South Fork Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the South Fork Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	6% 4 1 3 2 2 2 2 3 5% 69%	139685
	Night	7% 4 1 3 2 2 2 2 4 5% 67%	139587
Transparency	Day	20% 5% 6% 19% 29% 21%	134757
	Night	21% 5% 8% 21% 27% 18%	135490
Seeing	Day	9% 13% 32% 32% 10% 3	17113
	Night	8% 5% 21% 34% 25% 7%	26800

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	11% 7% 3 6% 3 3 4 3 6% 6% 48%	4722
	11th to 20th	Day	6% 4 2 4 3 3 3 3 5% 6% 62%	5040
	21st to end	Day	9% 8% 2 6% 3 4 4 4 6% 8% 47%	5843
February	1st to 10th	Day	14% 10% 2 3 2 3 3 3 4 6% 49%	2089
	11th to 20th	Day	13% 10% 2 4 3 3 4 4 6% 7% 46%	2050
	21st to end	Day	14% 11% 2 5% 4 3 3 3 5% 6% 43%	1730
March	1st to 10th	Day	14% 16% 3 5% 4 3 3 4 5% 7% 36%	4618
	11th to 20th	Day	12% 9% 1 4 3 3 3 3 5% 7% 50%	4877
	21st to end	Day	15% 10% 2 5% 4 4 4 3 5% 6% 42%	6073
April	1st to 10th	Day	10% 7% 2 5% 4 4 3 2 5% 6% 53%	2413
	11th to 20th	Day	10% 9% 2 4 3 3 3 3 4 5% 54%	2369
	21st to end	Day	9% 7% 1 4 2 2 3 3 4 6% 59%	2400
May	1st to 10th	Day	6% 5% 1 3 2 2 2 2 3 4 70%	5650
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 75%	5841
	21st to end	Day	3 1 2 1 1 2 1 3 4 80%	6676
June	1st to 10th	Day	3 3 1 2 2 3 3 3 4 6% 70%	2800
	11th to 20th	Day	1 1 2 1 1 1 1 3 4 86%	2800
	21st to end	Day	1 1 1 1 1 2 1 2 2 86%	2800
July	1st to 10th	Day	1 1 1 1 1 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6320
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	6546
August	1st to 10th	Day	1 1 1 1 1 2 1 2 2 88%	5346
	11th to 20th	Day	1 1 1 1 94%	5364
	21st to end	Day	1 1 1 1 1 1 2 2 3 86%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 1 2 3 86%	2400
	11th to 20th	Day	2 1 1 1 1 2 2 2 2 3 83%	2305
	21st to end	Day	2 1 1 2 1 1 2 2 3 4 82%	2200
October	1st to 10th	Day	3 2 1 2 1 2 2 3 3 5% 77%	3521
	11th to 20th	Day	2 2 1 3 2 2 3 2 3 3 77%	3261
	21st to end	Day	7% 4 1 3 3 3 3 2 4 6% 65%	3472
November	1st to 10th	Day	7% 5% 2 3 2 2 3 3 4 7% 61%	1854
	11th to 20th	Day	8% 4 1 3 2 3 4 2 4 6% 63%	1890
	21st to end	Day	8% 6% 2 3 2 3 3 4 4 7% 58%	1890
December	1st to 10th	Day	7% 3 1 3 2 3 3 3 4 6% 64%	3022
	11th to 20th	Day	13% 7% 1 5% 3 4 4 3 6% 8% 46%	3108
	21st to end	Day	15% 7% 2 5% 4 4 5% 4 5% 9% 40%	4092
January	1st to 10th	Night	15% 7% 2 5% 3 2 2 2 4 6% 52%	8172
	11th to 20th	Night	8% 5% 2 3 2 2 3 2 4 6% 63%	7918
	21st to end	Night	11% 7% 2 5% 3 3 4 3 5% 7% 48%	8371
February	1st to 10th	Night	14% 9% 2 4 3 3 3 2 5% 5% 50%	2529
	11th to 20th	Night	13% 7% 2 3 2 3 2 3 4 7% 54%	2458
	21st to end	Night	14% 8% 2 4 2 3 3 3 4 6% 52%	2076
March	1st to 10th	Night	16% 10% 2 5% 3 3 3 3 5% 7% 44%	5496
	11th to 20th	Night	9% 7% 2 4 3 3 3 3 5% 6% 55%	5340
	21st to end	Night	16% 7% 2 5% 3 3 3 3 5% 7% 47%	5131
April	1st to 10th	Night	11% 4 1 3 3 2 3 3 5% 6% 59%	2010
	11th to 20th	Night	9% 5% 1 3 3 2 3 3 4 6% 62%	1979
	21st to end	Night	7% 4 1 2 2 3 2 3 4 8% 65%	2000
May	1st to 10th	Night	7% 3 1 3 2 2 2 1 3 4 73%	4337
	11th to 20th	Night	5% 1 2 1 1 1 2 3 4 79%	4074
	21st to end	Night	3 1 1 1 1 1 1 2 3 86%	4205
June	1st to 10th	Night	4 2 1 3 2 1 2 2 3 6% 75%	1600
	11th to 20th	Night	1 1 1 1 2 1 1 1 2 4 86%	1600
	21st to end	Night	1 1 1 1 1 1 2 2 3 89%	1600
July	1st to 10th	Night	1 1 1 2 94%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	4299
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4486
	11th to 20th	Night	1 1 2 95%	4506
	21st to end	Night	1 1 1 2 2 3 3 86%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 1 2 2 89%	2000
	11th to 20th	Night	2 1 2 1 2 2 1 3 3 83%	2094
	21st to end	Night	1 1 2 1 2 1 2 3 3 84%	2200
October	1st to 10th	Night	2 1 1 1 1 2 1 2 4 84%	3663
	11th to 20th	Night	2 1 2 2 2 2 2 3 4 80%	3897
	21st to end	Night	4 2 1 2 2 2 2 2 2 4 77%	4334
November	1st to 10th	Night	7% 5% 2 3 3 2 2 2 4 5% 66%	2665
	11th to 20th	Night	7% 4 2 4 3 3 3 2 5% 7% 62%	2730
	21st to end	Night	9% 5% 2 4 3 3 3 3 5% 7% 58%	2730
December	1st to 10th	Night	6% 3 1 4 3 3 4 3 4 5% 64%	4370
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 7% 9% 47%	5316
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 6% 9% 40%	7106

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	32% 6% 12% 26% 18% 5%	4723
	11th to 20th	Day	21% 5% 8% 14% 24% 28%	5040
	21st to end	Day	33% 6% 8% 18% 28% 8%	5843
February	1st to 10th	Day	35% 5% 7% 24% 19% 10%	2089
	11th to 20th	Day	37% 6% 8% 21% 21% 7%	2060
	21st to end	Day	41% 6% 10% 26% 13% 4	1730
March	1st to 10th	Day	43% 9% 8% 15% 19% 6%	4618
	11th to 20th	Day	32% 11% 16% 25% 10% 5%	4877
	21st to end	Day	39% 10% 13% 22% 15% 2	6073
April	1st to 10th	Day	32% 8% 11% 27% 18% 4	2413
	11th to 20th	Day	32% 8% 12% 23% 21% 4	2372
	21st to end	Day	26% 8% 10% 35% 18% 1	2400
May	1st to 10th	Day	18% 6% 9% 31% 35% 2	5650
	11th to 20th	Day	14% 5% 6% 35% 37% 2	5844
	21st to end	Day	11% 4 4 31% 47% 3	6677
June	1st to 10th	Day	17% 5% 11% 26% 34% 8%	2800
	11th to 20th	Day	7% 3 5% 20% 42% 23%	2800
	21st to end	Day	8% 3 2 11% 42% 35%	2800
July	1st to 10th	Day	3 1 1 5% 22% 68%	6251
	11th to 20th	Day	6% 2 2 6% 32% 52%	6245
	21st to end	Day	5% 2 2 16% 39% 36%	6356
August	1st to 10th	Day	8% 2 1 8% 38% 41%	3890
	11th to 20th	Day	4 2 1 7% 31% 54%	3880
	21st to end	Day	5% 2 1 12% 36% 45%	4311
September	1st to 10th	Day	7% 3 3 11% 37% 39%	2400
	11th to 20th	Day	10% 4 2 20% 41% 23%	2305
	21st to end	Day	9% 5% 3 21% 39% 24%	2200
October	1st to 10th	Day	13% 4 3 11% 24% 45%	3521
	11th to 20th	Day	15% 3 3 25% 35% 19%	3261
	21st to end	Day	22% 4 3 16% 34% 21%	3472
November	1st to 10th	Day	23% 5% 7% 21% 28% 15%	1854
	11th to 20th	Day	24% 4 8% 24% 28% 11%	1890
	21st to end	Day	28% 6% 7% 17% 27% 15%	1890
December	1st to 10th	Day	23% 5% 5% 23% 27% 17%	3022
	11th to 20th	Day	37% 6% 9% 17% 24% 6%	3108
	21st to end	Day	39% 6% 14% 22% 14% 5%	4092
January	1st to 10th	Night	33% 4 15% 24% 18% 6%	8172
	11th to 20th	Night	21% 5% 6% 17% 28% 23%	7918
	21st to end	Night	33% 6% 7% 19% 27% 8%	8371
February	1st to 10th	Night	37% 5% 6% 23% 21% 9%	2529
	11th to 20th	Night	33% 5% 9% 21% 23% 8%	2459
	21st to end	Night	34% 6% 11% 27% 17% 5%	2076
March	1st to 10th	Night	39% 9% 8% 18% 21% 5%	5496
	11th to 20th	Night	29% 8% 15% 30% 12% 6%	5340
	21st to end	Night	37% 8% 18% 22% 13% 2	5131
April	1st to 10th	Night	29% 7% 13% 30% 15% 6%	2010
	11th to 20th	Night	25% 8% 11% 29% 23% 4	1979
	21st to end	Night	20% 7% 13% 40% 19% 1	2000
May	1st to 10th	Night	17% 6% 8% 40% 27% 2	4334
	11th to 20th	Night	11% 4 8% 42% 34% 1	4075
	21st to end	Night	8% 3 5% 39% 43% 3	4205
June	1st to 10th	Night	14% 4 11% 27% 36% 8%	1600
	11th to 20th	Night	7% 3 6% 21% 39% 24%	1600
	21st to end	Night	6% 1 2 12% 43% 36%	1600
July	1st to 10th	Night	2 1 1 5% 23% 68%	3600
	11th to 20th	Night	5% 1 2 7% 32% 53%	3592
	21st to end	Night	5% 2 2 17% 34% 39%	4178
August	1st to 10th	Night	8% 2 2 12% 35% 41%	3256
	11th to 20th	Night	3 1 1 10% 24% 61%	3256
	21st to end	Night	4 1 1 10% 40% 43%	3608
September	1st to 10th	Night	5% 2 2 12% 33% 46%	2000
	11th to 20th	Night	9% 3 4 19% 42% 23%	2094
	21st to end	Night	8% 3 4 20% 39% 25%	2200
October	1st to 10th	Night	9% 3 3 12% 24% 49%	3663
	11th to 20th	Night	11% 3 3 24% 40% 19%	3897
	21st to end	Night	14% 3 3 18% 39% 22%	4334
November	1st to 10th	Night	21% 4 9% 21% 28% 17%	2665
	11th to 20th	Night	23% 5% 10% 19% 29% 14%	2730
	21st to end	Night	28% 5% 8% 17% 29% 13%	2730
December	1st to 10th	Night	24% 4 5% 23% 26% 17%	4370
	11th to 20th	Night	34% 7% 12% 19% 20% 8%	5316
	21st to end	Night	38% 7% 13% 24% 14% 4	7106

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	23% 12% 35% 22% 8% 1	220
	11th to 20th	Day	13% 10% 31% 30% 13% 4	325
	21st to end	Day	18% 11% 31% 26% 13% 2	473
February	1st to 10th	Day	19% 13% 32% 24% 9% 3	535
	11th to 20th	Day	20% 17% 36% 21% 5% 1	535
	21st to end	Day	16% 16% 41% 21% 5%	446
March	1st to 10th	Day	18% 22% 36% 18% 6% 1	540
	11th to 20th	Day	16% 19% 31% 24% 8% 2	540
	21st to end	Day	19% 22% 31% 20% 6% 1	594
April	1st to 10th	Day	16% 19% 30% 26% 8% 1	505
	11th to 20th	Day	21% 17% 30% 26% 5% 1	492
	21st to end	Day	10% 20% 31% 26% 10% 2	500
May	1st to 10th	Day	12% 23% 31% 23% 8% 3	503
	11th to 20th	Day	10% 16% 34% 28% 10% 2	528
	21st to end	Day	11% 13% 32% 32% 10% 3	573
June	1st to 10th	Day	4 12% 28% 38% 15% 3	530
	11th to 20th	Day	3 13% 34% 34% 12% 3	530
	21st to end	Day	2 11% 35% 36% 12% 5%	530
July	1st to 10th	Day	1 4 34% 44% 13% 5%	526
	11th to 20th	Day	1 1 20% 56% 14% 8%	524
	21st to end	Day	2 2 24% 54% 12% 7%	575
August	1st to 10th	Day	2 7% 37% 38% 9% 7%	527
	11th to 20th	Day	2 5% 32% 43% 10% 9%	517
	21st to end	Day	1 4 33% 43% 12% 7%	583
September	1st to 10th	Day	2 6% 34% 41% 13% 4	530
	11th to 20th	Day	3 12% 36% 37% 11% 2	530
	21st to end	Day	4 13% 35% 36% 11% 1	530
October	1st to 10th	Day	4 14% 31% 37% 13%	530
	11th to 20th	Day	6% 12% 36% 35% 10% 1	530
	21st to end	Day	7% 13% 31% 34% 14% 1	533
November	1st to 10th	Day	9% 13% 30% 32% 15% 1	330
	11th to 20th	Day	10% 15% 32% 27% 13% 4	330
	21st to end	Day	13% 13% 29% 32% 11% 2	330
December	1st to 10th	Day	12% 13% 32% 29% 13% 2	327
	11th to 20th	Day	13% 8% 37% 24% 14% 4	220
	21st to end	Day	23% 10% 36% 21% 9% 2	242
January	1st to 10th	Night	17% 7% 28% 24% 17% 8%	840
	11th to 20th	Night	9% 8% 28% 34% 17% 5%	840
	21st to end	Night	15% 7% 28% 29% 15% 4	924
February	1st to 10th	Night	17% 7% 25% 31% 15% 4	840
	11th to 20th	Night	15% 7% 26% 29% 17% 6%	824
	21st to end	Night	16% 8% 27% 33% 13% 2	692
March	1st to 10th	Night	15% 10% 24% 32% 15% 3	840
	11th to 20th	Night	12% 7% 27% 28% 20% 6%	840
	21st to end	Night	15% 7% 26% 35% 16% 2	924
April	1st to 10th	Night	12% 6% 25% 37% 17% 3	804
	11th to 20th	Night	11% 8% 25% 31% 21% 5%	791
	21st to end	Night	7% 7% 24% 35% 22% 6%	800
May	1st to 10th	Night	7% 5% 26% 35% 24% 3	707
	11th to 20th	Night	6% 6% 21% 35% 28% 3	591
	21st to end	Night	7% 5% 23% 35% 26% 3	537
June	1st to 10th	Night	5% 3 16% 40% 31% 6%	400
	11th to 20th	Night	1 2 20% 39% 31% 7%	400
	21st to end	Night	2 15% 37% 39% 7%	400
July	1st to 10th	Night	1 10% 34% 46% 9%	390
	11th to 20th	Night	1 5% 36% 48% 10%	388
	21st to end	Night	1 1 10% 32% 43% 14%	576
August	1st to 10th	Night	2 1 11% 38% 40% 9%	796
	11th to 20th	Night	1 8% 36% 43% 12%	792
	21st to end	Night	1 8% 32% 47% 12%	880
September	1st to 10th	Night	1 1 9% 37% 40% 12%	800
	11th to 20th	Night	2 1 12% 41% 33% 10%	800
	21st to end	Night	2 2 16% 36% 34% 10%	800
October	1st to 10th	Night	3 2 18% 45% 25% 7%	800
	11th to 20th	Night	4 2 21% 37% 28% 9%	800
	21st to end	Night	5% 2 19% 35% 28% 11%	880
November	1st to 10th	Night	8% 4 17% 37% 24% 10%	820
	11th to 20th	Night	8% 5% 22% 35% 22% 8%	840
	21st to end	Night	11% 4 23% 35% 21% 5%	840
December	1st to 10th	Night	11% 6% 24% 34% 18% 6%	840
	11th to 20th	Night	15% 6% 25% 35% 15% 4	840
	21st to end	Night	16% 9% 26% 31% 14% 4	924

Page generated at 2026-03-29 23:18:40. Version ac07

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