Updated: Aug 28, 2022
Have you ever used an app that can tell you whether rain is headed your way?
An app where you can see where it’s raining and where it’s going, so you know if you should bring your laundry inside, or skip that afternoon run?
Or maybe you’ve used one before… have you ever wondered how it works?
Today we’re going to learn about a particular piece of technology that allows us to locate rain from a distance.
When we hear the word “radar”, what comes to mind?
If images of a screen on a warship where a green line is going in circles and detecting enemy ships come to mind, you’re not far off! In fact, that’s where it all started.
Radars were originally developed during the 2nd world war, for precisely that purpose: detecting enemy planes and ships. However, they noticed that whenever there is a big storm, it would show up on the radars as well and block their view of the planes. The meteorologists saw a new opportunity there—they can see storms and where it’s going!
Typhoon Cobra as seen on a ship's radar screen in December 1944
Fast forward to now—we’ve taken that technology much further and now we have radars dedicated to just looking out for rain, snow, storms, and other kinds of precipitation.
How does it work?
A weather radar works by sending out invisible signals from the transmitter into the atmosphere. The signals are reflected by objects in the atmosphere—birds, insects, and more importantly raindrops—which are then received by the receiver.
The signals are reflected in all directions, and some of those make their way back to the radar. We can tell by these signals where the raindrops are, how much raindrops there are (light/strong rains), if it’s actually rain or ice (hail/snow), and how fast it’s going.
The radar transmitter then rotates 360 degrees to get information from all directions.
It’s like taking a snapshot of the atmosphere with information that is beyond what we can see. Radars collect information every few minutes, so that when we put all these snapshots together, we can watch it happen, and sometimes even predict what’s going to happen.
How does it see different types of precipitation?
In certain types of radars called polarimetric radars, the signals come in two directions: vertical and horizontal. When sent out by the transmitter, the horizontal and vertical signals are the same strength. The shape of the object that it hits determines how strong the signals are reflected back in both orientations.
A smaller raindrop tends to be more spherical (imagine a ball), where the vertical and horizontal extents are the same, so that the signal it reflects also have very similar strengths for vertical and horizontal. Bigger raindrops tend to be wider (imagine a burger bun), so that the reflected signals along the horizontal are stronger than along the vertical.
The difference in the strengths between the reflected horizontal and vertical signals give us clues on the shape of the raindrop.
Do we have some in the Philippines?
Our weather agency, PAGASA, maintains a network of 19 weather radars. There are two that have a direct view over NCR: the one in Subic and the one in Tagaytay.
You can view their data here: https://bagong.pagasa.dost.gov.ph/index.php?id=226
Why are weather radars important?
It scans the atmosphere in high-resolution. This means that it has detailed information in the area it covers, such that it can tell differences in rainfall within kilometers. For example, an instrument with low resolution will say it’s raining in the whole Metro Manila, while an instrument with high resolution can tell if it’s raining in Fairview but not in Cubao.
High temporal resolution is also an advantage. This means that the radar is able to tell us the rainfall conditions every 15 minutes, so that when we look at an hour’s worth of data, we can deduce where rainfall (if a large enough system is present) is going.
Combining the information of location (where it is raining), intensity (is it light rain or heavy rain? How much?), and having it over time, helps in weather prediction. Our local meteorologists use radar, among their many instruments and their own skills, to predict weather in different areas of the country.
This dense information is also very important in understanding and reacting to severe weather. If we can see extreme heavy rainfall (for example from a typhoon) coming towards a city, and we see that it is a big system that will dump rain in one area over several hours, then we can give residents and the local government an early warning. This allows them to decide what to do—do they simply have to move their furniture upstairs, or do they need to evacuate the area?
Flocks of birds and insects can look like heavy rain in a radar image, so most meteorologists want to filter them out of the data. But like meteorologists finding use in seeing storms in warship radars, scientists studying birds and insects are finding importance in seeing these creatures in weather radars!
There is a whole field of study using weather radars in tracking bird migratory patterns that falls under Radar Aeroecology. Because polarimetric radars can give an idea about the shape (as well as other characteristics) of the object that reflects the signals, it can differentiate between rain and flying animals. And due to its very frequent data collection we get very rich information about migration patterns.