Pollen rain refers to the fact that pollen which is produced by all plants that have flowers and conifers is ubiquitous in nature.
Pollen is found on almost everyone, clinging on to hair, skin and clothing. This has made it a tool for forensic investigation and though not as popular and developed as other methods, is gaining popularity and rapid advances are being made in its use. A couple of cases have been solved using pollen rain present on potential suspects.
As pollen is not destroyed over a long period of time it can be used even when investigating murders of people where only the bones remain. The presence of a unique kind of pollen everywhere which varies with the plants that grow there and the difficulty that a person under investigation would have in eliminating all the samples present on him also make it an attractive forensic tool.