Warmer weather and/or air conditioning

Warmer weather will not affect your basal body temperature as your body is very good at maintaining itself at its optimal temperature. Meaning that you can still keep measuring as always even if the temperature increases/decreases due to seasonal changes.

If you're experiencing a big increase in outdoor temperature, such as in the event of a heat wave or traveling to a new destination, it may be best to skip measuring the very first morning if you're feeling warmer. After the first morning, you can continue measuring as usual. 

When traveling to a new country and/or starting to use air conditioning during the night, it may also be best to skip measuring the first morning as the cooler air may very well be cooler than at your home and normal environment.