In the past many wine books have been written about the regions around the world where grapes are grown and thrive for the production of wine. From the days when Winkler separated the regions by heat degree days and through to growing degree days and ripening temperature suggested by Gladstone.
Having worked all over the world to say one particular climate is more suited to a grape variety than another ignores the complexities of wine. For at every stage their are so many different combinations and possibilities that to say region A makes better wine than region B with a variety is sometimes more about marketing than actualities.
What can be said, that if your starting template for high quality wine is balanced fruit , sugar, acid and ripe tannin accumulation low disease and freshness. Then comparing your region , or ideally your site, with different regions and sites around the world can give an indication into what will grow well. In some regions there may be many varieties that grow well in others there may be a few based on limiting factors like temperature, rainfall and suitable sites.
So where do you start?
1/Find out where the closest or most representative climate station in your region is located and obtain the information from the latest 30 year period (1981-2010).
Steer clear of any major city information as invariably they tend to be heat sinks and will have a higher temperature than surrounding farmland.
2/ From this data get the following figures:
A/ Heat degree days latitude adjusted :
# this is the mean temperatures of the seven months of the growing season (Oct-Apr or Apr-Oct) minus ten degrees
# then multiplied by a day length factor (See Gladstones) above and below 40 degrees latitude (40 degrees being one)
# then multiply each monthly figure buy the days of the month
# add the monthly figures together to get HDD latitude adjusted
B/ Mean temperature of the warmest month
# this will tend to be mid summer unless you are in a strongly maritime region where sometimes late summer will be warmer.
# This is useful in ranking the apex heat load of the region in comparison to other regions
C/ Maximum Temperature of the warmest month
# This is important in differentiating between a milder acting climate and a more extreme one with similar mean temperatures. The vines can 'shut down' in higher temperatures typically above 35 degrees compared to being actively photosynthesizing at lower temperatures particularly if their is low water availablility.
# higher temperatures will require more moisture demand and typically regions with higher maximums to mean temperatures have much lower relative humidities so are more likely to suffer damaging stress unless soil is a mitigating factor.
# Colour development can also be delayed in the berry skins with higher temperatures.
D/ Average growing season rainfall (mm)
# the mean rainfall is the most readily available figure for comparison
# however the median rainfall is the better indicator of moisture status
# regions with similar rainfall will more than likely have similar disease pressure if not accounting for wind factors
3/ Collate these figures together and compare it to the table below.
#The HDD figure is more involved but is critical in placing your region in the right heat zone (including daylength).
# Use the HDD as Temperature (HDD / no. of days in the season plus 10) match the regions within 1 degree
# match regions within 1 degree of your region for MTWM and Maximum temperature of the warmest month
# Match regions within 100mm of rainfall to your region
# Regions with three to four matches are similar
4/ The closest matching regions to yours will be a useful starting point on what is suitable to grow in your region.
# The first three indices A, B and C are more critical in regards to wine style being about temperature, but D (rainfall) is important in regards to disease pressure and can be useful in deciding what soils are appropriate for what varieties.