Ground-based cloud observation system

Conditions of diversified physical quantities in the atmosphere could be reflected by the change of cloud state. Hence, cloud observation could accomplish indirect understanding on the variation of meteorological elements and conditions of atmospheric movement, thereby precisely estimating when it will be windy and when it will rain.

Cloud form indicates information beyond shape characteristics. Cloud height refers to the perpendicular distance between the cloud base and the observation station. The height of cloud could be varied in the process of occurring, blossoming and transmuting. Cloud cover means the percentage of cloud taking up in the sky. It incorporates total cloud cover and low cloud cover. Total cloud cover presents the sum of cloud cover in the sky whereas low cloud cover is described as the portions of low clouds covering in the sky.

From the perspective of cloud height, observers could estimate cloud height with black or dead color glasses. Average cloud height and common scope are dominating reference data for visual measurement on cloud height.In respect of cloud cover observation, observers generally estimate total cloud cover at first and therein conducting low cloud cover estimation. Observing sites are normally selected in those spacious and permanent locales or terraces where sight of the whole sky and horizon can be caught.

Although the precision is not highly achieved, observers would take synthetic factors into account with the integration of early weather system features as well as future trend, so that the reasonable and holistic observation results could be arrived at.

There are three primary instrumental observation methods on cloud height containing balloon, cloud searchlight and laser ceilometer.

When taking advantage of balloon measurement, observers would sent a rubber balloon filled with hydrogen or helium from the ground, so that data on cloud height could be acquired by the devices aboard on balloon when ascending from the ground to the cloud base. Hereinto, cloud base is the point that the balloon enters into clouds but not ultimately vanished. This method is mainly applied to the measurement of clouds in low altitude.

As for cloud searchlight, the light from powerful source of cloud searchlight could vertically illuminate cloud base and forms a distinct light-spot. Therefore, cloud height could be determined by the measurement of this light-spot’s elevation angle from the observation site where it generates foregone horizontal distance with cloud searchlight.

The theory of laser ceilometer is emitting laser to the cloud via transmitter-telescope, then the laser pulse would be refracted by massive cloud droplets when it encounters clouds with back scattering portions received by reception telescope. Based on the computation of time interval between transmission and reception, the laser ceilometer could obtain the elapsed time of a round trip from the laser ceilometer to the target objects by laser. Ultimately, observation data on cloud height could be received through calculating relations among distance, time and velocity of light.

With regards to the instrumental measurement of cloud cover, data are acquired by means of pixel recognition of sky images received from sensors with visible light and infrared light.

Except that a few automatic observation instruments are equipped in the observation stations, most of stations implement observation on the manual basis at present. Nevertheless, in the near future, objective observation of cloud would be achieved with the layout of automation in basic weather stations.

Editor Chen Meiqi