nethoncho/Arduino-DHT22 — pull request #1 by capswiz (2011-12-30 10:17:08+00:00)
Title: BUG : return DHT_ERROR_CHECKSUM when temperature is negative.

So I removed "currentTemperature &= 0x7FFF" line 208 and put "& 0x7FFF" in the expression line 209.
Now it works with negative temperatures.