dfc Subarctic Severe winter, no dry season, cool summer
    Dfd Subarctic Severe, very cold winter, no dry season, cool summer
    Dwc Subarctic Severe, dry winter, cool summer
    Dwd Subarctic Severe, very cold and dry winter, cool summer
E Polar ET Tundra Polar tundra, no true summer
    EF Ice Cap Perennial ice
H Highland   Mountain climate Contains all highland areas not easily categorized by other climate types

Light (Solar Radiation Amount and Photoperiod)

Plant growth responses to solar radiation can be separated into those due to wavelength or color, light intensity, and duration (photoperiod). Radiation in the visible range is most active in photosynthesis and is referred to as photosynthetically active radiation (PAR; 400-700 nm). The relative ratio of radiation in the red and far-red regions of the spectrum control plant photoperiod responses such as flowering. Radiation density is measured in energy units (µmoles photons/m2/s). Although the growth rate of plants with adequate nutrition and water is directly related to radiation density, forage growth rate is more often related to percent radiation interception by leaves. A full canopy of leaf blades intercepts maximum radiation. Thus, moderate defoliation optimizes plant growth.

Duration of the photoperiod (the time from sunrise to sunset) changes with latitude and season due to the tilt of the earth relative to its orbital path around the sun. Minimal seasonal change occurs at the equator whereas large changes occur at the poles. Most temperate grasses and legumes flower during long photoperiods; the flowering of perennial grasses also requires induction caused by cold temperatures, a process called vernalization.


Growth rate and other processes depend on the temperature pattern, including diurnal variation. Daytime temperatures should be near optimum for photosynthesis and growth, whereas lower temperatures at night conserve energy by reducing respiration. Cool-season forages have optimal growth temperatures near 70 ⁰F but can still grow slowly near 35 ⁰F. Warm-season forages have growth optima around 90 ⁰F and grow little below 60 ⁰F. Higher temperatures increase the rate of plant development and decrease time from seeding to flowering. This is one reason forage yields of cool-season species such as alfalfa and red clover are lower during hot summer periods. These forage types have less time to produce stem and leaf growth when warmer temperatures speed flowering.

Temperatures above or below the optimal range stress plants. High-temperature stress often occurs concurrently with moisture stress. Excessively high temperatures can induce flower sterility, especially pollen abortion, and lead to poor seed production.

Low-temperature stress can cause chilling injury in warm-season grasses and some legumes, but most cool-season legumes and grasses are not sensitive to above-freezing temperatures. Most cool-season grasses accumulate the storage carbohydrate fructan in cell vacuoles, whereas legumes and warm-season grasses store starch in chloroplasts. At low temperatures, plants continue to synthesize and break down fructan more readily than starch. This difference in metabolism between the cool- and warm-season grasses is the reason that each type thrives in its preferred temperature range.

Forage species differ widely in their ability to withstand cold temperatures. Differences within species (among types and cultivars) also exist. Overwintering species gradually develop cold resistance with the onset of the shorter days and colder temperatures of autumn. Forages lose cold resistance much faster than they gain it. Most winter killing in the northern areas occurs during late winter and early spring when insulating snow cover has disappeared and plants are exposed to severe temperature fluctuations above and below freezing.

Water Relations

Seasonal distribution patterns of precipitation, total quantity of precipitation, and evapotranspiration demands affect water availability and adaptation of forage species.  In addition, available water in the soil depends on soil texture and plant rooting depth. Solar radiation is the primary driver of transpiration.

Limited water. Shoot growth slows well before water stress becomes severe enough to cause stomatal closure and a decline in photosynthesis. Sugars from photosynthesis often accumulate during mild to moderate drought stress because growth is slowed. Some forage species gain drought tolerance by accumulating solutes like sugars, amino acids, and ions that hold water in plant tissues and prevent injury.

Forage species differ in response to drought stress. Many plants reduce shoot growth but maintain root growth under moderate drought stress conditions. Deep-rooted plants avoid drought stress due to greater access to stored water in the soil profile.

Excessive water. Poorly drained soils provide an unfavorable environment for growth of many forage species, especially legumes. (See Soils section)

Climate information slightly revised from: Volenec, Jeffrey J. and C. Jerry Nelson. 2003. Environmental Aspects of Forage Management. p. 99-124. Chapter 4 In: Robert F. Barnes, C. Jerry Nelson, Michael Collins, and Kenneth J. Moore (eds.) Forages: An Introduction to Grassland Agriculture. 6th ed. Vol. 1. Iowa State Press.

Clover Species Climate Tolerance Table

USDA Plant Hardiness Zone Map


PRISM Conterminous US Climate Maps: 30-Year Normals