5.4: Other factors affecting Earth’s Energy Budget

Reflection by Clouds and Earth's Surface - Albedo

Radiation that isn’t absorbed by the atmosphere is either absorbed by the surface of the earth, reflected by the surface of the earth, or reflected by clouds. With the exception of some surface covers, which we will cover below, the Earth’s surface absorbs almost 100% of the radiation that reaches it. However, many objects both in the atmosphere and on Earth’s surface are good reflectors of sunlight. Every object has a property called Albedo that represents the percentage of sunlight it reflects. While you can measure albedo by calculating the ratio of sunlight reflecting off an object to sunlight hitting the object, you can actually estimate the albedo simply by looking at how dark/light colored the object is. If you’ve ever worn a dark-colored t-shirt, or driven in a car with dark interiors, you probably know that dark objects don’t reflect much of the sunlight that hits them… and thus have a low albedo. Objects with a low albedo absorb the radiation instead, causing them to heat up. On the other hand, if you’ve ever gone skiing and been snow-blinded, or had a bad sunburn, you probably know that light-colored objects have higher albedos, thus reflecting sunlight and keeping them cool. A simple guide to albedo is found in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Relationship between Albedo and Darkness. Lighter colors have a high albedo while darker colors have a low albedo (CC BY 4.0; Alicia Mullens)

Simply put, the lighter an object's color is, the higher the albedo. Let's put this to the test. Figure \(\PageIndex{2}\) is a typical snow-covered mountainside and Figure \(\PageIndex{3}\) is a typical building in an urban area.

Figure \(\PageIndex{2}\): A snow capped mountain (Public Domain; Wikimedia Commons)

Figure \(\PageIndex{3}\): Dark buildings and street typical of an urban area. (CC-BY-SA 2.0; Wikimedia Commons)

We know that objects that reflect sunlight are cooler, while objects that absorb sunlight are warmer. The phenomenon covered above is called the Urban Heat Island Effect.

13. Ice and Snow, as shown in Figure \(\PageIndex{2}\), would likely have a _______ albedo.
    1. high
    2. middle
    3. low
14. On the other hand, urban areas, as shown in Figure \(\PageIndex{3}\), most likely have a _______ albedo.
    1. high
    2. low
15. As a result, urban areas should absorb __________ radiation than non-urban areas.
    1. more
    2. less
16. Meanwhile, ice-covered areas should absorb ____________ radiation than other areas.
    1. more
    2. less
17. Therefore, urban areas are likely to be _______ than ice-covered locations.
    1. warmer
    2. colder

Ice-Albedo Feedback

A particularly concerning consequence of Earth warming is the melting of Earth’s ice. As air temperatures continue to rise, the edges of ice sheets (especially in the Northern Hemisphere, where the ice sheet is floating above warmer water, and is therefore more vulnerable to melting) begin to melt. We can see the dramatic effects of this at NASA’s Images of Change website, which features the Northern Polar Ice Cap in 1984 vs 2012.

18. Ice Coverage in 2012 (slide the “curtain” all the way to the right) was significantly _____________________ than in 1984 (slide the “curtain all the way to the left).
    1. Less
    2. More
    3. About the Same

What’s concerning about this is that ice has a very high albedo while ocean and land have a very low albedo (Ocean has between 5-10% and Land between 20-30%). So by ice melting, higher albedo surfaces are replaced with lower albedo surfaces.

19. Suppose Earth Warms, and Ice Melts. Earth’s Average Albedo would:
    1. Increase
    2. Decrease
20. Therefore, Earth would _______________________ more solar radiation.
    1. Absorb
    2. Reflect
21. The result is that Earth would:
    1. Continue to Warm
    2. Begin to Cool
    3. Not experience a change in temperature

The scenario mentioned in the past three questions is is what is called a Positive Feedback Loop. In this nightmare scenario, the initial change (Earth warming) is amplified by further processes (such as ice melting). Thankfully there are also Negative Feedback Loops, which can include changes in cloud cover that could counteract such warming. We will investigate that next.

Clouds and Radiation

The last topic we’ll cover in this investigation is in regards to cloud cover and temperature. You can usually observe that typical clouds are a bright white, meaning they reflect most of the sunlight that hit them, keeping Earth cool during the day time. This is called the Cloud Albedo Effect. However, what is often times not thought of is the insulating properties of clouds; Clouds have a large amount of water vapor, and are thus can absorb much of the longwave radiation escaping Earth. This keeps Earth Warm at night (when there is no incoming sunlight, thus the Albedo Effect has no effect). This is called the Cloud Greenhouse Effect. Figure 4.4.4 is a visualization of these two effects.

Figure \(\PageIndex{4}\): Cloud Albedo and Greenhouse Effects. (Public Domain; Alicia Mullens). Alternative Description of Image.

Let’s consider weather conditions at a location on a cloudy vs. sunny day, and investigate these effects. Table 4.4.1 is data taken from San Jose International Airport on Tuesday, April 16th (A cloudy/rainy day) and Wednesday, April 17th (A clear day).

Table \(\PageIndex{1}\): Conditions for April 16th and 17th, 2023 at San Jose International Airport

Date	April 16th (Cloudy)	April 17th (Clear)
High (°F)	66.9°F	73.9°F
Low (°F)	48.9°F	46.9°F

22. Based on the data above, we could expect that the temperature range (difference between high and low temperature) for a day would be ___________________ on Cloudy days than on Clear days.
    1. Lower
    2. Higher

However, the presence of Clouds could come with a bad side effect. There are two types of clouds: High Clouds and Low Clouds.

Figure \(\PageIndex{5}\): Cirrus Clouds are a type of High Cloud (Public Domain, NOAA via Wikimedia Commons)