CLIMATE

Climate is the long-term state of the atmosphere; weather is the instantaneous state of the atmosphere.

The differences in climate from place to place (geographical) and time to time (seasonal) result from (1) differential heating of the surface and atmosphere, and (2) the spin of the earth on its axis, and (3) the tilt of the earth's axis.

Important features of climate include:

• light
• temperature
• humidity
• precipitation
• wind

Climate includes both:

• Resources [light, precipitation (actually, soil water is the resource)]
• Conditions (e.g., temperature, day length, humidity, wind)

In this unit I want to concentration on climate as a set of conditions affecting crop growth; the next set of notes examines resources.

I. LIGHT (Solar Radiation)

The seasonal distribution of light is controlled by latitude. [How does the light environment of tropical latitudes differ from that of temperate and boreal latitudes?]

Plants (including many crops) show photoperiodic responses to day length, particularly in their phenology. Phenology has been defined as "the sequence of development events during the plant's life cycle as it is determined by environmental conditions" (Hall, 2001); these include flowering, bolting, tuber formation, etc. Either increasing or decreasing day length (actually, the length of the dark period) can trigger certain phenological responses.

"Long day" (LD) plants flower in response in increasing day lengths (summer); "short-day" (SD) plants flower in response to decreasing day lengths (summer and fall); and "day neutral" (DN) plants flower in either LD or SD conditions, depending on their maturity.

Day length can be an important barrier for growing tropical plants in temperate zones, or growing temperate crops in tropical zones. Many traditional tropical crops are SD plants while modern varieties are DN plants.

Energy balance.

The energy balance of any surface-for instance, a piece of ground or a section of a leaf-can be summarized in a relatively simple equation:

I - R - T = A + P + LE + dH

where:

I = intercepted radiation

R = Reflected radiation

T = Transmitted radiation

A = convective energy loss

P = Photosynthesis (ca. 5.3% of I under ideal conditions)

LE = Latent heat exchange

dH = Change in sensible heat (storage of heat as measured by temperature)

Applying the energy balance equation to plant leaf surfaces provides insight into the temperature stresses experienced by leaves (or crop canopies).

Latent heat exchange refers to:

• the heat lost by the evaporation of water or the gain of heat by condensation of water (latent heat of vaporization = 2442 J g^-1), and
• the heat exchange that occurs with the freezing of water and thawing of ice (latent heat of fusion = 334 J g^-1).

Plant leaves lose significant amounts of heat by transpiration. When plants close their stomata in response to water stress, they no longer can transfer heat to the atmosphere by transpiration. This increases the amount of heat stored in the leaf (dH) increasing leaf temperature.

High leaf temperatures can be fatal; plant strategies to avoid lethal temperature in hot, dry environments where they must experience water stress during part of their growing season include:

• small leaf size (increases A, convective energy loss)
• Higher reflectance (wax, resin, trichomes) (decreases I)
• High leaf angle (decreases I)
• Short life span (lowers LAD)

These are traits of plants native to dry regions, however, not necessarily traits of crops grown in dry regions. Plant scientists often talk about transferring traits conferring "drought tolerance" to crops, however. How would the traits listed above affect the yield potential of crop plants?

II. TEMPERATURE

The seasonal and diurnal variation in temperature increase with latitude. Increasing temperature in the temperate and boreal zones results from the increasing solar radiation in the spring and summer. In humid areas this increase tends to lag behind the increase in solar radiation; in arid regions there is a greater correlation between daylength and temperature.

Temperature also decreases with increasing altitude (according to the Universal Gas Law); the rate of temperature change with altitude is called the lapse rate and is about 1 C 100 m^-1 for dry air and about 0.6 C 100 m^-1 for wet air [Why the difference?]

Most plant processes have an optimum temperature. At the cellular level, physiological minimum and maximum temperate limits are determined by membrane structure and function, which is in turn determined in large part by the fatty-acid composition of the membrane phospholipids.

Respiration increases monotonically with increasing temperature.

Plant development is mostly controlled by temperature. Plants sense environmental temperature in terms of degree days --the cumulative number of the degrees above a base or threshold temperature. The threshold temperature depends on the species and the developmental process, but base temperatures are often higher in tropical crops.

Crop plants exposed to higher than normal temperatures develop at a more rapid rate (for example, flower earlier), which could decrease yield.

III. PRECIPITATION

Mechanism of condensation/precipitation

• The amount of water vapor the atmosphere can hold is called the saturation vapor pressure (e*) and is a function of temperature: e* (mbar) = 6.11 exp[(17.27T)/(T+237.3)]. Study the graph showing this relationship provided in class.
• Relative humidity = actual vapor pressure/saturation vapor pressure
• Dew point is the temperature at which the actual vapor pressure = saturation vapor pressure; at temperatures below the dew point, water condenses.
• Rising air cools, resulting in condensation of water vapor (cloud formation); sinking air warms, resulting in evaporation of water particles suspended in the atmosphere (clouds). Precipitation is thus always associated with the lifting of air masses.

There are three mechanisms that can result in the lifting of air masses; these will be illustrated and discussed further in class:

• Convectional
• Orographic
• Frontal

Global Circulation Patterns

Consider first the globe on the spring or fall equinox, i.e., the two times of year when the sun is directly over the equator. The amount of solar radiation reaching the surface of the earth (per unit) area is then highest at the equator.

Convectional transport of heat from the surface of the earth to the lower atmosphere causes that air to heat, decreasing its density, and thus causing it to rise. The equator at this time of year is thus a zone of rising air or low pressure. Air rising from the equatorial latitudes into the upper atmosphere is replaced at the surface by air flowing toward the equator from the poles.

At about 30^o N and S latitude, air in the upper atmosphere that has been moving pole-ward cools, condenses, and, due to crowding of the air masses, is forced downward. This zone thus becomes one of subsiding air or high pressure.

The low-pressure zone in the tropics is called the Intertropical Convergence Zone (ITCZ) or Equatorial Low; the high-pressure zones above the tropics is called the Subtropical High-pressure Zones (STHZs).

The ITCZ is always a zone of humid climates, while the STHZs are associated with arid climates.

Because the earth is tilted on its axis, ITCZ and STHZs shift during the year, with the ITCZ moving into the northern hemisphere during the our summer, and into the southern hemisphere in our winter. During each hemisphere's winter period, the STHZ moves toward the equator, creating dry conditions in the tropics during the winter winter months.

Given these basic circulation patterns, combined with other fundamental features of the earth's geography, you should be able to explain:

• Why the dry season in the tropics and subtropics is always in the (climatic) winter.
• What accounts for the monsoonal climate of south Asia.
• Why Mediterranean climates, with wet winters and dry summers, are located on the west sides of all continents at about 40^o latitude.

URL: http://ag.arizona.edu/~spmcl/lecturenotes/climatenotes.html