While the Gimblett Gravels Winegrowing District maybe slightly dryer than other localities within Hawke's Bay during the summer and autumn the major climatic factor is, in fact, air temperature. The amount of sunshine varies little across the region.
What really matters is how hot is gets in summer, simply put for us we need the warmest sites possible. Without getting into the physiology of the vine and grape quality we need to ripen the grapes earlier in the warmest conditions possible. If that happens every aspect of red wine will be better. And that means to produce high quality wines from the Bordeaux red varieties and Syrah the mercury needs to hit at least 25°C during the summer more times than it doesn't, and summer nights need to be warm enough to keep the vine working. Growing Degree Days during berry development need to be above 1000.
Many parts of the Hawke's Bay region may be slightly cool for the ripening of the 'Bordeaux' varieties and Syrah. Unfortunately these varieties are still planted in some of these localities. Gimblett Gravels Winegrowing District has enough summer heat to do the trick in most years with maximum daytime temperatures between 2°C and 3°C warmer than many other viticultural localities on a classic Hawke's Bay summers day. Significantly these high temperatures are reached earlier in the day and last longer into the evening. On cooler, cloudy days there is very little difference between the Gimblett Gravels Winegrowing District and other localities during summer except slightly higher night temperatures. Also in vintages where the whole region experiences hot dry conditions, such as 1998, the Gimblett Gravels Winegrowing District is still significantly warmer than other localities. However the effect on the wine may not be as great as in more normal years such as 1999 as these normally cooler localities have reached the 25°C base required in warmer years such as 1998.
The higher temperatures are due to an interaction between the districts sheltered location some 15km from the sea where the sea breeze is significantly warmer; its relatively low altitude of only 30m above sea level; the south-westerly shelter of Roys Hill; and, very importantly, the soil type.
How does soil type affect air temperature? The gravely soils warm up early in the spring, dry out rapidly and act as a big thermic blanket under the vines of the Gimblett Gravels Winegrowing District with soil temperatures at 30cm below ground level being some 5°C higher than other localities. Warm soil temperatures have a direct impact on hormonal triggers in the vine during ripening, nice warm soils in climates such as ours giving the most desirable effect. This thermic blanket also releases heat in the evening to the surrounding air, meaning the air temperatures stay warmer throughout the night sometimes into the early hours of the morning.
Even with these higher temperatures the Gimblett Gravels Winegrowing District still only reaches the bottom of what we regard is a desirable maximum daytime temperature for producing high quality wines from the 'Bordeaux' red varieties. We rely on terroir manipulation to get us right into that zone in more years than not!
Having said that however, when viewed over a 24hr period, vines in the Gimblett Gravels Winegrowing District receive significantly more heat than other localities, that is not adequately represented by the standard climatic indices of maximum and minimum temperatures and growing degree days. We believe this is an important factor in the GIMBLETT GRAVELS terroir without having any scientific basis for the opinion. There's something to be said for the gut feel of a farmer, and that's all we are, fancy farmers! Maybe the proof is in the wines.
There are likely other sheltered localities within the Hawke's Bay region that also have these warmer daytime maximum temperatures, however the Gimblett Gravels Winegrowing District is the only area where this occurs in association with the gravely soils. Sheltered sloping sites in the Havelock Hills and around Bay View have proven to have equal ripening capacity to the Gimblett Gravels Winegrowing District . And of course in the hot vintages such as 1998, many localities in the region are able to ripen these varieties fully.