From Where Comes Our Air?

Winter air masses in Cincinnati tell the meteorologist a great deal about coming weather.  If I can identify the air mass that will arrive in the Tri-state I can describe the weather and give you a good idea of what it will be like and how it will impact your day without forecasting specifics.

In winter air masses arrive from the north.  They are either maritime polar, continental polar or the "Bringer of Below Zero" air mass we call arctic air.

Occasionally, actually rarely air arrives from the central Pacific and we have a nice mild spell. But in most cases we get mP (maritime Polar) air from the north Pacific, cP (continental Polar) from central and northern Canada or cA (continental Arctic) air from Siberia, the North Pole and the northern fringe of Canada.

The jet stream comes into play by being the steering current that guides the air masses from the various source regions.  A major arctic air outbreak like January 1977, December 1989 or January 1994  we need the "Arctic Express" an over-the-pole jet stream flow which brings air from the interior of Siberia, the coldest region of the northern hemisphere, right over the pole and into the Ohio River Valley.

Continental Polar air is "normal" cold air and we get typical winter temperatures from its arrival.

When the jet stream is from the Pacific Northwest our air is cool to chilly and near or just above normal.

The air this winter should predominately be from the north Pacific meaning close to normal temperatures and the potential for a large amount of precipitation.

Recipe For a Raw, Wet Day

Cut off lows form occasionally when a pool of cold air is isolated south of the main jet stream. These lows move slowly because they are not part of the main jet stream and they are notorious for thick cloud cover and chilly days with rain. Basically it is raw weather that is not pleasant.

The video below explains it.

Cooler and Wet For A Few Days

 Before we get to the main topic, take a look at the picture of the dust storm in Lubbock, TX today as it approaches a subdivision.

We have had two previous occasions to look into "Cut Off Lows" at jet stream level in the atmosphere.  When Hurricane Irene left behind a upper cut off low and a few weeks later when one sat over top of us for more than a week we enjoyed extended periods of wet weather.

Because these lows are pools of cool air isolated from the main jet stream they travel only slowly until they are re-absorbed by the jet. 

Now as the video below shows another cutoff low will make a visit to the tristate but this one will not linger long.

Winter 2011 - 2012 Forecast

It is that time of year when we wonder if we should stock up on de-icer and buy a new snow shovel.  You want to know: "Is the winter going to be a rough one?"

Right now it looks like a repeat of last year as a La Niña is developing along the equator over the Pacific from the coast of South America to 180° longitude.  The video below explains it.

La Niña is a global sloshing of pressure systems that results in colder than normal water along the equator from South America to the center of the ocean at 180° longitude. The cold water affects the jet stream systems over the U.S. during winter and there fore the weather. The opposite pattern is the better known El Niño which occurs when water temperature is warmer than normal.

The warmer or colder than normal the water temperature is -  the stronger the system and the greater the effects around the globe. In Cincinnati and the surrounding area La Niña causes a bull's eye of wet weather right over us and puts us in the battle ground between constant cold and persistent warmth.

The maps below are from NOAA's Climate Prediction Center, the official forecast.

Fog



Fog in Appalachian Mountain Valleys (NOAA)


Fog is a cloud on or close to the ground and like all clouds there are a number of items which must be present for the cloud to form.

For any cloud to form the relative humidity much reach 100%. And the two ways the relative humidity can be increased are by adding moisture to the air and by cooling the air.

Adding moisture has little to do with the fog we had this morning or the fog I expect tomorrow morning because those fogs are all about cooling the air.

Water can be added to  the air and form fog when air passes over a large body of water  like the ocean or the Great Lakes  or very wet soil or rain falls through the air. The water that is evaporated in each of these situations increases the relative humidity and when it reaches 100% condensation takes place and the formerly invisible gaseous becomes very small visible liquid drops.

If the temperature is below freezing the small liquid drops freeze on contact with tree limbs, wires, your windshield and deck railings and form a milky ice called rime. The fog is then called freezing fog.


When air cools the molecules in the air, including water vapor, slow down and eventually some slow enough so that when they collide or pass just close enough to each other the forces of attraction between molecules are stronger than the energy of motion.  When two water vapor molecules "stick" together we call it condensation and the formerly independent water vapor molecules are now part of a tiny water drop. The are now liquid water molecules.

A relative humidity of 100% means all the available energy has been used to do the work of evaporation and no more evaporation can take place and as air cools further condensation must take place.

The only differences between water vapor molecules and liquid water molecules is how fast they move and water vapor molecules are independent of others because they move too fast to stick together.  Liquid molecules stick together in drops because they have less energy.

So on clear, cool, calm nights the molecules slow as the temperature drops. When the molecules start to stick together we call it condensation. Eventually enough stick so a visible drop has formed and when trillions and trillions of drops are floating in the air we call it fog.

To be complete water condenses easily on substances that are said to be hygroscopic or water attracting. Water will not spontaneously condense in very clean air when the relative humidity reaches 100% but will quickly if bits of dust, sea salt, nitrate fertilizer etc. that are hygroscopic are present and there is plenty of all of these in the air over the tristate.  Yes even microscopic bits of sea salt are easy to find high in the atmosphere here.

Little Bundles of Jet Stream Energy (Stayin' Away)

TV meteorologists are always yapping about "THE JET STREAM".  Jet stream systems are not as simple as a river of air like most meteorologists call them. Another often used description is a "current" of air like an ocean current.

Embedded within the jet are "upper level disturbances" - the real weather makers. Some meteorologists go on and on about vorticity, or spin in the atmosphere. Vorticity does nothing, but it tells us where the energy to lift the air is and where clouds and rain are likely to form.

The graphic above shows those bundles of energy in the jet stream in yellow and red. As you can see the pattern is complicated. And generally when a red area passes over you there is upward motion and IF there is enough moisture clouds and rain develop.

No bundles of energy  move into the tristate until Monday, but on Monday the air is very dry still so I do not have rain in the forecast until Tuesday night and Wednesday and then it is a chance.

Not Quite Indian Summer! A Killing Frost Comes First.

If you like Indian Summer thank John Hector St. John de Crevecoeur. In 1778 he wrote about a period of smoke and mildness after the first frost and before the big snows of winter in Upstate New York.

The term may have originated because the last of the Indian raids came during this time. Soon snow would slow travel and raids were impossible.  This is also when Indians harvested squash and corn so it may have been how the term originated.

The true origin is likely lost in history but we all (at least all of us in the Eastern U.S.) know well what it means.

The weather this week will be warm and dry, much like  Indian Summer. But we have not had a widespread killing frost.  So while you will enjoy great weather this week it is not that traditional folklore favorite.

What is causing this warm, dry spell is a big ridge. A ridge is a northward bulge of the jet stream. the air in the ridge is sinking and it is warm. The ridge  over the central U.S. is headed our way.

The jet stream forms along the boundary between the cold polar air north of the jet stream and the warmer air to the south.  When the wind current bulges northward  the warm air to the south is surging northward. The entire bulge will move eastward and we will enjoy warm days, cool nights and plenty of sun into and through the weekend.