Albedo:Albedo:Its effects on global warming

in Polar Regions

Presented by Morton SternheimPresented by Morton Sternheim

2008 Batom Inc.

Big Idea

• Why are Polar Regions more affected by global warming than other parts of the globe?

• One reason (there are others) is that as sea ice melts and more open water appears, more energy is absorbed, and warming accelerates.

• This is a form of positive feedback and it makes the polar climate change faster than the climate in temperate areas.

• Increasing vegetation on land also has a similar positive feedback effect.

Albedo

• This effect is a change in the albedo – the fraction of the incident sunlight that is reflected back to space.

• The albedo is much higher for snow and ice than for water. Fresh snow can have an albedo as high as 0.95. This is why skiers wear dark glasses. Ice and old snow typically have albedos from 0.2 to 0.45.

• However, liquid water has an albedo of only 0.03 for light incident at right angles to the water, and the albedo remains small until light comes in almost at a grazing angle.

Examples

• A black car is much hotter to the touch than a white car in the summer sun

• If you clear the snow from a patch of dark colored driveway, the adjoining snow melts faster than snow further away. – The driveway absorbs more solar energy and

heats the nearby snow, speeding up the melting process.

At what time of year to the southern oceans have the higher albedo? The Arctic?

How does the albedo of north Africa compare with that of the southern part? What is the reason?

http://isccp.giss.nasa.gov/products/browsesurf1.html

Typical Albedos (approx.)

Fresh snow 0.8 – 0.9 Bare soil 0.17

New concrete 0.55 Worn asphalt 0.12

Desert sand 0.40 New asphalt 0.04

Green grass 0.25 Conifer forest (summer)

0.08

See Wikipedia for sources

Albedo of Water

http://en.wikipedia.org/wiki/Image:Water_reflectivity.jpg#file

How Can We Measure Albedo?

• Pyranometer – measures temperature of a dark absorbing material, pointing alternately at surface and sky. Expensive.

• Digital camera and free imaging software (ImageJ) – compare reflected light from surfaces. Free given a camera, computer. Somewhat complicated; write-up available.

• Light meters – fast, easy to use, inexpensive. • Measure temperature changes. Effects of

surface color

The Sun’s Radiation Spectrum• ~ 43% of energy is

in the visible range

• ~ 49% in near infrared range

• ~ 7% in ultraviolet range

• < 1% in x-rays, gamma waves, and radio waves

.

Source: Adapted from http://www.ucar.edu/learn/imgcat.htm

A Caveat

• Light meters designed for illumination measurements (and most digital cameras) only detect visible light.

• Light meters are also designed to be most sensitive to green light, matching sensitivity of human eye.

• The fraction of the light that is reflected may depend on the frequency.

• Thus albedo results from a light meter (or digital camera) may not be really accurate.

Light Meter

• Mastech Digital 4-Range 200,000 Lux Luxmeter, LX1330B, Sold by: Kaito Electronics, Inc

• $49.95 + shipping from Amazon.com

• 4 ranges, lux or ft-candles; ± 3% accuracy.

• Lux – measures intensity of light per unit area, weighted according to human brightness perception

Typical Lux Values

Brightest sunlight 200,000 lux

Well lit office 400 lux

Full moon 0.25 lux

Minimum value for meter 0.1 lux

Two Experiments Today

1. Measuring albedo with light meters

2. Measuring temperature changes:

A two part experiment. Heat lamps, thermometers, etc.

a. Albedo and heat absorption

b. Angle of incidence and heat absorption

Questions

• Does the albedo really matter – does it tell us about energy transfer?

• What effect does the angle of incidence of sunlight have?– How do sunlight overhead and low in the sky

compare in terms of the amount of energy absorbed?