A recent medical-journal article reported research showing that there was a direct correlation between higher temperatures and lower blood pressure. We should expect this to be the case, since it is very-well established that higher vitamin D levels lead to a dramatically reduced risk of hypertension (high blood pressure). (See my previous post for more information on the relationship of blood pressure to vitamin D levels. http://drsorenson.blogspot.com/search?q=hypertension)
In summer, when temperatures are higher, the sun stimulates the production of vitamin D very efficiently; in winter, there is little or no production of vitamin D by sunlight, because the ultraviolet B light (UVB) is filtered out. It is also true that people like to go outside more in summer than winter, and that they use less clothing, thereby exposing more skin to the sunlight and making more vitamin D.
Therefore, if you suffer from hypertension, get outdoors in the summer (never sunburn). In winter, use a tanning bed (never burn), which is exceptionally efficient at stimulating vitamin D production, or take supplements of 3,000-5,000 IU daily.
One of the biggest lies in medicine is that a person must stay on hypertensive drugs for a lifetime if he/she is hypertensive. It is simply not true. In most people, sunlight and a plant-based nutrition program can easily bring blood pressure levels to normal. Ask your physician before changing any prescribed medication, of course.
 Alpérovitch, A. et al. Relationship between blood pressure and outdoor temperature in a large sample of elderly individuals: the Three-City study. Arch Intern Med. 2009 Jan 12;169(1):75-80.