Jet stream lab

Overlying theme: wind is the result of a horizontal difference in pressure

-         Wind always blows initially from high to low pressure (in the absence of all other forces) and this is due to the pressure gradient force (PGF)

-         Once the parcel starts to move (as a result of the PGF) the Coriolis force begins to act to the right of the wind (in the northern hemisphere), balancing the PGF (when there is no friction)

o       Near the surface, another force, Friction, acts in the opposite direction as the wind. When this happens, the wind turns into the low and out of the high. The reason why it turns is because of a force imbalance. When Friction slows the wind, the Coriolis force is weakened BUT the PGF is not. Therefore the actual wind vector (as a result of all these forces) will turn in toward the center of a low and out of the center of a high.

 

 

 

 

 

Jet stream

-         Why do we care? In the mid-latitudes (IN BOTH HEMISPHERES), winds generally blow from W-E, and storms usually track from west to east. The Jet stream is what guides these storms (cyclones and anticyclones) through the mid-latitudes. Also, all the weather we have at the surface is due to the distribution of winds aloft. The jet stream plays the most important role in the development of surface high and low-pressure systems.

-         Definition = a narrow band of fast moving winds in the upper levels of the atmos.

-         Dimensions = several hundreds even thousands of miles long, several hundreds of miles wide but only a couple of miles thick. In the core, wind speeds usually are 100-200kts.

-         Found in the upper levels between 10-14 km (at the tropopause)

 

 

-         What causes the jet stream? Jet streams form because of lower level (including surface) temperature gradients.

o      Remember that the warmer the atmosphere the thicker it is. For example, in the figure above, the thickness of the troposphere is near 16-18km in the Tropics while only 9km at the North Pole.

o      Therefore, if there is a tight temperature gradient at the surface (temperature changes rapidly over a short horizontal distance) the pressure contours above the temperature gradient will have slope to them (because the warm side will be thicker than the cold side in the vertical).

o      The slope of the pressure contours tells us how fast the wind blows because the more sloped the pressure contours the stronger the PGF. Therefore, the more sloped the pressure contours, the faster the jet stream!

 

***Weak temperature gradient weakly sloped pressure contours weak winds aloft***

 

***Tight temperature gradient very sloped pressure contours fast jet stream***

 

-         The Polar Jet is strongest in the winter because of the larger (tighter) temperature gradient at the surface and weakest in the summer because of the very weak surface temperature gradient/

o      To understand this, think about the temperatures in North Dakota and Florida in the winter and then in the summer.

       In the winter, there can be a 100F temperature difference between the two while in the summer, North Dakota is sometimes hotter the Florida

-         North of the jet temperatures are cold and south of jet they are warm

o       Plays a major role in the global transport of heat

 

Blue arrows show cold air coming from the N, while red show warm air coming up from the S.

 

 

-        HOW TO PICK OUT A JET ON A MAP

o       First, looking at a surface or 850mb Temperature map, find the region where the temperature changes the most. The Jet Stream will be located near the tropopause directly above the tight temperature gradient in the lower levels/surface (behind the cold front usually)

On the Left is the 300mb map with winds shaded. The black line represents the axis of the Jet. Everywhere north of that black line is cold and everywhere south is warm. This is evident by looking at the 850mb map on the Right. The black line here goes along the area with the tightest temperature gradient. The color shading in the 850mb map is temperature.