Rainfall is a familiar phenomenon that we often associate with the dark clouds looming above, warning us of an impending downpour. Meteorologists and weather enthusiasts alike use various tools and techniques to predict, track, and study rainfall. One such tool is weather radar, which is crucial in detecting rainfall and other precipitation. However, there are times when we experience rainfall that radar doesn’t seem to detect. This can be perplexing, especially when we expect the technology to be foolproof. One key factor to this anomaly is “low-level moisture.” Let’s delve into what it is and why it might cause rain to fall undetected by radar.
What is Low-Level Moisture?
At its core, low-level moisture refers to the presence of water vapor close to the earth’s surface. This moisture can come from various sources like evaporation from water bodies, transpiration from plants, or even the general humidity in the air. The atmosphere is a multi-layered structure, and low-level moisture primarily affects the layer closest to the ground, known as the boundary layer or the troposphere’s lower part.
How Does Radar Work?
Before we can understand why radar might not detect certain rain events, we need to briefly touch upon how radar works. Weather radars send out pulses of radio waves. These waves, when they encounter objects like raindrops, snowflakes, or even hail, get scattered in various directions. Some of these waves bounce back to the radar. By analyzing these returned signals, the radar determines the location, intensity, and movement of precipitation.
Why Isn’t the Rain Detected?
Now that we understand the basic principle of radar and the concept of low-level moisture, let’s connect the dots.
- Radar Beam Elevation: The radar beam doesn’t scan right at the ground level but is slightly elevated. As the beam travels away from the radar source, it gets higher above the ground due to the curvature of the Earth. Therefore, if the precipitation is restricted to very low levels, especially within a few hundred feet from the ground, the radar beam might overshoot it, leading to undetected rainfall.
- Rain Intensity and Droplet Size: Light rainfall or drizzle consists of tiny water droplets. These droplets might be too small for the radar to detect effectively. Since low-level moisture can lead to the formation of such fine droplets, the rain might be falling right under our noses but stay invisible to the radar.
- Temperature Inversions: At times, the atmosphere experiences a phenomenon called a temperature inversion, where instead of the temperature decreasing with altitude (which is the norm), it increases. This can trap moisture and pollutants close to the surface. When this moisture condenses and falls as precipitation, it might be in the form of light drizzle or rain that radar overlooks.
While technology has come a long way in predicting and understanding weather patterns, there are still nuances and atmospheric conditions that challenge our tools and systems. Low-level moisture and its contribution to undetected rainfall is just one of these complexities. The next time you find yourself caught in a light drizzle that wasn’t on the radar, you’ll have a better understanding of the atmospheric mischief at play!
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3 Responses
I do love your forecasting Nick. And I was telling someone your forecasts are actually very accurate compared to the central New Jersey forecasts I masochistically keep coming back to from the Weather Channel. It was laughable this summer here. Forecasts changed from the extremes within minutes this spring and summer. (I.e. forecasted torrential rain all day (checked all the way until the supposed start of the rain) then abruptly changing to partly cloudy the rest of the day because it dissipated to nothing. Maybe there was a difference in accuracy between northern Ocean County and west/south Jersey?
Also I had to comment because (as I’m sure you are already aware- 24gHz spectrum (used by Verizon) for 5G is where the issue is a major concern. A quick google search of these keywords will give many many hits in this subject.
Here is a fairly recent article from Scientific American- https://www.scientificamerican.com/article/5g-wireless-could-interfere-with-weather-forecasts/
its ever since 5G that moisture is not detected correctly….how many times in last 4 months, did we expect tons of rain, overnight and never got a drop or vice versa…….i dont remember weather forecasting ever being so off, regarding moisture, in my lifetime.
That’s just simply not true. Low level moisture has ALWAYS been a thing. Radar technology hasn’t changed in 20 years and has nothing to do with 5G. Every forecast I can think of, especially recently has been as close to perfect as possible with minor deviations here and there.