Last Friday, June 28, we had the opportunity to visit the facilities of the Atmospheric Optics Group of UVa (GOA-UVa), guided by Roberto Román.
This university group focuses on studying the atmosphere (composed of gases, aerosols, and clouds). Specifically, they concentrate on studying aerosols and use various tools and methods to do so. In this article, we will discuss four instruments that GOA uses daily to carry out their work.
Entrance to the Faculty of Sciences at UVa. Photo by Lucas Pérez.
Photometer
Solar photometers (sun photometers) are used to measure the intensity of solar radiation reaching the Earth at a specific point (where the sun is located). By comparing the received values with the radiation that should arrive, they can calculate how much radiation has been lost along the way. The greater this amount, the more particles (aerosols) are present between the Sun and the photometer, obstructing the radiation.
Photometer. Photo by Lucas Pérez.
Some data we can infer from the radiation received are as follows:
- If the same radiation is lost across all colors, it means the particles are large, attenuating all colors equally.
- If a lot of blue radiation is lost but little red, it means the particles are fine, as they attenuate shorter wavelengths more.
These photometers are calibrated once a year by intercomparison with parameters provided by the AERONET network (AErosol RObotic NETwork), established by NASA, and PHOTONS (PHOtométrie pour le Traitement Opérationnel de Normalisation Satellitaire). There are over 500 photometers worldwide, and GOA-UVa manages about 60 of them.
They have also attempted to conduct similar studies using moonlight instead of sunlight. Although determining the radiation from the Moon is more complex, as it reflects sunlight rather than emitting its own, the group’s efforts in recent years have enabled accurate aerosol estimation at night.
Pyranometer
Pyranometer. Photo by Lucas Pérez.
The pyranometer also measures solar radiation, but unlike the photometer, it measures radiation from all directions, more akin to what we receive. In contrast, the photometer measures a specific point in the sky, usually in the direction of the sun. This instrument has a peculiar shape, often referred to as “the fried egg.”
Ceilometer
In English, “ceil” means “ceiling,” which is fitting since this instrument measures cloud height. In fact, the latest models can also measure aerosols.
Exterior of a ceilometer. Photo by Lucas Pérez.
It is a Lidar and works similarly to an ultrasonic sensor, with two compartments:
- Compartment 1: sends laser light into the sky, which “bounces” back when it hits clouds.
- Compartment 2: receives the energy sent by Compartment 1 after bouncing off the cloud.
By measuring the time it takes to receive the energy (from when the laser light is sent to when it is received), we can calculate the height, knowing its speed.
One drawback of this instrument is that if clouds are very low, it won’t detect them correctly, as the two compartments are not aligned and require at least 200 meters of cloud-free height to send and receive energy properly.
Knowing the height of a cloud allows us to:
- Identify the type of cloud.
- Determine if a weather front is approaching.
- Predict the weather.
Sky Cameras
Sky camera. Photo by Lucas Pérez.
These instruments are used to take, literally, pictures of the sky. They also provide information about aerosols. Each of these cameras includes:
- A fisheye lens with a dome, specifically to protect the lens from rain and other weather conditions.
- A heating system in the housing to prevent water droplets and ice on the dome, with humidity and temperature sensors.
Open sky camera. Photo by Lucas Pérez.
These cameras are mainly used in the Proyecto Presente of this same group, which we discuss in this article.
Acknowledgments
We would like to thank Roberto Román for showing us the GOA-UVa facilities and assisting us with the publication of this article.
