If you've ever been listening to your radio during a rain storm, and then kept it on after the clouds subsided and the leaves stopped fluttering, you might have noticed a sudden increase in radio reception. DX radio fans, who thrill at the challenge of pulling in distant radio signals, look forward to the end of a rain storm when lightening static stops. That's when the recent rainfall increases the relative humidity at the surface and the temperature drops several degrees. The combination of an increase in humidity and a decrease in temperature at the surface results in a phenomenon called tropospheric ducting.
It's called tropospheric because the ducting occurs in the lowest of the three layers of the earth's atmosphere. This includes the stratosphere, the middle layer (which holds little water vapor and has little affect on radio waves) and the ionosphere (the top layer) which is used for long-distance radio wave propagation. The ducting takes place below a temperature inversion; the point where a warm air mass passes over a cooler air mass. Basically, the radio signals bounce off the duct top at the temperature inversion and then reflect again off the surface of the earth. At times like this, it's possible to pull in stations you've never heard before and it's a great way to expand the boundaries of your local world.
In addition to ducting, recent rains also increase soil conductivity in a lot of areas. Though you might have heard of soil conductivity mostly in terms of agriculture, it can also have a pretty big impact on your radio reception. If you have highly conductive soil, the combination of water and the minerals present in the soil help reduce the amount of attenuation (or weakening of the surface wave) that usually affects a radio signal. The combination of increased soil conductivity and tropospheric ducting makes for some rare and exciting distant radio listening.
Depending on where you live in the country, you may be hoping for rain, or hoping to get a break from it. Rain is something so common and so seemingly ordinary, that we often don't give it much thought unless it acts in extreme ways such as causing floods and damage, or being threateningly absent, letting crops dry up. When scientists talk about life on Mars, they talk about water. They look for evidence of dried rivers and patterns of water flow. Historically, when villages were settled throughout the world, they settled first near abundant sources of water such as rivers or lakes. People depended on rain. These days it's easy for people who live in cities or suburbs to forget about the importance of rain. Sure we'll think about it when our cars get hit with acid rain, or we'll check the Farmer's Almanac to see if rain is forecast for a wedding day. We'll hardly give it a thought except to go inside and watch the water fall and listen to it patter on a window or through the trees. Few people know that they can look forward to the rain, even in a city, because the conditions after a rainfall create a rare opportunity to hear radio signals as far away as Ohio even if you're in New Jersey.
If you live on the West coast, you can also take advantage of the high conductivity of the salt in the ocean to pull in signals as far away as Japan. Yes! Japan. No need to wait for a rainstorm (though the best time to tune in is after midnight on a Sunday night) when other stations shut down for the evening. If you try it, let us know what stations you bring in. By the way, it's much easier to tune in those distant stations on a radio with a digital display. You can get started by trying to tune in 954 AM and listening for some Japanese.
Now for a bit on rain itself. Basically, it rains when warm air lifts water vapor up into the air and clouds form. Depending on their height and temperature, these clouds are made up of either water or ice crystals. Sometimes dust particles from the earth (like pollen or even the dust of human skin) get carried up into the clouds with the water vapor. There, these dust particles become something called condensation nuclei. It's around these condensation nuclei that raindrops start to form (and depending on weather conditions, snow, hail or sleet). Once the raindrops get too heavy to stay in the clouds, they fall as rain. Eventually, they'll evaporate again, getting pulled up into the air as water vapor, and start the process all over again.
If you're fascinated by rain, one of the most interesting things you can do is measure and record rainfall in your area. There's always weather going on, rain or shine, night or day. Tracking it helps us learn a lot about ourselves as well as our surroundings. Measuring humidity and temperature in your area might also help you predict if conditions are right for tropospheric ducting and the accompanying distant radio reception.