Wednesday, January 30, 2013

A Strange Warm Rain

3 PM View In Seattle
 Today was a day of warm, light drizzle from sunrise to sunset.  Visibility was minimal and air was permeated by an ethereal mist.  Biking home in the drizzle tonight on the dark Burke Gilman trail, I could not remember when I had less visibility.

View from the KIRO Tower could barely see the ground!
But although it was drizzling lightly all day, the Camano National Weather Service radar showed nothing for most of the day (see example):

Radar at 3:17 PM

How could that be?

And consider that the origin of the light rain was very different from nearly all of the rain we get here during the winter.

Most rain in the Northwest starts in clouds as ice crystals aloft, often in an environment that also contains supercooled water (water below freezing, but still liquid).   Precipitation develops through the cold-cloud process in which ice crystals collide and grow (aggregation) into snowflakes or ice assemblages, or collect supercooled water that freezes on to the crystal.  As the ice crystals get larger and heavier they fall faster and collect more supercooled droplets and ice crystals.  If the temperatures at low levels are cold we get snow, if warm, the ice crystals melt and we get rain.

Since the precipitation falls from high up and the precipitation particles are fairly large, they show up well on radar.

But today we got some precipitation, the drizzle, from an entirely different process:  the warm cloud process.  In this process, clouds that are entirely above freezing can produce rain.  Shallow warm clouds produce very light rain with small droplets:  drizzle.

Imagine a shallow warm cloud made up of many, many small cloud droplets.  Some droplets are bigger than others and fall a bit faster.  They collect some of the droplets below them and get bigger and thus fall even faster.  Thus, they get bigger and fall faster still, eventually leaving the cloud and falling as drizzle.

Today, the atmosphere was above freezing and saturated at low levels; here are the observations of temperature and dewpoint at Salem, Oregon up to about 7000 ft this morning at 4 AM.  The X axis is temperature (degrees C) and the Y axis is height in pressure.  The rest of the lines you don't want to know about! When temperature (right most line) and dew point (the other line) are the same, the air is saturated (relative humidity of 100%).  You can see that the air was saturated and above freezing up to about 850 hPa (that is a pressure)---about 1500 meters or 5000 ft.

So why did the radar have trouble seeing the drizzle?   Two reasons.  One is that very small droplets don't show up well on radar.  Another is that the radar beam was already quite high when it got to Seattle (about 2000 ft), so it was above the largest drizzle drops, which are found lower in the shallow cloud.

Although warm rain is not rare in the Northwest it produces only a few percent of the precipitation.  So enjoy it...I am sure a dermatologist would tell you it is good for your skin.  And it tends to keep pesky Californians away.


  1. It was a completely dry day on Sinclair Island yesterday and very springlike. First a light wind came from the southeast and then it switched to the north bringing with it a fog bank that dissipated.

  2. Thanks Cliff!

    I commute on my bike as well. I don't read much about the weather in regards to forecasts (nothing personal), but I usually glance at the radar every morning.

    I've wondered about being so wet when I arrive at work when there is nothing on the radar.

    Now I know.


  3. Cliff, I enjoy your blog but struggle to understand many of the non-radar charts and graphs.

    Do you have a blog entry or could you write one that explains the units on the x and y axis?

  4. It got very warm on the east slopes of the Cascades yesterday, and not a single weather forecast that I saw predicted it. Places in the Methow ranged from 45-53. Leavenworth got to at least 55. Even after nearly 24 hrs the standard NOAA forecast was still really off for last night's predicted low (forecast was for 15 where I live and it only dropped to 31). Why is that, and what happened? Thanks!

  5. cliff,
    i'm also a year-round bike commuter. i am often perplexed by the reverse of what you noted with the radar: biking in the dry while the radar shows light to moderate rain. what's going on there?

  6. Mary, I can answer your question. The warm temperatures along the Cascade east slopes were due to compressional warming. The air coming over the Cascades descends, which causes it to warm. This in itself isn't unusual. What was unusual is that warming made it all the way to the surface and valleys. Typically in the winter, the cold air is locked into the valleys and tough to mix out. So the warmer descending air just rides up and over the cold air, leaving folks in the Methow still in the cold. But in this case, the cold air was mixed out, which is rare. And being that it is rare, we missed it. But the UW-WRF didn't. Looking back in hindsight, it forecast mid 40s to low 50s.

  7. Cliff, I was amazed yesterday at how many continuous hours we got steady warm-rain drizzle. I would have emailed wx about it if you hadn't posted! What I'm still not totally clear on is, where did the lift come from? As far as I can tell, we were in steady northwesterly flow aloft, under a ridge, and no indication of any synoptic-scale forcing for asent (which wouldn't help much anyway to lift air below 1 km). So it would likely have to be orographic. But being half-way between the Olympics and Cascades, Seattle is typically not in an orographic ascent zone, except when the PSCZ sets up. The wind direction is roughly correct for a PSCZ, but I didn't see any indication of a PSCZ in the UW WRF forecasts, especially not as far south as Seattle. Perhaps it's Cascade orographic ascent, pushed upstream by stability and blocking, into the PS basin?

  8. It was less drizzly, and better viz in Tacoma. We even had a few brief appearances of that strange light in the sky. Noticed that we were in the lee of the general moisture pattern, so that must have helped us. My rain gauge picked up about 0.13 inches of rain, and noted much higher amounts (up to and over one inch) north of us.

  9. Cliff, you said:
    "...Some droplets are bigger than others and fall a bit faster. They collect some of the droplets below them and get bigger and thus fall even faster. Thus, they get bigger and fall faster still, eventually leaving the cloud and falling as drizzle."

    I don't get this. How can this be? This flies in the face of the old experiment of dropping something light and something heavy! Any object's (big or small) downward velocity will increase by 9.81 m/s because of gravity. All objects fall at the same speed! The only exception is that in the atmosphere, things reach a terminal velocity due to atmospheric drag- so a bigger droplet in volume, if anything, could only fall a tiny tad slower than a smaller (lighter or heavier-mass should not matter)droplet due to greater drag. What am I missing here? Do rain drops really mess with gravitational acceleration?

    There must be some other explanation?

  10. I'm teaching Earth Science to middle school-aged homeschooled children. We've been checking your blog since beginning our weather unit. The inversion a couple of weeks ago, the shipping cloud tracks, and now the warm cloud precipitation (which they have learned as the collision-coalescence process) have all been very fascinating for us as we get to see our science book illustrated for us when we walk outside. Thanks for taking the time to educate through your blog and pointing out the far from dull weather we enjoy here in the NW.

  11. Tim,
    No I am not talking about changing the rules of physics. This is about terminal velocity, which is the result of the balance between the acceleration of gravity and drag. All droplets DO NOT fall at the same speed, because of the effects of drag.

  12. Cliff,
    OK, but aerodynamic drag is a function of frontal area. The larger the frontal area, the more drag. Seems like a larger droplet will have larger frontal area and should be slowed MORE by drag than a smaller droplet and thus fall slower than a small droplet.

    Is there any observational evidence that clearly shows that large droplets fall faster than small ones?

    Not trying to be a pain here, but it seems like there is an inconsistancy in your description.

  13. Tim,
    What I told you is consistent with both theory and observations. A lot of supporting literature, e.g.,

    But surely this is consistent with your own experience...drizzle drops fall slowly while big drops fall fast!


  14. Cliff, you're the man!
    Thank you so much for those links, that clears it up for me. A few equations and a few graphs are worth a thousand words! It is interesting that due to mechanical instability, there appears to be an upper limit on droplet size. Anyway, its facinating.

    RE my experience, To be honset, I have never even thought about droplet velocity -I am more prone to curse drops than measure their falling speed!

  15. "aerodynamic drag is a function of frontal area"

    right - and area is proportional to the square of droplet diameter while droplet volume and hence mass and hence gravitational force is proportional to the cube of droplet diameter.

    So for bigger droplets the balance of gravitational and drag forces (aka terminal velocity) happens at a higher speed.

    AFAIK that's about the simplest explanation of bigger droplets falling faster

  16. Tim, big drops of course have more drag, but they have yet more weight.

    For a sphere, volume (and hence droplet weight) is proportional to the cube of the radius while surface area (and hence drag) is proportional to the square of the radius.

    Weight growing faster than drag means big drops fall faster.

  17. Hi Cliff,

    Will you please address Sudden Stratospheric Warming in one of your posts? Is this why the Midwest and Northeast are currently experiencing such variability in their weather patterns? If the jet stream drops South of the PNW will we get the same type of Arctic blast Michigan has been getting? What are the chances for this happening in our neck of the woods any time soon? Thanks so much for your time! I love what you do :-).

    Tim Stedman
    Science Teacher
    Ballard High

  18. Wonderful post. I had not known this about the Pacific NW. Thank you!