Diurnal temperature variation is a meteorological (also a viticultural) term that relates to the variation in temperature that occurs from the highest to the lowest during the day. Temperature lag is an important factor in diurnal temperature variation: the peak daily temperature occurs after noon, as air keeps net absorbing heat even after this time and similarly the minimum daily temperature occurs substantially after midnight, happening during early morning in the hour just before dawn, since heat is lost all night long.
This variation in temperatures has a significant impact on a grape's ripening pattern; the heat of the day promotes sugar accumulation, while the cooler night-time temperature preserves acidity levels.

 

On a warm summer’s day, for example, air temperatures may vary by 16-17°C from just above the ground to waist height - (around the height of the berry bunch growing zone). Incoming solar radiation exceeds outgoing heat energy for many hours after noon and equilibrium is usually reached from 3-5 p.m. but this may be affected by a variety of different things such as large bodies of water, soil type and cover, wind, cloud cover/water vapour and moisture on and in the ground.
Other natural effects on temperature variations include strong winds which can alter the air temperature between 10°C and 20°C - also the presence of large bodies of water and mountain ranges can have effects on the climate and vines. Nearby lakes and rivers can serve as protection for drastic temperature drops at night by releasing heat stored during the day to warm the vines, Rivers may raise the ambient temperature 1-2 degrees.
Diurnal temperature variation (DTV) is of particular importance in viticulture. Wine regions situated at high altitude experience the most dramatic swing in temperature variation during the course of a day. In grapes, this variation has the effect of producing high acid and high sugar content as the grapes exposed to sunlight increases the ripening qualities while the sudden drop in temperature at night preserves the balance of natural acids inside the grape.
Photosynthesis finishes with the start of darkness, but, if the night-time temperature does not fall below a certain level, respiration and flavour and tannin synthesis will continue, resulting in more rapid and complete phenolic ripening of the fruit at lower sugar levels. Diurnal variation in arid regions, allows production of high levels of sugar during the daylight hours and the cool nights take the vines out of the effective metabolic range. This slows the phenolic ripening process thus allowing the accumulation of higher sugar levels over the longer ripening period and an unbalanced wine.
In arid regions, cool nights are essential for the prevention of rapid acid metabolism. Different wine styles are produced in high-DTV areas versus narrow-DTV areas. Areas with narrow-DTVs generally gain phenolic ripeness at lower sugar levels than do their high-DTR counterparts.
This Diurnal Shift - one of the greatest natural phenomena for grape growing which end up balanced between ripe sugars (which will equate to alcohol in the wine) and crisp acidity is a difference between day time and night time temperatures - or, diurnal shift. Washington State in the US and Central Otago in New Zealand - have some of the most dramatic fluctuations of any wine region in the world.
Malic Acids generally dissipate through respiration from the grape in constant warm temperatures. Cool evenings preserve the acid, which translate through fermentation to wine and adds freshness and balance. The resultant acid in the wines is natural - and appears more integrated than the types added in the winery. A continental climate leads to large diurnal temperature shifts, the difference between day and night temperatures can range from 35ºC+ during the day and down to 0-5ºC at night. The hot days and cool nights are one of Central Otago's best attributes in growing grapes as this aids in developing flavour and colour intensity and complexity as the grapes ripen slowly and allows a long hang time. This slow ripening also helps retain acidity, which aids the stability of the wines.