Are Earth’s weather and climate systems merely the result of chance? Do they manifest the elements of exquisitely designed systems? Temperature is but one of many weather-defining factors. Our ability to forecast temperatures connects our knowledge of astronomy with the topic of weather. The tilt of Earth’s axis of rotation is central to understanding weather-related temperature variation. Our home planet’s axis of rotation is not perpendicular to its plane of its revolution. Instead, it tilts away from the 90º configuration by the familiar 23.5º. In terms of the welfare of Earth’s population of over 7.5 billion, we should be thankful for its deviation from the perpendicular.
Considering the reliability or inaccuracy of long term weather forecasting, we may profitably understand factors which cause the overall warm to cold and cold to warm seasonal cycles. In the writer’s neighborhood of extreme northwest Illinois, each year around July 11 Earth begins a slow descent from an 82.2ºF average high temperature to a January 11 average high of only 26.3ºF, a drop of 55.9ºF. Then a reversal occurs. After a week or two of virtually no change, temperatures stage a comeback, recovering slowly at first, then more rapidly before leveling off after reaching their July peak. The running averages are a source of satisfaction for statistics-minded weather addicts.
Next day, three-day, five-day, and ten-day weather forecasts become progressively less reliable. Thirty-day forecasts may reflect already established weather trends. For example, some locations experience a rainy season or dry season. Most weather-aware residents note how much above or below average the temperature, precipitation, and wind speed will be for the current day or the immediate future. Modern technology has refined the accuracy of forecasts.
Most observers are more interested in local weather forecasts than in causes of warm to cold or cold to warm seasonal transitions. Those transitions, however, are fundamental to understanding life conditions on our planet. The most important factor determining temperature conditions at any location is the angle of insolation—the angle at which the sun’s rays strike the Earth’s surface. This determines whether residents live in a tropical (warm) climate, a temperate (moderate) climate, or a polar (cold) climate. In terms of cause of our varying temperature conditions on Planet Earth, we pinpoint the tilt of Earth’s axis as the main cause of variations in the angle at which Sun rays strike the surface of Earth. If we could alter the 23.5º axis tilt to 0º we would not alter Earth’s generally warm, moderate, or cold zones. We would, however, remove seasons from our experience. Life would be very different for Earth dwellers:
Seasonal temperature variation depends on the angle of sunlight striking the earth’s surface. This factor is important wherever we are on the planet. The tilt of Earth’s axis dictates that direct noontime sun rays strike the Tropic of Cancer on June 21. Those residents would see the Sun at their zenith (straight overhead) at noon. Three months later on September 21 the Sun’s direct noon time rays strike directly at the Equator but Tropic of Cancer residents would notice the Sun’s noontime rays have descended to 23.5º south of their zenith. Three months later on December 21 residents at the Tropic of Capricorn would observe their noontime sun directly overhead, but people living at the Tropic of Cancer would observe their noontime Sun 47º south of their zenith while residents directly at the Equator would observe the Sun 23.5º south of their zenith. Dwellers in tropical regions are the only Earth residents ever to observe the Sun directly at their zenith on some occasions. Residents in tropical regions experience fairly direct noon solar rays all year long. This explains why their weather forecasts almost always use adjectives “warm” or “hot.”
North of the Tropic of Cancer in the temperate zone between 23.5º and 66.5º N. Lat. people are more impacted by seasonal temperature variations owing to the indirect angle of insolation. In a previous post we illustrated by reporting that Ft. Myers FL has an average high temperature difference of only 21ºF from June to December while Dubuque IA has a June to December difference of 55ºF. The primary reason is the greater difference in the angle of solar insolation. This angle differs more greatly from the generally vertical the farther north one travels. Specifically, the amount of radiant energy from sun rays striking Earth’s surface at 30º is 50% of the radiant energy received from rays striking at 90º.
Seasonal temperatures depend slightly on a few other factors in addition to sun angle. For instance, at 41º north latitude June has 15 hours of daylight and 9 hours of night darkness. In December the figures are reversed. In June there are more hours of heating; in December there are less. Other interesting phenomena occur seasonally. In June the Sun rises about 23º north of due east, attains an altitude of 72º above the horizon at noon, and sets about 23º north of due west. The Sun takes a very long trip across the sky on summer days. These conditions supply warmer, more direct summer sunlight, long summer days, and short summer nights. In December the Sun rises about 23º south of due east, attains an altitude of only about 26º above the horizon, and sets 23º south of due west. In short, the December sun is never very high, not very warm, not as bright as summer sun, and not above the horizon many hours.
Proximity to large water and land bodies, the large scale circulation of water and atmosphere, and occurrence of wind and precipitation are just a few other issues of concern to weather prognosticators. Without energy from the Sun there would be no weather at all. Earth would be a lifeless, cold rock. Habitation of this planet is sustained by dynamic life conditions accessible to human discovery. We are gifted with the ability to contemplate the existence of both design and the Divine Designer of our planetary system. We speak of the God of Creation. One design feature, orbital tilt (let’s call it the cause) enables us to discover many interesting and beneficial effects for prosperous human life.