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 2025-02-21.

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% 2 6% 4 4 4 4 6% 7% 42%	108927
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	109690
Transparency	Day	39% 4 10% 23% 19% 5%	112689
	Night	38% 4 10% 22% 21% 6%	114067
Seeing	Day	15% 11% 24% 24% 18% 8%	15415
	Night	16% 9% 25% 28% 19% 4	25232

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	16% 11% 3 9% 3 2 3 2 3 5% 42%	3617
	11th to 20th	Day	15% 7% 2 10% 3 2 3 2 3 5% 48%	3960
	21st to end	Day	19% 11% 3 6% 3 3 3 3 4 6% 40%	4972
February	1st to 10th	Day	18% 12% 3 7% 3 3 3 3 4 7% 38%	2000
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 3 5% 39%	1948
	21st to end	Day	20% 10% 3 6% 3 4 4 3 4 6% 37%	1570
March	1st to 10th	Day	21% 11% 3 7% 5% 4 4 3 4 5% 32%	3120
	11th to 20th	Day	13% 8% 2 6% 3 3 3 4 6% 7% 45%	3269
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 5% 6% 39%	4165
April	1st to 10th	Day	13% 8% 2 5% 4 4 4 3 6% 7% 45%	2280
	11th to 20th	Day	15% 7% 2 5% 4 4 3 4 5% 7% 45%	2254
	21st to end	Day	11% 6% 3 6% 5% 4 4 4 6% 8% 45%	2280
May	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 40%	3921
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 4 6% 8% 38%	4081
	21st to end	Day	12% 6% 3 7% 5% 4 5% 4 6% 8% 39%	4476
June	1st to 10th	Day	9% 7% 2 6% 5% 5% 5% 5% 7% 10% 39%	2497
	11th to 20th	Day	5% 5% 2 5% 5% 5% 5% 6% 8% 10% 44%	2660
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 6% 7% 10% 44%	2660
July	1st to 10th	Day	6% 6% 2 7% 6% 7% 7% 6% 9% 11% 33%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 6% 7% 6% 10% 12% 31%	4408
	21st to end	Day	13% 10% 3 9% 6% 6% 6% 4 8% 8% 25%	4249
August	1st to 10th	Day	3 2 1 4 3 5% 5% 4 6% 10% 56%	3785
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 5% 7% 10% 47%	3729
	21st to end	Day	7% 4 2 4 5% 5% 4 5% 6% 7% 50%	4180
September	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 8% 42%	2280
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 4 6% 7% 42%	2114
	21st to end	Day	11% 5% 2 5% 4 4 4 3 6% 7% 50%	2090
October	1st to 10th	Day	10% 5% 1 5% 4 3 3 3 4 5% 57%	2090
	11th to 20th	Day	8% 5% 2 5% 4 3 3 3 5% 6% 56%	1977
	21st to end	Day	15% 5% 2 4 3 3 4 3 4 5% 52%	2005
November	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 4 56%	1762
	11th to 20th	Day	12% 7% 2 4 4 3 4 2 4 6% 52%	1800
	21st to end	Day	14% 8% 2 6% 4 3 4 3 5% 6% 45%	1800
December	1st to 10th	Day	15% 9% 3 7% 4 3 3 3 4 5% 45%	2930
	11th to 20th	Day	20% 10% 4 6% 4 3 3 2 4 4 39%	3373
	21st to end	Day	16% 10% 3 7% 4 4 4 3 5% 7% 37%	4185
January	1st to 10th	Night	17% 12% 3 9% 3 3 3 3 4 4 39%	6348
	11th to 20th	Night	17% 9% 4 7% 3 2 2 2 3 4 48%	5994
	21st to end	Night	20% 11% 2 5% 3 3 2 2 3 5% 43%	5984
February	1st to 10th	Night	18% 11% 3 6% 4 4 3 3 4 4 41%	2400
	11th to 20th	Night	21% 9% 3 6% 4 3 4 3 3 4 40%	2338
	21st to end	Night	22% 9% 2 6% 4 4 4 3 4 5% 37%	1885
March	1st to 10th	Night	22% 10% 3 6% 4 4 3 2 4 5% 37%	3733
	11th to 20th	Night	13% 8% 2 5% 3 4 4 3 5% 6% 46%	3717
	21st to end	Night	16% 8% 3 6% 5% 4 4 4 4 5% 41%	3466
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 8% 46%	1900
	11th to 20th	Night	14% 6% 2 4 3 4 4 3 5% 7% 49%	1875
	21st to end	Night	12% 6% 1 5% 4 3 4 3 5% 7% 48%	1900
May	1st to 10th	Night	14% 7% 3 7% 4 4 3 3 5% 7% 43%	2970
	11th to 20th	Night	14% 7% 2 7% 4 4 4 4 5% 7% 42%	2812
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 5% 7% 10% 44%	3079
June	1st to 10th	Night	7% 5% 3 6% 6% 4 5% 4 6% 9% 45%	1676
	11th to 20th	Night	3 3 1 4 3 4 5% 5% 7% 12% 54%	1520
	21st to end	Night	4 3 2 4 5% 4 4 4 7% 10% 52%	1520
July	1st to 10th	Night	6% 5% 2 5% 5% 6% 6% 5% 8% 13% 38%	2530
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 5% 8% 13% 44%	2556
	21st to end	Night	9% 9% 4 9% 7% 6% 6% 5% 8% 10% 27%	3298
August	1st to 10th	Night	2 2 1 3 3 4 4 4 7% 10% 62%	3144
	11th to 20th	Night	3 3 1 4 3 4 5% 4 7% 11% 53%	3100
	21st to end	Night	5% 3 1 5% 4 4 5% 4 6% 9% 55%	3476
September	1st to 10th	Night	9% 5% 2 5% 4 5% 4 3 6% 8% 49%	1900
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 6% 8% 47%	2044
	21st to end	Night	9% 4 2 5% 3 2 3 3 5% 6% 56%	2090
October	1st to 10th	Night	8% 3 1 4 3 3 3 3 4 6% 63%	2090
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 5% 54%	2199
	21st to end	Night	16% 6% 1 4 3 3 3 3 4 4 53%	2590
November	1st to 10th	Night	12% 5% 2 4 4 3 3 3 5% 6% 53%	2536
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 5% 52%	2600
	21st to end	Night	17% 6% 2 5% 4 3 3 3 4 6% 49%	2600
December	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 45%	4250
	11th to 20th	Night	22% 9% 3 6% 3 3 3 2 4 4 42%	4822
	21st to end	Night	19% 10% 4 6% 4 4 4 3 5% 6% 35%	6748

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	46% 2 6% 24% 20% 2	3618
	11th to 20th	Day	41% 3 5% 23% 26% 2	3960
	21st to end	Day	47% 2 5% 21% 20% 5%	4972
February	1st to 10th	Day	48% 3 7% 19% 19% 4	2000
	11th to 20th	Day	51% 2 5% 20% 16% 5%	1958
	21st to end	Day	50% 3 7% 15% 19% 7%	1570
March	1st to 10th	Day	55% 3 7% 12% 14% 10%	3120
	11th to 20th	Day	39% 4 8% 25% 18% 7%	3269
	21st to end	Day	45% 3 11% 20% 18% 3	4165
April	1st to 10th	Day	38% 5% 10% 23% 18% 6%	2280
	11th to 20th	Day	39% 4 8% 21% 23% 6%	2254
	21st to end	Day	36% 5% 9% 28% 20% 2	2280
May	1st to 10th	Day	41% 4 12% 26% 14% 2	3956
	11th to 20th	Day	44% 3 15% 25% 12% 1	4085
	21st to end	Day	42% 4 12% 26% 15% 1	4489
June	1st to 10th	Day	39% 5% 14% 23% 17% 2	2497
	11th to 20th	Day	32% 7% 15% 26% 18% 2	2660
	21st to end	Day	31% 9% 17% 22% 16% 4	2660
July	1st to 10th	Day	39% 10% 21% 22% 7% 1	4371
	11th to 20th	Day	41% 8% 21% 23% 6% 1	4333
	21st to end	Day	54% 7% 14% 19% 6%	4113
August	1st to 10th	Day	23% 6% 13% 36% 21% 1	3773
	11th to 20th	Day	31% 6% 16% 28% 16% 3	3765
	21st to end	Day	32% 6% 13% 21% 25% 4	4179
September	1st to 10th	Day	37% 7% 14% 20% 18% 5%	2280
	11th to 20th	Day	41% 5% 12% 17% 22% 4	2114
	21st to end	Day	34% 5% 9% 24% 22% 7%	2090
October	1st to 10th	Day	20% 3 5% 15% 37% 20%	3480
	11th to 20th	Day	20% 2 5% 20% 34% 19%	3269
	21st to end	Day	26% 3 4 24% 34% 10%	3282
November	1st to 10th	Day	33% 3 8% 20% 27% 9%	1762
	11th to 20th	Day	35% 2 7% 18% 29% 9%	1800
	21st to end	Day	41% 4 7% 22% 22% 6%	1800
December	1st to 10th	Day	43% 2 4 24% 23% 4	2930
	11th to 20th	Day	51% 2 5% 21% 16% 5%	3370
	21st to end	Day	46% 3 9% 25% 14% 2	4185
January	1st to 10th	Night	50% 2 6% 17% 21% 3	6348
	11th to 20th	Night	43% 2 5% 19% 25% 6%	5994
	21st to end	Night	47% 2 3 19% 21% 7%	5984
February	1st to 10th	Night	48% 3 7% 20% 19% 5%	2400
	11th to 20th	Night	49% 3 6% 18% 18% 6%	2339
	21st to end	Night	50% 3 5% 17% 18% 7%	1885
March	1st to 10th	Night	50% 3 9% 14% 16% 8%	3733
	11th to 20th	Night	39% 3 8% 21% 19% 9%	3717
	21st to end	Night	47% 3 7% 19% 22% 2	3466
April	1st to 10th	Night	37% 4 10% 23% 20% 5%	1900
	11th to 20th	Night	35% 4 8% 21% 23% 9%	1875
	21st to end	Night	35% 4 10% 26% 21% 3	1900
May	1st to 10th	Night	41% 4 12% 23% 18% 2	3000
	11th to 20th	Night	41% 5% 16% 24% 13% 2	2813
	21st to end	Night	33% 5% 16% 28% 16% 2	3087
June	1st to 10th	Night	34% 7% 14% 24% 18% 3	1676
	11th to 20th	Night	23% 6% 16% 32% 21% 3	1520
	21st to end	Night	27% 7% 15% 29% 19% 4	1520
July	1st to 10th	Night	34% 10% 24% 22% 8% 2	2490
	11th to 20th	Night	29% 8% 27% 27% 7% 2	2508
	21st to end	Night	50% 8% 16% 18% 8%	3193
August	1st to 10th	Night	17% 5% 14% 39% 24% 2	3132
	11th to 20th	Night	22% 7% 24% 28% 17% 2	3130
	21st to end	Night	25% 6% 16% 22% 26% 5%	3476
September	1st to 10th	Night	32% 6% 13% 23% 19% 6%	1900
	11th to 20th	Night	36% 6% 13% 17% 25% 4	2044
	21st to end	Night	29% 5% 9% 26% 22% 9%	2090
October	1st to 10th	Night	17% 2 6% 18% 36% 21%	3480
	11th to 20th	Night	21% 2 4 16% 33% 24%	3646
	21st to end	Night	25% 2 4 26% 27% 15%	4263
November	1st to 10th	Night	33% 4 6% 22% 24% 11%	2536
	11th to 20th	Night	35% 3 7% 18% 27% 11%	2600
	21st to end	Night	38% 4 6% 22% 24% 7%	2600
December	1st to 10th	Night	43% 2 5% 24% 23% 3	4250
	11th to 20th	Night	47% 2 5% 19% 24% 3	4824
	21st to end	Night	50% 4 8% 24% 12% 3	6748

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% 23% 35% 14% 2	300
	11th to 20th	Day	26% 24% 37% 9% 3 1	300
	21st to end	Day	23% 25% 37% 13% 2	330
February	1st to 10th	Day	23% 23% 35% 15% 4 1	300
	11th to 20th	Day	37% 18% 34% 8% 2 1	300
	21st to end	Day	34% 23% 35% 6% 2	225
March	1st to 10th	Day	24% 22% 33% 17% 3 1	270
	11th to 20th	Day	20% 23% 41% 14% 1 1	303
	21st to end	Day	25% 17% 35% 17% 7%	506
April	1st to 10th	Day	22% 17% 31% 23% 7%	460
	11th to 20th	Day	19% 17% 32% 20% 10% 1	452
	21st to end	Day	13% 15% 33% 25% 12% 2	460
May	1st to 10th	Day	16% 12% 29% 27% 11% 5%	463
	11th to 20th	Day	11% 6% 29% 27% 20% 7%	487
	21st to end	Day	10% 6% 24% 27% 21% 11%	529
June	1st to 10th	Day	10% 5% 19% 31% 23% 12%	517
	11th to 20th	Day	6% 3 18% 31% 28% 14%	680
	21st to end	Day	6% 3 19% 34% 24% 13%	680
July	1st to 10th	Day	7% 4 12% 32% 31% 14%	671
	11th to 20th	Day	8% 3 9% 29% 29% 21%	649
	21st to end	Day	9% 2 13% 25% 29% 22%	532
August	1st to 10th	Day	7% 2 9% 29% 34% 19%	486
	11th to 20th	Day	8% 3 11% 30% 36% 12%	479
	21st to end	Day	14% 3 9% 22% 34% 19%	539
September	1st to 10th	Day	10% 3 11% 31% 33% 11%	490
	11th to 20th	Day	12% 4 11% 26% 30% 17%	490
	21st to end	Day	13% 4 14% 31% 27% 9%	490
October	1st to 10th	Day	12% 2 21% 33% 23% 10%	490
	11th to 20th	Day	8% 11% 35% 30% 15% 1	377
	21st to end	Day	18% 14% 26% 28% 11% 2	330
November	1st to 10th	Day	19% 11% 27% 27% 13% 2	300
	11th to 20th	Day	13% 18% 40% 23% 6% 1	300
	21st to end	Day	16% 21% 31% 20% 9% 3	300
December	1st to 10th	Day	26% 22% 31% 17% 4	300
	11th to 20th	Day	21% 20% 35% 15% 8%	300
	21st to end	Day	24% 21% 37% 13% 4	330
January	1st to 10th	Night	23% 18% 35% 21% 4	1000
	11th to 20th	Night	24% 17% 33% 20% 6%	977
	21st to end	Night	20% 16% 40% 22% 2	880
February	1st to 10th	Night	22% 14% 38% 22% 5%	800
	11th to 20th	Night	27% 18% 32% 21% 2	783
	21st to end	Night	27% 18% 34% 19% 1	629
March	1st to 10th	Night	21% 15% 32% 27% 5%	760
	11th to 20th	Night	20% 13% 37% 23% 6% 1	751
	21st to end	Night	18% 13% 34% 24% 10%	627
April	1st to 10th	Night	15% 12% 36% 27% 10% 1	570
	11th to 20th	Night	15% 10% 34% 29% 10% 1	563
	21st to end	Night	15% 9% 32% 29% 13% 1	570
May	1st to 10th	Night	13% 5% 29% 32% 18% 2	570
	11th to 20th	Night	12% 6% 22% 34% 22% 4	561
	21st to end	Night	7% 2 19% 37% 28% 6%	617
June	1st to 10th	Night	8% 3 14% 36% 31% 7%	570
	11th to 20th	Night	4 1 12% 36% 38% 10%	570
	21st to end	Night	5% 2 11% 34% 38% 10%	570
July	1st to 10th	Night	8% 2 9% 29% 40% 13%	555
	11th to 20th	Night	6% 1 8% 30% 40% 16%	552
	21st to end	Night	7% 2 8% 29% 39% 15%	624
August	1st to 10th	Night	4 1 7% 26% 49% 14%	567
	11th to 20th	Night	5% 2 9% 32% 43% 9%	564
	21st to end	Night	9% 3 9% 29% 41% 10%	627
September	1st to 10th	Night	11% 3 10% 31% 38% 6%	570
	11th to 20th	Night	13% 2 13% 31% 35% 7%	714
	21st to end	Night	10% 3 16% 33% 36% 3	760
October	1st to 10th	Night	9% 3 21% 32% 30% 5%	760
	11th to 20th	Night	14% 8% 23% 36% 17% 2	760
	21st to end	Night	18% 6% 26% 30% 18% 2	836
November	1st to 10th	Night	15% 8% 23% 35% 17% 2	780
	11th to 20th	Night	14% 10% 35% 29% 10% 1	800
	21st to end	Night	21% 11% 31% 27% 10% 1	800
December	1st to 10th	Night	23% 13% 30% 27% 6% 1	800
	11th to 20th	Night	21% 12% 33% 26% 7%	800
	21st to end	Night	23% 15% 33% 24% 4	995

Page generated at 2025-02-18 02:26:28. Version ac07

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