Definitely a manga written by someone that had never used glasses once in their life... your glasses get fogged when the weather is cold and you enter a hot place. Also, they defog in a couple of seconds.
The answer is probably humidity, regardless if it's hot or cold. In their case, because of the hot weather, the air could be very humid and/or maybe because their glasses catch their body heat and moisture.
Ah, I also remembered when I get off a bus with very cool air-conditioning, the fogs stays for a long time lol