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The sea breeze and its cooling effect along the coast

April 22, 2014/0 Comments/by Doug Simonian

Today was another one of those classic days where inland areas soared into the upper 70s, and coastal areas had trouble exceeding 60 degrees, with plenty of wind.

The explanation for this is the sea breeze. Whenever we have a warm, springtime airmass, the temperatures over the land warm up quickly. However, the sea is still relatively cold this time of year, especially considering how cold of a winter we had. The ocean has a high specific heat capacity, which means that it can absorb the same amount of heat as the land does, but will not warm up nearly as much as the land. Thus, during the day, the land will often be much warmer than the ocean.

The sea breeze circulation (top) and land breeze circulation (bottom). (Free Online Pilot Ground School).

Once the land heats up, thermodynamics dictate that the warm air must rise, since it is less dense. However, the relatively cold ocean does not have this same heat, as it is more dense, so the air does not rise. The rising air over the land creates a slightly lower surface pressure, since the air is escaping the surface, rather than exerting a force on it. The lack of rising air over the sea thus creates a slightly higher surface pressure, relative to the land. The atmosphere always wants to balance itself out, so to compensate for the air over the land escaping the surface, air from higher pressure has to flow towards it. This air happens to be coming from the colder ocean, which helps to create colder temperatures and windier conditions along the coast.

The circulation gets reinforced because when the air over the land rises, it creates an area of higher pressure at the upper levels of the atmosphere, since lots of air is rushing towards the upper levels. However, over the sea, there is no air rising into the upper levels, so the pressure is relatively lower there. The air with higher pressure aloft on the land thus flows towards the area with lower pressure above the sea. Now that this air is above the sea — where air at the surface is already heading towards the land — the air has to sink to replace the air that is leaving the sea. This sinking air towards the sea surface reinforces the high pressure, which reinforces the source of air that is blowing towards the land, which reinforces the sea breeze.

Temperature and wind observations taken from Monday (4/14/14) afternoon at 4:48pm. Notice temperatures approaching 80 in New Jersey with relatively light winds, yet much cooler and windier conditions on Long Island. Also notice how in some areas, the southerly winds are stronger on Long Island compared to areas further west (Weather Underground).

Storm Prediction Center releases details on outlook changes

April 22, 2014/0 Comments/by John Homenuk

The Storm Prediction Center has used the same risk categories for a large part of most of our meteorological careers. Although they have adjusted the probabilities, percentages, and yes even the colors — the categorical risk areas have remained the same. In Spring 2014, that will change. The Storm Prediction Center currently uses a simple categorical risk system which highlights the potential for severe thunderstorms across the country. The risk areas of Slight, Moderate and High are triggered by percentage chance of a type of severe weather within 25 miles of any point. And while the newer outlooks also highlight the potential for general non-severe thunderstorms across the country, Spring 2014 will mark a dramatic change in the outlooks.

The main change will be the addition of two risk levels, both falling on opposite sides of the “Slight” risk of severe thunderstorms. “Marginal”, a new risk category, will highlight the potential for severe thunderstorms that includes a 5-10″ chance of wind and hail and a 2-5% chance of tornadoes — but does not trigger a slight risk. “Enhanced” falls on the other side of “Slight”; in other words the risk levels are enhanced (30-45% risk) but not organized or widespread enough to trigger a Moderate Risk. Below, the Storm Prediction Center has released an example of the changes in the outlooks using an example in the Northeast US from 2011. You can visualize the new outlook area with an “ENH” or Enhanced risk running throughout the majority of our forecast area. Previously, the outlooks would only show this area as “Slight”despite there being an enhanced percentage of severe weather within that slight risk.

An example of proposed changes to the Storm Prediction Center’s categorical outlook and graphics.

What causes storms to strengthen?

March 28, 2014/1 Comment/by Doug Simonian

In light of Wednesday’s meteorological bomb of a storm that was just offshore, we have decided to write an article that explains why storms strengthen to begin with, and how they can get to be as strong as this storm got. Wednesday’s storm went under what we call “bombogenesis” (yes, that is a real meteorological term), meaning that its pressures dropped more than 24 millibars in 24 hours. At once point, surface analysis showed the storm being as strong as 955mb, which is equivalent to a category 3 hurricane!

As most of you probably know, a lower pressure means a stronger storm, and a higher pressure means a weaker storm — or if the pressure is high enough, an area of tranquil weather. Now the question becomes, what causes pressures to fall in a certain area, and why do they sometimes fall so rapidly?

The most important meteorological aspect for pressure falls is an area of upward vertical motion. If air is being lifted vertically, then pressure within that column of air has to decrease, because air is escaping that column when it is moving vertically. Naturally, if less air exists within a column, the pressure in that column will be less.

The atmosphere always wants to maintain balance, so to accommodate for the air that is being lifted vertically, there is a need for air to converge at the surface to replace what is lost at the surface, and to generate the lift to fill the void in that column of air as well. This is one reason why air converges at the surface in areas of lower pressures; it is all part of the balancing act of the atmosphere. Air also flows from higher pressures to lower pressures, being that lower pressures are an area of least resistance; another aspect of this balancing act. All areas of relatively higher pressures essentially shove air away, and it all converges where the lowest pressure is. This surface convergence leads to upward vertical motion, which leads to storm development, precipitation, and an additional lowering of pressure.

To illustrate this further, think about the opposite scenario: wouldn’t it make sense for pressure at the surface to be higher if there were downward vertical motion, meaning that air is being pressed downward towards the ground?

A water vapor animation taken yesterday afternoon, beautifully illustrates the size and strength of the storm system (wx.rutgers.edu).

A water vapor animation taken Wednesday afternoon, beautifully illustrates the size and strength of the storm system (wx.rutgers.edu). You may need to click to animate.

Let’s go over the factors that cause upward vertical motion:

Ending the developing misconceptions regarding the Polar Vortex

February 25, 2014/1 Comment/by John Homenuk

It has been a media frenzy. Since January, when a piece of the Polar Vortex made its dramatic trip through the Central and Eastern United States, the misconceptions regarding its origins and actual definition have continued. Whether made in a joking manner or not, some of the ideas regarding the Polar Vortex’s actual characteristics have created a bit of an issue for meteorologists. When using it to describe the pattern, we now have to keep in mind the potential media impacts. To be frank: That is not the way it should be.

The Polar Vortex, in our hemisphere, is a persistent cyclone which is located near the North Pole. It features a counter-clockwise rotating pool of cold, dense air — some of the coldest on the globe, in fact. The vortex moves around to a certain extent, but generally remains in the vicinity of the poles. What separates and moves southward (sometimes towards our area) in anomalous patterns, are pieces of the Polar Vortex which feature characteristics of the vortex itself and similar air masses.

A significant piece of the Polar Vortex is forecast to drop into Southeast Canada later this week. Here, the GFS model forecasts its position at 500mb at 84 hours.

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