Chapter 4: Atmospheric Circulation

The next variable we’ll consider in determining the general climate on a model planet, is it’s atmospheric circulation system.

The surface wind systems with their distribution of atmospherically absorbed solar energy and powers of precipitation and evaporation, are immensely important in creating climate and determining basic direction in which biological adaptations must be made.

At the beginning of this section, I should point out that my use of the terms, ‘tropic’ and ‘subtropic’ are not meant to suggest jungle or climatic conditions similar to those found on Earth, but as geographic locations and ‘conditions relative’ to the given average planetary surface temperature.

The tropic region is that area extending about 20º latitude on either side of the point perpendicular to the planet’s parent star, for most of the models, this is above or somewhat near the equator.

The average temperature of tropic (usually, but not always equatorial) regions on the planet models used herein, extend between 41ºF and 151ºF, depending on the selected model.

The sub tropic region, is an area about 25º latitude above and below the point perpendicular to the planet’s sun. Let’s consider the basic planetary climatic zones, seen in the illustration below.

The tropical, subtropical, temperate and frost zones on our model planets may be nearly stationary and relatively tranquil, as in Figure A; or they may move up and down across the latitudes with specific seasons, seen in Figure B; or they may move across longitudes with sweeping change, as in Figure C.

Air movement
Air close to the ground in the tropic zone absorbs ‘relatively’ large amounts of heat. As this occurs, the capacity of the air to hold moisture increases and the warm mass of air begins to rise in the atmosphere. The rising air is replaced by cooler air as it moves onto progressively cooler regions.  As the warm air cools, it looses it’s ability to carry moisture, where upon the excess moisture is lost through precipitation.

The cooling air mass continues to move away from the tropic zone toward those areas which receive a lesser amount of seasonal sunlight/ As the air moves, it further cools and begins to sink.

In order to gain a background understanding of atmospheric circulation we’ll now discuss only those planetary models with relatively low angles of axial inclination.

Around 35º latitude, the relatively cool and drier air finally reaches the ground and creates a general band of high pressure; from this point, the now cooler air begins to move back toward the tropic zone.

Cool air does not carry much moisture, so as the cool air mass moves from about 35º latitude to 20º latitude, it will be crossing relatively arid terrain. Since these latitudes still receive about 80% of the sunlight available in the tropics, this zone will be relatively hot, as well as dry. This portion of the atmospheric circulation system can be seen in the associated illustration, Tropic and Sub tropic Zone Circulation.

On the whole, the circulation system of the tropic and sub tropic zones will be quite large and fairly steady, except on worlds with high axial inclination,

Biologically, conditions in the tropic zone depend on temperatures and the availability of water.

On planets with low axial inclination and low to mid range APST (about 0ºC-30ºC), I suspect we would find something between temperate and steamy tropical forests as temperatures were elevated. Between an APST of 45ºC and extending up toward 60ºC, I believe the tropical forest would break down to be replaced by simpler plant communities with smaller individuals. Under these conditions, hot temperatures become a limiting factor in much the same way as cold temperatures limit growth in Earth’s tundra.

Around 50º to 60º latitude, on either side of the central tropic zone, there is a low pressure area. This region is cooler than the tropic low since it receives only about one half the solar energy. Being cooler, this mid latitude area is less humid and receives less precipitation.

The asterisks at 1.0 in the above graph, Relative Precipitation and Latitude axis, indicates the average precipitation level for the model world. Once the average precipitation and evaporation rate has been calculated a good model can be made for precipitation.

The following precipitation table was computed using data from the table in Chapter 3: Climatic Factors, Table: Data for Computing precipitation and Evaporation Rates used in either the associated equation or the BASIC computer program.

Table:  Average Inches of Precipitation and Evaporation for Planets with the given Mass and APST

Planetary Mass   Ê = 1
   APST ↓ 0.25 0.5 1.0 1.5 2.0 3.0
APST ºC 0ºC 55.1 50.4 41.7 35.2 29.5 18.2
15ºC 56.5 49.6 41.2 34.6 28.6 15.9
30ºC 55.8 49.6 40.6 33.6 27.4 13.0
45ºC 55.2 48.7 40.8 31.9 25.0 7.8
60ºC 55.0 48.4 38.9 30.4 19.2 5.2

In order to see if the preceding table had relevancy, I also ran calculations for Earth, where the averages are fairly well known, see following table:

Table: Average Inches of PPT & Evaporation on an Earth Mass Planet with 24 Hr Rate of Rotation

APST ºC 0 15 30 45 60
Inches Precipitation 41.7 42.07 40.6 38.9

Earth, with al 20ºC APST, has an actual  average annual precipitation rate of 40 inches.  The theoretical (calculated) Earth mass planet with 20ºC APST and 24 hour rate of rotation  has 41.6 inches of precipitation, a 4% difference between the actual and theoretical. Graph the above table (using the Chapter 3- Table: Cosine of planetary latitudes)

Although the theoretical precipitation rate works out well for an Earth type planet, meaningful equation results seem to degenerate as we approach the much smaller 0.25 Ê mass or the larger 3.0 Ê mass planets.

On the 0.25 Earth mass planet: At 0ºC to 15ºC there is a fairly small habitable zone which may indeed receive the theoretical amount of precipitation; however, most of what water exists on the planet will be locked up as ice in higher latitudes. Also, because there is a small water to land ratio, there is a very large area which will be arid-cool arid and cold arid.

In the 30ºC to 60ºC range, we’d find the higher latitude ice and frost caps either gone or greatly diminished. This would not lead to larger bodies of liquid water. The 0.25 Earth mass planet is a planet losing its moisture. The surface gravity is such that at higher temperatures, water molecules in the atmosphere simply drift out into space, resulting in a planet that is slowly drying. Therefore, in the higher APST range, I would suspect the precipitation and evaporation figures to break down. Several billion year old planets of this small size and high temperature would have relatively dry atmospheres.

On the large 3.0 Earth mass planets there maybe only small land masses, perhaps only island chains as opposed to ‘continents’. Warm moist ocean air might not move out onto large cool land masses.

At 0ºC to 15ºC, there would exist in the north and south latitudes, very large ice caps. Under cooler conditions the oceans would have reduced depth and the island land masses may have expanded to form small continents. This scenario would tend to increase the precipitation level for those areas, above the data given in the Precipitation and Evaporation table.

In the 30ºC to 60ºC APST range, we would find the ice caps diminishing to frost caps and finally entirely absent. As the APST increases, the atmosphere would become more and more saturated with moisture. Instead of true precipitation, we may find states of continual condensation occurring on terrestrial objects, especially at night and in latitudes experiencing change of season. Humidity would be extreme.

Wind deflection
Because all of the planet models rotate on their axis, we can expect wind systems to be deflected at various latitudes on either side of the tropic zone. The basic global air circulation for models, with a low to mid range axial inclination may be illustrated as follows.

As we study the Basic Global Circulation model above and recall the concepts derived from Tropic and Sub tropic Zone Circulation, illustration, page 21 and Relative Precipitation and Latitude graph, page 21, we are able to predict some interesting bits of information about continental climates.

Looking at the Subtropical Highs, we see the winds have a tendency to move from east to west as they return to the low pressure area in the tropic zone. This moving air mass is relatively cool and dry, and as previously mentioned, the area of the planet’s surface crossed will tend to be warm and arid, relative to the APST and the average planetary precipitation rate. The biotic community, which extends across these bands of high pressure depend on the APST, so that on worlds with an

APST  0ºC: The planet may carry cold/dry adaptive, coniferous forest type vegetation

APST  30ºC: Zone may be a hot desert.

APST 60ºC: Plant life is sparse and terrestrial animal life nocturnally active.

Local Climates
At any latitude, where high mountain ranges are situated perpendicular to the prevailing winds, we could expect higher moisture on the windward side and more arid conditions on the lee side.

On planetary models with some degree of axial inclination, that hemisphere undergoing Spring would have its land masses heating up faster than the oceans. This results in a low pressure system developing over the land and a high pressure system developing over the adjacent ocean. Conversely, in the Fall, the land would cool faster than the ocean and the high and low pressure systems would be reversed.

Water has a greater capacity to hold heat than does soil. Therefore the temperature variation between land and ocean would remain more stable in  the equatorial region that at higher latitudes on worlds with much free surface water.

Inland areas of continents have a far greater annual variation in temperature than coastal areas of the same latitude.

The planet’s surface temperature range
Planets with small axial inclinations have small variations in seasonal temperatures. As the axial inclination increases, the total range of temperatures for each latitude expands.

Model planet A, at left, has  0º axial inclination. An observer at 0º latitude would note his Sun rises to a point directly overhead every day and that the average temperatures through out the year is, let’s say, 81ºF.

Model B has a 15º axial inclination. An observer at 15º N will note that the sun is directly overhead at noon during the summer and that his summer average  temperature is perhaps 81ºF. Half a year later, the Sun has apparently moved to a point 15º below the equator and is now 30º lower in the sky to our observer. Recording his average Winter temperature of 70ºF, the observer notes that there is a 11ºF difference between his Summer and Winter temperatures averages.

Model C has a 45º axial inclination. This time, our observer is located at 45º N latitude. He notices the sun directly over head at noon and a temperature of 81ºF during the summer. Half a year later, the Sun has moved to a point 45º below the equator and is now 90º lower in the sky to the observer. At noon during the winter, the Sun will rise to a point where it just peeks over the southern horizon and the average temperature will be about -25ºF,

For the sake of comparison and to develop a better understanding of axial inclination and the subsequent temperature range, see the following table:

Table: Approximate Change in Temperature Range  (from the concept explanation above)

        0º     81      81        0
       15º     81      70        11
       45º     81     -25       106
For a planet of 1.0 Earth mass and 20ºC APST (Earth like); the axial inclination has been varied in the table to show simplified temperature effects. Our imaginary observer is located at a latitude above the equator which is equal to the planets axial inclination. For any given location on a planet, the greater the axial inclination, the greater the temperature range for that location and therefore, the greater the variation in its seasonal climate.
The chart shows, the greater the axial inclination, the more pronounced are the ‘seasons’ of the year.

All the planets whose mass and APST fall within the parameters of this study will have thunderstorms, with possible exception of the 0.25 Earth mass planet. The ingredients necessary for thunderstorm development are unstable warm air and humidity. These conditions exist in at least some latitudes on all the other, larger habitable planets. About the only area that would not be at least occasionally affected by thunderstorm activity would be the ice caps and other extensive desert (arid) regions

Continued in Chapter 5: Atmospheric Retention


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Filed under SRAPO (Stellar Radii and Planetary Orbits), __Chapter 4: Atmospheric Circulation

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