San Bernardino County Museum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Bernardino County Museum clear sky chart from 2005-11-06 to 2025-11-18.

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	5% 3 1 3 2 2 2 2 4 5% 71%	135948
	Night	6% 4 1 4 2 2 2 2 3 5% 68%	132002
Transparency	Day	16% 4 6% 15% 28% 31%	131037
	Night	19% 4 6% 16% 27% 27%	127887
Seeing	Day	7% 6% 23% 37% 20% 7%	17128
	Night	7% 3 12% 35% 34% 9%	27096

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	6% 2 1 3 2 2 2 2 3 4 73%	4098
	11th to 20th	Day	6% 2 1 3 1 2 2 2 3 6% 72%	4282
	21st to end	Day	10% 4 2 4 3 4 4 3 5% 9% 52%	5016
February	1st to 10th	Day	11% 4 1 3 3 3 2 2 4 5% 61%	2000
	11th to 20th	Day	12% 5% 1 3 2 2 3 2 5% 6% 58%	1950
	21st to end	Day	12% 6% 2 5% 3 3 3 2 4 5% 55%	1650
March	1st to 10th	Day	15% 7% 2 4 2 3 3 3 6% 7% 47%	4518
	11th to 20th	Day	10% 4 1 4 3 3 3 4 5% 7% 55%	4749
	21st to end	Day	10% 4 1 5% 3 3 4 3 5% 7% 54%	5827
April	1st to 10th	Day	7% 5% 1 4 3 3 3 3 4 7% 60%	2400
	11th to 20th	Day	5% 5% 1 3 3 3 3 3 4 5% 64%	2369
	21st to end	Day	6% 5% 1 4 2 2 2 2 3 6% 66%	2400
May	1st to 10th	Day	6% 4 1 5% 3 2 3 2 3 5% 66%	5748
	11th to 20th	Day	2 3 1 6% 3 2 2 2 4 6% 67%	5841
	21st to end	Day	2 3 1 7% 3 2 2 2 3 6% 68%	6423
June	1st to 10th	Day	3 3 1 4 2 2 2 2 4 6% 70%	2600
	11th to 20th	Day	2 3 1 3 1 1 2 1 2 4 80%	2600
	21st to end	Day	1 2 1 2 1 2 2 2 2 3 83%	2792
July	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	6236
	11th to 20th	Day	1 1 2 1 1 2 2 3 3 84%	5880
	21st to end	Day	1 1 2 2 2 2 2 3 4 82%	6198
August	1st to 10th	Day	1 1 1 1 2 2 4 87%	5346
	11th to 20th	Day	1 1 1 1 1 2 3 90%	5364
	21st to end	Day	2 1 1 1 2 2 2 3 4 83%	5984
September	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 78%	2400
	11th to 20th	Day	2 1 1 3 2 1 1 1 3 4 82%	2200
	21st to end	Day	2 1 1 2 1 1 2 2 3 4 80%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 2 4 80%	3600
	11th to 20th	Day	3 3 1 2 2 1 1 1 3 5% 77%	3519
	21st to end	Day	3 2 1 2 2 2 3 3 4 7% 72%	3372
November	1st to 10th	Day	4 3 1 2 2 2 2 2 4 5% 73%	1854
	11th to 20th	Day	5% 3 1 3 3 2 2 3 4 5% 69%	1863
	21st to end	Day	6% 3 1 4 3 3 3 3 4 7% 62%	1800
December	1st to 10th	Day	4 2 1 2 2 2 3 3 4 9% 67%	2950
	11th to 20th	Day	9% 4 2 3 3 4 4 4 6% 8% 54%	3409
	21st to end	Day	8% 5% 2 6% 4 3 4 4 6% 9% 49%	4510
January	1st to 10th	Night	6% 3 1 3 2 2 2 2 3 4 72%	5867
	11th to 20th	Night	5% 5% 1 5% 3 2 2 2 3 5% 66%	5704
	21st to end	Night	13% 5% 2 6% 3 3 3 3 5% 5% 50%	5984
February	1st to 10th	Night	12% 6% 2 3 3 2 2 3 4 4 59%	2400
	11th to 20th	Night	11% 5% 1 4 2 2 3 2 4 5% 61%	2338
	21st to end	Night	14% 7% 1 4 2 3 3 3 4 4 55%	1980
March	1st to 10th	Night	21% 9% 2 5% 3 3 3 2 4 6% 42%	5376
	11th to 20th	Night	9% 7% 2 6% 4 3 4 4 5% 6% 50%	5249
	21st to end	Night	11% 10% 2 6% 3 2 3 2 4 5% 51%	5143
April	1st to 10th	Night	9% 6% 2 4 4 3 3 3 5% 6% 57%	2000
	11th to 20th	Night	8% 6% 1 3 3 2 2 3 3 5% 63%	1979
	21st to end	Night	9% 6% 1 4 2 2 3 2 4 5% 61%	2000
May	1st to 10th	Night	10% 6% 2 6% 2 2 2 1 2 3 64%	4221
	11th to 20th	Night	4 5% 1 6% 3 2 2 2 4 4 68%	4074
	21st to end	Night	4 5% 1 6% 2 2 2 2 3 5% 70%	4474
June	1st to 10th	Night	2 4 1 4 2 2 2 2 3 6% 73%	1800
	11th to 20th	Night	2 2 1 3 2 1 1 2 2 5% 79%	1794
	21st to end	Night	1 2 1 1 1 1 2 3 3 86%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 93%	3704
	11th to 20th	Night	1 1 1 1 1 1 2 4 87%	4100
	21st to end	Night	1 1 2 2 1 1 1 2 3 86%	4667
August	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	4486
	11th to 20th	Night	1 1 1 1 1 2 3 90%	4506
	21st to end	Night	1 1 2 1 1 2 1 2 3 86%	5015
September	1st to 10th	Night	2 2 2 2 2 2 1 3 4 80%	2000
	11th to 20th	Night	2 2 1 3 2 1 1 1 2 3 82%	2200
	21st to end	Night	2 3 1 2 1 1 1 1 2 3 82%	2200
October	1st to 10th	Night	2 2 2 2 1 1 1 2 3 83%	3600
	11th to 20th	Night	3 5% 1 4 5% 2 2 1 3 5% 70%	3634
	21st to end	Night	3 2 1 3 2 2 2 2 3 4 75%	4432
November	1st to 10th	Night	5% 3 1 2 3 2 2 2 3 6% 71%	2665
	11th to 20th	Night	4 3 2 4 2 2 2 3 4 6% 68%	2691
	21st to end	Night	8% 5% 1 4 4 2 3 3 6% 7% 58%	2600
December	1st to 10th	Night	6% 3 1 3 2 2 2 2 5% 6% 68%	4240
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 6% 7% 53%	4811
	21st to end	Night	8% 5% 2 6% 3 4 4 3 6% 9% 51%	6468

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	16% 3 3 20% 35% 23%	4099
	11th to 20th	Day	15% 5% 8% 17% 23% 31%	4282
	21st to end	Day	27% 6% 5% 14% 25% 23%	5016
February	1st to 10th	Day	26% 4 5% 12% 26% 27%	2000
	11th to 20th	Day	27% 4 4 15% 27% 23%	1960
	21st to end	Day	32% 5% 5% 14% 21% 23%	1650
March	1st to 10th	Day	32% 7% 8% 14% 26% 13%	4518
	11th to 20th	Day	26% 6% 11% 25% 21% 11%	4749
	21st to end	Day	25% 8% 11% 28% 18% 10%	5827
April	1st to 10th	Day	24% 6% 8% 23% 24% 15%	2400
	11th to 20th	Day	21% 5% 6% 20% 27% 21%	2372
	21st to end	Day	20% 5% 8% 20% 35% 12%	2400
May	1st to 10th	Day	19% 6% 7% 20% 29% 19%	5748
	11th to 20th	Day	14% 3 10% 23% 37% 12%	5844
	21st to end	Day	12% 6% 8% 20% 40% 14%	6424
June	1st to 10th	Day	14% 4 6% 23% 35% 18%	2600
	11th to 20th	Day	11% 2 4 12% 29% 42%	2600
	21st to end	Day	9% 3 3 7% 25% 54%	2792
July	1st to 10th	Day	4 1 1 4 17% 72%	6169
	11th to 20th	Day	7% 3 3 9% 36% 42%	5811
	21st to end	Day	6% 5% 4 12% 30% 43%	6019
August	1st to 10th	Day	7% 2 2 12% 39% 38%	3890
	11th to 20th	Day	4 2 3 13% 24% 56%	3880
	21st to end	Day	6% 2 3 6% 26% 58%	4311
September	1st to 10th	Day	10% 3 4 8% 31% 44%	2400
	11th to 20th	Day	7% 3 3 10% 33% 44%	2200
	21st to end	Day	10% 3 3 13% 28% 43%	2200
October	1st to 10th	Day	11% 2 3 9% 19% 56%	3600
	11th to 20th	Day	10% 4 3 10% 33% 39%	3519
	21st to end	Day	12% 4 2 10% 28% 44%	3372
November	1st to 10th	Day	14% 3 5% 13% 30% 35%	1854
	11th to 20th	Day	17% 4 5% 11% 23% 39%	1863
	21st to end	Day	24% 5% 6% 12% 22% 32%	1800
December	1st to 10th	Day	15% 4 4 17% 25% 34%	2950
	11th to 20th	Day	26% 7% 8% 13% 29% 17%	3408
	21st to end	Day	30% 5% 9% 22% 24% 9%	4510
January	1st to 10th	Night	17% 3 4 19% 33% 24%	5867
	11th to 20th	Night	19% 6% 8% 19% 20% 28%	5704
	21st to end	Night	30% 7% 7% 12% 23% 21%	5984
February	1st to 10th	Night	29% 4 6% 14% 21% 27%	2400
	11th to 20th	Night	26% 5% 7% 16% 21% 25%	2339
	21st to end	Night	33% 4 5% 14% 21% 23%	1980
March	1st to 10th	Night	40% 7% 9% 14% 19% 12%	5376
	11th to 20th	Night	30% 7% 11% 22% 20% 10%	5249
	21st to end	Night	31% 8% 12% 21% 21% 7%	5143
April	1st to 10th	Night	27% 6% 10% 22% 20% 16%	2000
	11th to 20th	Night	23% 5% 7% 22% 24% 19%	1979
	21st to end	Night	23% 6% 7% 23% 29% 13%	2000
May	1st to 10th	Night	24% 5% 8% 24% 23% 17%	4218
	11th to 20th	Night	16% 4 10% 29% 31% 10%	4075
	21st to end	Night	14% 6% 12% 24% 33% 11%	4474
June	1st to 10th	Night	14% 3 8% 23% 32% 21%	1800
	11th to 20th	Night	9% 3 5% 16% 27% 40%	1794
	21st to end	Night	7% 2 3 8% 27% 54%	1600
July	1st to 10th	Night	2 1 1 5% 20% 71%	3663
	11th to 20th	Night	4 2 3 12% 47% 31%	4046
	21st to end	Night	6% 3 3 17% 37% 33%	4535
August	1st to 10th	Night	5% 2 2 17% 42% 31%	3256
	11th to 20th	Night	3 1 3 17% 28% 47%	3256
	21st to end	Night	4 1 2 7% 36% 50%	3608
September	1st to 10th	Night	10% 3 3 9% 31% 44%	2000
	11th to 20th	Night	9% 2 3 12% 33% 41%	2200
	21st to end	Night	11% 2 3 13% 30% 41%	2200
October	1st to 10th	Night	10% 2 3 10% 21% 55%	3600
	11th to 20th	Night	12% 9% 3 9% 35% 31%	3634
	21st to end	Night	13% 3 3 11% 25% 45%	4432
November	1st to 10th	Night	16% 3 5% 14% 26% 36%	2665
	11th to 20th	Night	19% 3 3 13% 23% 38%	2691
	21st to end	Night	26% 4 6% 13% 19% 32%	2600
December	1st to 10th	Night	18% 3 4 15% 28% 32%	4240
	11th to 20th	Night	27% 6% 8% 15% 30% 14%	4811
	21st to end	Night	27% 6% 12% 24% 21% 9%	6468

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	15% 4 24% 36% 17% 4	300
	11th to 20th	Day	12% 6% 21% 35% 22% 3	378
	21st to end	Day	10% 9% 26% 34% 18% 3	550
February	1st to 10th	Day	15% 7% 27% 31% 16% 4	500
	11th to 20th	Day	14% 8% 31% 35% 9% 2	495
	21st to end	Day	14% 12% 28% 32% 13% 1	414
March	1st to 10th	Day	18% 12% 29% 28% 11% 1	500
	11th to 20th	Day	15% 9% 31% 32% 10% 3	500
	21st to end	Day	11% 14% 33% 31% 10% 1	550
April	1st to 10th	Day	10% 13% 25% 31% 16% 5%	500
	11th to 20th	Day	6% 14% 34% 31% 12% 4	492
	21st to end	Day	6% 13% 31% 27% 16% 8%	500
May	1st to 10th	Day	10% 8% 31% 33% 14% 5%	503
	11th to 20th	Day	9% 9% 28% 33% 16% 5%	528
	21st to end	Day	8% 6% 28% 36% 16% 6%	573
June	1st to 10th	Day	3 3 22% 36% 27% 8%	530
	11th to 20th	Day	2 2 26% 40% 22% 9%	530
	21st to end	Day	2 3 18% 42% 23% 12%	530
July	1st to 10th	Day	1 1 12% 44% 28% 15%	526
	11th to 20th	Day	1 7% 43% 32% 17%	524
	21st to end	Day	1 1 8% 45% 27% 18%	575
August	1st to 10th	Day	2 1 13% 45% 25% 14%	527
	11th to 20th	Day	1 1 10% 45% 26% 17%	517
	21st to end	Day	2 1 11% 47% 24% 15%	583
September	1st to 10th	Day	3 2 19% 43% 21% 12%	530
	11th to 20th	Day	3 3 22% 46% 19% 7%	530
	21st to end	Day	3 4 25% 37% 24% 7%	530
October	1st to 10th	Day	3 5% 25% 39% 22% 5%	530
	11th to 20th	Day	5% 5% 22% 38% 25% 4	530
	21st to end	Day	5% 4 22% 36% 24% 9%	472
November	1st to 10th	Day	5% 2 21% 41% 25% 6%	330
	11th to 20th	Day	6% 4 22% 40% 22% 5%	321
	21st to end	Day	6% 6% 26% 39% 19% 4	300
December	1st to 10th	Day	9% 3 23% 38% 22% 4	300
	11th to 20th	Day	7% 3 26% 32% 24% 8%	300
	21st to end	Day	14% 8% 27% 32% 16% 3	330
January	1st to 10th	Night	12% 3 18% 39% 21% 8%	800
	11th to 20th	Night	10% 3 18% 40% 22% 7%	800
	21st to end	Night	13% 4 19% 39% 20% 5%	880
February	1st to 10th	Night	13% 3 17% 39% 21% 7%	800
	11th to 20th	Night	11% 5% 17% 42% 19% 6%	784
	21st to end	Night	15% 5% 19% 38% 19% 4	660
March	1st to 10th	Night	15% 5% 17% 41% 18% 4	800
	11th to 20th	Night	12% 5% 17% 35% 24% 7%	800
	21st to end	Night	11% 5% 17% 40% 22% 5%	880
April	1st to 10th	Night	10% 5% 14% 43% 25% 4	800
	11th to 20th	Night	9% 3 16% 42% 25% 5%	791
	21st to end	Night	9% 5% 12% 41% 26% 7%	800
May	1st to 10th	Night	8% 4 14% 38% 31% 5%	656
	11th to 20th	Night	7% 2 13% 31% 39% 7%	591
	21st to end	Night	6% 3 11% 41% 34% 6%	650
June	1st to 10th	Night	2 1 9% 36% 45% 7%	600
	11th to 20th	Night	2 1 6% 40% 46% 5%	594
	21st to end	Night	2 1 2 34% 54% 9%	400
July	1st to 10th	Night	1 21% 60% 18%	414
	11th to 20th	Night	2 1 24% 63% 11%	582
	21st to end	Night	2 1 2 16% 63% 15%	738
August	1st to 10th	Night	1 2 26% 54% 17%	796
	11th to 20th	Night	2 1 17% 58% 23%	792
	21st to end	Night	1 1 20% 56% 21%	880
September	1st to 10th	Night	3 2 25% 53% 17%	800
	11th to 20th	Night	4 1 4 31% 47% 13%	800
	21st to end	Night	3 7% 36% 41% 13%	800
October	1st to 10th	Night	4 2 8% 45% 35% 7%	800
	11th to 20th	Night	5% 2 11% 37% 34% 10%	800
	21st to end	Night	6% 2 13% 36% 31% 13%	880
November	1st to 10th	Night	6% 2 11% 41% 31% 10%	820
	11th to 20th	Night	6% 2 16% 39% 28% 10%	828
	21st to end	Night	9% 3 16% 40% 25% 8%	800
December	1st to 10th	Night	10% 3 16% 38% 26% 7%	800
	11th to 20th	Night	13% 3 17% 39% 23% 6%	800
	21st to end	Night	12% 3 21% 39% 19% 6%	880

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