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For all that we complain about weather forecasts, they’re actually pretty amazing.
A 5-day forecast today is about as reliable as a 24-hour forecast was in 1980.
It’s not just about the convenience of having your umbrella, either.
Good weather forecasts save lives.
Hurricane forecasts, for example,
can give people days instead of hours to get to safety.
So it’s understandable that meteorologists are up in arms about a big potential threat to forecasting:
the way the US Federal Communications Commission, the FCC,
is handling the next generation of cell phone service, known as fifth-generation or 5G.
The issue is that the FCC wants to let cellular carriers use some radio frequencies
that are right next door to the frequencies measured by weather satellites.
Those weather measurements need to be very sensitive for models to do their work well.
So scientists are worried that unless the FCC changes its guidelines,
5G transmissions could drown out the measurements,
setting weather forecasts back decades.
5G technologies promise something we all want: to make our phones faster and more reliable.
One of the biggest changes 5G makes to get there is to move to
new radio frequencies not previously used for cell service.
Different frequency ranges come with different advantages and disadvantages.
With higher frequencies, it’s easier to get faster speeds and lots of devices on the same network.
That’s because each range of cellular frequencies gets divided up into channels,
thin slices of the radio spectrum.
Like a pipe carrying water, each channel can only carry a limited amount of data.
The wider a channel’s band of frequencies, the more data it can carry.
This is actually why we use the word bandwidth to mean capacity.
When a cell tower and a mobile device communicate,
they take up capacity on one of the available channels.
And in lower frequency ranges, capacity is a valuable commodity.
These frequencies tend to be very in-demand,
so the cellular industry gets relatively small blocks of spectrum to work with.
But there’s much less competition for higher frequencies.
So it’s easier to allocate big blocks and carve them into lots of wide channels.
The network can then support more devices, and more bandwidth for each device.
Now, the downside of higher frequencies is, well,
there are reasons they’ve historically been snubbed.
For one thing, researchers will tell you it’s just harder to engineer the equipment.
A more fundamental issue is that higher-frequency radio signals don’t travel nearly as far.
They quickly fade as they pass through air,
and even more so through trees, and walls, and rain, and fog.
To balance these tradeoffs for different use cases,
5G standards are designed around 3 frequency ranges:
Below 1 gigahertz, for long-distance links that can tolerate lower speeds;
1 to 6 gigahertz, for a balance of range and bandwidth;
And above 24 gigahertz, for super-fast communication in areas like cities
where small cell towers can be placed every few hundred meters to compensate for that signal fading.
It’s that highest range that’s causing the meteorology ruckus.
In the US, the rights to use each band in each geographic area are auctioned off by the FCC.
As part of the government’s big push to get 5G off the ground,
the FCC recently ran its first few auctions for 5G bands,
including one that starts at 24.25 gigahertz.
That band is appealing to telecoms, because it’s the furthest-traveling
and the easiest-to-engineer among the uncrowded higher frequencies.
But it is also uncomfortably close to a critical frequency for weather forecasting: 23.8 gigahertz.
The 23.8 gigahertz channel is used by weather satellites
to measure water vapor in the atmosphere, a pretty key variable for forecasting.
Roger Saunders, a meteorologist with the UK Met Office, explained to SciShow
that that’s because both the ground and the water vapor in the air
are constantly giving off and absorbing radio waves all over the frequency spectrum.
And water vapor happens to absorb and emit particularly strongly at 23.8 gigahertz.
And it’s not a super-intense effect.
The water vapor doesn’t absorb that much
of what’s emitted by the ground, and it doesn’t emit too much itself.
So in most places, water vapor has only a very small impact on the signal detected by satellites.
But, even though it’s a small signal, it’s a very sensitive signal:
changes in the amount of water vapor make a noticeable difference
in how much 23.8 gigahertz radiation filters up to the satellites.
So if a satellite passes over some point on Earth and sees
an unusually weak or strong 23.8 signal, that can indicate a change
in the amount of water vapor in that column of atmosphere.
If it is water vapor, it’s specifically low-altitude water vapor,
since that’s where the water vapor gives off the strongest 23.8 signal.
This isn’t the only frequency that we can use to detect water vapor
throughout our atmosphere, but scientists argue that the 23.8 channel is crucial.
Not only does it tell us a lot about vapor at lower altitudes, it’s also important for
checking the assumptions behind measurements at other heights and of other variables.
By helping to paint this whole picture,
the satellite instruments that observe this channel slash the error rate of forecasts worldwide.
Now, theoretically those measurements should be unaffected by 5G.
Remember, the 5G band starts at 24.25 gigahertz, which is a different number!
The catch is that no radio transmits at one precise frequency.
A transmission on any given channel is more of a smear across different frequencies,
with a peak at the target frequency.
So the concern is that some of the off-target noise from 5G transmitters
could bleed over into the 23.8 channel.
The FCC does impose limits on how much off-target noise a transmitter is allowed to produce.
But those limits are much higher than almost every other country
outside of the U.S. is recommending for this band.
With lots of 5G radios hollering away,
the noise bleeding over from 24.25 into 23.8 could add up to a lot.
Scientists from NASA, and NOAA, and even the US Navy are warning that
satellites could pick up this noise and think it was emitted by water vapor.
That would make measuring the 23.8 signal like
trying to listen to your friend in the middle of a concert.
A study by NOAA and others suggests that satellites would lose
77% of certain microwave data, which would set forecasts back by 40 years.
However, it is not totally clear how serious this risk is.
Unsurprisingly, telecom industry advocates claim everything is fine.
They claim, for example, that current satellite instruments are
less susceptible to interference than those in the NOAA study.
Also, 5G transmitters could probably be designed and deployed carefully enough
that they don’t send too much off-target noise upward toward the satellites.
But NOAA hasn’t made its study data public yet, and until it does,
it's the claims of the cell phone industry and the FCC
against those of NOAA, NASA, and the Navy, and other scientists.
Meanwhile, many scientists are alarmed, and say the FCC should at least
tighten the limits on spillover, even though that would mean lower-power cell towers.
So that’s the science so far behind this argument.
Now it is time for the FCC to come to some sort of agreement with
NOAA, NASA, Congress, and the rest of the forecast-loving world.
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