The Doppler effect is a phenomenon that can be observed with a moving object which is emitting waves. Many objects emit waves: cars and trains emit sound waves, and stars emit light waves, just to name a few examples. Imagine a time when you were standing outdoors and a vehicle (such as a fire truck with its sirens on) drove by you. The sound you hear from the moving vehicle becomes higher-pitched as the vehicle moves toward you, and the sound becomes lower-pitched after the vehicle passes by and drives away from you. Why does this happen?

This visual could help you picture the fire truck example.

We can imagine wave-emitting objects like a siren-emitting fire truck as “point sources” of waves. This means the object can be modeled as a single point that emits circular waves that drift outward from itself, sort of like what you see when you drop a pebble into a pond. Keeping this in mind, if the point source moves, then the circular waves become more densely-packed in the direction that the object is moving in, and the waves become more sparse in the direction behind the object’s motion. For example, if we have a fire truck traveling to the left emitting the sound of a siren, then to the left of the fire truck the sound waves are closely-packed together, and to the right they are farther apart from each other. See if you can picture this, because this is the key to understanding the Doppler effect. This is why if you’re standing in front of a moving vehicle, you hear a high-pitched sound (high frequency, sound waves closer together), and if you’re standing behind a moving vehicle, the sound is low-pitched (low frequency, sound waves farther apart).

So, how does the Doppler effect affect weather forecasting? Doppler radars are devices that emit waves, which get reflected from objects in the sky such as raindrops, snowflakes, or even dust. The reflected waves can be interpreted in several ways. The number of reflected waves can tell weather forecasters how intense a storm is, and the frequency of the reflected waves can reflect (no pun intended) which direction the storm is traveling in relative to the direction that the Doppler radar is pointing in. These things are all useful information for weather forecasters, which is why you sometimes hear the term “Doppler radar” during weather forecasts.

(Image from Wikipedia.)

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.)