Sinking air warms – example of Santa Ana wind flow

Yesterday’s post was about how rising air chills out when forced up and over denser air.  As the air rises, it expands as the air pressure decreases and it cools.

What happens when air sinks?  It warms as its pressure increases.  If the warmer air keeps the same amount of moisture as it warms, its relative humidity decreases.

When winds blow across a mountain range, on the downwind side where the air moves downhill the temperature can increase dramatically.  And relative humidity can decrease to skin-cracking, suck-the-moisture-out-of-plants levels.

You’ve heard about the dry conditions in California.  They don’t get much rain outside of the winter season, so it gets dry.   In addition, in southern California winds from the east blow air over the mountains toward the coast.  As the air goes up the mountain range, it cools and as the humidity comes to saturation, rain or snow can fall.  This takes moisture out of the air.  Then the wind comes down over the mountain this air’s relative humidity decreases and now you have warm, very dry air, drying out plants and surfaces and increasing the risk of fire spreading.  The winter rains may arrive next week, but until then, there is a red flag warning for Southern California.

Here’s an example where the winds and the moisture levels changed from minute to minute at the John Wayne/Santa Ana airport south of Los Angeles.  When the wind is from the west – bringing in a moist air mass from the ocean, the dew point and humidity readings increase.  When the wind is from the east – the drier air mass from over the mountains, the dew point and humidity crash to single digits.

Data every five minutes – listed as going back in time. . .

Date and Time Temp. (deg F) Dew Point (deg. F) RelativeHumidity


Wind direction Wind speed (miles per hour) Visibility



 Weather observed Cloud cover
11 Nov 5:00 pm 75 0 5 NE 9 10.00 CLR
11 Nov 4:55 pm 75 0 5 NE 8 10.00 CLR
11 Nov 4:53 pm 75 0 5 NE 10G20 10.00 CLR
11 Nov 4:50 pm 75 0 5 ENE 12 10.00 CLR
11 Nov 4:45 pm 75 0 5 ENE 10 10.00 CLR
11 Nov 4:40 pm 75 0 5 ENE 13 10.00 CLR
11 Nov 4:35 pm 75 1 5 ENE 14G20 10.00 CLR
11 Nov 4:30 pm 77 1 5 ENE 15 10.00 CLR
11 Nov 4:25 pm 77 1 5 ENE 20G25 10.00 CLR
11 Nov 4:20 pm 77 1 5 ENE 17 10.00 CLR
11 Nov 4:15 pm 79 1 5 ENE 15 10.00 CLR
11 Nov 4:10 pm 79 1 5 ENE 16 10.00 CLR
11 Nov 4:05 pm 79 1 5 ENE 16 10.00 CLR
11 Nov 4:00 pm 79 1 5 ENE 20G29 10.00 CLR
11 Nov 3:55 pm 79 1 5 ENE 20 10.00 CLR
11 Nov 3:53 pm 79 2 5 ENE 20G25 10.00 CLR
11 Nov 3:50 pm 79 3 5 ENE 16G23 10.00 CLR
11 Nov 3:45 pm 79 10 7 E 12 10.00 CLR
11 Nov 3:40 pm 79 9 7 ENE 13 10.00 CLR
11 Nov 3:35 pm 79 36 21 ESE 9 10.00 CLR
11 Nov 3:30 pm 77 39 26 SSE 5 10.00 CLR
11 Nov 3:25 pm 75 SW 5 10.00 CLR
11 Nov 3:20 pm 75 45 34 NW 3 10.00 CLR
11 Nov 3:15 pm 73 46 38 S 10 10.00 CLR
11 Nov 3:10 pm 75 E 8 10.00 CLR
11 Nov 3:05 pm 73 48 41 SSE 7 10.00 CLR
11 Nov 3:00 pm 73 48 41 S 10 10.00 CLR
11 Nov 2:55 pm 75 48 39 SW 9 10.00 CLR
11 Nov 2:53 pm 75 48 39 SSW 7 10.00 CLR
11 Nov 2:50 pm 73 46 38 S 5 10.00 FEW090
11 Nov 2:45 pm 73 48 41 SW 5 10.00 FEW090
11 Nov 2:40 pm 73 48 41 SSW 8 10.00 FEW090
11 Nov 2:35 pm 73 48 41 SW 9 10.00 FEW090
11 Nov 2:30 pm 73 50 44 SW 7G18 10.00 CLR
11 Nov 2:25 pm 73 50 44 CALM 10.00 CLR
11 Nov 2:20 pm 75 48 39 SSW 8 10.00 CLR
11 Nov 2:15 pm 81 7 6 SSE 7 10.00 CLR
11 Nov 2:10 pm 81 7 6 ENE 9 10.00 CLR
11 Nov 2:05 pm 81 7 6 E 7 10.00 CLR
11 Nov 2:00 pm 82 9 6 ENE 9 10.00 CLR
11 Nov 1:55 pm 81 7 6 ENE 9 10.00 CLR
11 Nov 1:53 pm 81 7 6 ENE 9G21 10.00 FEW100
11 Nov 1:50 pm 81 7 6 NE 12 10.00 CLR
11 Nov 1:45 pm 81 9 6 ENE 12 10.00 CLR
11 Nov 1:40 pm 81 9 6 ENE 9 10.00 CLR
11 Nov 1:35 pm 81 9 6 ENE 12 10.00 CLR
11 Nov 1:30 pm 81 9 6 ENE 9 10.00 CLR
11 Nov 1:25 pm 82 10 6 NE 13G18 10.00 CLR

Other examples of sudden heating are when the wind direction switches from coming from across a plain to coming from over higher elevation.  Temperatures in the northern plains of the United States can rise or fall tens of degrees in minutes.


That Cold Air Damming

Yes, cold air damming is a thing.  This map shows a high pressure sitting over the northeastern United States, blocking the way of the low pressure storm creeping up the Atlantic coast. This storm setting up for Thursday/Friday for the east USA.


Weather map for the lower 48 United States Thursday morning, Nov. 15th.                                   Light blue is a chance of snow. Orange is icefall, green is rain.

Since cold air is denser than warm air, the warm air advancing northward gets forced over the cold air.  This does two things – one, the rising air cools and wrings out precipitation.  Two, the cold air is harder to move out.  The damming occurs when cold air is trapped by mountain ridges and stays in the lower lying areas.  Often the cold air will linger long after it seem like it should have been forced out by the warmer air.  That’s cold air damming.

It can be very local, it can be stubborn, and make a rain forecast look silly as cold air, and sometimes, with snow or freezing rain, seems to refuse to depart.

The National Weather Service is noting the possibility of snowfall amounts for the first part of this storm, with snow moving up the coast Thursday and changing to ice early Thursday night.  This will be earlier in Washington metro and later in New England.


Temperature (red) and dew point (green) with height (in units of pressure, decreasing with height).  Right side shows where the wind is coming from. More flags on the staff means faster wind.

Above is a forecast of temperature in the atmosphere for midday Thursday for just north of New York City.  We weather folks love to skew the temperature plot; tipping to the right.  When the temperature line is straight up, temperature is decreasing, not staying the same.  When the temperature goes to the upper right, it’s staying the same. So, this graph shows temperatures decreasing as we go up in the air and increasing slightly further above us.

The less dense warmer air aloft is trapping the denser colder air below.    Notice the winds from the east-northeast becoming more southerly further aloft.  Rain or snow falling into this cooler, drier layer will cool the air below it by evaporation (just like sweat evaporates and cools you on a hot day).  The northeast winds supply colder air.

This all combines to keep cold air around longer as it is trapped below the warmer air and the evaporative cooling  keeps the precipitation as snow longer.  Cold air damming is when the terrain helps blockade the cooler air .  Then, southerly winds aloft bring the moisture and rising air wrings out the rain or snow.









When a plan comes together

Monday’s Morning Maps: Not much good for astronomy this week, but if you do get some gaps in the clouds, perhaps Wednesday night ahead of the second storm, look out to the south.  Halfway up in the south, Mars is the brightest object in our evening sky (after our Moon, which is going from banana phase to past half-lit this week).  Folks in Puerto Rico and the Virgin Islands can scroll down to the latest tropical Sahara Dust/cloud maps at the end of the post.


As we say in our family, ‘Don’t you just love it when a plan comes together?’

Well, computer forecast models love it, too.  Of course, the models don’t have feelings!  They don’t enjoy it or hate it like humans do when all the packets of upper air energy line up and the resulting deep bend in the jet stream shapes a storm moving up the eastern seaboard of the United States.

This week, the packets of energy (a.k.a. vorticity maximums or short waves) don’t quite come together and the slight variations among models and differences in input data from computer run to computer run gives widely varying results for the potential coastal storm for later this week.


The red and purple are areas of high vorticity around 18,000 feet above sea level.  Often there are northern and southern branches of the jet stream in winter. This map of the first 12 hours of the North American Model shows streams moving across Canada, the USA’s mountain states and even a stream from Mexico. But only the middle stream forms a dip in the jet stream called a ‘trough’.

Let’s look later in the week at the longer range USA Global Forecast Model:


USA Global Forecast Model for last night through Thursday Nov 16. Shows the southern branch of the jet stream with enough energy to spin in a tight circle, which eventually opens up and moves northeastward. When this circle ‘cutoff low’ spins out and moves toward the eastern United States’ coast.  How its energy spreads out will determine the strength and path of Thursday’s storm.

Moisture (in green on the computer forecast map, below) moves up the eastern seaboard with Thursday’s storm:


So, let’s see what the GFS gives for weather we may see this week:


GFS rainfall and surface pressure model through Friday.

The first storm on Tuesday explodes off of Southern New England and the Thursday storm forms over the Carolinas early Thursday and moves off the coast bringing rain (color areas – key on the bottom in inch of rainfall) to the mid-Atlantic and New England states.  We’ll see if that late-forming blob of rain on Friday morning actually happens.

In case it clears out, here’s Wednesday evening’s star map.  It’s good for the whole week, except our Moon’s position moves eastward in the sky each night.


And for our friends in the tropics, the Virgin Islands had a deluge of rain last week from a low pressure trough (not a tropical storm-type situation).  An unrelated batch of spin and a bunch of moisture will try to crack a wall of drier air this week northeast of Puerto Rico.  You might have noticed it creeping into the lower right of the forecast models’ maps later in the week.  Whatever forms is likely to get swept out to sea before reaching the coastal United States.

For our fans in Puerto Rico, here’s the link to the past five days’ satellite composite of dry air (red) and clouds (gray) for the tropical Atlantic, and the latest static graphic:


Keep looking up – and let people know what you see!



Heads UP! for November 2018

This month the early morning Moon falls from overhead, Venus rockets into sky, bright as it ever is. What’s with Comet 46P? Bright, but not so bright?

Morning Moons

Hang on as the abrupt transition from Daylight Time hits us on Sunday morning November 4th.  Watch the crescent moon fall from its late October heights in the morning sky.  The path to new moon makes a large angle with the horizon, making these waning moons easier to track as we approach new moon on the 7th.

Venus Goes Vertical

Venus rises to meet the morning Moon on Election Day, the 6th, but at only 17 degrees from the Sun, we’ll be hard pressed to see them rising just after 5:15am EST.  If you can find Venus with binoculars, you might see Spica just above, closest on the 14th.  Venus makes an incredible vertical leap into the dark sky this month.  By the end of the month, Venus is more than 30 degrees from the Sun and almost minus fifth magnitude in brightness.  Ironically, Venus will appear smaller over time in a telescope, but more of the planet is in sunlight as seen from Earth, so overall, it gets brighter, about as bright as it can get.

Hiding in Broad Twilight

The evening sky keeps its sweep of planets low in the southwestern sky.  Mercury and Jupiter are equally low and hard to see on the 4th, joined by the Moon on the 8th as Jupiter departs the evening sky.  Mercury’s greatest elongation from the Sun is 23 degrees on the 6th, but doesn’t get high enough to be easily seen in northern hemisphere skies.

Low-down on Saturn

Saturn is still up after the end of twilight until early December, but also suffers from ‘low ecliptic syndrome’.  The Moon faces Saturn on the 11th as Luna seemingly struggles to get out of the twilight’s last gleaming.  Iapetus is brightest around mid-month, 2.9 Titan-orbit widths ahead of Saturn, but harder to view though our thick atmosphere. Brighter Titan is on the same side of Saturn as Iapetus from the 12th to 15th.  With the latest research on Saturn’s radiation belts, I’m nominating Iapetus as the place to live in the outer solar system.

Mars Out Standing in the Evening

Mars is getting smaller and smaller in the telescope, but is high in the southern sky and actually getting higher and easier to see, especially in comparison to the other planets lower on the ecliptic. The reddish planet will be even higher in our sky at the next closest approach in October 2020, so use this month as a small (literally) preview.  The Moon saunters past on the 15th.  People who haven’t noticed Mars lately will be surprised by the red ‘spacecraft’ soaring over the Moon’s north pole that night.

Way Out

Uranus and Neptune provide useful targets for the planet-starved astronomer. Both are up in the middle of the evening, but need a bit of hunting to find.  Uranus is near the Ares/Pisces border.  Neptune is in Aquarius.

Uranus and Neptune up in the evening sky to the east (left) of Mars. The Moon passes south of Neptune in this diagram from 9pm Fri Nov 16.

MobileObsScreenshot_181025_173331 3

Uranus (upper left on this map) and Neptune (with Moon nearby on the 16th) in the southern sky.

Pluto Plays Hide and Hide

If you want to know where super-faint Pluto is this month, the Moon crosses in front of the Kuiper belt object during the afternoon of the 12th.

Deep Southern Sky

The constellations Eridanus and Fornax don’t usually get much press, but an asteroid and a comet, respectively, make noticeable appearances there in November.  Asteroid 3Juno is closest to Earth on the 17th, at magnitude plus 7.5 all month in Eridanus.  Follow Orion’s bow westward to get to Eridanus and Fornax.

We may be able to see Comet 46P Wirtanen in Fornax in binoculars in the second half of November.  The 46th periodic comet to have its orbit calculated will be only 7 million miles away from Earth in mid-December.  The comet may flare to magnitude plus 7½ or more compared to its more likely maximum at +8½ in December, so it’s worth keeping an eye on. Could be a pretty picture with the Pleiades December 17th and 18th.  This loopy comet could be an article all by itself, and it does in November’s Sky and Telescope.  Some estimates peg 46P maxing out at magnitude +3 when it comes into northern skies in December.  A magnitude +3 fuzzy comet close to us with its light spread out over a large area is often much fainter than a +3 magnitude star.

MobileObsScreenshot_181025_172336 2

Crumbs of Tempel-Tuttle

The Leonid meteor shower peaks on the evening of the 16th.  As the Earth plows through the shower, leading with our sunrise side, the peak of a dozen or so meteors an hour will be after midnight, but pieces of Comet 55P/Tempel-Tuttle might show up anytime of night. They often leave persistent trails behind.

Minimum Sun

The Sun is deep in solar minimum.  If solar observatories report any sizable sunspot groups, check them out while you can (with the proper solar filter).  Solar eruptions, and strong aurora produced by them, still happen, but less frequently during solar minimum.


Prime satellite-spotting time for the brightest satellites in November post-DST is after 4am in morning twilight and until 7pm in evening twilight.   The ISS, with three souls aboard at this writing, is visible in morning skies through the 16th and evening skies starting on the 21st.

Last morning moon for me for awhile- storms a comin’

Caught a waning moon this morning.

IMG_9431cont brigh sharp

It’ll be the last for the northeastern USA for a few days.  Based on the extended range weather computer model output, the only clear morning between now and Wednesday’s new moon for my house (and probably yours in the northeastern USA) is Sunday morning.  By all means, click on the link highlighted above!  It’s a cool chart translating the output of the models into actual conditions (with some met tech parameters, as well).  You may want to change the units to degrees F, miles per hours and inches of precipitation.   Saturday night’s star party may be a race against time for the clouds to clear while the temperature drops and winds gust over 20 miles an hour.

Here’s the story: Another strong pulse of atmospheric energy will be diving into a dip in the jet stream and swinging toward us.  Look at the blues and purples in this computer output of airflow and atmospheric energy (“vorticity”) on this animation of the short-range North American Mesoscale Model (NAM):


500mb pressure altitude (approx. 18,000 feet above sea level). Map (gray lines) of the lower 48 United States with wind flow (black lines) and vorticity (colors, with purple and blue the most extreme).  Time is in the upper right corner, in ‘Z’ time a/k/a Universal Coordinated Time.  Subtract 4 hours for EDT or 5 hours for EST (the USA Eastern Time Zone clocks change Sunday morning).

The thing that makes storms go ‘pop’ is when the amount of vorticity increases as it follows the black lines.  It’s not the absolute amount of vorticity.  So when you see the black lines (winds blowing from left to right) crossing out of the areas of brightest colors (highest vorticity), that’s where the upper motion of the air is the greatest and the greatest tendency for air to spin and rise to make showers or storms.  Decreasing vorticity brings sinking air and clearer skies.

Here’s what the forecast looks like on a surface weather forecast map:


Weather forecast map for Friday evening 8pm (00Z or midnight Universal Coordinated Time). Dark green is ‘rain likely’.  Blue and red lines are cold and warm fronts, respectively.

Check your local forecast for details.


Morning Moons

This morning’s last quarter moon, Wed Oct 31, (at bottom) compared to yesterday’s slightly more lit moon (first photo). Same exposure (1/1600 sec, f7.1, ISO800 Canon XS with 250mm zoom lens), cropped to same size. Can you see the Apennines Mountains extending like a line into the dark portion of the moon in today’s photo?

IMG_9398 crop 8x8IMG_9405 crop 8x8

Also, the crater Copernicus’ bright rays seem muted today, perhaps because they show up better when the sun is higher over craters with bright splashes around them.  Check again tomorrow!

Moon Morning Photos

Two shots with the Canon XS and my trusty zoom telephoto lens.  More pictures – through the telescope – will be added to this entry later. Second photo is moon in blue sky after sunrise.  I love how beautiful the moon is in a blue sky.  When I see it, I feel like saying ‘what are you doing there?’  Moon will be up through lunchtime, setting a little after 1pm Local Daylight Time.


Last year’s photos (they will look a lot like this year’s) 🙂