Solar radiation is converted to sensible and latent heat at Earth’s surface. A change in sensible heat results in a change in the temperature of a medium, whereas energy stored as latent heat is used to drive a process, such as a phase change in a substance from its liquid to its gaseous state, and does not produce a change in temperature. Thus, the daily maximum surface temperature at a given location is dependent on the amount of radiant energy converted to sensible heat. Water available for evaporation increases latent heating by adding water vapour to the atmosphere. As a result, relatively little energy remains to heat the air, and thus the sensible heating of the air near the ground is minimized. In addition, daily maximum temperatures are not as high in locations with strong latent heating.

As day length increases from winter to summer, sensible heating and maximum surface temperatures rise. In the U.S. Midwest, prior to the leafing out of vegetation in the springtime and the resulting rise in evaporation and transpiration, sensible heating causes an average increase in maximum surface temperatures of only about 0.3 °C (0.5 °F) per day. The process of leaf production creates a surge in evaporation and transpiration and results in increased latent heating and reduced sensible heating. After leafing, since most of the available thermal energy is used to convert liquid water to water vapour rather than to heat the air, the average day-to-day rise in daily maximum temperatures is reduced to about 0.1 °C (0.2 °F) per day.

This effect extends upward through the atmosphere. Prior to leafing out, the one-kilometre-thick layer occurring between the 850-to-750-millibar pressure level (which typically occurs between 1,650 and 2,750 metres [5,400 and 9,000 feet]) in the Midwest warmed at the rate of 0.1 °C (0.2 °F) per day. Following leafing out, the warming rate fell to 0.02 °C (0.04 °F) per day. Scientists have used computer models of the atmosphere to study the effect of transpiration from vegetation on maximum surface air temperatures. In these models, the variable controlling transpiration by vegetation was “turned off,” and the character of the resulting modeled climate was studied. By subtracting the effect of transpiration, temperatures in central North America and on the other continents were predicted to equilibrate at a very hot 45 °C (113 °F). Such warming is nearly realized in desert areas where moisture is unavailable for transpiration.

Biosphere controls on minimum temperatures

During the late 1860s, British experimental physicist John Tyndall, based on his studies of the infrared radiation absorption by atmospheric gases, concluded that nighttime minimum temperatures were dependent on the concentration of trace gases in the atmosphere. Of these gases, water vapour had the greatest impact. To emphasize the significance of water vapour on decreases in air temperature during the night, he wrote that if all the water vapour in the air over England was removed even for a single night, it would be “attended by the destruction of every plant which a freezing temperature could kill.” As a result, it follows that the greater the water content of the atmosphere, the lower the radiative loss of energy to the sky and the less the surface atmosphere is cooled. Thus, locations with substantial amounts of water vapour experience reduced nocturnal cooling.

Water vapour in the atmosphere also limits the extent to which temperatures fall at night. This limiting temperature is known as the dew point, which is defined as the temperature at which condensation begins. Over North America east of the 100th meridian (a line of longitude traditionally dividing the moist eastern part of North America from drier western areas), average nighttime minimum temperatures are within a degree or two of the dew point temperature. Upon nocturnal cooling, the dew point is reached, condensation begins, and latent energy is converted to heat. Additional temperature falls are retarded by this release of heat to the atmosphere. A significant fraction of the water in the atmosphere over the continents comes from the evaporation of water from soils and the transpiration from vegetation. Transpired water directly moderates temperature by increasing humidity and thus raising the dew point. As a consequence, the amount of outgoing terrestrial radiation released to space is reduced. This results in the elevation of the minimum temperature of the air above what it would otherwise be.

The effect of spring leafing on the buildup of humidity in the lower atmosphere has received the attention of researchers in recent years. In the late 1980s, American climatologists M.D. Schwartz and T.R. Karl used the superimposed epoch method to study the climate before and after the leafing out of lilac plants in the spring in the U.S. Midwest. (This method uses time series data from multiple locations, which can be compared to one another by adjusting each data set around the respective onset date of lilac blooming.) In the illustration, the x-axis marks days before and after leafing, whereas the y-axis shows the related changes in the vapour pressure of the atmosphere. A second y-axis follows the day-by-day changes in minimum temperatures. Prior to the average date of leafing, the atmospheric humidity (vapour pressure) is relatively constant and minimum temperatures hover near freezing. At leafing, there is an abrupt increase in atmospheric humidity. Following leafing, daily minimum temperatures also increase abruptly. Although frosts are possible until June 10 in many parts of the Midwest, the chances of frost decline as the atmosphere is humidified.