Washington DC in Sept.?
 

For a visit to Washington DC in September, how would the weather be.? Would there be less crowds than in the summer?

Hot and humid weather. Fewer crowds after Labor Day than in the summer.

Typically hot/humid in early to mid sept. later in the month some "possible" relief, not quite as hot/humid. Crowds are less, but congress comes back into session after Labor Day and metro/traffic can be daunting during peak rush hour periods. Most attractions are not as crowded, but will need tickets for Wash. monument and capital. if you want to visit the ACTUAL White house, you must contact local senator/representative office to arrange, otherwise you can visit the White House visitor center

I love DC in late September. I've been several times during that time and it's always been warm but not unbearble. Nice weather for walking around.

I think the temps in Sept are just fine, one of the nicest months in DC. early on it's generally in the low to mid 80's and humidity drops considerably in Sept. Late Sept more likely to find temps in the high-mid 70's. Typically there will be a couple hot days, but it doesn't last.
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Tourists decrease in Sept

Have been numerous times and I find Sept to be still too hot for a lot of touring and walking. Highs are usually still in the 80s and you can easily get some days over 90 - although it does cool down at night.

IMHO the weather is much better for visiting in October.