Sky Camera

Both of the models of sky-camera used at the CDAO were designed for use in closed-circuit television (CCTV) systems. They automatically adjust their aperture size and exposure duration in response to the available light levels. The values of these settings are not recorded. The front plate of the camera housing is heated mildly in order to evaporate any water that accumulates on it.

Owing to the low light levels between dusk and dawn, the image quality is generally poor. Clouds can only be seen if they are suitably illuminated from below by street lights or from above by the Moon. The Moon, the planets, and some of the brighter stars can sometimes be seen in the images.

The sky-camera is mounted on the north-facing wall of the bungalow at the CDAO. Its pointing direction and focal length have been adjusted in order to maximise the capture of sky conditions. This primarily means pointing down the valley (slightly to the north of west) since this is the direction for which the sky is least obscured by the landscape. An elevation angle was chosen so that the images capture some of the ground (for reference purposes), but not too much. The focal length of the zoom lens (which can give a horizontal angle of view of between 27° and 67° in the case of the first sky-camera and of between 25° and 65° in the case of the second) was adjusted to give a wide angle, but not so wide that the images capture the edge of the bungalow. The pointing direction and focal length of the second sky-camera were adjusted to give approximately the same field of view, although the images capture the edge of the bungalow. The elevation angle of the second sky-camera drooped slowly over time and had to be re-adjusted.

The pointing direction of the first sky camera and the angular extent of the images (quoted at the top of the page) have been estimated from the known positions of the Sun and the star Arcturus as they pass through the field of view. Although the pointing angles should be accurate to within a degree or so, estimating the angular extent is complicated by the fact that the camera has a non-zero elevation angle. This leads to so-called perspective distortion, which causes vertical lines to converge to a point above the top of the centre of the image. This means that the azimuthal extent at the top of the image is wider than that at the bottom of the image. Note that the elevation angle of the second sky-camera varies over time.

Not applicable for this instrument.

Refer to the Access to the data section of the Capel Dewi Atmospheric Observatory page for general details about:

• Licensing and acknowledgements
• Access to data files/plots
• Availability of data files/plots
• Centre for Environmental Data Analysis (CEDA) file naming convention and archive structure

Individual images are in JPG format. These are available in a single “tar” archive file for each hour. In a Linux/Unix environment, the contents of these files can be extracted using the command: tar xf file_path, where file_path is the path of the tar file. On a Windows system the individual files can be extracted using an application such as WinZip.

• CEDA catalogue page
• File name format: sky-camera_capel-dewi_YYYYMMDD_HHMM.tar (note that this does not conform to the current CEDA file naming convention)
• CEDA archive location: /badc/mst/data/sky-camera/
• Web access to CEDA archive (requires a CEDA account)

A log of interesting atmospheric conditions and phenomena seen in the sky-camera images is available for the first 18 months of operations. This has been created by a succession of school pupils (thanks to Joseph Prentice, Emily Mackintosh, Daniel Khosla, Kathryn Stone, Kieran Cook, Natasha Bierrum, Amelia Gilio, Alice Ardis, and Daniel McGregor) during their work experience placements at the STFC Rutherford Appleton Laboratory. This is a useful resource for anyone searching for examples of particular phenomena. A number of openly-accessible time-lapse videos based on the more spectacular sequences available below.

The time-lapse videos covered in this section are published under a Creative Commons Attribution License. Anyone may download, view, and redistribute copies.

For each video, follow the link to the Zenodo repository in order to download a copy.

The videos cover the following phenomena:

• Cumulus clouds (humilis, mediocris, and congestus)
• Cumulonimbus clouds
• Wave clouds (asperitas, fluctus, lenticularis, unudulatus)
• Miscellaneous (22° Halo, Crepuscular Rays, Poster)

---

Cumulus clouds

---

• Description: A time-lapse video showing Cumulus humilis clouds, which are also known as fair weather cumulus.
• Video start datetime: 2008-10-05 11:00 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Cumulus mediocris clouds (with occasional Cirrus clouds and contrails at a higher level). The Cumulus clouds are in a continuous state of change. Many can be seen to form and/or to evaporate within the field of view. The evaporating clouds give rise to ragged Cumulus fractus formations.
• Video start datetime: 2007-10-04 05:30 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing the development of Cumulus congestus clouds, leading to a Rain Shower and a Rainbow.
• Video start datetime: 2007-05-29 03:00 UTC
• Location of the video in the Zenodo repository

---

Cumulonimbus clouds

---

• Description: A rapidly-growing Towering Cumulus [actually a Cumulonimbus] cloud, which develops an Anvil.
• Video start datetime: 2007-07-16 18:40 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Cumulonimbus cloud development at sunset. Most of the first Cumulonimbus cloud is obscured by the smaller Cumulus clouds in the foreground. Its anvil is clearly visible. Three further Cumulonimbus clouds cast crepuscular rays from the light of the setting sun. Their anvils merge together.
• Video start datetime: 2010-09-07 16:50 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing widespread development of Cumulonimbus clouds, some of which have extensive Anvils. A variety of other cumuloform clouds can be seen. The origin of the widespread cirrus cloud seen during the second half of the sequence is probably outflow from Cumulonimbus anvils.
• Video start datetime: 2010-09-07 05:30 UTC
• Location of the video in the Zenodo repository

---

Wave clouds

---

• Description: A time-lapse video showing Stratocumulus clouds that develop wave-like asperitas features. During the early part of the sequence, there appear to be layers of cloud at two distinct levels. These are lit differently by the setting sun. The asperitas features begin to appear approximately half way through the sequence. Altocumulus undulatus clouds can be seen towards the end.
• Video start datetime: 2007-09-04 17:50 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Altocumulus fluctus and Altocumulus undulatus clouds followed by Altocumulus lenticularis. The short-lived fluctus cloud feature is caused by Kelvin-Helmholtz wave activity, which is generated where the wind speed changes sharply with altitude. The initial telephoto image shows the characteristic breaking-wave pattern. This was taken (at 17:37 UTC) at the same location as the camera used to to generate the video sequence, but shows a narrower field of view. The wave activity is also revealed by the Altocumulus undulatus cloud elements, which form in parallel bands. There is evidence of mountain wave activity throughout the sequence and Altocumulus lenticularis becomes the dominant cloud type towards the end. These clouds remain stationary relative to the landscape rather moving with the wind. Occasional contrails and Cirrus clouds can be seen at a higher level (one of the contrails develops undulatus features). These remain illuminated for longer than the Altocumulus clouds as the sun sets.
• Video start datetime: 2009-03-25 17:25 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Altocumulus lenticularis clouds revealing Mountain Wave activity. Notice how the positions of the clouds remain fixed relative to the landscape. This contrasts with the lower-level (Cumulus) clouds, seen at the beginning of the sequence, which move downstream with the wind.
• Video start datetime: 2008-10-02 15:44 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Altocumulus undulatus clouds. The cloud layer initially appears to be of Stratocumulus type, with relatively poorly-defined undulatus elements. However, the layer soon breaks up to give well-defined Altocumulus undulatus. Cirrus clouds and contrails can be seen at a higher level towards the end of the sequence.
• Video start datetime: 2007-05-21 03:00 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Altocumulus clouds displaying both undulatus and lenticularis features. A lower cloud layer can be seen moving in a different direction at the beginning of the sequence. Contrails and cirrus clouds can be seen at higher levels.
• Video start datetime: 2007-09-30 14:30 UTC
• Location of the video in the Zenodo repository

---

Miscellaneous

---

• Description: A time-lapse video showing Cirrostratus clouds giving rise to a 22° Halo around the Sun. A number of aircraft Contrails pass through the field of view and a Sun Dog can be seen briefly towards the end of the sequence.
• Video start datetime: 2008-07-13 18:00 UTC
• Location of the video in the Zenodo repository

---

• Description: A time-lapse video showing Crepuscular Rays. These are shafts of light that appear to radiate from the sun. The effect is caused by the sunlight scattering from aerosols, i.e. small particles such as dust and smoke that are suspended in the atmosphere. It is usually seen when broken clouds are present, which leads to alternating shafts of light and shadow. The rays of light from the sun are actually parallel, but they appear to converge because of the effects of perspective. This is the same effect that makes roads appear to become more narrow as they recede into the distance. The word crepuscular implies twilight and so the term crepuscular rays is sometimes reserved for the effect that is seen when the sun is just below the horizon. However, crepuscular rays can potentially be seen at any time of day. They tend to be most dramatic when the sun is low in the sky and when the concentration of aerosols is particularly high. Although the clouds in this video look like Cumulus, the cloud base (detected by a laser ceilometer at the same location as the camera) is above 2 km suggesting that they are Altocumulus castellanus.
• Video start datetime: 2008-05-07 15:00 UTC
• Location of the video in the Zenodo repository

---

• Description: An educational poster, together with brief explanatory notes, illustrating some of the many ways in which atmospheric conditions can vary.
• Location of the resource in the Zenodo repository

---