The antonyms for the word "semi-tropical" include terms like polar, arctic, frigid, subarctic, and icy. Contrary to the warm and humid climate of a semi-tropical region, areas with these antonyms experience extreme cold and harsh winter conditions. Polar and arctic zones are characterized by long periods of darkness and temperatures that can drop below zero degrees Celsius. The frigid and subarctic zones, on the other hand, also experience cold weather conditions, but not as severe as that found in polar or arctic regions. Lastly, icy refers to areas covered in ice or experiencing icy conditions, such as glaciers, frozen lakes, and snow-covered mountains.