Total Precipitable Water Values Research Articles

Evaluation of PWV products derived from satellite-based and radiosonde retrievals for the eastern anatolia observatory (DAG)

In this study, we have presented the results of the precipitable water vapor (PWV) for the Eastern Anatolia Observatory (in Turkish: Doğu Anadolu Gözlemevi, the acronym is DAG) site in Erzurum, Türkiye. The DAG has Türkiye’s largest and the first near infrared (NIR) telescope with a mirror diameter of 4 meters at the altitude of 3170 m. The DAG telescope is going to take the first light in the end of summer 2024. This study is focused on the examining of the precipitable water vapor data for the NIR observations at the DAG. In this context, the NWC SAF Total Precipitable Water (TPW) data obtained by both the satellite based and the radiosonde balloon validated with six radiosonde stations were examined by temporal, vertical and horizontal analyses for the DAG site between June 2019 to December 2020. The results obtained from these analyzes indicate that the mean and median TPW values at the DAG site were approximately 7 mm and the minimum and maximum values were 0.59 mm and 24.12 mm, respectively. The monthly median TPW values at the DAG site varied between approximately 3-10 mm, with a decreasing trend from June to January and an increase in the first seven months of 2020. These results also indicate that the TPW data obtained by its 15 minutes temporal resolution, aligns closely with the radiosonde measurements. Furthermore, the values of PWV at both lower and upper levels of the atmosphere are minimal while the values increase slightly in the middle layer of the atmosphere. As a result, the effective monitoring of the PWV in a site would result in the generation of higher quality astronomical IR observations and be important in terms of the optimum operating cost for an observatory.

Precipitable water characterization using global navigation satellite system data: A case study in Nghia Do area, Vietnam

This paper evaluates the estimated total precipitable water (TPW) and studies its characteristics in the Nghia Do area (Hanoi) using Global Positioning System (GPS) data and the Canadian Spatial Reference System-Precise Point Positioning (CSRS-PPP) processing technique and calculation tool. The TPW was estimated from GPS data from September 22, 2022 to March 31, 2023 using CSRS-PPP provided by Natural Resources Canada. The calculated TPW was validated with TPW products from Aeronet data and radiosonde data. Taking advantage of its high time resolution, the TPW estimated from GPS data was used to analyze the temporal variation of TPW during cold surges affecting the Nghia Do area. The results indicate a strong agreement between the estimated TPW from GPS data and TPW products derived from Aeronet and radiosonde data. The mean error (ME), root mean square error (RMSE), and correlation coefficient between the estimated TPW from GPS data and the Aeronet-derived product are 0.68 mm, 2.05 mm, and 0.988, respectively. The corresponding values between the estimated TPW and the radiosonde-derived product are -3.01 mm, 3.24 mm, and 0.996, respectively. The study of TPW variation over the Nghia Do area during the research period revealed that, generally, TPW values in December and January were lower than those in the other months. Before the arrival of a specific cold surge at a station, TPW tends to increase by about 6 mm within 12 hours as convection activities ahead of the cold front intensify. After the cold air passes over the station, the TPW value decreases by about 8 mm within 12 hours due to the influx of cold and dry winter air brought by the prevailing winds. This distinct TPW variation pattern suggests that GPS data can be effectively employed to define the arrival of cold surges in the station area.

Variations of Pre-monsoon season related atmospheric parameters over Kakinada region

Pre-monsoon showers occur before the beginning of the rainy season. From the months of March to May, they take place, and can be anything from little drizzles to powerful thunderstorms. March, April and May are known as the pre-monsoon season (PRMS). The precipitation patterns recorded in PRMS are critical because they have an impact on a wide range of crop-related operations across the country. During PRMS, the maximum temperature (TMAX), minimum temperature (TMIN), soil moisture, relative humidity, latent heat, convective available potential energy (CAPE), and total precipitable water (TPW) were analysed at Kakinada region. These variables were collected using daily ERA5 reanalysis data for the PRMS from 1981 to 2021. Studying the convection-related characteristics over the Kakinada station during the PRMS was our goal. During the study period, the five years with the highest PRMS rainfall were 1990, 1995, 2008, 2010 and 2016 and the five years with the lowest rainfall were 1990, 1995, 2008, 2010 and 2016. In the months leading up to the monsoon, the Kakinada station experiences CAPE values between 1000 and 6000 J/kg, while TPW values are between 25 and 60 mm. The PRMS values for CAPE and TPW both show that the prerequisites for moderate to severe convection activity have been fulfilled. TPW, soil moisture, relative humidity, and CAPE parameters during PRMS were well estimated using the ARMA and ARIMA models.

Improvement of GOCI-II Water Vapor Absorption Correction through Fusion with GK-2A/AMI Data

In remote sensing of the ocean color, in particular, in coarse-resolution global model simulations, atmospheric trace gases including water vapor are generally treated as auxiliary data, which create uncertainties in atmospheric correction. The second Korean geostationary satellite mission, Geo-Kompsat 2 (GK-2), is unique in combining visible and infrared observations from the second geostationary ocean color imager (GOCI-II) and the advanced meteorological imager (AMI) over Asia and the Pacific Ocean. In this study, we demonstrate that AMI total precipitable water (TPW) data to allow realistic water vapor absorption correction of GOCI-II color retrievals for the ocean. We assessed the uncertainties of two candidate TPW products for GOCI-II atmospheric correction using atmospheric sounding data, and then analyzed the sensitivity of four ocean-color products (remote sensing reflectance [Rrs], chlorophyll-a concentration [CHL], colored dissolved organic matter [CDOM], and total suspended sediment [TSS]) for GOCI-II water vapor transmittance correction using AMI and global model data. Differences between the TPW sources increased the mean absolute percentage error (MAPE) of Rrs from 2.97% to 6.43% in the blue to green bands, higher than the global climate observing system requirements (<5%) at 412 nm. By contrast, MAPE values of 3.53%, 6.18%, and 7.71% were increased to 6.63%, 13.53%, and 16.14% at high sun and sensor zenith angles for CHL, CDOM, and TSS, respectively. Uncertainty analysis provided similar results, indicating that AMI TPW produced approximately 3-fold lower error rates in ocean-color products than obtained using TPW values from the National Centers for Environmental Prediction. These results imply that AMI TPW can improve the accuracy and ability of GOCI-II ocean-color products to capture diurnal variability.

In‐Depth Evaluation of MiRS Total Precipitable Water From NOAA‐20 ATMS Using Multiple Reference Data Sets

AbstractThe National Oceanic and Atmospheric Administration (NOAA) Microwave Integrated Retrieval System (MiRS) algorithm has been producing retrieval products from NOAA‐20 Advanced Technology Microwave Sounder (ATMS) data since shortly after launch in late 2017. Retrieval products from the Suomi National Polar‐orbiting Partnership (SNPP) ATMS data have been similarly available since 2012. Among the products routinely generated is Total Precipitable Water (TPW), based on the vertical integral of the MiRS retrieved water vapor profile. In this study, TPW data from NOAA‐20 ATMS have been the subject of an in‐depth evaluation. The evaluation included characterization of dependence on orbital node (ascending/descending), season, surface type, scan angle, and meridional (zonally averaged) variation. Due to the similarity between NOAA‐20 and SNPP derived TPW, the emphasis here is on the NOAA‐20 evaluation. The globally averaged values of TPW for 2019 are 25.41 and 25.39 mm from NOAA‐20 and SNPP ATMS, respectively, for combined orbits (ascending and descending). For combined orbits, the bias and standard deviation of MiRS NOAA‐20 ATMS TPW with respect to European Centre for Medium‐Range Weather Forecasts/Global Data Assimilation System (ECMWF/GDAS) analyses are 0.79/1.76 mm and 3.65/3.55 mm, respectively, at 10°N where part of annual mean Intertropical Convergence Zone (ITCZ) is located, while at 40°N, corresponding bias and standard deviation values are 0.84/0.67 mm and 3.00/2.55 mm, respectively. The yearly averaged horizontal distribution of MiRS TPW retrieved from NOAA‐20 ATMS is highly correlated with that of ECMWF and GDAS analyses data, with a correlation coefficient of ∼1.

Intercomparison of Total Precipitable Water Derived From COSMIC-2 and Three Different Microwave Radiometers Over the Ocean

Total precipitable water (TPW) values derived from Constellation Observing System for Meteorology, Ionosphere and Climate-2 (COSMIC-2) are compared with those derived from Special Sensor Microwave Imager Sounder (SSMIS), Global Precipitation Measurement (GPM) Microwave Imager (GMI), and Advanced Microwave Scanning Radiometer-2 (AMSR-2) over the ocean from October 1, 2019 to February 16, 2020. The overall comparison results indicate that TPW values derived from SSMIS, AMSR-2, and GMI have a good correlation and agreement with COSMIC-2 TPW values with the correlation coefficients greater than 0.99 and root mean square (rms) no greater than 2.7 mm. We compare TPW derived from three different microwave radiometers with COSMIC-2 TPW over the subtropical and tropical oceans. The differences illustrate that TPW values derived from three different microwave radiometers are more consistent with COSMIC-2 TPW values over the subtropical ocean than those over the tropical ocean. In addition, we also analyze the relationship between the TPW retrieval accuracy derived from three different microwave radiometers and environmental factors, including cloud, rain rate, wind speed, and surface temperature. The results indicate that four environmental factors have an important influence on the TPW retrieval from three different microwave radiometers.

An Empirical Model for Rainfall Maximums Conditioned to Tropospheric Water Vapor Over the Eastern Pacific Ocean

One of the most difficult weather variables to predict is rain, particularly intense rain. The main limitation is the complexity of the fluid dynamic equations used by predictive models with increasing uncertainties over time, especially in the description of brief, local and high intensity precipitation events. Although computational, instrumental and theoretical improvements have been developed for models, it is still a challenge to estimate high intensity rainfall events, especially in terms of determining the maximum rainfall rates and the location of the event. Within this context, this research presents a statistical and relationship analysis of rainfall intensity rates, total precipitable water (TPW), and sea surface temperature (SST) over the ocean. An empirical model to estimate the maximum rainfall rates conditioned to TPW values is developed. The performance of the maximum rainfall rate model is spatially evaluated for a case study. High-resolution TRMM 2A12 satellite data with a resolution of 5.1 km x 5.1 km and 1.67 seconds was used from January 2009 to December 2012, over the Eastern Pacific Nino area in the tropical Pacific Ocean (0-5°S; 90-81°W), comprising 326,092 rain pixels. After applying the model selection methodology, i.e. the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC), an empirical exponential model between the maximum possible rain rates conditioned to TPW was found with R2 = 0.96, indicating that the amount of TPW determines the maximum amount of rain that the atmosphere can precipitate exponentially. Spatially, this model unequivocally locates the rain event, however the rainfall intensity is underestimated in the convective nucleus of the cloud. Thus, these results provide an additional constraint for maximum rain intensity values that should be adopted in dynamic models, improving the quantification of heavy rainfall event intensities and the correct location of these events.

An Optimal Estimation Retrieval Algorithm for Microwave Humidity Sounding Channels with Minimal Scan Position Bias

AbstractA flexible and physical optimal estimation-based inversion algorithm for retrieving atmospheric water vapor and cloud liquid water path from passive microwave radiometers over the global oceans is presented. The algorithm’s main strength lies in its ability to explicitly account for forward model errors that depend on the Earth incidence angle (EIA) at which a given radiometer measurement is made. Validation of total precipitable water (TPW) retrieved from Microwave Humidity Sounder (MHS) measurements against near-coincident estimates of TPW from SuomiNet GPS ground stations shows that retrieved TPW values agree closely with SuomiNet estimates, and somewhat better than values from the Microwave Integrated Retrieval System that are retrieved from the same MHS instruments. More importantly, it is found that the inclusion of appropriate forward model error assumptions, which are tailored to the EIA and sea surface temperature of the scene being considered, are able to almost entirely eliminate EIA-dependent biases in retrieved TPW. This result holds true across all satellites currently carrying an MHS instrument, despite the fact that only measurements from one satellite are used to estimate forward model errors. The consistency achieved by the retrieval algorithm across all view angles suggests that other inversion algorithms, particularly those for cross-track-scanning radiometers and potential future constellations of small satellites, would benefit from the inclusion of nuanced error assumptions that consider the effect of EIA.

Potential of INSAT-3D sounder-derived total precipitable water product for weather forecast

Abstract. The objectives of the INSAT-3D satellite are to enhance the meteorological observations and to monitor the Earth's surface for weather forecasting and disaster warning. One of the weather-monitoring capabilities of the INSAT-3D sounder is the estimation of water vapour in the atmosphere. The amount of water vapour present in the atmospheric column is derived as the total precipitable water (TPW) product from the infrared radiances measured by the INSAT-3D sounder. The present study is based on TPW derived from INSAT-3D sounder, radiosonde (RS) observations and the corresponding National Oceanic and Atmospheric Administration (NOAA) satellite. To assess retrieval performances of INSAT-3D sounder-derived TPW, RS TPW observations are considered for the validation from May to September 2016 from 34 stations belonging to the India Meteorological Department (IMD). The analysis is performed on daily, monthly, and subdivisional bases over the Indian region. The comparison of INSAT-3D TPW with RS TPW on daily and monthly bases shows that the root mean square error (RMSE) and correlation coefficients (CC) are ∼8 mm and 0.8, respectively. However, on subdivisional and overall scales, the RMSE found to be in the range of 1 to 2 mm and CC was around 0.9 in comparison with RS and NOAA. The spatial distribution of INSAT-3D TPW with actual rainfall observation is also investigated. In general, INSAT-3D TPW corresponds well with rainfall observation; however, it has found that heavy rainfall events occur in the presence of high TPW values. In addition, the cases of thunderstorm events were assessed using TPW from INSAT-3D and network of Global Navigation Satellite System (GNSS) receiver. This shows the good agreement between TPW from INSAT-3D and GNSS during the mesoscale activity. The improvement in the estimation of TPW is carried out by applying the GSICS calibration corrections (Global Space-based Inter-Calibration System) to the radiances from infrared (IR) channels of the sounder, which is used by IMDPS (INSAT Meteorological Data Processing System). The current TPW from INSAT-3D satellite can be utilized operationally for weather monitoring and forecast purposes. It can also offer substantial opportunities for improvement in nowcasting studies.

Interannual and intraannual variations in total precipitable water over the North Atlantic from satellite microwave measurements

Based on the long-term data of satellite microwave radiometers, the estimates are obtained of interannual and intraannual variations in monthly mean values of total precipitable water in the atmosphere over the North Atlantic from 1988 to 2011, in particular, in the regions characterized by the maximum rate of heat and moisture exchange between the ocean and atmosphere such as the Gulf Stream, Newfoundland, and Norway-Greenland energy-active zones. Long-term trends in total precipitable water in the atmosphere over these regions are estimated. The variations in total precipitable water in the atmosphere in 2010 are noted which were caused by oil spills in the Gulf of Mexico (the spring of 2010) and by the severe drought in the European part of Russia (the summer of 2010).

The comparison of IR and MW ground-based measurements of total precipitable water

Water vapor is one of the basic climate gases playing a key role in various processes at different altitudes of the Earth’s atmosphere. An intercomparison and validation of different total precipitable water (TPW) measurement methods are important for determining the true accuracy of these methods, the shared use of data from multiple sources, the creation of data archives of different measurements, etc. In this paper, the TPW values obtained from measurements of solar IR spectral radiation (~8–9 μm absorption band) and thermal MW radiation of the atmosphere (1.35 cm absorption line) for 138 days of observation are compared. Measurements have been carried out from March 2013 to June 2014 at Peterhof station of the St. Petersburg State University in (59.88° N, 29.82° E). It is shown that MW measurements usually give higher TPW values than IR measurements. The bias between the two methods varies from 1 to 8% for small and large TPW values, respectively. With increasing TPW values, the bias reduces and for TPW > 1 cm it is ~1%. Standard deviation (SD) between the two methods reaches 7% for TPW 1 cm. These data show the high quality of both remote sensing methods. Moreover, the IR measurements have a higher accuracy than MW measurements for small TPW values.

Observations of the convective environment in developing and non‐developing tropical disturbances

AbstractAnalyses of thermodynamic data gathered from airborne dropwindsondes released from the upper troposphere during the Pre‐Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment are presented. We focus on two systems that finally became hurricanes Karl and Matthew, and one system (Gaston) that attained tropical storm status, but subsequently weakened and never redeveloped during five days of monitoring. Data for all events show that the largest values of total precipitable water are collocated with the surface trough and with values of convective available potential energy that seem high enough to support convective organization. These values coincide mostly with low values of convective inhibition. Vertical profiles of virtual potential temperature show little variability between soundings on a particular day, but the system means from day to day show a slight warming. In contrast, vertical profiles of pseudo‐equivalent potential temperature, θe, show much more variability between soundings on a particular day on account of the variability in moisture.In all systems, there was is a tendency for the lower troposphere to moisten, but in the non‐developing system, the troposphere became progressively drier in the height range between approximately 2 and 9 km during the five days of observations. In the developing systems, the troposphere moistened. The most prominent difference between the non‐developing system and the two developing systems was the much larger reduction of θe between the surface and a height of 3 km, typically 25 K in the non‐developing system, compared with only 17 K in the developing systems. Conventional wisdom would suggest that, for this reason, the convective downdraughts would be stronger in the non‐developing system and would thereby act to suppress the development. Here we propose an alternative hypothesis in which the drier air weakens the convective updraughts and thereby the convective amplification of absolute vorticity necessary for development. Copyright © 2012 Royal Meteorological Society

Water Vapor Profiling From CoSSIR Radiometric Measurements

Previous water vapor profiling by millimeterwave radiometry using the 183-GHz absorption line is generally limited to an altitude range of 0-11 km. The additional measurements at the frequencies of 380.2 0.8, 380.2 1.8, 380.2 3.3, and 380.2 6.2 GHz by the new airborne compact scanning submillimeterwave imaging radiometer (CoSSIR) reported in this paper can extend this profiling capability up to an altitude of about 15 km. This is demonstrated by recent CoSSIR measurements onboard the NASA WB-57 aircraft in a flight from Texas to Costa Rica on January 14, 2006. Retrievals of water vapor mixing ratio were performed at eight altitudes of 1, 3, 5, 7, 9, 11, 13, and 15 km from the CoSSIR data set acquired at observational angles of 0 and 53.4. The results were compared with other available measurements from near-concurrent satellites. A very good agreement was found between the collocated values of total precipitable water (TPW) derived from the CoSSIR-retrieved water vapor profiles and those estimated from Tropical Rainfall Measuring Mission Microwave Imager; the average TPW differences range between 0.30 and 0.64 cm, depending on CoSSIR's observational angles. The accuracy of the retrievals was inferred from an analysis of inflight CoSSIR radiometric signal fluctuations.

Evaluation of daily and diurnal signals of total precipitable water (TPW) over the Indian Ocean based on TMI retrieved 3‐day composite estimates and radiosonde data

AbstractTropical rainfall measuring mission (TRMM) TRMM microwave imager (TMI) retrieved total precipitable water (TPW) has been shown in previous studies to be useful for characterizing energy transport and rainfall variability over water bodies. In this study, 3‐day composite TPW values during the monsoon months of June–September in 1999 and 2003 were evaluated against radiosonde derived TPW over the coastal regions of Indian Ocean. The radiosonde measurements were from coastal locations, island stations and ship‐based observations made during the 1999 Bay of Bengal Monsoon Experiment (BOBMEX). Our study is one of the first attempts to compare TMI‐derived TPW with the BOBMEX data. Root mean square (RMS) error of 8.1 mm and a bias of −0.76 mm were obtained for TMI 3‐day composite TPW data with respect to the radiosonde data over the Indian coastal regions. TMI overestimated (underestimated) radiosonde measurements under dry (wet) atmospheric conditions. Daily TMI‐derived TPW was also validated with radiosonde measurements from the BOBMEX data for the period 21 July–28 August 1999. Diurnal signals of TPW over Bay of Bengal were found to be comparable to the in situ measurements during the same period. Copyright © 2006 Royal Meteorological Society

Evaluation of water vapor distribution in general circulation models using satellite observations

This paper presents a comparison of the water vapor distribution obtained from two general circulation models, the European Centre for Medium‐Range Weather Forecasts (ECMWF) model and the National Center for Atmospheric Reseach (NCAR) Community Climate Model (CCM), with satellite observations of total precipitable water (TPW) from SSM/I and upper tropospheric relative humidity (UTH) from GOES. Overall, both models are successful in capturing the primary features of the observed water vapor distribution and its seasonal variation. For the ECMWF model, however, a systematic moist bias in TPW is noted over well‐known stratocumulus regions in the eastern subtropical oceans. Comparison with radiosonde profiles suggests that this problem is attributable to difficulties in modeling the shallowness of the boundary layer and large vertical water vapor gradients which characterize these regions. In comparison, the CCM is more successful in capturing the low values of TPW in the stratocumulus regions, although it tends to exhibit a dry bias over the eastern half of the subtropical oceans and a corresponding moist bias in the western half. The CCM also significantly overestimates the daily variability of the moisture fields in convective regions, suggesting a problem in simulating the temporal nature of moisture transport by deep convection. Comparison of the monthly mean UTH distribution indicates generally larger discrepancies than were noted for TPW owing to the greater influence of large‐scale dynamical processes in determining the distribution of UTH. In particular, the ECMWF model exhibits a distinct dry bias along the ITCZ and a moist bias over the subtropical descending branches of the Hadley cell, suggesting an underprediction in the strength of the Hadley circulation. The CCM, on the other hand, demonstrates greater discrepancies in UTH than are observed for the ECMWF model, but none that are as clearly correlated with well‐known features of the large‐scale circulation.

Water vapor profiling over ocean surface from airborne 90 and 183 GHz radiometric measurements under clear and cloudy conditions

Radiometric measurements at 90 GHz and three sideband frequencies near the water vapor absorption line at 183.3 GHz were made with the Advanced Microwave Moisture Sounder (AMMS) aboard the NASA DC-8 aircraft over some regions of the Pacific Ocean during November 1989. These measurements were used to retrieve atmospheric water vapor profiles over ocean surface using the algorithm developed by T.T. Wilheit (1979). The algorithm incorporates a mechanism to estimate cloud liquid water when the estimated relative humidity is greater than 95%. The results are compared with the estimated values from the measurements of Special Sensor Microwave Imager (SSMI) and TIROS Operational Vertical Sounder (TOVS). The water vapor profiles estimated from AMMS are generally higher at low altitudes and lower at high altitudes compared to those from the TOVS measurements. Values of total precipitable water estimated from the AMMS and SSM/I are in general agreement. Cloud liquid water vapor profiles retrieved from the AMMS show more fluctuations than those from SSM/I.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Determination of oceanic total precipitable water from the SSM/I

Results are presented of calibration/validation studies showing the ability of the Special Sensor Microwave/Imager (SSM/I) to measure total precipitable water in the atmosphere over the ocean. Comparisons between radiosondes and the SSM/I are presented for three different algorithms. The results show the possibility of a distinct improvement in the retrieval of total precipitable water over the ocean. The global, nonlinear algorithm is more sensitive to cloud liquid water content, rain, and sea ice. The additional sensitivity is due to the screening of rain and sea ice from the dependent data and the squared term in the retrieval algorithm. Thus, it will be very important to have good screening procedures for identifying these conditions. The linear algorithm overestimates in the mid-range and underestimates at large values of total precipitable water. The explanation for this effect is probably related to the selection of the center of the water vapor line as the operating frequency of the SSM/I water vapor channel. The line center is most likely to exhibit a saturation effect at large water vapor amounts, and pressure and temperature effects can also be important, depending on the distribution of water vapor in the atmosphere. >

Humidity and Turbidity Parameters in Sahel: A Case Study for Niamey (Niger)

Abstract Relationships between the daily values of total precipitable water in the atmosphere and the surface humidity parameters are presented from sunphotometric investigations made daily during the year 1981 and 1982. Turbidity parameters are given in terms of their frequency of occurrence.

The Relationship between Total Precipitable Water and Surface Dew Point

Mean monthly and annual values of total precipitable water are compared with mean monthly and annual values of surface dew point for each 5° latitude circle from the equator to 85°S. The correlation coefficients are lower for latitudes 0–20°S than for latitudes 25–85°S. The degree of relationship between mean monthly values of total precipitable water and mean monthly values of surface dew point is good in winter for all latitudes but much weaker in warmer months for latitudes 0–20°S. The computed regression equations give excellent estimates of monthly mean zonally averaged precipitable water from surface dew point for middle and higher latitudes in cooler seasons. Zonally averaged values of the moisture profile power index λ for the Southern Hemisphere are presented. This power index is highly variable (−0.26 to +4.57) even for mean monthly moisture profiles. These results are useful for the zonally averaged climatic models.

A Spectrophotometer for the Determination of the Water Vapor in a Vertical Column of the Atmosphere

There is described a simple photoelectric spectrophotometer, developed to determine the precipitable water in a vertical column of the atmosphere. A transmission replica grating is used as the dispersing element and an infra-red-sensitive photo-tube as the energy receiver. The photo-tube current is amplified by a negative feed-back d.c. amplifier and is read either by means of a 500 microampere ammeter or recorded by a high speed potentiometer recorder. Measurements are obtained by comparing the radiant flux in the 0.94μ water vapor absorption band with that at 1.01μ where no absorption occurs. A single-mirror heliostat is utilized to keep the spectrophotometer focused on the sun. An approximate calibration was obtained by plotting the spectrophotometer ratios against the total precipitable water values in a vertical column of the atmosphere as calculated from radiosonde moisture data.