CBBHO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CBBHO clear sky chart from 2005-11-06 to 2026-09-29.

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% 7% 3 7% 4 4 4 4 6% 8% 47%	158409
	Night	4 6% 3 8% 5% 5% 5% 4 6% 8% 47%	150589
Transparency	Day	32% 7% 14% 26% 17% 5%	153187
	Night	31% 7% 15% 26% 17% 4	146823
Seeing	Day	14% 22% 34% 21% 8% 1	18578
	Night	10% 21% 36% 25% 8% 1	28446

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 7% 5% 11% 6% 5% 6% 4 6% 8% 40%	4720
	11th to 20th	Day	1 6% 4 16% 6% 5% 6% 3 4 7% 41%	4740
	21st to end	Day	5% 4 10% 6% 4 4 4 6% 8% 49%	5285
February	1st to 10th	Day	2 11% 5% 15% 8% 6% 5% 4 5% 5% 34%	1890
	11th to 20th	Day	2 12% 5% 12% 5% 4 5% 4 6% 7% 39%	1970
	21st to end	Day	2 10% 5% 12% 5% 5% 5% 5% 5% 6% 40%	1730
March	1st to 10th	Day	2 9% 4 9% 5% 3 3 3 5% 6% 52%	4600
	11th to 20th	Day	3 9% 2 6% 4 3 3 3 6% 10% 50%	4828
	21st to end	Day	9% 16% 4 9% 4 4 4 4 5% 7% 34%	6070
April	1st to 10th	Day	7% 14% 4 7% 5% 4 4 4 6% 7% 38%	2520
	11th to 20th	Day	10% 14% 5% 9% 6% 5% 4 3 5% 6% 33%	2491
	21st to end	Day	13% 14% 5% 8% 4 4 4 4 4 7% 32%	2635
May	1st to 10th	Day	8% 12% 4 7% 5% 4 5% 4 5% 5% 40%	7971
	11th to 20th	Day	14% 9% 2 5% 3 4 4 4 5% 7% 43%	8191
	21st to end	Day	12% 7% 3 6% 4 5% 5% 4 6% 8% 41%	9026
June	1st to 10th	Day	9% 4 2 6% 5% 4 5% 4 7% 9% 45%	2940
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 7% 8% 46%	2940
	21st to end	Day	6% 4 1 5% 3 4 4 4 6% 7% 57%	2940
July	1st to 10th	Day	3 3 2 5% 4 5% 6% 6% 8% 10% 47%	8249
	11th to 20th	Day	5% 3 1 4 3 4 4 4 6% 8% 58%	8250
	21st to end	Day	4 4 2 4 4 5% 5% 5% 8% 10% 49%	8957
August	1st to 10th	Day	6% 3 1 4 3 4 5% 4 6% 8% 57%	8142
	11th to 20th	Day	5% 3 2 5% 4 4 6% 5% 8% 9% 50%	7254
	21st to end	Day	6% 5% 2 5% 3 4 4 4 7% 8% 51%	7751
September	1st to 10th	Day	7% 4 1 4 2 2 3 3 5% 6% 61%	2520
	11th to 20th	Day	7% 5% 1 5% 3 3 4 4 5% 7% 56%	2462
	21st to end	Day	9% 5% 2 5% 3 3 3 3 5% 6% 57%	2288
October	1st to 10th	Day	6% 5% 1 3 2 2 2 2 3 4 68%	3345
	11th to 20th	Day	7% 7% 3 5% 3 3 2 2 4 6% 59%	3235
	21st to end	Day	7% 9% 3 6% 4 4 4 3 6% 8% 48%	3190
November	1st to 10th	Day	4 7% 3 6% 5% 4 6% 4 7% 8% 46%	1853
	11th to 20th	Day	2 7% 3 8% 4 4 4 3 6% 9% 49%	1848
	21st to end	Day	3 6% 4 7% 4 4 5% 6% 7% 9% 45%	1680
December	1st to 10th	Day	1 5% 3 13% 5% 4 3 4 5% 7% 50%	2700
	11th to 20th	Day	1 4 5% 6% 5% 5% 4 4 7% 10% 47%	3106
	21st to end	Day	2 8% 4 11% 6% 6% 4 4 5% 7% 43%	4092
January	1st to 10th	Night	2 9% 4 12% 6% 5% 6% 4 6% 8% 39%	8201
	11th to 20th	Night	4 3 12% 7% 6% 5% 5% 6% 9% 43%	8250
	21st to end	Night	1 3 3 12% 7% 5% 4 3 4 6% 50%	8986
February	1st to 10th	Night	2 8% 5% 13% 5% 6% 5% 5% 7% 8% 36%	2730
	11th to 20th	Night	2 7% 3 10% 7% 6% 6% 5% 7% 7% 39%	2535
	21st to end	Night	1 8% 4 9% 6% 6% 5% 3 6% 7% 44%	2076
March	1st to 10th	Night	2 8% 4 8% 5% 4 4 3 5% 6% 50%	5494
	11th to 20th	Night	3 7% 3 7% 5% 4 4 4 6% 8% 50%	5365
	21st to end	Night	4 11% 5% 10% 6% 6% 6% 5% 7% 10% 32%	5125
April	1st to 10th	Night	9% 10% 4 7% 5% 5% 6% 5% 7% 7% 36%	2100
	11th to 20th	Night	9% 11% 3 8% 5% 4 5% 4 5% 6% 40%	2077
	21st to end	Night	11% 9% 2 7% 5% 5% 4 4 5% 7% 41%	1985
May	1st to 10th	Night	6% 9% 3 7% 5% 5% 5% 4 5% 6% 46%	4961
	11th to 20th	Night	10% 9% 3 7% 4 3 5% 4 5% 8% 43%	4701
	21st to end	Night	9% 10% 3 6% 4 4 5% 5% 7% 8% 40%	5181
June	1st to 10th	Night	6% 6% 3 6% 4 4 4 5% 7% 10% 46%	1680
	11th to 20th	Night	9% 5% 3 6% 4 4 5% 5% 7% 9% 44%	1680
	21st to end	Night	5% 4 1 4 3 5% 5% 3 6% 8% 55%	1680
July	1st to 10th	Night	4 3 2 6% 4 5% 5% 5% 7% 9% 50%	4740
	11th to 20th	Night	3 3 1 4 3 3 4 4 7% 9% 58%	4740
	21st to end	Night	5% 4 2 6% 4 5% 6% 5% 7% 8% 49%	5152
August	1st to 10th	Night	4 3 2 4 3 4 5% 4 6% 7% 57%	4680
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 7% 10% 50%	5597
	21st to end	Night	5% 4 2 7% 4 5% 5% 4 6% 9% 48%	6465
September	1st to 10th	Night	5% 4 2 4 3 3 3 3 4 6% 62%	2100
	11th to 20th	Night	6% 5% 2 4 4 4 5% 4 6% 8% 53%	2158
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 6% 58%	2288
October	1st to 10th	Night	6% 5% 2 3 2 2 2 2 3 4 68%	3839
	11th to 20th	Night	6% 6% 2 5% 4 3 4 3 5% 7% 54%	3898
	21st to end	Night	6% 9% 3 6% 3 3 4 4 6% 8% 50%	4591
November	1st to 10th	Night	4 7% 2 7% 4 5% 5% 4 6% 9% 46%	2665
	11th to 20th	Night	3 7% 3 8% 5% 5% 5% 4 5% 8% 46%	2770
	21st to end	Night	2 6% 3 8% 4 4 5% 4 7% 9% 48%	2940
December	1st to 10th	Night	1 4 3 10% 4 4 5% 5% 7% 8% 50%	4710
	11th to 20th	Night	1 5% 3 9% 6% 6% 6% 5% 7% 8% 44%	5321
	21st to end	Night	1 8% 4 10% 7% 6% 6% 4 6% 8% 42%	7128

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	36% 7% 15% 26% 11% 4	4723
	11th to 20th	Day	37% 8% 17% 18% 16% 5%	4740
	21st to end	Day	29% 6% 16% 26% 17% 6%	5285
February	1st to 10th	Day	47% 7% 12% 20% 11% 3	1890
	11th to 20th	Day	40% 6% 15% 22% 14% 4	1979
	21st to end	Day	40% 8% 13% 22% 13% 4	1730
March	1st to 10th	Day	31% 5% 8% 31% 17% 7%	4600
	11th to 20th	Day	29% 5% 14% 33% 19% 1	4828
	21st to end	Day	47% 7% 15% 25% 7%	6070
April	1st to 10th	Day	42% 6% 17% 22% 10% 3	2520
	11th to 20th	Day	49% 5% 12% 20% 13% 1	2494
	21st to end	Day	49% 7% 12% 24% 8% 1	2635
May	1st to 10th	Day	41% 8% 10% 27% 13% 1	7971
	11th to 20th	Day	38% 7% 10% 26% 17% 3	8195
	21st to end	Day	37% 8% 13% 29% 13%	9027
June	1st to 10th	Day	31% 9% 19% 28% 11% 2	2940
	11th to 20th	Day	31% 8% 17% 25% 16% 3	2940
	21st to end	Day	24% 7% 13% 31% 21% 5%	2940
July	1st to 10th	Day	25% 10% 13% 27% 21% 4	8181
	11th to 20th	Day	21% 8% 11% 26% 20% 13%	8175
	21st to end	Day	26% 8% 13% 23% 20% 10%	8759
August	1st to 10th	Day	22% 7% 12% 21% 25% 13%	6446
	11th to 20th	Day	31% 9% 15% 26% 15% 5%	5724
	21st to end	Day	19% 7% 20% 21% 27% 6%	6078
September	1st to 10th	Day	23% 4 10% 28% 27% 9%	2520
	11th to 20th	Day	26% 5% 10% 27% 24% 7%	2462
	21st to end	Day	28% 3 10% 34% 22% 3	2288
October	1st to 10th	Day	21% 3 10% 23% 32% 11%	3345
	11th to 20th	Day	28% 4 15% 28% 23% 3	3235
	21st to end	Day	33% 5% 15% 28% 17% 2	3190
November	1st to 10th	Day	30% 9% 15% 23% 20% 3	1853
	11th to 20th	Day	28% 7% 17% 27% 17% 4	1848
	21st to end	Day	31% 6% 18% 28% 13% 3	1680
December	1st to 10th	Day	30% 6% 23% 25% 15% 1	2700
	11th to 20th	Day	27% 7% 21% 35% 9% 1	3104
	21st to end	Day	35% 7% 10% 25% 17% 5%	4092
January	1st to 10th	Night	37% 8% 17% 25% 11% 2	8202
	11th to 20th	Night	31% 9% 22% 18% 14% 6%	8250
	21st to end	Night	29% 8% 16% 25% 16% 6%	8986
February	1st to 10th	Night	40% 8% 17% 20% 12% 2	2730
	11th to 20th	Night	37% 8% 14% 23% 15% 3	2536
	21st to end	Night	34% 7% 13% 24% 16% 5%	2076
March	1st to 10th	Night	31% 6% 11% 27% 17% 8%	5494
	11th to 20th	Night	30% 5% 19% 27% 18% 2	5365
	21st to end	Night	42% 8% 21% 21% 7%	5125
April	1st to 10th	Night	41% 8% 15% 24% 10% 2	2100
	11th to 20th	Night	40% 6% 12% 25% 15% 1	2077
	21st to end	Night	40% 6% 15% 26% 12% 1	1985
May	1st to 10th	Night	34% 7% 12% 30% 17% 1	4962
	11th to 20th	Night	36% 7% 13% 31% 12% 1	4702
	21st to end	Night	37% 9% 13% 27% 13%	5182
June	1st to 10th	Night	33% 8% 23% 25% 11% 1	1680
	11th to 20th	Night	33% 8% 20% 24% 13% 2	1680
	21st to end	Night	24% 6% 17% 27% 22% 4	1680
July	1st to 10th	Night	26% 8% 12% 29% 22% 3	4700
	11th to 20th	Night	19% 6% 13% 29% 21% 11%	4692
	21st to end	Night	30% 8% 15% 24% 19% 5%	5038
August	1st to 10th	Night	23% 6% 12% 24% 27% 8%	3700
	11th to 20th	Night	28% 10% 20% 24% 15% 3	4403
	21st to end	Night	23% 7% 19% 25% 23% 3	5069
September	1st to 10th	Night	23% 4 8% 31% 26% 8%	2100
	11th to 20th	Night	27% 7% 12% 26% 23% 5%	2158
	21st to end	Night	27% 5% 9% 30% 24% 5%	2288
October	1st to 10th	Night	21% 3 9% 23% 34% 10%	3839
	11th to 20th	Night	27% 5% 10% 31% 24% 4	3898
	21st to end	Night	31% 6% 13% 27% 18% 5%	4591
November	1st to 10th	Night	31% 7% 18% 22% 18% 5%	2665
	11th to 20th	Night	32% 6% 15% 28% 15% 4	2770
	21st to end	Night	28% 6% 20% 26% 16% 3	2940
December	1st to 10th	Night	26% 8% 22% 27% 15% 2	4710
	11th to 20th	Night	30% 9% 18% 32% 10% 1	5322
	21st to end	Night	36% 7% 10% 27% 14% 5%	7128

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	12% 23% 36% 19% 7% 3	330
	11th to 20th	Day	21% 31% 27% 12% 6% 3	330
	21st to end	Day	15% 39% 32% 10% 4 1	363
February	1st to 10th	Day	15% 36% 38% 9% 3	330
	11th to 20th	Day	10% 34% 43% 11% 2	330
	21st to end	Day	18% 33% 36% 11% 1	273
March	1st to 10th	Day	13% 36% 32% 15% 2 2	330
	11th to 20th	Day	20% 29% 34% 12% 4 2	414
	21st to end	Day	23% 29% 29% 15% 4 1	594
April	1st to 10th	Day	23% 31% 32% 11% 2	540
	11th to 20th	Day	25% 35% 27% 10% 2	532
	21st to end	Day	22% 23% 31% 20% 4	655
May	1st to 10th	Day	21% 22% 30% 18% 8% 2	753
	11th to 20th	Day	21% 19% 30% 18% 10% 2	775
	21st to end	Day	19% 22% 32% 18% 8% 1	844
June	1st to 10th	Day	9% 19% 32% 26% 12% 2	780
	11th to 20th	Day	13% 24% 36% 20% 6% 1	780
	21st to end	Day	10% 18% 36% 25% 10% 1	780
July	1st to 10th	Day	5% 10% 37% 33% 14% 2	771
	11th to 20th	Day	5% 11% 38% 33% 10% 1	768
	21st to end	Day	5% 9% 34% 35% 14% 2	849
August	1st to 10th	Day	5% 7% 37% 35% 14% 1	776
	11th to 20th	Day	7% 10% 35% 35% 12% 1	582
	21st to end	Day	6% 14% 36% 30% 12% 2	627
September	1st to 10th	Day	9% 16% 36% 28% 8% 4	570
	11th to 20th	Day	12% 17% 34% 23% 10% 4	512
	21st to end	Day	16% 18% 31% 23% 8% 4	354
October	1st to 10th	Day	18% 18% 27% 20% 13% 4	330
	11th to 20th	Day	22% 26% 33% 11% 5% 2	330
	21st to end	Day	15% 30% 32% 16% 6% 2	363
November	1st to 10th	Day	16% 30% 33% 15% 5% 1	330
	11th to 20th	Day	15% 31% 35% 14% 4 1	330
	21st to end	Day	13% 29% 33% 19% 6% 1	330
December	1st to 10th	Day	15% 32% 34% 15% 2 1	330
	11th to 20th	Day	19% 32% 34% 12% 2	330
	21st to end	Day	17% 31% 34% 13% 5% 1	363
January	1st to 10th	Night	9% 27% 36% 20% 8% 1	1050
	11th to 20th	Night	12% 31% 34% 15% 6% 1	1050
	21st to end	Night	10% 32% 36% 16% 5%	1141
February	1st to 10th	Night	9% 30% 40% 15% 6%	840
	11th to 20th	Night	8% 25% 40% 19% 7% 1	824
	21st to end	Night	7% 24% 42% 22% 5%	692
March	1st to 10th	Night	8% 25% 37% 25% 5% 1	840
	11th to 20th	Night	12% 18% 40% 24% 5% 1	840
	21st to end	Night	13% 21% 35% 23% 7% 1	711
April	1st to 10th	Night	16% 23% 32% 25% 4	630
	11th to 20th	Night	18% 26% 32% 20% 4	623
	21st to end	Night	17% 18% 31% 26% 8%	630
May	1st to 10th	Night	15% 16% 34% 25% 10% 1	630
	11th to 20th	Night	16% 16% 29% 27% 11%	621
	21st to end	Night	13% 20% 36% 23% 6%	683
June	1st to 10th	Night	8% 18% 34% 29% 10%	630
	11th to 20th	Night	11% 20% 42% 20% 6%	630
	21st to end	Night	6% 14% 40% 32% 8% 1	630
July	1st to 10th	Night	6% 11% 33% 33% 15% 1	615
	11th to 20th	Night	5% 8% 41% 38% 8% 1	612
	21st to end	Night	3 5% 37% 43% 11%	690
August	1st to 10th	Night	5% 9% 33% 41% 12%	627
	11th to 20th	Night	6% 7% 29% 43% 14% 1	624
	21st to end	Night	5% 11% 30% 40% 13% 1	693
September	1st to 10th	Night	7% 10% 30% 37% 14% 1	630
	11th to 20th	Night	10% 12% 30% 32% 14% 1	630
	21st to end	Night	11% 17% 30% 30% 11% 1	624
October	1st to 10th	Night	16% 17% 30% 23% 12% 1	745
	11th to 20th	Night	16% 19% 32% 23% 9% 1	800
	21st to end	Night	13% 23% 33% 23% 7% 1	1081
November	1st to 10th	Night	11% 24% 38% 19% 6% 1	1025
	11th to 20th	Night	11% 25% 40% 19% 5% 1	1050
	21st to end	Night	8% 26% 38% 24% 4	1050
December	1st to 10th	Night	8% 32% 37% 18% 5%	1050
	11th to 20th	Night	8% 28% 38% 21% 5%	1050
	21st to end	Night	10% 30% 36% 19% 5%	1155

Page generated at 2026-09-26 02:18:43. Version ac07

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