Bucknell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bucknell Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	16% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	144363
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 37%	137842
Transparency	Day	45% 6% 11% 22% 13% 2	147954
	Night	45% 6% 12% 22% 14% 2	142339
Seeing	Day	21% 14% 25% 23% 13% 5%	17132
	Night	20% 10% 28% 30% 11% 1	26432

By Month

Percentage Cloud Cover NE (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	15% 11% 4 9% 5% 5% 5% 3 5% 6% 34%	4661
	11th to 20th	Day	11% 11% 6% 13% 6% 7% 6% 4 5% 6% 26%	5004
	21st to end	Day	15% 10% 3 9% 6% 5% 4 3 5% 6% 33%	5780
February	1st to 10th	Day	15% 11% 3 8% 6% 6% 5% 4 5% 6% 29%	1890
	11th to 20th	Day	14% 10% 3 9% 6% 5% 4 3 6% 8% 31%	1879
	21st to end	Day	18% 9% 4 9% 5% 4 5% 3 4 6% 32%	1730
March	1st to 10th	Day	20% 9% 3 6% 4 4 4 3 4 4 38%	4847
	11th to 20th	Day	13% 8% 3 8% 5% 5% 4 3 5% 7% 39%	5090
	21st to end	Day	14% 9% 3 7% 5% 5% 4 4 5% 6% 39%	5743
April	1st to 10th	Day	18% 8% 3 8% 6% 5% 5% 3 6% 6% 34%	2520
	11th to 20th	Day	17% 8% 2 6% 5% 4 5% 4 5% 6% 38%	2494
	21st to end	Day	17% 6% 2 6% 5% 5% 4 3 6% 7% 37%	2525
May	1st to 10th	Day	23% 6% 2 7% 4 5% 5% 4 6% 7% 30%	7774
	11th to 20th	Day	20% 7% 3 7% 5% 4 5% 4 6% 8% 32%	8221
	21st to end	Day	23% 6% 2 6% 4 4 5% 4 6% 7% 34%	9060
June	1st to 10th	Day	12% 6% 3 7% 6% 5% 5% 4 6% 8% 39%	2940
	11th to 20th	Day	14% 5% 2 6% 5% 5% 5% 4 6% 9% 38%	2940
	21st to end	Day	11% 5% 2 6% 5% 4 5% 4 7% 10% 40%	2940
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 9% 38%	8280
	11th to 20th	Day	7% 5% 3 7% 5% 6% 6% 5% 8% 11% 36%	8280
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	8560
August	1st to 10th	Day	19% 6% 3 7% 5% 4 5% 4 7% 8% 31%	4364
	11th to 20th	Day	15% 6% 3 6% 5% 5% 6% 5% 8% 8% 34%	4173
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 9% 43%	4420
September	1st to 10th	Day	16% 5% 2 5% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 5% 7% 50%	2400
	21st to end	Day	22% 7% 3 6% 4 4 3 3 5% 6% 37%	2112
October	1st to 10th	Day	18% 6% 2 6% 4 4 3 3 5% 6% 42%	2000
	11th to 20th	Day	17% 5% 2 7% 4 5% 4 3 6% 8% 38%	2000
	21st to end	Day	18% 7% 3 6% 3 5% 6% 4 5% 6% 37%	2200
November	1st to 10th	Day	12% 7% 2 7% 4 4 4 3 5% 7% 46%	1861
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 4 6% 6% 37%	1801
	21st to end	Day	17% 8% 3 7% 4 5% 4 4 7% 6% 35%	1680
December	1st to 10th	Day	16% 11% 5% 9% 6% 5% 5% 4 5% 7% 28%	2680
	11th to 20th	Day	23% 10% 2 6% 4 5% 4 4 4 6% 32%	3066
	21st to end	Day	20% 11% 3 9% 6% 6% 5% 4 5% 6% 24%	4048
January	1st to 10th	Night	12% 11% 4 11% 6% 5% 5% 4 5% 6% 32%	8234
	11th to 20th	Night	12% 10% 5% 10% 5% 5% 5% 4 6% 6% 32%	7878
	21st to end	Night	15% 9% 3 8% 5% 5% 4 4 5% 5% 37%	8401
February	1st to 10th	Night	15% 10% 3 8% 6% 4 5% 4 5% 6% 34%	2730
	11th to 20th	Night	16% 9% 4 8% 5% 4 6% 4 5% 7% 32%	2626
	21st to end	Night	17% 11% 3 7% 6% 5% 4 3 5% 5% 34%	2076
March	1st to 10th	Night	20% 10% 3 7% 4 3 3 3 4 5% 37%	5208
	11th to 20th	Night	11% 9% 3 6% 4 3 4 3 5% 7% 45%	5089
	21st to end	Night	16% 9% 3 7% 5% 4 4 3 4 5% 40%	5430
April	1st to 10th	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	2100
	11th to 20th	Night	16% 6% 3 6% 5% 4 4 4 6% 8% 38%	2075
	21st to end	Night	18% 5% 2 5% 4 4 4 4 5% 7% 41%	2100
May	1st to 10th	Night	24% 7% 3 7% 4 5% 4 3 5% 7% 31%	5146
	11th to 20th	Night	20% 7% 3 7% 4 4 5% 4 5% 6% 35%	4639
	21st to end	Night	24% 7% 2 6% 4 3 4 3 5% 6% 36%	5114
June	1st to 10th	Night	15% 6% 2 5% 5% 4 4 3 5% 8% 43%	1680
	11th to 20th	Night	13% 6% 2 7% 4 4 5% 4 7% 8% 41%	1680
	21st to end	Night	11% 4 1 6% 5% 4 5% 4 8% 9% 42%	1680
July	1st to 10th	Night	12% 6% 2 7% 5% 4 4 4 6% 9% 40%	4680
	11th to 20th	Night	9% 6% 3 7% 5% 5% 7% 6% 8% 10% 35%	4680
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	5500
August	1st to 10th	Night	19% 7% 2 7% 4 5% 4 4 6% 8% 34%	3005
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 5% 7% 41%	3062
	21st to end	Night	15% 4 2 5% 4 4 4 4 7% 8% 43%	3677
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 8% 51%	2000
	21st to end	Night	20% 7% 3 5% 3 3 4 3 5% 7% 40%	2283
October	1st to 10th	Night	19% 7% 2 5% 4 3 3 3 5% 6% 43%	2400
	11th to 20th	Night	17% 6% 2 5% 4 3 4 3 5% 7% 44%	2400
	21st to end	Night	19% 7% 2 6% 4 4 4 3 5% 6% 41%	2640
November	1st to 10th	Night	11% 6% 2 6% 4 3 4 4 5% 5% 49%	2657
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 5% 6% 38%	2814
	21st to end	Night	16% 9% 2 7% 4 5% 5% 4 6% 7% 35%	2940
December	1st to 10th	Night	13% 10% 4 7% 5% 5% 6% 4 5% 7% 35%	4720
	11th to 20th	Night	23% 8% 3 6% 4 4 4 3 4 7% 33%	5348
	21st to end	Night	21% 12% 4 8% 5% 4 5% 3 5% 6% 27%	7150

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	51% 4 13% 21% 10% 1	4662
	11th to 20th	Day	57% 5% 9% 15% 11% 3	5037
	21st to end	Day	51% 5% 12% 20% 11%	5780
February	1st to 10th	Day	53% 6% 11% 19% 10% 1	1890
	11th to 20th	Day	53% 5% 10% 23% 9% 1	1888
	21st to end	Day	54% 4 10% 22% 8% 2	1730
March	1st to 10th	Day	49% 4 10% 22% 10% 5%	4850
	11th to 20th	Day	43% 5% 13% 25% 10% 4	5090
	21st to end	Day	46% 5% 10% 18% 18% 3	5743
April	1st to 10th	Day	50% 4 10% 19% 13% 4	2520
	11th to 20th	Day	46% 6% 11% 20% 15% 3	2494
	21st to end	Day	45% 6% 9% 22% 14% 4	2525
May	1st to 10th	Day	51% 7% 11% 20% 10% 1	7783
	11th to 20th	Day	48% 7% 11% 23% 11% 1	8225
	21st to end	Day	48% 7% 9% 22% 12% 2	9061
June	1st to 10th	Day	42% 6% 11% 23% 16% 2	2940
	11th to 20th	Day	42% 7% 13% 22% 14% 2	2940
	21st to end	Day	38% 8% 14% 26% 15% 1	2940
July	1st to 10th	Day	41% 9% 11% 24% 13% 1	8210
	11th to 20th	Day	38% 9% 14% 27% 10% 1	8205
	21st to end	Day	41% 7% 11% 22% 19% 1	8372
August	1st to 10th	Day	49% 7% 13% 19% 11%	4351
	11th to 20th	Day	43% 9% 13% 25% 9% 1	4212
	21st to end	Day	36% 7% 12% 31% 13% 2	4420
September	1st to 10th	Day	39% 6% 11% 24% 18% 2	2400
	11th to 20th	Day	34% 5% 11% 26% 22% 3	2400
	21st to end	Day	49% 5% 10% 17% 18% 2	2112
October	1st to 10th	Day	40% 5% 9% 24% 18% 4	3260
	11th to 20th	Day	36% 6% 8% 27% 21% 2	3240
	21st to end	Day	32% 4 14% 31% 17% 1	3538
November	1st to 10th	Day	38% 4 13% 26% 17% 3	1861
	11th to 20th	Day	47% 5% 11% 23% 12% 2	1801
	21st to end	Day	47% 5% 12% 26% 8% 1	1680
December	1st to 10th	Day	56% 6% 12% 20% 6% 1	2680
	11th to 20th	Day	52% 5% 15% 21% 7%	3066
	21st to end	Day	58% 5% 14% 14% 7% 1	4048
January	1st to 10th	Night	52% 5% 15% 18% 9% 2	8234
	11th to 20th	Night	51% 6% 12% 15% 12% 4	7920
	21st to end	Night	47% 5% 9% 21% 16% 2	8401
February	1st to 10th	Night	50% 6% 12% 23% 10% 1	2730
	11th to 20th	Night	52% 5% 10% 21% 11% 2	2628
	21st to end	Night	52% 4 10% 19% 13% 2	2076
March	1st to 10th	Night	50% 4 10% 20% 11% 4	5218
	11th to 20th	Night	39% 5% 15% 26% 10% 5%	5090
	21st to end	Night	47% 5% 7% 17% 20% 4	5430
April	1st to 10th	Night	52% 4 9% 19% 13% 2	2100
	11th to 20th	Night	44% 6% 9% 20% 17% 3	2075
	21st to end	Night	42% 6% 9% 24% 16% 3	2100
May	1st to 10th	Night	53% 6% 10% 19% 10% 2	5153
	11th to 20th	Night	48% 8% 12% 19% 11% 1	4640
	21st to end	Night	48% 6% 11% 20% 12% 3	5115
June	1st to 10th	Night	39% 7% 10% 25% 18% 1	1680
	11th to 20th	Night	40% 7% 13% 22% 16% 2	1680
	21st to end	Night	36% 8% 13% 28% 14% 2	1680
July	1st to 10th	Night	41% 8% 13% 21% 15% 2	4640
	11th to 20th	Night	40% 10% 14% 25% 11% 1	4632
	21st to end	Night	39% 8% 12% 21% 20% 1	5374
August	1st to 10th	Night	46% 9% 13% 21% 10% 1	2994
	11th to 20th	Night	42% 7% 13% 24% 14%	3089
	21st to end	Night	36% 7% 12% 31% 14% 1	3677
September	1st to 10th	Night	38% 6% 10% 25% 18% 3	2000
	11th to 20th	Night	31% 5% 10% 30% 22% 2	2000
	21st to end	Night	44% 5% 9% 22% 20% 1	2283
October	1st to 10th	Night	42% 5% 11% 20% 21% 1	3920
	11th to 20th	Night	32% 5% 11% 28% 23% 2	3899
	21st to end	Night	32% 4 13% 30% 19% 1	4252
November	1st to 10th	Night	36% 5% 13% 25% 18% 3	2657
	11th to 20th	Night	46% 4 9% 24% 14% 3	2814
	21st to end	Night	47% 5% 14% 24% 9% 1	2940
December	1st to 10th	Night	48% 5% 14% 26% 7%	4720
	11th to 20th	Night	52% 4 14% 20% 10% 1	5348
	21st to end	Night	57% 5% 11% 17% 9% 1	7150

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	28% 23% 28% 16% 3 1	220
	11th to 20th	Day	31% 24% 28% 16% 1	285
	21st to end	Day	23% 25% 31% 14% 6% 1	363
February	1st to 10th	Day	27% 22% 33% 14% 4	330
	11th to 20th	Day	29% 23% 34% 13% 1	366
	21st to end	Day	29% 18% 31% 19% 3	446
March	1st to 10th	Day	27% 22% 26% 21% 3 1	540
	11th to 20th	Day	28% 21% 29% 17% 4 2	540
	21st to end	Day	27% 24% 24% 18% 4 2	594
April	1st to 10th	Day	30% 19% 26% 17% 7% 1	540
	11th to 20th	Day	24% 21% 27% 15% 8% 4	532
	21st to end	Day	25% 15% 30% 19% 9% 2	540
May	1st to 10th	Day	29% 15% 28% 17% 8% 3	543
	11th to 20th	Day	20% 15% 27% 24% 10% 4	567
	21st to end	Day	22% 8% 25% 23% 15% 7%	617
June	1st to 10th	Day	15% 13% 25% 26% 17% 5%	570
	11th to 20th	Day	18% 12% 25% 26% 16% 4	570
	21st to end	Day	15% 9% 27% 30% 15% 4	570
July	1st to 10th	Day	12% 4 22% 31% 23% 8%	565
	11th to 20th	Day	7% 6% 21% 34% 25% 6%	564
	21st to end	Day	12% 6% 24% 30% 22% 5%	620
August	1st to 10th	Day	13% 5% 20% 27% 25% 9%	566
	11th to 20th	Day	15% 5% 20% 30% 19% 11%	559
	21st to end	Day	13% 6% 22% 28% 19% 12%	619
September	1st to 10th	Day	17% 9% 19% 23% 19% 12%	530
	11th to 20th	Day	14% 5% 16% 26% 25% 13%	530
	21st to end	Day	26% 11% 18% 24% 15% 6%	530
October	1st to 10th	Day	22% 8% 21% 23% 15% 10%	530
	11th to 20th	Day	19% 16% 25% 25% 13% 2	530
	21st to end	Day	22% 14% 30% 23% 10% 1	583
November	1st to 10th	Day	20% 12% 27% 25% 13% 2	430
	11th to 20th	Day	25% 17% 26% 23% 9% 1	341
	21st to end	Day	24% 24% 30% 16% 5%	220
December	1st to 10th	Day	30% 21% 29% 15% 5% 1	220
	11th to 20th	Day	30% 16% 35% 15% 3	220
	21st to end	Day	29% 19% 29% 17% 6%	242
January	1st to 10th	Night	20% 17% 39% 21% 3	840
	11th to 20th	Night	24% 14% 33% 26% 3	840
	21st to end	Night	19% 14% 39% 26% 2	924
February	1st to 10th	Night	23% 11% 34% 29% 2	840
	11th to 20th	Night	23% 16% 41% 19% 1	823
	21st to end	Night	24% 14% 39% 21% 3	692
March	1st to 10th	Night	23% 15% 33% 25% 4	840
	11th to 20th	Night	25% 14% 31% 25% 4	840
	21st to end	Night	23% 12% 38% 22% 5% 1	924
April	1st to 10th	Night	25% 15% 31% 24% 4	840
	11th to 20th	Night	21% 13% 33% 26% 7%	831
	21st to end	Night	21% 12% 29% 33% 6%	840
May	1st to 10th	Night	25% 7% 28% 29% 10% 1	595
	11th to 20th	Night	17% 6% 32% 37% 7%	414
	21st to end	Night	19% 6% 20% 37% 17% 1	455
June	1st to 10th	Night	15% 6% 21% 39% 19%	420
	11th to 20th	Night	14% 6% 25% 42% 14%	420
	21st to end	Night	12% 2 22% 49% 15%	420
July	1st to 10th	Night	12% 3 20% 40% 23% 2	410
	11th to 20th	Night	10% 2 14% 47% 25% 1	408
	21st to end	Night	12% 5% 16% 42% 24% 1	606
August	1st to 10th	Night	14% 3 10% 41% 30% 2	627
	11th to 20th	Night	14% 3 12% 38% 31% 2	682
	21st to end	Night	15% 4 14% 36% 29% 3	917
September	1st to 10th	Night	17% 4 20% 34% 24% 2	800
	11th to 20th	Night	15% 4 14% 32% 31% 4	800
	21st to end	Night	21% 6% 19% 32% 20% 2	800
October	1st to 10th	Night	23% 5% 20% 28% 21% 3	800
	11th to 20th	Night	20% 7% 28% 35% 11% 1	800
	21st to end	Night	23% 9% 29% 32% 8%	880
November	1st to 10th	Night	17% 9% 29% 34% 11% 1	820
	11th to 20th	Night	23% 12% 29% 28% 8% 1	840
	21st to end	Night	22% 12% 31% 29% 6%	840
December	1st to 10th	Night	25% 14% 32% 24% 5%	840
	11th to 20th	Night	24% 12% 32% 28% 4	840
	21st to end	Night	24% 13% 31% 28% 4	924

Page generated at 2026-08-27 04:07:16. Version ac07

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