NASA has a service where they will read the temperature anywhere on the globe, to an accuracy of .0002 degrees. The only trick is you have to have your jug outside when the satellite is going over. Plus, it costs $110,000 per, but for good beer it's worth it.
Like ncbrewer said: swamp cooler, thermo strip, towel (or t-shirt). Unless you're turning pro, there's no need to overthink it.