First, before today’s topic, let us all take a moment to remember all of those whose gave their lives protecting our country and freedoms as well as those who lost their lives protecting us in our everyday lives. Memorial Day is the traditional beginning of summer yet there will be stories on June 20th marking it as the official start of summer. Which is correct? The reality is that both are. The question is why?
• We are dealing with two different areas of study – Climatology and Astronomy. Specifically the generalized climate of the planet and the track of the sun and the location of the Earth. It isn’t that the sun, its track and the amount of daylight don’t have an effect on climate but it’s how they are defined.
• Astronomically for the Northern Hemisphere, summer is considered to start when the sun reaches its farthest northern track, 23.5o North latitude, the Tropic of Cancer, on the summer solstice. This occurs on June 20 or 21 each year. Because the Earth tilts on its axis at 23.5o that is the furthest north in the sky the sun can track and corresponds to the longest period of daylight for the year. If the planet wasn’t tilted on its axis, day length would be constant. Increased daylight results in more incoming solar radiation and so the period is hotter and that helps define summer. From this day forward day length decreases a bit each day.
• Astronomical summer lasts until the autumnal equinox which is Sept. 22, this year. On the equinox, we experience equal periods of light and dark. This corresponds to the “equator passing through the center of the sun.” After this date day length continues to decrease until the winter solstice around December.
• Climatological summer starts June 1. Meteorologists consider summer to be all of June, July, and August. Autumn starts with the first of September and lasts until Dec. 1. So climate focuses more on the temperature and weather patterns, not specifically the location of the sun.
• We all know that “summer conditions often start earlier than the above dates and may end earlier. If we are picky, we could define the solstice as the midpoint, not the start of summer. But it really doesn’t matter to most of us.
• What makes this time of year so “active” meteorologically is after exiting winter and moving towards summer, the pressure and temperature differences between the equator and North Pole are more extreme and this steep gradient which nature seeks to equalize results in severe weather. This is especially true when the jet stream allows the inflow of Gulf moisture into our area.
What makes the prediction of weather and climate patterns more difficult today is that scientists don’t yet have a good handle on how climate change, especially shrinking Arctic ice and its effect on warming will have on our weather.