Introduction

The program CHECK performs consistency checks on the data in the database. To check the data of a certain day the observations from 18 UTC of the previous day until 12 UTC of the next day are needed. Consequently, CHECK can only be used after the data of the next day are added to the database with the program EXDEC. Only the data of the stations which will be part of the database (i.e. the stations marked with S) are subjected to the consistency checks. The values of the following observation elements are checked: air temperature, dew-point temperature, pressure at sea level, wind speed, amount of precipitation, amount of C_L clouds (or C_M clouds if no C_L clouds are present) and duration of sunshine. Obvious errors in the observations are automatically corrected and a message is written to the log file REALDATA:CHECK.LOG. Other possible errors are also written to this log file. These latter messages can be read by the OBSCON program, which also offers an operator the possiblity of modifying the data. After the observations of a station are checked (and if necessary corrected) the derived parameters are recalculated and the data are written back to the database DECODED.

Air temperature, maximum temperature and minimum temperature

The minimum temperature T_nT_nT_n is first checked on possible sign errors using the observed air temperatures between 18 and 6 UTC. When (a) the minimum value of the observed air temperatures is positive while T_nT_nT_n is negative, (b) the difference between these two values is more than 4°C, and (c) -T_nT_nT_n is less than the minimum value of the observed air temperatures, the minus sign of T_nT_nT_n is changed in a plus sign. In case the maximum value of the observed air temperatures between 18 and 6 UTC is less than 0°C while T_nT_nT_n is positive, the plus sign of T_nT_nT_n is changed in a minus sign. Next the value of T_nT_nT_n is checked by comparing it with the air temperatures between 18 and 6 UTC. When there are two or more values of TTT below the minimum temperature the value of T_nT_nT_n is changed to NOT DEFINED ('/'). In case only one observation of TTT is below the minimum temperature the value of T_nT_nT_n is changed to this value when the difference is less than 0.5°C. For differences of 0.5°C or more an error message is written to the log file.
Similarly, the maximum temperature T_xT_xT_x is compared with the observed air temperatures between 6 and 18 UTC. The value of T_xT_xT_x is changed to NOT DEFINED when there are two or more values of TTT which exceed the observed maximum temperature. In case only one value of air temperature is higher than the maximum temperature and the difference is less than 0.5°C T_xT_xT_x is changed to this value. When the difference is larger an error message is written to the log file.
Finally, the minimum and maximum temperature are compared with the values of air temperature before and near sunrise (three values of TTT) and around high noon (four values of TTT), respectively. When T_nT_nT_n is more than 5°C below all three values of TTT or T_xT_xT_x exceeds all four values of TTT (or three when the fourth observation is not present or not defined) with more than 5°C an error message is written to the log file.
The values of the air temperature TTT between 6 UTC of the day under consideration and 6 UTC of the next day are checked using a temperature curve. This curve is based on the two minimum temperatures and the maximum temperature reported in this period. When one or more of these values are not available they are calculated as follows:

• the minimum temperature T_nT_nT_n is set at a value of 1°C below the air temperature TTT which is observed at the observation time closest to the time of sunrise plus half an hour. When this value is not available T_nT_nT_n is set at 1°C below the value observed three hours earlier. Furthermore, T_nT_nT_n must be lower than all values of TTT between 18 and 6 UTC and if necessary the value of T_nT_nT_n is changed to the lowest observed value. In case both values of TTT near sunrise are not available the minimum temperature is set at NOT DEFINED;
• the maximum temperature T_xT_xT_x is set at 1°C above the air temperature TTT which is observed at the observation time closest to the time of high noon plus two hours. When this value is not available T_xT_xT_x is set at 2°C above the value of TTT observed three hours later. Furthermore, T_xT_xT_x must exceed all values of TTT between 6 and 18 UTC and if necessary the value of T_xT_xT_x is changed to the highest observed value. In case both values of TTT near high noon plus two hours are not available the maximum temperature is set at NOT DEFINED.

Note that the computed values of T_nT_nT_n and T_xT_xT_x are only used for the construction of the temperature curve, but are not stored in the database.
The temperature curve is only constructed when values for both minimum temperatures and the maximum temperature could be determined (i.e. none of these values is NOT DEFINED). From the whole hour closest to the time of sunrise plus half an hour (t 1 ) until the whole hour closest to half way between high noon plus two hours and sunset (t 2 ) a sine function is used:

where TTT(t) is the air temperature at time t (t₁ ≤ t ≤ t₂) and T_nT_nT_n is the minimum temperature observed at 6 UTC of the day under consideration. An exponential curve is used from time t₂ until time t₁ + 24 hours (t3):

where t is the observation time (t₂ ≤ t ≤ t₃) and T_nT_nT_n ' is the minimum temperature observed at 6 UTC of the day after the day under consideration.
The observed values of air temperature TTT are compared with the values obtained from the temperature curve. In case an observed value of TTT deviates more than 10°C from the computed value while the minimum and maximum temperatures were found to be correct or were automatically corrected the value of TTT is changed to NOT DEFINED. When the observed air temperature deviates between 6°C and 10°C from the value obtained from the curve and this difference can not be explained by the presence of heavy showers or thunderstorms (ww-codes above 80) an error message is written to the log file. An error message is also written to the log file when the deviation is more than 6°C and errors were found in the minimum and/or maximum temperature which could not automatically be corrected.

Dew-point temperature

The dew-point temperature T_dT_dT_d is first compared with the dew-point temperatures at the same station observed three hours before and three hours later. When the difference with both of these values is more than 8°C and the difference between these two values is less than 8°C T_dT_dT_d is changed to NOT DEFINED ('/'). In case one of the neighbouring observations of dew-point temperature is missing and the difference of the observation with the only other neighbouring observation present is more than 8°C an error message is written to the log file. Furthermore, the dew-point temperature is compared with the air temperature at the same time: T_dT_dT_d must be less than or equal to TTT. When T_dT_dT_d exceeds TTT with less than 0.3°C T_dT_dT_d changed to the value of TTT. For differences of 0.3°C or more an error message is written to the log file.

Amount of C_L clouds (or C_M clouds if no C_L clouds present)

The program checks whether the amount of C_L clouds (or C_M clouds if no C_L clouds are present) N_h is less than or equal to the total cloud cover N. When this is not the case the value of N_h is changed to the value of N.

Amount of precipitation

First the program checks whether the amount of precipitation RRR reported for the six hour period (i.e. t_R = 6) is equal to the amount reported for the twelve hour period (t_R = 12). When the reported amounts are equal no further checking is done, because it is unlikely that the same mistake is made twice. An exception is made for precipitation amounts above 30 millimetres: these amounts must always be checked manually using OBSCON and therefore an error message is written to the log file. In all other cases the observed weather conditions (i.e. ww, W₁ and W₂ ) are used for some more checks.
If there has only been fog in the period t_R and no other source of precipitation (i.e. all ww-codes present are between 40 and 49 and all W₁ -codes are less than or equal to 4) no more than 1 millimetre of precipitation is allowed. When RRR exceeds this value an error message is written to the log file.
In table III.1 relative precipitation intensities are given for all values of ww, W₁ and W₂ , which are used when fog is not a source of precipitation or not the only source in the period t_R .

For every observation of a station in the period tR for which at least the present weather ww or the past weather W₁ is available the total precipitation intensity is found by adding the intensities corresponding with ww, W₁ and W₂. Furthermore, the maximum intensity is determined from ww, W₁ or W₂. When ww and W₁ are not observed or NOT DEFINED ('/') the total intensity is set at 1 for that particular observation, while the maximum intensity remains 0. Next the total intensities of all observations of a station in the period tR are added to give the overall intensity and the maximum intensity during this period is determined. The amount of precipitation reported is allowed when one of the following criteria is met:

• the overall intensity is 8 or more for amounts between 20 and 30 millimetres
• the overall intensity is 7 or more for amounts between 15 and 20 millimetres
• the overall intensity is 6 or more for amounts between 10 and 15 millimetres
• the overall intensity is 5 or more for amounts between 8 and 10 millimetres
• the overall inentsity is 4 or more for amounts between 6 and 8 millimetres
• the overall intensity is 3 or more for amounts between 4 and 6 millimetres
• the overall intensity is 2 or more for amounts between 3 and 4 millimetres
• the overall intensity is 1 or more for amounts between 1 and 3 millimetres
• the maximum intensity is 4 for amounts between 20 and 30 millimetres
• the maximum intensity is 3 or more for amounts between 8 and 20 millimetres
• the maximum intensity is 2 or more for amounts between 3 and 8 millimetres
• the maximum intensity is 1 or more for amounts between 1 and 3 millimetres
• the amount of precipitation is less than 1 millimetre

When the observed value of RRR is does not correspond to the precipitation intensity or when the amount of precipitation exceeds 30 millimetres an error message is written to the log file. Finally, the program compares the amount of precipitation observed in the six hour period with the amount observed in the twelve hour period: the amount observed in the latter period must be equal to or exceed the amount reported six hours before. If this is not the case an error message is written to the log file.

Pressure at sea level

The value of the pressure at sea level PPPP must lie between 950 hP_a and 1060 hP_a. When PPPP is less than 950 hP_a or more than 1060 hP_a the value of PPPP is changed to NOT DEFINED ('/'). Furthermore, the values of the amount of pressure tendency ppp and of the pressure at station level P_oP_oP_oP_o are also changed to NOT DEFINED.
If the pressure at sea level is between 950 and 1060 hP_a the pressure tendency ppp is checked: when the absolute value of ppp exceeds 15 hP_a per three hours ppp is changed to NOT DEFINED. Since it is very likely that there are also errors in the pressure at sea level and the pressure at station level, the values of PPPP and P_oP_oP_oP_o are also changed to NOT DEFINED.
Finally, if the pressure at sea level is between 950 and 1060 hP_a and the pressure tendency is less than 15 hP_a per three hours, PPPP is compared with the observation of three hours before. When the difference between the PPPP of three hours before and the PPPP under consideration exceeds the pressure tendency ppp more than 2 hP_a the values of PPPP, P_oP_oP_oP_o and ppp are changed to NOT DEFINED.

Wind speed

The observed wind speed ff is compared with the observed value of three hours before at the same station. The program checks on an increase in ff of more than 20 knots. Such an increase is only allowed if (a) heavy showers or thunderstorms are coded in the observation (i.e. ww-codes above 80), (b) the pressure tendency is more than 3 hP_a per three hours or (c) the difference in wind speed with the observation of three hours later is less than 20 knots. However, when the increase in wind speed exceeds 40 knots the value of ff is changed to NOT DEFINED ('/'). In all other cases (i.e. increases between 20 and 40 knots which can not be explained) an error message is written to the log file.
In case the wind speed of three hours before is not known ff is only checked on its value: when the wind speed is higher than 80 knots an error message is written to the log file.

Duration of sunshine

The program checks whether the observed duration of sunshine 55j₂j₃j₄ exceeds the calculated time between sunrise and sunset. When the observed sunshine duration is more than 25% longer than this computed day length the sunshine duration is set at NOT DEFINED ('/'). In all other cases an error message is written to the log file.

Messages

The messages generated by the CHECK program are written to the file REALDATA:CHECK.LOG. In table III.2 the possible messages are described. Note the difference between %I-messages (information) and %E-messages (errors).