We all can recall this past winter's short daylight hours and weak noontime sun that was low in the sky. Now, thoughts of the upcoming spring evoke memories of longer daylight and strong rays from a midday sun that is higher in the sky. The Study Guide Part A: Narrative shows the paths of the sun on the first days of the astronomical seasons in the midlatitudes where most of us live. We are now in the part of the year when the combination of the inclination of the Earth's axis to its orbital plane and the movement of our planet along its orbital path are tilting the Northern Hemisphere more and more toward the Sun. Our daylight periods are getting longer and the noon sun is ever higher in the sky.
We are rapidly approaching the Spring Equinox (on Saturday, 20 March 1999), where we all experienced 12 hours of sunlight (plus a few minutes - see Thursday's optional Supplemental Summary Information file), the length of daytime is increasing by several minutes from one day to the next. For Washington, DC, this increase is about 3 minutes more possible sunshine each day. This effect is accentuated the higher the latitude. On the first day of winter, 21 December, International Falls, MN on the Canadian border had only 8 hours and 17 minutes of possible sunlight compared to Key West, FL at 10 hours and 37 minutes. (Barrow, AK had NO sunlight, being north of the Arctic Circle.) By the summer solstice on 21 June, residents in International Falls should bask in a lengthy 16 hours and 9 minutes of possible sunshine, while Key West would have 13 hours and 40 minutes of possible sunshine (Barrow conceivably could have 24 hours of sunshine, barring no clouds). A portfolio of sunrise photos shows the beginning of the path of midlatitude sun over year.
The more direct solar rays and the longer intervals of daylight combine to increase solar heating in the Northern Hemisphere at this time of year. This increase in daytime energy gain dominates the exchange of energy between Earth and space. The result is a warming of the Northern Hemisphere.
More details are provided in DataStreme Activity 3B and Thursday's optional Supplemental Summary Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 3 Chapter Progress Response Form, under section B. Daily Summary.