Stone Bridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stone Bridge clear sky chart from 2005-11-06 to 2025-12-03.

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	2 4 2 5% 4 4 5% 4 7% 8% 56%	135687
	Night	2 4 2 4 3 3 4 3 6% 8% 62%	132472
Transparency	Day	22% 8% 12% 24% 24% 10%	130636
	Night	19% 7% 14% 28% 25% 8%	128527
Seeing	Day	7% 18% 31% 26% 15% 3	16904
	Night	4 11% 37% 33% 14% 1	26982

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	4 2 7% 2 3 3 2 5% 8% 63%	3638
	11th to 20th	Day	1 3 3 4 5% 3 7% 8% 65%	4054
	21st to end	Day	3 2 5% 3 4 3 3 5% 6% 65%	4510
February	1st to 10th	Day	4 2 5% 3 3 3 3 6% 8% 64%	1800
	11th to 20th	Day	2 1 3 3 3 3 3 6% 10% 66%	1890
	21st to end	Day	3 2 6% 4 4 3 3 6% 7% 63%	1650
March	1st to 10th	Day	4 2 5% 3 3 4 3 5% 7% 65%	4637
	11th to 20th	Day	1 12% 3 6% 4 4 5% 4 6% 8% 48%	5000
	21st to end	Day	1 4 2 5% 3 3 4 4 7% 11% 55%	5700
April	1st to 10th	Day	1 5% 2 5% 3 3 4 4 6% 8% 60%	2400
	11th to 20th	Day	3 6% 2 5% 3 4 4 4 6% 9% 53%	2371
	21st to end	Day	2 5% 2 5% 4 5% 4 4 8% 9% 51%	2400
May	1st to 10th	Day	4 7% 2 7% 5% 5% 5% 5% 7% 9% 43%	5516
	11th to 20th	Day	3 5% 2 6% 4 4 5% 4 7% 10% 50%	6298
	21st to end	Day	1 3 1 4 4 5% 6% 5% 7% 8% 56%	6925
June	1st to 10th	Day	2 3 1 4 5% 5% 5% 6% 8% 10% 50%	2800
	11th to 20th	Day	3 3 2 4 4 4 5% 5% 7% 7% 58%	2800
	21st to end	Day	2 3 1 4 5% 6% 7% 6% 8% 9% 51%	2800
July	1st to 10th	Day	2 2 1 5% 4 4 6% 5% 7% 8% 57%	6339
	11th to 20th	Day	2 3 2 6% 6% 8% 9% 7% 8% 9% 40%	6340
	21st to end	Day	3 4 2 7% 6% 7% 7% 6% 8% 8% 42%	6408
August	1st to 10th	Day	3 3 2 7% 6% 6% 7% 6% 8% 11% 42%	5779
	11th to 20th	Day	3 2 2 5% 4 6% 7% 6% 8% 10% 48%	5417
	21st to end	Day	3 4 2 7% 6% 7% 7% 6% 8% 9% 41%	5984
September	1st to 10th	Day	4 4 2 5% 5% 5% 5% 5% 8% 8% 50%	2400
	11th to 20th	Day	3 3 1 6% 3 4 5% 4 7% 7% 57%	2400
	21st to end	Day	3 4 1 4 3 3 3 3 5% 7% 64%	2165
October	1st to 10th	Day	2 2 1 3 3 2 3 3 5% 6% 70%	3270
	11th to 20th	Day	2 2 1 3 3 3 3 4 6% 9% 63%	3250
	21st to end	Day	1 3 1 2 2 2 2 2 4 7% 73%	3548
November	1st to 10th	Day	3 1 2 2 1 2 1 3 7% 77%	1878
	11th to 20th	Day	2 1 2 1 2 2 2 3 7% 77%	1890
	21st to end	Day	1 2 1 3 2 2 2 3 4 7% 71%	1764
December	1st to 10th	Day	2 1 2 2 2 2 2 3 6% 77%	2636
	11th to 20th	Day	2 1 2 1 2 2 2 5% 8% 75%	3026
	21st to end	Day	2 2 5% 2 3 3 3 6% 9% 64%	4004
January	1st to 10th	Night	3 4 7% 3 4 3 3 5% 8% 61%	6327
	11th to 20th	Night	1 1 1 2 3 3 5% 8% 74%	5956
	21st to end	Night	4 2 5% 3 3 3 3 5% 7% 64%	6468
February	1st to 10th	Night	3 2 4 3 2 3 3 4 7% 69%	2600
	11th to 20th	Night	2 2 4 3 3 3 3 4 6% 69%	2396
	21st to end	Night	3 2 5% 3 3 4 3 4 7% 67%	1980
March	1st to 10th	Night	6% 3 5% 4 3 3 3 4 6% 63%	5228
	11th to 20th	Night	1 10% 2 4 4 3 4 3 5% 7% 57%	5000
	21st to end	Night	1 4 2 4 3 3 4 3 6% 10% 62%	5265
April	1st to 10th	Night	1 3 2 4 2 3 3 3 5% 8% 65%	2000
	11th to 20th	Night	3 7% 2 5% 2 3 3 3 4 7% 61%	1977
	21st to end	Night	3 7% 2 5% 4 3 4 4 5% 6% 58%	2000
May	1st to 10th	Night	4 8% 3 7% 5% 4 4 3 6% 8% 48%	4431
	11th to 20th	Night	5% 6% 2 4 3 3 4 3 5% 7% 57%	3617
	21st to end	Night	3 6% 2 5% 3 3 4 3 5% 6% 60%	3972
June	1st to 10th	Night	2 3 2 4 3 2 3 4 6% 9% 62%	1600
	11th to 20th	Night	1 3 1 2 2 2 4 3 5% 7% 69%	1600
	21st to end	Night	2 2 1 3 3 4 4 5% 7% 9% 59%	1600
July	1st to 10th	Night	4 4 1 4 4 4 4 4 7% 7% 58%	3640
	11th to 20th	Night	2 3 2 3 4 4 5% 6% 9% 13% 50%	3640
	21st to end	Night	3 4 2 5% 4 5% 5% 6% 8% 10% 48%	4461
August	1st to 10th	Night	2 4 2 5% 4 4 5% 4 7% 11% 51%	4033
	11th to 20th	Night	2 2 1 3 3 4 6% 5% 10% 12% 53%	4430
	21st to end	Night	3 5% 2 7% 5% 6% 6% 6% 8% 11% 41%	4993
September	1st to 10th	Night	4 4 2 5% 4 4 5% 5% 7% 9% 52%	2000
	11th to 20th	Night	4 4 2 3 3 4 4 3 6% 8% 61%	2000
	21st to end	Night	4 3 2 4 3 3 3 3 5% 6% 65%	2231
October	1st to 10th	Night	3 3 1 3 3 2 2 2 4 6% 71%	3920
	11th to 20th	Night	4 4 2 3 2 2 3 2 4 7% 68%	3898
	21st to end	Night	2 3 1 3 2 2 2 2 4 6% 75%	4252
November	1st to 10th	Night	1 3 1 2 2 2 2 2 4 6% 74%	2620
	11th to 20th	Night	1 1 2 2 2 2 2 3 6% 79%	2730
	21st to end	Night	1 5% 2 3 1 2 2 3 5% 7% 69%	2853
December	1st to 10th	Night	1 1 2 2 2 2 2 3 6% 79%	4599
	11th to 20th	Night	1 2 1 2 1 2 2 2 5% 7% 75%	5181
	21st to end	Night	3 2 5% 3 3 4 4 6% 8% 62%	6974

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	19% 5% 13% 32% 27% 4	3639
	11th to 20th	Day	16% 6% 15% 23% 32% 9%	4054
	21st to end	Day	18% 8% 9% 29% 26% 10%	4510
February	1st to 10th	Day	19% 6% 17% 30% 20% 9%	1800
	11th to 20th	Day	15% 6% 14% 31% 27% 8%	1900
	21st to end	Day	20% 5% 11% 25% 29% 11%	1650
March	1st to 10th	Day	18% 6% 8% 19% 33% 16%	4637
	11th to 20th	Day	31% 7% 8% 28% 22% 3	5000
	21st to end	Day	20% 9% 13% 40% 15% 2	5700
April	1st to 10th	Day	21% 8% 12% 25% 23% 10%	2400
	11th to 20th	Day	26% 7% 12% 27% 22% 6%	2374
	21st to end	Day	27% 8% 14% 26% 22% 4	2400
May	1st to 10th	Day	33% 11% 12% 20% 18% 7%	5516
	11th to 20th	Day	25% 9% 11% 24% 22% 8%	6302
	21st to end	Day	21% 9% 11% 20% 31% 8%	6926
June	1st to 10th	Day	24% 10% 15% 20% 22% 9%	2800
	11th to 20th	Day	21% 7% 11% 16% 25% 20%	2800
	21st to end	Day	24% 9% 11% 19% 23% 14%	2800
July	1st to 10th	Day	20% 8% 9% 13% 20% 29%	6271
	11th to 20th	Day	30% 12% 18% 23% 13% 5%	6265
	21st to end	Day	33% 12% 15% 15% 13% 12%	6223
August	1st to 10th	Day	37% 12% 19% 13% 13% 6%	4208
	11th to 20th	Day	29% 11% 14% 14% 21% 11%	3921
	21st to end	Day	32% 12% 14% 17% 19% 6%	4311
September	1st to 10th	Day	28% 9% 11% 18% 22% 12%	2400
	11th to 20th	Day	24% 8% 12% 18% 26% 13%	2400
	21st to end	Day	19% 7% 8% 23% 33% 10%	2165
October	1st to 10th	Day	15% 6% 6% 19% 34% 21%	3270
	11th to 20th	Day	15% 7% 10% 31% 28% 9%	3250
	21st to end	Day	12% 4 7% 27% 39% 11%	3548
November	1st to 10th	Day	10% 5% 11% 25% 39% 10%	1878
	11th to 20th	Day	10% 4 8% 38% 31% 8%	1890
	21st to end	Day	15% 4 12% 31% 28% 11%	1764
December	1st to 10th	Day	9% 4 11% 40% 28% 8%	2636
	11th to 20th	Day	10% 4 12% 38% 31% 4	3024
	21st to end	Day	15% 7% 12% 30% 25% 11%	4004
January	1st to 10th	Night	21% 5% 11% 35% 23% 5%	6327
	11th to 20th	Night	10% 4 13% 31% 29% 13%	5956
	21st to end	Night	17% 6% 13% 28% 27% 9%	6468
February	1st to 10th	Night	17% 3 16% 32% 23% 9%	2600
	11th to 20th	Night	16% 5% 14% 32% 26% 6%	2396
	21st to end	Night	18% 4 11% 27% 30% 9%	1980
March	1st to 10th	Night	21% 5% 8% 19% 40% 7%	5228
	11th to 20th	Night	23% 7% 10% 27% 30% 4	5000
	21st to end	Night	17% 7% 19% 40% 16% 1	5265
April	1st to 10th	Night	17% 6% 15% 30% 25% 7%	2000
	11th to 20th	Night	24% 5% 12% 28% 24% 6%	1977
	21st to end	Night	24% 7% 14% 31% 20% 3	2000
May	1st to 10th	Night	29% 10% 15% 21% 21% 4	4428
	11th to 20th	Night	25% 6% 13% 29% 20% 7%	3618
	21st to end	Night	21% 9% 15% 17% 33% 6%	3972
June	1st to 10th	Night	18% 9% 18% 23% 21% 11%	1600
	11th to 20th	Night	14% 8% 11% 21% 25% 22%	1600
	21st to end	Night	18% 8% 13% 23% 24% 14%	1600
July	1st to 10th	Night	22% 9% 12% 17% 23% 17%	3600
	11th to 20th	Night	22% 11% 32% 22% 10% 4	3592
	21st to end	Night	25% 14% 23% 19% 8% 9%	4334
August	1st to 10th	Night	32% 13% 26% 13% 10% 6%	2928
	11th to 20th	Night	20% 15% 23% 15% 19% 7%	3202
	21st to end	Night	28% 14% 17% 24% 14% 3	3597
September	1st to 10th	Night	26% 10% 15% 22% 20% 9%	2000
	11th to 20th	Night	20% 8% 13% 23% 23% 13%	2000
	21st to end	Night	21% 5% 10% 26% 29% 9%	2231
October	1st to 10th	Night	14% 6% 9% 26% 31% 13%	3920
	11th to 20th	Night	16% 5% 13% 33% 26% 8%	3898
	21st to end	Night	12% 3 7% 35% 34% 9%	4252
November	1st to 10th	Night	12% 4 11% 29% 34% 9%	2620
	11th to 20th	Night	9% 4 11% 37% 30% 9%	2730
	21st to end	Night	16% 4 10% 30% 30% 9%	2853
December	1st to 10th	Night	10% 3 12% 45% 26% 4	4599
	11th to 20th	Night	10% 4 12% 37% 33% 4	5182
	21st to end	Night	17% 6% 15% 28% 25% 8%	6974

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% 19% 39% 21% 13% 1	300
	11th to 20th	Day	9% 21% 37% 29% 4 1	300
	21st to end	Day	3 14% 41% 33% 8% 1	330
February	1st to 10th	Day	7% 33% 41% 16% 2 1	300
	11th to 20th	Day	9% 33% 35% 19% 3 1	300
	21st to end	Day	8% 37% 36% 16% 4	249
March	1st to 10th	Day	8% 35% 31% 19% 8%	359
	11th to 20th	Day	11% 28% 34% 19% 7% 1	500
	21st to end	Day	10% 38% 34% 13% 3	550
April	1st to 10th	Day	12% 33% 33% 18% 4 1	500
	11th to 20th	Day	16% 36% 30% 13% 4 1	492
	21st to end	Day	10% 34% 34% 15% 6% 1	500
May	1st to 10th	Day	15% 28% 34% 16% 7% 1	508
	11th to 20th	Day	11% 16% 31% 26% 13% 2	725
	21st to end	Day	8% 21% 27% 28% 15% 2	789
June	1st to 10th	Day	4 12% 28% 32% 21% 3	730
	11th to 20th	Day	4 12% 36% 30% 15% 3	730
	21st to end	Day	2 12% 31% 30% 21% 4	730
July	1st to 10th	Day	5% 7% 21% 32% 29% 6%	721
	11th to 20th	Day	4 3 17% 32% 36% 8%	718
	21st to end	Day	8% 5% 21% 35% 25% 6%	582
August	1st to 10th	Day	5% 6% 33% 33% 19% 4	527
	11th to 20th	Day	7% 4 28% 34% 19% 7%	517
	21st to end	Day	8% 5% 28% 33% 22% 5%	583
September	1st to 10th	Day	7% 7% 28% 30% 21% 7%	530
	11th to 20th	Day	4 10% 37% 32% 14% 3	530
	21st to end	Day	5% 13% 26% 27% 23% 5%	355
October	1st to 10th	Day	5% 16% 24% 25% 21% 8%	330
	11th to 20th	Day	6% 22% 29% 26% 13% 3	330
	21st to end	Day	9% 19% 32% 25% 13% 2	363
November	1st to 10th	Day	5% 23% 29% 29% 12% 1	330
	11th to 20th	Day	5% 20% 31% 31% 11% 2	330
	21st to end	Day	4 19% 37% 29% 10% 1	330
December	1st to 10th	Day	8% 21% 33% 25% 12% 2	306
	11th to 20th	Day	4 22% 42% 24% 8% 1	300
	21st to end	Day	7% 30% 36% 18% 8% 1	330
January	1st to 10th	Night	2 19% 41% 28% 8% 2	1000
	11th to 20th	Night	2 18% 45% 27% 7% 1	837
	21st to end	Night	1 11% 52% 27% 8%	880
February	1st to 10th	Night	2 14% 59% 21% 4	800
	11th to 20th	Night	1 16% 55% 23% 4	784
	21st to end	Night	2 19% 57% 21% 2	660
March	1st to 10th	Night	2 16% 50% 26% 5% 1	800
	11th to 20th	Night	3 11% 44% 33% 9%	800
	21st to end	Night	3 16% 51% 26% 5%	785
April	1st to 10th	Night	3 18% 45% 29% 5%	600
	11th to 20th	Night	7% 17% 41% 28% 6% 1	593
	21st to end	Night	5% 16% 38% 34% 6% 1	600
May	1st to 10th	Night	7% 13% 36% 34% 10% 1	600
	11th to 20th	Night	10% 5% 31% 38% 15% 1	591
	21st to end	Night	7% 9% 31% 36% 16% 1	650
June	1st to 10th	Night	3 5% 25% 43% 23% 2	600
	11th to 20th	Night	2 6% 29% 48% 14% 1	600
	21st to end	Night	2 3 23% 41% 30% 2	600
July	1st to 10th	Night	7% 2 13% 42% 33% 3	585
	11th to 20th	Night	5% 1 8% 39% 42% 5%	582
	21st to end	Night	7% 2 8% 32% 49% 3	657
August	1st to 10th	Night	7% 2 13% 39% 37% 3	597
	11th to 20th	Night	3 3 11% 46% 34% 3	594
	21st to end	Night	5% 2 12% 44% 32% 4	660
September	1st to 10th	Night	5% 2 14% 48% 28% 3	600
	11th to 20th	Night	7% 3 22% 45% 22% 3	600
	21st to end	Night	5% 5% 25% 42% 20% 4	656
October	1st to 10th	Night	4 5% 33% 39% 17% 2	800
	11th to 20th	Night	3 8% 39% 34% 14% 2	800
	21st to end	Night	4 8% 41% 35% 11% 1	880
November	1st to 10th	Night	2 11% 45% 31% 11% 1	980
	11th to 20th	Night	2 15% 46% 28% 7% 1	1050
	21st to end	Night	4 14% 41% 34% 7%	1050
December	1st to 10th	Night	2 17% 46% 28% 6%	1011
	11th to 20th	Night	3 17% 49% 24% 7% 1	1000
	21st to end	Night	2 21% 46% 25% 5%	1100

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