An effort should be made to observe most of the elements as close to the reporting hour as possible. Wave observations require a slightly longer period than most and many observers maintain a continuous watch on the waves. Visual estimates of wind speed and direction are based on the appearance of the sea surface, using the Beaufort scale. Air temperature and humidity readings are usually taken with a sling psychrometer. Sea surface temperature is taken either with a bucket thermometer or by reading the temperature at the engine water intake. Wave period and height are computed from the average values of the better formed waves. Swells are waves that have outrun the wind and are characterized by more uniformity and consistency. Cloud observations require careful study of the cloud descriptions and photographs which are provided to the ship. Visibility is determined by the appearance of the horizon, of other ships, of targets on the radar screen and of objects at known distances on the vessel. The presence of ice on the sea surface or on the surface of the ship is also reported.