The temperature you ferment at depends on the style of beer you are brewing.
For instance
Ale fermentation (using and ale yeast) average range 18-22c
Lagers average 10-12c (using a lager yeast) some funky belgians might go to 25 or even higher. Yeast themselves prefer a higher temperature but the beer quality suffers
The easiest way to make the whole weather thing go away is to ferment in a temp controlled fridge.
Also try a wet towell wrapped around the fermenter keep it wet and in a breeze (fan?) works best on low humidity days. You may want some sanitiser in the water as the towel can go mouldy.
From White labs-
The optimum temperature for yeast growth is 32 c. Yeast cell death occurs above 38 c. Why then, dont we ferment our beers then at 32 C, decreasing the time that it takes to make beer? Why do we make yeast work slower? Because what is best for yeast, is not best for beer. As they grow and multiply, yeast produces many compounds, the most noticeable of which are esters. As the temperature fermentation rises, more yeast growth occurs, and consequently more esters are produced. At 32 C yeast produce so much acetaldehyde (which tastes like apples) that the beer becomes undrinkable. The optimum ale fermentation temperature has been found to be 20 C (18-22). This temperature strikes the best balance between yeast growth and ester levels for most ale strains. For Hefeweizen-style beers, some brewers like to ferment above 26 C, which increases the level of banana- flavored esters produced by these strains. Most ale strains are unable to ferment or grow at 12 C, which is the most common lager fermentation temperature. This fermentation temperature greatly reduces the ester-forming ability of most lager strains, creating the clean flavour associated with lager beers. Ester levels are kept low, placing the emphasis on malt and hop flavors.
