i've found a lot of thermometers to be pretty unreliable under typical astro conditions.. once the dew/frost kicks in they often show temperatures that are clearly way lower than real ambient temp.
i'd mount your thermometer under your observing table or something similar so that it stays dry (less dew) but still gives you a good reading of 'ambient'.
there's quite an art to getting an accurate measure even of something that seems as simple as ambient temp.. a stevenson screen from a weather station would be a good start
