Why are thermos’s better at keeping things cold rather then keeping things hot ?
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The *rate* of heat transfer is based on the difference in temperature between two places.
Let’s assume it’s normally around 70F outside of a thermos.
A cold drink might be around 35-40F. That’s an approximately a 40F temperature difference at *most*.
A hot drink might served around 180-190F. That’s approximately a 110F temperature difference at *least.*
Since the difference in temperature affects the rate of cooling, and hot drinks are usually much hotter than room temp than cold drinks are cooler than room temp, hot drinks cool off faster.
Well for one, and I suspect the main reason, cold drinks are a lot closer to room/outside temperature than hot drinks.
Insulation (the blocking between the hot and the cool sides in a thing) works by slowing down the speed of the heat getting to the cold side. It will get there, the heat just goes slower the better the insulation AND it also goes slower if the difference between the sides is smaller! It is like a ramp and a fuzzy ball. If the ramp is not steep the ball’s fuzz can slow it. The steeper the ramp, the less the fuzz helps slow the ball.
Now. A hot drink at 145 inside the thermos is in a 70 degree room so the “ramp” is 75 degrees of heat difference. But a cold thing is probably 36 F compared to the 70 degree room. A difference “ramp” of just 34 degrees. An easier job for the insulation to accomplish is to keep heat from rolling down that smaller ramp.
The larger the temperature difference between your drink and room temperature, the longer it takes for the drink to reach room temperature.
The thing here, is more of your perception. When you get a hot drink like a coffee, it’s going to be really hot. It will cool fast, but is still quite warm (still warmer than room temperature). But you will feel like it isn’t as hot anymore (you are right of course), but it isn’t room temperature. The temperature difference will still be quite high.
A cold drink really isn’t that much colder than room temperature. It’s a drink out of your fridge. Because the difference in temperature isn’t that much, the drink warms more slowly. Even then, you still perceive it as ‘cold’ even if it does warm up a fair amount.
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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!
Basically, because hot drinks are more hot than cold drinks are cold.
You see, the heat lost (or gained) depends on how different the temperature of the liquid is from the temperature of the air outside it. The greater the difference, the quicker your stuff will heat up or cool down.
A cold drink might be around 32 degrees if it has ice in it (using US temps), and the air is around 72. That’s 40 degrees difference. A hot drink, like coffee, should be around 175 degrees, which is 103 degrees hotter than the air. It will lose heat over twice as fast as the cold drink heats up. So when your cold drink is 42 degrees, which is about the temperature inside your refrigerator, your coffee might be 150, which is a bit colder than most people like their coffee.
The bigger the temperature difference between 2 objects, the faster the heat will flow between them and the temperature equalize. (Very sloppy phrasing, but it’s ELI5 right?)
There is not that big of a difference between “cold” and room temp, while the difference between “hot” and room temp is significantly larger. The difference between “hot” and room temp is roughly 3x or 4x larger than the difference between “cold” and room temp, thus thermoses will be able to keep something cold much longer than hot.
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