Although Uranus exhibits a clear zonal wind structure, no regular cyclonic or anticyclonic eddies have been observed. Small white clouds are occasionally observed at midlatitudes (at approximately ±30°), which have been interpreted as localized methane clouds forming in restricted areas of rapid upwelling, rather like the equatorial brightenings seen on Saturn, and these have become particularly numerous during the run-up to the Northern Spring Equinox in 2007. Observations of these clouds (Figure 5.38) with HST are reported by Karkoschka (1998b, 2001), Sromovsky et al. (2000), and more recently by Hammel et al. (2005a, b) and Sromovsky et al. (2007). A particularly bright cloud was observed by HST in 2004 by Hammel et al. (2005b) at 36°S. Such clouds, if associated with vigorous enough convection may help to transport methane through the cold trap of the tropopause and on into the stratosphere. However, an observation of an even brighter, higher cloud by the Keck Observatory in 2006 (Sromovsky and Fry, 2007) put the cloud top at 200 mbar, which is still below the tropopause and estimates of the abundance of methane in the stratosphere lie close to the "cold trap'' value. HST observations in 2000 found that the northern midlatitudes just coming into view as Uranus approached equinox were considerably more convectively active then southern mid-latitudes. However, during the equinox in 2007 both hemispheres appeared equally convectively active.
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