San Juan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Juan Observatory clear sky chart from 2005-11-06 to 2025-12-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	4 8% 3 7% 4 4 5% 4 5% 6% 50%	135600
	Night	4 8% 3 6% 4 3 4 3 5% 7% 54%	132930
Transparency	Day	34% 4 7% 19% 24% 11%	130478
	Night	31% 4 7% 21% 25% 12%	129055
Seeing	Day	15% 16% 29% 25% 12% 3	16642
	Night	11% 10% 26% 31% 18% 3	26895

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	2 9% 5% 11% 4 4 5% 3 5% 6% 46%	4098
	11th to 20th	Day	4 8% 3 9% 8% 4 4 3 5% 6% 46%	4100
	21st to end	Day	3 11% 3 7% 4 3 3 3 4 5% 55%	4510
February	1st to 10th	Day	4 17% 4 8% 4 4 2 2 4 5% 46%	1800
	11th to 20th	Day	5% 15% 4 10% 4 4 4 4 4 6% 41%	1764
	21st to end	Day	4 15% 7% 10% 4 3 3 3 4 4 44%	1700
March	1st to 10th	Day	3 10% 5% 8% 3 4 4 4 5% 7% 45%	4954
	11th to 20th	Day	2 16% 7% 12% 6% 5% 6% 3 5% 5% 34%	5000
	21st to end	Day	2 16% 6% 9% 5% 4 4 3 5% 6% 39%	5486
April	1st to 10th	Day	5% 11% 3 7% 4 4 4 3 5% 6% 49%	2370
	11th to 20th	Day	4 11% 3 8% 5% 5% 5% 4 6% 7% 44%	2371
	21st to end	Day	6% 10% 2 6% 4 4 4 4 5% 6% 48%	2400
May	1st to 10th	Day	6% 10% 3 7% 5% 5% 6% 4 7% 6% 41%	5866
	11th to 20th	Day	3 6% 2 6% 5% 5% 6% 5% 6% 7% 50%	5841
	21st to end	Day	3 5% 2 5% 4 4 4 4 5% 6% 59%	6743
June	1st to 10th	Day	5% 4 1 4 3 5% 4 3 5% 7% 59%	2800
	11th to 20th	Day	5% 3 1 3 2 3 3 3 4 4 69%	2800
	21st to end	Day	3 3 1 4 4 4 4 4 7% 6% 60%	2800
July	1st to 10th	Day	4 3 1 4 3 4 4 3 4 5% 66%	6019
	11th to 20th	Day	3 3 1 6% 6% 7% 8% 7% 9% 9% 41%	5880
	21st to end	Day	5% 4 2 6% 5% 5% 5% 4 6% 8% 49%	6401
August	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 5% 7% 8% 45%	5779
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 6% 7% 48%	5693
	21st to end	Day	6% 6% 2 7% 6% 6% 8% 6% 9% 8% 36%	5984
September	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 5% 6% 49%	2400
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 5% 6% 51%	2329
	21st to end	Day	7% 5% 1 5% 3 3 3 3 3 5% 61%	2141
October	1st to 10th	Day	7% 4 1 2 2 2 2 3 3 5% 69%	3270
	11th to 20th	Day	5% 7% 3 6% 5% 4 5% 3 5% 6% 51%	3250
	21st to end	Day	6% 6% 3 5% 2 2 3 2 4 5% 63%	3548
November	1st to 10th	Day	4 5% 2 5% 3 2 3 3 4 6% 64%	2059
	11th to 20th	Day	4 8% 1 3 4 2 2 2 3 7% 63%	1888
	21st to end	Day	4 10% 4 5% 3 3 3 2 4 5% 57%	1680
December	1st to 10th	Day	4 8% 3 5% 2 2 2 2 4 7% 62%	2700
	11th to 20th	Day	4 9% 4 6% 3 3 3 3 5% 8% 52%	3082
	21st to end	Day	3 12% 4 8% 3 3 3 2 5% 6% 52%	4094
January	1st to 10th	Night	2 6% 4 15% 5% 4 4 3 3 6% 46%	5867
	11th to 20th	Night	3 12% 5% 7% 4 3 3 2 4 5% 52%	5880
	21st to end	Night	3 10% 4 8% 3 3 3 2 4 6% 54%	6468
February	1st to 10th	Night	3 13% 4 7% 4 3 2 2 4 6% 52%	2600
	11th to 20th	Night	4 12% 5% 7% 5% 3 3 3 4 5% 49%	2522
	21st to end	Night	4 13% 4 9% 4 4 4 3 5% 5% 46%	1925
March	1st to 10th	Night	2 13% 4 7% 3 3 4 3 5% 6% 49%	4940
	11th to 20th	Night	2 16% 5% 9% 5% 4 4 3 4 7% 41%	5000
	21st to end	Night	5% 18% 4 7% 4 3 4 3 5% 6% 42%	5489
April	1st to 10th	Night	7% 12% 4 5% 4 3 4 3 5% 6% 48%	2030
	11th to 20th	Night	5% 12% 3 5% 4 4 4 3 4 6% 49%	1977
	21st to end	Night	7% 9% 2 5% 5% 3 3 3 5% 7% 51%	2000
May	1st to 10th	Night	4 7% 3 5% 4 4 4 4 6% 8% 52%	4110
	11th to 20th	Night	4 6% 2 5% 3 3 4 4 5% 8% 55%	4074
	21st to end	Night	5% 4 1 3 2 2 2 2 4 6% 68%	4137
June	1st to 10th	Night	2 3 1 4 3 4 4 4 5% 8% 63%	1600
	11th to 20th	Night	5% 2 1 4 2 3 3 3 4 6% 67%	1600
	21st to end	Night	3 2 1 3 4 4 4 4 6% 7% 64%	1600
July	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 5% 66%	3954
	11th to 20th	Night	2 3 1 4 4 5% 5% 6% 10% 11% 48%	4100
	21st to end	Night	4 3 2 5% 4 5% 5% 5% 8% 9% 51%	4472
August	1st to 10th	Night	3 3 1 5% 5% 6% 6% 5% 9% 11% 47%	4033
	11th to 20th	Night	2 3 1 4 3 4 5% 5% 8% 9% 56%	4156
	21st to end	Night	5% 5% 2 7% 6% 5% 6% 4 7% 10% 42%	4993
September	1st to 10th	Night	5% 4 2 5% 4 5% 5% 4 6% 7% 53%	2000
	11th to 20th	Night	5% 4 1 4 4 4 4 3 5% 6% 59%	2068
	21st to end	Night	6% 3 1 4 2 2 3 2 4 6% 67%	2257
October	1st to 10th	Night	5% 3 1 3 2 2 2 2 3 4 73%	3920
	11th to 20th	Night	4 5% 2 5% 4 4 4 4 5% 6% 57%	3898
	21st to end	Night	6% 7% 2 4 2 2 2 2 3 4 65%	4252
November	1st to 10th	Night	5% 5% 2 4 3 2 3 2 4 5% 64%	2460
	11th to 20th	Night	5% 7% 1 3 2 2 3 2 4 5% 67%	2706
	21st to end	Night	5% 10% 3 5% 3 3 3 3 4 5% 57%	2940
December	1st to 10th	Night	3 7% 3 4 2 2 2 2 4 6% 67%	4710
	11th to 20th	Night	3 9% 3 6% 4 3 3 2 5% 7% 54%	5280
	21st to end	Night	3 11% 4 8% 4 3 3 2 4 5% 51%	6912

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 7% 10% 24% 19% 4	4099
	11th to 20th	Day	40% 4 9% 18% 16% 13%	4100
	21st to end	Day	33% 3 4 22% 28% 10%	4510
February	1st to 10th	Day	42% 3 12% 17% 18% 8%	1800
	11th to 20th	Day	45% 4 5% 20% 20% 6%	1773
	21st to end	Day	45% 4 3 16% 25% 7%	1700
March	1st to 10th	Day	39% 3 5% 19% 26% 7%	4954
	11th to 20th	Day	51% 5% 6% 20% 15% 2	5000
	21st to end	Day	46% 4 8% 25% 15% 1	5486
April	1st to 10th	Day	38% 3 6% 23% 23% 8%	2370
	11th to 20th	Day	39% 5% 6% 20% 24% 6%	2374
	21st to end	Day	36% 5% 7% 24% 25% 4	2400
May	1st to 10th	Day	40% 6% 6% 20% 22% 6%	5866
	11th to 20th	Day	30% 6% 5% 23% 30% 6%	5844
	21st to end	Day	25% 4 3 19% 39% 10%	6744
June	1st to 10th	Day	25% 4 6% 21% 30% 15%	2800
	11th to 20th	Day	20% 3 4 12% 32% 29%	2800
	21st to end	Day	23% 5% 4 16% 27% 26%	2800
July	1st to 10th	Day	22% 3 5% 7% 16% 46%	5956
	11th to 20th	Day	36% 4 10% 21% 18% 10%	5811
	21st to end	Day	33% 3 11% 16% 23% 14%	6217
August	1st to 10th	Day	47% 5% 18% 10% 11% 9%	4208
	11th to 20th	Day	39% 5% 11% 13% 20% 13%	4116
	21st to end	Day	41% 5% 9% 16% 21% 9%	4311
September	1st to 10th	Day	36% 5% 6% 15% 25% 13%	2400
	11th to 20th	Day	34% 4 6% 16% 25% 15%	2329
	21st to end	Day	27% 4 3 18% 36% 12%	2141
October	1st to 10th	Day	21% 2 3 16% 33% 24%	3270
	11th to 20th	Day	36% 2 2 25% 28% 8%	3250
	21st to end	Day	26% 2 4 24% 39% 5%	3548
November	1st to 10th	Day	23% 4 7% 27% 32% 7%	2059
	11th to 20th	Day	25% 2 6% 28% 30% 7%	1888
	21st to end	Day	32% 2 8% 24% 22% 13%	1680
December	1st to 10th	Day	25% 2 7% 28% 30% 8%	2700
	11th to 20th	Day	33% 4 7% 23% 29% 5%	3080
	21st to end	Day	36% 3 8% 22% 20% 11%	4094
January	1st to 10th	Night	39% 6% 7% 23% 20% 7%	5867
	11th to 20th	Night	36% 4 7% 18% 19% 16%	5880
	21st to end	Night	34% 3 4 22% 24% 14%	6468
February	1st to 10th	Night	36% 3 10% 19% 22% 10%	2600
	11th to 20th	Night	39% 4 6% 22% 22% 7%	2523
	21st to end	Night	39% 5% 4 17% 27% 8%	1925
March	1st to 10th	Night	36% 4 4 16% 29% 11%	4940
	11th to 20th	Night	42% 6% 5% 20% 22% 4	5000
	21st to end	Night	43% 5% 8% 27% 16% 2	5489
April	1st to 10th	Night	37% 5% 7% 20% 25% 7%	2030
	11th to 20th	Night	37% 4 5% 19% 29% 7%	1977
	21st to end	Night	32% 4 5% 26% 28% 4	2000
May	1st to 10th	Night	30% 4 9% 27% 26% 4	4107
	11th to 20th	Night	27% 4 6% 27% 32% 5%	4075
	21st to end	Night	18% 4 3 20% 42% 13%	4137
June	1st to 10th	Night	18% 5% 5% 23% 33% 16%	1600
	11th to 20th	Night	19% 3 4 15% 27% 32%	1600
	21st to end	Night	20% 3 5% 16% 25% 31%	1600
July	1st to 10th	Night	22% 2 7% 10% 11% 48%	3909
	11th to 20th	Night	27% 3 10% 27% 17% 15%	4046
	21st to end	Night	28% 4 13% 19% 20% 15%	4344
August	1st to 10th	Night	36% 6% 23% 12% 11% 12%	2928
	11th to 20th	Night	28% 5% 20% 12% 21% 14%	3009
	21st to end	Night	34% 4 10% 21% 20% 11%	3597
September	1st to 10th	Night	29% 4 10% 17% 23% 17%	2000
	11th to 20th	Night	26% 5% 8% 18% 25% 19%	2068
	21st to end	Night	22% 3 4 20% 36% 16%	2257
October	1st to 10th	Night	18% 3 4 12% 40% 24%	3920
	11th to 20th	Night	29% 2 5% 26% 29% 9%	3898
	21st to end	Night	26% 2 4 20% 39% 11%	4252
November	1st to 10th	Night	24% 3 7% 25% 32% 9%	2460
	11th to 20th	Night	23% 3 6% 27% 33% 9%	2706
	21st to end	Night	31% 3 5% 22% 24% 15%	2940
December	1st to 10th	Night	22% 2 6% 27% 31% 11%	4710
	11th to 20th	Night	32% 3 5% 26% 24% 10%	5281
	21st to end	Night	36% 3 7% 21% 22% 11%	6912

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 18% 34% 17% 9% 2	300
	11th to 20th	Day	18% 22% 32% 19% 8%	300
	21st to end	Day	13% 23% 35% 20% 7% 2	330
February	1st to 10th	Day	24% 23% 34% 15% 3	300
	11th to 20th	Day	23% 22% 37% 13% 4 1	300
	21st to end	Day	22% 30% 28% 16% 3	299
March	1st to 10th	Day	24% 24% 26% 20% 6%	500
	11th to 20th	Day	20% 24% 27% 19% 9% 1	500
	21st to end	Day	27% 27% 29% 12% 5%	550
April	1st to 10th	Day	17% 24% 31% 20% 7% 1	500
	11th to 20th	Day	22% 28% 30% 13% 7% 1	492
	21st to end	Day	21% 25% 30% 19% 5% 1	500
May	1st to 10th	Day	22% 24% 32% 17% 5%	503
	11th to 20th	Day	17% 18% 32% 21% 10% 2	528
	21st to end	Day	14% 19% 28% 24% 11% 4	711
June	1st to 10th	Day	8% 11% 31% 31% 15% 3	730
	11th to 20th	Day	9% 16% 32% 27% 13% 3	730
	21st to end	Day	6% 12% 31% 31% 15% 4	730
July	1st to 10th	Day	5% 6% 27% 39% 19% 5%	588
	11th to 20th	Day	7% 3 20% 35% 26% 9%	524
	21st to end	Day	12% 4 18% 36% 22% 8%	575
August	1st to 10th	Day	9% 7% 22% 36% 21% 5%	527
	11th to 20th	Day	10% 4 23% 40% 17% 6%	517
	21st to end	Day	9% 6% 25% 36% 18% 5%	583
September	1st to 10th	Day	8% 7% 27% 31% 21% 5%	530
	11th to 20th	Day	12% 15% 30% 28% 11% 4	530
	21st to end	Day	13% 14% 28% 27% 13% 4	471
October	1st to 10th	Day	17% 19% 19% 24% 17% 5%	330
	11th to 20th	Day	15% 16% 29% 27% 11% 2	330
	21st to end	Day	17% 20% 29% 23% 10% 2	363
November	1st to 10th	Day	16% 21% 27% 25% 11%	330
	11th to 20th	Day	13% 17% 37% 21% 11% 1	330
	21st to end	Day	17% 18% 34% 21% 8% 2	330
December	1st to 10th	Day	16% 16% 36% 22% 9% 1	330
	11th to 20th	Day	16% 17% 33% 23% 9% 2	321
	21st to end	Day	22% 22% 31% 19% 5% 2	330
January	1st to 10th	Night	12% 10% 28% 31% 15% 4	800
	11th to 20th	Night	12% 10% 34% 29% 13% 3	800
	21st to end	Night	13% 12% 31% 29% 12% 3	880
February	1st to 10th	Night	14% 12% 35% 29% 9% 2	800
	11th to 20th	Night	12% 14% 38% 26% 8% 2	784
	21st to end	Night	13% 13% 35% 32% 7%	660
March	1st to 10th	Night	17% 17% 31% 25% 9% 2	800
	11th to 20th	Night	10% 13% 27% 29% 19% 2	800
	21st to end	Night	14% 12% 32% 30% 11% 1	880
April	1st to 10th	Night	15% 13% 34% 31% 7% 1	630
	11th to 20th	Night	14% 18% 30% 28% 10% 1	593
	21st to end	Night	14% 16% 30% 26% 12% 2	600
May	1st to 10th	Night	12% 10% 33% 31% 13% 2	600
	11th to 20th	Night	13% 10% 25% 31% 16% 4	591
	21st to end	Night	12% 11% 25% 30% 18% 3	650
June	1st to 10th	Night	3 10% 24% 35% 26% 2	600
	11th to 20th	Night	8% 8% 22% 37% 22% 3	600
	21st to end	Night	3 5% 23% 38% 26% 5%	600
July	1st to 10th	Night	7% 2 14% 37% 34% 6%	585
	11th to 20th	Night	5% 1 10% 35% 40% 9%	582
	21st to end	Night	8% 1 7% 32% 42% 9%	657
August	1st to 10th	Night	6% 1 8% 35% 40% 10%	597
	11th to 20th	Night	5% 2 12% 37% 37% 7%	594
	21st to end	Night	6% 4 10% 37% 35% 9%	660
September	1st to 10th	Night	5% 3 12% 44% 32% 4	600
	11th to 20th	Night	6% 7% 21% 36% 26% 6%	668
	21st to end	Night	10% 7% 19% 35% 23% 7%	800
October	1st to 10th	Night	12% 10% 24% 36% 16% 3	800
	11th to 20th	Night	8% 10% 30% 33% 16% 4	800
	21st to end	Night	10% 9% 31% 29% 16% 4	880
November	1st to 10th	Night	10% 9% 30% 31% 16% 4	820
	11th to 20th	Night	11% 10% 35% 28% 13% 3	971
	21st to end	Night	14% 14% 27% 28% 13% 3	1050
December	1st to 10th	Night	14% 10% 32% 32% 11% 2	1050
	11th to 20th	Night	15% 13% 32% 26% 12% 2	1036
	21st to end	Night	17% 15% 29% 28% 10% 1	1077

Page generated at 2025-12-14 23:13:55. Version ac07

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