Beech Lick Branch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beech Lick Branch Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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 7% 4 4 4 4 6% 8% 41%	137374
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	134716
Transparency	Day	40% 5% 11% 22% 18% 4	141100
	Night	39% 4 10% 21% 19% 6%	139067
Seeing	Day	15% 11% 24% 24% 17% 8%	17202
	Night	16% 9% 26% 28% 18% 3	26912

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	18% 11% 3 9% 3 2 3 2 3 5% 40%	4691
	11th to 20th	Day	13% 6% 2 8% 3 2 3 2 3 5% 55%	5161
	21st to end	Day	20% 10% 3 7% 5% 3 3 3 4 5% 37%	6436
February	1st to 10th	Day	17% 12% 3 7% 3 2 3 3 4 6% 39%	2100
	11th to 20th	Day	19% 11% 3 7% 4 3 4 2 3 5% 39%	2048
	21st to end	Day	19% 10% 3 6% 4 4 4 3 4 6% 40%	1730
March	1st to 10th	Day	19% 11% 3 8% 5% 4 4 3 4 5% 33%	4580
	11th to 20th	Day	12% 8% 3 6% 4 4 3 4 5% 6% 46%	4774
	21st to end	Day	12% 8% 3 7% 5% 4 4 4 5% 6% 42%	6099
April	1st to 10th	Day	12% 7% 2 5% 4 4 5% 3 6% 7% 45%	2520
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 5% 7% 47%	2494
	21st to end	Day	12% 6% 3 6% 5% 4 4 4 6% 8% 43%	2520
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 9% 33%	7315
	11th to 20th	Day	12% 8% 3 9% 5% 5% 5% 4 6% 9% 35%	7626
	21st to end	Day	18% 10% 4 9% 5% 5% 5% 4 5% 7% 29%	8391
June	1st to 10th	Day	9% 7% 2 6% 5% 5% 5% 5% 7% 10% 39%	2760
	11th to 20th	Day	5% 5% 2 5% 5% 6% 6% 6% 8% 10% 42%	2871
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 6% 8% 11% 43%	2800
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 7% 6% 8% 11% 37%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 6% 7% 6% 11% 13% 33%	6314
	21st to end	Day	9% 7% 3 8% 6% 6% 6% 5% 9% 12% 28%	6109
August	1st to 10th	Day	3 3 1 5% 3 5% 5% 4 6% 10% 55%	3905
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 5% 7% 10% 47%	3849
	21st to end	Day	7% 4 2 4 5% 5% 5% 5% 6% 7% 50%	4312
September	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 6% 7% 8% 43%	2400
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 7% 43%	2225
	21st to end	Day	11% 5% 2 5% 4 4 5% 3 5% 7% 49%	2200
October	1st to 10th	Day	10% 5% 2 5% 4 3 3 2 4 5% 57%	2200
	11th to 20th	Day	8% 5% 2 5% 4 3 3 3 4 6% 57%	2081
	21st to end	Day	16% 5% 2 4 3 3 4 3 4 5% 52%	2110
November	1st to 10th	Day	11% 5% 2 6% 3 3 3 3 4 5% 56%	1852
	11th to 20th	Day	12% 7% 2 4 3 3 3 2 4 6% 53%	1890
	21st to end	Day	14% 8% 2 6% 4 3 4 3 5% 6% 45%	1890
December	1st to 10th	Day	15% 9% 3 7% 4 3 3 3 4 5% 44%	3020
	11th to 20th	Day	20% 10% 4 6% 4 3 3 2 4 4 40%	3463
	21st to end	Day	16% 10% 3 7% 4 4 4 3 5% 7% 37%	4278
January	1st to 10th	Night	18% 11% 3 8% 3 3 4 3 4 5% 39%	8232
	11th to 20th	Night	14% 7% 3 6% 3 3 3 2 3 4 51%	7802
	21st to end	Night	21% 10% 3 7% 4 3 3 2 3 4 40%	7745
February	1st to 10th	Night	17% 10% 3 6% 4 4 3 3 4 4 42%	2520
	11th to 20th	Night	21% 9% 3 6% 4 3 4 3 3 4 40%	2458
	21st to end	Night	21% 8% 2 6% 4 4 5% 3 4 5% 39%	2076
March	1st to 10th	Night	19% 11% 3 7% 4 4 4 3 5% 5% 36%	5493
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 6% 49%	5433
	21st to end	Night	17% 8% 3 7% 4 4 3 3 3 5% 44%	5072
April	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 6% 8% 45%	2100
	11th to 20th	Night	13% 6% 2 4 3 4 4 3 5% 8% 50%	2075
	21st to end	Night	12% 6% 2 5% 4 3 4 3 6% 7% 47%	2100
May	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 5% 7% 37%	5534
	11th to 20th	Night	13% 7% 3 8% 5% 4 5% 4 5% 7% 38%	5234
	21st to end	Night	15% 10% 3 8% 5% 5% 5% 4 6% 8% 31%	5761
June	1st to 10th	Night	7% 5% 3 6% 6% 4 5% 4 6% 9% 45%	1852
	11th to 20th	Night	4 3 1 4 4 4 5% 5% 7% 12% 53%	1640
	21st to end	Night	4 3 2 4 5% 4 4 4 7% 10% 51%	1600
July	1st to 10th	Night	5% 5% 2 5% 4 6% 6% 6% 9% 12% 40%	3618
	11th to 20th	Night	5% 5% 2 5% 4 4 5% 5% 8% 14% 42%	3656
	21st to end	Night	7% 7% 3 7% 6% 5% 6% 6% 10% 13% 30%	4736
August	1st to 10th	Night	2 2 1 3 3 4 4 4 7% 10% 61%	3244
	11th to 20th	Night	3 3 1 4 3 4 5% 4 7% 11% 53%	3200
	21st to end	Night	5% 3 2 5% 4 4 5% 4 6% 9% 54%	3586
September	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 6% 8% 49%	2000
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 6% 8% 47%	2152
	21st to end	Night	9% 4 2 5% 3 2 3 3 5% 6% 55%	2200
October	1st to 10th	Night	8% 3 1 4 3 3 3 3 4 6% 62%	2200
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 5% 56%	2315
	21st to end	Night	16% 7% 1 4 3 3 3 3 3 4 53%	2727
November	1st to 10th	Night	12% 5% 2 4 4 3 3 3 4 5% 54%	2666
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 5% 53%	2730
	21st to end	Night	17% 6% 2 5% 4 3 3 3 4 6% 48%	2730
December	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 44%	4380
	11th to 20th	Night	21% 9% 3 6% 3 3 3 2 3 4 43%	4952
	21st to end	Night	19% 10% 3 6% 4 4 4 3 5% 6% 35%	6897

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	49% 2 7% 24% 16% 2	4692
	11th to 20th	Day	36% 2 5% 22% 32% 4	5161
	21st to end	Day	51% 2 4 20% 17% 5%	6436
February	1st to 10th	Day	47% 3 7% 20% 19% 4	2100
	11th to 20th	Day	50% 2 5% 20% 17% 5%	2058
	21st to end	Day	48% 3 6% 15% 21% 8%	1730
March	1st to 10th	Day	54% 3 5% 13% 16% 9%	4580
	11th to 20th	Day	40% 3 7% 20% 19% 11%	4774
	21st to end	Day	43% 3 10% 24% 19% 2	6101
April	1st to 10th	Day	38% 5% 10% 24% 17% 6%	2520
	11th to 20th	Day	36% 4 8% 24% 23% 6%	2494
	21st to end	Day	38% 5% 9% 27% 19% 2	2520
May	1st to 10th	Day	47% 5% 12% 21% 12% 2	7350
	11th to 20th	Day	46% 5% 14% 21% 13% 1	7630
	21st to end	Day	55% 5% 9% 17% 12% 2	8405
June	1st to 10th	Day	38% 5% 14% 22% 18% 2	2760
	11th to 20th	Day	33% 7% 15% 25% 18% 2	2871
	21st to end	Day	31% 9% 17% 23% 16% 4	2800
July	1st to 10th	Day	36% 9% 18% 25% 10% 1	6291
	11th to 20th	Day	37% 10% 24% 24% 4 1	6239
	21st to end	Day	44% 9% 23% 19% 4	5934
August	1st to 10th	Day	24% 6% 14% 36% 20% 1	3893
	11th to 20th	Day	31% 6% 16% 29% 15% 3	3885
	21st to end	Day	31% 6% 12% 21% 24% 4	4311
September	1st to 10th	Day	36% 7% 14% 21% 18% 5%	2400
	11th to 20th	Day	39% 5% 12% 18% 22% 4	2225
	21st to end	Day	34% 5% 9% 23% 22% 7%	2200
October	1st to 10th	Day	20% 3 5% 15% 37% 20%	3590
	11th to 20th	Day	19% 2 5% 20% 34% 19%	3373
	21st to end	Day	27% 3 4 24% 34% 9%	3387
November	1st to 10th	Day	33% 2 8% 20% 28% 8%	1852
	11th to 20th	Day	34% 2 8% 19% 28% 8%	1890
	21st to end	Day	41% 3 7% 22% 22% 5%	1890
December	1st to 10th	Day	44% 2 4 23% 23% 4	3020
	11th to 20th	Day	50% 2 5% 21% 16% 6%	3460
	21st to end	Day	46% 3 9% 25% 14% 2	4278
January	1st to 10th	Night	49% 2 9% 21% 17% 3	8233
	11th to 20th	Night	39% 2 4 18% 28% 8%	7802
	21st to end	Night	50% 2 3 18% 18% 8%	7745
February	1st to 10th	Night	47% 3 7% 20% 18% 4	2520
	11th to 20th	Night	50% 3 6% 18% 18% 6%	2459
	21st to end	Night	49% 3 5% 16% 19% 8%	2076
March	1st to 10th	Night	51% 2 7% 16% 17% 7%	5493
	11th to 20th	Night	36% 3 7% 18% 20% 16%	5433
	21st to end	Night	45% 3 7% 22% 20% 4	5072
April	1st to 10th	Night	37% 4 10% 23% 20% 5%	2100
	11th to 20th	Night	34% 4 9% 22% 23% 8%	2075
	21st to end	Night	36% 4 11% 25% 20% 3	2100
May	1st to 10th	Night	48% 4 11% 20% 15% 1	5564
	11th to 20th	Night	45% 5% 14% 22% 12% 3	5235
	21st to end	Night	51% 7% 12% 19% 11% 1	5771
June	1st to 10th	Night	34% 7% 14% 24% 19% 3	1852
	11th to 20th	Night	23% 6% 16% 30% 21% 3	1640
	21st to end	Night	27% 7% 15% 30% 18% 3	1600
July	1st to 10th	Night	33% 10% 22% 24% 9% 1	3580
	11th to 20th	Night	29% 9% 29% 27% 5% 1	3611
	21st to end	Night	41% 9% 26% 19% 5%	4597
August	1st to 10th	Night	17% 6% 14% 39% 23% 2	3232
	11th to 20th	Night	22% 7% 24% 28% 16% 2	3230
	21st to end	Night	26% 6% 15% 22% 27% 4	3586
September	1st to 10th	Night	32% 6% 14% 24% 19% 6%	2000
	11th to 20th	Night	35% 6% 13% 19% 25% 3	2152
	21st to end	Night	29% 5% 9% 25% 23% 9%	2200
October	1st to 10th	Night	17% 3 6% 18% 36% 21%	3590
	11th to 20th	Night	21% 2 4 16% 34% 23%	3762
	21st to end	Night	25% 2 4 26% 27% 15%	4400
November	1st to 10th	Night	33% 4 7% 22% 25% 10%	2666
	11th to 20th	Night	34% 3 7% 19% 27% 11%	2730
	21st to end	Night	39% 4 6% 21% 24% 7%	2730
December	1st to 10th	Night	44% 2 5% 24% 23% 3	4380
	11th to 20th	Night	47% 2 5% 19% 24% 4	4954
	21st to end	Night	49% 4 7% 24% 12% 3	6897

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	27% 22% 35% 14% 2	330
	11th to 20th	Day	25% 25% 38% 8% 3 1	330
	21st to end	Day	25% 25% 35% 12% 2	363
February	1st to 10th	Day	23% 24% 36% 13% 3 1	330
	11th to 20th	Day	37% 18% 35% 8% 2 1	330
	21st to end	Day	33% 21% 35% 9% 2	273
March	1st to 10th	Day	27% 20% 35% 15% 2 1	330
	11th to 20th	Day	22% 25% 39% 12% 1 1	367
	21st to end	Day	23% 18% 37% 16% 6%	594
April	1st to 10th	Day	20% 18% 32% 23% 6%	540
	11th to 20th	Day	18% 19% 33% 20% 9% 1	532
	21st to end	Day	15% 15% 32% 25% 11% 1	540
May	1st to 10th	Day	18% 15% 28% 25% 10% 4	543
	11th to 20th	Day	13% 8% 30% 26% 17% 6%	567
	21st to end	Day	15% 6% 25% 26% 19% 9%	617
June	1st to 10th	Day	10% 6% 21% 31% 22% 11%	600
	11th to 20th	Day	7% 4 19% 31% 27% 13%	756
	21st to end	Day	6% 3 18% 33% 26% 14%	730
July	1st to 10th	Day	7% 4 12% 33% 31% 13%	721
	11th to 20th	Day	8% 3 9% 30% 30% 20%	698
	21st to end	Day	9% 2 12% 26% 30% 21%	576
August	1st to 10th	Day	7% 3 10% 29% 34% 17%	526
	11th to 20th	Day	8% 3 10% 29% 36% 13%	519
	21st to end	Day	13% 3 10% 23% 33% 17%	583
September	1st to 10th	Day	11% 3 12% 31% 33% 11%	530
	11th to 20th	Day	11% 4 11% 28% 30% 16%	530
	21st to end	Day	13% 5% 15% 31% 26% 9%	530
October	1st to 10th	Day	12% 2 21% 33% 22% 9%	530
	11th to 20th	Day	8% 10% 34% 32% 14% 1	411
	21st to end	Day	21% 14% 28% 25% 10% 2	363
November	1st to 10th	Day	19% 12% 28% 27% 13% 2	330
	11th to 20th	Day	12% 17% 42% 22% 6% 1	330
	21st to end	Day	17% 21% 32% 19% 8% 2	330
December	1st to 10th	Day	26% 22% 32% 16% 3	330
	11th to 20th	Day	21% 20% 36% 16% 7%	330
	21st to end	Day	25% 21% 37% 13% 4	363
January	1st to 10th	Night	23% 18% 35% 20% 4	1050
	11th to 20th	Night	24% 18% 33% 20% 5%	1026
	21st to end	Night	20% 16% 40% 22% 2	924
February	1st to 10th	Night	21% 14% 39% 21% 4	840
	11th to 20th	Night	28% 18% 31% 21% 2	823
	21st to end	Night	27% 18% 33% 21% 2	692
March	1st to 10th	Night	21% 16% 32% 26% 5%	840
	11th to 20th	Night	21% 14% 37% 23% 6%	830
	21st to end	Night	18% 13% 36% 23% 9%	693
April	1st to 10th	Night	16% 12% 35% 27% 9%	630
	11th to 20th	Night	14% 10% 35% 29% 10% 1	623
	21st to end	Night	16% 9% 31% 31% 12% 1	630
May	1st to 10th	Night	15% 6% 29% 32% 16% 2	630
	11th to 20th	Night	13% 6% 24% 33% 20% 4	621
	21st to end	Night	10% 3 19% 35% 27% 6%	683
June	1st to 10th	Night	8% 3 16% 37% 30% 6%	630
	11th to 20th	Night	4 1 13% 37% 35% 9%	615
	21st to end	Night	5% 2 11% 34% 38% 10%	600
July	1st to 10th	Night	8% 2 9% 29% 40% 13%	585
	11th to 20th	Night	6% 1 7% 30% 41% 15%	582
	21st to end	Night	7% 2 8% 29% 40% 14%	657
August	1st to 10th	Night	4 1 7% 26% 48% 13%	597
	11th to 20th	Night	5% 2 9% 31% 44% 9%	594
	21st to end	Night	9% 3 10% 29% 40% 9%	660
September	1st to 10th	Night	11% 3 11% 33% 37% 6%	600
	11th to 20th	Night	12% 2 13% 31% 35% 7%	752
	21st to end	Night	10% 3 17% 32% 35% 3	800
October	1st to 10th	Night	9% 3 21% 32% 30% 5%	800
	11th to 20th	Night	14% 8% 22% 37% 18% 2	800
	21st to end	Night	19% 6% 27% 30% 17% 1	880
November	1st to 10th	Night	15% 8% 23% 35% 17% 2	820
	11th to 20th	Night	14% 11% 35% 30% 10% 1	840
	21st to end	Night	22% 11% 31% 27% 10% 1	840
December	1st to 10th	Night	23% 13% 30% 27% 6% 1	840
	11th to 20th	Night	21% 13% 33% 26% 7%	840
	21st to end	Night	23% 15% 34% 24% 4	1045

Page generated at 2026-06-13 01:31:41. Version ac07

Copyright © 2026 Attilla Danko.