Climate
Ukraine is in the mid-latitudes, and generally has a continental climate, except for its southern coasts, which have cold semi-arid and humid subtropical climates.[1] The period between April and October is usually very comfortable and is when most volunteers stay in Ukraine. From November to March it can be a bit chilly outside, but if you wear the right thermal underwear, beanie, face mask, scarf and winter jacket you'll be fine. The number of volunteers is usually smaller from November to March, so I encourage volunteers to stay in Ukraine from November to March, as this is also when Ukraine needs help the most.
Useful links
Ukrainian Hydrometeorological Center