San Pedro Martir Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Pedro Martir Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	1 1 1 2 2 2 3 3 5% 7% 73%	140288
	Night	1 1 1 2 1 2 2 2 4 6% 78%	138572
Transparency	Day	9% 6% 6% 12% 26% 41%	135380
	Night	8% 5% 5% 11% 25% 46%	134455
Seeing	Day	7% 16% 31% 29% 12% 4	17414
	Night	4 7% 19% 32% 30% 9%	27816

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	1 3 1 2 1 2 3 3 6% 7% 72%	5318
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 6% 77%	5557
	21st to end	Day	1 3 2 5% 3 3 4 3 5% 8% 64%	6500
February	1st to 10th	Day	1 2 1 2 3 4 3 3 7% 8% 67%	2100
	11th to 20th	Day	1 3 1 3 4 3 3 3 7% 8% 63%	2050
	21st to end	Day	1 2 1 3 2 2 4 4 8% 8% 66%	1730
March	1st to 10th	Day	1 1 1 1 2 2 4 4 6% 8% 70%	4555
	11th to 20th	Day	2 3 2 4 2 2 2 2 5% 7% 70%	5000
	21st to end	Day	1 1 2 2 2 3 4 5% 6% 75%	5486
April	1st to 10th	Day	1 2 2 2 2 2 3 5% 7% 73%	2318
	11th to 20th	Day	1 1 1 1 2 3 2 4 6% 80%	2369
	21st to end	Day	1 1 1 1 2 2 2 3 4 84%	2400
May	1st to 10th	Day	1 1 1 2 1 1 1 1 3 4 84%	5867
	11th to 20th	Day	1 1 1 1 2 2 4 6% 82%	5841
	21st to end	Day	1 1 1 1 1 2 4 87%	6423
June	1st to 10th	Day	1 1 1 2 2 2 2 4 5% 80%	2600
	11th to 20th	Day	1 1 1 1 1 2 4 88%	2600
	21st to end	Day	2 2 1 2 2 2 3 3 4 5% 76%	2600
July	1st to 10th	Day	1 1 2 1 2 4 3 5% 6% 75%	5879
	11th to 20th	Day	1 2 1 3 3 4 5% 5% 7% 8% 63%	5880
	21st to end	Day	2 2 1 3 3 4 5% 4 7% 8% 62%	6277
August	1st to 10th	Day	1 1 1 3 3 4 4 4 6% 8% 64%	5346
	11th to 20th	Day	1 1 1 2 2 3 4 4 6% 9% 66%	5364
	21st to end	Day	2 3 1 4 3 3 4 4 7% 7% 63%	5984
September	1st to 10th	Day	5% 3 2 4 3 4 4 3 5% 8% 58%	2264
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 5% 74%	2200
	21st to end	Day	1 1 1 3 2 3 3 3 3 4 77%	2200
October	1st to 10th	Day	1 1 2 4 1 2 2 2 3 4 78%	3600
	11th to 20th	Day	1 2 1 1 2 2 4 4 84%	3579
	21st to end	Day	1 1 1 1 2 4 6% 83%	3614
November	1st to 10th	Day	1 2 2 2 3 4 5% 81%	1859
	11th to 20th	Day	1 2 1 2 2 2 3 3 5% 7% 72%	1890
	21st to end	Day	3 3 1 3 2 3 3 3 5% 8% 66%	1890
December	1st to 10th	Day	1 2 1 2 2 2 2 2 3 5% 78%	3040
	11th to 20th	Day	1 1 1 2 2 3 3 3 6% 9% 69%	3499
	21st to end	Day	1 2 2 2 3 3 6% 10% 69%	4609
January	1st to 10th	Night	1 2 1 2 2 2 3 4 7% 8% 68%	7605
	11th to 20th	Night	1 1 1 1 2 2 2 4 6% 79%	7441
	21st to end	Night	1 2 1 4 3 4 5% 4 4 8% 62%	7747
February	1st to 10th	Night	1 3 1 3 3 3 3 3 5% 7% 69%	2520
	11th to 20th	Night	2 2 1 3 3 2 4 5% 6% 9% 64%	2458
	21st to end	Night	1 3 1 4 4 3 3 3 6% 6% 66%	2076
March	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 7% 73%	5383
	11th to 20th	Night	1 2 1 2 2 2 2 2 5% 8% 74%	5000
	21st to end	Night	1 1 1 3 2 3 3 4 6% 7% 70%	5489
April	1st to 10th	Night	1 1 1 3 2 2 3 2 4 5% 76%	2076
	11th to 20th	Night	1 1 2 1 1 2 1 3 5% 83%	1979
	21st to end	Night	1 1 1 1 3 5% 87%	2000
May	1st to 10th	Night	1 1 1 1 1 1 2 91%	4110
	11th to 20th	Night	1 1 2 2 4 88%	4074
	21st to end	Night	1 1 1 1 3 4 89%	4474
June	1st to 10th	Night	1 1 2 2 2 1 3 4 85%	1800
	11th to 20th	Night	1 1 1 1 1 2 2 92%	1800
	21st to end	Night	1 1 1 2 1 1 1 2 4 87%	1800
July	1st to 10th	Night	1 1 1 1 2 3 91%	4100
	11th to 20th	Night	1 1 1 2 5% 87%	4100
	21st to end	Night	1 1 1 1 1 1 2 2 4 7% 81%	4597
August	1st to 10th	Night	1 1 1 1 2 3 7% 84%	4486
	11th to 20th	Night	1 1 1 2 3 6% 86%	4506
	21st to end	Night	1 1 1 1 1 2 1 3 4 85%	5015
September	1st to 10th	Night	2 1 1 2 1 2 2 2 4 4 80%	2135
	11th to 20th	Night	2 1 1 1 1 2 1 2 3 87%	2200
	21st to end	Night	1 1 2 1 1 1 2 2 3 87%	2200
October	1st to 10th	Night	1 1 1 1 1 1 1 2 2 88%	3600
	11th to 20th	Night	1 1 1 1 1 1 3 91%	3579
	21st to end	Night	1 1 1 1 2 3 5% 84%	4185
November	1st to 10th	Night	1 1 2 2 3 2 4 6% 78%	2655
	11th to 20th	Night	1 2 1 3 2 3 2 3 5% 5% 74%	2730
	21st to end	Night	2 3 1 3 2 2 3 3 6% 7% 68%	2730
December	1st to 10th	Night	1 2 1 2 1 2 2 3 3 5% 79%	4370
	11th to 20th	Night	1 2 1 3 3 4 4 3 6% 9% 65%	4941
	21st to end	Night	1 2 1 1 2 3 3 3 6% 8% 71%	6611

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	9% 7% 7% 12% 24% 40%	5321
	11th to 20th	Day	8% 6% 4 9% 23% 50%	5557
	21st to end	Day	13% 10% 5% 10% 27% 36%	6500
February	1st to 10th	Day	13% 5% 8% 10% 24% 40%	2100
	11th to 20th	Day	17% 7% 7% 11% 27% 30%	2060
	21st to end	Day	13% 9% 7% 11% 25% 35%	1730
March	1st to 10th	Day	9% 7% 6% 9% 21% 48%	4555
	11th to 20th	Day	9% 8% 7% 13% 36% 28%	5000
	21st to end	Day	7% 9% 8% 16% 32% 27%	5486
April	1st to 10th	Day	11% 4 6% 9% 35% 35%	2318
	11th to 20th	Day	8% 3 5% 8% 27% 50%	2372
	21st to end	Day	7% 2 3 8% 33% 47%	2400
May	1st to 10th	Day	5% 3 4 9% 30% 50%	5867
	11th to 20th	Day	3 3 4 9% 32% 50%	5844
	21st to end	Day	4 2 3 5% 24% 63%	6424
June	1st to 10th	Day	7% 2 3 5% 25% 57%	2600
	11th to 20th	Day	4 1 2 3 18% 72%	2600
	21st to end	Day	11% 2 5% 6% 15% 61%	2600
July	1st to 10th	Day	8% 5% 6% 14% 19% 49%	5816
	11th to 20th	Day	13% 9% 10% 23% 24% 21%	5811
	21st to end	Day	13% 11% 13% 16% 20% 28%	6095
August	1st to 10th	Day	15% 9% 19% 26% 14% 18%	3890
	11th to 20th	Day	15% 7% 11% 20% 31% 16%	3880
	21st to end	Day	12% 7% 8% 15% 24% 34%	4311
September	1st to 10th	Day	22% 7% 9% 13% 19% 31%	2264
	11th to 20th	Day	12% 3 4 8% 26% 47%	2200
	21st to end	Day	11% 4 5% 10% 27% 43%	2200
October	1st to 10th	Day	6% 9% 4 7% 21% 53%	3600
	11th to 20th	Day	4 4 2 6% 30% 53%	3579
	21st to end	Day	5% 3 3 7% 26% 56%	3614
November	1st to 10th	Day	7% 4 4 12% 28% 46%	1859
	11th to 20th	Day	12% 4 6% 10% 27% 40%	1890
	21st to end	Day	16% 6% 7% 10% 22% 39%	1890
December	1st to 10th	Day	10% 4 4 7% 29% 45%	3040
	11th to 20th	Day	11% 6% 6% 14% 24% 39%	3498
	21st to end	Day	8% 8% 8% 19% 29% 28%	4609
January	1st to 10th	Night	10% 8% 7% 12% 29% 35%	7605
	11th to 20th	Night	6% 5% 3 8% 21% 55%	7441
	21st to end	Night	14% 12% 5% 8% 23% 38%	7747
February	1st to 10th	Night	13% 6% 8% 10% 21% 42%	2520
	11th to 20th	Night	15% 8% 7% 11% 23% 36%	2459
	21st to end	Night	17% 6% 8% 9% 21% 39%	2076
March	1st to 10th	Night	9% 7% 5% 6% 20% 53%	5383
	11th to 20th	Night	6% 6% 6% 12% 34% 36%	5000
	21st to end	Night	8% 8% 9% 10% 35% 29%	5489
April	1st to 10th	Night	11% 3 5% 9% 30% 42%	2076
	11th to 20th	Night	7% 2 3 6% 24% 57%	1979
	21st to end	Night	4 2 3 8% 30% 53%	2000
May	1st to 10th	Night	3 1 2 6% 25% 62%	4107
	11th to 20th	Night	2 4 2 7% 30% 55%	4075
	21st to end	Night	2 2 3 5% 18% 70%	4474
June	1st to 10th	Night	6% 1 2 6% 21% 64%	1800
	11th to 20th	Night	3 1 1 3 15% 77%	1800
	21st to end	Night	6% 1 3 6% 15% 70%	1800
July	1st to 10th	Night	3 1 4 15% 21% 56%	4055
	11th to 20th	Night	3 2 6% 28% 30% 31%	4046
	21st to end	Night	6% 3 12% 24% 24% 31%	4467
August	1st to 10th	Night	5% 2 20% 33% 20% 20%	3256
	11th to 20th	Night	5% 3 7% 24% 39% 22%	3256
	21st to end	Night	6% 2 3 16% 33% 40%	3608
September	1st to 10th	Night	10% 3 7% 16% 26% 38%	2135
	11th to 20th	Night	6% 2 3 8% 30% 52%	2200
	21st to end	Night	5% 2 4 12% 28% 49%	2200
October	1st to 10th	Night	5% 2 3 6% 22% 62%	3600
	11th to 20th	Night	4 1 2 6% 29% 58%	3579
	21st to end	Night	5% 3 2 5% 23% 62%	4185
November	1st to 10th	Night	8% 4 5% 10% 29% 44%	2655
	11th to 20th	Night	10% 5% 6% 10% 23% 46%	2730
	21st to end	Night	14% 6% 6% 11% 24% 38%	2730
December	1st to 10th	Night	9% 5% 4 6% 26% 50%	4370
	11th to 20th	Night	14% 9% 5% 10% 22% 41%	4941
	21st to end	Night	9% 7% 7% 15% 26% 36%	6611

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	14% 21% 34% 22% 8% 2	330
	11th to 20th	Day	8% 15% 39% 25% 10% 2	404
	21st to end	Day	10% 26% 37% 21% 4 1	594
February	1st to 10th	Day	10% 24% 37% 20% 8% 2	540
	11th to 20th	Day	14% 28% 36% 18% 3	535
	21st to end	Day	11% 32% 35% 15% 6%	446
March	1st to 10th	Day	15% 29% 36% 14% 4 1	509
	11th to 20th	Day	15% 26% 35% 17% 6% 1	500
	21st to end	Day	13% 33% 35% 14% 5% 1	550
April	1st to 10th	Day	9% 28% 33% 20% 7% 4	500
	11th to 20th	Day	8% 28% 37% 18% 8% 1	492
	21st to end	Day	9% 22% 35% 23% 8% 3	500
May	1st to 10th	Day	5% 26% 33% 25% 8% 3	503
	11th to 20th	Day	5% 15% 42% 28% 8% 3	528
	21st to end	Day	3 18% 44% 25% 8% 2	573
June	1st to 10th	Day	2 16% 39% 30% 10% 3	530
	11th to 20th	Day	2 13% 38% 32% 11% 5%	530
	21st to end	Day	4 8% 30% 38% 15% 6%	530
July	1st to 10th	Day	2 4 23% 45% 18% 8%	526
	11th to 20th	Day	5% 2 16% 41% 25% 10%	524
	21st to end	Day	5% 5% 13% 41% 25% 11%	575
August	1st to 10th	Day	4 2 18% 42% 23% 10%	527
	11th to 20th	Day	3 2 11% 44% 29% 11%	517
	21st to end	Day	3 3 15% 42% 26% 11%	583
September	1st to 10th	Day	8% 3 24% 40% 18% 6%	530
	11th to 20th	Day	5% 9% 26% 41% 15% 3	530
	21st to end	Day	4 12% 29% 34% 16% 6%	530
October	1st to 10th	Day	4 15% 36% 31% 12% 2	530
	11th to 20th	Day	2 13% 36% 38% 9% 2	530
	21st to end	Day	5% 10% 32% 32% 18% 3	405
November	1st to 10th	Day	4 13% 31% 35% 14% 2	330
	11th to 20th	Day	10% 15% 32% 25% 15% 2	330
	21st to end	Day	8% 23% 34% 22% 10% 3	330
December	1st to 10th	Day	7% 19% 35% 26% 11% 2	330
	11th to 20th	Day	4 16% 38% 30% 11% 2	330
	21st to end	Day	15% 24% 33% 17% 8% 3	363
January	1st to 10th	Night	7% 13% 25% 30% 20% 5%	840
	11th to 20th	Night	4 9% 27% 33% 20% 6%	840
	21st to end	Night	8% 14% 28% 31% 17% 2	924
February	1st to 10th	Night	5% 12% 28% 30% 19% 6%	840
	11th to 20th	Night	8% 13% 30% 32% 15% 3	824
	21st to end	Night	7% 14% 29% 31% 16% 3	692
March	1st to 10th	Night	7% 15% 30% 31% 14% 3	808
	11th to 20th	Night	6% 14% 19% 30% 22% 8%	800
	21st to end	Night	7% 11% 24% 35% 19% 4	880
April	1st to 10th	Night	5% 9% 28% 34% 21% 4	800
	11th to 20th	Night	3 8% 31% 31% 23% 4	791
	21st to end	Night	1 9% 24% 36% 24% 6%	800
May	1st to 10th	Night	1 6% 25% 40% 23% 5%	610
	11th to 20th	Night	2 4 20% 38% 28% 8%	591
	21st to end	Night	2 4 21% 38% 30% 5%	650
June	1st to 10th	Night	1 2 20% 37% 35% 6%	600
	11th to 20th	Night	1 3 12% 41% 36% 7%	600
	21st to end	Night	2 10% 31% 46% 12%	600
July	1st to 10th	Night	1 1 6% 24% 53% 15%	585
	11th to 20th	Night	2 1 25% 57% 15%	582
	21st to end	Night	2 3 21% 54% 21%	707
August	1st to 10th	Night	1 3 23% 53% 20%	796
	11th to 20th	Night	1 1 1 17% 53% 27%	792
	21st to end	Night	3 4 19% 50% 24%	880
September	1st to 10th	Night	3 1 6% 28% 45% 18%	800
	11th to 20th	Night	3 1 8% 32% 46% 10%	800
	21st to end	Night	1 2 14% 32% 41% 11%	800
October	1st to 10th	Night	2 4 17% 38% 32% 7%	800
	11th to 20th	Night	1 2 18% 38% 31% 10%	800
	21st to end	Night	3 3 16% 37% 31% 11%	880
November	1st to 10th	Night	2 7% 18% 35% 28% 10%	820
	11th to 20th	Night	5% 7% 19% 33% 25% 11%	840
	21st to end	Night	6% 7% 27% 32% 20% 7%	840
December	1st to 10th	Night	5% 7% 26% 33% 21% 7%	840
	11th to 20th	Night	4 9% 26% 37% 21% 3	840
	21st to end	Night	7% 14% 27% 31% 15% 6%	924

Page generated at 2026-02-27 22:40:24. Version ac07

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