Professional Pilot, October 2018

1 Cross sectional view of a cold front The central low advances the cold air into the region of warmer air forcing the warm air aloft and often forming lines of thunderstorms energy reaching the high latitudes This expands the tropical heat northward while reducing the strength and extent of the cold arctic air This moves the boundary between the 2 air masses to higher latitudes and allows the middle latitudes to bake in hot summer conditions After June 22 however as the sun begins its trek back to the southern hemisphere the amount of energy received in the northern hemisphere decreases most markedly in the higher latitudes The result is a strengthening and expansion of the cold arctic air mass and a cooling and retreat of the warmer subtropical air As the boundary pushes southward it does not do so evenly but rather in a series of advances and retreats as the polar front is subjected to various troughs and ridges Fall weather patterns In the Tropics the migration of the sun between the solstices is a season not of heat but of rain The presence of a high noon day sun throughout the year ensures ample energy for evaporating water Even at its lowest noon angle the sun provides 68 of its total overhead energy concentration and the large quantity of already warm water in the Tropical ocean ensures an ample supply to evaporate Combined these factors mean that 100 PROFESSIONAL PILOT October 2018 tropical seasons are divided into rainy and less rainy The core of the rain a band of convection surrounding the earth known as the intertropical convergence zone follows the suns track between the tropics so fall in the northern hemisphere September November marks the departure of rains and the arrival of the rains in the southern hemisphere The farther from the equator one goes the less regular those rains will be Pilots routinely flying in this region are undoubtedly familiar with this pattern but those who only transit the region occasionally should closely study the rainfall and storm patterns relative to the time of year Fall above the Arctic Antarctic Circles is a time where the nights account for much of the 24 hr day length with the sun not rising above the horizon on the winter solstice and for days or weeks on either side of it as one goes farther poleward Overall this means that the net radiation budget of the region the amount of radiation emitted by the surface subtracted from the amount of incoming solar radiation is negative producing a net heat loss and cooling of the area While this is good news for resolidifying remote permafrost runways that may have thawed during the summer months it also means that the air will continue to chill Pilots should make arrangements to ensure that engine lubricants and even fuel doesnt freeze while their aircraft sits on the ramp overnight Freezing fog and ice fog also become much more of a concern to aviation once fall begins Importantly the increased density of the cold air can produce strong high pressure cells over the region occasionally exceeding an altimeter of 3100 inches of Mercury During these events pilots are normally advised to set their altimeters to 2992 and rely on pressure altitudes to avoid collisions Middle latitudes have the most changes It is fall in the middle latitudes that brings the most diverse weather and flying conditions The middle latitudes between around 35 and 55 degrees of latitude is where the polar front is normally found The polar front which generally resides beneath the jet stream is the boundary between the cold polar air and the warm subtropical air Uneven heating of the atmosphere mountain ranges and the spin of the earth itself all play a role in generating continental scale troughs and ridges in the front allowing warmer air to push poleward and cold air to move equatorward Along the front localized variations in heating produce a vertical motion that becomes surface lows Cold front Altitude km Warm air Wind Cold air 50 km X 0 N 55 50 42 39 32 F 25 Cb Cs Ci Ac X Source Thomson Higher Education