Builderbug Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Builderbug Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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% 8% 3 6% 4 4 4 3 5% 6% 50%	135542
	Night	7% 8% 3 6% 4 4 4 3 5% 6% 50%	132966
Transparency	Day	31% 6% 11% 19% 20% 13%	130276
	Night	32% 6% 12% 20% 20% 11%	129235
Seeing	Day	12% 10% 24% 32% 18% 3	16325
	Night	12% 5% 21% 37% 22% 3	26902

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	6% 15% 6% 12% 6% 4 4 4 6% 7% 30%	4098
	11th to 20th	Day	7% 14% 5% 14% 8% 5% 4 3 5% 5% 31%	4100
	21st to end	Day	6% 14% 6% 9% 4 3 3 2 3 5% 45%	4510
February	1st to 10th	Day	10% 16% 5% 9% 6% 5% 4 3 4 5% 33%	1800
	11th to 20th	Day	9% 16% 4 10% 5% 4 5% 3 5% 5% 32%	1828
	21st to end	Day	8% 17% 5% 10% 5% 4 4 3 6% 6% 32%	1650
March	1st to 10th	Day	5% 16% 6% 9% 5% 4 4 4 6% 6% 36%	4840
	11th to 20th	Day	5% 12% 3 9% 7% 6% 6% 4 6% 6% 35%	5000
	21st to end	Day	8% 20% 6% 12% 5% 4 4 3 4 5% 28%	5486
April	1st to 10th	Day	9% 11% 4 8% 4 5% 4 4 6% 7% 38%	2219
	11th to 20th	Day	9% 11% 3 8% 5% 4 5% 4 6% 6% 40%	2518
	21st to end	Day	9% 11% 3 7% 5% 3 4 4 5% 7% 43%	2600
May	1st to 10th	Day	8% 10% 3 6% 4 4 4 3 6% 6% 45%	5880
	11th to 20th	Day	7% 6% 2 7% 5% 5% 4 4 6% 8% 45%	5841
	21st to end	Day	6% 6% 2 5% 3 4 4 4 6% 7% 53%	6423
June	1st to 10th	Day	4 5% 1 5% 3 4 3 4 6% 9% 54%	2601
	11th to 20th	Day	6% 4 1 4 3 3 3 3 4 6% 62%	2734
	21st to end	Day	3 3 2 3 2 3 3 2 4 5% 70%	2600
July	1st to 10th	Day	2 2 1 3 3 2 3 3 4 4 74%	5879
	11th to 20th	Day	2 3 1 3 4 4 5% 4 6% 8% 58%	5880
	21st to end	Day	2 2 1 3 3 3 4 3 5% 6% 68%	6401
August	1st to 10th	Day	2 3 1 3 3 3 4 3 5% 6% 67%	5779
	11th to 20th	Day	2 2 1 4 3 3 3 3 4 5% 70%	5798
	21st to end	Day	4 4 1 5% 4 4 4 4 6% 7% 56%	6407
September	1st to 10th	Day	3 3 1 3 2 3 3 3 5% 7% 66%	2258
	11th to 20th	Day	5% 4 1 4 3 3 4 4 5% 6% 62%	2200
	21st to end	Day	6% 3 1 3 2 3 3 2 4 6% 67%	2200
October	1st to 10th	Day	6% 4 1 3 3 3 3 2 3 4 67%	3436
	11th to 20th	Day	7% 6% 2 5% 3 3 3 3 5% 6% 56%	3250
	21st to end	Day	6% 9% 3 5% 2 3 3 3 4 5% 56%	3548
November	1st to 10th	Day	11% 7% 3 6% 4 3 3 3 5% 6% 49%	1855
	11th to 20th	Day	12% 9% 3 6% 4 4 4 3 5% 7% 43%	1890
	21st to end	Day	8% 11% 4 9% 5% 5% 4 3 4 6% 42%	1759
December	1st to 10th	Day	7% 7% 2 5% 3 2 3 4 6% 8% 54%	2700
	11th to 20th	Day	9% 11% 4 10% 5% 6% 5% 3 6% 9% 33%	3082
	21st to end	Day	11% 14% 4 9% 6% 6% 5% 4 5% 5% 31%	4492
January	1st to 10th	Night	6% 12% 4 10% 5% 5% 4 4 7% 8% 35%	5867
	11th to 20th	Night	9% 14% 4 10% 4 4 4 3 6% 8% 34%	5880
	21st to end	Night	5% 13% 4 7% 3 3 3 3 4 6% 49%	6468
February	1st to 10th	Night	9% 15% 3 7% 5% 4 5% 4 5% 7% 36%	2600
	11th to 20th	Night	8% 13% 3 9% 5% 4 4 2 4 6% 41%	2458
	21st to end	Night	7% 13% 4 9% 5% 4 5% 4 6% 6% 38%	1980
March	1st to 10th	Night	7% 16% 5% 9% 4 4 4 3 4 6% 37%	5052
	11th to 20th	Night	5% 10% 3 9% 6% 4 5% 4 7% 8% 38%	5000
	21st to end	Night	9% 15% 4 7% 4 4 4 4 5% 5% 38%	5489
April	1st to 10th	Night	12% 12% 3 5% 3 3 3 3 5% 6% 46%	2180
	11th to 20th	Night	7% 9% 3 5% 5% 4 6% 4 5% 6% 46%	1826
	21st to end	Night	10% 8% 3 6% 5% 5% 4 3 5% 5% 46%	1800
May	1st to 10th	Night	8% 8% 3 5% 4 4 4 4 6% 6% 48%	4100
	11th to 20th	Night	6% 4 2 4 4 4 4 3 6% 8% 55%	4074
	21st to end	Night	7% 5% 1 5% 3 3 3 3 5% 7% 58%	4474
June	1st to 10th	Night	7% 5% 2 4 3 4 4 4 5% 7% 55%	1777
	11th to 20th	Night	6% 4 1 3 3 3 2 2 4 5% 65%	1641
	21st to end	Night	4 3 2 3 2 2 3 3 5% 7% 66%	1800
July	1st to 10th	Night	4 3 2 4 4 3 3 2 3 4 68%	4100
	11th to 20th	Night	3 4 2 6% 4 5% 5% 4 7% 8% 53%	4100
	21st to end	Night	3 4 1 4 3 4 3 3 5% 6% 64%	4472
August	1st to 10th	Night	2 2 1 4 3 3 4 3 5% 7% 65%	4033
	11th to 20th	Night	2 3 2 5% 4 4 5% 4 4 5% 63%	4051
	21st to end	Night	6% 5% 2 6% 4 4 5% 3 5% 5% 56%	4570
September	1st to 10th	Night	3 4 1 4 3 3 4 2 4 5% 66%	2138
	11th to 20th	Night	6% 4 1 4 3 3 3 3 5% 7% 62%	2200
	21st to end	Night	6% 4 1 4 2 2 2 2 4 5% 68%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 3 2 4 5% 70%	3757
	11th to 20th	Night	9% 6% 2 4 3 3 3 2 5% 5% 58%	3898
	21st to end	Night	6% 5% 2 6% 3 4 4 3 5% 6% 56%	4252
November	1st to 10th	Night	10% 8% 2 4 3 3 3 3 6% 7% 51%	2660
	11th to 20th	Night	10% 6% 3 5% 4 3 3 3 5% 8% 50%	2730
	21st to end	Night	8% 10% 3 7% 4 3 4 3 5% 7% 48%	2835
December	1st to 10th	Night	6% 6% 2 4 3 2 3 3 5% 6% 60%	4710
	11th to 20th	Night	10% 13% 3 6% 4 4 4 3 5% 6% 42%	5280
	21st to end	Night	9% 11% 4 10% 6% 5% 5% 4 6% 7% 33%	6514

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	47% 13% 17% 14% 8% 1	4099
	11th to 20th	Day	50% 9% 15% 15% 9% 2	4100
	21st to end	Day	40% 10% 13% 15% 19% 2	4510
February	1st to 10th	Day	51% 10% 14% 14% 10% 1	1800
	11th to 20th	Day	50% 6% 15% 18% 10% 1	1837
	21st to end	Day	47% 5% 15% 20% 11% 2	1650
March	1st to 10th	Day	41% 8% 14% 21% 12% 5%	4840
	11th to 20th	Day	36% 11% 13% 28% 11% 1	5000
	21st to end	Day	53% 4 19% 16% 6% 1	5486
April	1st to 10th	Day	41% 6% 13% 20% 16% 5%	2219
	11th to 20th	Day	40% 4 14% 22% 17% 3	2521
	21st to end	Day	36% 4 12% 25% 20% 2	2600
May	1st to 10th	Day	33% 6% 11% 24% 22% 5%	5880
	11th to 20th	Day	28% 8% 9% 26% 26% 4	5844
	21st to end	Day	24% 6% 9% 27% 30% 4	6424
June	1st to 10th	Day	24% 4 9% 22% 31% 11%	2601
	11th to 20th	Day	22% 3 7% 17% 29% 23%	2734
	21st to end	Day	16% 2 3 11% 29% 38%	2600
July	1st to 10th	Day	13% 2 4 6% 17% 57%	5816
	11th to 20th	Day	18% 5% 7% 12% 27% 31%	5811
	21st to end	Day	14% 3 7% 10% 26% 41%	6217
August	1st to 10th	Day	19% 4 7% 21% 25% 24%	4208
	11th to 20th	Day	20% 4 6% 17% 28% 25%	4197
	21st to end	Day	17% 3 5% 15% 30% 29%	4614
September	1st to 10th	Day	16% 3 7% 17% 28% 28%	2258
	11th to 20th	Day	20% 3 9% 19% 27% 22%	2200
	21st to end	Day	18% 4 6% 21% 32% 20%	2200
October	1st to 10th	Day	20% 3 5% 16% 38% 18%	3436
	11th to 20th	Day	26% 6% 8% 28% 27% 6%	3250
	21st to end	Day	28% 6% 12% 27% 23% 4	3548
November	1st to 10th	Day	35% 5% 14% 24% 19% 4	1855
	11th to 20th	Day	38% 6% 14% 27% 14% 1	1890
	21st to end	Day	40% 8% 16% 22% 13% 2	1759
December	1st to 10th	Day	26% 6% 15% 38% 13% 1	2700
	11th to 20th	Day	44% 10% 16% 19% 10% 1	3080
	21st to end	Day	48% 11% 15% 15% 11%	4492
January	1st to 10th	Night	42% 11% 19% 16% 10% 2	5867
	11th to 20th	Night	45% 9% 14% 19% 12% 1	5880
	21st to end	Night	36% 9% 16% 17% 21% 1	6468
February	1st to 10th	Night	45% 9% 18% 18% 8% 2	2600
	11th to 20th	Night	43% 6% 14% 22% 14% 2	2459
	21st to end	Night	42% 5% 15% 23% 12% 3	1980
March	1st to 10th	Night	44% 6% 11% 23% 12% 4	5052
	11th to 20th	Night	37% 8% 11% 27% 15% 2	5000
	21st to end	Night	43% 6% 17% 19% 14% 1	5489
April	1st to 10th	Night	37% 4 12% 22% 18% 6%	2180
	11th to 20th	Night	34% 4 13% 21% 22% 5%	1826
	21st to end	Night	36% 5% 14% 22% 21% 2	1800
May	1st to 10th	Night	30% 6% 11% 22% 27% 4	4097
	11th to 20th	Night	23% 6% 10% 28% 28% 6%	4075
	21st to end	Night	23% 5% 8% 26% 35% 3	4474
June	1st to 10th	Night	24% 5% 9% 20% 29% 13%	1777
	11th to 20th	Night	19% 3 7% 17% 29% 24%	1641
	21st to end	Night	17% 2 5% 9% 29% 38%	1800
July	1st to 10th	Night	19% 3 3 7% 12% 56%	4055
	11th to 20th	Night	24% 5% 6% 10% 22% 33%	4046
	21st to end	Night	19% 3 6% 10% 23% 38%	4344
August	1st to 10th	Night	20% 5% 9% 19% 23% 25%	2928
	11th to 20th	Night	23% 5% 7% 13% 26% 25%	2928
	21st to end	Night	19% 3 6% 12% 29% 31%	3294
September	1st to 10th	Night	20% 4 6% 14% 26% 31%	2138
	11th to 20th	Night	21% 4 7% 18% 27% 24%	2200
	21st to end	Night	20% 3 6% 16% 32% 24%	2200
October	1st to 10th	Night	18% 3 6% 16% 36% 22%	3757
	11th to 20th	Night	27% 3 6% 29% 26% 8%	3898
	21st to end	Night	25% 5% 10% 30% 26% 4	4252
November	1st to 10th	Night	30% 5% 11% 26% 21% 6%	2660
	11th to 20th	Night	32% 5% 16% 29% 15% 3	2730
	21st to end	Night	34% 7% 16% 27% 13% 3	2835
December	1st to 10th	Night	25% 5% 14% 36% 19% 1	4710
	11th to 20th	Night	40% 7% 20% 23% 10%	5281
	21st to end	Night	46% 9% 19% 17% 9%	6514

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% 10% 23% 22% 14% 5%	300
	11th to 20th	Day	14% 12% 26% 29% 17% 2	300
	21st to end	Day	15% 6% 29% 32% 15% 3	330
February	1st to 10th	Day	26% 9% 30% 24% 9% 2	300
	11th to 20th	Day	19% 10% 31% 28% 11% 2	373
	21st to end	Day	15% 11% 33% 28% 11% 1	414
March	1st to 10th	Day	17% 14% 28% 27% 13% 2	500
	11th to 20th	Day	17% 11% 26% 30% 16% 1	500
	21st to end	Day	20% 15% 29% 26% 10%	550
April	1st to 10th	Day	16% 15% 24% 29% 14% 2	500
	11th to 20th	Day	15% 17% 30% 26% 10% 2	492
	21st to end	Day	15% 16% 26% 25% 14% 3	500
May	1st to 10th	Day	11% 14% 30% 30% 12% 3	503
	11th to 20th	Day	13% 9% 25% 31% 17% 4	528
	21st to end	Day	9% 11% 23% 30% 21% 5%	573
June	1st to 10th	Day	5% 10% 22% 37% 23% 4	531
	11th to 20th	Day	9% 10% 27% 31% 22% 2	664
	21st to end	Day	5% 10% 26% 38% 18% 4	530
July	1st to 10th	Day	3 6% 23% 40% 24% 4	526
	11th to 20th	Day	5% 3 22% 44% 22% 3	524
	21st to end	Day	3 5% 16% 43% 26% 6%	575
August	1st to 10th	Day	8% 4 21% 39% 24% 3	527
	11th to 20th	Day	5% 5% 23% 34% 26% 6%	517
	21st to end	Day	6% 6% 18% 38% 26% 5%	583
September	1st to 10th	Day	4 5% 20% 36% 28% 7%	530
	11th to 20th	Day	9% 8% 23% 31% 24% 5%	530
	21st to end	Day	8% 9% 19% 34% 25% 5%	530
October	1st to 10th	Day	11% 10% 24% 28% 23% 5%	431
	11th to 20th	Day	11% 14% 21% 26% 22% 7%	330
	21st to end	Day	12% 9% 28% 33% 16% 2	363
November	1st to 10th	Day	18% 12% 24% 25% 17% 4	330
	11th to 20th	Day	15% 12% 22% 33% 12% 6%	330
	21st to end	Day	15% 11% 21% 28% 22% 4	330
December	1st to 10th	Day	15% 10% 24% 30% 17% 5%	330
	11th to 20th	Day	15% 9% 26% 36% 12% 2	321
	21st to end	Day	25% 9% 20% 33% 11% 1	330
January	1st to 10th	Night	18% 5% 22% 32% 19% 4	800
	11th to 20th	Night	16% 5% 22% 34% 20% 4	800
	21st to end	Night	15% 5% 21% 38% 18% 3	880
February	1st to 10th	Night	19% 5% 22% 38% 13% 3	800
	11th to 20th	Night	14% 6% 28% 36% 15% 1	784
	21st to end	Night	13% 7% 27% 39% 13% 1	660
March	1st to 10th	Night	18% 7% 25% 35% 13% 3	800
	11th to 20th	Night	12% 6% 23% 39% 19% 2	800
	21st to end	Night	15% 9% 26% 35% 12% 2	880
April	1st to 10th	Night	18% 8% 22% 36% 14% 1	780
	11th to 20th	Night	10% 7% 30% 39% 13% 1	593
	21st to end	Night	13% 7% 25% 34% 19% 2	600
May	1st to 10th	Night	10% 7% 24% 42% 17% 1	600
	11th to 20th	Night	10% 5% 19% 35% 28% 2	591
	21st to end	Night	12% 6% 20% 36% 24% 3	650
June	1st to 10th	Night	8% 7% 18% 38% 28% 2	600
	11th to 20th	Night	8% 6% 25% 40% 20% 2	600
	21st to end	Night	5% 3 18% 42% 30% 3	600
July	1st to 10th	Night	6% 3 18% 31% 37% 4	585
	11th to 20th	Night	5% 1 13% 47% 29% 4	582
	21st to end	Night	6% 1 11% 34% 47% 2	657
August	1st to 10th	Night	5% 2 16% 43% 33% 2	597
	11th to 20th	Night	4 2 13% 42% 35% 3	594
	21st to end	Night	6% 4 10% 43% 35% 2	660
September	1st to 10th	Night	5% 3 14% 41% 34% 4	738
	11th to 20th	Night	8% 3 18% 33% 32% 5%	800
	21st to end	Night	8% 3 17% 33% 32% 7%	800
October	1st to 10th	Night	11% 4 22% 36% 24% 4	800
	11th to 20th	Night	11% 5% 19% 36% 25% 4	800
	21st to end	Night	11% 5% 22% 38% 22% 3	880
November	1st to 10th	Night	13% 6% 22% 35% 21% 4	820
	11th to 20th	Night	14% 6% 21% 39% 16% 4	840
	21st to end	Night	13% 6% 23% 35% 19% 4	945
December	1st to 10th	Night	16% 5% 25% 36% 17% 2	1050
	11th to 20th	Night	19% 6% 22% 39% 12% 2	1036
	21st to end	Night	20% 7% 23% 38% 11% 1	900

Page generated at 2025-12-14 23:13:54. Version ac07

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