The term "remote sensing" means exactly what it says--the sensing (or measuring) of something remotely (that is, not being in the midst of what is being measured). In the context of the atmosphere, remote sensing most often refers to observations collected by radar or satellite that involve the measurement of electromagnetic radiation in a narrow region of the EM spectrum. This radiation either originates from the object itself or is sent out from the instrument and "bounced off" the thing being measured.
To make an analogy, did you know that you have some very sophisticated remote sensing instruments? It's true! Consider your eyes. Your eyes allow you to observe and measure things from great distances. You can also "see" wavelengths over a relatively large band of the EM spectrum (something that most remote sensors can't do). Now, imagine yourself in the center of a large, dark room. How can you remotely sense what's around you? Well, first you might look around for any emission sources within the band of wavelengths that you can detect. This might include a small nightlight over in one corner, or a hot burner (perhaps you're in a kitchen) that is glowing slightly red. These are types of "passive" remote sensing - you are just measuring what is coming to your eye without doing anything else. Now imagine that you have a flashlight. The light is emitted from the flashlight, bounces off objects in the room and then travels to your eye (where you "see" it). This is an example of "active" remote sensing (you are sending out radiation) and then measuring how it comes back to you.
Unfortunately, there is no perfect remote sensing instrument. Each one has its own limitations. For one thing, each type of remote sensor only measures a very limited portion of the electromagnetic spectrum and thus may not be able to "see" everything that we wish to measure. Again, think about your eye--although you can see in the visible spectrum, you cannot see in the infrared spectrum. That being the case, you will likely miss seeing a cat hiding behind a box in your dark room. Even a flashlight won't help you (the cat is behind the box, remember). However, if you had an infrared camera, you could spot the cat by its heat signature. A second limitation is that often (especially for passive remote sensors), what is "seen" by the sensor is not really what is happening. Think about optical illusions involving forced perspective (here's a cool example)--why do they work? Usually, they work because we make certain assumptions about how light travels to our eyes, and those assumptions are hard to break, even though our brain says, "Hey, that can't be happening!" If we go back into our darkened room example, our eyes can play all sorts of "tricks" on us... remember the shadow monsters that you used to "see" in your room at night when you were a young child.
So, remote sensing can be a very powerful tool for measuring things in our vast atmosphere, but we need to be careful. We need to: 1) understand how the remotely sensed measurement was collected and what it means (and what it doesn't mean), 2) use the right tool for the right measurement, and 3) be careful in situations where the instrument may not be correctly measuring what is really going on. Think about these key points as you dive into this week's lesson material.