Big Bear Solar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Bear Solar Observatory 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	2 3 1 3 4 3 4 4 5% 6% 66%	74990
	Night	1 2 1 3 4 2 4 4 5% 6% 68%	74718
Transparency	Day	18% 4 5% 14% 28% 31%	74719
	Night	17% 3 4 14% 28% 33%	74511
Seeing	Day	5% 15% 33% 32% 11% 4	14474
	Night	4 9% 23% 33% 25% 7%	24675

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 5% 2 5% 5% 3 5% 4 7% 8% 54%	1708
	11th to 20th	Day	2 4 1 4 4 3 5% 6% 5% 7% 59%	1665
	21st to end	Day	3 5% 3 5% 5% 3 4 3 5% 5% 57%	1980
February	1st to 10th	Day	2 4 2 4 3 4 5% 4 5% 6% 61%	1800
	11th to 20th	Day	2 4 2 4 4 4 5% 4 5% 7% 59%	1750
	21st to end	Day	1 4 3 6% 5% 3 4 3 6% 7% 58%	1480
March	1st to 10th	Day	3 4 1 5% 5% 3 5% 5% 7% 10% 53%	1800
	11th to 20th	Day	2 4 2 4 4 3 4 5% 6% 9% 56%	1890
	21st to end	Day	1 3 2 3 4 3 5% 4 6% 8% 61%	2283
April	1st to 10th	Day	2 4 2 4 5% 4 3 4 5% 7% 61%	2160
	11th to 20th	Day	2 2 1 3 4 2 2 3 4 5% 72%	2129
	21st to end	Day	2 2 1 2 3 3 3 3 5% 8% 69%	2160
May	1st to 10th	Day	2 2 2 2 3 2 4 3 5% 6% 70%	2317
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 7% 73%	2301
	21st to end	Day	2 2 1 2 3 2 3 3 4 5% 72%	2530
June	1st to 10th	Day	2 1 1 3 3 2 3 3 5% 7% 71%	2340
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 83%	2340
	21st to end	Day	2 2 1 2 3 1 2 2 2 3 80%	2426
July	1st to 10th	Day	2 2 1 3 3 2 3 3 4 6% 71%	2403
	11th to 20th	Day	3 3 1 2 4 2 4 4 6% 8% 62%	2340
	21st to end	Day	4 3 2 4 5% 3 5% 5% 6% 9% 54%	2513
August	1st to 10th	Day	2 1 1 3 4 2 3 3 4 6% 70%	2149
	11th to 20th	Day	2 1 1 3 3 2 4 5% 5% 7% 68%	2122
	21st to end	Day	2 2 1 3 3 3 4 4 5% 6% 68%	2376
September	1st to 10th	Day	3 2 1 3 3 3 4 4 5% 6% 66%	2143
	11th to 20th	Day	2 1 2 2 1 2 2 2 3 83%	1980
	21st to end	Day	1 2 1 2 2 1 3 3 3 5% 77%	1980
October	1st to 10th	Day	2 3 2 2 3 2 3 2 4 7% 72%	1980
	11th to 20th	Day	1 1 1 2 2 1 3 2 4 5% 78%	1972
	21st to end	Day	1 1 1 1 2 2 2 2 4 6% 78%	1854
November	1st to 10th	Day	1 1 1 3 3 2 4 4 5% 5% 71%	1674
	11th to 20th	Day	1 3 1 3 4 3 4 4 4 6% 67%	1710
	21st to end	Day	3 3 1 3 4 3 5% 4 5% 7% 61%	1710
December	1st to 10th	Day	3 4 2 6% 5% 3 5% 5% 6% 8% 54%	1710
	11th to 20th	Day	2 5% 3 5% 7% 4 6% 6% 7% 9% 46%	2169
	21st to end	Day	3 5% 2 6% 7% 5% 7% 6% 6% 6% 46%	3146
January	1st to 10th	Night	2 3 3 5% 5% 3 6% 6% 5% 8% 53%	2457
	11th to 20th	Night	2 4 2 4 5% 3 5% 4 6% 6% 59%	2313
	21st to end	Night	3 5% 2 6% 6% 3 5% 3 5% 6% 55%	2376
February	1st to 10th	Night	3 5% 2 4 5% 3 4 4 5% 6% 59%	2160
	11th to 20th	Night	1 4 2 4 5% 3 6% 5% 4 7% 59%	2098
	21st to end	Night	1 3 2 4 6% 4 6% 6% 8% 7% 53%	1776
March	1st to 10th	Night	1 4 1 4 6% 4 6% 6% 7% 9% 51%	2160
	11th to 20th	Night	1 4 2 4 5% 2 6% 7% 7% 7% 56%	2070
	21st to end	Night	1 3 2 3 4 4 6% 4 6% 8% 58%	2070
April	1st to 10th	Night	1 3 2 4 4 2 4 4 6% 8% 60%	1800
	11th to 20th	Night	1 2 2 2 3 2 4 3 4 5% 72%	1779
	21st to end	Night	1 1 1 2 2 2 4 4 5% 8% 71%	1800
May	1st to 10th	Night	1 1 1 2 3 1 3 3 6% 7% 72%	1642
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 5% 77%	1594
	21st to end	Night	1 1 1 1 1 2 2 4 7% 79%	1746
June	1st to 10th	Night	1 1 1 1 2 3 3 6% 80%	1620
	11th to 20th	Night	1 1 1 1 1 1 2 3 87%	1620
	21st to end	Night	1 1 1 1 1 1 2 2 2 3 85%	1512
July	1st to 10th	Night	1 1 2 1 1 2 2 2 4 83%	1538
	11th to 20th	Night	2 1 1 3 3 2 2 4 3 5% 73%	1620
	21st to end	Night	2 2 1 3 3 2 3 4 5% 6% 69%	1800
August	1st to 10th	Night	2 2 1 1 2 1 3 2 3 4 80%	1790
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 82%	1780
	21st to end	Night	1 2 1 2 2 1 1 2 2 4 82%	1980
September	1st to 10th	Night	1 1 1 2 2 2 2 2 3 4 80%	1815
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 89%	1980
	21st to end	Night	1 1 1 1 1 1 2 2 3 87%	1980
October	1st to 10th	Night	1 1 1 3 2 1 2 3 4 4 76%	1980
	11th to 20th	Night	1 1 1 1 2 2 2 2 3 3 82%	1984
	21st to end	Night	2 1 2 3 2 2 2 3 5% 78%	2501
November	1st to 10th	Night	1 1 1 3 3 2 4 3 6% 6% 71%	2405
	11th to 20th	Night	1 2 1 3 4 2 4 4 4 7% 68%	2470
	21st to end	Night	2 2 1 3 5% 3 6% 5% 5% 7% 59%	2470
December	1st to 10th	Night	3 3 2 5% 5% 4 5% 3 7% 9% 55%	2470
	11th to 20th	Night	2 5% 2 5% 6% 4 5% 6% 7% 11% 48%	3041
	21st to end	Night	2 3 3 5% 6% 5% 6% 7% 6% 7% 50%	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	24% 5% 6% 18% 28% 19%	1709
	11th to 20th	Day	22% 5% 8% 14% 30% 22%	1665
	21st to end	Day	28% 4 5% 18% 25% 20%	1980
February	1st to 10th	Day	23% 4 4 15% 26% 29%	1800
	11th to 20th	Day	23% 4 4 18% 27% 25%	1760
	21st to end	Day	24% 4 7% 14% 30% 20%	1480
March	1st to 10th	Day	24% 6% 6% 16% 28% 20%	1800
	11th to 20th	Day	22% 6% 4 20% 29% 19%	1890
	21st to end	Day	20% 6% 6% 22% 30% 17%	2283
April	1st to 10th	Day	24% 3 6% 21% 30% 17%	2160
	11th to 20th	Day	16% 3 3 19% 33% 27%	2132
	21st to end	Day	15% 3 6% 19% 39% 18%	2160
May	1st to 10th	Day	16% 3 6% 17% 34% 25%	2317
	11th to 20th	Day	12% 3 4 15% 42% 24%	2304
	21st to end	Day	16% 2 5% 17% 38% 23%	2530
June	1st to 10th	Day	13% 2 4 12% 31% 37%	2340
	11th to 20th	Day	6% 2 2 8% 24% 57%	2340
	21st to end	Day	12% 2 2 4 18% 62%	2426
July	1st to 10th	Day	14% 2 3 7% 18% 55%	2339
	11th to 20th	Day	18% 5% 8% 17% 22% 31%	2271
	21st to end	Day	22% 5% 10% 18% 21% 23%	2373
August	1st to 10th	Day	15% 3 6% 9% 19% 47%	2135
	11th to 20th	Day	14% 5% 4 14% 20% 42%	2122
	21st to end	Day	17% 3 3 11% 31% 36%	2376
September	1st to 10th	Day	18% 4 5% 7% 24% 43%	2143
	11th to 20th	Day	9% 2 2 6% 31% 51%	1980
	21st to end	Day	11% 2 3 11% 31% 41%	1980
October	1st to 10th	Day	15% 3 3 13% 27% 39%	1980
	11th to 20th	Day	10% 2 2 12% 32% 42%	1972
	21st to end	Day	10% 3 3 8% 32% 44%	1854
November	1st to 10th	Day	15% 4 3 15% 33% 31%	1674
	11th to 20th	Day	17% 3 5% 13% 26% 36%	1710
	21st to end	Day	22% 4 5% 14% 28% 27%	1710
December	1st to 10th	Day	27% 5% 7% 16% 22% 24%	1710
	11th to 20th	Day	29% 9% 6% 18% 22% 16%	2168
	21st to end	Day	31% 9% 5% 17% 24% 14%	3146
January	1st to 10th	Night	27% 4 6% 16% 26% 21%	2457
	11th to 20th	Night	22% 5% 5% 16% 27% 26%	2313
	21st to end	Night	29% 2 6% 13% 25% 25%	2376
February	1st to 10th	Night	25% 3 5% 12% 24% 31%	2160
	11th to 20th	Night	23% 4 4 14% 26% 29%	2099
	21st to end	Night	23% 7% 5% 14% 26% 25%	1776
March	1st to 10th	Night	25% 6% 5% 17% 24% 22%	2160
	11th to 20th	Night	22% 5% 6% 17% 29% 23%	2070
	21st to end	Night	21% 4 5% 19% 33% 17%	2070
April	1st to 10th	Night	20% 4 6% 22% 30% 19%	1800
	11th to 20th	Night	15% 4 3 15% 34% 29%	1779
	21st to end	Night	11% 3 6% 15% 43% 23%	1800
May	1st to 10th	Night	10% 3 4 18% 35% 30%	1642
	11th to 20th	Night	10% 2 5% 13% 42% 28%	1595
	21st to end	Night	8% 2 5% 18% 40% 28%	1746
June	1st to 10th	Night	8% 2 3 12% 30% 45%	1620
	11th to 20th	Night	6% 2 2 8% 25% 58%	1620
	21st to end	Night	8% 1 2 4 18% 67%	1512
July	1st to 10th	Night	8% 2 2 6% 21% 60%	1493
	11th to 20th	Night	14% 3 6% 17% 22% 38%	1566
	21st to end	Night	17% 4 9% 20% 27% 24%	1700
August	1st to 10th	Night	10% 3 5% 10% 23% 49%	1780
	11th to 20th	Night	8% 2 3 16% 27% 44%	1780
	21st to end	Night	10% 1 2 9% 32% 45%	1980
September	1st to 10th	Night	11% 3 3 8% 24% 52%	1815
	11th to 20th	Night	4 2 2 7% 29% 56%	1980
	21st to end	Night	6% 1 2 11% 33% 47%	1980
October	1st to 10th	Night	12% 3 3 12% 30% 40%	1980
	11th to 20th	Night	9% 2 2 10% 32% 45%	1984
	21st to end	Night	11% 2 3 7% 31% 46%	2501
November	1st to 10th	Night	13% 4 3 11% 35% 34%	2405
	11th to 20th	Night	16% 3 4 12% 29% 36%	2470
	21st to end	Night	22% 4 4 14% 27% 28%	2470
December	1st to 10th	Night	23% 4 6% 14% 26% 27%	2470
	11th to 20th	Night	25% 9% 7% 19% 22% 19%	3041
	21st to end	Night	27% 5% 7% 15% 26% 20%	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	10% 23% 34% 25% 8%	270
	11th to 20th	Day	12% 13% 33% 31% 9% 1	279
	21st to end	Day	9% 22% 34% 26% 7% 2	462
February	1st to 10th	Day	7% 23% 35% 24% 9% 3	420
	11th to 20th	Day	11% 24% 37% 23% 5%	415
	21st to end	Day	12% 28% 36% 20% 4	346
March	1st to 10th	Day	15% 28% 35% 18% 5%	420
	11th to 20th	Day	12% 21% 38% 21% 6% 1	420
	21st to end	Day	11% 29% 37% 18% 5%	462
April	1st to 10th	Day	9% 21% 37% 24% 7% 1	420
	11th to 20th	Day	8% 29% 36% 19% 6% 1	412
	21st to end	Day	7% 24% 33% 25% 8% 4	420
May	1st to 10th	Day	6% 21% 38% 25% 6% 4	423
	11th to 20th	Day	5% 18% 38% 28% 8% 4	448
	21st to end	Day	6% 16% 38% 28% 8% 4	485
June	1st to 10th	Day	2 14% 34% 33% 12% 5%	450
	11th to 20th	Day	1 8% 37% 38% 10% 7%	450
	21st to end	Day	3 8% 32% 38% 12% 6%	450
July	1st to 10th	Day	2 2 26% 47% 14% 8%	446
	11th to 20th	Day	2 21% 48% 19% 10%	444
	21st to end	Day	2 2 18% 47% 19% 11%	487
August	1st to 10th	Day	2 4 31% 42% 14% 8%	447
	11th to 20th	Day	2 1 22% 45% 21% 8%	437
	21st to end	Day	2 1 26% 46% 17% 8%	495
September	1st to 10th	Day	3 4 33% 40% 17% 3	450
	11th to 20th	Day	1 7% 38% 39% 14% 1	450
	21st to end	Day	2 13% 29% 36% 17% 2	450
October	1st to 10th	Day	4 13% 37% 36% 10% 1	450
	11th to 20th	Day	1 10% 39% 36% 13% 1	450
	21st to end	Day	3 12% 33% 36% 14% 3	369
November	1st to 10th	Day	5% 12% 35% 34% 12% 1	270
	11th to 20th	Day	4 10% 37% 33% 11% 4	270
	21st to end	Day	4 19% 33% 31% 11% 1	270
December	1st to 10th	Day	7% 14% 37% 31% 9% 2	270
	11th to 20th	Day	5% 14% 36% 33% 11% 1	270
	21st to end	Day	8% 26% 36% 20% 8% 2	297
January	1st to 10th	Night	8% 13% 27% 28% 17% 7%	760
	11th to 20th	Night	6% 10% 27% 32% 20% 5%	724
	21st to end	Night	7% 16% 30% 27% 17% 3	792
February	1st to 10th	Night	7% 11% 30% 30% 16% 6%	720
	11th to 20th	Night	4 17% 30% 28% 16% 5%	704
	21st to end	Night	7% 18% 32% 26% 14% 3	592
March	1st to 10th	Night	7% 14% 33% 28% 15% 3	720
	11th to 20th	Night	5% 16% 25% 28% 20% 7%	720
	21st to end	Night	4 19% 29% 30% 15% 3	792
April	1st to 10th	Night	4 13% 34% 30% 16% 4	720
	11th to 20th	Night	2 15% 33% 26% 19% 4	711
	21st to end	Night	3 13% 28% 30% 19% 7%	720
May	1st to 10th	Night	2 11% 30% 37% 18% 3	562
	11th to 20th	Night	1 10% 24% 38% 23% 4	531
	21st to end	Night	2 9% 27% 35% 23% 4	584
June	1st to 10th	Night	1 5% 17% 47% 26% 4	540
	11th to 20th	Night	1 2 25% 44% 26% 3	540
	21st to end	Night	1 2 20% 42% 31% 4	432
July	1st to 10th	Night	1 6% 40% 44% 10%	443
	11th to 20th	Night	3 5% 47% 40% 5%	522
	21st to end	Night	2 5% 38% 48% 6%	618
August	1st to 10th	Night	2 10% 35% 41% 12%	716
	11th to 20th	Night	1 1 4 33% 47% 13%	712
	21st to end	Night	3 6% 32% 45% 15%	792
September	1st to 10th	Night	1 2 8% 31% 45% 13%	720
	11th to 20th	Night	1 14% 37% 36% 12%	720
	21st to end	Night	1 3 17% 34% 36% 9%	720
October	1st to 10th	Night	2 5% 23% 41% 24% 4	720
	11th to 20th	Night	2 4 22% 33% 29% 10%	720
	21st to end	Night	2 6% 25% 30% 25% 11%	792
November	1st to 10th	Night	3 8% 23% 33% 24% 9%	740
	11th to 20th	Night	2 5% 24% 36% 24% 8%	760
	21st to end	Night	3 9% 29% 30% 23% 6%	760
December	1st to 10th	Night	5% 11% 31% 29% 20% 5%	760
	11th to 20th	Night	9% 11% 30% 31% 16% 4	760
	21st to end	Night	6% 16% 31% 27% 15% 5%	836

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