English Articles

A Satellite Picture Explaining Our Weather

The European weather satellite Meteosat, circles the Earth on a geostationary orbit (36.000 km altitude) providing daily current views of our planet. On this color infrared recording of  May 31th 2009,you can see some important phenomena of global weather patterns.

Dynamical Weather Systems

Weather on earth-like planets is driven by temperature differences between equator and poles, caused by different sun´s irradiance. In mid – latitudes, where warm tropical and cool polar air masses encounter each other, gradient of temperature (and thereby gradient of pressure) is sufficient to generate a high altitude air current (called tropospheric polar jetstream) on both hemispheres, turning eastward under influence of earth’s rotation.

Breaking a critical speed limit, the jetstream forms Rossby waves with troughs and ridges(wave peaks). A lot of shear forces emerge. The waves break and roll up to vortices. These are the high pressure und low pressure systems, enabled to intermix the warm tropical and cool polar air masses.

The high pressure vortices (anticyclones) are spinning downward and clockwise (counterclockwise) on northern (southern) hemisphere, whereas the low pressure vortices (cyclones) are spinning upward and counterclockwise (clockwise) on northern (southern) hemisphere.


Weather at June 11th, 2009, 11:00 UTC . The ITCZ, the deserts in the Subtropical High Presure Belt and the Low Pressure Systems (Cyclones) of the mid – latidudes are easily dercernable. Natural Color RGB images makes use of three solar channels: red, green and blue. In this color scheme vegetation appears greenish because of its large reflectance in the green beam channel compared to the red and blue beam channels. Water clouds with small droplets have large reflectance at all three channels and hence appear whitish, while snow and ice clouds appears cyan because ice strongly absorbs in red. Bare ground appears brown because of the larger reflectance in the red beam channel than at the blue one, and the ocean appears black because of the low reflectance in all three channels. Source: Meteosat, EUMETSAT

Inside low pressure systems the air rises and cools, so that water vapor condenses, forming clouds made of tiny water droplets or ice crystals (bad weather). Latent heat (thermal energy of condensation) thereby released powers cloud formation on her part warming the rising air.

Inside high pressure systems the air sinks and clouds decay, because water in the condensed form tends to evaporate into water vapor (fair weather).

Cyclones derive their energy not only from the jetstream, but also from latent heat liberated during formation of clouds. In turn they transmitted back a portion of their energy to jetstream.

The pathways of cyclones are affected by the behaviour of the jetstream.But sometimes the high air current slow down or breaks actually, so that the cyclones are able to seperate from jetstream. These cut off lows move slowly and won’t exit a region until they are captured by a trough of a new jetstream, which meanwhile has usually formed. 


Bjerknes total

Low Pressure Systeme (Cyclone) Source: Bjerknes (1922)

The high pressure systems (anticyclones) often swerve equatorward, forming a belt of subtropical high pressure on both hemispheres. In this reagions precipitation happens very rarely (desert climate).

Tropical Hadley – Circulation

Away from this areas of high pressure the air masses move equatorially along the surface (tradewinds), where´s a buildup of low pressure (Innertropical Convergence Zone, ITCZ) : These tradewinds turn westward due to earth´s  rotation. Heated by the sun,  equatorial air rises and cools, forcing whatever water vapor it holds to condense into clouds. The ascended air moves poleward , but it is turned eastward by the earth´s rotation. As moving  polewards, the air current contracts closer to the axis of earth’s rotation. So it must spin faster, creating subtropical jetstreams that rotate more rapidly than the Earth itself..In parts however, the air descends in the belt of subtropical pressure, closing the air circulation. This so called Hadley-Circulation.partions in a row of convective cells around the whole planet.

Stratosphere and Polar Vortex

The stratosphere is the next layer of atmosphere above the troposphere, in which most weather processes play. The stratosphere contains little water vapor, but larger quantities of ozone, protecting life by absorption of dangerous solar ultraviolet radiation. Therefore the stratosphere is much warmer than the upper troposphere.

If the stratosphere over the poles is cold enough during the polar night, a polar vortex forms due to a sufficient gradient of temperature to build up an eastward stratospheric jetstream, which is a propulsion engine of tropospheric polar jetstream (see above).

A strong polar vortex favors a poleward, zonal circulation (along the lines of latitude), a weak, often divided polar vortex, however, favors a meridional circulation with pronounced troughs and ridges (along the lines of longitude).

Jens Christian Heuer



Temperature Cooling During Cereal Harvest Periods

English Abstract:

 There is a dramatic evidence that worldwide global warming is especially pronounced during the past 20 years. There are however significant spatial and temporal changes in recent global warming trends. Even periods of climate cooling occurred during the last decades and were explained in the literature for example by dust and sulfate aerosols emitted by industrial activities and volcanic eruptions and also by conversion of natural historical land cover to croplands and residential areas. We present comparisons between historical mean monthly temperature records (1900 to 1929) and those of the last two decades, collected within in the center of huge areas of crop production worldwide (Canada, Midwest-US, Russia, Australia, Pakistan/India and China). We find a significant cooling for most of the analyzed weather station records that align exactly to periods of harvest and subsequent field cultivation. These periods reveal significantly lower mean temperatures in comparison to the months before and after harvest and cultivation. In some regions the temperatures even absolutely decreased for periods of 2 to 3 months during the last century in spite of general global warming. A general feature of our analysis is the expected temperature increase for all the months of the year with the exception of harvest periods. The citations in the literature, that land use changes during the last century could be an explanation for the temperature cooling does not seem to be effectual enough to us, since the cooling phenomenon only occurs during harvest and cultivation periods, whilst the other months are widely unaffected. An analysis of smoothed time series of mean temperature for some stations reveal a change in the general course to a cooler atmosphere typically at the end of the last century, when modern harvest and cultivation equipment was developed and started to be applied in agriculture. This feature is significantly pronounced for harvest times. We surmise that in combination with albedo effects, emissions of smog by open field burning after harvest, aerosols and dust emerging especially from combine harvesters and acre cultivation with modern equipment as farm tractors could be reasonable factors for the quoted cooling effects (Fig. 1).


The earth climate is an extremely sensitive system with regard to aerosols of natural and anthropogenic origin. Aerosols may have an influence in two different ways: a direct effect by scatter and absorption of solar radiation, and indirect effect by their influence in cloud formation changing the optical properties and lifetimes of clouds. Some examples will be given. Dust storms in China are strongly related to decreased air temperatures especially in the winter season and to highly frequent cyclone activity in northern parts of China (1). Volcanic eruptions lead to intense worldwide but temporary atmospheric cooling (2) and are able to produce oceanic tropical precipitation changes (3). Sahel dust layers affect the downward IR-flux at the surface during the night and increases the radiative cooling rate of the atmosphere (4). The absence of condensation trails during the three day grounding of all commercial aircraft in the US (after the terrorist attacks on 11 September 2001) lead to an anomalous increase in the average diurnal temperature range across the United States (5). Local land use practices on regional climate may overshadow larger-scale temperature changes, associated with observed increases in CO2 and other greenhouse gases (6). Several modeling studies have shown that land use changes in North America result in a cooling in surface climate (7,8). Most of the published studies, however, apply to 3 months averages. By this procedure the temperature signal associated with this land use is generally small and hard to detect. In our paper we use historical and actual monthly data of weather stations within the different crop regions and adjust them to the corresponding periods of cereal harvest periods. In a recent study by researchers at the University of California, Berkely (10) it was found that global warming may include some periods of cooling. This effect may be due to mixing of manmade pollutants with natural compounds emitted from forests and vegetation, forming secondary aerosols that reflect sunlight. Data and methods Monthly temperature records, which are open to the public (9) for 6 huge crop regions of the world, mentioned above, were analyzed. Historical homogeneous and complete meteorological data bases for at least 100 years are rare and are only sufficient and of higher quality in the United States, Canada and also in Europe, which we used for comparison. For the other regions only a few stations could be used and are the basis of our investigations. Few cases of missing data in the records, were interpolated by means of adjacent values. Most of the results discussed below refer to the first 3 decades of the last century and are compared to temperature data sets of the last 20 years (1988 – 2007). Only in a few cases (e.g. China) we had to use periods that differ slightly from those of the other data sets. During the 20th century, cereal output worldwide expanded by about 3 to 5 -fold, most of which occurred after 1950 due to technological innovations and scientific crop management with synthetic nitrogen fertilizers, crop breeding and better germination ability (Fig. 2). In the early 20th century a significant part of the harvested crop (up to 30 per cent) had to be retained by the farmers for the next year’s seed. By 2000, however, the global averaged seed use e.g. for wheat was only about 5 per cent of output. Increases in yields are due mainly to the level of intensive farming techniques by agricultural implements such as modern tractors, harrows, ploughs and especially the combine harvester which was introduced in the mid 20th century.

Wamser H1

Fig. 1: A: Dust cloud behind harvesters at work. B: Huge cloud of smoke (length of some tenths of kilometers) over the Baltic Sea, spreading from crop fields by straw burning (Photo G. Peters)

Modern techniques in agriculture significantly reduced the required manpower especially in North America from about 2 to 0.05 workers per hectare within the last century. With the rapid rise of mechanization, together with the common practice of burning the stubble left behind by harvesters in the fields, results in great emissions of dust and smoke aerosols into the atmosphere. By this the short wave radiation budget could at least be changed locally and temporally.

Wamser H2

Fig. 2: Canada – seeded acres (solid line – left axis) and wheat production (dashed line – right axis)

During the last century there is strong evidence (although quantitative data cannot be found everywhere in the literature), that there are significant changes in land use – especially by deforestation into croplands. A complete data set is found for free access in ‚Historical Statistics of Canada – 11-516-XIE‘. Figure 2 gives an example for the increase of the seeded area (dashed line) and wheat production (solid line) in Canada for the period 1900 to 2007 in billion acres resp. billion bushels per year. By comparison of the first three decades with the last two we find an almost doubling of the seeded area and a 3.5-fold increase of the wheat production in Canada. Although no corresponding complete data sets for the 6 other world corn regions are available to us, a nearly comparable increase of crop areas and wheat productions in these regions during the last century can be assumed, since within all the other investigated areas wheat is the main produced type of grain (together with maize specially in the last two decades) and the changes and improvements in agricultural techniques and fertilization during the last century show a similar development. Results This chapter describes results of our investigations with additional information, remarks, preliminary concepts, and ideas. Since the cooling periods presented below, coincide convincingly with the harvest months, we will describe some ideas that have to be considered to explain this fact. The following questions arise: What are the characteristic differences in agriculture at the beginning of the 20th century and ‚today‘ leading to the considerable cooling during harvest.

1) Due to the significant increase of corn production the sowing is much more compact today. This fact could also lead to an albedo increase of the stubble fields after harvest (for this, however, we did not find any convincing data in the literature).

2) Most important is of course the doubling of the cereal area and the corresponding significant albedo increase of the bright, golden-yellow corn fields just before the beginning of the harvest.

3) The use of modern agricultural equipment and the emissions of dust and biogenetic aerosols may also contribute to cooling.

4) At the beginning of the 20th century the farmers normally used the straw for their cattle etc. Straw burning was generally a method applied later, when monoculture came into vogue.

5) Emissions by fertilizers and by liquid manure may escape as gases into the Atmosphere.

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Fig. 3: CANADA: Temperature differences for the periods (1988 to 2007) minus (1900 to 1929). The differences refer to the 4 Canadian stations Saskatoon, Regina, Edmonton and Winnipeg in the center of the Canadian corn belt. The dashed line below the curves indicates the harvest period in Canada. This is relatively late in the year, since most of the harvested cereal there is spring wheat, which is seeded not before May and harvested in Autumn

Wamser H4

Fig. 4: As in Fig. 3 but for 4 other main crop regions of the world A: Lahore (frontier area of Pakistan/India) B: Two stations in NW-China: Tanjin – left axis, solid line, Shenyan – right axis, dotted line. C: Mean of 5 Australian stations north of Melbourne (data 1900 to 1929 and 1973 to 1990 – no free data after 1990 were available to us) D: Russia: Omsk (dotted line left), Barnaul solid line right)}

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Fig. 5: Moving averages of the surface temperature – Lahore left – Canada (mean of 4 stations) right

The harvest periods (Lahore: June, Canada: October) are given as solid lines, left axis. The straight lines present the linear regression for the second period of the 20th century, clearly showing a significant decrease of the mean surface temperature of about 2 degrees centigrade within the harvest months in both cases. For comparison the dotted curves show the mean temperatures for the periods two months before the beginning of the harvest. In spite of strong variations (typical period of about 20 years) it is clearly seen, that in both cases the cooling starts about in the middle of the 20th century, which nearly corresponds with the increase of the seeded area, shown in Fig. 2. (Similar behavior can be given for other crop regions).

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Fig. 6: Comparison (temperature differences and standard deviations) between 5 US stations in Kansas and Nebraska (Horton, Topeka, Beatrice, Syracuse, Auburn) and 13 stations in Europe.

Here are significant differences evident: the general mean climate warming is much more pronounced in Europe during the last century (+0.85 +/-0.2°C) compared to the 5 US stations (0.42 +/-0.55°C). Within the US-crop regions there is a gradual cooling between July and November (a relatively long crop growing and harvesting period due to winter and spring seed). Although there are also many crop regions in Europe, they are much smaller and widespread than within the crop regions discussed in this paper. So, no significant temperature cooling is to be expected (only a week indication for the harvest period in September is indicated.

Literature (1) Qian W, Quan L, and Shi S, (2002) Variations of the Dust Storm in China and its Climatic Control. Journal of Climate 15, 1216-1229. (2) Hansen J, et al. (1992) Potential Climate Impact of Mount Pinatubo Eruption. Geophysical Research Letters 19, 215-218. (3) Spencer RW, et al. (1997) Tropical Oceanic Precipitation Changes after the 1991 Pinatubo Eruption. Journal of Atmospheric Sciences, 1707-1713. (4) Guedalia D, Estournel C, and Vehil R, (1984) Effects of Sahel Dust Layers upon Nocturnal Cooling of the Atmosphere (ECLATS Experiment). Journal of Applied Meteorology 23, 644-650. (5) Travis DJ, et al. (2002) Contrails reduce daily temperature range. Nature 418, 601. (6) Stohlgren TJ et al. (1998) Evidence that local land use practices influence regional climate, vegetation, and stream flow patterns in adjacent natural areas. Global Change Biology 4, 495-504. (7) Bonan GB (2000) Observational Evidence for Reduction of Daily Maximum Temperature by Croplands in the Midwest United States. Journal of Climate 14, 2430-2442. (8) Bonan GB Ecological Climatology – Concept and Applications 2nd Edition National Center for Atmospheric Research, Boulder, Colorado, (ISBN- 13-9780521872218). (9) Climex.knmi.nl and climateweatheroffice.ec.gc.ca (10) Kerry A. Pratt, Paul J. DeMott, Jeffrey R. French, Zhien Wang, Douglas L. Westphal, Andrew J. Heymsfield, Cynthia H. Twohy, Anthony J. Prenni & Kimbely A. Prather (2009). In situ detection of biological particles in cloud ice-crystals. Nature Geosciences, 2009; DOI: 10.1038/ngeo521.

Christian Wamser and Dörte Wamser, Beverstedt Germany

Address (main author): Dr. rer.nat. Christian Wamser (Dipl. Met.) Kampstr. 11 D27616 Beverstedt Germany

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