How is the relative humidity parameter calculated

[pvandewater]

I have used hysplit for some time and recently was messing around with generating hourly relative humidity data for specific locations. What I was getting was highly variable at times and other times somewhat respectable numbers (meaning that the were what I expected not necessarily what actually occurs). I have not done an exhaustive search but could not find any quick explanations of how the data is generated within the model. Any help out there?

[glenn.rolph]

Are you referring to the ability of HYSPLIT to output meteorological variables along a trajectory? Or are you referring to how is RH calculated in HYSPLIT from the meteorological fields and used in chemistry routines in the model?

[pvandewater]

I was generating RH values above a single spot on a bi-hourly basis and they seemed at times to be highly variable. The variability went against what i expected, however I must admit that there may be nothing wrong with it. That is why I asked about how the calculations were made.

Cheers

[redgee34]

Could you tell me please what is the download size of the data required to run HYSPLIT ? Also are subsequent periodic or ad hoc downloads or updates required ?