East Fork Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the East Fork Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	13% 6% 2 6% 4 4 4 3 5% 7% 46%	132175
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	130927
Transparency	Day	38% 4 10% 23% 20% 7%	135784
	Night	34% 3 9% 21% 23% 10%	135382
Seeing	Day	15% 9% 24% 27% 20% 5%	17164
	Night	15% 9% 25% 31% 17% 2	27063

By Month

Percentage Cloud Cover SE (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	15% 10% 2 4 3 3 3 2 4 6% 48%	5289
	11th to 20th	Day	11% 5% 2 3 2 3 3 3 5% 6% 58%	5310
	21st to end	Day	26% 7% 2 6% 3 2 3 2 3 4 42%	5812
February	1st to 10th	Day	21% 9% 3 6% 5% 4 3 3 2 4 41%	1890
	11th to 20th	Day	21% 9% 3 6% 4 3 3 2 3 5% 41%	1990
	21st to end	Day	22% 7% 2 4 3 3 3 2 4 4 45%	1903
March	1st to 10th	Day	20% 10% 3 6% 3 3 2 2 4 5% 42%	5046
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 5% 5% 52%	5090
	21st to end	Day	16% 8% 3 6% 4 4 4 3 4 6% 43%	5583
April	1st to 10th	Day	14% 7% 3 7% 4 4 4 2 4 6% 45%	2373
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 5% 6% 42%	2494
	21st to end	Day	14% 6% 2 6% 5% 4 4 4 6% 7% 41%	2520
May	1st to 10th	Day	21% 8% 2 6% 4 4 4 3 5% 7% 36%	5731
	11th to 20th	Day	15% 6% 3 8% 5% 4 4 3 5% 7% 39%	5941
	21st to end	Day	13% 7% 3 7% 5% 5% 6% 4 6% 9% 36%	6434
June	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 7% 10% 41%	2600
	11th to 20th	Day	6% 5% 1 6% 5% 5% 6% 5% 7% 9% 45%	2600
	21st to end	Day	5% 3 2 5% 6% 5% 5% 5% 7% 10% 48%	2600
July	1st to 10th	Day	8% 6% 2 7% 6% 6% 6% 5% 9% 11% 34%	5900
	11th to 20th	Day	5% 3 1 4 4 3 4 4 6% 10% 56%	5900
	21st to end	Day	9% 7% 2 6% 5% 5% 5% 4 7% 9% 42%	6422
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 7% 66%	4234
	11th to 20th	Day	5% 2 1 4 4 4 5% 4 7% 10% 54%	4017
	21st to end	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 9% 42%	4312
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 5% 8% 48%	2359
	11th to 20th	Day	11% 4 2 6% 5% 6% 4 3 5% 8% 45%	2200
	21st to end	Day	8% 3 1 4 4 4 4 3 5% 7% 56%	2200
October	1st to 10th	Day	7% 3 1 3 3 3 3 2 5% 8% 62%	2000
	11th to 20th	Day	14% 5% 1 4 2 3 3 3 4 5% 56%	2000
	21st to end	Day	16% 5% 2 4 3 3 3 2 3 4 54%	2200
November	1st to 10th	Day	13% 6% 2 5% 3 4 3 2 4 6% 51%	2058
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 4 6% 54%	2100
	21st to end	Day	19% 6% 3 5% 3 3 3 2 4 5% 47%	1969
December	1st to 10th	Day	20% 6% 2 5% 3 3 2 2 3 4 50%	3030
	11th to 20th	Day	18% 9% 3 6% 4 3 3 2 3 5% 43%	3481
	21st to end	Day	18% 9% 3 6% 3 4 3 2 4 4 44%	4587
January	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 3 5% 49%	7634
	11th to 20th	Night	10% 4 1 3 2 2 2 2 3 5% 65%	7680
	21st to end	Night	25% 10% 2 6% 3 3 2 2 3 3 40%	8402
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 4 5% 47%	2730
	11th to 20th	Night	21% 8% 2 5% 3 3 3 2 4 5% 44%	2532
	21st to end	Night	18% 6% 2 5% 3 2 3 2 3 5% 50%	1903
March	1st to 10th	Night	22% 8% 2 5% 3 3 3 2 4 5% 42%	5029
	11th to 20th	Night	13% 5% 1 5% 4 3 4 3 5% 7% 51%	5090
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	5588
April	1st to 10th	Night	13% 4 2 5% 3 4 4 4 5% 7% 50%	2245
	11th to 20th	Night	12% 7% 2 5% 4 3 4 4 5% 7% 47%	2075
	21st to end	Night	12% 6% 2 5% 3 3 4 3 4 7% 50%	2100
May	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 7% 37%	4376
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	4107
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 8% 45%	4443
June	1st to 10th	Night	5% 3 2 5% 3 4 5% 4 6% 9% 56%	1800
	11th to 20th	Night	5% 4 1 4 3 4 3 3 5% 8% 60%	1800
	21st to end	Night	4 4 1 4 4 3 4 4 8% 10% 56%	1800
July	1st to 10th	Night	6% 4 2 7% 5% 5% 5% 4 9% 13% 41%	4078
	11th to 20th	Night	3 2 1 3 3 3 4 3 7% 10% 60%	4077
	21st to end	Night	6% 5% 2 5% 4 4 4 4 7% 10% 48%	4449
August	1st to 10th	Night	2 1 2 2 2 3 3 4 7% 74%	2926
	11th to 20th	Night	4 2 1 3 3 3 4 4 6% 11% 60%	3037
	21st to end	Night	4 3 1 5% 4 4 5% 5% 8% 9% 52%	3586
September	1st to 10th	Night	8% 4 1 5% 3 4 4 3 5% 7% 55%	2035
	11th to 20th	Night	9% 4 2 5% 4 3 4 3 6% 8% 52%	2200
	21st to end	Night	6% 4 1 4 3 3 2 3 5% 7% 62%	2200
October	1st to 10th	Night	5% 3 1 3 3 3 3 3 5% 6% 66%	2380
	11th to 20th	Night	11% 4 1 3 2 3 3 2 4 4 62%	2400
	21st to end	Night	15% 5% 2 4 3 2 2 2 3 5% 58%	2640
November	1st to 10th	Night	12% 5% 1 4 3 3 2 3 4 5% 57%	2460
	11th to 20th	Night	9% 4 2 6% 4 3 3 3 4 6% 57%	2520
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	2646
December	1st to 10th	Night	17% 6% 3 5% 3 3 3 2 4 4 50%	4380
	11th to 20th	Night	16% 8% 3 6% 4 3 3 3 4 4 45%	4946
	21st to end	Night	16% 10% 2 5% 3 3 3 2 4 4 48%	6633

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	40% 1 8% 29% 19% 2	5290
	11th to 20th	Day	29% 2 4 29% 30% 6%	5310
	21st to end	Day	49% 1 3 14% 22% 11%	5812
February	1st to 10th	Day	49% 3 4 16% 18% 9%	1890
	11th to 20th	Day	49% 3 5% 19% 18% 7%	1999
	21st to end	Day	44% 3 5% 14% 23% 10%	1903
March	1st to 10th	Day	47% 3 4 14% 20% 12%	5046
	11th to 20th	Day	35% 3 5% 17% 21% 19%	5090
	21st to end	Day	44% 3 7% 23% 19% 4	5585
April	1st to 10th	Day	41% 4 8% 22% 17% 8%	2373
	11th to 20th	Day	42% 5% 8% 17% 20% 8%	2494
	21st to end	Day	42% 5% 10% 19% 20% 4	2520
May	1st to 10th	Day	49% 4 9% 25% 11% 2	5766
	11th to 20th	Day	45% 4 10% 19% 19% 2	5945
	21st to end	Day	45% 5% 12% 21% 14% 3	6447
June	1st to 10th	Day	36% 7% 15% 26% 15% 1	2600
	11th to 20th	Day	34% 6% 15% 24% 18% 3	2600
	21st to end	Day	29% 7% 14% 30% 18% 2	2600
July	1st to 10th	Day	43% 5% 21% 22% 8%	5836
	11th to 20th	Day	24% 4 19% 36% 16% 1	5831
	21st to end	Day	40% 4 14% 29% 12% 1	6240
August	1st to 10th	Day	17% 4 14% 32% 27% 6%	4220
	11th to 20th	Day	26% 5% 20% 37% 12% 1	4056
	21st to end	Day	36% 5% 16% 27% 14% 2	4311
September	1st to 10th	Day	35% 5% 11% 20% 19% 10%	2359
	11th to 20th	Day	38% 6% 10% 20% 19% 7%	2200
	21st to end	Day	28% 5% 9% 22% 25% 11%	2200
October	1st to 10th	Day	14% 2 7% 13% 26% 38%	3260
	11th to 20th	Day	20% 2 3 17% 33% 24%	3240
	21st to end	Day	31% 2 3 23% 32% 9%	3538
November	1st to 10th	Day	36% 3 7% 19% 26% 9%	2058
	11th to 20th	Day	32% 4 7% 20% 26% 11%	2100
	21st to end	Day	41% 4 5% 17% 24% 9%	1969
December	1st to 10th	Day	41% 2 5% 25% 23% 5%	3030
	11th to 20th	Day	46% 2 4 16% 22% 9%	3479
	21st to end	Day	45% 2 4 19% 25% 5%	4587
January	1st to 10th	Night	40% 2 6% 21% 26% 4	7635
	11th to 20th	Night	25% 2 5% 19% 36% 13%	7680
	21st to end	Night	51% 2 4 14% 18% 11%	8402
February	1st to 10th	Night	42% 3 6% 20% 19% 11%	2730
	11th to 20th	Night	45% 3 6% 18% 21% 8%	2533
	21st to end	Night	39% 3 7% 16% 22% 14%	1903
March	1st to 10th	Night	46% 3 4 12% 20% 15%	5029
	11th to 20th	Night	34% 3 6% 16% 19% 21%	5090
	21st to end	Night	40% 3 8% 21% 22% 6%	5588
April	1st to 10th	Night	34% 5% 10% 22% 18% 11%	2245
	11th to 20th	Night	37% 5% 8% 18% 21% 11%	2075
	21st to end	Night	35% 4 10% 23% 24% 4	2100
May	1st to 10th	Night	47% 5% 11% 22% 12% 4	4406
	11th to 20th	Night	42% 5% 13% 18% 20% 3	4108
	21st to end	Night	36% 5% 15% 24% 14% 6%	4452
June	1st to 10th	Night	25% 7% 15% 30% 20% 3	1800
	11th to 20th	Night	24% 6% 13% 31% 22% 5%	1800
	21st to end	Night	22% 6% 15% 28% 25% 4	1800
July	1st to 10th	Night	35% 4 21% 31% 8% 1	4035
	11th to 20th	Night	20% 4 20% 37% 17% 2	4026
	21st to end	Night	32% 5% 15% 33% 12% 2	4320
August	1st to 10th	Night	11% 4 13% 30% 34% 6%	2916
	11th to 20th	Night	19% 4 19% 38% 17% 3	3064
	21st to end	Night	26% 6% 16% 26% 24% 2	3586
September	1st to 10th	Night	29% 7% 12% 19% 23% 10%	2035
	11th to 20th	Night	30% 6% 10% 24% 21% 9%	2200
	21st to end	Night	23% 4 9% 23% 28% 13%	2200
October	1st to 10th	Night	14% 2 5% 15% 21% 42%	3887
	11th to 20th	Night	18% 2 3 15% 34% 27%	3899
	21st to end	Night	27% 2 4 20% 31% 17%	4252
November	1st to 10th	Night	31% 4 8% 17% 28% 12%	2460
	11th to 20th	Night	30% 3 8% 17% 27% 14%	2520
	21st to end	Night	37% 4 6% 18% 25% 9%	2646
December	1st to 10th	Night	40% 3 5% 20% 27% 6%	4380
	11th to 20th	Night	44% 2 7% 14% 23% 10%	4947
	21st to end	Night	42% 2 6% 18% 26% 6%	6633

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	27% 16% 36% 16% 4	330
	11th to 20th	Day	25% 21% 35% 17% 2	330
	21st to end	Day	27% 23% 34% 13% 3	363
February	1st to 10th	Day	27% 19% 41% 10% 3 1	330
	11th to 20th	Day	32% 23% 32% 10% 2	330
	21st to end	Day	34% 24% 32% 7% 3	273
March	1st to 10th	Day	26% 20% 34% 16% 3	330
	11th to 20th	Day	26% 25% 38% 8% 2	330
	21st to end	Day	20% 23% 37% 16% 4	363
April	1st to 10th	Day	23% 16% 36% 19% 6%	393
	11th to 20th	Day	24% 18% 33% 17% 8%	532
	21st to end	Day	20% 15% 33% 23% 7% 2	540
May	1st to 10th	Day	20% 10% 38% 26% 6%	632
	11th to 20th	Day	16% 7% 32% 27% 15% 3	756
	21st to end	Day	12% 7% 29% 33% 15% 3	789
June	1st to 10th	Day	8% 4 19% 36% 28% 5%	730
	11th to 20th	Day	7% 2 18% 42% 26% 5%	730
	21st to end	Day	6% 2 15% 36% 32% 9%	730
July	1st to 10th	Day	8% 2 14% 33% 33% 10%	721
	11th to 20th	Day	4 1 7% 40% 37% 10%	718
	21st to end	Day	8% 2 9% 34% 35% 12%	794
August	1st to 10th	Day	4 1 8% 29% 47% 11%	726
	11th to 20th	Day	9% 1 7% 30% 44% 10%	626
	21st to end	Day	11% 1 6% 25% 39% 17%	583
September	1st to 10th	Day	10% 1 11% 28% 38% 12%	489
	11th to 20th	Day	6% 4 12% 29% 36% 13%	330
	21st to end	Day	11% 4 14% 38% 25% 8%	330
October	1st to 10th	Day	5% 4 22% 43% 22% 5%	330
	11th to 20th	Day	12% 7% 32% 32% 13% 4	330
	21st to end	Day	24% 12% 28% 23% 11% 1	363
November	1st to 10th	Day	20% 7% 31% 32% 9% 1	330
	11th to 20th	Day	15% 11% 40% 26% 7% 1	330
	21st to end	Day	22% 19% 32% 18% 8% 1	330
December	1st to 10th	Day	22% 15% 38% 18% 6% 2	330
	11th to 20th	Day	25% 18% 34% 16% 5% 1	330
	21st to end	Day	22% 21% 37% 17% 3 1	363
January	1st to 10th	Night	23% 14% 31% 27% 5%	1050
	11th to 20th	Night	23% 16% 31% 25% 6%	1050
	21st to end	Night	23% 15% 33% 26% 3	1155
February	1st to 10th	Night	22% 14% 37% 23% 4	1050
	11th to 20th	Night	29% 14% 34% 19% 3	898
	21st to end	Night	22% 21% 33% 21% 3	519
March	1st to 10th	Night	24% 12% 32% 26% 5%	630
	11th to 20th	Night	20% 14% 33% 28% 4 1	630
	21st to end	Night	17% 12% 40% 26% 6%	693
April	1st to 10th	Night	14% 15% 37% 27% 6%	630
	11th to 20th	Night	17% 15% 33% 26% 9%	623
	21st to end	Night	15% 10% 36% 28% 11% 1	630
May	1st to 10th	Night	12% 9% 35% 34% 10% 1	630
	11th to 20th	Night	13% 6% 31% 32% 16% 2	609
	21st to end	Night	9% 5% 28% 37% 19% 2	650
June	1st to 10th	Night	5% 2 14% 40% 37% 2	600
	11th to 20th	Night	6% 2 17% 42% 31% 2	600
	21st to end	Night	4 12% 41% 36% 7%	600
July	1st to 10th	Night	5% 1 8% 35% 44% 6%	585
	11th to 20th	Night	4 1 7% 38% 45% 6%	582
	21st to end	Night	5% 1 7% 36% 41% 10%	657
August	1st to 10th	Night	4 1 7% 37% 45% 7%	597
	11th to 20th	Night	7% 7% 40% 39% 7%	594
	21st to end	Night	6% 1 7% 36% 42% 8%	660
September	1st to 10th	Night	10% 1 10% 33% 40% 6%	600
	11th to 20th	Night	10% 1 10% 33% 41% 5%	600
	21st to end	Night	8% 3 13% 37% 35% 4	600
October	1st to 10th	Night	6% 2 14% 43% 29% 5%	780
	11th to 20th	Night	12% 7% 23% 38% 19% 1	800
	21st to end	Night	18% 8% 26% 30% 17% 2	880
November	1st to 10th	Night	14% 6% 26% 38% 15% 1	820
	11th to 20th	Night	11% 11% 31% 37% 10% 1	840
	21st to end	Night	18% 14% 31% 26% 10% 1	966
December	1st to 10th	Night	19% 12% 31% 31% 6%	1050
	11th to 20th	Night	22% 11% 31% 29% 6%	1050
	21st to end	Night	23% 14% 30% 27% 5%	1155

Page generated at 2026-05-14 00:58:55. Version ac07

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