Tuesday 18 December 2018:

After 1.5 inches of rain fell over the weekend the ground here in Pittsburgh is super soggy.  Meanwhile it’s been warm since last Thursday but the temperature dropped below freezing last night.  These are perfect conditions for needle ice.

Needle ice forms when the soil is warm and the air is freezing.  While ice is forming on top of the soil it draws up subsurface water by capillary action. The ice grows from the bottom creating a structure that looks like needles or even tiny barricades. Since there’s little or no soil on top of the ice, it stands up in the air.

From the top it doesn’t look interesting — basically a frozen lump — but seen from the side it’s very cool.  In the patch shown below some ice needles keeled over so I captured both the top and sides in one shot.

When the soil itself freezes, needle ice can form underground and create a frost heave on the surface.  I’ve never opened a frost heave but here’s what one looks like in a photo from Vermont.

As you watch your footing today, look for the delicate structures of needle ice.

(photos by Kate St. John and from Wikimedia Commons; click on the caption to see the original)

We don’t have flowers in December but we do have ice and it comes in many forms.  Here’s the first in a series on ice, starting with my favorite:  Rime.

The word “rime” comes from Old English hrim which meant hoarfrost or a chill mist or fog.  Nowadays hoarfrost and rime are not the same thing. Hoarfrost doesn’t form in fog. To get rime there has to be cold fog or the location has to be in the clouds. For that reason, “Rime shouts, Mountains!”  It’s easy to find rime at high altitudes.

Rime forms when super-cooled water drops (in fog or a cloud) crystallize on cold objects.  Soft rime is feathery and so lightweight that it can’t break the trees.  Hard rime is denser and comb-like.  Both are white because they contain trapped air.

The best part about rime is that it can form in a light wind. When it does it points toward the wind because each new crystal is deposited on the windward side. This is counter-intuitive; it’s the opposite of rain.

What direction did the wind blow through this fence?

Rime can even coat snowflakes. Graupel, which looks like hail, is actually a rime-coated snowflake.  I wonder if these pop when you step on them.

This month is a good time to find rime in the Laurel Highlands. My best experience with it was during an east wind at the Allegheny Front Hawk Watch.  No one else was up there to enjoy it.  It was way too foggy!

Click here to see more rime photos including a “rime doughnut” at Summitpost.

(photos from Wikimedia Commons; click on the captions to see the originals)

The sun will rise an hour early tomorrow in Pittsburgh.

At 2:00am on 4 Nov 2018 the cell networks will push out the new time (1:00am Standard Time) to our mobile phones.  We’ll gain an hour of sleep and will be surprised when the sun sets just after 5pm. Rush hour will start at dusk on Monday.

Now that we’ve passed the autumnal equinox we’re losing daylight every day.  Tomorrow we’ll lose two minutes! 

Meanwhile, birds are unaffected by our trick with clocks and will still cue on daylight.  When the crows show up in Oakland at rush hour, lots of people will be surprised. 

Monday will be the first time people and crows are on the same schedule.  😉

(photo credits: Pittsburgh sunrise by David Fulmer, CC license on Flickr, screenshots of SunDroid app & photo of crows by Kate St. John)

Sunday morning, 21 Oct 2018:

In early October, warm days and cool clear nights are the perfect combination for dew on the grass.  One of these nights the temperature of the grass will drop below freezing and the dew will form as frost.

Have you had frost in your neighborhood yet? 

Tonight might be the night.

p.s. If the air is below freezing but not humid, the plants freeze without a frost. 

(photo by Kate St. John)

9 October 2018

Last week the National Weather Service published an analysis of six of the many tornadoes that hit our area on Tuesday, October 2.  There were 14 tornadoes in Pennsylvania that day!

The most damaging tornado hit a nursing home in Conneautville, PA.  The most unusual one touched down in the Valley Green Road area of Westmoreland County.  NWS Pittsburgh describes what made the Valley Green Road EF1 tornado so interesting:

This tornado was unusual not only for its northward movement in an eastward-moving storm, but especially because it formed on the northern side of the parent thunderstorm, removed from the typical hook echo region.

NWS Damage Surveys for 10/2/2018 tornado event, National Weather Service Pittsburgh

So what is the hook echo region?

According to Wikipedia, “A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms.”  

USTornadoes.com describes how it forms: “This “hook-like” feature occurs when the strong counter-clockwise winds circling the mesocyclone (rotating updraft) are strong enough to wrap precipitation around the rain-free updraft area of the storm.”

The annotated radar image below shows the hook at bottom left, curling around the back of the storm with a tornado at the tip. 

Tornadoes usually form in the hook echo and they move with the storm.  Storm chasers use these facts to find and safely chase tornadoes.

But not at Valley Green Road.  That tornado formed on the north edge and traveled north (See region on the annotated example of an Oklahoma tornado below.)

Sneaky tornado!  Fortunately it was not very powerful (EF1) and there were only trees in its path.

(photo and hook echo images from Wikimedia Commons. The tornado photo was taken in North Carolina (not in PA); click on the captions to see the originals)

In case you haven’t heard, it’s been raining here in Pittsburgh.  A lot! 

This is mostly from the remnant of Tropical Storm Gordon that came ashore in Mississippi on September 4 and arrived here September 8.  We were in a drought but now it’s been raining for three days.

Yesterday set a record of 3.73 inches at the National Weather Service in Moon Township but it’s rained even more in the South Hills. Since Friday morning 7.5 inches have fallen at the Allegheny County Airport in West Mifflin (as of 10 Sept 2018, 7:53a). And it’s still raining!

Basements are flooded and roads are closed.  Two screenshots from PennDOT traffic cameras show:

• On the left, the 10th Street Bypass is flooded and closed (hard to see through the rain-splashed cam).
• On the right, the I-376 “bathtub” at Stanwix Street hasn’t filled yet but it’s going to, so it’s closed.

And the rivers are rising.  Here’s the National Weather Service’s Ohio River prediction for Pittsburgh.  Uh oh!

Fortunately our house and street are fine (photo at top) but I feel bad for those who live in the valleys.

Rain, Rain Go Away!

(photo by Kate St. John, PennDOT traffic camera screenshots, Ohio River gauge from the NWS Advanced Hydrological Prediction Service)

Pittsburgh’s weather had a tropical flavor this month with a chance of thunderstorms predicted every afternoon.

On clear days I could see individual thunderheads form and sometimes hear them rumble in the distance.  This late morning cloud on August 13 had some interesting features. Was it going to rain or thunder?

It was already a two-part cloud: Cumulus is the bright white top, nimbus is the darker lower section.

While I watched, two updrafts rapidly pushed moisture higher in the sky. If I’d made a video you would see both columns expanding upward and growing fast. The small cloud on the left dissolved moments after I took this photo. The big one kept growing.

The nimbus section looked threatening with dark lumpy areas (photo retouched to add contrast).  I’m not sure what the lumps mean but they probably weren’t good.

I watched this cloud as I walked home and, sure enough, it thundered after I took the photo.  Fortunately I was indoors before it rained.

(photos by Kate St. John)