What is the solar declination number referred to in climatic studies?

The solar declination number is a significant concept in climatic studies referring to the angle between the rays of the sun and the plane of the Earth's equator. This angle changes throughout the year as the Earth orbits the Sun, resulting in varying amounts of solar radiation received at different latitudes, which in turn influences the seasons.

The correct answer, which is +/- 23.5 degrees, represents the maximum solar declination values reached during the solstices. Specifically, during the summer solstice, the solar declination is approximately +23.5 degrees, indicating that the sun is directly overhead at the Tropic of Cancer. Conversely, during the winter solstice, the solar declination is around -23.5 degrees, where the sun is directly overhead at the Tropic of Capricorn. These values are crucial for understanding solar insolation patterns and how they affect climate and weather in different regions across the globe.

The other options do not accurately represent the solar declination. For instance, 0 degrees corresponds to the equinoxes, where day and night are of equal length, while +/- 54 degrees and +/- 90 degrees represent angles that are not relevant to solar declination in climatic contexts.