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THE NERC MST RADAR FACILITY AT ABERYSTWYTH
MOUNTAIN WAVES
picture of a lee wave cloud The influence of the Earth's surface on winds is typically confined to the lowest kilometre or so of the atmosphere; the effects of friction cause the wind speed to reduce closer to the ground and the direction to rotate by 20° - 30° (anticlockwise when viewed from above in the northern hemisphere and clockwise in the southern hemisphere) compared to what it is in the 'free atmosphere'. However, where the winds flow over hills or mountains, the effects of the Earth's surface can (under suitable conditions) have an influence right the way up through the atmosphere. This is owing to the generation of a class of internal gravity waves, known as mountain waves or orographic waves, which cause the air to oscillate in the vertical direction with amplitudes of up to a few m s-1. Where the relative humidity of the air is close to 100%, the (adiabatic) cooling within the rising portions of the waves can be sufficient for condensation to occur; this gives rise to the formation of characteristically smooth lenticular (i.e. lens-shaped) clouds such as those pictured above. Both pictures (from different occaissions) were taken looking northwards from the Aberystwyth sea-front, 6 km to the West of the NERC MST Radar Facility. The MST radar often detects the presence of such waves when the low-level wind direction is from between North and South through East, i.e. the directions in which the largest hills are situated.
picture of a lee wave cloud
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The terrain around the MST Radar site
MST Radar vertical velocities
Page maintained by David Hooper
Last updated 24th September 2002