Also, if you increase the speed and intensity too quickly, and the muscles do not have time for repair, because you may also be increasing the frequency, then people will begin to experience a systematic response from over training, which includes lack of concentration, fatigue, and loss of desire to run. Probably the two most important things that need to change with the changing weather are how you dress, and how you hydrate. In the winter, it's very important to wear clothes that keep moisture away from the skin but also provide insulation from the cold. In the summer, it is more important to allow sweat to evaporate, which enhances the cooling process. So in the summer most people are advised to wear a light top and shorts, and most often, it is recommended that the top be a light mesh material to allow air to flow through and enhance cooling as well.
Hydration is also important.
In the winter because it's cold, people don't feel as hot, even though they are sweating. Also, because the air is dry, moisture is drawn from the mucus membranes. Both of these factors can cause dehydration. So it is important to have fluids available for long runs. In the summer, as the humidity rises.