The spring equinox marks the beginning of the spring season in the Northern Hemisphere. Also called the vernal equinox, it is the time of year when the sun rises due east and sets due west, no matter where you live. Earth’s axis of spin is tilted at an angle of 23.5 degrees from its orbital plane and always points to the North Star. The orientation of Earth’s axis with respect to the sun changes throughout the year and is the fundamental cause of our seasons.

On the equinoxes, the axis is not pointed at or away from the sun. This results in all areas on Earth experiencing 12 hours of direct sunlight. On the equinox and at the equator, the sun appears directly overhead at noon.

The equinoxes occur when the sun’s rays strike the equator at noon at an angle of 90 degrees. During the spring and fall equinoxes, the sun is above the horizon for all locations on Earth for 12 hours. During the vernal equinox, the sun is moving from south to north as it crosses above the equator.

For a long time, people have marked the changing of seasons and the sun’s track across our skies. Stonehenge was constructed in a way that marks its relation to the Sun’s position in the sky and in different seasons. At Machu Picchu, stones mark the four cardinal directions so that exactly at noon on the equinox, no shadow is cast. In Chaco Canyon, the ancestral Puebloan people carved spiral designs into rock that track the seasons.

The sun crosses the equator today at 4:24 p.m. Central Daylight Time, a good time to go outside and celebrate the event.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Since Dec. 1, Madison and Dane County have received approximately 40 inches of snow — 3 inches above normal for that span.

Snow is a form of solid water, and water is the only substance that occurs naturally in all three phases — solid, liquid and invisible gas — in the Earth’s atmosphere.

Of course, that means that the 40 inches of snow began as the equivalent amount of water in the invisible vapor, or gas, phase before being transformed into solid water.

Everyone knows that melting ice into liquid water requires energy. Not surprisingly, energy is also required to transform liquid water into water vapor, the familiar process of evaporation.

The particular amounts of energy needed to accomplish these changes of phase are known as latent heats — the latent heat of melting for the first one and the latent heat of evaporation for the second.

When a cloud of invisible water vapor condenses into a puddle of liquid water, the latent heat of condensation (equal to the latent heat of evaporation) is released to the environment. Also, when that puddle freezes into ice, the latent heat of fusion (equal to the latent heat of melting) is similarly released, incoherently, to the environment.

Since we know the depth of liquid equivalent precipitation involved in delivering us 40 inches of snow since Dec. 1, the area of Dane County, and the latent heats of condensation and fusion, we can calculate how much energy has been released to the atmosphere in the production of that much snow. Without providing the details of the calculation, we can report that the amount of energy involved could power the entire Madison metro area for approximately 8.8 years. Clearly, there are huge amounts of energy involved.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.