WEATHER VERSUS PROPAGATION

I recall a here sometime back (and more than once) where we were discussing the affects of weather conditions on our signals....Well, the other night I somehow managed to fall into a US Navy training manual about Radio Wave Propagation, and a specific topic "WEATHER VERSUS PROPAGATION", which I'll get to in just a moment.

But first I want to jump back (in the manual) to chapter 1  http://www.navymars.org/national/training/nmo_courses/NMO1/module10/14182_ch1.pdf which worded what I consider to be a rather delightful introduction to their course:

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CHAPTER 1
WAVE PROPAGATION

INTRODUCTION TO WAVE PROPAGATION

Of the many technical subjects that naval personnel are expected to know, probably the one least susceptible to change is the theory of wave propagation. The basic principles that enable waves to be propagated (transmitted) through space are the same today as they were 70 years ago. 
One would think, then, that a thorough understanding of these principles is a relatively simple task. For the electrical engineer or the individual with a natural curiosity for the unknown, it is indeed a simple task. 
Most technicians, however, tend to view wave propagation as something complex and confusing, and would just as soon see this chapter completely disappear from training manuals. This attitude undoubtedly stems from the fact that wave propagation is an invisible force that cannot be detected by the sense of sight or touch. Understanding wave propagation requires the use of the imagination to visualize the associated concepts and how they are used in practical application. This manual was developed to help you visualize and understand those concepts. Through ample use of illustrations and a step-by-step transition from the simple to the complex, we will help you develop a better understanding of wave propagation. In this chapter, we will discuss propagation theory on an introductory level, without going into the technical details that concern the engineer. However, you must still use thought and imagination to understand the new ideas and concepts as they are presented.

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I really liked the way whoever wrote that wrote it! -- The whole manual is written in such a way that even someone like me who has little more than a hazy understanding about the mechanics of radio can comprehend.

I jumped around the manual reading for about an hour, then downloaded both chapters 1 and 2 so I could come back to it later.

Anyway the following about how weather affects radio waves, is found in the middle of chapter 2 on page Page 2-34, here's part of it..
http://www.navymars.org/national/training/nmo_courses/NMO1/module10/14182_ch2.pdf

 WEATHER VERSUS PROPAGATION

Weather is an additional factor that affects the propagation of radio waves. In this section, we will explain how and to what extent the various weather phenomena affect wave propagation.

Wind, air temperature, and water content of the atmosphere can combine in many ways. Certain combinations can cause radio signals to be heard hundreds of miles beyond the ordinary range of radio communications. Conversely, a different combination of factors can cause such attenuation of the signal that it may not be heard even over a normally satisfactory path. Unfortunately, there are no hard and fast rules on the effects of weather on radio transmissions since the weather is extremely complex and subject to frequent change. We will, therefore, limit our discussion on the effects of weather on radio waves to general terms....

PRECIPITATION ATTENUATION

Calculating the effect of weather on radio wave propagation would be comparatively simple if there were no water or water vapor in the atmosphere. However, some form of water (vapor, liquid, or solid) is always present and must be considered in all calculations. Before we begin discussing the specific effects that individual forms of precipitation (rain, snow, fog) have on radio waves, you should understand that attenuation because of precipitation is generally proportionate to the frequency and wavelength of the radio wave. For example, rain has a pronounced effect on waves at microwave frequencies. However, rain hardly affects waves with long wavelengths (hf range and below). You can assume, then, that as the wavelength becomes shorter with increases in frequency, precipitation has an increasingly important attenuation effect on radio waves. Conversely, you can assume that as the wavelength becomes longer with decreases in frequency, precipitation has little attenuation effect.

Rain

Attenuation because of raindrops is greater than attenuation because of other forms of precipitation. Attenuation may be caused by absorption, in which the raindrop, acting as a poor dielectric, absorbs power from the radio wave and dissipates the power by heat loss or by scattering (fig. 2-24). Raindrops cause greater attenuation by scattering than by absorption at frequencies above 100 megahertz. At frequencies above 6 gigahertz, attenuation by raindrop scatter is even greater. 

 It continues on from there discussing fog, snow, and hail.. But I got tired of reading, so I saved the pdf for another day. Thought some of you might be interested in grabbing a copy as well.

Links to the pdfs are above