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 2024-11-12.

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	5% 6% 2 6% 5% 5% 5% 5% 7% 9% 46%	105925
	Night	4 6% 3 7% 5% 5% 5% 4 7% 9% 46%	101983
Transparency	Day	30% 9% 15% 24% 16% 5%	105671
	Night	30% 10% 16% 24% 15% 4	101827
Seeing	Day	8% 10% 34% 31% 14% 2	15892
	Night	8% 7% 31% 39% 14% 1	25606

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 7% 4 10% 7% 7% 7% 7% 8% 12% 31%	2538
	11th to 20th	Day	2 6% 2 9% 8% 7% 8% 7% 9% 9% 32%	2608
	21st to end	Day	1 5% 2 6% 4 4 6% 5% 8% 10% 47%	3146
February	1st to 10th	Day	4 10% 5% 10% 7% 7% 6% 6% 8% 9% 27%	1710
	11th to 20th	Day	2 8% 3 12% 8% 7% 8% 5% 9% 8% 30%	1665
	21st to end	Day	3 9% 3 8% 8% 6% 8% 6% 8% 9% 31%	1450
March	1st to 10th	Day	2 6% 3 8% 5% 5% 6% 5% 8% 10% 44%	3384
	11th to 20th	Day	2 5% 3 6% 5% 5% 5% 4 7% 8% 49%	3490
	21st to end	Day	6% 12% 5% 10% 8% 7% 7% 5% 7% 8% 25%	3841
April	1st to 10th	Day	6% 9% 3 6% 5% 5% 5% 5% 8% 9% 38%	2280
	11th to 20th	Day	7% 8% 3 8% 6% 5% 5% 5% 7% 9% 37%	2290
	21st to end	Day	9% 10% 3 9% 5% 4 6% 5% 6% 9% 36%	2470
May	1st to 10th	Day	9% 11% 4 10% 5% 6% 5% 5% 6% 8% 32%	4238
	11th to 20th	Day	6% 5% 2 5% 4 5% 5% 5% 8% 9% 46%	4388
	21st to end	Day	10% 9% 3 7% 5% 5% 5% 4 7% 9% 36%	4825
June	1st to 10th	Day	7% 6% 2 5% 5% 5% 6% 6% 9% 10% 40%	2660
	11th to 20th	Day	6% 6% 2 5% 5% 6% 5% 5% 8% 9% 42%	2708
	21st to end	Day	3 3 1 3 3 4 5% 5% 7% 8% 58%	2850
July	1st to 10th	Day	3 2 1 3 3 4 5% 4 7% 8% 60%	4595
	11th to 20th	Day	1 1 1 3 3 4 5% 5% 8% 10% 58%	4430
	21st to end	Day	2 2 1 4 4 5% 4 5% 7% 8% 57%	4688
August	1st to 10th	Day	5% 4 2 6% 4 4 5% 4 8% 9% 48%	4092
	11th to 20th	Day	3 3 1 4 4 4 5% 5% 7% 8% 57%	4028
	21st to end	Day	4 2 1 4 3 3 4 3 5% 7% 64%	4353
September	1st to 10th	Day	5% 3 1 3 2 4 3 2 5% 7% 64%	2280
	11th to 20th	Day	6% 4 1 4 3 4 5% 3 6% 8% 56%	2220
	21st to end	Day	8% 4 2 4 4 4 3 3 4 6% 58%	2036
October	1st to 10th	Day	6% 6% 1 5% 3 3 2 2 3 5% 63%	3170
	11th to 20th	Day	7% 5% 2 5% 3 3 3 3 3 5% 61%	3150
	21st to end	Day	5% 7% 2 5% 4 4 4 5% 7% 8% 48%	3438
November	1st to 10th	Day	5% 8% 3 6% 6% 6% 6% 5% 6% 7% 43%	1616
	11th to 20th	Day	3 7% 2 7% 5% 5% 6% 5% 8% 9% 43%	1528
	21st to end	Day	4 9% 4 8% 5% 6% 6% 5% 7% 8% 40%	1520
December	1st to 10th	Day	3 7% 3 9% 7% 7% 8% 5% 8% 8% 36%	1520
	11th to 20th	Day	2 4 2 8% 6% 6% 8% 7% 8% 12% 37%	1926
	21st to end	Day	3 6% 3 8% 6% 6% 7% 5% 7% 9% 40%	2794
January	1st to 10th	Night	2 5% 3 9% 7% 7% 6% 6% 8% 10% 37%	4417
	11th to 20th	Night	1 8% 3 10% 9% 7% 7% 6% 7% 11% 30%	4391
	21st to end	Night	2 6% 3 7% 5% 5% 6% 5% 8% 10% 41%	4521
February	1st to 10th	Night	4 10% 6% 12% 6% 5% 6% 5% 7% 9% 32%	2470
	11th to 20th	Night	3 9% 6% 11% 7% 6% 7% 5% 8% 9% 30%	2402
	21st to end	Night	2 8% 4 9% 7% 5% 6% 5% 7% 9% 36%	2003
March	1st to 10th	Night	2 8% 3 7% 5% 4 5% 4 7% 8% 47%	3477
	11th to 20th	Night	2 5% 2 6% 4 4 5% 4 7% 11% 49%	3490
	21st to end	Night	5% 10% 4 10% 7% 7% 7% 6% 7% 10% 27%	3811
April	1st to 10th	Night	7% 9% 3 7% 6% 5% 6% 4 6% 6% 40%	1900
	11th to 20th	Night	6% 11% 3 9% 6% 4 5% 4 7% 8% 36%	1836
	21st to end	Night	8% 8% 3 8% 6% 4 5% 5% 6% 8% 39%	1710
May	1st to 10th	Night	9% 12% 3 6% 6% 5% 4 3 6% 7% 39%	2732
	11th to 20th	Night	7% 4 1 5% 5% 5% 5% 5% 7% 10% 46%	2517
	21st to end	Night	9% 7% 3 6% 5% 4 5% 4 7% 10% 39%	2762
June	1st to 10th	Night	9% 6% 1 5% 5% 5% 6% 5% 8% 9% 42%	1520
	11th to 20th	Night	7% 5% 2 6% 5% 4 6% 4 7% 9% 45%	1463
	21st to end	Night	3 3 2 4 3 4 4 3 6% 8% 61%	1330
July	1st to 10th	Night	4 2 1 3 2 3 4 5% 6% 9% 62%	2366
	11th to 20th	Night	2 2 1 4 3 4 5% 4 7% 9% 59%	2540
	21st to end	Night	3 2 1 3 3 3 3 4 7% 9% 61%	2872
August	1st to 10th	Night	3 3 2 7% 5% 4 5% 4 8% 9% 51%	2847
	11th to 20th	Night	4 3 2 4 4 4 5% 5% 6% 9% 54%	2811
	21st to end	Night	3 3 1 4 3 3 3 3 4 7% 66%	3301
September	1st to 10th	Night	5% 4 1 3 3 2 3 3 5% 7% 64%	1900
	11th to 20th	Night	7% 4 2 4 3 3 4 4 6% 7% 58%	1957
	21st to end	Night	8% 5% 2 5% 3 2 3 3 5% 6% 59%	2143
October	1st to 10th	Night	6% 4 1 3 2 3 2 3 4 7% 65%	3800
	11th to 20th	Night	8% 7% 2 4 3 3 3 3 4 5% 57%	3778
	21st to end	Night	5% 7% 3 5% 4 4 4 3 6% 6% 54%	4120
November	1st to 10th	Night	5% 8% 3 5% 5% 5% 5% 4 7% 9% 44%	2657
	11th to 20th	Night	3 7% 3 7% 5% 5% 5% 4 7% 9% 44%	2675
	21st to end	Night	3 8% 3 7% 6% 5% 5% 5% 6% 8% 43%	2660
December	1st to 10th	Night	2 8% 4 9% 6% 6% 6% 6% 8% 9% 36%	2660
	11th to 20th	Night	2 6% 3 7% 6% 5% 5% 5% 8% 10% 43%	3271
	21st to end	Night	2 7% 3 7% 5% 5% 6% 5% 9% 10% 42%	4873

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	37% 14% 19% 27% 3	2539
	11th to 20th	Day	33% 17% 23% 19% 7% 1	2608
	21st to end	Day	27% 17% 27% 21% 8%	3146
February	1st to 10th	Day	48% 9% 17% 20% 5%	1710
	11th to 20th	Day	46% 11% 19% 17% 7% 1	1674
	21st to end	Day	40% 11% 17% 20% 11% 1	1450
March	1st to 10th	Day	25% 10% 19% 28% 15% 3	3384
	11th to 20th	Day	28% 11% 22% 29% 9% 1	3490
	21st to end	Day	46% 16% 22% 12% 3	3841
April	1st to 10th	Day	38% 11% 19% 22% 9% 1	2280
	11th to 20th	Day	39% 9% 18% 26% 8%	2293
	21st to end	Day	41% 10% 16% 24% 9% 1	2470
May	1st to 10th	Day	45% 10% 13% 22% 9% 2	4238
	11th to 20th	Day	28% 10% 17% 30% 13% 2	4392
	21st to end	Day	38% 11% 18% 25% 7%	4825
June	1st to 10th	Day	33% 10% 21% 23% 11% 2	2660
	11th to 20th	Day	33% 10% 16% 22% 17% 4	2708
	21st to end	Day	20% 8% 14% 26% 22% 10%	2850
July	1st to 10th	Day	20% 6% 9% 29% 28% 7%	4525
	11th to 20th	Day	16% 6% 7% 22% 33% 15%	4355
	21st to end	Day	21% 7% 8% 19% 29% 17%	4540
August	1st to 10th	Day	28% 7% 13% 21% 19% 12%	4078
	11th to 20th	Day	21% 8% 9% 27% 29% 6%	4067
	21st to end	Day	19% 5% 9% 25% 32% 11%	4352
September	1st to 10th	Day	19% 6% 10% 24% 30% 12%	2280
	11th to 20th	Day	24% 6% 12% 27% 25% 6%	2220
	21st to end	Day	27% 4 12% 29% 23% 5%	2036
October	1st to 10th	Day	25% 4 8% 18% 34% 11%	3170
	11th to 20th	Day	27% 4 13% 32% 21% 4	3150
	21st to end	Day	29% 8% 18% 33% 12% 1	3438
November	1st to 10th	Day	37% 8% 15% 26% 12% 1	1616
	11th to 20th	Day	31% 11% 22% 23% 11% 2	1528
	21st to end	Day	36% 9% 21% 24% 10% 1	1520
December	1st to 10th	Day	42% 10% 19% 20% 8% 1	1520
	11th to 20th	Day	31% 13% 30% 21% 5%	1924
	21st to end	Day	35% 9% 14% 26% 12% 3	2794
January	1st to 10th	Night	33% 15% 22% 22% 7% 1	4417
	11th to 20th	Night	38% 15% 22% 17% 7% 1	4391
	21st to end	Night	32% 17% 23% 21% 6%	4521
February	1st to 10th	Night	44% 9% 19% 21% 7%	2470
	11th to 20th	Night	44% 11% 20% 18% 6% 1	2403
	21st to end	Night	38% 11% 16% 21% 12% 1	2003
March	1st to 10th	Night	29% 8% 16% 26% 17% 4	3477
	11th to 20th	Night	24% 13% 24% 29% 9% 2	3490
	21st to end	Night	41% 21% 20% 14% 4 1	3811
April	1st to 10th	Night	40% 9% 18% 22% 9% 2	1900
	11th to 20th	Night	41% 10% 16% 27% 7%	1836
	21st to end	Night	40% 8% 17% 25% 11%	1710
May	1st to 10th	Night	38% 11% 14% 22% 12% 3	2732
	11th to 20th	Night	26% 12% 16% 33% 10% 2	2518
	21st to end	Night	34% 12% 18% 30% 7%	2762
June	1st to 10th	Night	34% 11% 18% 24% 11% 2	1520
	11th to 20th	Night	31% 9% 18% 21% 16% 5%	1463
	21st to end	Night	19% 7% 13% 30% 22% 8%	1330
July	1st to 10th	Night	17% 6% 12% 33% 28% 4	2327
	11th to 20th	Night	18% 6% 8% 24% 31% 13%	2492
	21st to end	Night	14% 6% 8% 23% 34% 14%	2782
August	1st to 10th	Night	27% 6% 14% 21% 20% 11%	2838
	11th to 20th	Night	23% 7% 11% 26% 24% 9%	2838
	21st to end	Night	18% 4 8% 23% 32% 14%	3301
September	1st to 10th	Night	19% 6% 9% 22% 30% 14%	1900
	11th to 20th	Night	25% 7% 10% 25% 26% 7%	1957
	21st to end	Night	26% 5% 10% 27% 26% 7%	2143
October	1st to 10th	Night	20% 5% 9% 23% 30% 14%	3800
	11th to 20th	Night	30% 4 11% 30% 21% 3	3778
	21st to end	Night	30% 6% 11% 33% 13% 6%	4120
November	1st to 10th	Night	32% 9% 17% 25% 13% 3	2657
	11th to 20th	Night	33% 9% 19% 28% 9% 2	2675
	21st to end	Night	34% 9% 21% 22% 14% 1	2660
December	1st to 10th	Night	37% 11% 22% 20% 8% 2	2660
	11th to 20th	Night	31% 11% 26% 27% 5%	3272
	21st to end	Night	31% 10% 19% 26% 12% 2	4873

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	6% 13% 41% 31% 9%	270
	11th to 20th	Day	6% 13% 46% 24% 9% 2	270
	21st to end	Day	5% 10% 51% 26% 6% 2	297
February	1st to 10th	Day	7% 15% 50% 25% 3 1	270
	11th to 20th	Day	6% 12% 53% 25% 4 1	270
	21st to end	Day	3 12% 49% 28% 6% 1	225
March	1st to 10th	Day	5% 12% 45% 32% 5% 1	422
	11th to 20th	Day	6% 10% 39% 36% 8% 1	460
	21st to end	Day	10% 16% 35% 26% 10% 2	506
April	1st to 10th	Day	14% 13% 44% 25% 3	460
	11th to 20th	Day	10% 19% 44% 23% 4	452
	21st to end	Day	17% 14% 29% 27% 12% 2	460
May	1st to 10th	Day	14% 14% 28% 27% 15% 3	539
	11th to 20th	Day	12% 10% 27% 31% 16% 4	675
	21st to end	Day	16% 10% 22% 28% 20% 4	734
June	1st to 10th	Day	7% 9% 26% 34% 20% 4	680
	11th to 20th	Day	10% 14% 31% 33% 12% 1	680
	21st to end	Day	4 7% 27% 42% 19% 2	680
July	1st to 10th	Day	4 5% 22% 43% 23% 2	671
	11th to 20th	Day	3 4 25% 43% 22% 2	668
	21st to end	Day	3 4 20% 42% 27% 4	674
August	1st to 10th	Day	7% 6% 24% 37% 22% 5%	487
	11th to 20th	Day	6% 6% 23% 36% 26% 3	477
	21st to end	Day	5% 7% 27% 36% 19% 6%	539
September	1st to 10th	Day	6% 5% 30% 33% 22% 4	490
	11th to 20th	Day	10% 7% 29% 33% 17% 4	490
	21st to end	Day	11% 9% 30% 33% 14% 2	436
October	1st to 10th	Day	10% 7% 34% 30% 16% 3	300
	11th to 20th	Day	9% 13% 38% 26% 11% 3	300
	21st to end	Day	10% 16% 43% 22% 8% 2	330
November	1st to 10th	Day	9% 12% 47% 24% 7% 1	300
	11th to 20th	Day	7% 16% 51% 23% 3 1	273
	21st to end	Day	10% 11% 51% 23% 4	270
December	1st to 10th	Day	7% 10% 55% 22% 6% 1	270
	11th to 20th	Day	8% 12% 54% 22% 4	270
	21st to end	Day	7% 11% 49% 26% 7%	297
January	1st to 10th	Night	5% 9% 41% 36% 8% 1	950
	11th to 20th	Night	8% 10% 43% 30% 8% 1	924
	21st to end	Night	7% 7% 40% 37% 8% 1	836
February	1st to 10th	Night	7% 10% 41% 33% 8% 1	760
	11th to 20th	Night	8% 7% 42% 34% 8% 1	744
	21st to end	Night	5% 7% 39% 36% 11% 2	628
March	1st to 10th	Night	6% 6% 37% 40% 10% 1	760
	11th to 20th	Night	6% 7% 34% 39% 13% 1	760
	21st to end	Night	11% 9% 25% 40% 14% 1	831
April	1st to 10th	Night	11% 10% 29% 41% 9% 1	570
	11th to 20th	Night	10% 10% 35% 37% 7% 1	563
	21st to end	Night	11% 8% 27% 40% 12% 2	570
May	1st to 10th	Night	14% 10% 22% 34% 19% 2	570
	11th to 20th	Night	10% 7% 20% 39% 22% 1	561
	21st to end	Night	13% 7% 22% 38% 18% 1	617
June	1st to 10th	Night	9% 7% 23% 37% 22% 2	570
	11th to 20th	Night	10% 8% 28% 39% 15%	513
	21st to end	Night	3 5% 16% 53% 22% 1	380
July	1st to 10th	Night	3 2 16% 46% 31% 1	447
	11th to 20th	Night	3 3 20% 52% 20% 1	552
	21st to end	Night	3 2 15% 53% 25% 1	624
August	1st to 10th	Night	5% 3 15% 50% 26% 2	567
	11th to 20th	Night	4 4 14% 49% 27% 2	564
	21st to end	Night	5% 5% 18% 46% 23% 2	627
September	1st to 10th	Night	6% 4 16% 45% 25% 4	570
	11th to 20th	Night	7% 4 22% 42% 22% 3	627
	21st to end	Night	10% 6% 21% 40% 20% 3	760
October	1st to 10th	Night	13% 4 26% 37% 17% 3	760
	11th to 20th	Night	9% 8% 30% 36% 15% 2	760
	21st to end	Night	9% 8% 34% 35% 11% 2	836
November	1st to 10th	Night	9% 8% 37% 36% 9% 1	954
	11th to 20th	Night	6% 11% 41% 36% 5%	956
	21st to end	Night	7% 8% 42% 36% 7%	950
December	1st to 10th	Night	5% 8% 40% 37% 8% 1	950
	11th to 20th	Night	6% 7% 44% 34% 8% 1	950
	21st to end	Night	7% 10% 42% 33% 7% 1	1045

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