Answer:
thermometer(more likely) or barometer(less likely)
Explanation:
Weather vane is obviously incorrect, because it is used to show the direction of the wind.
Thermometer is more likely, because you indicated that the change was gradual, and it is cold when it is snowy.
A barometer is less likely, because it measures atmospheric pressures. It is also used to measure the wind. In general, if a low pressure system is on its way, be prepared for warmer weather with storms and rain. If a high pressure system is coming, you can expect clear skies and cooler temperatures.
