Professional Pilot, October 2018

Idealized cross section of a typical warm front The central low gradually draws warm air in over cooler air creating widespread stratoform cloud cover and light to moderate rain PROFESSIONAL PILOT October 2018 99 across the sky The tilt of the earth on its axis coupled with its revolution around the sun ensures that the sun appears to migrate its position throughout the year The midpoints and extremes of the suns path are the equinoxes and solstices respectively Around December 21 22 each year the noon sun is directly above the Tropic of Capricorn at 235 degrees south latitude Correspondingly that means that while it is at 90 degrees above the horizon there it is 665 degrees 90 235 above the horizon at the equator and only 43 degrees above the horizon at the Tropic of Cancer at 235 degrees North Latitude 90 235 235 At the Arctic Circle 43 degrees latitude further north the noon sun is exactly zero degrees above the horizon This situation is reversed around June 21 22 the summer solstice when the tilt of the earth points the northern hemisphere toward the sun the noon sun is 90 degrees above the horizon at the Tropic of Cancer and the Antarctic Circle doesnt see the noonday sun at all At the vernal and autumnal equinoxes around March 22 and September 21 respectively the earths axial tilt is perpendicular to the sun with the noon sun at 90 degrees directly above the equator 665 degrees above the horizon at the 2 Tropics and 235 degrees above the horizon at the Arctic and Antarctic Circles At the 2 poles on these dates the sun is at the horizon the full day Between the 2 solstices at any point on earth except the equator and the poles the length of the day and length of night is either lengthening or shortening until the next solstice when the situation reverses Understanding seasons This geometric relationship between the earth and sun is crucial to understanding seasons such as fall The atmosphere is driven almost entirely by the solar radiation it receives The earth is tiny compared to the sun it would take 1300000 earths to fill the volume of the sun and at 93 million miles distant the solar radiation received at the top of the atmosphere is more or less equal over the entire half of the earth facing the sun However to be more precise the dynamics of the lower atmosphere are driven by the concentration of energy available at any given location The amount of solar radiation received by the surface is measured in energy per area such as watts per square meter W m2 On any given date and time that quantity is determined by the angle at which the suns rays strike the earth If the sun is directly overhead the radiation is most concentrated while if the sun is just a degree or so above the horizon that same amount of radiation is spread over a much wider area The top of the atmosphere receives around 1368 W m2 of energy If the sun is directly overhead on a clear day a square meter of earths surface will receive around 1000 of those watts and the rest is scattered away or absorbed by the atmosphere But as the angle at which the sunlight hits the surface decreases the same amount of light is spread over a larger area meaning any square meter of that area gets only a fraction of the total energy For example if the sun is 235 degrees above the horizon the amount of energy hitting a square meter is just around 40 of what it would be if the sun were completely overhead roughly 400 W m2 You can see this behavior by shining a flashlight directly down at the floor and then tilting the flashlight a few degrees The light now illuminates a greater area but the illumination of any part of the floor is not quite as bright This difference in solar radiation relative to latitude and the position of the sun through the year drives the concentration of heat in the tropics and the concentration of cold at the poles It is also the change in that relative solar position through the year that creates the seasons In the northern hemisphere the opposite is true for the southern hemisphere the summer brings a sun that is directly overhead at 235 degrees north and more Nimbostratus Ns Warm air Warm front Cool air Moderate precipitation Altostratus As Cirrostratus Cs Cirrus Ci Source Lutgens and Tarbuck 2004 Warm front