Atmospheric turbidity of urban and desert areas of the Nile Basin in the aftermath of Mt. Pinatubo’s eruption

Summary  The Linke T _L , ?ngstr?m β and Unsworth-Monteith δ _a turbidity parameters are investigated for two sites in Egypt: Cairo, a densely populated urban area, and Aswan, an arid unpolluted area. These three turbidity parameters are calculated from broadband pyrheliometric measurements recorded hourly over the period 1992–96. Monthly averages of T _L , β and δ _a show relatively flat and identical seasonal variations with a marked main maxima during spring at both sites, due to Khamsin depressions coming from the Great Sahara. A secondary maximum is observed at Aswan in summer, due to dust haze which prevails during that season, and at Cairo in autumn, due to the northern extension of the Sudan monsoon trough, which is accompanied by small scale depressions with dust particles. Annual mean values of T _L , β and δ _a (5.59, 0.250 and 0.372, respectively) at Cairo are larger than at Aswan (3.89, 0.139 and 0.213, respectively). In the same way, the seasonal mean values of T _L , β and δ _a at Cairo are larger than at Aswan. More generally, the monthly and yearly average turbidity values are significantly larger in Cairo than in Aswan for the whole period 1992–96, which is attributable in part to the urbanization/industrialization effect of Cairo. An estimate of the corresponding overburden is obtained by comparison between the present data and older T _L data from 1922–27. It is also shown that turbidity over both sites is largest during 1992, just after the eruption of Mount Pinatubo in 1991. The dependence of β on some meteorological parameters such as wind speed and direction, precipitable water, relative humidity, temperature and visibility, is also analyzed. This reveals in particular that visibility is not a good predictor of turbidity at either site. Conversely, the wind direction and speed have a definite effect on turbidity, and consequently, largest turbidities occur when the wind carries aerosols from the main industrial particle source areas around Cairo. For any season of the year, the average turbidity at the latter site is larger than that at other big cities such as Athens, Rome, and Toronto, but is lower than at Dhahran, Saudi Arabia. Received February 3, 2000 Revised August 25, 2000     

Variability of the Linke and Unsworth-Monteith turbidity parameters in Athens, Greece

Summary  This work studies the variability of the Linke (T _L ) and Unsworth-Monteith (T _U ) turbidity parameters in the urban atmosphere of Athens. Beam irradiance observations performed at the Actinometric Station of the National Observatory of Athens are used in the period 1975–1995. This study examines (i) the inter-annual variation of T _L and T _U , (ii) their mean seasonal variability, (iii) their monthly average variation, and (iv) their mean daily variation. Also, for various air-mass origins, the mean seasonal variations of T _L and T _U are given. The frequency of occurrence of the parameters in various ranges is also shown. An innovative interpretation of the impact of the Saharan dust on the radiative properties in the atmosphere of Athens is attempted. Received May 19, 2000 Revised October 17, 2000     

A Study of Linke Turbidity Factor over Qena / Egypt

Data on instantaneous atmospheric Linke turbidity factor TL (m) are reported for clear days at Qena/Egypt in the period from June 1992 to May 1993.TL(m) is determined using the values of irradiance of direct solar radiation (I),which are calculated from global (G) and diffuse (D) - solar radiation measurements.Monthly and seasonally variations of both diurnal and daily average values of TL (m) increases steadily in the direction of sunset in the months from June to December 1992 as well as Summer and Autumn seasons,while it falls generally in this direction for the months from January to March and Winter season.In April and May,TL (m) fluctuates obviously through the day hours,it is also shown that the average values of TL(m) are particularly large during Summer months compared to other months of the year.This behavior of TL(m) is discussed in view of the variations of some weather elements,which affect the content of water vapor and dust particle in the atmosphere of the study region.It seems t be of s     

Analysis of atmospheric turbidity levels at Taichung Harbor near the Taiwan Strait

In this study, two universal turbidity parameters, the Angstrom turbidity coefficient and Linke turbidity factor, are applied to study the atmospheric turbidity characteristics of Taichung Harbor. Meteorological parameter values were measured during 2004 and 2005 at the Wuchi weather station of the Taiwan Central Weather Bureau, near the Taiwan Strait. Results based on the Angstrom turbidity models (β_Lou, β_Pin, and β_Vis) indicated that annual mean values of the Angstrom turbidity coefficients were 0.174, 0.21 and 0.201, respectively. Four sets of Linke turbidity factors (T_Lin, T_Lou, T_Pin and T_Vis) were calculated using the original Linke method and the Dogniaux method, incorporating the computed Angstrom turbidity coefficients (β_Lou, β_Pin and β_Vis); the resultant values were 4.30, 6.40, 7.10 and 6.95, respectively. The monthly average values, frequency of occurrence, and cumulative frequency distributions were calculated using different models to describe the clear-sky atmospheric conditions at Taichung Harbor. The frequency results show that for over 50% of the dataset, three sets of Angstrom turbidity coefficients fell between 0.15 and 0.18, and four sets of Linke turbidity factors (T_Lin, T_Lou, T_Pin and T_Vis) fell between 4.0 and 6.5. Thus, for 50% of cloudless days, the sky can be between turbid and clear over Taichung Harbor. Furthermore, the results reveal that for 30% of the dataset, three Angstrom sets of turbidity coefficients (β_Lou, β_Pin, and β_Vis) exceed 0.2 and four sets of Linke turbidity factors (T_Lin, T_Lou, T_Pin and T_Vis) exceed 5.0. This indicates that 30% of cloudless sky conditions can be considered turbid to very turbid.     

Trend analysis of rainfall time series for Sindh river basin in India

The main goal of this paper is to estimate a set of optimal seasonal, daily, and hourly values of atmospheric turbidity and surface radiative parameters Ångström’s turbidity coefficient (β), Ångström’s wavelength exponent (α), aerosol single scattering albedo (ω_o), forward scatterance (F_c) and average surface albedo (ρ_g), using the Brute Force multidimensional minimization method to minimize the difference between measured and simulated solar irradiance components, expressed as cost functions. In order to simulate the components of short-wave solar irradiance (direct, diffuse and global) for clear sky conditions, incidents on a horizontal surface in the Metropolitan Area of Rio de Janeiro (MARJ), Brazil (22° 51′ 27″ S, 43° 13′ 58″ W), we use two parameterized broadband solar irradiance models, called CPCR2 and Iqbal C, based on synoptic information. The meteorological variables such as precipitable water (u_w) and ozone concentration (u_o) required by the broadband solar models were obtained from moderate-resolution imaging spectroradiometer (MODIS) sensor on Terra and Aqua NASA platforms. For the implementation and validation processes, we use global and diffuse solar irradiance data measured by the radiometric platform of LabMiM, located in the north area of the MARJ. The data were measured between the years 2010 and 2012 at 1-min intervals. The performance of solar irradiance models using optimal parameters was evaluated with several quantitative statistical indicators and a subset of measured solar irradiance data. Some daily results for Ångström’s wavelength exponent α were compared with Ångström’s parameter (440–870 nm) values obtained by aerosol robotic network (AERONET) for 11 days, showing an acceptable level of agreement. Results for Ångström’s turbidity coefficient β, associated with the amount of aerosols in the atmosphere, show a seasonal pattern according with increased precipitation during summer months (December–February) in the MARJ.     

Recent trends of mean maximum and minimum air temperatures in the western half of Iran

In this study, the trends of the annual, seasonal and monthly maximum (T _max) and minimum (T _min) air temperatures time series were investigated for 20 stations in the western half of Iran during 1966?C2005. Three statistical tests including Mann?CKendall, Sen??s slope estimator and linear regression were used for the analysis. The annual T _max and T _min series showed a positive trend in 85% of the stations and a negative trend in 15% of the stations in the study region. The highest increase of T _max and T _min values were obtained over Kermanshah and Ahwaz at the rates of (+)0.597°C/decade and (+)0.911°C/decade, respectively. On the seasonal scale, the strongest increasing trends were identified in T _max and T _min data in summer. The highest numbers of stations with positive significant trends occurred in the monthly T _max and T _min series in August. In contrast, the lowest numbers of stations with significant positive trends were observed between November and March. Overall, the results showed similar increasing trends for the study variables, although T _min generally increased at a higher rate than T _max in the study period.     

Analysis and trends of atmospheric turbidity parameters over Cairo

Summary This work deals with the Linke turbidity factor, based on total spectrum observations of the direct solar beam and aerosol turbidity parametersa _a , , and based on observations in broad spectral bands. Diurnal and seasonal variations of these turbidity parameters were analyzed for the period 1975 to 1991.Annual variations of these parameters show low values in winter and high values in both spring and summer. The extinction coefficients decrease with increase of both wavelength and optical airmass. Trend analysis shows an increase in aerosol extinction coefficient below 0.63 m, and a slight decrease for longer wave-lengths.Linear regression relations are also constructed to estimate botha _a and whenT _L is available. The relations show thata _a can be estimated with errors below 20%. The relation with the parameter, may give better results when it is estimated by assigning a fixed value of .Nomenclature AV Monthly and total average of the measured parameter - a Atmospheric optical thicknes - a _a Aerosol optical thickness - a _r Mean value of optical thickness of. Rayleigh atmosphere over all wavelengths - a _o Ozone optical thickness - a _o Ozone absorption coefficient - a _w Water vapor optical thickness - COR Correlation coefficient of the linear relation in percentage - Ex₁, Ex₂, Ex₃ Aerosol extinction coefficients in the bands .2–.53, .53–.63, .63–.695, respectively - I _(o) Normal incident direct solar radiation, under clear sky condition - I _o Extraterrestrial insolation at normal incidence - m _r Relative (optical) air mass - NO Number of the observations used in either making the relation or the verification - RMSE Root mean square error of the linear relation - RMSE% Percentage value of the root mean square error relative to the average measured value AV - T _L Linke turbidity factor - T Dry bulb temperature in °C - u _o Ozone layer thickness, cm - Z Zenith angle - Ångström wavelength exponent - Ångström turbidity coefficient - Wavelength - Y The year number after 1975 With 5 Figures     

Comparison of mean radiant temperature from field experiment and modelling: a case study in Freiburg,Germany

Mean radiant temperature (T _mrt) based on two measurement methods and outputs from three models are compared in this study. They are the six direction radiation method, globe thermometer method, RayMan model, ENVI-met model and SOLWEIG model. The comparison shows that globe thermometer method may overestimate the T _mrt since wind velocity is a key variable in the estimation based on this method. For better estimation, T _mrt measured by the globe-thermometer method be corrected by the imported wind speed (stable, low and assuming wind speed) and validated by the six-direction radiation method. The comparison of models shows that the RayMan model’s evaluation of T _mrt involving global radiation with fine time resolution was better than the corresponding evaluations under the other two models (ENVI-met and SOLWEIG) in this case. However, the RayMan model can only assess T _mrt for a one-point one-time context, whereas the other two models can evaluate two-dimensional T _mrt. For two-dimensional evaluations of T _mrt, SOLWEIG have a better prediction of T _mrt than ENVI-met, and ENVI-met can simulate several different variables, which are wind field, particle distribution, CO₂ distribution and the other thermal parameters (T _a, surface temperature and radiation fluxes), that SOLWEIG cannot.     

Watershed-wide trend analysis of temperature characteristics in Karun-Dez watershed, southwestern Iran

Trend estimation of climatic characteristics for a watershed is required to determine developing compatible strategies related to design, development, and management of water resources. In this study, the trends of the annual maximum (T _max), minimum (T _min), and mean (T _mean) air temperature; temperature anomaly (T _anomaly); and diurnal temperature range (DTR) time series at 13 meteorological stations located in the Karun-Dez watershed were analyzed using the Mann–Kendall and linear regression trend tests. The pre-whitening method was used to eliminate the influence of serial correlation on the Mann–Kendall test. The result showed increasing trends in the T _min, T _mean, and T _anomaly series at the majority of stations and decreasing trend in the T _max and DTR series. A geographical analysis of the trends revealed a broad warming trend in most of the watershed, and the cooling trends were observed only in the southern parts. Furthermore, the geographical pattern of the trends in the T _mean and T _anomaly series was similar, and the T _max data did not show any dominant trend for the whole watershed. This study provides temperature change scenarios that may be used for the design of future water resource projects in the watershed.     

Comparison of artificial neural network and multivariate linear regression methods for estimation of daily soil temperature in an arid region

Soil temperature (T _S) strongly influences a wide range of biotic and abiotic processes. As an alternative to direct measurement, indirect determination of T _S from meteorological parameters has been the focus of attention of environmental researchers. The main purpose of this study was to estimate daily T _S at six depths (5, 10, 20, 30, 50 and 100?cm) by using a multilayer perceptron (MLP) artificial neural network (ANN) model and a multivariate linear regression (MLR) method in an arid region of Iran. Mean daily meteorological parameters including air temperature (T _a), solar radiation (R _S), relative humidity (RH) and precipitation (P) were used as input data to the ANN and MLR models. The model results of the MLR model were compared to those of ANN. The accuracy of the predictions was evaluated by the correlation coefficient (r), the root mean-square error (RMSE) and the mean absolute error (MAE) between the measured and predicted T _S values. The results showed that the ANN method forecasts were superior to the corresponding values obtained by the MLR model. The regression analysis indicated that T _a, RH, R _S and P were reasonably correlated with T _S at various depths, but the most effective parameters influencing T _S at different depths were T _a and RH.     

The impact of warm pool SST and general circulation on increased temperature over the Tibetan Plateau

In this paper, the possible reason of Tibetan Plateau (TP) temperature increasing was investigated. An increase in T min (minimum temperature) plays a robust role in increased TP temperature, which is strongly related to SST over the warm pool of the western Pacific Ocean, the subtropical westerly jet stream (SWJ), and the tropical easterly upper jet stream (TEJ), and the 200-hPa zonal wind in East Asia. Composite analysis of the effects of SST, SWJ, and TEJ on pre- and post-abrupt changes in T a (annual temperature) and T min over the TP shows remarkable differences in SST, SWJ, and TEJ. A lag correlation between T a /T min , SST, and SWJ/TEJ shows that changes in SST occur ahead of changes in T a /T min by approximately one to three seasons. Partial correlations between T a /T min , SST, and SWJ/TEJ show that the effect of SWJ on T a /T min is more significant than the effect of SST. Furthermore, simulations with a community atmospheric model (CAM3.0) were performed, showing a remarkable increase in T a over the TP when the SST increased by 0.5 ? C. The main increase in T a and T min in the TP can be attributed to changes in SWJ. A possible mechanism is that changes in SST force the TEJ to weaken, move south, and lead to increased SWJ and movement of SWJ northward. Finally, changes in the intensity and location of the SWJ cause an increase in T a /T min . It appears that TP warming is governed primarily by coherent TEJ and SWJ variations that act as the atmospheric bridges to remote SSTs in warm-pool forcing.     

A model for inferring canopy and underlying soil temperatures from multi-directional measurements

Thermal emission is modeled from a canopy/soil surface, where the soil and the leaves are at different temperatures,T _g andT _c respectively. The temperatureT _m corresponding to a radiometer reading is given by $$B_\lambda (T_m ) = \chi B_\lambda (T_g ) + (1 - \chi )B_\lambda (T_c ) ,$$ whereB _λ denotes the Planck blackbody function at wavelength λ, χ specifies the fraction of the field of view occupied by the soil at a given view direction, and an emissivity of 1.0 is assumed for the plants and the soil. The dependence of the soil-fraction χ on the view direction and the structure is expressed by the viewing-geometry parameter, which allows for concise and simple formulation. We observe from our model that at large view zenith angles, only the plants are effectively seen (that is, χ tends to zero), and thereforeT _c can be determined from observations at large zenith angles, to the extent that such observations are practical. Viewing from the zenith, χ = exp(-L _hc), whereL _hc is the projection of the canopy leaf-area (per unit surface area) on a horizontal plane. For off-zenith observations, the soil-fraction χ depends on the distribution in the azimuth of the projected areas of various leaf categories, in addition to the dependence on the sum total of these projections,L _hc.L _hc, rather than the leaf-area index, emerges as the parameter characterizing the optical thickness of the canopy. Inferring bothT _c andT _g from observations from the zenith and from large zenith angles is possible ifL _hc is known from other measurements. Drooping of leaves under water-stress conditions affects the observed temperatureT _m in a complicated way because a leaf-inclination change produces a change inL _hc (for the same leaf area) and also a change in the dependence of χ on the view direction. Water stress can produce an increase of the soil-fraction χ and thus tends to produce an exaggerated increase in the observed temperature compared to the actual increase in canopy temperature. These effects are analyzed for a simulated soybean canopy.     

Ground-based GPS measurements: time behavior from half-hour to years

Ground-based GPS and weather stations time series for the period 2010–2012 of precipitable water vapor (PWV), relative humidity (RH), and surface temperature (T) of half-hourly resolution are analyzed to demonstrate their value for dynamical analyses and weather forecasting. Three sample stations in the USA from the SoumiNet network are considered, which have rather continuous data for the last 3 years and a few missing values. Results for the three stations reveal the following features: (1) PWV time behavior is dominated by the annual cycle superimposed on high-frequency fluctuations with missing daily cycle, indicating a prevailing large-scale transport source of precipitable water at these sites; (2) RH is characterized by the daily cycle and high-frequency variability, while the annual cycle is missing; (3) T mainly varies following the annual and diurnal cycles; and (4) all variables show similar scaling properties of their variance spectra, S(f)?～?f ^–β , with a high-frequency regime of red noise type scaling (β?～?2) up to a day and long-term persistence beyond a week (β?～?0.5), with a week-long frequency interval of transition. Detrended fluctuation analysis of relative humidity indicates a clear long-term persistence scaling covering more than three decades. Implications of these findings on weather forecasting and climate modeling are discussed.     

Comprehensive modeling of monthly mean soil temperature using multivariate adaptive regression splines and support vector machine

Soil temperature (T _s) and its thermal regime are the most important factors in plant growth, biological activities, and water movement in soil. Due to scarcity of the T _s data, estimation of soil temperature is an important issue in different fields of sciences. The main objective of the present study is to investigate the accuracy of multivariate adaptive regression splines (MARS) and support vector machine (SVM) methods for estimating the T _s. For this aim, the monthly mean data of the T _s (at depths of 5, 10, 50, and 100 cm) and meteorological parameters of 30 synoptic stations in Iran were utilized. To develop the MARS and SVM models, various combinations of minimum, maximum, and mean air temperatures (T _min, T _max, T); actual and maximum possible sunshine duration; sunshine duration ratio (n, N, n/N); actual, net, and extraterrestrial solar radiation data (R _s, R _n, R _a); precipitation (P); relative humidity (RH); wind speed at 2 m height (u ₂); and water vapor pressure (V_p) were used as input variables. Three error statistics including root-mean-square-error (RMSE), mean absolute error (MAE), and determination coefficient (R ²) were used to check the performance of MARS and SVM models. The results indicated that the MARS was superior to the SVM at different depths. In the test and validation phases, the most accurate estimations for the MARS were obtained at the depth of 10 cm for T _max, T _min, T inputs (RMSE = 0.71 °C, MAE = 0.54 °C, and R ² = 0.995) and for RH, V _p, P, and u ₂ inputs (RMSE = 0.80 °C, MAE = 0.61 °C, and R ² = 0.996), respectively.     

Thermals over the sea and gull flight behavior

The flight performance of Herring Gulls relative to specific atmosphere and ocean conditions over the western North Atlantic indicates that large groups of gulls are able, through cooperative flight maneuvers, to induce ascending convective flow (thermals) in which they make extended soaring flights. These group flights in gull-induced thermals are limited to winds of 0 to ~ 1 m s^?1 and to sea-minus-air temperature differences (δT) of ~3 to 6?C. As wind speed increases from ~ 2 to 5 m s^?1, thermals are naturally induced, and the minimum δT required for soaring is inversely related to wind speed. At higher winds (~5 to 13 ms^?1), the minimum positive δT and minimum wind speed required for thermal soaring are directly related, thus indicating an apparent maximum efficiency for the natural production of thermals at wind speeds of about 5 m s^?1 and δT of 1 to 2 ?C.     

Analysis of solar radiation over Egypt

Summary  The database utilized in this analysis consisted of daily sunshine duration and hourly global and diffuse radiation on a horizontal surface (for Matruh Cairo, and Aswan), and normal incidence beam radiation (for Cairo and Aswan only). Monthly-average hourly and daily values are reported for each of these three types of measured radiation, together with the calculated monthly-average daily values for the components of global radiation, horizontal beam and diffuse radiation. The monthly-average hourly and daily clearsky index values have also been calculated and analyzed. Monthly-average daily frequency distributions of the clearsky index values are reported for each month. The annual-average daily global irradiation values are 19.4, 18.67, and 21.78 MJ/m² and for diffuse irradiation they are 6.34, 6.65 and 6.23 MJ/m² for Matruh, Cairo and Aswan, respectively. For the normal incidence beam irradiation the annual-average daily values are 16.94 and 24.46 MJ/m² for Cairo and Aswan, respectively. The annual-average daily fractions of the direct component of horizontal global radiation are 0.70, 0.61 and 0.72 for the three stations, respectively. The annual-average daily values for the clearsky index are 0.585, 0.566, and 0.648, and the average frequency of clear days annually are 67.3, 42.3 and 77.6% respectively. The annual variations and trend analysis were analyzed for daily global, direct, and diffuse radiation on a horizontal surface, daily sunshine duration, and for the daily ratios G/G₀, and D/G for the stations Matruh, Cairo and Aswan. The distribution of these components of radiation and their ratios over the study stations in Egypt is also discussed. The results show that Egypt is characterized by relatively high average-daily radiation rates, both global and direct, and a relatively high frequency of clear days. Cairo, due to its urbanization and high pollution, has relatively low average-daily radiation rates, particularly in direct radiation, and the frequency of clear days. Received February 26, 1998 Revised February 4, 1999     

Turbulence characteristics in the atmospheric surface layer during summer monsoon of 1997 over a semi-arid location in India

A land surface processes experiment (LASPEX) was conducted in the semi-arid region of Northwest India during January 1997–February 1998. Analysis of turbulent components of wind and air temperature collected in the surface layer (SL) at Anand (22°35′N, 72°55′E) during the Indian summer monsoon season from June to September 1997 is presented. Turbulent fluctuation of wind components and air temperature observed at Anand varied as a function of terrain features and stability of the surface layer. Under neutral conditions, the standard deviation of vertical velocity (σ _w ) and temperature (σ _T ) were normalized using respective surface layer scaling parameter u _* and T _* which fitted the expressions σ _w /u _* = 1.25 and σ _T /T _* ≈ 4. Micrometeorological spectrum of wind and temperature at 5 m above ground level (AGL) at Anand showed peaks at time scale of 1–3 min at the low-frequency end. The inertial sub-range characteristics (?2/3 slope) of the spectrum are exhibited mostly. However, in some occasions, slope of ?1 denoting brown noise was depicted by the wind and temperature spectrum, which indicated anisotropy in turbulence.     

Day-of-the-week variations of urban temperature and their long-term trends in Japan

Temperature differences among days of the week and their long-term trends were evaluated using 29 years of hourly data from the Automated Meteorological Data Acquisition System network of Japan. Stations were categorized with respect to the population density around each site, and an urban temperature anomaly (δT*) was defined as a departure from the spatial average of nearby rural stations. On Saturdays and holidays (Sundays and national holidays), δT* was lower than on weekdays by 0.2–0.25°C at Tokyo, by 0.1–0.2°C at Osaka, and by about 0.02°C at stations where the population density was 300 to 1,000 km^–2. Moreover, δT* showed a relative decreasing trend over the long term on Mondays and an increasing trend on Fridays, at a rate of about 0.05–0.1°C decade^–1 at Tokyo and about 0.02°C decade^–1 at stations where the population density was 100 to 1,000 km^–2, but no significant difference in δT* trends was observed between weekdays and weekend days.     

横断山脉中段云岭白茫雪山的大气混浊度

本文根据1983和1984年横断山脉中段云岭白茫雪山东坡三个不同拔海高度(4292m、3760m和2025m)上测量的太阳直接辐射资料,计算并分析了云岭白茫雪山林克大气混浊因子T_P。T_P随拔海高度的升高而减小,不同拔海高度上T_p的年变化趋势基本一致,最大值出现在7、8月份,最小值在11月份。若将T_P分成气溶胶混浊因子T_R和水汽混浊因子T_W(T_P=1+T_R+T_W),水汽混浊因子T_W的年变化趋势与降水分布相吻合,气溶胶混浊因子T_R呈相反的变化趋势。地处干热河谷的奔子拦,由于局地气候的影响,其混浊度较大,接近海拔50m的北京,年平均值为3.09。     

Changes in integral and aerosol atmospheric turbidity in Trans-Baikal and Central Siberia

The analysis of time series of the Linke turbidity factors is performed for the atmosphere mass m = 2 (T ₂) and atmospheric aerosol optical thickness for the wavelength λ₀ = 0.55 μm (AOT) from the data of 14 actinometrical stations of Central Siberia and Trans-Baikal territory. It is shown that over the period from 1976 to 2006, the increased atmospheric transparency is observed in the region. Quantitative estimates of changes in multiyear mean annual variations of T ₂ and AOT at different periods of averaging and for different time periods are derived.