ABSTRACT
Perhaps the most distinctive visual aspect of planetary atmospheres is the presence offeaturesdrawninthecloud˜eld.Manyformswithdifferentspatialandtemporal scalesofevolutionareseeninimagesofVenus,Earth,Mars,thegiantplanets,and thesatelliteTitan.Theyemergeasaresultofalocalizedinstability,growingand evolvinguntilthey˜nallyinteractandmixwithotherfeaturesordissipatewithinthe atmospheric ¥ow. Different types of measurements distinguish them as deviations of the physical or chemical atmospheric properties, relative to the mean values (e.g., pressure,temperature,geopotential,composition,andvelocity).Inthecloud˜eld, theyshowindifferentways,althoughinasimpleschemetheycanbegroupedinto twoclasses.Onetypehasirregulartexturesandgrowthsandevolvesrapidlyintime. The other type manifests as vortices with atmospheric parcels circulating around a centralregion.The˜rstgroupistypicallythemanifestationoftheconvectiveinstabilitywithassociatedintenseverticalmotions.Thesecondgrouprespondstoavarietyofhydrodynamicinstabilitymechanismswithdominanthorizontaldevelopment. Inthischapter,we˜rstclassifythemaintypesofinstabilities,andthenwediscuss their physical fundamentals and the properties of the different kinds of features that are observed in the planetary atmospheres.
