All-Sky Cloud Imager

All-Sky Imager Cloud Data

The all-sky imager consists of a 2π hemispheric mirror and a webcamera in a protective housing, which captures images every 30 seconds of the sky’s reflection in the domed mirror. It was installed on Deck 7 on top of the port side container where the view was least obscured by overhead antennae, masts, and other structures. Data are collected 24 hours a day; though, the only images that can be analyzed are those where sufficient daylight is present for the camera to resolve the sky reflection. The data are analyzed in the lab post-cruise by a meteorologist, Mr. Robert Jones, who reviews 10 minutes’ worth of images to assess the cloud amount and type at standard meteorological heights. By looking through multiple images over the 10 minute interval, he is able to view cloud movement and see clouds at higher levels through breaks in low-level clouds. Instantaneous analysis of static imagery does not allow for this determination of multiple cloud layers, thereby biasing cloud amount time series to the lowest visible layer. Mr. Jones has performed all of our cloud analyses, removing the random observer bias that results when using multiple analysts.

The cloud time series is presented as an Excel file, with columns of:

1 date ( UTC; m/dd/yy )
2 time ( UTC; hh:mm )
3 low-level cloud fraction ( oktas )
4 low-level cloud type ( see key below )
5 mid-level cloud fraction ( oktas )
6 mid-level cloud type ( see key below )
7 high-level cloud fraction ( oktas )
8 high-level cloud type ( see key below )
9 sun obscuration flag ( 0 means that clouds are in front of the sun, diffusing
radiation; 1 means that the clouds are not in front of the sun)
10 commentary

Key to Cloud Types:

St = Stratus
Cu = Cumulus
Sc = Stratocumulus

As = Altostratus
Ac = Altocumulus

Ci = Cirrus
Cs = Cirrostratus
Cc = Cirrocumulus

While other cloud types may occur, these are the only types identified within this dataset. The amount of each cloud type is identified for that atmospheric height (low, middle, or high) and is quantified in oktas, or eights of the sky. Because of cloud layering, it is possible to have 8 oktas of stratus as low levels and 8 oktas of altostratus at mid-levels. If the cell is blank, it means that no cloud was identified at that level. If the cells under low, mid, and high cloud fraction and type are all blank, and there is no commentary in column 10 indicating instrument failure, darkness, or weather issues, then the sky is considered clear.

Since the camera is imaging the entire sky, it is possible to identify weather events within the daylight data. The commentary column provides indicators of snow flurries, icing events, fog, hoarfrost, and rain, if those occurred during daylight hours during the cruise. It can also be used to determine sunrise and sunset, as it changes over the course of the cruise.

ASCOS2008.xls917 KB