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 2026-10-29.

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 6% 8% 40%	149605
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	142490
Transparency	Day	41% 7% 12% 22% 17% 2	153134
	Night	41% 6% 12% 22% 17% 3	147050
Seeing	Day	20% 15% 30% 22% 10% 3	17578
	Night	20% 13% 32% 27% 8% 1	26707

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% 3 5% 4 5% 4 4 5% 6% 47%	4661
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 4 6% 44%	4650
	21st to end	Day	16% 7% 3 6% 4 4 5% 4 5% 6% 40%	5698
February	1st to 10th	Day	15% 7% 3 7% 5% 4 4 3 5% 6% 41%	1890
	11th to 20th	Day	16% 8% 3 8% 4 4 4 3 4 5% 41%	2042
	21st to end	Day	18% 9% 2 6% 4 4 3 4 4 5% 39%	1730
March	1st to 10th	Day	21% 6% 2 4 3 3 3 3 4 6% 44%	4576
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 3 6% 8% 42%	4945
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 6% 43%	6008
April	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 39%	2520
	11th to 20th	Day	15% 7% 2 6% 4 4 3 4 6% 9% 40%	2494
	21st to end	Day	18% 6% 2 6% 4 4 4 3 6% 6% 40%	2520
May	1st to 10th	Day	23% 7% 2 6% 4 4 4 3 5% 7% 33%	7615
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 39%	8214
	21st to end	Day	24% 7% 2 6% 4 4 4 3 6% 7% 33%	9060
June	1st to 10th	Day	14% 5% 2 5% 5% 5% 5% 4 6% 10% 38%	2940
	11th to 20th	Day	14% 7% 2 6% 6% 5% 4 4 6% 8% 38%	2940
	21st to end	Day	11% 6% 2 6% 5% 6% 6% 4 5% 8% 40%	2940
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 9% 37%	8280
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 5% 8% 11% 36%	8280
	21st to end	Day	12% 6% 2 7% 5% 4 5% 4 6% 9% 38%	8993
August	1st to 10th	Day	15% 6% 2 8% 6% 6% 6% 5% 7% 11% 28%	6025
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 7% 10% 41%	5488
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 9% 44%	6080
September	1st to 10th	Day	13% 6% 2 6% 4 3 5% 4 6% 6% 45%	2520
	11th to 20th	Day	13% 5% 2 4 3 4 3 3 5% 9% 48%	2447
	21st to end	Day	19% 7% 3 5% 4 5% 4 4 5% 6% 37%	2310
October	1st to 10th	Day	18% 6% 2 4 3 4 4 4 4 6% 44%	2164
	11th to 20th	Day	15% 6% 2 4 3 3 4 2 5% 6% 48%	2100
	21st to end	Day	17% 6% 2 4 3 4 4 3 5% 6% 47%	2147
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 4 6% 52%	1851
	11th to 20th	Day	16% 6% 2 5% 3 3 4 4 6% 5% 47%	1890
	21st to end	Day	16% 6% 2 5% 4 4 4 3 5% 7% 44%	1793
December	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 6% 7% 45%	2680
	11th to 20th	Day	21% 7% 2 4 3 2 3 3 4 6% 44%	3066
	21st to end	Day	17% 7% 3 6% 4 5% 5% 4 6% 7% 38%	4048
January	1st to 10th	Night	12% 8% 3 6% 3 3 4 3 5% 6% 47%	8234
	11th to 20th	Night	15% 8% 3 6% 3 3 4 3 4 6% 45%	8235
	21st to end	Night	16% 8% 2 6% 3 4 4 3 4 6% 44%	8479
February	1st to 10th	Night	16% 9% 2 5% 4 3 5% 3 6% 6% 41%	2730
	11th to 20th	Night	18% 9% 3 6% 4 4 3 3 5% 5% 40%	2463
	21st to end	Night	18% 8% 3 5% 3 4 4 3 4 5% 42%	2076
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 5% 43%	5484
	11th to 20th	Night	13% 8% 2 6% 4 3 3 3 4 6% 48%	5230
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 5% 41%	5166
April	1st to 10th	Night	19% 7% 2 6% 4 4 4 4 5% 7% 37%	2100
	11th to 20th	Night	17% 7% 3 6% 4 5% 4 3 5% 7% 40%	2075
	21st to end	Night	18% 6% 2 5% 4 3 4 3 5% 7% 43%	2100
May	1st to 10th	Night	22% 8% 3 8% 4 5% 4 3 5% 8% 31%	5283
	11th to 20th	Night	20% 8% 3 6% 4 4 4 3 5% 7% 36%	4701
	21st to end	Night	24% 8% 2 7% 4 3 4 3 5% 7% 32%	5114
June	1st to 10th	Night	15% 6% 2 6% 4 3 3 4 6% 8% 43%	1680
	11th to 20th	Night	13% 7% 2 6% 4 5% 5% 4 6% 9% 39%	1680
	21st to end	Night	11% 5% 2 6% 4 4 5% 5% 7% 10% 40%	1680
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 5% 4 6% 10% 38%	4680
	11th to 20th	Night	11% 8% 3 7% 6% 5% 5% 5% 7% 10% 34%	4680
	21st to end	Night	13% 7% 2 6% 4 4 4 4 6% 9% 41%	5084
August	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 4 6% 8% 30%	4072
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 6% 8% 39%	4557
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 7% 47%	5045
September	1st to 10th	Night	14% 6% 3 7% 4 5% 5% 3 5% 6% 43%	2100
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 4 7% 51%	2172
	21st to end	Night	19% 7% 2 6% 4 4 3 3 4 7% 40%	2310
October	1st to 10th	Night	17% 6% 2 5% 4 3 4 4 5% 5% 45%	2452
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 4 6% 50%	2520
	21st to end	Night	17% 6% 3 5% 3 3 3 3 5% 5% 48%	2868
November	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 6% 53%	2667
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 5% 48%	2730
	21st to end	Night	14% 7% 2 5% 4 3 4 3 6% 7% 45%	2825
December	1st to 10th	Night	13% 8% 2 6% 4 3 3 3 5% 6% 47%	4720
	11th to 20th	Night	24% 6% 2 5% 3 3 3 3 4 5% 43%	5348
	21st to end	Night	20% 9% 3 6% 4 3 4 3 4 5% 40%	7150

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	37% 5% 9% 28% 19% 2	4662
	11th to 20th	Day	40% 4 9% 22% 19% 6%	4680
	21st to end	Day	43% 5% 11% 23% 16% 1	5698
February	1st to 10th	Day	44% 5% 13% 23% 13% 2	1890
	11th to 20th	Day	46% 5% 8% 23% 15% 3	2051
	21st to end	Day	46% 5% 10% 22% 14% 4	1730
March	1st to 10th	Day	41% 5% 10% 21% 15% 7%	4579
	11th to 20th	Day	38% 6% 12% 26% 13% 5%	4945
	21st to end	Day	42% 5% 9% 15% 23% 6%	6008
April	1st to 10th	Day	44% 6% 9% 20% 15% 5%	2520
	11th to 20th	Day	40% 6% 13% 21% 17% 3	2494
	21st to end	Day	42% 7% 10% 20% 17% 4	2520
May	1st to 10th	Day	50% 6% 13% 17% 14% 1	7627
	11th to 20th	Day	45% 6% 10% 23% 15% 1	8218
	21st to end	Day	49% 6% 10% 21% 13% 1	9061
June	1st to 10th	Day	39% 8% 13% 21% 17% 1	2940
	11th to 20th	Day	42% 8% 13% 22% 12% 2	2940
	21st to end	Day	40% 9% 13% 21% 16% 1	2940
July	1st to 10th	Day	42% 9% 13% 19% 17% 1	8210
	11th to 20th	Day	39% 9% 16% 23% 13% 1	8205
	21st to end	Day	38% 8% 11% 21% 21% 1	8795
August	1st to 10th	Day	46% 11% 16% 18% 8% 1	6011
	11th to 20th	Day	36% 8% 17% 28% 11% 1	5524
	21st to end	Day	35% 7% 11% 26% 19% 2	6080
September	1st to 10th	Day	38% 8% 12% 21% 20% 3	2520
	11th to 20th	Day	33% 6% 12% 24% 22% 3	2447
	21st to end	Day	47% 7% 9% 17% 18% 3	2310
October	1st to 10th	Day	35% 7% 11% 18% 25% 5%	3463
	11th to 20th	Day	25% 4 11% 24% 30% 6%	3340
	21st to end	Day	26% 4 14% 25% 27% 5%	3398
November	1st to 10th	Day	34% 4 13% 24% 22% 3	1851
	11th to 20th	Day	38% 5% 9% 24% 21% 3	1890
	21st to end	Day	39% 4 13% 29% 12% 2	1793
December	1st to 10th	Day	39% 4 15% 27% 13% 1	2680
	11th to 20th	Day	42% 5% 12% 26% 13% 2	3066
	21st to end	Day	43% 6% 10% 23% 15% 2	4048
January	1st to 10th	Night	37% 4 12% 24% 18% 4	8234
	11th to 20th	Night	40% 4 11% 22% 16% 7%	8280
	21st to end	Night	41% 5% 9% 22% 20% 4	8479
February	1st to 10th	Night	43% 5% 12% 24% 13% 2	2730
	11th to 20th	Night	45% 5% 9% 19% 17% 4	2465
	21st to end	Night	43% 4 8% 24% 17% 4	2076
March	1st to 10th	Night	43% 4 9% 22% 12% 9%	5494
	11th to 20th	Night	37% 7% 14% 24% 14% 4	5231
	21st to end	Night	45% 5% 7% 15% 23% 5%	5166
April	1st to 10th	Night	44% 6% 10% 19% 17% 4	2100
	11th to 20th	Night	43% 6% 10% 18% 20% 2	2075
	21st to end	Night	40% 6% 10% 21% 19% 5%	2100
May	1st to 10th	Night	50% 7% 12% 18% 12% 1	5289
	11th to 20th	Night	47% 7% 13% 20% 13% 1	4702
	21st to end	Night	51% 7% 11% 20% 10% 1	5115
June	1st to 10th	Night	39% 7% 11% 21% 20% 2	1680
	11th to 20th	Night	40% 8% 15% 24% 12% 1	1680
	21st to end	Night	36% 9% 15% 23% 15% 1	1680
July	1st to 10th	Night	42% 9% 13% 22% 13% 1	4640
	11th to 20th	Night	41% 10% 15% 23% 11% 1	4632
	21st to end	Night	38% 7% 15% 21% 18% 1	4967
August	1st to 10th	Night	49% 9% 16% 16% 10% 1	4062
	11th to 20th	Night	40% 9% 14% 28% 9% 1	4587
	21st to end	Night	35% 6% 11% 27% 19% 1	5045
September	1st to 10th	Night	40% 7% 11% 22% 18% 2	2100
	11th to 20th	Night	34% 5% 11% 22% 24% 4	2172
	21st to end	Night	44% 6% 8% 20% 19% 2	2310
October	1st to 10th	Night	36% 6% 11% 23% 21% 2	3933
	11th to 20th	Night	25% 4 11% 26% 29% 5%	4019
	21st to end	Night	27% 4 11% 30% 24% 3	4567
November	1st to 10th	Night	33% 5% 12% 24% 23% 4	2667
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2730
	21st to end	Night	38% 4 13% 26% 16% 1	2825
December	1st to 10th	Night	39% 4 12% 30% 12% 2	4720
	11th to 20th	Night	44% 5% 10% 24% 14% 4	5348
	21st to end	Night	45% 5% 11% 19% 16% 3	7150

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% 28% 32% 14% 3 1	220
	11th to 20th	Day	29% 32% 28% 10% 1	220
	21st to end	Day	23% 29% 31% 13% 3 1	348
February	1st to 10th	Day	25% 25% 36% 11% 3 1	330
	11th to 20th	Day	28% 23% 37% 11% 1	530
	21st to end	Day	27% 23% 30% 17% 3	446
March	1st to 10th	Day	25% 24% 32% 16% 3	540
	11th to 20th	Day	24% 25% 33% 14% 4 1	540
	21st to end	Day	27% 25% 31% 12% 4 1	594
April	1st to 10th	Day	28% 22% 32% 12% 6% 1	540
	11th to 20th	Day	22% 24% 32% 13% 8% 2	532
	21st to end	Day	26% 19% 31% 18% 4 2	540
May	1st to 10th	Day	29% 19% 30% 15% 5% 2	543
	11th to 20th	Day	21% 14% 35% 22% 7% 1	567
	21st to end	Day	24% 9% 25% 26% 14% 2	617
June	1st to 10th	Day	15% 11% 31% 29% 11% 2	570
	11th to 20th	Day	18% 12% 30% 27% 11% 2	570
	21st to end	Day	15% 8% 33% 30% 12% 2	570
July	1st to 10th	Day	12% 6% 25% 33% 19% 5%	565
	11th to 20th	Day	8% 6% 26% 34% 20% 5%	564
	21st to end	Day	11% 7% 29% 33% 15% 4	620
August	1st to 10th	Day	11% 7% 22% 37% 17% 6%	566
	11th to 20th	Day	12% 5% 23% 35% 21% 4	559
	21st to end	Day	12% 7% 29% 30% 15% 7%	627
September	1st to 10th	Day	15% 8% 31% 24% 16% 6%	570
	11th to 20th	Day	14% 7% 26% 30% 14% 8%	570
	21st to end	Day	24% 12% 24% 26% 10% 4	570
October	1st to 10th	Day	23% 8% 27% 22% 14% 6%	570
	11th to 20th	Day	20% 14% 30% 24% 10% 2	570
	21st to end	Day	20% 14% 32% 21% 10% 3	540
November	1st to 10th	Day	15% 18% 32% 20% 12% 2	425
	11th to 20th	Day	24% 19% 31% 20% 6%	430
	21st to end	Day	22% 21% 33% 16% 7% 1	333
December	1st to 10th	Day	26% 25% 31% 10% 6%	220
	11th to 20th	Day	34% 21% 29% 12% 4	220
	21st to end	Day	26% 19% 33% 14% 7%	242
January	1st to 10th	Night	20% 21% 38% 18% 3	840
	11th to 20th	Night	25% 18% 34% 20% 3	840
	21st to end	Night	20% 21% 39% 20% 1	924
February	1st to 10th	Night	22% 17% 36% 23% 2	840
	11th to 20th	Night	26% 20% 38% 15% 1	823
	21st to end	Night	25% 20% 35% 18% 2	692
March	1st to 10th	Night	24% 19% 33% 21% 3	840
	11th to 20th	Night	23% 20% 34% 18% 4	840
	21st to end	Night	22% 15% 40% 18% 4	924
April	1st to 10th	Night	23% 20% 35% 18% 5%	840
	11th to 20th	Night	20% 16% 38% 21% 4	831
	21st to end	Night	20% 13% 38% 25% 4	840
May	1st to 10th	Night	24% 14% 35% 22% 5%	641
	11th to 20th	Night	22% 9% 36% 27% 6%	438
	21st to end	Night	18% 8% 26% 36% 11% 2	455
June	1st to 10th	Night	16% 7% 30% 37% 10%	420
	11th to 20th	Night	15% 7% 31% 37% 11%	420
	21st to end	Night	10% 4 30% 44% 12%	420
July	1st to 10th	Night	11% 4 26% 42% 16% 1	410
	11th to 20th	Night	12% 3 22% 43% 19% 1	408
	21st to end	Night	10% 3 27% 43% 17%	460
August	1st to 10th	Night	13% 3 18% 44% 20% 2	590
	11th to 20th	Night	13% 4 20% 38% 23% 2	830
	21st to end	Night	15% 3 19% 41% 19% 2	924
September	1st to 10th	Night	15% 4 28% 34% 17% 2	840
	11th to 20th	Night	14% 5% 23% 33% 20% 4	840
	21st to end	Night	22% 6% 24% 33% 14% 1	840
October	1st to 10th	Night	20% 8% 28% 27% 15% 3	840
	11th to 20th	Night	17% 13% 29% 31% 10%	840
	21st to end	Night	21% 12% 32% 26% 8% 1	913
November	1st to 10th	Night	15% 11% 35% 27% 11% 1	820
	11th to 20th	Night	21% 18% 32% 22% 7%	840
	21st to end	Night	18% 18% 35% 23% 6%	840
December	1st to 10th	Night	26% 16% 33% 19% 5% 1	840
	11th to 20th	Night	23% 16% 36% 22% 3	840
	21st to end	Night	25% 15% 35% 23% 3	924

Page generated at 2026-10-26 04:37:47. Version ac07

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