Robert A. Schommer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert A. Schommer Observatory clear sky chart from 2005-11-06 to 2025-07-06.

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 4 5% 7% 40%	119872
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	118927
Transparency	Day	41% 7% 12% 22% 16% 3	123444
	Night	40% 6% 11% 22% 17% 3	123516
Seeing	Day	20% 15% 30% 22% 10% 3	15827
	Night	20% 13% 32% 27% 8% 1	25032

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% 6% 2 5% 3 4 4 3 4 6% 50%	3597
	11th to 20th	Day	13% 8% 3 7% 4 4 5% 3 4 6% 42%	3570
	21st to end	Day	19% 9% 4 7% 4 4 4 3 4 5% 40%	4398
February	1st to 10th	Day	16% 7% 3 7% 5% 4 4 3 5% 6% 40%	1800
	11th to 20th	Day	16% 8% 3 7% 3 4 4 3 4 5% 41%	1942
	21st to end	Day	19% 8% 2 7% 4 4 3 4 4 5% 39%	1650
March	1st to 10th	Day	21% 6% 2 4 3 3 3 3 4 6% 44%	4476
	11th to 20th	Day	12% 8% 2 6% 4 4 4 4 6% 7% 42%	4838
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 6% 43%	5878
April	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 5% 6% 38%	2400
	11th to 20th	Day	16% 7% 2 6% 4 4 3 4 6% 8% 40%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 3 6% 6% 40%	2400
May	1st to 10th	Day	26% 8% 2 6% 4 4 4 4 5% 8% 29%	5876
	11th to 20th	Day	24% 9% 2 6% 4 4 4 2 4 6% 34%	6311
	21st to end	Day	20% 6% 2 6% 4 4 4 4 7% 8% 35%	6949
June	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 5% 6% 10% 37%	2800
	11th to 20th	Day	15% 7% 2 6% 6% 5% 4 4 6% 7% 37%	2800
	21st to end	Day	11% 6% 2 6% 5% 6% 6% 4 5% 8% 40%	2800
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 7% 10% 35%	4510
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 9% 38%	4440
	21st to end	Day	13% 8% 3 8% 6% 5% 6% 5% 7% 8% 30%	4814
August	1st to 10th	Day	19% 6% 2 7% 5% 5% 4 4 6% 9% 31%	4115
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 40%	3730
	21st to end	Day	11% 5% 2 5% 3 4 4 4 5% 8% 48%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 6% 45%	2280
	11th to 20th	Day	13% 6% 2 4 3 4 3 3 5% 8% 49%	2214
	21st to end	Day	19% 6% 3 5% 4 5% 4 4 5% 6% 39%	2090
October	1st to 10th	Day	19% 7% 2 5% 3 4 5% 4 4 6% 41%	1958
	11th to 20th	Day	14% 6% 2 4 3 3 4 3 6% 6% 48%	1900
	21st to end	Day	18% 6% 2 5% 3 4 4 3 5% 6% 45%	1967
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 4 6% 52%	1761
	11th to 20th	Day	16% 6% 2 5% 3 2 3 3 6% 5% 48%	1800
	21st to end	Day	16% 6% 2 4 4 3 4 3 5% 7% 45%	1708
December	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 6% 7% 45%	2600
	11th to 20th	Day	22% 8% 2 4 3 3 3 3 4 6% 43%	2986
	21st to end	Day	17% 6% 3 6% 4 5% 5% 4 6% 7% 38%	3960
January	1st to 10th	Night	13% 7% 2 5% 3 3 4 3 5% 6% 50%	6349
	11th to 20th	Night	14% 8% 2 6% 3 4 4 3 4 6% 46%	6315
	21st to end	Night	18% 9% 2 6% 3 4 4 3 4 6% 41%	6554
February	1st to 10th	Night	17% 10% 2 5% 4 3 4 3 6% 6% 41%	2600
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 5% 5% 41%	2343
	21st to end	Night	19% 7% 3 5% 3 4 4 3 4 6% 43%	1980
March	1st to 10th	Night	19% 8% 2 5% 4 3 4 3 4 5% 44%	5364
	11th to 20th	Night	13% 9% 2 6% 4 3 3 3 4 6% 48%	5117
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 5% 41%	5054
April	1st to 10th	Night	18% 7% 3 6% 4 4 4 4 5% 7% 37%	2000
	11th to 20th	Night	18% 7% 2 6% 3 5% 4 3 5% 6% 40%	1975
	21st to end	Night	18% 6% 2 5% 4 3 4 3 6% 7% 43%	2000
May	1st to 10th	Night	25% 8% 3 8% 4 4 4 3 5% 8% 27%	4064
	11th to 20th	Night	24% 8% 2 7% 4 4 4 3 4 6% 34%	3624
	21st to end	Night	22% 8% 2 6% 3 3 4 3 5% 8% 35%	3926
June	1st to 10th	Night	16% 6% 2 6% 4 3 3 4 6% 9% 41%	1600
	11th to 20th	Night	13% 7% 2 6% 4 5% 5% 4 6% 9% 39%	1600
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 7% 10% 41%	1600
July	1st to 10th	Night	8% 6% 2 8% 5% 6% 5% 5% 7% 10% 38%	2560
	11th to 20th	Night	13% 6% 2 6% 5% 4 5% 5% 7% 11% 36%	2520
	21st to end	Night	15% 8% 3 7% 5% 4 5% 5% 7% 8% 35%	2739
August	1st to 10th	Night	23% 6% 3 6% 5% 5% 5% 4 6% 8% 31%	2777
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 7% 7% 39%	3097
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 8% 50%	3476
September	1st to 10th	Night	14% 6% 3 7% 4 5% 5% 3 5% 6% 44%	1900
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 5% 7% 51%	1965
	21st to end	Night	18% 7% 2 6% 4 4 3 3 4 7% 42%	2090
October	1st to 10th	Night	18% 7% 2 6% 4 3 4 4 5% 5% 44%	2218
	11th to 20th	Night	14% 4 2 5% 4 3 3 3 4 6% 51%	2280
	21st to end	Night	18% 7% 3 5% 3 3 3 3 5% 5% 46%	2629
November	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 5% 54%	2537
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 4 48%	2600
	21st to end	Night	14% 7% 2 4 4 4 3 3 6% 7% 45%	2690
December	1st to 10th	Night	14% 8% 2 6% 4 3 3 3 5% 6% 47%	4580
	11th to 20th	Night	24% 6% 2 5% 3 3 3 3 4 5% 43%	5208
	21st to end	Night	20% 9% 3 6% 4 3 4 3 4 5% 40%	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	34% 6% 9% 29% 21% 3	3598
	11th to 20th	Day	42% 5% 9% 22% 17% 5%	3600
	21st to end	Day	47% 5% 10% 22% 15% 1	4398
February	1st to 10th	Day	45% 5% 13% 23% 13% 2	1800
	11th to 20th	Day	45% 5% 9% 23% 15% 3	1951
	21st to end	Day	46% 5% 10% 22% 14% 4	1650
March	1st to 10th	Day	40% 5% 10% 22% 15% 8%	4479
	11th to 20th	Day	37% 6% 12% 26% 13% 5%	4838
	21st to end	Day	42% 5% 9% 15% 23% 6%	5878
April	1st to 10th	Day	45% 6% 9% 20% 15% 5%	2400
	11th to 20th	Day	40% 6% 12% 21% 17% 3	2374
	21st to end	Day	42% 7% 10% 19% 18% 4	2400
May	1st to 10th	Day	53% 7% 14% 15% 10% 1	5887
	11th to 20th	Day	50% 5% 10% 20% 13% 1	6315
	21st to end	Day	44% 7% 10% 22% 15% 1	6949
June	1st to 10th	Day	40% 8% 13% 21% 15% 2	2800
	11th to 20th	Day	42% 8% 13% 22% 12% 2	2800
	21st to end	Day	39% 9% 13% 22% 16% 1	2800
July	1st to 10th	Day	41% 11% 14% 19% 14% 1	4440
	11th to 20th	Day	39% 10% 16% 21% 14% 1	4365
	21st to end	Day	47% 9% 12% 16% 15% 1	4661
August	1st to 10th	Day	47% 9% 14% 20% 9% 1	4101
	11th to 20th	Day	38% 8% 14% 26% 13% 1	3766
	21st to end	Day	32% 7% 11% 26% 22% 2	4180
September	1st to 10th	Day	38% 8% 11% 20% 20% 3	2280
	11th to 20th	Day	33% 6% 11% 23% 23% 3	2214
	21st to end	Day	45% 6% 9% 17% 19% 3	2090
October	1st to 10th	Day	36% 7% 11% 17% 24% 5%	3257
	11th to 20th	Day	24% 4 11% 24% 31% 6%	3140
	21st to end	Day	26% 4 14% 25% 27% 4	3218
November	1st to 10th	Day	34% 4 13% 24% 22% 3	1761
	11th to 20th	Day	38% 4 9% 24% 22% 3	1800
	21st to end	Day	38% 4 14% 30% 12% 2	1708
December	1st to 10th	Day	39% 4 15% 27% 13% 2	2600
	11th to 20th	Day	43% 5% 11% 26% 13% 2	2986
	21st to end	Day	43% 6% 11% 23% 15% 2	3960
January	1st to 10th	Night	34% 4 9% 26% 22% 5%	6349
	11th to 20th	Night	39% 4 12% 23% 15% 7%	6360
	21st to end	Night	44% 5% 10% 20% 17% 4	6554
February	1st to 10th	Night	44% 6% 13% 24% 12% 2	2600
	11th to 20th	Night	44% 4 9% 20% 18% 5%	2345
	21st to end	Night	43% 4 8% 23% 18% 4	1980
March	1st to 10th	Night	43% 4 10% 22% 12% 9%	5374
	11th to 20th	Night	37% 7% 14% 24% 14% 4	5118
	21st to end	Night	45% 5% 7% 15% 23% 5%	5054
April	1st to 10th	Night	45% 6% 9% 19% 16% 4	2000
	11th to 20th	Night	44% 6% 10% 18% 20% 2	1975
	21st to end	Night	39% 6% 10% 21% 19% 5%	2000
May	1st to 10th	Night	54% 7% 13% 15% 10% 2	4070
	11th to 20th	Night	50% 7% 13% 15% 14% 2	3625
	21st to end	Night	46% 7% 13% 21% 11% 2	3927
June	1st to 10th	Night	40% 7% 11% 20% 20% 2	1600
	11th to 20th	Night	40% 8% 15% 24% 12% 1	1600
	21st to end	Night	36% 9% 15% 23% 15% 2	1600
July	1st to 10th	Night	40% 10% 16% 19% 12% 2	2520
	11th to 20th	Night	39% 10% 15% 23% 12% 1	2472
	21st to end	Night	44% 8% 16% 16% 15% 2	2651
August	1st to 10th	Night	49% 8% 17% 16% 8% 1	2767
	11th to 20th	Night	40% 8% 12% 27% 12% 1	3127
	21st to end	Night	31% 7% 11% 29% 21% 1	3476
September	1st to 10th	Night	40% 7% 11% 21% 19% 2	1900
	11th to 20th	Night	33% 5% 11% 23% 24% 4	1965
	21st to end	Night	42% 6% 7% 21% 21% 2	2090
October	1st to 10th	Night	37% 6% 11% 23% 21% 2	3699
	11th to 20th	Night	24% 4 11% 27% 30% 5%	3779
	21st to end	Night	28% 4 11% 30% 24% 3	4328
November	1st to 10th	Night	33% 5% 12% 24% 23% 4	2537
	11th to 20th	Night	40% 5% 8% 24% 20% 5%	2600
	21st to end	Night	38% 4 14% 27% 16% 1	2690
December	1st to 10th	Night	39% 4 12% 30% 12% 2	4580
	11th to 20th	Night	44% 5% 10% 24% 14% 4	5208
	21st to end	Night	45% 5% 11% 19% 17% 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	24% 27% 31% 15% 3 1	200
	11th to 20th	Day	27% 33% 29% 11% 1	200
	21st to end	Day	22% 27% 32% 14% 4 1	316
February	1st to 10th	Day	26% 25% 36% 10% 2	300
	11th to 20th	Day	28% 23% 37% 11% 1	490
	21st to end	Day	27% 22% 30% 17% 3	414
March	1st to 10th	Day	24% 25% 32% 16% 3	500
	11th to 20th	Day	22% 26% 34% 14% 4 1	500
	21st to end	Day	27% 23% 32% 13% 4 1	550
April	1st to 10th	Day	28% 22% 31% 12% 6% 1	500
	11th to 20th	Day	23% 23% 33% 13% 8% 2	492
	21st to end	Day	26% 20% 31% 18% 4 2	500
May	1st to 10th	Day	30% 18% 29% 16% 5% 2	503
	11th to 20th	Day	22% 15% 34% 21% 7% 1	527
	21st to end	Day	22% 9% 25% 27% 15% 2	573
June	1st to 10th	Day	16% 11% 31% 28% 12% 2	530
	11th to 20th	Day	18% 11% 30% 28% 12% 2	530
	21st to end	Day	15% 8% 33% 30% 12% 2	530
July	1st to 10th	Day	11% 6% 26% 32% 20% 6%	505
	11th to 20th	Day	9% 5% 25% 35% 21% 5%	484
	21st to end	Day	12% 7% 28% 33% 16% 3	532
August	1st to 10th	Day	11% 6% 22% 37% 17% 6%	486
	11th to 20th	Day	13% 4 22% 35% 22% 5%	479
	21st to end	Day	12% 7% 26% 31% 16% 8%	539
September	1st to 10th	Day	16% 9% 27% 24% 17% 7%	490
	11th to 20th	Day	14% 7% 26% 30% 14% 9%	490
	21st to end	Day	24% 11% 25% 25% 10% 4	490
October	1st to 10th	Day	25% 7% 25% 23% 14% 6%	490
	11th to 20th	Day	17% 14% 31% 25% 11% 3	490
	21st to end	Day	21% 15% 31% 21% 9% 2	474
November	1st to 10th	Day	14% 17% 33% 21% 13% 2	395
	11th to 20th	Day	22% 19% 32% 21% 6% 1	400
	21st to end	Day	21% 20% 33% 17% 7% 1	308
December	1st to 10th	Day	25% 26% 31% 11% 7% 1	200
	11th to 20th	Day	34% 21% 29% 13% 5%	200
	21st to end	Day	24% 18% 35% 15% 7%	220
January	1st to 10th	Night	21% 21% 38% 17% 3	800
	11th to 20th	Night	24% 17% 35% 20% 3	800
	21st to end	Night	20% 20% 39% 20% 1	880
February	1st to 10th	Night	23% 17% 36% 23% 1	800
	11th to 20th	Night	27% 21% 37% 14% 1	783
	21st to end	Night	25% 20% 34% 18% 2	660
March	1st to 10th	Night	24% 19% 34% 20% 3	800
	11th to 20th	Night	23% 20% 34% 18% 4	800
	21st to end	Night	22% 15% 41% 18% 4	880
April	1st to 10th	Night	22% 20% 36% 18% 4	800
	11th to 20th	Night	21% 16% 38% 21% 4	791
	21st to end	Night	20% 13% 37% 25% 4 1	800
May	1st to 10th	Night	24% 14% 34% 22% 6%	610
	11th to 20th	Night	22% 9% 36% 27% 6%	417
	21st to end	Night	18% 7% 26% 36% 11% 2	433
June	1st to 10th	Night	17% 7% 30% 36% 10% 1	400
	11th to 20th	Night	15% 6% 30% 38% 12%	400
	21st to end	Night	9% 4 29% 46% 12% 1	400
July	1st to 10th	Night	11% 4 26% 42% 16% 2	380
	11th to 20th	Night	11% 4 22% 43% 19% 1	368
	21st to end	Night	10% 3 27% 43% 16%	416
August	1st to 10th	Night	13% 3 19% 45% 18% 2	532
	11th to 20th	Night	14% 3 19% 39% 23% 2	750
	21st to end	Night	15% 4 17% 41% 20% 2	836
September	1st to 10th	Night	16% 3 27% 34% 18% 2	760
	11th to 20th	Night	15% 5% 22% 32% 21% 4	760
	21st to end	Night	21% 6% 23% 33% 14% 1	760
October	1st to 10th	Night	22% 8% 27% 26% 14% 3	760
	11th to 20th	Night	16% 12% 30% 32% 10%	760
	21st to end	Night	23% 11% 32% 26% 8%	836
November	1st to 10th	Night	14% 11% 36% 27% 11% 1	780
	11th to 20th	Night	21% 16% 33% 23% 7%	800
	21st to end	Night	18% 17% 36% 22% 6% 1	800
December	1st to 10th	Night	26% 16% 32% 19% 6% 1	800
	11th to 20th	Night	23% 15% 37% 22% 4	800
	21st to end	Night	24% 15% 35% 24% 3	880

Page generated at 2025-07-03 01:30:13. Version ac07

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