Temperature is the amount of kinetic energy in a system. Are we feeling the force of the molecules hitting our skin when we detect something as being hot? Do we feel the kinetic energy in our skin decreasing when something feels cold? What do we actually feel when we detect temperature differences between our bodies and the objects with which we come into contact?
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What we feel is the transfer of heat energy between our skin and the object we are touching. If energy is transferring from the object to our skin out body interprets it as feeling hot, while if energy is transferring from our skin to the object our body interprets it as feeling cold.
We are feeling the increased temperature of our skin/body, and more specifically we are feeling the rate of heat flow into/out of our body.
The questions “what is “temperature” and how do we measure it” and “how do humans feel temperature” are two different issues, one is physics, the other biology.
Since you appear to be asking about human sense – we don’t feel temperature we feel *changes in temperature* and sometimes our senses are cheated by the presence of chemicals that interact with our nerve endings.
This is why being in 70F air feels so different from being in 70F water. Both are the same temperature but what we are feeling is the flow of heat outwards from our skin into our surroundings. Air is a great thermal insulator so it pulls heat out slowly, so we feel ‘comfortable’ in 70F air. But water sucks up heat like Greta Van Sustern goes through hotdogs so we feel a huge outflow of heat into 70F and it’s chilly to be in.
Now human thermal comfort is also tied to our ability to sweat, so 80F air on a dry fall day feels nice where as 80F in 100% humidity could very well be life threateningly hot (a “wet bulb” event)
Finally, some of our temperature sensors can be fooled by chemicals. Capsaicin (from hot peppers) and menthol (from mint-family plants) both trick our hot and cold nerve ends respectively. That’s why ‘hot’ peppers feel ‘hot’ (even though they might actually be cold) and why mint gum or soothing chest rubs might feel cold, even though you are actually hot.
The thermoreceptors in the skin are nerve endings that respond to changes in temperature – some to warming, and some to cooling. (The exact mechanism isn’t fully understood.) But it is primarily change in temperature that triggers them, not absolute temperature. So if you grab a cold can from the fridge it will feel a lot colder than the (actually equal temperature) plastic bottle because the aluminum conducts heat energy away from your skin a lot faster than the plastic does. When you detect something as hot I guess it is sort of correct to say that you are feeling the force of the molecules hitting your skin, but only in a very roundabout way – the energetic vibration of those molecules is what heat is, and a hot thing will transfer some heat energy into your skin, changing the temperature of your skin, which is what activates the warming thermoreceptors. It’s sort of like saying that when you hear the sound of a motor running, that sound is the kinetic energy of the motor being detected by your body – which of course, it is, but in a roundabout way
You pretty much got it from a physics perspective. A couple of things to note. Temperature measures the average kinetic energy of a system, not the total. How hot/cold something feels is ordinarily determined by the rate of the kinetic energy being transfered. The rate of heat transfer will be based on the materials’ thermal conductivity.
The actual “feeling” of hot and cold is more of a biological question. Biology could give an explanation involving nerves, the brain, and whatever else may be involved. However, your post seems more targeted to the physics of what’s happening.
Heat/cold is transferred via matter. Different states of matter conduct heat better than other. Say when you go outside and touch a metal pole it feels really cold even though it’s the same temperature as the air, but metal conducts heat really well. Air conducts heat less than metal and is a great insulator because of that. Liquid transfers heat much better than air.
There does have to be physical contact with matter to conduct heat transfer.
You can’t actually feel outside temperature. If you hold your hand in water that is at 20°C / 68°F, it will feel quite cool, whereas air of the same temperature feels pretty neutral (or at least not nearly as cool). Similarly, a piece of metal that’s colder than your skin feels much colder than a piece of wood that is at the same temperature.
This is because, what you actually feel is the temperature (and changes in temperature) in your skin. That may sound the same, but it actually makes a big difference, because it means that for something to feel cold, it has to cool your skin. And the thing is, not all materials are equally good at transferring heat. Water, for instance, is quite good at this. Cold water next to your skin will readily absorb heat and cool your skin, while warm water will readily transfer heat to your skin. Metal is also very good at this. This why we make pans out of metal (among other materials), and boil things in water. But something like wood is very bad at this, which is why on a hot, sunny day you can safely sit down on a wooden bench. Air also does not transfer heat very well, which is very fortunate for us because it means we can tolerate quite a wide range of air temperatures around our bodies.
(Strictly speaking, there are actually two relevant properties: thermal conductivity and heat capacity. Thermal conductivity refers to how easily a material transfers its heat. Heat capacity refers to how much heat the material can hold. For instance, you need to add much more heat energy to a kg of water to heat it by 1°C than you need to heat a kg of air the same amount (about 4 times more). Water actually doesn’t conduct heat all that well, but it does have a very high heat capacity, so it still ends up delivering or absorbing a lot of heat when it is at a different temperature to your body.)
If you train yourself, you can get very good at estimating the temperature of a given material (say, water) based on how it feels, but that skill won’t transfer to judging the temperatures of other materials, because they transfer heat at different rates.
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