Washington DC average August - 31/22C, Cairns January 31/24C, humidity range comparable. So it's a couple of degrees cooler at night in DC, but during the day/active hours, it can be downright awful, especially if you're outside queuing up to visit some monument.

I think DC and the northeast (NYC can also be very hot and humid, but unlike DC, it wasn't built in a swamp) are marvelous places, and I won't discourage you from visiting them.

However, with only three weeks, I'll just raise the question about priorities. If you skipped the east coast this trip, there's more than enough to see and do in the west that I'd just bet you wouldn't go home unsatisfied.

Allocate the east coast days to - I don't know, Vancouver BC and Whistler, or hell, even think about a cruise to Alaska (7 days return from Vancouver or Seattle) - or just a long weekend (Fri-Mon) taking the Alaska ferry from Bellingham WA (90 min. from Seattle) through the Inside Passage to Juneau, then fly back.

Use your "city" time in Vancouver, Seattle, San Francisco or LA, all of them far more pleasant in terms of weather than the east coast.

What is there to see between Seattle (or Vancouver) and San Francisco? Well, take your pick... idyllic islands with whales swimming around, dormant and active volcanoes with glaciers, temperate rain forest, "old west" sagebrush country, mile-wide rivers flowing through enormous gorges, rocky and stormy beaches, vineyards and frozen-in-time small towns, the redwoods - typically the highlight of most overseas visitors' trips - with farmers markets, county fairs, brewpubs, festivals... crikey. And in Vancouver, Seattle, Portland and San Francisco, four of North America's most beautiful, cosmopolitan and fascinating cities.

Three weeks is barely enough time to scratch the surface, much less see things in depth.

We have Israel-in-summer places, too - they don't call it Death Valley for nothing, you know.

Just a couple of teaser pics -

Seattle at last light



Olympic mountains from Seattle



Mount Rainier



Columbia Gorge, near Portland



Oregon coast



Redwoods (car for scale)