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 2024-01-12.

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	6% 3 1 3 3 2 3 3 4 6% 68%	75154
	Night	7% 4 1 3 3 2 3 3 4 5% 66%	74589
Transparency	Day	19% 4 6% 15% 27% 30%	74882
	Night	21% 4 6% 16% 25% 29%	74383
Seeing	Day	7% 6% 23% 38% 20% 7%	14537
	Night	7% 2 11% 35% 34% 10%	24605

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	10% 3 1 4 3 2 4 4 4 7% 58%	1708
	11th to 20th	Day	8% 3 1 3 2 2 3 4 4 8% 62%	1703
	21st to end	Day	10% 4 2 4 3 2 3 3 5% 5% 59%	1980
February	1st to 10th	Day	10% 3 2 3 3 2 2 2 3 6% 65%	1800
	11th to 20th	Day	11% 5% 1 2 3 1 3 3 4 6% 60%	1750
	21st to end	Day	13% 6% 2 4 4 2 3 3 3 6% 56%	1480
March	1st to 10th	Day	11% 6% 2 3 3 2 4 5% 6% 7% 52%	1800
	11th to 20th	Day	10% 4 2 3 3 2 3 3 4 8% 59%	1872
	21st to end	Day	7% 4 1 3 3 3 5% 4 5% 7% 58%	2312
April	1st to 10th	Day	8% 4 1 3 4 3 3 4 4 8% 59%	2160
	11th to 20th	Day	5% 4 2 3 3 2 3 3 4 6% 65%	2129
	21st to end	Day	7% 4 1 3 3 1 2 2 3 6% 68%	2160
May	1st to 10th	Day	6% 5% 2 4 4 2 3 3 4 6% 61%	2286
	11th to 20th	Day	5% 5% 1 4 3 1 2 3 3 7% 67%	2301
	21st to end	Day	6% 4 1 4 4 2 2 4 4 6% 64%	2530
June	1st to 10th	Day	3 3 1 3 2 1 2 2 4 6% 71%	2340
	11th to 20th	Day	2 3 1 3 1 1 2 2 2 4 79%	2340
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 82%	2512
July	1st to 10th	Day	1 1 1 2 2 1 3 3 3 5% 79%	2489
	11th to 20th	Day	1 2 1 2 2 1 2 3 4 5% 77%	2340
	21st to end	Day	3 2 1 2 2 2 3 4 4 8% 69%	2469
August	1st to 10th	Day	1 1 2 2 1 2 3 3 5% 82%	2149
	11th to 20th	Day	1 1 1 1 1 2 2 3 2 4 83%	2122
	21st to end	Day	1 1 2 2 1 2 3 3 4 80%	2376
September	1st to 10th	Day	2 1 1 2 2 1 2 2 3 5% 78%	2160
	11th to 20th	Day	2 1 1 2 2 1 1 2 3 4 83%	1980
	21st to end	Day	2 1 2 2 1 2 2 2 4 81%	1980
October	1st to 10th	Day	4 2 2 3 3 2 2 3 3 5% 70%	1980
	11th to 20th	Day	4 3 1 2 1 1 2 2 3 4 78%	1946
	21st to end	Day	5% 2 1 2 2 1 2 3 3 4 75%	1881
November	1st to 10th	Day	4 2 1 2 2 2 2 3 4 5% 72%	1674
	11th to 20th	Day	4 2 2 2 3 1 2 4 4 5% 70%	1710
	21st to end	Day	6% 3 2 3 3 2 3 3 4 7% 63%	1710
December	1st to 10th	Day	8% 4 1 3 3 2 3 4 3 8% 60%	1710
	11th to 20th	Day	14% 5% 3 4 4 2 4 3 6% 8% 47%	2169
	21st to end	Day	12% 6% 2 6% 5% 3 4 4 6% 6% 46%	3146
January	1st to 10th	Night	11% 4 2 4 3 2 3 3 4 7% 58%	2457
	11th to 20th	Night	9% 4 2 3 3 2 3 3 3 6% 63%	2294
	21st to end	Night	11% 5% 2 3 3 2 2 2 4 6% 60%	2376
February	1st to 10th	Night	10% 4 2 3 3 1 3 3 3 5% 63%	2160
	11th to 20th	Night	11% 4 1 3 2 1 3 3 4 4 63%	2098
	21st to end	Night	14% 7% 1 4 2 2 3 3 4 5% 54%	1776
March	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 4 8% 49%	2160
	11th to 20th	Night	12% 5% 1 3 3 1 2 3 4 7% 58%	2086
	21st to end	Night	10% 7% 2 3 4 2 3 3 4 6% 57%	2039
April	1st to 10th	Night	9% 5% 2 3 4 2 3 4 5% 6% 56%	1800
	11th to 20th	Night	7% 6% 1 3 3 1 2 3 3 5% 65%	1779
	21st to end	Night	9% 5% 1 4 2 1 3 3 4 5% 63%	1800
May	1st to 10th	Night	7% 5% 1 6% 3 2 2 2 3 6% 62%	1671
	11th to 20th	Night	5% 4 2 5% 3 2 2 2 3 5% 67%	1594
	21st to end	Night	6% 5% 1 4 2 1 2 2 4 6% 66%	1746
June	1st to 10th	Night	3 4 1 3 3 1 2 2 3 6% 74%	1620
	11th to 20th	Night	2 2 1 2 2 1 1 2 3 5% 79%	1616
	21st to end	Night	1 1 1 1 1 1 1 2 2 3 85%	1440
July	1st to 10th	Night	1 1 1 1 2 1 2 4 86%	1462
	11th to 20th	Night	1 1 1 1 1 1 2 2 4 6% 78%	1620
	21st to end	Night	2 2 1 2 3 2 3 4 4 5% 72%	1845
August	1st to 10th	Night	2 1 1 2 2 2 2 3 4 81%	1790
	11th to 20th	Night	1 1 1 1 1 1 2 2 5% 85%	1780
	21st to end	Night	1 1 2 2 1 1 2 2 3 85%	1980
September	1st to 10th	Night	2 2 2 2 1 2 2 3 4 79%	1800
	11th to 20th	Night	1 2 1 2 2 1 2 1 2 3 84%	1980
	21st to end	Night	2 2 1 2 1 1 1 1 2 3 82%	1980
October	1st to 10th	Night	4 3 1 3 3 2 2 2 3 4 73%	1980
	11th to 20th	Night	4 3 1 3 2 1 2 2 2 3 77%	2009
	21st to end	Night	5% 3 1 4 2 1 2 2 3 4 73%	2474
November	1st to 10th	Night	5% 3 1 2 3 1 3 3 3 6% 71%	2405
	11th to 20th	Night	3 3 1 3 2 2 2 3 4 6% 70%	2470
	21st to end	Night	7% 4 2 3 4 1 3 4 6% 6% 59%	2470
December	1st to 10th	Night	10% 5% 1 4 3 2 3 3 4 8% 57%	2470
	11th to 20th	Night	14% 7% 2 4 4 2 4 4 4 7% 47%	3041
	21st to end	Night	10% 6% 2 7% 4 3 4 4 5% 6% 48%	4521

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	26% 5% 5% 18% 25% 21%	1709
	11th to 20th	Day	22% 5% 6% 14% 28% 25%	1703
	21st to end	Day	28% 3 6% 15% 21% 27%	1980
February	1st to 10th	Day	24% 3 5% 12% 27% 30%	1800
	11th to 20th	Day	26% 4 3 13% 28% 25%	1760
	21st to end	Day	32% 5% 6% 14% 22% 22%	1480
March	1st to 10th	Day	30% 6% 7% 16% 22% 20%	1800
	11th to 20th	Day	25% 4 9% 17% 25% 19%	1872
	21st to end	Day	24% 7% 9% 22% 22% 15%	2312
April	1st to 10th	Day	25% 6% 8% 24% 23% 13%	2160
	11th to 20th	Day	20% 5% 6% 20% 27% 21%	2132
	21st to end	Day	20% 4 8% 20% 35% 13%	2160
May	1st to 10th	Day	23% 5% 8% 19% 26% 19%	2286
	11th to 20th	Day	19% 3 8% 22% 33% 16%	2304
	21st to end	Day	20% 4 9% 27% 28% 11%	2530
June	1st to 10th	Day	15% 3 6% 22% 35% 20%	2340
	11th to 20th	Day	11% 2 4 13% 29% 40%	2340
	21st to end	Day	10% 3 3 7% 26% 52%	2512
July	1st to 10th	Day	10% 2 3 10% 21% 54%	2421
	11th to 20th	Day	11% 4 6% 17% 28% 34%	2271
	21st to end	Day	14% 5% 8% 21% 29% 23%	2332
August	1st to 10th	Day	7% 3 4 11% 28% 47%	2135
	11th to 20th	Day	6% 3 3 15% 29% 44%	2122
	21st to end	Day	9% 3 4 10% 34% 41%	2376
September	1st to 10th	Day	11% 3 4 8% 31% 43%	2160
	11th to 20th	Day	7% 1 3 11% 32% 46%	1980
	21st to end	Day	10% 3 2 13% 30% 42%	1980
October	1st to 10th	Day	18% 3 5% 16% 24% 35%	1980
	11th to 20th	Day	13% 2 5% 10% 29% 40%	1946
	21st to end	Day	13% 4 4 9% 23% 47%	1881
November	1st to 10th	Day	15% 3 6% 13% 29% 34%	1674
	11th to 20th	Day	16% 5% 5% 11% 23% 40%	1710
	21st to end	Day	23% 5% 5% 12% 23% 33%	1710
December	1st to 10th	Day	24% 6% 6% 16% 20% 29%	1710
	11th to 20th	Day	33% 8% 10% 13% 20% 17%	2168
	21st to end	Day	37% 7% 7% 19% 18% 13%	3146
January	1st to 10th	Night	28% 4 5% 15% 26% 23%	2457
	11th to 20th	Night	24% 4 6% 14% 24% 27%	2294
	21st to end	Night	28% 4 7% 13% 20% 28%	2376
February	1st to 10th	Night	26% 3 6% 14% 22% 30%	2160
	11th to 20th	Night	26% 4 6% 14% 22% 28%	2099
	21st to end	Night	33% 4 4 15% 22% 22%	1776
March	1st to 10th	Night	34% 6% 6% 17% 19% 19%	2160
	11th to 20th	Night	26% 5% 9% 16% 25% 19%	2086
	21st to end	Night	29% 5% 9% 22% 22% 14%	2039
April	1st to 10th	Night	27% 5% 10% 23% 18% 16%	1800
	11th to 20th	Night	23% 4 7% 22% 23% 20%	1779
	21st to end	Night	22% 5% 7% 22% 30% 14%	1800
May	1st to 10th	Night	22% 5% 9% 22% 22% 20%	1671
	11th to 20th	Night	21% 4 9% 25% 27% 15%	1595
	21st to end	Night	20% 5% 12% 26% 26% 11%	1746
June	1st to 10th	Night	14% 3 7% 22% 32% 22%	1620
	11th to 20th	Night	10% 2 5% 16% 26% 40%	1616
	21st to end	Night	7% 2 3 8% 27% 53%	1440
July	1st to 10th	Night	6% 2 2 8% 28% 54%	1421
	11th to 20th	Night	8% 3 7% 20% 28% 34%	1566
	21st to end	Night	14% 4 7% 20% 33% 21%	1742
August	1st to 10th	Night	8% 4 4 12% 32% 41%	1780
	11th to 20th	Night	5% 2 3 17% 32% 41%	1780
	21st to end	Night	7% 2 3 12% 34% 42%	1980
September	1st to 10th	Night	11% 3 3 9% 30% 44%	1800
	11th to 20th	Night	8% 2 3 12% 31% 43%	1980
	21st to end	Night	10% 2 3 13% 31% 41%	1980
October	1st to 10th	Night	17% 3 5% 17% 27% 32%	1980
	11th to 20th	Night	15% 2 5% 11% 31% 36%	2009
	21st to end	Night	16% 3 4 9% 20% 48%	2474
November	1st to 10th	Night	17% 4 5% 13% 26% 35%	2405
	11th to 20th	Night	17% 3 3 13% 23% 40%	2470
	21st to end	Night	26% 4 5% 14% 19% 33%	2470
December	1st to 10th	Night	27% 4 5% 14% 20% 30%	2470
	11th to 20th	Night	34% 7% 8% 13% 22% 17%	3041
	21st to end	Night	34% 7% 10% 19% 18% 13%	4521

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	17% 4 23% 38% 16% 3	270
	11th to 20th	Day	13% 6% 20% 35% 22% 4	315
	21st to end	Day	10% 9% 26% 35% 17% 3	462
February	1st to 10th	Day	12% 6% 29% 32% 17% 5%	420
	11th to 20th	Day	13% 9% 33% 33% 9% 1	415
	21st to end	Day	15% 12% 29% 29% 14%	346
March	1st to 10th	Day	17% 12% 30% 28% 12% 1	420
	11th to 20th	Day	15% 8% 30% 32% 11% 3	420
	21st to end	Day	11% 14% 33% 31% 10% 1	462
April	1st to 10th	Day	10% 12% 26% 32% 17% 3	420
	11th to 20th	Day	6% 16% 35% 30% 11% 3	412
	21st to end	Day	6% 13% 32% 26% 15% 7%	420
May	1st to 10th	Day	10% 9% 31% 32% 13% 5%	423
	11th to 20th	Day	9% 8% 27% 35% 16% 5%	448
	21st to end	Day	9% 7% 27% 35% 16% 6%	485
June	1st to 10th	Day	4 3 24% 35% 26% 8%	450
	11th to 20th	Day	2 2 25% 40% 21% 10%	450
	21st to end	Day	2 3 17% 42% 24% 12%	450
July	1st to 10th	Day	1 1 11% 46% 27% 14%	446
	11th to 20th	Day	1 8% 43% 32% 16%	444
	21st to end	Day	2 1 8% 45% 28% 17%	487
August	1st to 10th	Day	2 1 13% 47% 24% 13%	447
	11th to 20th	Day	1 1 10% 46% 26% 15%	437
	21st to end	Day	2 1 9% 50% 25% 14%	495
September	1st to 10th	Day	3 2 20% 45% 21% 9%	450
	11th to 20th	Day	2 2 22% 48% 20% 6%	450
	21st to end	Day	2 4 25% 37% 24% 7%	450
October	1st to 10th	Day	3 6% 27% 41% 22% 1	450
	11th to 20th	Day	5% 6% 23% 39% 24% 3	450
	21st to end	Day	6% 4 23% 37% 24% 6%	396
November	1st to 10th	Day	6% 3 21% 44% 23% 3	270
	11th to 20th	Day	3 4 23% 41% 23% 5%	270
	21st to end	Day	6% 6% 27% 38% 19% 4	270
December	1st to 10th	Day	10% 4 25% 37% 21% 3	270
	11th to 20th	Day	7% 3 28% 33% 23% 6%	270
	21st to end	Day	15% 9% 29% 28% 16% 3	297
January	1st to 10th	Night	13% 3 17% 38% 20% 8%	760
	11th to 20th	Night	10% 3 18% 38% 23% 8%	728
	21st to end	Night	12% 3 19% 39% 21% 5%	792
February	1st to 10th	Night	10% 3 17% 39% 23% 8%	720
	11th to 20th	Night	11% 5% 15% 42% 21% 7%	704
	21st to end	Night	16% 5% 18% 37% 20% 5%	592
March	1st to 10th	Night	13% 4 17% 42% 20% 4	720
	11th to 20th	Night	11% 5% 13% 36% 27% 8%	720
	21st to end	Night	10% 4 16% 40% 24% 5%	792
April	1st to 10th	Night	10% 5% 12% 44% 26% 4	720
	11th to 20th	Night	9% 4 16% 41% 26% 5%	711
	21st to end	Night	8% 4 11% 42% 26% 8%	720
May	1st to 10th	Night	8% 4 14% 38% 31% 6%	591
	11th to 20th	Night	8% 1 11% 31% 41% 8%	531
	21st to end	Night	6% 2 11% 40% 33% 6%	584
June	1st to 10th	Night	2 1 9% 35% 46% 7%	540
	11th to 20th	Night	2 1 4 39% 48% 5%	536
	21st to end	Night	1 1 2 34% 53% 9%	360
July	1st to 10th	Night	1 1 20% 60% 18%	371
	11th to 20th	Night	2 1 22% 65% 10%	522
	21st to end	Night	2 1 2 14% 65% 16%	663
August	1st to 10th	Night	1 2 24% 55% 17%	716
	11th to 20th	Night	2 15% 60% 23%	712
	21st to end	Night	2 19% 57% 22%	792
September	1st to 10th	Night	3 2 25% 52% 18%	720
	11th to 20th	Night	2 1 3 31% 48% 15%	720
	21st to end	Night	3 6% 35% 42% 14%	720
October	1st to 10th	Night	5% 2 8% 44% 34% 7%	720
	11th to 20th	Night	5% 2 11% 36% 35% 11%	720
	21st to end	Night	6% 2 14% 34% 30% 14%	792
November	1st to 10th	Night	6% 3 11% 41% 30% 10%	740
	11th to 20th	Night	5% 1 15% 39% 29% 10%	760
	21st to end	Night	8% 3 16% 39% 26% 9%	760
December	1st to 10th	Night	10% 4 17% 36% 26% 7%	760
	11th to 20th	Night	14% 3 18% 39% 22% 6%	760
	21st to end	Night	12% 3 21% 39% 18% 6%	836

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