This is the major factor in determining the suitability of a region to viticulture. Sufficient heat for a sufficient length of time is required by the vine to complete its yearly growth cycle. However, most quality wines are produced in climates that are only just suitable for vine culture. This may be because slow berry ripening produces the finest tastes and aromas.

Climatic indices are useful for comparing potential and existing vineyard sites and determining which varieties are most suitableThe most commonly used is Amerine & Winkler’s heat summation system.

Amerine and Winkler (1974) found that the quality of wines produced in California reflected the heat summation of the sites on which they were produced.The heat summation for vines is usually calculated by measuring the mean temperature for the month, subtracting 10ºC (the minimum temperature for vine growth), and multiplying the result by the number of days in the month. For example, if the average temperature for June is 15ºC, the heat summation for that month is

(15 - 10) x 30 days = 150 Growing Degree Days (GDD)

The heat summation for every month is then added together to obtain the heat summation for the year. Average heat summations for major vine-growing regions are:

England has a very cool climate indeed; with heat summations averaging about 800 GDDs (Growing Degree Days), but the trend over the last 20 years has been for a significant increase.

Category I vineyards (less than 1350 degree days) are said to produce the finest dry white wines. Category II & III (1351-1930 degree-days) are said to produce the finest red wines. Vine varietals can also be classified by the same sort of categories, allowing them to be matched to their appropriate areas.

There are considerable inaccuracies in the Heat Summation system:

• Relationship between vine growth & temp is not linear
• Vines can be actively growing in months when the mean temp < 10ºC
• Soil temperatures are not taken into account
• Doesn’t take cold nights into account

There are other indices that can be used:

• Continentality - The difference between the average mean temperatures of the hottest and the coldest months.
• Latitude-temperature index - LTI = Mean temperature of the warmest month (60 – latitude)
• The average mean temperature for the ripening month.

Very low winter temperatures will injure vines: freeze injury to dormant vinifera buds and wood begins at -15ºC, is very serious at -20ºC, and -25ºC is usually fatal unless the vine is insulated by snow. It is reckoned that a site will not be successful for vine culture if its temperature falls below 20ºC as often as every 20 years or if the mean temperature for the coldest month is less than -1ºC.