Long-term trends in ice nucleus concentrations

Ice nucleus concentrations are listed for four areas for which measurements have been made for longer than 25 years. These are Sydney (Australia), Antarctica, Tasmania and Hawaii. With the exception of the last two years in Sydney associated with unusually wet weather and a period in Tasmania when there was extensive cloud seeding, there has been an apparent general decrease over that time. It should have been sufficiently large to have climatic implications, perhaps leading to less frequent but more intense precipitation. However, it would be a difficult task to distinguish between dynamic and microphysical causes of any such change in precipitation. There is a need to establish quantitatively the relation between ice nucleus concentrations and cloud-radiative climatic forcing. Then, if the changes suggested by the present results are significant they will need to be verified and the sources of the ice nuclei established.