In most cases, conductive heat transfer happens more rapidly than convective heat transfer. That is, heat transfer through solid materials is more profound than that in liquids or gases. You’ve probably experienced this in your everyday lives, knowing that we can “sense” heat transfer when we feel warm or cold ourselves. The heat or cold that we feel is known as heat flux, which is heat transfer per unit area. Therefore, in most cases if we have a metallic object and a roomful of air at the same temperature, touching the metallic object will feel warmer (be careful of burns!) than simply standing in the room and absorbing the ambient temperature.

Wear gloves when touching hot metal surfaces!

Why does this happen? Intuitively, solids are denser than liquids and gases, meaning the molecules in solids are more closely-packed. This means that it is easier for heat to be transferred from molecule to molecule in solids, which would explain why heat transfers faster in solids.

When designing an apparatus for heat transfer purposes, one must consider two things: cost and effectiveness. Natural convection (such as with air) is relatively inexpensive because air is everywhere, but it isn’t as effective as using a metallic solid for heat transfer purposes. However, metals can be expensive. Therefore, some form of middle-ground is often desireable. This can be seen in computers, where fins conduct heat away from, say, a processor, and a fan blows the heat away in a process called forced convection.

(Image from WellBake)

Fallen Snow in Ithaca, NY

Last week, the eastern half of the United States was hit by a extreme cold front which brought temperatures down to low temperatures as low as 30 degrees below zero Fahrenheit with the wind chill in some places.

Isn’t wind a peculiar thing? It can make a decently chilly day to freezing cold. And in the summertime, a breeze in a terribly hot day can make the day seem bearable. There isn’t anything different in the air. And there isn’t anything magical about it. It is purely heat transfer via convection and conduction. In both instances, the wind stirs and mixes the air and removes heat from the body.

Let’s take a look at conduction only. You’ve probably noticed that a typical metal pot with boiling water can get hot very quickly on the outside because metals have relatively high termal conductivity. (In fact, this is also what makes metals so amazing with electric circuits, with copper and gold having the highest electric conductivities around. It has something to do with their highly unique metallic bonds.) Sometimes, though, we’d like to insulate things as best we can. This might be your morning coffee in a Styrofoam cup, or your home during the winter.

Units for Thermal Conductivity

Thermal conductivity for copper is about 380 W/m-K, where glass is about 1.1 W/m-K. Water is about 0.6 and air is 0.025. Arguably, air is the best all-around insulator that isn’t also synthetic or a rare material.

Isn’t that just a little weird? Air is the best at retaining heat, but thinking back to 20 mph wind conditions and we might find something wrong with this picture. Because air is a gas, once it’s moving, it can get messy and turblent; wind makes it feel colder than it really is. Standing air prevents the mixing of hot and cold air. That’s why a calm 25 degree day is more bearable than a 35 degree windy day.

Let’s take a quick look at our glass windows. Have you noticed that good, insulated windows are double glazed or triple glazed. Essentially, there is a layer of standing air in between two or three glass panes. We can picture little packets of heat traveling from inside the house to the outside, where it’s cold. While it can walk through glass pretty easily, but once it hits the layer of standing air, the “rate of heat escape” becomes over 40 times slower through the window surface. (Keep in mind we are not considering gaps and drafty windows.)

That is not to say that the window glass will not feel cold. The cold that you might feel is the heat escaping your hand and into the glass and has little to do with the standing air.

So, whenever and wherever you are trying to insulate from the cold this winter, make sure you put as much standing air in between you and the cold. Just make sure you don’t have any leaks though.

Stay warm!

(Fallen snow image from my Flickr.)