Temp/Humidity sensor enclosures

For those that have homebuilt sensor enclosures. I've noticed that almost all of the radiation shields that are made from PVC have a 90 degree turn on the top air inlet. Is there a reason for this or is it simply the "style" as the NWS Mobile Mesonet's had?

I finally broke down and got a temp/humidity sensor for the Davis WM II that I use and the only way to get "good" measurements is to enclose the sensor puck. Since I'm building from PVC I would like to know if there is a reason behind this, or will the straight PVC work just as well.

John Diel
Ok, since no one decided to take up the glove on my first question (except one person via PM), I'll try another one.

Those of you that do have a PVC NOAA style Radiation shield (and I know that some of you do!), how many are passive (no fan or other mechanism to "force air movement") and does this skew your temp/humidity values?

If you have an "active" Radiation Shield (meaning you have a fan or other means of force air movement over the sensors), what are you using and how well does it work?

C'mon folks, I know there's a few out there.

John Diel
Good questions...I'd like to hear some dialog on both of these as well.