Frosty creations

With almost continuously sub-zero temperatures at Halley (it did get to +0.6 °C in summer) we get an array of ice structures when the conditions are right. The two main types of frost here are rime and hoar frost. Rime occurs when water droplets from fog or mist freeze to the surface of objects. Usually they are deposited on the windward side and form beautiful and often obstructing creations. It is as beautiful as it is annoying for the scientists and engineers at Halley, as it often blocks instruments and antennas. We recently had a long spell of calm stable weather with patches of sometimes dense fog come through. The prop vanes which measure wind speed and direction completely froze for several days making us rely on the more high-tech sonic anemometer for the same measurements. The reason we have both instruments is that the prop vane doesn’t do well in frost and low wind conditions, whereas the sonic anemometer struggles to make effective measurements in wind speeds over 40 kts.

The other type of frost, hoar frost, occurs during similar conditions as dew formation. In clear calm conditions when heat radiates away faster than it can be replaced objects become colder than the surrounding air. The water vapour in the air then resublimates and is deposited as ice crystals on the cold surfaces.

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