Giving other monsters some love~!
Alphys and her son with Undyne, Alpine.

seen from T1

seen from United States
seen from Indonesia

seen from United Kingdom
seen from Peru
seen from United States

seen from Australia

seen from Türkiye
seen from Indonesia
seen from Türkiye
seen from China

seen from United States
seen from Israel
seen from South Korea
seen from Russia
seen from United States
seen from United States
seen from China

seen from Jamaica
seen from China
Giving other monsters some love~!
Alphys and her son with Undyne, Alpine.
Twas thinking about it... So please take these reactions of some of the UT characters to the different weather and seasons on the Surface!
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Papyrus and Sans dont mind winter since Snowdin was super cold, but when it's any other season they're way too hot and can't handle it. Summer is notably worse for Papyrus because the heat is so bad he can't get any work done, which makes him especially annoyed. Spring is notoriously bad for Sans because it's more rainy, and not only does it mean his slippers would get absolutely soaked which inevitably means wet socks, but he also can't stand thunderstorms.
Undyne physically doesn't do well in most weather conditions unless it's humid or raining, but she forces herself to work normally regardless. When it's hot and dry, she works herself way too hard and it's Alphys who has to make her take a break. Undyne tries to combat the dryness by keeping full water bottles on herself at all times, but even still she still can get sick if not careful. Alphys practically has to force her to stay inside during hot, dry days in the summer.
Alphys doesn't mind most conditions, but prefers summer the most. Summer is nothing compared to the heat of Hotland, but it's comfortable, so she likes it the best. However, Alphys hates snow, so she usually stays inside during winter w hot chocolate and anime binge watching.
Mettaton, Mad Mew Mew, and Napstablook are unaffected by the weather due to being ghosts. Mettaton's body temperature, however, can't get too hot or cold or he'll crash until it goes back to normal state -- he usually operates at about 35°C (95°F), but if he gets below 20°C (68°F) or above 60°C (140°F) that's when his body is in critical condition. Mew Mew's body, being a doll, isn't affected by weather at all. Muffet lives in a similar condition as Mew Mew.
Grillby and any other elementals will remain the same temperature no matter the season. However, Grillby will be in extreme pain if he walks outside in the rain.
Toriel, Asgore, and Frisk have already before lived on the Surface, so they knew what to expect, but Toriel and Asgore being gone for so long took a while to readjust. Toriel notably hates rain because it soaks her fur and it takes forever to dry. Asgore does not like heat in any form.
Look at these nerds, I love them <3