Here's the second in the series of Things Ultrarunners Know.
This is pretty simple: if you are going to run long distances and be out there for hours, it helps to know what the weather is going to be. As a minimum, you need to know which local source of weather information is best and most likely to be accurate for your situation.
For example, one can always go to The Weather Channel, but I find that my local NBC affiliate, Channel 25 in Hagerstown, MD, is usually spot on and better than more national type resources.
Beyond the forecast, obviously, your personal observations are also critical--what's the humidity? Which direction is the wind coming from, and at what speed? I find that in the summer I tend to run east more often, so that I can enjoy the cooling westerly breezes as I finish my run. In the winter, I tend to run west first, into the teeth of the wind to get it over with, then cruise home with the wind at my back.
One final note: especially in the early morning or evening, I love to experience what I call micro pockets of air: areas where you feel a distinct change in temperature or humidity. Usually this is associated with a low spot or depression where cooler air tends to pool. But sometimes in the morning you can be running along in the coolness, then top a small rise, where the air has a noticeably warmer feel.