Brief very hot periods occur when modified dry continental air invades the area from the desert region of the southwestern U.S., often raising temperatures to 35 C or more. The growing of grapes requires careful consideration of the microclimate location. Growing conditions are determined largely by the solar radiation received at site. The sun is the primary energy source for practically all physical and biological processes on earth. Solar radiation has a direct effect on air and soil temperature, transpiration, soil moisture, atmospheric humidity, photosynthesis and photostimilus processes such as heliotropism, elongation, and flowering.
Solar radiation influences the level of ripeness a grape can achieve.
This can be compensated somewhat by cluster thinning, through use of trellis designs which allow maximum light penetration into the vines. The amount of solar radiation received at a site varies widely with slope and aspect, particularly in hilly or mountainous areas. The greatest amount of solar radiation is received on 20 to 40 degree south-facing slopes. North-facing slopes receive considerably less radiation while east and west slopes receive an intermediate amount. In general, the westerly slopes will be warmer than easterly slopes, because energy received in the morning is often used to evaporate dew as opposed to heating the air or soil.