Why are thermos’s better at keeping things cold rather then keeping things hot ?
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There are several comments on rate of thermal transfer being based on the temperature difference, which is true.
One more characteristic that comes into play is convection currents. Hot liquids are constantly circulating heat to the top of the liquid, transferring it to the air above. In many cases, the lid/opening to said container is on the top. The lid for vacuum insulated containers is typically the weakest spot for sealing heat in, resulting in thermal leakage.
On the other hand, cold liquid/ice becomes self insulating by developing a static covering of chilled air. This will slow the transfer of heat down into the liquid.
Anecdotally, I have a Yeti tumbler that I use open-top for hot and cold. Hot drinks tend to come to room temperature within a couple hours, whereas iced drinks will hold ice for a surprisingly long time (8+ hrs) as long as the cold air covering isn’t disturbed.
Bonus hack: if you get a Frosty from Wendy’s (or equivalent dessert), take the whole cup and nest it into an insulated tumbler. The ice cream stays cold way longer, plus your hand doesn’t get cold and wet from the condensation. Enjoy!
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