It is snowing where Jenna lives. It's been snowing all day, but now the snow has changed to sleet and then to freezing rain. What is happening to cause these changes?

The snow went through different pockets of temperature throughout the day.

Explanation:

Snow forms when the temperature of the atmosphere is at 32 degrees Fahrenheit or below, while it is raining.

When this snow changed to sleet, all that happened was the snow warmed a bit while falling, causing it to melt a little. After this, it then went through a pocket of cold air before reaching the ground, causing it to refreeze, which in result, is sleet.

Freezing rain is quite similar to sleet in terms of changing temperature. In this case, snow hits a large warm pocket of air, causing it to melt more than (snow turning to sleet), making it melt completely, and turning back to rain. This rain then hits a small pocket of cold air right before reaching the ground, but not a large enough pocket to turn to sleet. In result, you get freezing rain.