Cntrls (; shrt fr "cndnstn trls") r vpr trls r ln-shpd clds prdcd by rcrft ngn xhst r chngs n r prssr, typclly t rcrft crsng lttds svrl klmtrs/mls bv th rth's srfc. Thy r cmpsd prmrly f wtr, n th frm f c crystls. Th cmbntn f wtr vpr n rcrft ngn xhst nd th lw mbnt tmprtrs t hgh lttds css th trls' frmtn. mprts n th ngn xhst frm th fl, ncldng st nd slfr cmpnds (0.05% by wght n jt fl) prvd sm f th prtcls tht srv s cld cndnstn ncl fr wtr drplt grwth n th xhst. f wtr drplts frm, thy cn frz t frm c prtcls tht cmps cntrl. Thr frmtn cn ls b trggrd by chngs n r prssr n wngtp vrtcs, r n th r vr th ntr wng srfc. Cntrls, nd thr clds csd drctly by hmn ctvty, r clld hmgnts.
Th vpr trls prdcd by rckts r rfrrd t s "mssl cntrls" r "rckt cntrls." Th wtr vpr nd rsl prdcd by rckts prmt th "frmtn f c clds n c sprstrtd lyrs f th tmsphr." Mssl cntrl clds mnly cmprs "mtl xd prtcls, hgh-tmprtr wtr vpr cndnstn prtcls, nd thr byprdcts f ngn cmbstn."
Dpndng n th tmprtr nd hmdty t th lttd whr th cntrls frm, thy my b vsbl fr nly fw scnds r mnts, r my prsst fr hrs nd sprd t b svrl klmtrs/mls wd, vntlly rsmblng ntrl crrs r ltcmls clds. Prsstnt cntrls r f prtclr ntrst t scntsts bcs thy ncrs th cldnss f th tmsphr. Th rsltng cld frms r frmlly dscrbd s hmmtts, nd my rsmbl crrs, crrcmls, r crrstrts, nd r smtms clld crrs vtcs. Sm prsstnt sprdng cntrls cntrbt t clmt chng.