How convection works. At the bottom, the H (left building) has higher pressure than the L (right building), but also has less height than the building to the right. So the pressure gradient force at “street level” moves air from H to L pressure. As the air in the building to the left rises and reaches a level where the pressure in the bulding to the right has a higher pressure than the one on the left (since there is still more air above that point in the building to the right) and hence the pressure gradient force switches so that the air moves now from the building on the right to the one at the left. Hence the convection current at the street level moves from the building on the left to the one on the right, but at a higher level it reverses.

CORRECTING BAROMETRIC READINGS FOR THE ALTITUDE OF THE WEATHER STATION

Notice that the actual weather station reports are not connected as isobars, but rather that the isobars are drawn ever 4 mb. apart and the station reports are interpolated into them.

TWO COLUMNS OF AIR WITH EQUAL TEMPERATURES, PRESSURES AND DENSITIES

THE COLUMN ON THE RIGHT IS HEATED CAUSING IT TO EXPAND UPWARDS. IT HAS THE SAME AMOUNT OF MASS, BUT A LOWER DENISTY TO COMPENSATE FOR ITS GREATER HEIGHT NOTICE THAT THE PRESSURE DROPS TO 500mb AT 5700 METERS IN THE WARM AIR, WHEREAS THE PRESSURE DROPS TO 500mb AT 5640 METERS IN THE COOL AIR

CHANGES IN ALTITUDE The chart shows the lessening of the pressure as one goes higher in the atmosphere.

The chart shows that the 500 mb pressure is found at several levels – with an average height of 5600 meters. Note that the 500mb surface is lower in the colder north.

In this chart it is obvious that the 500 mb surface can be found as high as slightly above 5700 meters and as low as 5520 meters. The air temperature has a strong impact. In warm air the 500 mb isobar is found in the higher less dense air than it is in the cold more dense air.

NOTICE THE DIFFERENCE IN THE WIND PATTERNS IN THE SURFACE MAP AND THE 500mb MAP On the surface map, the winds blow ACROSS the isobars. On the upper air map they blow parallel to the isobars

The closer the spacing of the isobars, the faster the wind moves from the lower to higher pressure. IN HIGH PRESSURE AIR MOVES OUTWARD FROM AREAS OF HIGHER PRESSURE TO LOWER

Low pressure centers operate in the reverse with the center of the low having the lowest mb. so air moves inward. Notice too that as the isobars get closer the wind speed increases.

Pressure Gradient Force interacting with the Coriolis Force

The wind direction is plotted as the shaft of an arrow extending from the station circle toward the direction from which the wind is blowing. The wind speed is plotted as feathers and half-feathers representing 10 and 5 knots, on the shaft on the wind direction arrow. See the following table.

The most significant past weather since the last observation

The pressure change in the past 3 hours preceding the observation The pressure change in the past 3 hours preceding the observation. Plotted to the nearest tenth of a millibar.

ff= wind speed CH = high cloud type PPP=sea level pressure TT= temp dd=wind direction CM=Middle clouds type pp a=pressure tendency VV= visibility * ww=present weather N=cloud cover Td = dew point temperature CL= low cloud types W=past weather (solid dot=rain) h=cloud base height RR = six hour precipitation (mm) * Visibility in fractions of mile up to 10 miles. https://www.wpc.ncep.noaa.gov/dailywxmap/wxsymbols.html