Weather school: Reflecting on albedo

Longer-range spring weather forecasts aren’t all in lockstep just yet

If Earthshine increases, then Earth is reflecting more of the sun’s energy and as a result, the Earth should cool, and vice versa.

With COVID-19 forcing students to take a break from going to school, it doesn’t mean that they’re not supposed to still do schoolwork — so while I’m prepping online material for my students, I think it’s only fair we continue with my online weather course. This issue we’re going to continue our look at the atmosphere and energy transfer, but before we dive headfirst into looking at albedo and reflection of solar radiation, I thought we should take a quick look at what the latest medium- to long-range weather models are forecasting. I don’t know about you, but I am ready for spring to move in!

Looking at the latest forecast from the three main long-range forecasters/models (CFS, CanSIPS, NOAA) there is a little bit of disagreement. The CFS model, which seems to be over-forecasting warm temperatures is, you guessed it, forecasting warmer-than-average temperatures in both April and May. On the other hand, the CanSIPS model forecasts colder-than-average temperatures for April and May. NOAA is not helping to clarify things, as its forecast calls for above-average temperatures over eastern regions of Manitoba, near average in central areas, and below average in the far west for April. In May it calls for above-average temperatures over eastern and central regions and near average in the far western regions of Manitoba. Hopefully a clearer picture will materialize over the next week or two.

Upon further reflection

OK, now on to this week’s topic. A good chunk of the sun’s energy that reaches Earth is simply reflected away, never getting the chance to do any work. On average, over a year, the Earth reflects about 31 per cent of the sun’s energy back into space. This reflection is known as Earthshine. If we compare it to the moon, which reflects about six to eight per cent of the sun’s energy, and take into account that Earth is about four times wider than the moon, you can quickly see and understand that the Earth would appear very bright from space. In fact, astronauts often report just how startlingly bright Earth appears.

Scientists are also using Earthshine to help understand what might be causing long-term temperature changes. If Earthshine increases, then Earth is reflecting more of the sun’s energy and as a result, the Earth should cool, and vice versa — if Earthshine decreases, more energy is being absorbed and temperatures should increase. Of interest is that during the 1980s and ’90s, Earthshine was on a slow decrease, while during the last few years Earthshine has started to increase. On a side note, the 11-year solar cycle of increasing and decreasing energy output from the sun accounts for about a 0.1 per cent change in Earthshine.

So, Earth actually reflects a fair bit of the sun’s energy. This reflection of the sun’s energy is determined largely by the brightness of the Earth’s surface, and this is referred to as albedo.

Albedo is the percentage of the sun’s insolation that is reflected back into space. An object that absorbs all incoming solar radiation hitting it would have an albedo of zero per cent, while an object that reflects all of the radiation hitting it would have an albedo of 100 per cent.

The colour of an object has the biggest effect on albedo; the darker the object, the lower the albedo. Along with colour, the texture of the surface also affects albedo, with smooth, flat surfaces having a higher albedo. Finally, when it comes to water, the angle of the sun’s rays produces different amounts of albedo. Low sun angles produce more albedo compared to high sun angles — just think of the last time you watched the sun set over a lake.

The graphic here shows some average albedo values for different surfaces around the Earth. The one thing not shown in this graphic is the albedo of clouds. The amount of light reflected by clouds is relatively unpredictable and is one of the toughest things to figure in when making climate models. Clouds, as we all know from cloudy days, prevent a fair bit of the sun’s energy from reaching the Earth’s surface by reflecting this energy back into space. This process is known as cloud-albedo forcing. The more clouds covering the Earth, the greater Earth’s albedo and thus the cooler Earth will be.

Unfortunately, it is a little more complicated than this, and next class we will start to look at what happens to the sun’s energy once it reaches Earth’s surface and what effects clouds have on this.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.



Stories from our other publications