Some people with OI (orthostatic intolerance), POTS (Postural Orthostatic Tachycardia Syndrome), etc. do better in warm weather and others so better in cold weather.
Increased humidity, poor air quality and heat have very negative effects on the two patients I know best.
In 2011 we found out about cooling vests (and other cooling products) and a friend gave us the following list of links to some cooling products*:
We were pleased to discover that the cooling vests and neck wraps do indeed make a difference for our household (even indoors).
Consider the humidity levels of your area.
If high humidity is frequent in your area, products that use freezer/gel packs (while more expensive) cool much more effectively than evaporative products.
* This list is a sampling of sites that have these products. We will gladly make this list more extensive if legitimate links are sent to us. Legitimate links for such sites in countries outside the US are also hoped for.