The Winterometer is really just a giant thermometer which can measure the ambient air temperature between 0 and 35 degrees. While the temperature can go beyond these ranges (-20 to 40 degrees) but it covers the most important range. If your body temperature reaches below 0 degrees (which requires the air temperature to be under 0). Crops don't grow under 5 degrees but they grow.
The presence of fires or other heat sources can affect the Winterometer which allows it to be used to gauge the temperature at a certain distance from a fire but to get an accurate reading of the temperature, you'll have to place it beyond the range of the fire.