Sky & Telescope - March 2025 - 16

Science & Society
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pWIND-DRIVEN HALO Two infrared images taken with the Very Large Telescope's SPHERE-IRDIS instrument show the difference winds can make:
Both images are of the young star HR 4796A, which is surrounded by a debris disk. The edge-on disk is invisible around the star in the image with the
wind-driven halo (left, 96-second exposure) but visible in the other (right, 32-second exposure). The larger ring around the edge of each image is due
to adaptive-optics corrections.
prompted them to investigate potential climate impacts on
their own work by analyzing the observatory's weather data.
But first, they had to wrestle with those data. Despite having
weather stations for daily operations, observatories often
lack data readily usable for research. The data may not be
recorded or stored properly, or they're in a format incompatible
with later analysis. Sometimes, weather stations suffer
disruptions, such as relocation or construction of new facilities
nearby, creating misleading trends.
Nevertheless, after piecing together available information,
the researchers revealed some concerning patterns at
Paranal. Temperatures there have risen 1.5°C (2.7°F) in
the past 40 years, outpacing the global average of 0.2°C per
decade. They are projected to climb another 4°C by the end
of the century.
This increase is already impacting observations. Large
telescopes like the VLT must maintain the same temperature
inside the domes as will be outside at sunset, to prevent turbulence
from forming above the mirrors during operations.
However, the VLT's cooling system can only handle temperatures
below 16°C (61°F). As nights become warmer, there are
increasingly more nights where the outside air is hotter than
the inside of the domes.
" Turbulence is the number one enemy of optical and nearinfrared
astronomy, " says atmosphere scientist Angel Otárola
(European Southern Observatory). Turbulence can decrease
observations' quality, especially for high-precision endeavors
such as imaging and detecting exoplanets.
Problems also arise because the instruments themselves
operate best when cold. " We have cooling capacity, " Otárola
says. " The problem is letting things warm up [inside],
because the electronics inside the telescopes need to be relatively
cool. "
16 MARCH 2025 * SKY & TELESCOPE
Other trends are less obvious. One of the reasons Cantalloube
and Milli began exploring weather data was because
they'd seen a significant decrease in image sharpness during
strong El Niño events. El Niño is a natural but irregular
weather pattern that is produced by warmer-than-average sea
surface temperatures in the eastern Pacific Ocean. El Niño
events intensify the southern subtropical jet stream, an air
current that forms at an altitude of about 12 km with wind
speeds up to 60 m/s (130 mph). These winds can overwhelm
the adaptive optics system of the VLT by creating turbulent
patterns that change faster than the deformable mirrors can
compensate for. A noticeable smear in the direction the winds
are blowing appears up to 40% of the time above the VLT,
decreasing contrast by a factor of 10.
Based on computer simulations that combine real weather
data with physics-based models, Cantalloube and Milli found
that the speed of the subtropical jet stream has increased by 3
to 4 m/s in the last 40 years.
But such simulations rely on data with low spatial and
temporal resolution, often from satellite observations,
Otárola cautions. If he had a large enough budget, he says, he
would have real measurements at different altitudes, maybe
using recently developed drone technology for atmospheric
monitoring. " If we could monitor that for the next 10 or
20 years, my successors would have a good database to determine
what is happening. "
Cantalloube's work inspired Otárola and his colleagues to
look at long-term trends in the weather data from all three
European observatories in the Atacama Desert, paying extra
attention to the variability caused by El Niño and its cooler
counterpart, La Niña. They found that during El Niño periods,
roughly one-quarter of the year saw temperatures above
16°C at sunset. During La Niña, there are fewer hot days,
Contrast compared to target star
Contrast compared to target star
FAUSTINE CANTALLOUBE

Sky & Telescope - March 2025

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