Big Blue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Blue Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 7% 3 6% 4 4 4 4 6% 7% 43%	126439
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 43%	123188
Transparency	Day	38% 6% 12% 24% 17% 3	130035
	Night	40% 5% 11% 23% 18% 3	127668
Seeing	Day	17% 14% 28% 25% 13% 3	16916
	Night	18% 13% 30% 27% 12% 1	26653

By Month

Percentage Cloud Cover SE (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	13% 12% 6% 13% 5% 4 4 3 4 5% 31%	3617
	11th to 20th	Day	9% 10% 4 8% 5% 7% 4 3 5% 6% 40%	4062
	21st to end	Day	17% 12% 5% 7% 4 3 4 2 5% 5% 36%	4488
February	1st to 10th	Day	15% 11% 5% 9% 4 4 4 3 5% 6% 33%	1800
	11th to 20th	Day	14% 10% 3 8% 5% 4 4 3 4 6% 38%	1876
	21st to end	Day	14% 10% 3 8% 4 4 4 3 6% 5% 40%	1650
March	1st to 10th	Day	16% 10% 2 5% 3 3 3 2 3 4 49%	4705
	11th to 20th	Day	9% 6% 2 5% 3 3 4 3 5% 7% 52%	4980
	21st to end	Day	14% 9% 3 7% 5% 5% 5% 4 6% 8% 34%	5662
April	1st to 10th	Day	13% 7% 3 6% 5% 4 5% 4 4 7% 42%	2400
	11th to 20th	Day	16% 9% 3 5% 4 3 4 4 4 5% 42%	2374
	21st to end	Day	19% 8% 2 7% 5% 4 5% 3 5% 7% 35%	2400
May	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 9% 32%	5497
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 6% 8% 38%	6307
	21st to end	Day	15% 7% 3 6% 5% 5% 5% 4 5% 7% 41%	6935
June	1st to 10th	Day	9% 5% 2 4 4 4 5% 4 7% 9% 47%	2800
	11th to 20th	Day	7% 4 2 6% 4 5% 4 3 6% 10% 49%	2800
	21st to end	Day	4 4 2 4 5% 5% 6% 5% 7% 10% 48%	2800
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 45%	6360
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 5% 7% 11% 42%	6321
	21st to end	Day	7% 4 2 5% 5% 5% 6% 5% 9% 12% 39%	6422
August	1st to 10th	Day	5% 4 1 5% 4 4 5% 5% 7% 9% 51%	4234
	11th to 20th	Day	13% 5% 2 6% 3 4 5% 4 6% 8% 44%	3916
	21st to end	Day	6% 5% 2 5% 4 4 5% 4 6% 8% 52%	4312
September	1st to 10th	Day	11% 5% 1 5% 4 3 4 4 6% 8% 50%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 51%	2400
	21st to end	Day	9% 5% 2 5% 3 3 4 4 5% 7% 52%	2136
October	1st to 10th	Day	9% 4 1 6% 4 3 3 3 4 7% 55%	1930
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 4 5% 52%	1900
	21st to end	Day	17% 8% 3 5% 3 4 3 2 4 5% 47%	2090
November	1st to 10th	Day	13% 5% 2 4 4 3 2 3 5% 6% 54%	1800
	11th to 20th	Day	13% 7% 3 6% 4 3 3 2 6% 6% 47%	1800
	21st to end	Day	16% 11% 3 7% 3 2 2 3 4 5% 43%	1665
December	1st to 10th	Day	9% 10% 3 6% 4 3 3 2 3 5% 50%	2610
	11th to 20th	Day	12% 9% 5% 9% 5% 4 4 3 4 6% 40%	3008
	21st to end	Day	21% 18% 5% 10% 5% 4 4 3 4 4 23%	3982
January	1st to 10th	Night	15% 11% 7% 12% 5% 4 4 3 4 4 32%	6348
	11th to 20th	Night	11% 9% 4 13% 6% 6% 5% 3 3 5% 35%	5901
	21st to end	Night	18% 12% 4 7% 4 4 4 3 4 4 37%	6490
February	1st to 10th	Night	15% 11% 5% 8% 5% 4 4 4 5% 6% 34%	2600
	11th to 20th	Night	16% 9% 3 8% 4 3 4 3 4 6% 40%	2411
	21st to end	Night	15% 9% 4 7% 4 4 3 3 4 5% 42%	1980
March	1st to 10th	Night	18% 11% 3 5% 4 3 2 2 3 5% 43%	5191
	11th to 20th	Night	11% 8% 2 5% 4 3 3 3 5% 7% 49%	4980
	21st to end	Night	17% 9% 3 8% 5% 4 5% 4 5% 6% 34%	5266
April	1st to 10th	Night	11% 8% 3 6% 4 3 3 4 5% 7% 47%	2000
	11th to 20th	Night	14% 7% 2 7% 4 3 4 3 5% 7% 45%	1975
	21st to end	Night	18% 7% 2 6% 4 3 3 4 5% 7% 40%	2000
May	1st to 10th	Night	18% 8% 2 7% 4 4 4 4 5% 8% 36%	4386
	11th to 20th	Night	13% 8% 2 6% 5% 4 4 4 5% 8% 41%	3606
	21st to end	Night	14% 7% 2 6% 5% 4 4 4 6% 8% 40%	3942
June	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 6% 8% 49%	1600
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 5% 10% 52%	1600
	21st to end	Night	4 2 2 4 4 4 3 5% 7% 9% 56%	1600
July	1st to 10th	Night	9% 5% 2 8% 5% 4 5% 3 5% 9% 46%	3618
	11th to 20th	Night	6% 4 2 5% 4 5% 4 4 6% 10% 49%	3652
	21st to end	Night	5% 5% 2 6% 5% 5% 6% 4 7% 11% 44%	4449
August	1st to 10th	Night	5% 3 1 4 3 4 3 3 5% 8% 60%	2926
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 5% 7% 50%	3133
	21st to end	Night	6% 3 1 4 4 3 3 3 6% 9% 59%	3586
September	1st to 10th	Night	12% 6% 2 4 4 3 4 3 5% 6% 54%	2000
	11th to 20th	Night	8% 6% 2 6% 4 4 3 3 6% 7% 52%	2000
	21st to end	Night	8% 5% 2 5% 4 3 4 3 5% 5% 56%	2239
October	1st to 10th	Night	8% 4 1 4 3 3 4 2 5% 6% 60%	2317
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 53%	2280
	21st to end	Night	18% 6% 2 4 3 3 4 3 4 5% 48%	2508
November	1st to 10th	Night	10% 6% 2 6% 3 3 3 3 4 4 56%	2496
	11th to 20th	Night	13% 7% 3 6% 4 3 3 3 4 6% 50%	2600
	21st to end	Night	16% 10% 2 6% 3 3 3 3 5% 5% 44%	2733
December	1st to 10th	Night	10% 9% 3 6% 4 4 3 3 4 5% 47%	4580
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 5% 6% 41%	5199
	21st to end	Night	24% 17% 5% 9% 4 4 4 3 3 4 22%	6996

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	55% 4 11% 18% 10% 2	3618
	11th to 20th	Day	43% 5% 11% 22% 15% 4	4062
	21st to end	Day	50% 3 9% 15% 18% 4	4488
February	1st to 10th	Day	52% 5% 10% 21% 11% 1	1800
	11th to 20th	Day	48% 4 11% 19% 14% 4	1885
	21st to end	Day	45% 4 8% 23% 15% 4	1650
March	1st to 10th	Day	40% 3 6% 22% 24% 5%	4705
	11th to 20th	Day	31% 4 9% 24% 25% 6%	4980
	21st to end	Day	46% 7% 13% 26% 8%	5664
April	1st to 10th	Day	41% 5% 11% 24% 18% 1	2400
	11th to 20th	Day	44% 4 10% 23% 15% 3	2374
	21st to end	Day	48% 6% 11% 21% 13% 1	2400
May	1st to 10th	Day	47% 8% 12% 20% 13%	5532
	11th to 20th	Day	43% 7% 10% 24% 14% 2	6311
	21st to end	Day	42% 7% 10% 22% 19% 1	6949
June	1st to 10th	Day	32% 9% 13% 28% 16% 2	2800
	11th to 20th	Day	30% 7% 13% 28% 19% 4	2800
	21st to end	Day	27% 10% 17% 25% 16% 5%	2800
July	1st to 10th	Day	34% 7% 12% 28% 18% 2	6291
	11th to 20th	Day	34% 8% 17% 27% 13% 1	6246
	21st to end	Day	33% 9% 19% 28% 10% 1	6240
August	1st to 10th	Day	26% 7% 16% 36% 14%	4220
	11th to 20th	Day	37% 7% 13% 25% 16% 2	3953
	21st to end	Day	29% 7% 15% 32% 14% 3	4311
September	1st to 10th	Day	32% 6% 11% 26% 21% 4	2400
	11th to 20th	Day	31% 5% 9% 28% 24% 2	2400
	21st to end	Day	31% 6% 9% 26% 25% 3	2136
October	1st to 10th	Day	18% 3 6% 20% 47% 7%	3190
	11th to 20th	Day	23% 3 6% 21% 38% 9%	3140
	21st to end	Day	33% 3 7% 28% 26% 2	3428
November	1st to 10th	Day	32% 4 13% 25% 21% 5%	1800
	11th to 20th	Day	38% 4 9% 23% 21% 5%	1800
	21st to end	Day	44% 4 9% 27% 16%	1665
December	1st to 10th	Day	37% 3 12% 31% 16% 1	2610
	11th to 20th	Day	44% 5% 13% 22% 15% 1	3005
	21st to end	Day	65% 5% 11% 12% 7% 1	3982
January	1st to 10th	Night	57% 4 9% 21% 9%	6348
	11th to 20th	Night	54% 4 7% 21% 12% 2	5901
	21st to end	Night	51% 4 9% 19% 15% 2	6490
February	1st to 10th	Night	51% 6% 10% 19% 13% 1	2600
	11th to 20th	Night	47% 5% 10% 20% 16% 3	2412
	21st to end	Night	45% 4 10% 22% 15% 4	1980
March	1st to 10th	Night	46% 3 6% 21% 21% 2	5191
	11th to 20th	Night	35% 4 9% 23% 22% 7%	4980
	21st to end	Night	49% 7% 10% 23% 10% 1	5266
April	1st to 10th	Night	36% 5% 12% 26% 18% 3	2000
	11th to 20th	Night	38% 6% 10% 23% 18% 5%	1975
	21st to end	Night	43% 6% 10% 23% 16% 1	2000
May	1st to 10th	Night	46% 8% 15% 18% 13% 1	4416
	11th to 20th	Night	41% 6% 12% 23% 14% 4	3607
	21st to end	Night	40% 7% 15% 21% 17% 1	3950
June	1st to 10th	Night	30% 8% 15% 27% 17% 3	1600
	11th to 20th	Night	26% 7% 12% 28% 24% 4	1600
	21st to end	Night	22% 7% 17% 28% 19% 7%	1600
July	1st to 10th	Night	36% 6% 10% 28% 17% 2	3580
	11th to 20th	Night	29% 8% 16% 31% 15% 2	3607
	21st to end	Night	32% 9% 20% 28% 12% 1	4320
August	1st to 10th	Night	21% 6% 19% 33% 20% 2	2916
	11th to 20th	Night	34% 6% 14% 26% 17% 3	3162
	21st to end	Night	22% 6% 17% 34% 17% 3	3586
September	1st to 10th	Night	32% 5% 10% 24% 24% 5%	2000
	11th to 20th	Night	32% 6% 9% 25% 26% 4	2000
	21st to end	Night	30% 6% 8% 24% 28% 4	2239
October	1st to 10th	Night	16% 3 7% 19% 47% 8%	3837
	11th to 20th	Night	22% 3 7% 23% 35% 11%	3779
	21st to end	Night	32% 4 8% 26% 27% 3	4120
November	1st to 10th	Night	32% 5% 12% 25% 21% 6%	2496
	11th to 20th	Night	36% 4 8% 25% 23% 4	2600
	21st to end	Night	42% 4 11% 24% 17% 2	2733
December	1st to 10th	Night	38% 5% 9% 33% 14% 1	4580
	11th to 20th	Night	44% 4 12% 21% 17% 1	5201
	21st to end	Night	65% 6% 12% 12% 5% 1	6996

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	26% 26% 32% 12% 3	300
	11th to 20th	Day	30% 25% 33% 11% 1	300
	21st to end	Day	21% 29% 38% 12% 1	330
February	1st to 10th	Day	28% 24% 35% 12% 1	300
	11th to 20th	Day	34% 25% 27% 12% 1 1	300
	21st to end	Day	29% 22% 39% 8% 2	249
March	1st to 10th	Day	23% 29% 38% 8% 1	300
	11th to 20th	Day	33% 22% 33% 11% 1 1	300
	21st to end	Day	33% 24% 32% 11% 1	414
April	1st to 10th	Day	24% 18% 36% 17% 4 1	500
	11th to 20th	Day	28% 21% 30% 16% 5%	492
	21st to end	Day	26% 19% 33% 16% 5%	500
May	1st to 10th	Day	21% 17% 36% 20% 6% 1	543
	11th to 20th	Day	17% 18% 29% 24% 10% 2	725
	21st to end	Day	14% 12% 33% 25% 11% 4	789
June	1st to 10th	Day	10% 7% 28% 34% 17% 5%	730
	11th to 20th	Day	9% 5% 27% 37% 18% 3	730
	21st to end	Day	5% 9% 32% 36% 14% 4	730
July	1st to 10th	Day	10% 5% 18% 32% 26% 10%	721
	11th to 20th	Day	6% 6% 25% 38% 22% 4	718
	21st to end	Day	8% 5% 19% 34% 29% 5%	794
August	1st to 10th	Day	9% 3 18% 42% 23% 5%	726
	11th to 20th	Day	12% 4 22% 31% 23% 8%	562
	21st to end	Day	10% 3 19% 31% 29% 8%	583
September	1st to 10th	Day	11% 6% 17% 35% 24% 7%	530
	11th to 20th	Day	8% 5% 18% 32% 26% 11%	530
	21st to end	Day	13% 9% 22% 31% 18% 8%	451
October	1st to 10th	Day	12% 11% 33% 26% 14% 5%	309
	11th to 20th	Day	22% 19% 32% 19% 7% 1	300
	21st to end	Day	31% 21% 31% 15% 2	330
November	1st to 10th	Day	18% 14% 39% 19% 7% 3	300
	11th to 20th	Day	23% 21% 29% 20% 4 2	300
	21st to end	Day	29% 19% 29% 19% 3 1	300
December	1st to 10th	Day	18% 27% 33% 14% 6% 2	300
	11th to 20th	Day	24% 20% 37% 16% 3	300
	21st to end	Day	29% 27% 31% 10% 3	330
January	1st to 10th	Night	22% 21% 34% 19% 4	1000
	11th to 20th	Night	24% 22% 35% 17% 3	1000
	21st to end	Night	21% 20% 37% 20% 1	1100
February	1st to 10th	Night	25% 18% 36% 19% 2	1000
	11th to 20th	Night	25% 20% 35% 18% 2	856
	21st to end	Night	25% 20% 36% 17% 1	660
March	1st to 10th	Night	25% 17% 37% 18% 2	722
	11th to 20th	Night	24% 14% 37% 21% 4	600
	21st to end	Night	25% 18% 36% 18% 3	660
April	1st to 10th	Night	18% 22% 34% 23% 3 1	600
	11th to 20th	Night	21% 15% 34% 24% 5%	593
	21st to end	Night	22% 13% 38% 23% 5%	600
May	1st to 10th	Night	17% 13% 34% 28% 8%	600
	11th to 20th	Night	17% 11% 28% 30% 13% 1	591
	21st to end	Night	13% 8% 30% 32% 17% 2	650
June	1st to 10th	Night	10% 3 24% 39% 21% 2	600
	11th to 20th	Night	8% 3 27% 42% 19% 1	600
	21st to end	Night	3 4 29% 43% 20% 1	600
July	1st to 10th	Night	11% 2 13% 37% 32% 5%	585
	11th to 20th	Night	5% 2 19% 41% 29% 3	582
	21st to end	Night	7% 2 17% 41% 32% 2	657
August	1st to 10th	Night	7% 3 12% 46% 30% 2	597
	11th to 20th	Night	7% 3 16% 41% 31% 3	594
	21st to end	Night	7% 2 15% 40% 34% 3	660
September	1st to 10th	Night	13% 3 17% 37% 25% 5%	600
	11th to 20th	Night	10% 4 19% 33% 29% 5%	600
	21st to end	Night	10% 8% 23% 32% 23% 4	764
October	1st to 10th	Night	9% 7% 28% 28% 24% 3	773
	11th to 20th	Night	16% 15% 32% 27% 10% 1	760
	21st to end	Night	22% 16% 33% 22% 7%	836
November	1st to 10th	Night	14% 13% 38% 25% 9%	780
	11th to 20th	Night	20% 17% 32% 24% 7%	800
	21st to end	Night	23% 18% 32% 22% 5%	933
December	1st to 10th	Night	22% 18% 33% 22% 5% 1	1000
	11th to 20th	Night	25% 15% 32% 25% 3	1000
	21st to end	Night	29% 19% 31% 18% 2	1100

Page generated at 2025-10-01 00:39:06. Version ac07

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