These images show the vicinity of the Elburz Mountains, in northern Iran. Tehran lies in the south, and the Caspian Sea lies in the north.

The Elburz mountains run parallel to the southern coast of the Caspian Sea, and these mountains act as a barrier to rain clouds moving southward; as the clouds rise in altitude to cross the mountains they drop their moisture. This abundant rainfall supports a heavy rainforest (the bright red area) on the northern slopes. The valley to the south receives little precipitation because of this rain-shadow effect of the mountains.

Irrigated agriculture in the valley, shown by the red field patterns, has increased. This agriculture depends on rainfall captured in the mountains and channeled to the valley floor, as well as nearby rivers and drilled wells.

Question

Look at the in zoom by the coast. Can you tell the peaks (i.e., ridges) from the valleys? It may seem easy, but you may be getting tricked.

Hint

The white objects are clouds. What time of day is it? (See the answer below.)

References

Helen Chapin Metz (ed.), 1989, Iran: a country study: Federal Research Division, Library of Congress, Washington, D. C., 342 p.

Graves, William, and Blair, James, P., 1975, Iran: desert miracle: National Geographic Magazine, vol. 147, no. 1, January, p. 246.

Satellite images

LM2177035007719590 (Landsat 2 MSS, 14 July 1977)

LM5165035008725990 (Landsat 5 MSS, 16 September 1987)

Special Projects Image

A comparison image of 14 July 1977 and 16 September 1987 is available as Special Projects Image E-1857-810CT from U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS) Customer Services.

Map

Defense Mapping Agency, 1973, Operational Navigation Chart G-5: scale 1:1,000,000.

Ridge: dark | bright
Valley: bright | dark

There are several ways to figure this out.

First of all, you may know that all Landsat images are basically designed to be morning shots. (See the help article to learn why that is.) These 1977 and 1987 images were taken at 9:40 a.m. and 10:11 a.m. Tehran time (6:10 Greenwich Mean Time (GMT) and 6:41 GMT plus 3:30).

But you can see this anyway, by looking at the clouds (especially the small ones) and their shadows. The Sun must be to the east and slightly to the south. This agrees with what we know about the latitude, the time of year, and the time of day of these images.

So, basically, the western and northwestern slopes are dark because they face the sun less squarely, and so are less illuminated.

But there are other factors besides Sun angle. The widest variation in brightness we see here is in redness, representing the near-infrared band, and this band is affected by the health of surface vegetation. Healthy lush green leafy vegetation reflects infrared energy well. At this latitude, southern slopes get more sunshine, and this makes them warmer and drier than north-facing slopes. Depending on species and conditions, this can make vegetation thicker and healthier or it can make it more sparse and dried out. This in turn can make the red band brighter or darker-- exaggerating, diminishing or even counteracting the sun-angle effect.

You can also see that most of the highly reflective pieces of ground are at the peaks, not the valleys.