Custer Institute
Clear Sky Chart
History

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

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	16% 7% 2 6% 4 4 4 3 5% 7% 42%	126455
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	123584
Transparency	Day	27% 37% 8% 15% 11% 2	129999
	Night	29% 37% 7% 14% 11% 2	128127
Seeing	Day	18% 6% 22% 31% 18% 5%	16397
	Night	20% 8% 27% 30% 13% 2	25375

By Month

Percentage Cloud Cover NE (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	13% 7% 2 5% 4 4 3 3 5% 7% 48%	3597
	11th to 20th	Day	15% 7% 2 6% 3 4 4 3 5% 6% 45%	3570
	21st to end	Day	18% 8% 3 7% 4 4 4 3 4 5% 40%	4050
February	1st to 10th	Day	18% 7% 2 6% 4 5% 4 4 6% 7% 37%	1915
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 3 6% 6% 43%	1948
	21st to end	Day	20% 8% 3 5% 3 4 3 3 5% 7% 39%	1650
March	1st to 10th	Day	20% 8% 2 5% 3 3 3 2 4 7% 45%	4476
	11th to 20th	Day	12% 8% 2 6% 3 3 4 3 6% 7% 46%	4654
	21st to end	Day	19% 9% 3 6% 4 4 4 3 5% 6% 39%	5942
April	1st to 10th	Day	18% 8% 2 5% 4 3 4 3 5% 6% 41%	2400
	11th to 20th	Day	18% 6% 2 5% 4 3 3 3 5% 7% 44%	2374
	21st to end	Day	18% 6% 2 4 3 4 4 4 6% 6% 43%	2525
May	1st to 10th	Day	26% 9% 3 6% 3 3 3 3 5% 8% 31%	5886
	11th to 20th	Day	19% 10% 3 7% 4 3 3 2 4 6% 39%	5993
	21st to end	Day	18% 8% 2 6% 4 3 4 3 6% 7% 39%	6949
June	1st to 10th	Day	16% 6% 1 4 4 4 4 4 6% 8% 42%	2800
	11th to 20th	Day	13% 6% 3 6% 3 3 4 4 5% 8% 42%	2800
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 9% 45%	2800
July	1st to 10th	Day	12% 8% 3 8% 4 4 4 4 6% 9% 38%	6360
	11th to 20th	Day	11% 7% 3 7% 4 4 5% 4 7% 10% 36%	6360
	21st to end	Day	14% 6% 2 6% 3 4 4 4 5% 7% 44%	6926
August	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 6% 7% 38%	4313
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 6% 8% 46%	3894
	21st to end	Day	6% 4 1 5% 3 4 5% 4 6% 9% 53%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 48%	2400
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 7% 54%	2238
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Day	21% 5% 2 4 3 3 3 3 5% 7% 46%	2200
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 6% 7% 47%	1975
	21st to end	Day	18% 6% 2 7% 4 3 5% 3 5% 6% 42%	1980
November	1st to 10th	Day	14% 7% 2 4 4 3 4 4 4 5% 50%	1779
	11th to 20th	Day	16% 6% 2 5% 4 3 3 3 5% 6% 47%	1800
	21st to end	Day	16% 9% 2 4 3 4 3 4 5% 6% 43%	1800
December	1st to 10th	Day	15% 8% 3 6% 4 3 3 3 4 7% 44%	2643
	11th to 20th	Day	21% 8% 2 5% 3 2 2 2 4 5% 45%	2986
	21st to end	Day	19% 10% 3 8% 3 4 3 3 4 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 5% 6% 48%	6349
	11th to 20th	Night	15% 9% 3 7% 4 4 3 3 4 4 44%	6315
	21st to end	Night	18% 9% 3 6% 4 4 4 4 5% 7% 35%	6904
February	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 4 7% 37%	2483
	11th to 20th	Night	19% 8% 2 5% 4 4 4 3 6% 8% 37%	2338
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 6% 39%	1980
March	1st to 10th	Night	21% 10% 2 6% 3 3 3 3 4 5% 40%	5364
	11th to 20th	Night	16% 10% 3 7% 4 4 3 3 5% 5% 41%	5303
	21st to end	Night	20% 9% 2 6% 3 3 4 3 5% 6% 37%	4972
April	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 38%	2000
	11th to 20th	Night	18% 8% 2 5% 4 3 4 3 5% 6% 42%	1975
	21st to end	Night	19% 7% 3 7% 4 3 3 3 4 7% 40%	1874
May	1st to 10th	Night	23% 12% 4 6% 4 4 4 3 4 6% 28%	4054
	11th to 20th	Night	24% 10% 3 6% 4 4 4 3 5% 7% 31%	3941
	21st to end	Night	21% 10% 3 6% 4 4 4 3 6% 7% 31%	3926
June	1st to 10th	Night	17% 8% 3 5% 4 4 5% 3 6% 7% 39%	1600
	11th to 20th	Night	18% 6% 2 5% 4 4 3 4 6% 9% 39%	1600
	21st to end	Night	12% 7% 3 6% 4 5% 5% 5% 6% 9% 39%	1600
July	1st to 10th	Night	14% 10% 4 9% 4 4 4 4 5% 9% 32%	3600
	11th to 20th	Night	16% 10% 3 7% 5% 4 5% 4 7% 9% 30%	3600
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 8% 42%	3926
August	1st to 10th	Night	21% 7% 3 6% 4 5% 4 5% 6% 8% 31%	2802
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	3149
	21st to end	Night	7% 4 2 5% 3 3 3 4 6% 9% 54%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 5% 5% 4 6% 6% 42%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 51%	2161
	21st to end	Night	18% 7% 3 6% 4 4 4 3 5% 6% 38%	2200
October	1st to 10th	Night	19% 7% 3 5% 4 4 3 3 4 5% 42%	2200
	11th to 20th	Night	17% 6% 2 5% 3 3 3 2 4 6% 47%	2421
	21st to end	Night	19% 8% 3 7% 4 3 4 3 4 6% 40%	2860
November	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 47%	2564
	11th to 20th	Night	18% 8% 2 5% 4 3 3 3 4 5% 44%	2600
	21st to end	Night	15% 5% 3 5% 5% 4 3 3 5% 8% 43%	2600
December	1st to 10th	Night	16% 8% 3 7% 4 3 4 3 5% 5% 42%	4533
	11th to 20th	Night	24% 9% 3 6% 3 3 3 3 4 4 40%	5208
	21st to end	Night	18% 9% 3 6% 4 3 4 3 4 5% 41%	6996

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	28% 26% 3 24% 17% 2	3598
	11th to 20th	Day	27% 27% 7% 19% 13% 7%	3600
	21st to end	Day	37% 27% 8% 16% 11% 1	4050
February	1st to 10th	Day	18% 62% 8% 7% 4	1915
	11th to 20th	Day	18% 60% 2 9% 9% 1	1957
	21st to end	Day	17% 62% 5% 8% 6% 1	1650
March	1st to 10th	Day	32% 28% 7% 12% 11% 9%	4479
	11th to 20th	Day	27% 28% 13% 22% 10% 1	4654
	21st to end	Day	34% 28% 6% 11% 16% 4	5942
April	1st to 10th	Day	20% 61% 4 7% 6% 1	2400
	11th to 20th	Day	16% 62% 3 8% 10%	2374
	21st to end	Day	16% 62% 3 10% 8% 1	2525
May	1st to 10th	Day	41% 29% 15% 9% 5% 1	5897
	11th to 20th	Day	37% 28% 11% 15% 9% 1	5997
	21st to end	Day	35% 28% 7% 19% 11% 1	6949
June	1st to 10th	Day	16% 62% 4 10% 8% 1	2800
	11th to 20th	Day	16% 62% 5% 9% 7% 1	2800
	21st to end	Day	14% 61% 5% 10% 10% 1	2800
July	1st to 10th	Day	36% 29% 9% 13% 11% 1	6290
	11th to 20th	Day	34% 26% 15% 17% 7%	6285
	21st to end	Day	31% 25% 8% 19% 15% 1	6728
August	1st to 10th	Day	33% 36% 10% 15% 5%	4299
	11th to 20th	Day	23% 36% 11% 25% 6%	3931
	21st to end	Day	18% 35% 10% 23% 14% 1	4312
September	1st to 10th	Day	16% 57% 6% 13% 8%	2400
	11th to 20th	Day	15% 57% 4 11% 12% 1	2238
	21st to end	Day	22% 58% 5% 8% 7%	2200
October	1st to 10th	Day	20% 35% 7% 14% 19% 5%	3590
	11th to 20th	Day	15% 35% 5% 16% 26% 3	3191
	21st to end	Day	18% 35% 11% 19% 16% 2	3180
November	1st to 10th	Day	18% 56% 7% 9% 9% 1	1779
	11th to 20th	Day	15% 56% 3 11% 13% 2	1800
	21st to end	Day	17% 57% 6% 14% 6% 2	1800
December	1st to 10th	Day	26% 35% 10% 21% 7%	2643
	11th to 20th	Day	29% 31% 9% 12% 16% 2	2986
	21st to end	Day	37% 27% 3 16% 13% 3	3960
January	1st to 10th	Night	27% 26% 6% 21% 17% 2	6349
	11th to 20th	Night	30% 26% 9% 15% 15% 5%	6360
	21st to end	Night	40% 27% 6% 13% 11% 3	6904
February	1st to 10th	Night	18% 62% 5% 10% 3 1	2483
	11th to 20th	Night	18% 60% 3 9% 9% 1	2340
	21st to end	Night	19% 61% 3 8% 7% 2	1980
March	1st to 10th	Night	38% 28% 7% 11% 9% 7%	5374
	11th to 20th	Night	33% 28% 11% 17% 8% 3	5304
	21st to end	Night	38% 29% 3 7% 18% 5%	4972
April	1st to 10th	Night	20% 62% 3 8% 6% 2	2000
	11th to 20th	Night	15% 62% 5% 8% 8% 1	1975
	21st to end	Night	18% 62% 3 9% 7% 1	1874
May	1st to 10th	Night	45% 29% 12% 8% 5% 1	4060
	11th to 20th	Night	42% 28% 12% 10% 6% 2	3942
	21st to end	Night	41% 28% 9% 16% 6% 1	3927
June	1st to 10th	Night	18% 62% 2 8% 9% 1	1600
	11th to 20th	Night	16% 62% 5% 11% 6%	1600
	21st to end	Night	16% 60% 4 11% 8% 1	1600
July	1st to 10th	Night	39% 27% 8% 17% 9% 1	3560
	11th to 20th	Night	41% 26% 11% 14% 7%	3552
	21st to end	Night	33% 25% 10% 19% 14%	3809
August	1st to 10th	Night	38% 36% 9% 11% 5% 1	2792
	11th to 20th	Night	28% 36% 10% 21% 5%	3178
	21st to end	Night	14% 35% 9% 26% 15% 1	3586
September	1st to 10th	Night	18% 57% 4 13% 7% 1	2000
	11th to 20th	Night	16% 56% 4 10% 12% 1	2161
	21st to end	Night	23% 58% 4 8% 6%	2200
October	1st to 10th	Night	26% 35% 7% 16% 14% 2	3590
	11th to 20th	Night	13% 35% 6% 16% 27% 4	3944
	21st to end	Night	17% 35% 9% 19% 17% 2	4610
November	1st to 10th	Night	18% 56% 5% 10% 9% 1	2564
	11th to 20th	Night	17% 57% 3 9% 10% 3	2600
	21st to end	Night	17% 57% 5% 14% 6% 1	2600
December	1st to 10th	Night	27% 35% 7% 22% 8% 1	4533
	11th to 20th	Night	35% 31% 6% 14% 13% 2	5208
	21st to end	Night	35% 26% 7% 14% 14% 3	6996

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	18% 15% 36% 22% 9% 1	200
	11th to 20th	Day	17% 21% 36% 23% 3 1	200
	21st to end	Day	21% 10% 35% 23% 8% 3	240
February	1st to 10th	Day	22% 10% 34% 28% 6%	415
	11th to 20th	Day	19% 13% 40% 26% 2	496
	21st to end	Day	21% 12% 33% 26% 7% 1	414
March	1st to 10th	Day	19% 9% 32% 32% 7%	500
	11th to 20th	Day	17% 8% 36% 29% 9% 1	500
	21st to end	Day	22% 8% 34% 27% 8% 1	550
April	1st to 10th	Day	24% 8% 29% 29% 9% 2	500
	11th to 20th	Day	22% 4 26% 36% 11% 1	492
	21st to end	Day	21% 6% 22% 35% 14% 2	500
May	1st to 10th	Day	25% 6% 22% 32% 13% 2	503
	11th to 20th	Day	19% 3 20% 37% 19% 3	527
	21st to end	Day	21% 1 12% 32% 25% 8%	573
June	1st to 10th	Day	17% 2 15% 34% 25% 7%	530
	11th to 20th	Day	15% 2 14% 37% 25% 7%	530
	21st to end	Day	13% 2 12% 35% 30% 8%	530
July	1st to 10th	Day	16% 1 9% 31% 33% 10%	525
	11th to 20th	Day	11% 1 7% 32% 36% 12%	524
	21st to end	Day	12% 2 9% 39% 34% 5%	576
August	1st to 10th	Day	12% 2 9% 34% 33% 10%	526
	11th to 20th	Day	15% 1 8% 33% 32% 12%	519
	21st to end	Day	11% 2 11% 37% 28% 12%	583
September	1st to 10th	Day	18% 1 18% 27% 25% 12%	530
	11th to 20th	Day	13% 2 15% 30% 25% 14%	530
	21st to end	Day	22% 3 19% 32% 21% 4	530
October	1st to 10th	Day	24% 3 17% 25% 19% 11%	530
	11th to 20th	Day	16% 7% 25% 32% 16% 4	511
	21st to end	Day	22% 7% 27% 27% 14% 3	462
November	1st to 10th	Day	16% 7% 25% 34% 16% 3	401
	11th to 20th	Day	21% 13% 30% 27% 10% 1	400
	21st to end	Day	18% 14% 30% 27% 10% 1	400
December	1st to 10th	Day	20% 12% 35% 23% 10%	230
	11th to 20th	Day	24% 14% 33% 24% 6%	200
	21st to end	Day	22% 10% 32% 22% 10% 3	220
January	1st to 10th	Night	20% 17% 38% 20% 5%	800
	11th to 20th	Night	27% 14% 32% 23% 4	800
	21st to end	Night	20% 15% 38% 23% 3	880
February	1st to 10th	Night	22% 12% 37% 26% 4	800
	11th to 20th	Night	23% 14% 40% 21% 2	783
	21st to end	Night	24% 10% 37% 23% 5%	660
March	1st to 10th	Night	21% 10% 38% 26% 5%	800
	11th to 20th	Night	20% 8% 36% 27% 7% 2	800
	21st to end	Night	23% 8% 34% 27% 8% 1	880
April	1st to 10th	Night	24% 7% 30% 31% 8% 1	800
	11th to 20th	Night	22% 5% 29% 33% 11%	791
	21st to end	Night	22% 4 28% 35% 9% 1	674
May	1st to 10th	Night	25% 5% 22% 32% 15% 2	600
	11th to 20th	Night	23% 4 25% 37% 11% 1	552
	21st to end	Night	22% 2 14% 37% 24% 2	433
June	1st to 10th	Night	19% 4 19% 35% 22% 2	400
	11th to 20th	Night	20% 2 12% 41% 24% 3	400
	21st to end	Night	13% 1 19% 43% 22% 3	400
July	1st to 10th	Night	15% 2 12% 38% 28% 4	390
	11th to 20th	Night	13% 2 10% 41% 30% 4	388
	21st to end	Night	14% 1 13% 44% 25% 3	438
August	1st to 10th	Night	11% 1 12% 47% 26% 2	523
	11th to 20th	Night	14% 1 13% 35% 31% 6%	754
	21st to end	Night	13% 2 13% 38% 27% 6%	880
September	1st to 10th	Night	15% 3 20% 33% 24% 5%	800
	11th to 20th	Night	17% 2 14% 35% 25% 7%	800
	21st to end	Night	23% 2 19% 33% 20% 3	800
October	1st to 10th	Night	23% 7% 24% 25% 16% 5%	800
	11th to 20th	Night	19% 8% 29% 30% 13% 1	800
	21st to end	Night	25% 9% 31% 24% 10% 1	880
November	1st to 10th	Night	17% 8% 33% 27% 13% 2	789
	11th to 20th	Night	21% 12% 33% 25% 9% 1	800
	21st to end	Night	18% 16% 32% 27% 7% 1	800
December	1st to 10th	Night	24% 14% 32% 23% 6% 1	800
	11th to 20th	Night	24% 16% 33% 23% 4 1	800
	21st to end	Night	25% 13% 34% 23% 4	880

Page generated at 2025-10-31 02:02:37. Version ac07

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