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 2024-11-02.

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%	101498
	Night	4 7% 3 7% 5% 5% 5% 4 6% 8% 47%	97694
Transparency	Day	33% 7% 14% 26% 17% 4	101241
	Night	32% 7% 16% 26% 16% 3	97541
Seeing	Day	13% 22% 34% 22% 8% 2	15903
	Night	10% 21% 35% 25% 8% 1	25859

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% 4 8% 4 5% 5% 5% 7% 10% 45%	2538
	11th to 20th	Day	1 6% 4 11% 5% 5% 5% 5% 6% 10% 43%	2540
	21st to end	Day	1 6% 3 9% 5% 3 3 3 5% 7% 53%	2835
February	1st to 10th	Day	2 11% 5% 16% 7% 5% 5% 4 5% 5% 33%	1710
	11th to 20th	Day	2 13% 5% 12% 5% 4 5% 4 6% 7% 39%	1777
	21st to end	Day	2 10% 6% 12% 6% 5% 5% 5% 5% 6% 39%	1570
March	1st to 10th	Day	2 8% 4 9% 4 3 3 3 5% 7% 52%	3130
	11th to 20th	Day	4 9% 2 6% 4 3 4 4 7% 10% 47%	3298
	21st to end	Day	9% 15% 5% 9% 5% 4 4 4 6% 7% 33%	4148
April	1st to 10th	Day	8% 14% 4 7% 5% 4 4 4 6% 7% 38%	2280
	11th to 20th	Day	10% 14% 5% 10% 6% 5% 4 3 6% 5% 31%	2251
	21st to end	Day	13% 13% 5% 8% 4 4 4 4 5% 7% 33%	2384
May	1st to 10th	Day	11% 17% 5% 8% 6% 5% 5% 4 6% 5% 29%	4294
	11th to 20th	Day	11% 8% 3 7% 4 5% 4 4 6% 7% 40%	4388
	21st to end	Day	13% 8% 3 6% 4 5% 5% 4 7% 7% 40%	4825
June	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 8% 9% 45%	2660
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 8% 45%	2660
	21st to end	Day	5% 4 1 4 3 4 4 4 6% 7% 57%	2660
July	1st to 10th	Day	4 3 2 6% 5% 5% 6% 5% 7% 9% 48%	4430
	11th to 20th	Day	3 2 1 4 3 4 5% 5% 7% 9% 56%	4430
	21st to end	Day	3 3 1 4 3 4 5% 5% 7% 9% 55%	4802
August	1st to 10th	Day	8% 4 1 5% 4 4 5% 5% 6% 9% 50%	4411
	11th to 20th	Day	5% 3 2 4 4 4 6% 5% 7% 9% 50%	3858
	21st to end	Day	3 3 2 4 3 3 3 3 5% 6% 64%	4180
September	1st to 10th	Day	7% 5% 1 4 3 2 3 3 5% 6% 60%	2280
	11th to 20th	Day	7% 5% 1 4 3 3 4 4 4 6% 58%	2228
	21st to end	Day	9% 6% 2 5% 3 3 2 3 4 6% 58%	2090
October	1st to 10th	Day	10% 8% 2 5% 4 3 3 2 4 5% 54%	1949
	11th to 20th	Day	7% 10% 3 5% 3 3 3 2 4 6% 53%	1897
	21st to end	Day	6% 9% 3 6% 4 4 3 3 6% 8% 47%	1881
November	1st to 10th	Day	4 6% 3 7% 5% 4 5% 4 7% 8% 47%	1682
	11th to 20th	Day	2 8% 3 7% 5% 4 4 3 6% 9% 49%	1672
	21st to end	Day	3 7% 4 7% 5% 4 5% 5% 7% 9% 46%	1520
December	1st to 10th	Day	1 6% 3 11% 7% 6% 4 5% 7% 8% 42%	1520
	11th to 20th	Day	1 6% 2 5% 5% 5% 5% 4 7% 11% 47%	1926
	21st to end	Day	2 8% 4 10% 6% 6% 4 4 6% 7% 43%	2794
January	1st to 10th	Night	1 5% 3 10% 6% 5% 6% 5% 7% 8% 44%	4417
	11th to 20th	Night	1 6% 3 9% 7% 6% 7% 5% 8% 9% 40%	4430
	21st to end	Night	1 5% 3 9% 6% 5% 5% 4 5% 7% 51%	4861
February	1st to 10th	Night	3 8% 6% 14% 5% 5% 5% 5% 7% 8% 35%	2470
	11th to 20th	Night	2 7% 3 11% 7% 6% 6% 5% 8% 7% 39%	2288
	21st to end	Night	2 9% 4 10% 6% 6% 5% 3 6% 7% 42%	1884
March	1st to 10th	Night	2 8% 4 9% 5% 5% 5% 4 6% 6% 46%	3734
	11th to 20th	Night	3 8% 3 7% 5% 4 4 4 7% 9% 45%	3661
	21st to end	Night	4 10% 4 8% 7% 6% 6% 5% 7% 9% 33%	3493
April	1st to 10th	Night	9% 10% 3 7% 6% 5% 6% 5% 7% 7% 36%	1900
	11th to 20th	Night	9% 11% 4 9% 5% 4 5% 4 5% 6% 39%	1877
	21st to end	Night	11% 9% 2 7% 5% 5% 4 5% 6% 7% 41%	1796
May	1st to 10th	Night	8% 13% 4 9% 5% 5% 5% 4 4 5% 38%	2674
	11th to 20th	Night	6% 7% 2 7% 4 4 5% 4 6% 9% 46%	2517
	21st to end	Night	10% 11% 3 7% 5% 4 5% 5% 6% 8% 38%	2762
June	1st to 10th	Night	6% 6% 3 6% 4 4 4 5% 6% 10% 45%	1520
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 7% 9% 44%	1520
	21st to end	Night	5% 4 1 4 3 5% 5% 3 6% 9% 56%	1520
July	1st to 10th	Night	6% 4 2 6% 4 4 4 4 6% 7% 54%	2540
	11th to 20th	Night	3 2 1 4 3 3 5% 5% 8% 10% 56%	2540
	21st to end	Night	4 3 2 5% 3 4 4 4 6% 8% 58%	2761
August	1st to 10th	Night	6% 4 2 5% 4 5% 6% 5% 7% 8% 47%	2528
	11th to 20th	Night	5% 3 2 4 4 4 5% 5% 7% 11% 50%	2976
	21st to end	Night	3 3 1 5% 4 4 5% 4 5% 8% 58%	3487
September	1st to 10th	Night	6% 4 2 4 3 3 3 3 4 5% 62%	1900
	11th to 20th	Night	6% 5% 2 4 3 3 5% 4 6% 7% 56%	1952
	21st to end	Night	8% 6% 2 5% 3 3 3 3 4 6% 59%	2090
October	1st to 10th	Night	10% 9% 3 5% 4 4 3 3 5% 6% 50%	2228
	11th to 20th	Night	7% 7% 3 5% 4 4 4 3 5% 7% 53%	2280
	21st to end	Night	6% 10% 3 6% 4 3 4 4 6% 7% 49%	2699
November	1st to 10th	Night	4 7% 2 7% 4 5% 5% 4 6% 9% 46%	2419
	11th to 20th	Night	3 7% 4 8% 5% 5% 5% 4 5% 7% 46%	2506
	21st to end	Night	2 6% 3 8% 4 4 5% 5% 7% 9% 48%	2660
December	1st to 10th	Night	2 6% 4 8% 6% 5% 6% 5% 7% 8% 44%	2660
	11th to 20th	Night	1 5% 3 7% 5% 4 5% 5% 8% 9% 49%	3271
	21st to end	Night	1 7% 4 10% 7% 6% 6% 4 6% 7% 43%	4873

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	30% 7% 20% 27% 15% 2	2539
	11th to 20th	Day	31% 7% 17% 20% 20% 4	2540
	21st to end	Day	27% 6% 20% 29% 14% 4	2835
February	1st to 10th	Day	48% 7% 12% 20% 10% 2	1710
	11th to 20th	Day	40% 6% 16% 22% 12% 4	1786
	21st to end	Day	42% 8% 13% 21% 13% 4	1570
March	1st to 10th	Day	30% 6% 10% 28% 20% 6%	3130
	11th to 20th	Day	31% 5% 18% 34% 10% 1	3298
	21st to end	Day	48% 6% 18% 22% 5%	4148
April	1st to 10th	Day	42% 6% 17% 22% 10% 3	2280
	11th to 20th	Day	50% 6% 12% 20% 12% 1	2254
	21st to end	Day	48% 7% 13% 25% 8%	2384
May	1st to 10th	Day	52% 8% 11% 20% 8% 1	4294
	11th to 20th	Day	38% 8% 13% 29% 11% 1	4392
	21st to end	Day	38% 8% 16% 29% 9% 1	4825
June	1st to 10th	Day	32% 9% 20% 27% 10% 2	2660
	11th to 20th	Day	33% 8% 16% 25% 15% 3	2660
	21st to end	Day	23% 7% 13% 31% 21% 5%	2660
July	1st to 10th	Day	27% 9% 13% 28% 21% 3	4361
	11th to 20th	Day	21% 8% 12% 26% 25% 8%	4355
	21st to end	Day	20% 8% 12% 23% 26% 12%	4651
August	1st to 10th	Day	27% 8% 15% 23% 19% 7%	4396
	11th to 20th	Day	26% 8% 13% 28% 19% 6%	3897
	21st to end	Day	21% 5% 9% 21% 36% 8%	4179
September	1st to 10th	Day	24% 4 10% 27% 27% 9%	2280
	11th to 20th	Day	25% 6% 10% 27% 26% 7%	2228
	21st to end	Day	28% 3 11% 34% 22% 3	2090
October	1st to 10th	Day	34% 3 12% 26% 21% 4	1949
	11th to 20th	Day	32% 5% 14% 28% 17% 5%	1897
	21st to end	Day	33% 6% 18% 25% 15% 3	1881
November	1st to 10th	Day	30% 9% 15% 22% 21% 2	1682
	11th to 20th	Day	29% 7% 17% 27% 16% 5%	1672
	21st to end	Day	31% 6% 17% 28% 14% 3	1520
December	1st to 10th	Day	35% 7% 18% 22% 16% 2	1520
	11th to 20th	Day	26% 8% 20% 35% 9% 2	1924
	21st to end	Day	36% 7% 9% 21% 19% 8%	2794
January	1st to 10th	Night	33% 7% 14% 28% 16% 2	4417
	11th to 20th	Night	33% 10% 18% 20% 16% 4	4430
	21st to end	Night	29% 7% 20% 28% 14% 2	4861
February	1st to 10th	Night	41% 8% 17% 20% 11% 2	2470
	11th to 20th	Night	38% 8% 15% 24% 13% 3	2289
	21st to end	Night	37% 7% 13% 23% 15% 5%	1884
March	1st to 10th	Night	33% 7% 13% 26% 19% 2	3734
	11th to 20th	Night	32% 6% 22% 26% 12% 2	3661
	21st to end	Night	44% 7% 23% 20% 6% 1	3493
April	1st to 10th	Night	40% 8% 15% 24% 10% 2	1900
	11th to 20th	Night	42% 6% 13% 24% 13% 1	1877
	21st to end	Night	40% 6% 16% 25% 12% 1	1796
May	1st to 10th	Night	43% 8% 12% 22% 13% 2	2674
	11th to 20th	Night	30% 7% 15% 39% 8% 1	2518
	21st to end	Night	41% 9% 14% 28% 8% 1	2762
June	1st to 10th	Night	33% 7% 24% 25% 10% 1	1520
	11th to 20th	Night	33% 8% 20% 24% 12% 2	1520
	21st to end	Night	24% 6% 18% 27% 22% 3	1520
July	1st to 10th	Night	26% 8% 11% 30% 22% 4	2500
	11th to 20th	Night	19% 7% 13% 33% 22% 5%	2492
	21st to end	Night	22% 6% 13% 28% 25% 6%	2674
August	1st to 10th	Night	30% 8% 15% 24% 20% 4	2520
	11th to 20th	Night	24% 8% 16% 27% 21% 4	3003
	21st to end	Night	22% 6% 11% 24% 32% 5%	3487
September	1st to 10th	Night	24% 4 7% 31% 25% 9%	1900
	11th to 20th	Night	24% 6% 12% 28% 24% 6%	1952
	21st to end	Night	27% 5% 9% 30% 24% 5%	2090
October	1st to 10th	Night	35% 6% 12% 22% 22% 4	2228
	11th to 20th	Night	30% 4 11% 30% 20% 5%	2280
	21st to end	Night	32% 7% 16% 23% 18% 5%	2699
November	1st to 10th	Night	31% 7% 18% 22% 17% 5%	2419
	11th to 20th	Night	33% 5% 14% 28% 15% 4	2506
	21st to end	Night	29% 6% 19% 26% 17% 3	2660
December	1st to 10th	Night	31% 7% 20% 23% 16% 3	2660
	11th to 20th	Night	26% 8% 20% 34% 11% 1	3272
	21st to end	Night	35% 8% 12% 23% 14% 7%	4873

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	9% 24% 36% 20% 8% 4	270
	11th to 20th	Day	17% 28% 30% 15% 7% 4	270
	21st to end	Day	13% 38% 32% 12% 5% 1	297
February	1st to 10th	Day	13% 36% 38% 10% 3	270
	11th to 20th	Day	11% 36% 41% 10% 2	270
	21st to end	Day	15% 33% 38% 12% 1	225
March	1st to 10th	Day	10% 38% 33% 14% 3 1	270
	11th to 20th	Day	16% 27% 38% 13% 5% 2	346
	21st to end	Day	20% 29% 30% 16% 4 1	506
April	1st to 10th	Day	23% 31% 31% 12% 2	460
	11th to 20th	Day	25% 36% 29% 10% 1	452
	21st to end	Day	21% 21% 31% 21% 5% 1	564
May	1st to 10th	Day	22% 23% 28% 17% 8% 2	653
	11th to 20th	Day	19% 19% 30% 19% 11% 2	675
	21st to end	Day	19% 22% 31% 18% 9% 1	734
June	1st to 10th	Day	8% 17% 32% 28% 13% 2	680
	11th to 20th	Day	12% 23% 37% 20% 6% 1	680
	21st to end	Day	8% 17% 37% 26% 11% 2	680
July	1st to 10th	Day	5% 10% 35% 34% 14% 2	671
	11th to 20th	Day	5% 12% 38% 33% 11% 1	668
	21st to end	Day	5% 9% 33% 36% 15% 2	739
August	1st to 10th	Day	6% 7% 36% 36% 15% 1	676
	11th to 20th	Day	7% 11% 34% 35% 12% 1	500
	21st to end	Day	5% 15% 35% 30% 13% 2	539
September	1st to 10th	Day	9% 15% 35% 29% 8% 4	490
	11th to 20th	Day	12% 14% 33% 26% 11% 5%	438
	21st to end	Day	17% 19% 31% 22% 7% 4	300
October	1st to 10th	Day	17% 18% 27% 21% 13% 4	300
	11th to 20th	Day	20% 24% 35% 12% 6% 2	300
	21st to end	Day	14% 30% 33% 16% 5% 2	330
November	1st to 10th	Day	13% 30% 33% 16% 6% 1	273
	11th to 20th	Day	13% 31% 36% 16% 4	270
	21st to end	Day	11% 30% 33% 20% 5% 1	270
December	1st to 10th	Day	11% 31% 37% 16% 3 1	270
	11th to 20th	Day	13% 36% 34% 14% 3	270
	21st to end	Day	13% 31% 35% 14% 5% 1	297
January	1st to 10th	Night	7% 27% 36% 20% 9% 1	950
	11th to 20th	Night	10% 30% 35% 17% 7% 1	950
	21st to end	Night	9% 33% 36% 17% 6%	1033
February	1st to 10th	Night	9% 29% 39% 16% 7%	760
	11th to 20th	Night	8% 26% 39% 20% 6% 1	744
	21st to end	Night	7% 23% 42% 23% 5%	628
March	1st to 10th	Night	8% 25% 37% 25% 5% 1	760
	11th to 20th	Night	10% 17% 41% 25% 6% 1	760
	21st to end	Night	12% 19% 36% 24% 8% 1	643
April	1st to 10th	Night	16% 22% 32% 25% 5%	570
	11th to 20th	Night	18% 26% 32% 20% 4	563
	21st to end	Night	16% 19% 31% 26% 8% 1	570
May	1st to 10th	Night	16% 15% 33% 24% 10% 1	570
	11th to 20th	Night	14% 15% 30% 27% 12% 1	561
	21st to end	Night	13% 21% 36% 23% 7%	617
June	1st to 10th	Night	8% 17% 35% 29% 11%	570
	11th to 20th	Night	11% 20% 41% 21% 6% 1	570
	21st to end	Night	5% 12% 40% 33% 8% 1	570
July	1st to 10th	Night	7% 10% 33% 33% 16% 1	555
	11th to 20th	Night	4 8% 40% 38% 8% 1	552
	21st to end	Night	3 5% 36% 44% 12%	624
August	1st to 10th	Night	5% 8% 31% 42% 13% 1	567
	11th to 20th	Night	6% 6% 29% 43% 15% 1	564
	21st to end	Night	4 10% 30% 40% 15% 1	627
September	1st to 10th	Night	8% 9% 30% 38% 14% 1	570
	11th to 20th	Night	9% 12% 30% 33% 15% 1	570
	21st to end	Night	12% 18% 30% 29% 11% 1	570
October	1st to 10th	Night	16% 18% 29% 23% 13% 2	708
	11th to 20th	Night	14% 18% 33% 24% 10% 1	760
	21st to end	Night	12% 23% 34% 22% 7% 1	1027
November	1st to 10th	Night	10% 24% 38% 20% 7% 2	931
	11th to 20th	Night	10% 25% 41% 19% 5% 1	950
	21st to end	Night	8% 26% 38% 24% 4	950
December	1st to 10th	Night	8% 32% 37% 19% 5%	950
	11th to 20th	Night	5% 28% 38% 23% 6%	950
	21st to end	Night	9% 31% 36% 19% 6%	1045

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