A common effect of cold weather in regions where it is COLDER, is for people to do less physical exercise.
WHY is this?
However debatable, most people who live in winter climates suffer a form of depletion in energy (a conservation) whereby they go into "conservation mode". A natural adaptation for generations of people. This conservation mode allows for physical strength and exertion when asked, but tricks the mind into a depressive state. Partly due to lower vitamin D, partly due to immune system regeneration and support for fighting sicknesses.
There are the obvious reasons that people do not exercise as much in winter such as outside weather and lack of indoor facilities. We all know there are no real excuses.
Exercise itself, is great for the immune system but MUST be coupled with a regime of supportive nutrition. The exercise in the winter needs to be different than in the warmer months. For instance, focus on joint strengthening and spine exercises. Flexibility is important and can be a good time in the winter months to focus on. In cold temperatures those that suffer postural strain and joint pains normally will be amplified in the winter. Therefore keeping those joints in balance strength wise, and length wise, is very helpful to reducing pains and strains.
For the mind; exercise is extremely beneficial as supportive hormones are secreted to balance out depression feelings and aid motivation. To benefit do not leave long breaks in between exercises during the routine. Also exercise frequently with less focus on cardiovascular training.