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 2025-04-26.

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% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	113752
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 37%	114834
Transparency	Day	46% 6% 11% 22% 13% 2	117387
	Night	45% 6% 12% 22% 14% 2	119363
Seeing	Day	21% 14% 25% 23% 13% 5%	15337
	Night	20% 10% 28% 30% 11% 1	24841

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	14% 10% 4 10% 5% 5% 4 3 4 5% 36%	3597
	11th to 20th	Day	13% 12% 6% 14% 6% 6% 6% 4 5% 6% 23%	3840
	21st to end	Day	17% 12% 4 9% 6% 5% 4 3 5% 6% 30%	4466
February	1st to 10th	Day	16% 12% 4 9% 6% 6% 5% 4 5% 6% 28%	1800
	11th to 20th	Day	14% 10% 3 9% 6% 5% 4 3 6% 8% 32%	1787
	21st to end	Day	19% 8% 4 10% 5% 4 5% 3 4 6% 32%	1650
March	1st to 10th	Day	20% 9% 3 6% 4 4 4 3 4 4 38%	4742
	11th to 20th	Day	13% 8% 3 8% 5% 5% 4 3 5% 7% 40%	4980
	21st to end	Day	14% 9% 3 7% 5% 5% 4 4 5% 5% 39%	5619
April	1st to 10th	Day	17% 8% 3 8% 6% 5% 5% 3 6% 6% 34%	2400
	11th to 20th	Day	17% 9% 2 6% 5% 4 5% 4 5% 6% 38%	2374
	21st to end	Day	17% 6% 2 6% 6% 5% 4 4 6% 7% 37%	2345
May	1st to 10th	Day	27% 6% 2 6% 4 4 4 3 5% 7% 31%	4186
	11th to 20th	Day	22% 9% 3 8% 6% 5% 5% 4 5% 7% 27%	4398
	21st to end	Day	13% 5% 1 5% 4 4 5% 5% 6% 9% 42%	4837
June	1st to 10th	Day	12% 6% 3 7% 6% 5% 5% 4 6% 8% 37%	2660
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 38%	2660
	21st to end	Day	11% 5% 2 6% 5% 4 5% 4 7% 10% 41%	2660
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 5% 8% 11% 38%	4440
	11th to 20th	Day	7% 6% 3 7% 5% 6% 6% 5% 8% 10% 39%	4440
	21st to end	Day	10% 7% 3 9% 5% 6% 6% 5% 7% 8% 34%	4577
August	1st to 10th	Day	20% 6% 3 7% 5% 4 5% 4 7% 8% 31%	4104
	11th to 20th	Day	15% 6% 3 6% 5% 5% 6% 5% 8% 8% 34%	3921
	21st to end	Day	15% 5% 2 5% 4 4 5% 4 6% 8% 43%	4180
September	1st to 10th	Day	17% 5% 2 5% 4 4 4 4 4 6% 45%	2280
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 50%	2280
	21st to end	Day	23% 6% 2 6% 4 4 3 3 5% 7% 38%	2007
October	1st to 10th	Day	19% 7% 2 6% 4 4 3 3 5% 6% 41%	1900
	11th to 20th	Day	18% 6% 2 7% 4 4 4 3 6% 7% 38%	1900
	21st to end	Day	18% 7% 3 6% 3 5% 6% 4 5% 6% 37%	2090
November	1st to 10th	Day	12% 6% 3 6% 4 3 4 3 5% 7% 47%	1771
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 4 6% 6% 38%	1715
	21st to end	Day	17% 8% 3 7% 4 5% 4 4 7% 6% 36%	1600
December	1st to 10th	Day	16% 11% 5% 9% 6% 5% 4 3 5% 7% 28%	2600
	11th to 20th	Day	23% 10% 2 6% 4 5% 4 4 4 6% 33%	2986
	21st to end	Day	20% 12% 3 9% 6% 6% 5% 4 5% 6% 23%	3960
January	1st to 10th	Night	13% 9% 3 10% 6% 4 5% 4 5% 6% 35%	6349
	11th to 20th	Night	12% 10% 5% 9% 5% 5% 5% 4 5% 7% 32%	6042
	21st to end	Night	18% 9% 3 9% 5% 5% 4 4 5% 5% 33%	6490
February	1st to 10th	Night	16% 10% 3 8% 6% 4 5% 4 5% 6% 34%	2600
	11th to 20th	Night	16% 9% 4 8% 5% 5% 5% 4 5% 7% 33%	2498
	21st to end	Night	17% 10% 3 7% 6% 5% 4 3 5% 5% 34%	1980
March	1st to 10th	Night	19% 11% 3 7% 4 3 3 3 4 5% 37%	5094
	11th to 20th	Night	11% 9% 3 6% 4 3 4 3 5% 7% 45%	4979
	21st to end	Night	16% 9% 3 7% 5% 4 4 3 4 5% 40%	5312
April	1st to 10th	Night	20% 10% 3 7% 5% 4 4 3 5% 6% 33%	2000
	11th to 20th	Night	17% 6% 3 6% 5% 4 4 4 6% 8% 38%	1975
	21st to end	Night	18% 5% 2 6% 4 4 4 4 5% 7% 41%	1957
May	1st to 10th	Night	29% 7% 3 6% 4 4 3 3 4 6% 31%	2762
	11th to 20th	Night	23% 8% 3 7% 5% 4 5% 4 6% 6% 29%	2495
	21st to end	Night	12% 6% 3 7% 4 4 5% 4 6% 9% 42%	2738
June	1st to 10th	Night	15% 6% 2 5% 5% 4 4 3 6% 8% 42%	1520
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 7% 8% 42%	1520
	21st to end	Night	11% 4 1 5% 5% 4 5% 4 8% 9% 43%	1520
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 5% 5% 6% 8% 43%	2520
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 6% 8% 9% 40%	2520
	21st to end	Night	14% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	2959
August	1st to 10th	Night	19% 8% 2 7% 4 5% 4 4 6% 8% 34%	2825
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 4 7% 41%	2876
	21st to end	Night	15% 4 2 5% 4 4 5% 4 7% 8% 43%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 46%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 8% 51%	1900
	21st to end	Night	20% 7% 2 5% 3 3 4 3 5% 7% 41%	2168
October	1st to 10th	Night	20% 7% 2 5% 4 3 3 3 5% 6% 42%	2280
	11th to 20th	Night	17% 6% 2 5% 4 3 4 3 5% 7% 44%	2280
	21st to end	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	2508
November	1st to 10th	Night	11% 6% 2 6% 4 3 4 4 5% 5% 50%	2527
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 5% 6% 39%	2680
	21st to end	Night	16% 8% 2 7% 4 5% 5% 4 6% 7% 36%	2800
December	1st to 10th	Night	13% 10% 4 7% 4 5% 6% 4 5% 7% 35%	4580
	11th to 20th	Night	24% 8% 3 6% 4 4 4 3 4 7% 33%	5208
	21st to end	Night	21% 12% 4 8% 5% 4 5% 3 5% 6% 27%	6996

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% 4 11% 22% 12% 2	3598
	11th to 20th	Day	60% 5% 9% 14% 9% 3	3873
	21st to end	Day	54% 5% 11% 20% 10%	4466
February	1st to 10th	Day	55% 6% 11% 17% 10% 1	1800
	11th to 20th	Day	52% 5% 10% 23% 9% 1	1796
	21st to end	Day	54% 4 10% 21% 8% 2	1650
March	1st to 10th	Day	49% 4 10% 22% 10% 5%	4745
	11th to 20th	Day	43% 5% 13% 25% 10% 4	4980
	21st to end	Day	46% 5% 9% 18% 18% 3	5619
April	1st to 10th	Day	50% 4 9% 20% 13% 4	2400
	11th to 20th	Day	47% 6% 10% 20% 15% 3	2374
	21st to end	Day	45% 6% 10% 21% 15% 4	2345
May	1st to 10th	Day	52% 6% 8% 21% 11% 2	4198
	11th to 20th	Day	55% 8% 11% 17% 8% 2	4402
	21st to end	Day	36% 8% 12% 26% 16% 3	4837
June	1st to 10th	Day	43% 7% 12% 23% 14% 2	2660
	11th to 20th	Day	42% 7% 13% 22% 14% 2	2660
	21st to end	Day	37% 7% 14% 26% 15% 1	2660
July	1st to 10th	Day	38% 9% 14% 26% 11% 1	4370
	11th to 20th	Day	37% 9% 15% 24% 14% 1	4365
	21st to end	Day	45% 8% 11% 19% 16% 1	4431
August	1st to 10th	Day	50% 7% 14% 19% 10%	4091
	11th to 20th	Day	42% 9% 13% 26% 10% 1	3960
	21st to end	Day	37% 7% 12% 31% 13% 2	4180
September	1st to 10th	Day	40% 6% 11% 24% 17% 3	2280
	11th to 20th	Day	34% 5% 11% 25% 22% 3	2280
	21st to end	Day	47% 5% 10% 18% 18% 2	2007
October	1st to 10th	Day	40% 5% 9% 24% 17% 4	3160
	11th to 20th	Day	36% 6% 8% 27% 21% 2	3140
	21st to end	Day	31% 4 15% 31% 17% 1	3428
November	1st to 10th	Day	38% 4 13% 26% 17% 3	1771
	11th to 20th	Day	46% 5% 11% 24% 12% 2	1715
	21st to end	Day	47% 5% 12% 26% 8% 1	1600
December	1st to 10th	Day	56% 6% 12% 20% 6% 1	2600
	11th to 20th	Day	52% 5% 15% 21% 7%	2986
	21st to end	Day	58% 5% 14% 14% 8% 1	3960
January	1st to 10th	Night	48% 6% 13% 19% 12% 2	6349
	11th to 20th	Night	51% 7% 13% 16% 9% 4	6084
	21st to end	Night	52% 5% 10% 17% 14% 2	6490
February	1st to 10th	Night	51% 6% 12% 22% 8% 1	2600
	11th to 20th	Night	51% 5% 10% 21% 11% 2	2500
	21st to end	Night	52% 4 10% 19% 13% 2	1980
March	1st to 10th	Night	49% 4 10% 20% 11% 5%	5104
	11th to 20th	Night	39% 5% 15% 26% 10% 4	4980
	21st to end	Night	47% 5% 7% 17% 20% 4	5312
April	1st to 10th	Night	53% 4 8% 20% 13% 2	2000
	11th to 20th	Night	44% 6% 9% 21% 17% 3	1975
	21st to end	Night	42% 6% 9% 23% 16% 3	1957
May	1st to 10th	Night	55% 6% 7% 21% 9% 2	2769
	11th to 20th	Night	53% 10% 12% 14% 10% 1	2496
	21st to end	Night	38% 9% 15% 19% 14% 5%	2739
June	1st to 10th	Night	39% 7% 11% 24% 17% 1	1520
	11th to 20th	Night	38% 7% 14% 24% 15% 2	1520
	21st to end	Night	35% 8% 13% 29% 14% 2	1520
July	1st to 10th	Night	39% 8% 13% 19% 17% 4	2480
	11th to 20th	Night	36% 10% 14% 24% 15% 1	2472
	21st to end	Night	41% 8% 12% 17% 21% 2	2865
August	1st to 10th	Night	47% 9% 13% 21% 10% 1	2814
	11th to 20th	Night	42% 6% 12% 24% 15%	2903
	21st to end	Night	36% 6% 12% 31% 14% 1	3476
September	1st to 10th	Night	38% 6% 10% 25% 18% 3	1900
	11th to 20th	Night	32% 5% 9% 29% 22% 3	1900
	21st to end	Night	43% 5% 9% 22% 20% 1	2168
October	1st to 10th	Night	43% 5% 11% 20% 20% 1	3800
	11th to 20th	Night	31% 5% 11% 28% 23% 2	3779
	21st to end	Night	32% 4 13% 30% 19% 1	4120
November	1st to 10th	Night	36% 5% 13% 25% 19% 2	2527
	11th to 20th	Night	46% 4 9% 24% 14% 3	2680
	21st to end	Night	47% 5% 14% 24% 9% 1	2800
December	1st to 10th	Night	48% 6% 14% 26% 6%	4580
	11th to 20th	Night	52% 4 14% 20% 9% 1	5208
	21st to end	Night	57% 5% 11% 17% 9% 1	6996

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	29% 24% 27% 16% 4 2	200
	11th to 20th	Day	29% 24% 30% 17% 2	259
	21st to end	Day	22% 23% 33% 15% 6% 1	330
February	1st to 10th	Day	27% 23% 33% 14% 3	300
	11th to 20th	Day	28% 24% 35% 13% 1	334
	21st to end	Day	28% 18% 31% 20% 3	414
March	1st to 10th	Day	26% 22% 27% 21% 4 1	500
	11th to 20th	Day	25% 21% 30% 18% 4 2	500
	21st to end	Day	27% 23% 24% 19% 5% 3	550
April	1st to 10th	Day	28% 19% 26% 18% 7% 1	500
	11th to 20th	Day	25% 21% 25% 16% 8% 5%	492
	21st to end	Day	24% 15% 31% 19% 9% 2	480
May	1st to 10th	Day	29% 14% 26% 17% 10% 3	463
	11th to 20th	Day	19% 16% 26% 24% 11% 4	487
	21st to end	Day	19% 7% 24% 25% 17% 8%	529
June	1st to 10th	Day	15% 12% 25% 26% 17% 4	490
	11th to 20th	Day	16% 12% 23% 27% 18% 4	490
	21st to end	Day	14% 9% 28% 29% 16% 4	490
July	1st to 10th	Day	12% 4 22% 31% 22% 9%	485
	11th to 20th	Day	7% 4 21% 35% 27% 6%	484
	21st to end	Day	12% 6% 24% 30% 22% 5%	532
August	1st to 10th	Day	14% 5% 21% 27% 24% 9%	486
	11th to 20th	Day	15% 5% 19% 29% 20% 12%	479
	21st to end	Day	14% 6% 20% 28% 19% 12%	539
September	1st to 10th	Day	18% 9% 20% 22% 20% 12%	490
	11th to 20th	Day	14% 6% 16% 26% 25% 13%	490
	21st to end	Day	27% 11% 18% 24% 15% 6%	490
October	1st to 10th	Day	23% 8% 21% 23% 16% 9%	490
	11th to 20th	Day	19% 15% 26% 25% 13% 2	490
	21st to end	Day	23% 13% 30% 23% 11% 1	539
November	1st to 10th	Day	20% 12% 27% 26% 14% 2	400
	11th to 20th	Day	24% 16% 27% 24% 9% 1	315
	21st to end	Day	24% 22% 30% 18% 6% 1	200
December	1st to 10th	Day	30% 21% 29% 15% 5% 2	200
	11th to 20th	Day	30% 17% 34% 16% 3 1	200
	21st to end	Day	27% 20% 28% 18% 6%	220
January	1st to 10th	Night	20% 17% 39% 21% 3	800
	11th to 20th	Night	23% 14% 33% 27% 3	800
	21st to end	Night	19% 13% 40% 26% 2	880
February	1st to 10th	Night	24% 12% 34% 29% 2	800
	11th to 20th	Night	23% 16% 41% 19% 2	783
	21st to end	Night	24% 14% 39% 20% 3	660
March	1st to 10th	Night	22% 15% 33% 25% 5%	800
	11th to 20th	Night	25% 14% 32% 25% 4	800
	21st to end	Night	22% 12% 38% 22% 5% 1	880
April	1st to 10th	Night	25% 15% 31% 24% 4	800
	11th to 20th	Night	21% 13% 33% 26% 7% 1	791
	21st to end	Night	20% 12% 29% 33% 7%	783
May	1st to 10th	Night	24% 8% 27% 30% 11% 1	538
	11th to 20th	Night	18% 5% 30% 37% 8% 1	374
	21st to end	Night	16% 5% 20% 39% 18% 1	411
June	1st to 10th	Night	15% 6% 22% 38% 19%	380
	11th to 20th	Night	13% 5% 24% 43% 15%	380
	21st to end	Night	11% 2 22% 49% 15%	380
July	1st to 10th	Night	12% 3 21% 40% 22% 2	370
	11th to 20th	Night	10% 2 14% 47% 25% 2	368
	21st to end	Night	12% 4 16% 43% 24% 1	548
August	1st to 10th	Night	14% 3 11% 42% 29% 2	567
	11th to 20th	Night	15% 3 11% 37% 32% 2	616
	21st to end	Night	15% 4 13% 36% 30% 3	836
September	1st to 10th	Night	17% 3 20% 33% 24% 2	760
	11th to 20th	Night	15% 4 15% 31% 31% 4	760
	21st to end	Night	21% 6% 20% 32% 19% 2	760
October	1st to 10th	Night	23% 6% 21% 28% 20% 3	760
	11th to 20th	Night	19% 7% 27% 35% 11% 1	760
	21st to end	Night	24% 8% 28% 32% 8%	836
November	1st to 10th	Night	16% 9% 30% 33% 11% 1	780
	11th to 20th	Night	23% 11% 30% 28% 8% 1	800
	21st to end	Night	21% 12% 31% 30% 6%	800
December	1st to 10th	Night	25% 13% 32% 25% 5%	800
	11th to 20th	Night	23% 11% 32% 29% 5%	800
	21st to end	Night	24% 13% 31% 29% 4	880

Page generated at 2025-04-23 09:25:07. Version ac07

Copyright © 2025 Attilla Danko.