It wouldn't be good..., but the reaction would still be dependent on ambient temp and humidity. Bodies of water tend to have more violent reactions as they tend to trap or stall the release of hydrogen which doesn't mix with the metal when you have ignition or spark, so the explosions are more localized... the cesium in open air would have a reaction but it would not be explosive in most cases... The cesium would corrode almost instantly in open air... if the humidity was too high you could get ignition but it would not be explosive(or very little).