A predicate form (e.g., жарко, тепло, холодно, прохладно) is used as a state applied to some circumstances, so you can (in principle) say that you feel hot or that it was hot somewhere / at some moment in time:
- Мне жарко
- Здесь очень жарко.
- Вчера было жарко.
- Дома жарко.
If you instead say that something was hot it does not work.