East-West Maps | Description | North-South Maps |
Autumn(day) | Winter(day) | Spring(day) | Summer(day) | Summer(night) |
Autumn(day) | Winter(day) | Spring(day) | Summer(day) | Summer(night) |
In Eritrea many winds are produced by atmospheric temperature differences or gradients. Air of different temperatures has different densities. Changes in fluid density produce changes in hydrostatic pressures. And difference in pressure produce the forces which drive wind and other atmospheric motions.
Above, we illustrate the East-West component of horizontal temperature gradients (gradient = difference per unit length) during the middle of the day during the Eritrean winter (December to February inclusive). What we see in this image is a large area in the Central and Northern Eritrean highlands with strong East to West temperature gradients. The black outline approximates the area covered by the Eritrean Highlands (above about 1400 meters). And the while line traces the Eastern Crest of the Eritrean Highlands.
Generally, near-surface winds tend to flow in the same direction as the horizontal temperature gradients (when regional pressure gradients are weak). This is because for all other factors being equal, cold temperatures produce dense air. And dense air produces higher hydrostatic pressures. So low-level hydostatic pressures will be high where temperatures are low, and they will be low where temperatures are high. This means that pressure gradients produced by near-surface temperature differences will be proportional temperature gradients and in the opposite direction.
Also in Eritrea, near-surface winds are alligned with pressure gradients (rather than perpendicular like in higher latitudes). This is because the winds are in force balance with surface frictional forces rather than the Coriolis force (Note: the Coriolis force acts in a direction perpendicular to the velocity). Surface frictional forces are alligned with and in opposition to wind flow. And since the force pushing the wind will be balanced by the frictional force, the force pushing the wind (which is the negative of the pressure gradient) will be aligned with the wind direction.
All of this is to say that temperature-induced pressure gradients will be roughly proportional to horizontal temperature gradients, and that near-surface winds will be roughly aligned and proportional to these temperature gradients.
To interpret these temperature gradients there are several factors that we must consider.
We must first note that they are based on estimates of near-surface temperatures. They are therefore more indicative of air motions through passes and accross flatlands than they are of air motions at mountain-tops.
Secondly, we must note that topographic barriers have a big influence on whether or not the temperature-induced pressure gradients can be manifested in observed winds. For example, an area of Eastern temperature gradients (temperature increases from east to west) on the eastern side of some large hills may not be manifested in observed winds because those winds may be blocked by those hills.
And last but not lease there are significant errors in calculating these gradients. Strong gradients near the ocean are not physically significant because the temperatures at the ocean were not treated properly in the calculations. also small areas of temperature gradients are not significant because of the intrinsic erros in calculating gradients. A few degrees random error in temperature difference accross several kilometers can show up as a visible area of erroneous temperature gradients.
But with these caveats and observations in mind, we can provide the following interpretation of the observed temperature gradients.
During the Winter what we see is that the temperature gradients in the Western lowlands is fairly low, while there are large Eastern gradents in the highlands, especially in the Central and Northern Highlands. Meanwhile there appear to be some small areas of mixed Western and Eastern temperature gradients in the Eastern Lowlands.
These observed temperature gradient patterns roughly conform to the daytime wind patterns observed at this time of year. In the highlands winds are from the East to West generally. The winds peak at about 4 m/s in Asmara, but farther North in the darker blue areas daily wind peaks are 6 m/s to 10 m/s. Few if any systematic measurements exist for the far northern mountains, though accounts from visitors indicate that the areas at the Northern passes near Hishkib and Itaro experience very strong winds.
The mixed East/West gradients in the Eastern lowlands are indicative of the fact that there are very low amplitude converging and diverging winds in these areas during the day, in the Winter. Near the coast there are some areas of on-shore winds. But there is also some uplift which produces low day-time clouds. East-West motions are also blocked by the Highlands and a strong inversion layer which prevents low level air from rising up over the steep slopes.
Night-time temperature gradient maps indicate very low horizontal temperature gradients. This is in general agreement with the observation during this time of year that night-time winds are usually below 1 m/s.
On the Eastern lowlands we observe some West to East pressure gradients. This is because the highlands block the movement of lower layers of air. Meanwhile, the East to West upper-level flow pushes cold dense air up against the lower flanks of the highlands. Since the air is cool and dense, it cannot rise above the slopes and is in static equilibrium with the East to West pressure gradients which create the upper level flow. This means in the lower levels of the atmosphere there are West to East temperature gradients which create higher hydrostatic pressures along the lower slopes which stop the onshore winds as they approach the highlands from the Red Sea.
As for Winter, night time temperature gradients are very low during spring and fall, as are the corresponding wind speeds.
Unlike the rest of the year when night-time winds are relatively calm, during the summer the night-time temperature gradients are very significant. During summer there is a consistent West to East temperature gradient with cool western air flowing through the mountain passes into the warmer Red Sea basin air. This cool flow accelerates as it flows down the eastern side of the highlands and produces strong katabatic winds in the upper eastern slopes. This is reflected in certain stations along the upper slopes of the Eastern escarpments recording very high mean summertime winds (Faghena and Debresina record monthly average winds in excess of 7 m/s during this time of year).
This information has been prepared by Robert Van Buskirk. If you have any questions regarding some of this research please feel free to email me at robert@punchdown.org
Last update by rvb October 30, 1998.