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Keywords:

  • climate change;
  • cloud genera;
  • cloudiness;
  • global warming;
  • Krakow;
  • long-term variability in air temperature;
  • Poland

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References

The purpose of this article is to describe the effect of cloudiness on long-term changes in air temperature. The article attempts to assess the role of clouds in contemporary climate change. The article is based on archived air temperature and cloudiness data obtained from the Jagiellonian University in Krakow. The analyses include mean daily air temperature and cloud cover data for years 1863–2010 and noontime cloud genus observations for years 1906–2010. It was concluded that the increase in air temperature in Krakow is more strongly associated with changes in the structure of cloudiness than changes in the amount of cloudiness. The relationship between these two types of changes is more apparent during the second half of the 20th century, in an era of accelerating climate warming. Copyright © 2013 Royal Meteorological Society

1. Introduction

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References

Climate change is not only crucial but also one of the most complex challenges facing the world. Researchers, politicians and journalists argue about the causes and effects of contemporary climate change. Some climatologists (Bertrand and van Ypersele, 1999; Kejna, 2008) argue that the increase in air temperature recorded over the last few decades is not related to events in space including changes in the Earth's orbit and solar activity. Researchers who favour the notion of human impact as the cause of contemporary climate change consider the increase in the concentration of carbon dioxide in the Earth's atmosphere as the main factor in the global increase in air temperature (IPCC, 2007). It is important to note that the gas most responsible for absorbing long-wave radiation near the Earth is water vapour and not carbon dioxide. It is estimated that water vapour accounts for 36–66% of the greenhouse effect. When clouds are included, that number rises to 66–85% (IPCC, 2001). According to Kożuchowski (2005) and other climatologists, carbon dioxide is also less important than water vapour in terms of greenhouse properties. Water vapour is the chief means of energy exchange in the Earth's atmosphere. Water vapour content and the resulting cloudiness determine the quantity of solar energy reaching the Earth's surface and the amount of radiation being emitted by its surface. According to Norris and Slingo (2009), even small changes in cloudiness may cause a larger change in the Earth's radiation balance than corresponding changes in greenhouse gas content. Norris and Slingo state that an increase of 15–20% in low cloud cover causes a change in the Earth's radiation balance comparable to a twofold increase in carbon dioxide content.

Cloudiness is a key variable affecting Earth's radiation balance. Yet, the role of clouds in the Earth's climate is not understood sufficiently well. Researchers believe (Norris, 2000; Clement et al., 2009; Norris and Slingo, 2009) that one of the key problems in modern climatological research is the assessment of the significance of cloudiness in global warming, in particular its changes and links with other factors. This subject has been explored in a number of papers (Manabe and Wetherald, 1967; Schneider, 1972; Cess and Potter, 1987; Wetherald and Manabe, 1988; Sinik and Marki, 1996) but many questions remain as to the impact of cloudiness on air temperature.

It is well known that as the air temperature rises, the atmosphere's capacity to absorb water vapour also rises. This causes not only an increase in cloudiness, but also an initiation of a multitude of feedback systems both positive and negative. Increased cloudiness should theoretically limit insolation and lower the air temperature. However, clouds can retain heat and raise the air temperature, which is known as cloud greenhouse forcing, and also cool the Earth by reflecting solar radiation back into space. Hence, there are a number of questions worth asking. Is increased cloudiness produced by a warming climate? Does increased cloudiness simply imply greater cloud cover or a change in cloud cover structure? Does an increase in cloudiness produce thicker clouds or clouds that cover more area? Do changes in the amount and structure of cloudiness cause more solar radiation to be reflected or absorbed by the Earth's atmosphere?

The purpose of this article is to describe the effect of cloudiness on long-term changes in air temperature. The article attempts to assess the role of clouds in contemporary climate change using long-term meteorological observational data from Krakow, Poland.

Krakow is one of the few cities in Europe to possess a continuous high quality series of meteorological measurements and observations (Matuszko, 2007). The data represent climate conditions in an urban area in Central Europe at less than 300 m above sea level. The trends observed with this set of long-term cloudiness and air temperature data can be a sensitive indicator of climate fluctuations across larger geographic areas. According to many climatologists (IPCC, 2007), larger increases in air temperature are noted in the northern hemisphere, in inland areas and north of the 45th parallel. The likelihood of a positive effect of clouds on air temperature in these areas was shown by Schneider (1972) who investigated the feedback system between cloud and temperature.

2. Source materials and methods

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References

The article is based on archived air temperature and cloudiness data obtained from the Department of Climatology at the Institute of Geography and Spatial Management of the Jagiellonian University in Krakow (50°04′N, λ = 19°58′E, Hs = 206 m a.s.l.). The analyses include mean daily air temperature and cloud cover data for years 1863–2010 (148 time series) and noontime cloud genus observations for years 1906–2010 (105 time series).

The non-stationarity of time series was tested assuming linearity of changes, using the linear regression zero-slope test, or – in a more general case – the Mann–Kendall–Sneyers test (the MKS test) allowing to detect monotonicity and the point of its direction change. In order to study the relationship between the time series variables, the simple Pearson correlation coefficient was utilized.

The MKS test (Kendall and Stuart, 1968; Sneyers, 1975; Sneyers et al., 1998) is a set of Mann–Kendall tests testing the hypothesis H0 on the homogeneity of the consecutive subseries {x1, x2,…, xk}k=2,…,N and {xk+1, x2,…, xN}k=1,…,N-1 taken from the N-element time series {x1, x2,…, xN}, i.e. the hypothesis that these subseries are simple random samples, or, in other words, that their elements are independent and drawn from the same probability distribution. If for a certain k the hypothesis H0 is rejected, the graph of test statistics uk and uk versus k allows to specify the shape of non-stationarity, for example, as a monotonic trend or a so-called change point, that is, the time instant at which a trend changes the direction of its monotonicity.

If a sample comes from one population and the data are mutually independent, then the uk and uk lines should fluctuate around the zero-line within the (−ukryt(α), ukryt(α)) area, α being a fixed significance level. A monotonic trend in all time ranges will be shown on the uk and uk graph as two parallel irregular lines, rising or falling, reaching beyond the (−ukryt(α), ukryt(α)) area. If uk and uk lines cross above ukryt(α) or below −ukryt(α), this suggests that in the year (years) of the crossing a change point had occurred.

3. Long-term air temperature and cloudiness data

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References

According to the 2007 IPCC Report, the mean air temperature at the Earth's surface increased of 0.74 °C ± 0.18 °C during the 100-year period from 1906 to 2005. The mean rate of temperature increase was 0.07 °C ± 0.02 °C per 10 years. However, during the second 50 years the mean rate of temperature increase was 0.13 °C ± 0.32 °C per 10 years, which is double the mean for the 100-year period. The greatest warming was observed during the winter and spring seasons. According to many researchers (Brázdil et al., 1994; Klein Tank and Können, 2003; Beniston and Stephenson, 2004; Moberg and Jones, 2005; Brohan et al., 2006), the increase in mean annual air temperature is not synchronous either spatially or seasonally across the Earth, due to the fact that temperature changes are accompanied by changes in atmospheric circulation and oceanic circulation. There is also disagreement in the field as to whether global warming is associated with increasing maximum temperatures or increasing minimum temperatures. In short, is the world becoming more hot or less cold (Alexander et al., 2006)?

Long-term course of air temperature variability based on data from Krakow (Figure 1) confirms research results obtained in other locations and suggests an increase in climate warming. Existing research (Matuszko and Skublicka, 2010) does not unequivocally resolve the question whether the warming is caused by rising maximum temperatures or rising minimum temperatures. Both temperature extremes increase at almost the same rate, as shown by the virtually unchanging daily temperature fluctuations over the course of the study period. These fluctuations have only increased in the last 30 years due to a higher increase rate of maximum than minimum temperature. Data analysis shows that the number of hot and very hot days increased over the last five decades relative to the number of days with frost and strong frost.

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Figure 1. Long-term course of monthly air temperature means in Krakow (thin line); 11-year moving average (bold line); trend line (broken line).

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Air temperature in Krakow increased every month (Figures 1 and 2) but the largest increases were recorded in winter (above 1.5 °C per 100 years), especially in December, and in the spring, with the maximum in May. Temperature increases were smaller in the summer and the smallest in September (above 0.5 °C per 100 years). The trend line test (Figure 2) confirms that it is significant at a level of significance α better than 1% for every month except September, which has a p-value of 2.8%.

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Figure 2. Slope coefficients (a) of simple regressions of the long-term course of monthly air temperature means in Krakow (1863–2010), and p-values for H0 (real slope = 0).

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Long-term air temperature course differs substantially from the course of cloudiness in Krakow (Figure 3). The mean annual air temperature increases gradually (1.5°C per 100 years yearly average), while cloudiness is characterized by a large variation demonstrated by multi-year periods of increasing cloudiness followed by periods of decreasing cloudiness. The early years (from 1863 to 1880) are characterized by an increase in cloudiness from 65 to 70%. In the next period, there is a gradual decrease in cloudiness to 59% up to year 1907. The period between 1908 and 1941 is characterized by a gradual increase in cloudiness to a maximum of 78%. In the following 20 years substantial cloudiness was maintained – more than 70% on average per year. Since 1961 the cloudiness kept decreasing to a minimum of 56% in 1982, and was followed by a slow increase. Data from the beginning of the 21st century indicate a trend of decreasing cloudiness.

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Figure 3. Long-term course of (a) mean air temperature in consecutive months and (b) mean annual cloudiness (thin line), smoothed out using an 11-year moving average (bold line).

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The MKS test (Figure 4) shows a systematically increasing statistical significance of the rising trend of the mean annual air temperature and the expected change in the mean annual cloudiness trend around 1950. The change in the cloudiness trend from increasing to decreasing combined with the largest cloudiness over the long term (Figure 3) can be traced back to circulation factors and human impact. A cooling of ocean waters was noted in 1945–1970 on a global scale (Rayner et al., 2006), a factor that does not favour the formation of convective clouds. In addition, the ocean cooling effect could have halted the warming of the air. Furthermore, increased zonal circulation has been observed in Poland since the 1950s. This circulation has an eastern component, which increases cloudiness with a predominance of layered clouds that reduce solar radiation.

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Figure 4. The forward (bold solid line) and backward (bold broken line) MKS statistics for mean annual air temperature (inline image) and mean annual cloudiness (inline image). Thin grey lines (values not shown) depict the (inline image) and (inline image) time courses.

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The atmospheric circulation effect in Krakow could have been strengthened by human impact, since an iron foundry within the city boundaries started production in 1954, which was accompanied by industrial and geographic expansion of the city. Increased emission of dust and industrial gases contributed to the creation of layered clouds and the decrease in atmosphere transparency. Industrial emissions decreased in the 1980s as a result of reduced industrial production. Zonal circulation with a western component increased in the mid-1970s. In addition, more highs were observed than lows. This resulted in decreased cloudiness, increased occurrence of convective clouds and increase in the number of clear and cloudless days.

The MKS test for long-term temperature data for specific months (Figure 5) shows a number of different trends: from clearly increasing throughout the study period (e.g. August) to a lack of non-random changes in September.

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Figure 5. The forward (bold solid line) and backward (bold broken line) MKS statistics for the mean monthly air temperature (inline image) for consecutive months. Thin grey lines (values not shown) depict the (inline image) time courses.

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Long-term cloudiness patterns in specific months (Figure 6) show a statistically significant (at the 5% level) change in the overall trend in the 1950s. This change can be observed for the winter months (November to February) and the month of May. The remaining months of the year show complete or near-complete randomness, especially September.

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Figure 6. The forward (bold solid line) and backward (bold broken line) MKS statistics for mean monthly cloudiness (inline image) for consecutive months. Thin grey lines (values not shown) depict the (inline image) time courses.

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In the winter, the air usually flows from the west due to the activity of Atlantic lows moving across Europe. This type of circulation is usually accompanied by Altocumulus and Stratocumulus clouds (Figure 7), which are associated with polar-marine air and the cold fronts prevalent in Poland. Altocumulus and Stratocumulus clouds are characterized by considerable thickness and low radiation permeability (Matuszko, 2009), which leads to decreased radiation and air temperature during the day as well as increased long-wave radiation and increased temperature during the night. Altostratus and Stratus clouds (Figure 7), which often occur in winter have a similar effect on the temperature when they occur in Krakow even with polar continental air brought by highs from the southeast and east. Days with Stratus clouds are characterized by the smallest daily temperature fluctuations (about 1.9 °C) and relatively low temperatures. The occurrence of Stratus clouds has been clearly decreasing since 1956, most likely due to circulation factors. However, other causes may include warmer and drier air over Krakow due to the emission of heat by industrial plants and a urban development of wetlands in the Vistula Valley. The lower frequency of occurrence of Stratus clouds reduced cloud cover levels and increased the number of cloudless periods.

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Figure 7. Cloud genus percentage contribution for noontime in Krakow for the winter months (December to February) for the period 1906–2010 (Cl, cloudless).

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The change in the long-term cloudiness pattern noted in May can be explained by circulation factors and local factors determining the level of cloudiness in this month. A decrease in the frequency of occurrence of air influx from the north, which used to bring reoccurrence of cold temperatures and spring frost, has been observed in recent years. May is among the months with the largest increase in temperature over the long term (Figure 1). The number of hot days (tmax > 30 °C) in May has been increasing since the 1960s and they are occurring earlier every year. May has become a summer month with typical summertime convective clouds. The structure of cloudiness in Krakow has changed, and the degree of cloudiness decreased due to a lower frequency of occurrence of low layered clouds.

The completely random nature of temperature changes and changes in cloudiness in September can be explained by very stable weather created by highs, with the least amount of cloudiness during the year.

The annual course of the values of the correlation coefficient between mean daily air temperature and mean daily cloudiness (Figure 8) shows a significant relationship between the two variables. However, the sign of the coefficient changes is different between the cooler part of the year (January, February, November and December) and the remaining months. There is a negative correlation between cloudiness and temperature between March and October, indicating that the direction of changes in air temperature is different than the direction of changes in cloudiness. The effect of cloudiness on temperature is much more apparent during the summer (with a maximum in July) than during the spring and autumn. The sign of the coefficient of correlation is positive during the cooler months of the year (January, February, November and December), which indicates that the impact of the cloudiness on temperature is different than in the summer: the direction of air temperature changes is the same as the direction of changes in cloudiness.

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Figure 8. Coefficient of correlation ρ(T,Ovc) for daily air temperature and cloudiness values for selected months during the period 1863–2010.

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Each cloud genus differently impacts the Earth's radiation balance and air temperature due to its characteristics such as: structure, microstructure, size of cloud particles, thickness, temperature, water content and others, as well as in relation to time of day and season. Thin high clouds made of ice crystals substantially reduce long-wave radiation and are permeable to short-wave radiation. Thick low clouds such as Stratocumulus and Nimbostratus reflect short-wave radiation much more than they stop long-wave radiation (Norris and Slingo, 2009).

Low layered clouds, which are very common in the winter, increase the atmosphere's back radiation more and decrease the night cooling less than would be the case with a cloudless sky. Stratus clouds, which are particularly common in Krakow in the morning and in the evening, influence the increase in air temperature by reducing the Earth's back radiation. On the other hand, the compact cloud cover during the day limits insolation and lowers the temperature. Convective clouds, which are characteristic for the summer season, facilitate the influx of solar radiation during the day. However, at night, when convection stops, they often transform into Stratocumulus clouds, thus preventing the heat loss. Cumulus clouds contribute to the raise in temperature for other reasons as well. First, the heat of condensation is released when Cumulus clouds form, which raises the air temperature. Another reason is the increase in the intensity of total solar radiation, since the radiation that reaches the Earth's surface can be both direct radiation and radiation scattered from the cloud side surface. Hence, an increase in the frequency of occurrence of convective clouds contributes to a rise in temperature during the day as well as during the night.

4. Relationship between the frequency of occurrence of specific cloud genus and air temperature

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References

How it was shown previously, there exists mutual dependence between cloudiness and temperature. Modifying the Earth radiation balance, cloudiness influences temperature in night and daytime and temperature influences generation of particular cloud genera. High temperature favours development of convection and generation of cumuliform clouds whose occurrence raises air temperature. On the contrary, low stratiform clouds are formed as a result of cooling the lowest atmosphere layers and constitute a compact barrier to solar radiation hampering therefore heating of active area. So, cloudiness both depends on temperature and influences it.

Table 1 presents the association between mean monthly temperature and the monthly frequency occurrence of each specific genus of cloud. The analysis of the values in the Table 1 indicates that the presence of Cirrus clouds is associated with a rise in air temperature regardless of the time of year, because these clouds permit short-wave radiation to pass through and absorb long-wave radiation.

Table 1. Coefficient correlation r for mean monthly temperature and monthly frequency of a specific cloud genus. The coefficients are statistically significant at 5% level when the values are higher than 0.193 or lower than −0.193 (shaded fields)Thumbnail image of

In the cooler part of the year, Altocumulus clouds favour warming because they increase back radiation, while in July, August and September they contribute to decreasing air temperature as they limit the solar radiation flux. In the end of June the cloudiness in Krakow and other stations in Poland increases and its structure changes (Matuszko, 2009). Compared with the previous months, the contribution of Altocumulus clouds increases in July and reaches its annual maximum. The occurrence of this maximum is connected with the maximum of occurrence of polar maritime air accompanied by this type of clouds. In July, there also increases the contribution of Altostratus clouds characteristic for the frontal (stratiform) cloudiness. The simultaneous occurrence of Altocumulus and Altostratus forms compact cloud layer transforming into Nimbostratus from which long-lasting rain falls producing precipitation concentration that time in Poland. According to Kaszewski (1983), the cloudy turn of June and July is caused by a transformation of the circulation system in Europe and development of western and north-western advection favouring the increase and change of cloudiness.

The layered clouds Altostratus, Nimbostratus and Stratus have a negative correlation with temperature throughout the year, but the Cumulus clouds show a positive one. The intensity of total radiation reaching the Earth's surface is approximately twice of when vertical clouds are not blocking the Sun's disk as opposed to when layered clouds are present. In addition, the heat of condensation is released in the process of convective cloud formation, which increases the air temperature. A cloudless sky in the winter is associated with lower temperatures, while in the summer with higher temperatures. The strongest correlation between air temperature and cloud genus exists for Cirrus and Cumulus clouds (positive correlation) and Nimbostratus clouds (negative correlation).

The long-term frequency course of occurrence of selected clouds (Figures 9, 10 and 11) confirms the hypothesis that changes in the structure of cloudiness may impact climate change. Increases in frequency of vertical and high clouds, but decreases in occurrence of layered clouds have been observed in Krakow and other parts of the world, particularly during the last 40 years (Sun and Groisman, 2000; Matuszko, 2003; Wibig, 2008).

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Figure 9. Long-term frequency course of Ci cloud occurrence (alone or with other cloud genera) – given in percents for specific months. The continuous line is an 11-year moving average.

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Figure 10. Long-term frequency course of Cu cloud occurrence (alone or with other cloud genera) – given in percents for specific months. The continuous line is an 11-year moving average.

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Figure 11. Long-term frequency course of Ns cloud occurrence (alone or with other cloud genera) – given in percents for specific months. The continuous line is an 11-year moving average.

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For the whole Krakow data series, Cirrus, Altocumulus, Stratocumulus, Cumulus and Cumulonimbus clouds follow an increasing trend, while Cirrocumulus, Cirrostratus, Nimbostratus and Stratus clouds follow a decreasing trend. Among the middle clouds, the decrease in frequency of Altostratus clouds was replaced with a greater contribution of Altocumulus clouds. A particularly rapid increase in the frequency of occurrence of convective Cumulus and Cumulonimbus clouds was observed in the spring and summer months during the 1930s and since the 1980s. In contrast, the decrease in the occurrence of layered clouds has been gradual, with the highest rate of decrease being observed during the 1950s. The following trends, observed in Krakow over the long term, favour the increase of air temperature over the course of the study period: increased frequency of occurrence of Cirrus clouds and convective clouds, and decreased occurrence of layered clouds.

The change in the structure of cloudiness over the long-term causes an increase in the total amount of solar energy reaching the surface of the Earth and a corresponding rise in air temperature despite an increased coverage of sky with vertical clouds in some regions.

5. Summary and conclusions

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References

The analyses in this article allow to conclude that the increase in air temperature in Krakow is more strongly associated with changes in the structure of cloudiness than changes in the amount of cloudiness. The relationship between these two types of changes is more apparent during the second half of the 20th century, in an era of accelerating climate warming. The increased frequency of occurrence of vertical clouds, decreased frequency of occurrence of layered clouds, increased amount of low clouds during the summer, but decreased amount of low clouds during the winter, and increased frequency of occurrence of high clouds (Matuszko, 2009), possibly a result of high clouds being revealed by low layered clouds, all have been observed in Krakow, particularly in the last 40 years. The described changes in the structure of cloudiness cause increased insolation and air temperatures despite the increase in the degree of cloud coverage. Therefore, increase in cloudiness caused by elevated content of vapour in the air due to increase of air temperature is manifested in Krakow by the vertical cloud development, rather than a superficial cloud coverage of the sky.

As a consequence of these changes there are fewer cloudy days, fewer completely overcast days and shorter series of cloudy days. Lower degree of cloudiness, more clear days and a predominance of vertical clouds allow more solar radiation to reach the Earth's surface. A warmer active surface leads to more evaporation and increased convection. Cumulonimbus clouds form much more frequently than during the first half of the 20th century. They also form much more frequently during the cooler half of the year and are much larger in size than before. Research has shown (Matuszko, 2009) that Cumulus, Cumulonimbus, Cirrus, Cirrostratus and Cirrocumulus clouds are not very effective at limiting the influx of solar radiation due to their structure and properties. Therefore, it may be inferred that decrease in degree of cloudiness in winter and its changed structure in summer contributed to increased maximum temperature and climate warming. Layered clouds commonly encountered during the autumn and winter limit the amount of solar radiation reaching the Earth's surface but also limit back radiation, which results in relatively higher values of minimum temperature.

Changes in cloudiness in Krakow during the second half of the 20th century have resulted in more solar radiation reaching the Earth's surface, as evidenced, for example, by the largest over the long-term period monthly number of hours of sunshine in July and December of 2006 (Matuszko, 2009). One consequence of increased insolation associated with a predominance of Cumulus and Cirrus clouds was a rise in air temperature noted in 2006 at many European weather stations. In Krakow, the mean monthly air temperature during the cooler part of the year 2006 was one of the highest over the long term.

References

  1. Top of page
  2. ABSTRACT
  3. 1. Introduction
  4. 2. Source materials and methods
  5. 3. Long-term air temperature and cloudiness data
  6. 4. Relationship between the frequency of occurrence of specific cloud genus and air temperature
  7. 5. Summary and conclusions
  8. References
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