The Main Line has four seasons, for better or worse. Fall and spring are obviously the most pleasant times to be here. During the school year, you'll need all kinds of clothing, though, if you're pressed for packing space, it's best to take mostly summer clothes for the first semester. You'll only have six weeks until fall break, when you can go home to get your cold-weather stuff. As with every other season, winters vary considerably; you'll find everything from mild (around 45 degrees, with little snowfall) to absolutely frigid, and it's nearly impossible to predict what's coming next. During the coldest months, be prepared to bundle up and deal with some snow. If you're originally from a northern area, Haverford will almost certainly be an improvement; anyone from the west or south, however, may get a bit of a shock.
At the other end of the spectrum, it is also important to bring an electric fan or two when you first come to school. You can only get an air conditioner in your room if you have a medical need, so most dorms heat up quickly. Pennsylvania weather tends to vary between extremes, and most students find they either love or hate the local climate.