Readings & Meanings:
warm (to the touch); temperature
温める (transitive form)
温まる (intransitive form)
This onsen's not warm. The temperature is low.
Shall I warm up your gyoza?
I get warm from soaking in the onsen.
What, you don't know the pleasure of Japanese 温泉／onsens? Brief introduction: They are basically public baths which are filled with natural hot water (which they call by the generic term for hot water, お湯). The baths are smelly with whatever mineral the water has collected (usually sulfur), and you have to be naked in them with other people, unless you find a private one. Sounds terrible? Your loss; onsens are relaxing and reinvigorating, adn they're a big part of Japanese culture.
I recently went to Beppu, in the Kyushu part of Japan. It's a town famous for onsen. They have so many kinds of hot springs there, occasionally they get a little creative. Below is pictured a sign for a "どろ湯". Can you figure out what that is? Click the picture to go to my Flickr account to find out. (You can also check out my Beppu pictures with this link.)
Try not to get this kanji confused with 暖, the other あたたかい. Remember, 温 has a reading of オン, and it is for TANGIBLE warmth and temperature. 温 is for food, baths, things you can touch. It's even in the kanji: the three splashes of water on the side, and the plate on the bottom, will help you remember this kanji's for water and food.