Weather Statistics for CTIO

These pages describe average weather conditions at CTIO so that prospective observers can estimate the number of nights required for observing programs. Please note that the statistics that follow are based on specific definitions of photometric and spectroscopic night.

Contents

QUALITY OF NIGHTS

Figure 1 shows the average number of photometric hours per night for each month. The average number of observational hours (photometric plus spectroscopic) per night for each month is show in Figure 2.

A photometric night is defined here as one with over 6 continuos hours of cloudless sky.

A spectroscopic night as one with at most 3/8 cloud cover for 6 or more continuous hours.

These graphs have been calculated from data averaged over a period of 24 years (1965 - 1989, excluding 1968). Tabla 1 lists the number of hours per night available throughout the year.

Please note that these statistics are representative of the definitions as stated above and DO NOT reflect the number of hours in which the telescope were used.

TEMPERATURE

Figure 3 illustrates the temperature variations throughout the year averaged over data from a 23 year period (1965 to 1988). Maximum, minimum and average temperatures are included. The median temperature has been calculated for the years 1975 to 1988 only, as data from earlier years were not available.

PRECIPITATION

Figure 4 shows the total amount of precipitation for each year from 1965 to 1989.

GENERAL INFORMATION

Atmospheric conditions are excellent at CTIO. The months with the best seeing and least cloudiness extend from November through March. During these months 77.5% of th nights are normally of photometric quality and 88.6% are useful. From April through October 49.5% of the nights may be expected to be of photometric quality and 69.5% useful.

Diurnal temperature variations are normally 50 to 68 degrees fahrenheit and during the nighttime hours the variations are about 6 to 11 degrees fahrenheit throughout the year. For infrared observing the winter months are the best. The amount of precipitable water in the atmosphere is comparable then to that of the better known IR observing sites.

Tabla 1 : AVERAGE HOURS PER NIGHT AT CTIO * 

            Maximum Hrs.     Average Hrs.       Average Hrs.   
Month       Possible         Photometric        Observational **

January         8.5              6.7                 7.7
February        9.5              7.9                 8.9
March          10.5              8.1                 9.2
April          11.5              7.4                 9.5
May            12.1              5.2                 8.1
June           12.4              5.0                 8.3
July           12.3              5.2                 7.4
August         11.8              5.7                 8.0
September      10.9              5.7                 7.6
October         9.9              5.5                 7.2
November        8.9              6.0                 7.2
December        8.2              6.6                 7.3
* Determined from statistics from 1965 to 1989.

** Observational is spectroscopic plus photometric hours. See definitions in section on Quality of Nights.