In the preceding lessons, you have been introduced to all sorts of observations. One thing that these data have in common is that they are in situ observations -- meaning that they were collected by a sensor in direct contact with the medium we are measuring. Obviously this is not possible over the entire breadth and depth of the atmosphere. Therefore we need to be able to measure the state of the atmosphere from afar, or remotely. Remote sensing is just that... taking a measurement without having a sensor in direct contact with the medium that we are measuring. As an example, your body contains both in situ sensors (your skin) and remote sensors (your eyes). You don't have to physically touch a red-hot stove element to know that it is hot. Your eyes can sense the light coming from the heating coil and you then make an interpretation that the burner must be hot.
One of the most important things to keep in mind when using remote sensing data, is that there are no perfect, one-size-fits-all remote sensors. Each type of remote sensing instrument is designed to measure a specific thing (most often not even what you are interested in measuring). This measured quantity is then interpreted or converted into the observation that you really desire to have, but to make this conversion, we have to make assumptions. Sometimes these assumptions are perfectly appropriate, but in other cases, they are not. Ignoring the limitations of remote sensing data is a sure invitation for making an incorrect analysis. Optical illusions are a classic example of making incorrect assumptions about what your eyes (a remote sensor) are detecting.
So what types of remote sensing instruments do meteorologists use? I'm sure that you are very familiar with satellite and radar images available on the Internet and shown on TV weathercasts. These are two very important types of remote sensing observations -- and we will cover them in depth in this lesson. In addition to radar and common satellite imagery, there are many more types of remote sensing data that measure a vast array of atmospheric properties. Although many of these data lie beyond the scope of this course, they all have something in common.... All remote sensing data is based on measurements of electromagnetic radiation.
Meteorologists use a vast array of remote sensing instruments to measure the atmosphere. The key to properly interpreting each data set is to understand the advantages and limitations of the instrument. To aid in this understanding you must first familiarize yourself with the properties and laws of electromagnetic radiation.
Though the word "radiation" generally carries the tone of dire consequences for much of the public, meteorologists routinely and harmlessly harness part of the broad spectrum of electromagnetic radiation to help them diagnose the present state of the atmosphere and then make a prediction. So, what is electromagnetic radiation? Is all electromagnetic radiation invisible to the human eye? How do you as an apprentice forecaster use and interpret satellite and radar imagery derived from electromagnetic radiation?
To answer these and several other poignant questions, let's start with the basics of radiation. Though this topic may seem more like physics than meteorology to you, I stress that weather forecasters worth their salt must understand the underlying science behind satellite and radar imagery in order to effectively and correctly use them.