Ah, yes, heat and some other situations can make noise appear worse.
Also, keep in mind that the raw file has an embedded jpeg preview (a thumbnail) and a lot of software will show you only the preview jpeg, until you actually open the raw file in a raw converter software. I am not sure what the Bridge shows, but if you open the raw file and apply standard noise reduction, it might turn out that the noise isnt quite as bad

Btw, there are also some special plugins and NR software out there. Like Noise Ninja, Topaz Denoise, etc. These can be even better than the default raw converter's NR.
And for long exposures, you might want to enable "slow shutter NR" (or dark frame reduction), where the camera, after a long exposure, closes the shutter and takes a second exposure with the closed shutter, then subtracts the dark frame's noise from the original photo. This can help out a lot, if you have the time to take the second dark frame. Be sure to turn it off if you need to take rapid long exposures without missing a beat.