Friday, July 25, 2014

Troughs and Next weeks weather.

I've been getting a few questions on what exactly is a trough and want causes it to form. So I thought I would answer in it in the blog.

The weather Next Week
This morning places in New York, Pennsylvania,  and northern New England saw temps in the upper 30's and low 40's. make it feel like Fall.  Pennsylvania and Mid Atlantic States will see heat and humidity this weekend. New York State and New England will see temperatures and dewpoints rise this weekend...but not as much as those to the south.

 But as I said on my Facebook page, the warmer air will be temporary.  We're going to see a trough move in from the north and west. The trough will set up shop over the East Coast, allowing cooler than average July temperatures to invade even the Deep South.  The reason for this is the jet stream pattern, we're still basically in the same pattern we had during the Winter of 2013-2014.   

In The Great Lakes and  New England it will feel more like late September than July. Those around the Great Lakes and Higher Elevations of New York and northern New England.... a few days next week will see highs in the upper 50's into the 60's, with night time lows in the 30's-40's.
What is a trough?

A trough is an elongated area of low atmospheric pressure that can occur either at the Earth's surface or at higher altitudes.  Upper-level troughs influence many surface weather features, including the formation and movement of surface low pressure areas and the locations of clouds and precipitation.

Normally precipitation will fall to the east of the trough axis while colder, drier air tends to prevail to the west of the trough (in the Northern Hemisphere).

When the height contours bend strongly to the south, (as in the diagram above), it is called a trough. Strong troughs are typically preceded by stormy weather and colder air at the surface.

Troughs and Ridges deal with things like geopotential heights,  Vorticity Advection, Temperature Advection. These are very complex and deal with a lot of  physics, differential geometry and complicated math equations. I will try to keep the math and science out of the discussion .

The primary characteristic of a trough is that it is a region with relatively lower heights. Height is a primary function of the average temperature of the air below that height surface.

For example, if it is 500 mb heights then the 500 mb height is based on the average temperature between the surface and 500 millibars. The density of air changes with temperature. As the temperature of air cools down it becomes more dense and thus more compacted (takes up less volume). Therefore, as air cools the height lowers since the air is becoming more dense. Air will cool when it rises, thus a trough can be found where there is a lifting of air.

What is geopotential?

1 geopotential metre=0.98 dynamic metres

geopotential height is a better measure of height in the atmosphere as energy, is in general, lost or gained when air moves along a geometrically level surface. This is because geopotential depends on geometric height and gravity with mean sea level being selected as the zero potential height.

The unit of geopotential is the potential energy acquired by unit mass on being raised through unit distance in a field of gravitational force of unit strength.

Low geopotential height (compared to other locations at the same latitude) indicates the presence of a storm or trough in the mid-troposphere.

Relatively high geopotential height indicates a ridge, and nice quiet weather.

Decreasing geopotential height usually indicates an approaching or intensifying storm.

Increasing heights usually indicate clearing weather for the period.

Vorticity Advection:
The term advection refers to the transport of something from one area to another. In Meteorology advection deals with variables like temperature, moisture, and vorticity.

In Meteorology, vorticity is a property of the stream flow, namely the amount of localized rotation of the air, air that rotates counterclockwise, such as the low pressure associated with troughs, is said to have positive vorticity. Clockwise rotating air, such as in high pressure systems and ridges, has negative vorticity.  Vorticity advection is indicative of rising motion/falling pressures at the surface. In the atmosphere vorticity is caused by a change in wind speed (shear), change in wind direction (curvature), and the spin of the earth on its axis.

The advection of vorticity at high levels will result in a response at the surface which will attempt to offset the effects of the advection. More specifically, vorticity advection is indicative of rising motion/falling pressures at the surface.  

Vorticity advection does not amplify the wavetrain. But what it does do is allow the eastward movement of the  trough/ridge

Temperature Advection:

Temperature advection  occurs when  wind transfers heat energy horizontally through the atmosphere.  Cold air advection is when the wind is carrying cooler air into an area that already contains warmer air.  On the other hand, warm air advection is when the wind pushes warmer air into an area that already contains cooler air.  Temperature advection is measured in terms of the amount of temperature change that occurs in a given interval of time.   

Temperature advection  will deepen a trough, and build a ridge

So basically this all means.......As high and low pressure build up in the atmosphere, between places where the air rises (low) and sinks (high). There are also long elongated areas between those. Often we say that areas of low pressure are separated by a ridge of high pressure. it is not exactly a high pressure center but rather, a region of higher pressure between two areas of lower pressure.

Likewise, a trough is a region of lower pressure without being exactly a center of it. The difference is that, spin counter-clockwise along a trough, the air doesn't (in the northern hemisphere) but rather, influenced by the nearby clockwise motion of the surrounding high pressures.
It's a complex subject.... but I hope I made it more clear...................

No comments:

Post a Comment

Thank you for taking the time to comment, I will answer as soon as I can.