<P> The altitude of the troposphere most similar to Earth is near the tropopause--the boundary between troposphere and mesosphere . It is located slightly above 50 km . According to measurements by the Magellan and Venus Express probes, the altitude from 52.5 to 54 km has a temperature between 293 K (20 ° C) and 310 K (37 ° C), and the altitude at 49.5 km above the surface is where the pressure becomes the same as Earth at sea level . As manned ships sent to Venus would be able to compensate for differences in temperature to a certain extent, anywhere from about 50 to 54 km or so above the surface would be the easiest altitude in which to base an exploration or colony, where the temperature would be in the crucial "liquid water" range of 273 K (0 ° C) to 323 K (50 ° C) and the air pressure the same as habitable regions of Earth . As CO is heavier than air, the colony's air (nitrogen and oxygen) could keep the structure floating at that altitude like a dirigible . </P> <P> The circulation in Venus's troposphere follows the so - called cyclostrophic approximation . Its windspeeds are roughly determined by the balance of the pressure gradient and centrifugal forces in almost purely zonal flow . In contrast, the circulation in the Earth's atmosphere is governed by the geostrophic balance . Venus's windspeeds can be directly measured only in the upper troposphere (tropopause), between 60--70 km, altitude, which corresponds to the upper cloud deck . The cloud motion is usually observed in the ultraviolet part of the spectrum, where the contrast between clouds is the highest . The linear wind speeds at this level are about 100 ± 10 m / s at lower than 50 ° latitude . They are retrograde in the sense that they blow in the direction of the retrograde rotation of the planet . The winds quickly decrease towards the higher latitudes, eventually reaching zero at the poles . Such strong cloud - top winds cause a phenomenon known as the super-rotation of the atmosphere . In other words, these high - speed winds circle the whole planet faster than the planet itself rotates . The super-rotation on Venus is differential, which means that the equatorial troposphere super-rotates more slowly than the troposphere at the midlatitudes . The winds also have a strong vertical gradient . They decline deep in the troposphere with the rate of 3 m / s per km . The winds near the surface of Venus are much slower than that on Earth . They actually move at only a few kilometres per hour (generally less than 2 m / s and with an average of 0.3 to 1.0 m / s), but due to the high density of the atmosphere at the surface, this is still enough to transport dust and small stones across the surface, much like a slow - moving current of water . </P> <P> All winds on Venus are ultimately driven by convection . Hot air rises in the equatorial zone, where solar heating is concentrated, and flows to the poles . Such an almost - planetwide overturning of the troposphere is called Hadley circulation . However, the meridional air motions are much slower than zonal winds . The poleward limit of the planet wide Hadley cell on Venus is near ± 60 ° latitudes . Here air starts to descend and returns to the equator below the clouds . This interpretation is supported by the distribution of the carbon monoxide, which is also concentrated in the vicinity of ± 60 ° latitudes . Poleward of the Hadley cell a different pattern of circulation is observed . In the latitude range 60 °--70 ° cold polar collars exist . They are characterised by temperatures about 30--40 K lower than in the upper troposphere at nearby latitudes . The lower temperature is probably caused by the upwelling of the air in them and by the resulting adiabatic cooling . Such an interpretation is supported by the denser and higher clouds in the collars . The clouds lie at 70--72 km altitude in the collars--about 5 km higher than at the poles and low latitudes . A connection may exist between the cold collars and high speed midlatitude jets in which winds blow as fast as 140 m / s . Such jets are a natural consequence of the Hadley--type circulation and should exist on Venus between 55--60 ° latitude . </P> <P> Odd structures known as polar vortices lie within the cold polar collars . They are giant hurricane - like storms four times larger than their terrestrial analogs . Each vortex has two "eyes"--the centres of rotation, which are connected by distinct S - shaped cloud structures . Such double eyed structures are also called polar dipoles . Vortices rotate with the period of about 3 days in the direction of general super-rotation of the atmosphere . The linear wind speeds are 35--50 m / s near their outer edges and zero at the poles . The temperature at the cloud - tops in the each polar vortex is much higher than in the nearby polar collars reaching 250 K (− 23 ° C). The conventional interpretation of the polar vortices is that they are anticyclones with downwelling in the centre and upwelling in the cold polar collars . This type of circulation resembles a winter polar anticyclonic vortex on Earth, especially the one found over Antarctica . The observations in the various infrared atmospheric windows indicate that the anticyclonic circulation observed near the poles penetrates as deep as to 50 km altitude, i.e. to the base of the clouds . The polar upper troposphere and mesosphere are extremely dynamic; large bright clouds may appear and disappear over the space of a few hours . One such event was observed by Venus Express between 9 and 13 January 2007, when the south polar region became brighter by 30% . This event was probably caused by an injection of sulfur dioxide into the mesosphere, which then condensed forming a bright haze . The two eyes in the vortices have yet to be explained . </P>

What drives the high-speed winds in venus's atmosphere
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