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 2024-11-12.

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	5% 8% 3 6% 4 4 5% 4 5% 6% 49%	105030
	Night	5% 9% 3 6% 4 3 4 3 5% 7% 53%	102896
Transparency	Day	34% 5% 7% 19% 23% 11%	104795
	Night	32% 5% 8% 20% 24% 12%	102720
Seeing	Day	15% 16% 28% 25% 12% 3	15185
	Night	11% 10% 25% 32% 19% 4	25379

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	3 9% 4 11% 5% 4 5% 3 5% 6% 45%	2858
	11th to 20th	Day	6% 12% 5% 9% 6% 3 3 3 5% 7% 41%	2860
	21st to end	Day	4 13% 4 6% 4 3 3 3 5% 6% 48%	3146
February	1st to 10th	Day	4 17% 5% 8% 4 4 2 2 4 5% 45%	1710
	11th to 20th	Day	5% 15% 4 10% 4 4 4 4 4 6% 42%	1673
	21st to end	Day	4 16% 7% 10% 4 3 4 3 4 4 42%	1617
March	1st to 10th	Day	5% 11% 4 8% 3 4 4 4 6% 8% 43%	3444
	11th to 20th	Day	3 16% 5% 9% 6% 5% 6% 3 5% 6% 35%	3490
	21st to end	Day	3 20% 7% 9% 5% 4 4 3 5% 5% 35%	3825
April	1st to 10th	Day	5% 11% 3 7% 4 4 4 3 5% 6% 49%	2251
	11th to 20th	Day	4 11% 3 8% 5% 5% 5% 4 6% 6% 44%	2251
	21st to end	Day	6% 11% 2 6% 4 4 4 4 5% 6% 47%	2280
May	1st to 10th	Day	6% 9% 3 6% 5% 4 5% 4 6% 7% 46%	4096
	11th to 20th	Day	4 5% 1 5% 5% 5% 6% 5% 7% 8% 48%	4071
	21st to end	Day	4 6% 2 5% 4 4 4 4 5% 6% 58%	4699
June	1st to 10th	Day	5% 4 1 3 3 5% 4 3 5% 7% 60%	2660
	11th to 20th	Day	5% 3 1 3 3 3 3 3 4 4 68%	2660
	21st to end	Day	3 3 1 4 4 4 5% 4 7% 7% 58%	2660
July	1st to 10th	Day	4 3 1 3 3 4 4 3 5% 5% 65%	4208
	11th to 20th	Day	4 3 2 5% 6% 7% 8% 7% 10% 10% 39%	4110
	21st to end	Day	6% 4 2 6% 5% 5% 5% 5% 6% 7% 49%	4454
August	1st to 10th	Day	7% 5% 2 8% 7% 8% 9% 6% 8% 9% 30%	4092
	11th to 20th	Day	6% 5% 2 7% 6% 6% 6% 5% 6% 8% 43%	3949
	21st to end	Day	6% 6% 2 7% 5% 6% 7% 5% 8% 8% 40%	4180
September	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 5% 6% 49%	2280
	11th to 20th	Day	9% 4 2 6% 4 5% 4 4 5% 6% 51%	2213
	21st to end	Day	7% 5% 1 5% 3 3 3 3 3 5% 62%	2034
October	1st to 10th	Day	7% 4 1 3 2 2 2 3 3 5% 69%	3170
	11th to 20th	Day	5% 7% 3 6% 5% 4 5% 3 5% 6% 50%	3150
	21st to end	Day	6% 6% 3 5% 2 2 3 2 4 5% 63%	3438
November	1st to 10th	Day	4 6% 2 5% 3 2 3 3 4 6% 63%	1959
	11th to 20th	Day	4 8% 1 4 4 2 2 2 3 6% 63%	1720
	21st to end	Day	4 10% 5% 6% 3 3 2 2 5% 4 56%	1520
December	1st to 10th	Day	6% 12% 3 7% 3 3 3 3 3 6% 51%	1520
	11th to 20th	Day	7% 14% 4 7% 4 3 4 4 5% 6% 41%	1926
	21st to end	Day	4 12% 4 9% 4 3 3 2 4 5% 50%	2856
January	1st to 10th	Night	3 7% 4 16% 5% 4 4 3 3 6% 45%	4097
	11th to 20th	Night	4 16% 7% 8% 5% 3 3 3 4 6% 43%	4110
	21st to end	Night	4 13% 5% 8% 3 3 3 3 4 6% 50%	4521
February	1st to 10th	Night	4 13% 4 7% 3 3 2 3 4 6% 52%	2470
	11th to 20th	Night	4 12% 5% 7% 5% 3 3 3 4 5% 49%	2393
	21st to end	Night	4 13% 4 9% 4 4 4 3 5% 5% 44%	1832
March	1st to 10th	Night	3 16% 3 6% 3 3 4 3 5% 6% 47%	3430
	11th to 20th	Night	3 16% 4 7% 5% 5% 5% 4 4 6% 41%	3490
	21st to end	Night	5% 19% 5% 8% 4 4 4 3 5% 6% 36%	3828
April	1st to 10th	Night	8% 12% 3 5% 3 3 4 3 5% 6% 47%	1929
	11th to 20th	Night	5% 12% 3 5% 4 3 4 4 4 6% 49%	1877
	21st to end	Night	7% 9% 2 5% 5% 3 3 3 5% 6% 51%	1900
May	1st to 10th	Night	5% 8% 3 5% 4 3 3 3 5% 7% 55%	2870
	11th to 20th	Night	4 6% 2 5% 3 4 5% 4 6% 9% 53%	2834
	21st to end	Night	7% 6% 2 4 2 3 3 3 4 6% 62%	2871
June	1st to 10th	Night	2 3 1 4 3 4 4 4 5% 8% 64%	1520
	11th to 20th	Night	5% 2 1 4 2 3 3 3 4 6% 66%	1520
	21st to end	Night	3 2 1 3 4 4 4 4 6% 7% 63%	1520
July	1st to 10th	Night	4 3 1 3 3 3 3 3 4 5% 67%	2756
	11th to 20th	Night	2 3 1 4 4 5% 5% 5% 9% 11% 49%	2860
	21st to end	Night	5% 3 2 5% 4 4 5% 5% 7% 8% 52%	3109
August	1st to 10th	Night	4 4 2 6% 5% 7% 7% 6% 11% 12% 36%	2847
	11th to 20th	Night	3 4 1 5% 4 5% 5% 5% 9% 10% 48%	2890
	21st to end	Night	5% 5% 2 6% 5% 4 5% 4 7% 10% 46%	3487
September	1st to 10th	Night	5% 4 2 5% 4 5% 5% 5% 6% 7% 53%	1900
	11th to 20th	Night	5% 4 1 4 4 4 4 3 5% 6% 59%	1964
	21st to end	Night	6% 3 1 3 2 2 3 2 3 5% 68%	2144
October	1st to 10th	Night	5% 3 1 3 2 2 2 2 3 4 73%	3800
	11th to 20th	Night	4 5% 2 5% 4 4 4 4 5% 6% 57%	3778
	21st to end	Night	6% 7% 3 5% 2 2 2 2 3 4 65%	4120
November	1st to 10th	Night	5% 6% 2 4 3 3 3 2 5% 5% 62%	2340
	11th to 20th	Night	5% 7% 1 3 2 2 3 2 3 5% 67%	2459
	21st to end	Night	4 9% 3 5% 3 3 2 3 4 5% 57%	2660
December	1st to 10th	Night	6% 12% 5% 5% 3 2 2 2 3 5% 55%	2660
	11th to 20th	Night	6% 12% 3 7% 4 4 4 3 5% 8% 44%	3271
	21st to end	Night	4 11% 4 8% 4 3 3 2 4 6% 50%	4839

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	35% 9% 11% 23% 18% 3	2859
	11th to 20th	Day	42% 5% 13% 25% 11% 3	2860
	21st to end	Day	37% 5% 6% 26% 22% 4	3146
February	1st to 10th	Day	43% 3 12% 17% 17% 8%	1710
	11th to 20th	Day	44% 4 5% 20% 20% 6%	1682
	21st to end	Day	47% 4 3 17% 24% 5%	1617
March	1st to 10th	Day	39% 5% 8% 24% 21% 3	3444
	11th to 20th	Day	46% 8% 9% 23% 12% 2	3490
	21st to end	Day	51% 5% 6% 20% 15% 2	3825
April	1st to 10th	Day	38% 3 6% 23% 22% 8%	2251
	11th to 20th	Day	39% 5% 7% 20% 23% 6%	2254
	21st to end	Day	37% 5% 7% 24% 24% 4	2280
May	1st to 10th	Day	33% 9% 5% 18% 27% 8%	4096
	11th to 20th	Day	28% 9% 7% 22% 28% 6%	4074
	21st to end	Day	25% 6% 4 16% 38% 10%	4699
June	1st to 10th	Day	23% 4 5% 21% 31% 15%	2660
	11th to 20th	Day	21% 3 4 12% 32% 28%	2660
	21st to end	Day	24% 5% 4 16% 26% 25%	2660
July	1st to 10th	Day	21% 4 6% 8% 15% 46%	4144
	11th to 20th	Day	36% 5% 10% 22% 20% 7%	4041
	21st to end	Day	33% 5% 11% 17% 18% 17%	4314
August	1st to 10th	Day	48% 5% 19% 9% 10% 8%	4078
	11th to 20th	Day	39% 5% 11% 13% 20% 13%	3988
	21st to end	Day	41% 5% 9% 16% 21% 9%	4179
September	1st to 10th	Day	36% 5% 6% 14% 25% 13%	2280
	11th to 20th	Day	34% 5% 6% 16% 25% 15%	2213
	21st to end	Day	26% 4 3 18% 36% 12%	2034
October	1st to 10th	Day	21% 2 3 16% 33% 24%	3170
	11th to 20th	Day	36% 2 2 26% 27% 8%	3150
	21st to end	Day	26% 2 4 23% 39% 5%	3438
November	1st to 10th	Day	24% 4 7% 26% 32% 7%	1959
	11th to 20th	Day	25% 3 7% 28% 31% 7%	1720
	21st to end	Day	33% 2 8% 23% 22% 13%	1520
December	1st to 10th	Day	37% 4 11% 22% 19% 7%	1520
	11th to 20th	Day	43% 6% 7% 18% 22% 5%	1924
	21st to end	Day	37% 4 8% 15% 20% 16%	2856
January	1st to 10th	Night	39% 8% 6% 24% 18% 5%	4097
	11th to 20th	Night	43% 5% 10% 24% 14% 4	4110
	21st to end	Night	37% 5% 5% 27% 20% 5%	4521
February	1st to 10th	Night	35% 3 10% 19% 23% 9%	2470
	11th to 20th	Night	38% 4 6% 22% 22% 7%	2394
	21st to end	Night	41% 5% 4 17% 27% 7%	1832
March	1st to 10th	Night	36% 6% 6% 23% 26% 5%	3430
	11th to 20th	Night	41% 9% 8% 23% 17% 3	3490
	21st to end	Night	47% 7% 8% 19% 17% 2	3828
April	1st to 10th	Night	37% 5% 7% 20% 24% 7%	1929
	11th to 20th	Night	36% 4 5% 19% 29% 6%	1877
	21st to end	Night	33% 5% 6% 26% 27% 4	1900
May	1st to 10th	Night	28% 6% 6% 21% 34% 6%	2870
	11th to 20th	Night	26% 6% 8% 26% 28% 6%	2835
	21st to end	Night	24% 5% 4 17% 34% 16%	2871
June	1st to 10th	Night	17% 6% 4 23% 34% 16%	1520
	11th to 20th	Night	19% 4 4 15% 27% 31%	1520
	21st to end	Night	20% 3 5% 16% 26% 29%	1520
July	1st to 10th	Night	21% 4 7% 9% 11% 49%	2711
	11th to 20th	Night	26% 4 9% 29% 18% 13%	2806
	21st to end	Night	28% 6% 13% 21% 14% 18%	3011
August	1st to 10th	Night	37% 6% 24% 11% 11% 12%	2838
	11th to 20th	Night	29% 6% 20% 11% 20% 14%	2917
	21st to end	Night	33% 4 10% 21% 21% 11%	3487
September	1st to 10th	Night	29% 5% 10% 16% 23% 18%	1900
	11th to 20th	Night	26% 5% 8% 18% 24% 19%	1964
	21st to end	Night	21% 3 3 20% 36% 16%	2144
October	1st to 10th	Night	17% 3 4 12% 39% 25%	3800
	11th to 20th	Night	29% 2 5% 26% 29% 9%	3778
	21st to end	Night	26% 2 4 19% 39% 11%	4120
November	1st to 10th	Night	25% 3 8% 24% 31% 10%	2340
	11th to 20th	Night	23% 3 6% 27% 32% 9%	2459
	21st to end	Night	30% 3 6% 21% 25% 14%	2660
December	1st to 10th	Night	34% 3 11% 21% 22% 9%	2660
	11th to 20th	Night	40% 5% 8% 17% 23% 8%	3272
	21st to end	Night	35% 5% 6% 17% 22% 15%	4839

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% 17% 34% 18% 9% 2	270
	11th to 20th	Day	18% 23% 30% 20% 9%	270
	21st to end	Day	12% 22% 35% 22% 7% 2	297
February	1st to 10th	Day	21% 23% 35% 17% 4	270
	11th to 20th	Day	22% 21% 37% 14% 4 1	270
	21st to end	Day	24% 29% 27% 16% 3	272
March	1st to 10th	Day	24% 23% 26% 20% 7%	460
	11th to 20th	Day	19% 22% 27% 21% 9% 2	460
	21st to end	Day	28% 27% 28% 12% 5%	506
April	1st to 10th	Day	17% 25% 31% 19% 7% 1	460
	11th to 20th	Day	22% 28% 29% 13% 7% 1	452
	21st to end	Day	21% 24% 29% 19% 6% 1	460
May	1st to 10th	Day	21% 24% 33% 16% 5%	463
	11th to 20th	Day	15% 17% 32% 23% 10% 2	488
	21st to end	Day	15% 19% 27% 24% 10% 4	660
June	1st to 10th	Day	8% 11% 30% 31% 16% 3	680
	11th to 20th	Day	10% 16% 31% 26% 13% 4	680
	21st to end	Day	5% 11% 32% 32% 16% 4	680
July	1st to 10th	Day	5% 6% 25% 39% 20% 5%	545
	11th to 20th	Day	7% 3 19% 34% 27% 10%	484
	21st to end	Day	12% 4 17% 35% 23% 9%	531
August	1st to 10th	Day	9% 6% 19% 38% 23% 5%	487
	11th to 20th	Day	10% 4 23% 40% 16% 7%	477
	21st to end	Day	9% 6% 26% 36% 19% 5%	539
September	1st to 10th	Day	8% 7% 27% 30% 21% 6%	490
	11th to 20th	Day	12% 15% 29% 29% 11% 4	490
	21st to end	Day	13% 14% 28% 27% 14% 4	434
October	1st to 10th	Day	16% 18% 18% 24% 19% 5%	300
	11th to 20th	Day	13% 16% 28% 29% 12% 2	300
	21st to end	Day	18% 21% 28% 22% 10% 2	330
November	1st to 10th	Day	18% 20% 26% 25% 12%	300
	11th to 20th	Day	11% 18% 37% 21% 11% 1	273
	21st to end	Day	15% 20% 31% 23% 10% 2	270
December	1st to 10th	Day	17% 17% 33% 24% 10% 1	270
	11th to 20th	Day	19% 19% 28% 22% 10% 3	270
	21st to end	Day	23% 22% 29% 20% 5% 1	297
January	1st to 10th	Night	12% 10% 27% 32% 15% 4	760
	11th to 20th	Night	12% 9% 33% 29% 13% 3	760
	21st to end	Night	13% 11% 29% 31% 13% 3	836
February	1st to 10th	Night	13% 11% 34% 30% 9% 2	760
	11th to 20th	Night	12% 14% 36% 27% 9% 2	744
	21st to end	Night	14% 13% 33% 32% 7%	628
March	1st to 10th	Night	17% 17% 29% 26% 10% 2	760
	11th to 20th	Night	9% 13% 27% 30% 20% 2	760
	21st to end	Night	14% 13% 30% 30% 12% 1	836
April	1st to 10th	Night	16% 13% 33% 31% 7% 1	599
	11th to 20th	Night	14% 18% 29% 28% 10% 1	563
	21st to end	Night	14% 16% 28% 26% 13% 2	570
May	1st to 10th	Night	12% 11% 32% 30% 13% 2	570
	11th to 20th	Night	13% 10% 24% 32% 17% 5%	561
	21st to end	Night	13% 12% 24% 29% 19% 4	617
June	1st to 10th	Night	3 10% 22% 35% 27% 2	570
	11th to 20th	Night	8% 9% 20% 36% 23% 3	570
	21st to end	Night	3 5% 21% 39% 27% 5%	570
July	1st to 10th	Night	8% 2 13% 36% 36% 6%	555
	11th to 20th	Night	5% 1 8% 36% 41% 9%	552
	21st to end	Night	8% 1 6% 32% 43% 10%	624
August	1st to 10th	Night	6% 1 7% 34% 42% 10%	567
	11th to 20th	Night	5% 2 11% 37% 38% 7%	564
	21st to end	Night	6% 4 8% 36% 36% 9%	627
September	1st to 10th	Night	5% 3 12% 42% 33% 4	570
	11th to 20th	Night	6% 6% 19% 37% 27% 6%	634
	21st to end	Night	10% 8% 18% 34% 24% 7%	760
October	1st to 10th	Night	12% 9% 22% 38% 16% 3	760
	11th to 20th	Night	7% 9% 29% 34% 16% 4	760
	21st to end	Night	10% 9% 29% 30% 17% 4	836
November	1st to 10th	Night	11% 9% 28% 32% 17% 4	780
	11th to 20th	Night	11% 9% 32% 30% 15% 3	882
	21st to end	Night	14% 14% 24% 30% 15% 4	950
December	1st to 10th	Night	14% 10% 30% 33% 11% 2	950
	11th to 20th	Night	17% 12% 29% 27% 13% 2	950
	21st to end	Night	17% 15% 28% 29% 10% 1	1024

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