Do you know where that's been?

I meant to write a detailed Earth Day post about Iris' class trip to the West Basin Water Recycling Facility. It would have been a follow-on to Walking My Watershed. But, I am jet-lagged and all you get is this short post instead.

[Update, I subsequently posted the field trip report at Our Water Footprint.]

We bought 7-day NYC subway passes for the whole family during our visit. While Iris has taken the subway in LA, SF and DC, she doesn't do it habitually. Last week, she rode the subway as many as 7 times a day. I cringed when I saw how often she touched surfaces in the trains and stations, and then put her hands to her face. You could see the dirt on her hands and sleeves. Who knows how many germs there are? Who else touched their face and then that surface and where else have they touched? Yipes.

Moving right along...

Pennamite wrote on her Facebook wall that it was very windy outside this evening. Someone else noted that, not only is it windy, but it is cold (by LA standards).

My first thought was, of course it is windy; we are getting rain because the jet stream is parked over us.

Then I thought, hmm, this shows us the air that is headed towards us. From where did the airmass that is currently here originate? Well, NOAA has a handy web model interface that allows users to calculate both forward AND BACKWARD trajectories. So here's a 315 hour backward model for the air 500 meters aloft.

Yup, that's almost in Siberia. (The model only goes back 315 hours, but if it had a longer history, we might be able to definitively call it a Siberian airmass.) Whatever. It's from a very cold region.

Compare it to the source of our air from 3 days ago.

Does this help explain the weather?

If you want to run NOAA's HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model, read the tips in Fun with Finite Elements. Then visit NOAA ARL READY http://ready.arl.noaa.gov/HYSPLIT_traj.php.

• A user agreement will pop up.
• Read it, then go back to the original window and select "Compute forecast trajectories".
• Leave the first page of options at their default settings.
• On the second page, select Meteorological Data: GFS and City: Los Angeles, CA.
• The default forecast cycle is the most recent one. Leave it alone.
• On page 4, select BACKWARD trajectory direction and select your start time and total model run time (315 hours max)
• You can leave the plot options at the default settings, but I like to plot color trajectories and add distance circles. For a longer model run, you may want to use label intervals of 12 or 24 hours instead of the default 6.

You MUST select GFS on the second page because the web interface assumes that you know that
NAM is North American Mesoscale,
RUC is Rapid Update Cycle and
GFS is Global Forecast System.

NAM and RUC cover only North America. If you want a map that includes other areas, there is only one choice, GFS. Have fun and don't swamp the server.