GEOG 442

Biogeography

Lab: Solar Flux by Latitude

This lab familiarizes you with the distribution of insolation on Earth and, by way of comparison, on Mars at various times of year. Insolation is the energy source for nearly all ecosystems on Earth, so variations in its intensity by latitude and time of year set basic limits on the primary productivity of ecosystems. Insolation has varied over very deep time in response to orbital changes or Milankovitch cycles. These include:

• oscillations in the shape of the orbit from a more circular (Earth's now) to a more elliptical pattern (like Mars' now)
• obliquity or axial tilt (Mars' is slightly more tilted than ours, leading to more extreme seasonal differences)
• precession or a wobble in the axis that changes the relationship between the pole pointing toward the sun at different phases in the orbit (Earth's north pole and land-heavy hemisphere points toward the sun at aphelion and the south pole and water-dominated hemisphere points toward the sun at perihelion, which evens out seasonal disparities between the hemispheres on Earth -- on Mars, the two hemispheres have very different seasonality).

Your Data

Your data consist of two graphs that show insolation at the top of the atmosphere as it varies by latitude and by solar longitude or seasons. Insolation is given in calories per square centimeter per day for both planets. On each graph are horizontal lines representing the two solstices and the two equinoces. The March equinox on Earth corresponds to Equinox 1 on Mars and the September equinox on Earth corresponds to Equinox 2 on Mars. The June solstice is Solstice 1 and the December solstice is Solstice 2. We'll focus only on those four dates on each graph.

Your Data Entry Form (and what to do with it)

Download your data entry form here.

You'll work in pairs. Each pair will do one curve for each planet on the same chart: Equinox 1, Equinox 2, Solstice 1, Solstice 2. To do them, note that on the X axis of the chart is latitude. On the Y axis is solar flux in calories per day per square centimeter. Place a dot at the intersection of each latitude and its corresponding (interpolated) solar flux. Then, connect the nineteen dots for each date with a smooth line. Do each curve in a different color of pencil or pen, just so you can keep track of the two planets (and be sure to label which color goes with which planet). Each chart, then, will have two curves on it. Make sure both of you autograph your chart.

Each pair should now find the other two or three groups that did the same two curves. Compare your charts and make sure they all agree (this is the quality control part of the lab). The teams should then agree on one chart that looks the best for the next phase of the lab.

Now, each group of pairs that did Equinox 1 or Solstice 1 should find the other group that did the second date: Equinox 2 or Solstice 2. Comparing the best versions of the two (Equinox 1 to Equinox 2 or Solstice 1 to Solstice 2), describe which latitude has the highest value of insolation for each planet and then compare those high values: Which date for each planet generated that planet's highest insolation readings (Equinox 1 or Equinox 2; Solstice 1 or Solstice 2)? What was that reading? What is the disparity between that peak (equinoces at 0° and Solstice 1 at 90° and Solstice 2 at -90°) for each planet? Each pair should now put those three answers for each of the two planets back on their original chart (even if it wasn't the "prettiest" one you used for detailed comparisons).

At this point, everyone compare the distribution of energy between the solstices and the equinoces. Are the highest concentrations of energy found in the equinoces or in the solstices? Isn't that weird? Why do you see this, at first, counterintuitive pattern? Each pair should summarize that discussion on the back of their original chart.

Look closely at the slope of the various curves. For which situation(s) does latitude make very little difference in insolation? For which situation(s) does a change in latitude make a big difference in insolation? This helps account for the sometimes epic migrations that many animals make on Earth!

1. equinoces in either hemisphere on either planet
2. the northern hemisphere during Solstice 1
3. the southern hemisphere during Solstice 1

Now, express the difference in Equinox 1/Equinox 2 peak energies as a percentage of the larger peak energy receipt. Do the same for the difference in Solstice 1/Solstice 2. For which planet does it not make that much of a difference which equinox or which solstice you're comparing? For which planet does the particular equinox or solstice make a big difference in energy receipt? Why would you see such large differences on one planet and such trivial differences at the other? Summarize your discussion and conclusion on the back of your original chart.

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Last revision: 09/09/16