Bench Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bench Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	4 5% 2 6% 5% 5% 5% 5% 7% 9% 47%	137061
	Night	4 6% 2 6% 5% 5% 5% 4 7% 9% 46%	131760
Transparency	Day	30% 9% 15% 26% 16% 4	131821
	Night	30% 9% 16% 26% 15% 4	128004
Seeing	Day	8% 11% 34% 31% 14% 2	17431
	Night	7% 8% 32% 38% 13% 1	27210

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	2 9% 4 11% 8% 9% 7% 6% 7% 9% 27%	3646
	11th to 20th	Day	1 5% 2 9% 7% 7% 8% 8% 10% 10% 34%	3750
	21st to end	Day	1 4 2 6% 4 4 6% 6% 8% 10% 48%	4510
February	1st to 10th	Day	4 10% 5% 10% 7% 7% 7% 6% 9% 10% 26%	1800
	11th to 20th	Day	2 8% 3 12% 8% 7% 8% 5% 9% 8% 30%	1755
	21st to end	Day	3 8% 3 8% 7% 6% 7% 6% 8% 9% 32%	1524
March	1st to 10th	Day	2 7% 3 7% 5% 5% 5% 4 6% 9% 47%	4866
	11th to 20th	Day	2 6% 4 7% 6% 5% 6% 5% 7% 9% 44%	5000
	21st to end	Day	5% 11% 4 9% 7% 7% 7% 5% 8% 9% 27%	5502
April	1st to 10th	Day	6% 9% 3 7% 5% 5% 5% 5% 8% 9% 39%	2400
	11th to 20th	Day	7% 8% 3 8% 6% 5% 5% 5% 7% 9% 38%	2412
	21st to end	Day	9% 9% 3 9% 5% 5% 6% 5% 7% 9% 34%	2600
May	1st to 10th	Day	8% 8% 3 7% 4 5% 4 4 5% 8% 44%	6064
	11th to 20th	Day	8% 6% 2 6% 4 5% 5% 5% 7% 10% 42%	6298
	21st to end	Day	8% 7% 2 6% 5% 5% 5% 4 6% 8% 43%	6925
June	1st to 10th	Day	7% 6% 2 5% 5% 5% 6% 6% 9% 10% 41%	2800
	11th to 20th	Day	6% 6% 2 5% 5% 5% 5% 5% 8% 10% 43%	2851
	21st to end	Day	3 3 1 3 3 4 5% 5% 6% 9% 58%	3000
July	1st to 10th	Day	3 2 1 4 3 4 6% 5% 7% 7% 59%	6569
	11th to 20th	Day	1 1 1 3 3 4 4 5% 8% 11% 60%	6340
	21st to end	Day	2 2 1 4 4 5% 5% 5% 8% 10% 55%	6733
August	1st to 10th	Day	4 3 2 5% 3 4 5% 4 7% 10% 52%	5779
	11th to 20th	Day	2 2 1 4 4 4 5% 5% 7% 8% 57%	5798
	21st to end	Day	5% 4 2 5% 4 3 4 3 5% 7% 59%	6235
September	1st to 10th	Day	5% 3 1 3 2 4 3 2 5% 7% 64%	2400
	11th to 20th	Day	6% 4 2 4 3 4 5% 3 6% 8% 55%	2337
	21st to end	Day	8% 4 2 4 4 4 3 3 4 7% 58%	2143
October	1st to 10th	Day	6% 6% 1 5% 3 3 2 2 3 5% 63%	3270
	11th to 20th	Day	7% 5% 2 5% 3 3 3 3 4 5% 60%	3250
	21st to end	Day	5% 7% 2 5% 4 4 4 4 7% 8% 49%	3548
November	1st to 10th	Day	5% 8% 3 6% 5% 6% 6% 5% 6% 7% 43%	1698
	11th to 20th	Day	3 6% 2 7% 5% 5% 6% 5% 8% 8% 44%	1680
	21st to end	Day	4 9% 3 8% 5% 6% 6% 5% 7% 8% 40%	1680
December	1st to 10th	Day	2 5% 2 6% 5% 6% 7% 4 7% 7% 50%	2700
	11th to 20th	Day	2 4 2 7% 6% 6% 8% 7% 9% 11% 39%	3106
	21st to end	Day	4 7% 3 9% 6% 6% 7% 5% 8% 9% 36%	4092
January	1st to 10th	Night	1 7% 4 10% 9% 9% 8% 6% 8% 9% 31%	6342
	11th to 20th	Night	1 6% 3 8% 8% 7% 7% 6% 8% 11% 34%	6273
	21st to end	Night	2 5% 3 6% 4 4 7% 6% 8% 10% 46%	6468
February	1st to 10th	Night	4 10% 5% 12% 6% 6% 6% 5% 7% 9% 31%	2600
	11th to 20th	Night	3 10% 5% 11% 7% 6% 7% 5% 8% 9% 29%	2532
	21st to end	Night	2 8% 4 9% 7% 5% 6% 5% 7% 9% 38%	2105
March	1st to 10th	Night	2 8% 3 7% 5% 5% 7% 4 6% 7% 45%	5013
	11th to 20th	Night	3 7% 2 5% 4 4 5% 4 7% 11% 49%	5000
	21st to end	Night	4 10% 4 10% 7% 7% 7% 5% 7% 9% 30%	5472
April	1st to 10th	Night	7% 9% 3 8% 6% 5% 6% 4 6% 7% 40%	2000
	11th to 20th	Night	6% 11% 3 9% 6% 4 5% 4 7% 8% 36%	1933
	21st to end	Night	9% 9% 3 8% 6% 4 5% 5% 6% 8% 39%	1800
May	1st to 10th	Night	8% 9% 2 5% 5% 4 4 3 6% 7% 47%	3916
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 5% 8% 11% 39%	3617
	21st to end	Night	7% 6% 2 6% 4 4 5% 4 7% 10% 45%	3972
June	1st to 10th	Night	9% 6% 2 5% 5% 5% 6% 5% 8% 9% 42%	1600
	11th to 20th	Night	7% 5% 2 6% 4 4 6% 4 7% 9% 46%	1540
	21st to end	Night	4 3 2 4 3 4 4 4 6% 8% 61%	1400
July	1st to 10th	Night	3 2 1 3 3 3 4 4 7% 10% 59%	3382
	11th to 20th	Night	3 3 1 5% 4 4 5% 4 7% 8% 56%	3640
	21st to end	Night	3 2 1 4 3 4 4 4 8% 10% 59%	4123
August	1st to 10th	Night	2 3 2 6% 4 4 5% 4 7% 10% 53%	4033
	11th to 20th	Night	3 2 1 3 3 3 5% 5% 7% 9% 58%	4051
	21st to end	Night	5% 5% 2 5% 3 3 4 3 4 6% 60%	4729
September	1st to 10th	Night	5% 4 1 3 3 2 3 3 5% 7% 64%	2000
	11th to 20th	Night	6% 4 2 4 3 4 4 4 6% 7% 56%	2060
	21st to end	Night	8% 5% 2 4 3 2 3 3 5% 6% 59%	2256
October	1st to 10th	Night	6% 4 1 3 2 3 2 3 4 7% 64%	3920
	11th to 20th	Night	8% 7% 2 5% 3 3 4 3 4 5% 56%	3898
	21st to end	Night	5% 7% 2 5% 4 3 4 3 6% 6% 54%	4252
November	1st to 10th	Night	4 8% 3 5% 5% 5% 5% 4 7% 9% 45%	2794
	11th to 20th	Night	2 7% 3 7% 6% 5% 6% 5% 7% 9% 44%	2940
	21st to end	Night	3 8% 3 7% 6% 5% 5% 5% 6% 8% 44%	2940
December	1st to 10th	Night	1 6% 3 7% 5% 5% 5% 5% 7% 8% 50%	4710
	11th to 20th	Night	2 5% 3 7% 5% 5% 6% 6% 9% 10% 43%	5321
	21st to end	Night	2 7% 3 7% 5% 6% 7% 5% 9% 10% 39%	7128

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% 12% 17% 21% 4	3647
	11th to 20th	Day	31% 15% 22% 19% 13% 1	3750
	21st to end	Day	24% 14% 22% 28% 11%	4510
February	1st to 10th	Day	48% 9% 17% 19% 6%	1800
	11th to 20th	Day	46% 10% 18% 17% 7% 1	1764
	21st to end	Day	40% 10% 17% 21% 12% 1	1524
March	1st to 10th	Day	27% 9% 16% 29% 16% 3	4866
	11th to 20th	Day	30% 11% 18% 32% 9% 1	5000
	21st to end	Day	43% 15% 20% 19% 3	5502
April	1st to 10th	Day	37% 10% 18% 24% 9% 1	2400
	11th to 20th	Day	39% 9% 18% 26% 8%	2415
	21st to end	Day	42% 10% 16% 23% 8% 1	2600
May	1st to 10th	Day	36% 8% 14% 28% 12% 2	6064
	11th to 20th	Day	32% 9% 19% 26% 13% 2	6302
	21st to end	Day	33% 10% 16% 32% 10%	6926
June	1st to 10th	Day	33% 10% 21% 24% 11% 1	2800
	11th to 20th	Day	31% 10% 17% 22% 17% 4	2851
	21st to end	Day	20% 8% 14% 27% 22% 10%	3000
July	1st to 10th	Day	21% 7% 9% 29% 24% 10%	6500
	11th to 20th	Day	15% 7% 9% 26% 31% 11%	6265
	21st to end	Day	20% 8% 13% 22% 25% 13%	6538
August	1st to 10th	Day	28% 7% 13% 22% 18% 12%	4208
	11th to 20th	Day	21% 8% 9% 27% 28% 6%	4197
	21st to end	Day	19% 5% 8% 26% 32% 10%	4490
September	1st to 10th	Day	18% 6% 10% 24% 30% 11%	2400
	11th to 20th	Day	25% 6% 12% 28% 24% 6%	2337
	21st to end	Day	27% 4 11% 29% 24% 5%	2143
October	1st to 10th	Day	25% 4 8% 19% 33% 11%	3270
	11th to 20th	Day	27% 4 13% 32% 20% 3	3250
	21st to end	Day	28% 7% 18% 33% 12% 1	3548
November	1st to 10th	Day	36% 8% 15% 27% 12% 1	1698
	11th to 20th	Day	31% 11% 21% 25% 10% 2	1680
	21st to end	Day	36% 9% 21% 25% 9% 1	1680
December	1st to 10th	Day	31% 8% 22% 26% 11% 1	2700
	11th to 20th	Day	30% 11% 31% 23% 6%	3104
	21st to end	Day	38% 10% 13% 27% 11% 2	4092
January	1st to 10th	Night	40% 13% 20% 20% 7%	6342
	11th to 20th	Night	33% 14% 20% 21% 11% 1	6273
	21st to end	Night	28% 15% 20% 26% 11% 1	6468
February	1st to 10th	Night	44% 10% 18% 20% 8%	2600
	11th to 20th	Night	45% 11% 19% 18% 7% 1	2533
	21st to end	Night	37% 11% 16% 22% 13% 1	2105
March	1st to 10th	Night	33% 8% 14% 24% 17% 4	5013
	11th to 20th	Night	24% 11% 19% 32% 12% 2	5000
	21st to end	Night	41% 17% 19% 17% 6% 1	5472
April	1st to 10th	Night	40% 9% 17% 23% 9% 2	2000
	11th to 20th	Night	41% 9% 16% 27% 7%	1933
	21st to end	Night	41% 8% 17% 24% 10%	1800
May	1st to 10th	Night	33% 9% 15% 26% 15% 2	3913
	11th to 20th	Night	33% 11% 18% 26% 10% 2	3618
	21st to end	Night	29% 10% 17% 35% 9%	3972
June	1st to 10th	Night	34% 11% 18% 23% 12% 2	1600
	11th to 20th	Night	30% 9% 19% 22% 16% 5%	1540
	21st to end	Night	19% 7% 13% 30% 23% 8%	1400
July	1st to 10th	Night	17% 6% 11% 34% 27% 6%	3343
	11th to 20th	Night	21% 6% 8% 26% 28% 10%	3592
	21st to end	Night	16% 7% 12% 27% 27% 11%	4006
August	1st to 10th	Night	27% 6% 14% 22% 20% 11%	2928
	11th to 20th	Night	23% 7% 11% 26% 24% 9%	2928
	21st to end	Night	19% 4 8% 23% 32% 14%	3405
September	1st to 10th	Night	19% 6% 10% 22% 30% 13%	2000
	11th to 20th	Night	25% 7% 10% 25% 25% 7%	2060
	21st to end	Night	25% 5% 10% 27% 27% 7%	2256
October	1st to 10th	Night	21% 5% 9% 23% 29% 13%	3920
	11th to 20th	Night	30% 4 11% 30% 21% 3	3898
	21st to end	Night	29% 6% 11% 34% 13% 6%	4252
November	1st to 10th	Night	31% 9% 17% 26% 13% 3	2794
	11th to 20th	Night	33% 9% 18% 28% 10% 2	2940
	21st to end	Night	33% 9% 22% 23% 13% 1	2940
December	1st to 10th	Night	28% 9% 23% 30% 9% 1	4710
	11th to 20th	Night	30% 10% 26% 28% 6%	5322
	21st to end	Night	33% 9% 18% 28% 11% 2	7128

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	7% 14% 40% 31% 8%	303
	11th to 20th	Day	6% 14% 48% 23% 8% 2	300
	21st to end	Day	5% 12% 50% 26% 6% 2	330
February	1st to 10th	Day	8% 16% 50% 23% 2 1	300
	11th to 20th	Day	6% 13% 51% 25% 5% 1	300
	21st to end	Day	3 12% 49% 29% 6% 1	249
March	1st to 10th	Day	5% 13% 45% 30% 5% 2	460
	11th to 20th	Day	6% 11% 40% 34% 8% 1	500
	21st to end	Day	10% 16% 35% 27% 9% 2	550
April	1st to 10th	Day	13% 13% 43% 26% 4	500
	11th to 20th	Day	10% 20% 43% 23% 4	492
	21st to end	Day	17% 13% 31% 26% 11% 1	500
May	1st to 10th	Day	13% 13% 28% 27% 16% 3	583
	11th to 20th	Day	12% 11% 27% 30% 16% 3	725
	21st to end	Day	15% 10% 22% 30% 19% 4	789
June	1st to 10th	Day	7% 9% 26% 34% 21% 4	730
	11th to 20th	Day	10% 13% 31% 33% 12% 1	730
	21st to end	Day	4 7% 27% 41% 19% 2	730
July	1st to 10th	Day	4 5% 22% 43% 23% 2	721
	11th to 20th	Day	3 4 26% 43% 22% 2	718
	21st to end	Day	3 4 21% 41% 27% 4	725
August	1st to 10th	Day	6% 6% 24% 38% 21% 5%	527
	11th to 20th	Day	6% 6% 23% 38% 25% 3	517
	21st to end	Day	5% 7% 27% 35% 19% 6%	583
September	1st to 10th	Day	6% 5% 29% 33% 23% 4	530
	11th to 20th	Day	9% 8% 28% 34% 17% 5%	530
	21st to end	Day	11% 9% 30% 32% 16% 3	473
October	1st to 10th	Day	10% 8% 34% 31% 15% 3	330
	11th to 20th	Day	8% 15% 39% 25% 10% 3	330
	21st to end	Day	10% 16% 42% 22% 9% 1	363
November	1st to 10th	Day	8% 14% 48% 23% 7% 1	330
	11th to 20th	Day	8% 15% 49% 23% 4 1	330
	21st to end	Day	8% 13% 51% 23% 4	330
December	1st to 10th	Day	7% 12% 53% 22% 5% 1	330
	11th to 20th	Day	8% 16% 52% 21% 3	330
	21st to end	Day	7% 13% 50% 23% 6%	363
January	1st to 10th	Night	6% 9% 42% 35% 8%	1006
	11th to 20th	Night	8% 10% 44% 30% 8% 1	972
	21st to end	Night	6% 8% 40% 36% 8% 1	880
February	1st to 10th	Night	7% 11% 42% 33% 7% 1	800
	11th to 20th	Night	7% 7% 42% 34% 8% 1	784
	21st to end	Night	5% 7% 40% 36% 11% 2	660
March	1st to 10th	Night	6% 7% 37% 39% 10% 1	800
	11th to 20th	Night	6% 7% 35% 39% 13% 1	800
	21st to end	Night	10% 8% 27% 40% 13% 1	875
April	1st to 10th	Night	11% 10% 30% 40% 9% 1	600
	11th to 20th	Night	10% 10% 35% 37% 7% 1	593
	21st to end	Night	12% 8% 27% 40% 12% 2	600
May	1st to 10th	Night	14% 10% 22% 35% 18% 2	600
	11th to 20th	Night	11% 7% 21% 39% 20% 1	591
	21st to end	Night	12% 8% 22% 38% 19% 1	650
June	1st to 10th	Night	9% 7% 24% 37% 22% 2	600
	11th to 20th	Night	9% 8% 29% 39% 14%	540
	21st to end	Night	4 6% 15% 54% 21% 1	400
July	1st to 10th	Night	3 2 15% 48% 30% 1	471
	11th to 20th	Night	3 3 21% 51% 20% 1	582
	21st to end	Night	3 3 17% 51% 25% 1	657
August	1st to 10th	Night	4 4 15% 50% 25% 2	597
	11th to 20th	Night	4 4 14% 50% 27% 2	594
	21st to end	Night	5% 5% 18% 47% 23% 2	660
September	1st to 10th	Night	6% 4 16% 45% 26% 4	600
	11th to 20th	Night	7% 3 22% 43% 22% 3	660
	21st to end	Night	10% 5% 21% 41% 20% 3	800
October	1st to 10th	Night	13% 5% 25% 39% 16% 3	800
	11th to 20th	Night	10% 9% 30% 35% 14% 2	800
	21st to end	Night	9% 8% 34% 36% 11% 2	880
November	1st to 10th	Night	8% 8% 38% 35% 9% 1	1003
	11th to 20th	Night	6% 12% 40% 36% 6%	1050
	21st to end	Night	7% 8% 43% 35% 6%	1050
December	1st to 10th	Night	6% 9% 42% 35% 7% 1	1050
	11th to 20th	Night	6% 10% 45% 32% 7% 1	1050
	21st to end	Night	6% 10% 45% 31% 7% 1	1155

Page generated at 2025-12-29 23:14:08. Version ac07

Copyright © 2025 Attilla Danko.