Icon meaning

I need to replace icons from this wp-content/plugins/wp-forecast/icons/ folder with my own icons so that the style of the icons match the style of the website.

Could you please explain when is the thermometer icon used?
Does is appear on the website when the temperature is extremely hot or I misunderstand something?

Thermometer icon filenames:
30.gif, 31.gif
Hot thermometer: http://postimg.org/image/yuz365hav/
Cold thermometer: http://postimg.org/image/ffobd1o0n/

And also could someone to explain what type of weather does this 32.gif icon represent. Link: http://postimg.org/image/kd1vy5pzr/

http://wordpress.org/plugins/wp-forecast/