It is difficult to write a meaningful summary about when to travel where in the US as one has to attempt to summarise the weather of a continent. It is also difficult to be categorical about the weather by State as some states are huge and have very different weather patterns depending on exactly where you are.
Chicago can be extremely cold in the winter!
In general, spring and autumn are great times to visit much of the US and excellent times for a city break. Summer can be very warm and humid over a great deal of the US including the southern States, the Midwest and the East Coast. The West Coast, more northerly States and mountainous regions are cooler. Winters can be brutally cold in the Midwest and on the northern half of the East Coast. Temperatures are much milder in the southern Sates and on the West Coast.
To help give you some idea of the best places to head for at the time you are planning your trip we have summarised what we feel is the best time of year to visit different parts of the US by season in our Holiday Inspiration section.
If you want to check on the weather in a specific place, the best resource we have found for
Winters in Santa Fe are much milder
obtaining data on average temperatures and rainfall is weather.com. If you go on to their website it will automatically direct you to the UK site but for whatever reason this does not have the useful summary graphs for US destinations that appear on their main site, so make sure you are on weather.com rather than uk/weather.com.
From the landing page if you put your proposed destination in the search box it will take you to a page showing whatever the current weather for that destination is. On the left hand side you will see a tab that says ‘monthly’. Click on that and it will take you a page showing average temperature and rainfall by day for the month you are in. You have the ability to scroll through and get the daily detail for different months, but I find the most helpful information is obtained by hitting the ‘averages’ link which sits just below the temperature table. This takes you to a really helpful summary graph showing average monthly highs and lows and one showing average monthly rainfall. The link below will take you to the information for New York City as an example.
http://www.weather.com/weather/wxclimatology/monthly/graph/USNY0996