What is a total sky imager?
A total sky imager is an instrument which can take picture and record the hemispheric view of the sky at a particular time instance. In meteorology, through an image analyzing software, a total sky imager is a useful tool to estimate the fraction of the sky view covered by cloud other than manual observation. The instrument can also capture the occurrence of rare atmospheric optics phenomenon, such as sun halo.
How does a total sky imager work?
Traditionally, a total sky imager consists of a CCD camera facing downwards viewing the hemispheric skyview reflected by a convex mirror or surface. The instrument takes picture of the hemispheric skyview with a preset schedule, say once every hour, and stores the images. Through a computer software analyzing the image, cloud coverage can then be estimated.
Nowadays, total sky imager or all sky imagers available in the market are equipped with high-resolution fisheye camera facing directly to the sky. The high-resolution camera is now capable of determining the cloud coverage and cloud type. With two imagers in place, cloud base height can even be estimated using a dedicated software.
How to measure cloud coverage?
In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) to 8 oktas (completely overcast).
What are the challenges of using a total sky imager?
The Sun is one of the main concerns one has to consider when designing a total sky imager. Direct exposure to sunlight can damage the camera easily. Therefore, a blocking mechanism may have to be considered in the design. Extreme heat is also a factor to think about as the imager would be heat up under the Sun especially during scotching summer.
Hong Kong is a modernized city with lot of high-rise or skyscrapers which could block the view, hence affecting the estimation of cloud coverage. Moreover, under certain meteorological conditions, the existing of haze can obscure the sky and make the estimation of cloud cover even more challenging with considerable error.
Links
- Overview of the latest all sky images https://www.hko.gov.hk/en/gts/astronomy/site_all.htm
- Halo captured by the total sky imager http://www.weather.gov.hk/en/wxinfo/aws/imager/ttsd19art07.htm
- Wikipedia on Okta https://en.wikipedia.org/wiki/Okta
- EKO product on All Sky Imager https://eko-eu.com/products/environmental-science/all-sky-imager
- Atmospheric Radiation Measurement on TSI https://www.arm.gov/capabilities/instruments/tsi