Richard Schueller Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Richard Schueller Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	130583
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	130578
Transparency	Day	44% 6% 11% 22% 15% 2	134116
	Night	43% 5% 12% 21% 16% 3	135132
Seeing	Day	23% 17% 32% 18% 7% 2	16620
	Night	21% 14% 33% 24% 7% 1	25631

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	11% 6% 3 9% 5% 6% 5% 4 5% 6% 39%	4661
	11th to 20th	Day	15% 8% 3 6% 3 3 4 3 4 5% 45%	4650
	21st to end	Day	14% 8% 2 5% 3 3 3 3 5% 6% 47%	5124
February	1st to 10th	Day	17% 8% 3 5% 5% 4 5% 5% 5% 6% 37%	1941
	11th to 20th	Day	13% 8% 2 7% 4 4 6% 3 6% 7% 39%	2048
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 4 5% 38%	1730
March	1st to 10th	Day	21% 11% 3 6% 4 4 4 2 5% 5% 35%	4496
	11th to 20th	Day	17% 9% 2 6% 4 4 4 4 5% 5% 39%	4768
	21st to end	Day	25% 9% 2 6% 4 3 4 4 5% 6% 31%	5777
April	1st to 10th	Day	19% 10% 3 6% 5% 4 3 3 4 7% 36%	2400
	11th to 20th	Day	18% 7% 2 5% 4 5% 5% 4 6% 7% 38%	2394
	21st to end	Day	21% 6% 3 7% 5% 4 4 3 5% 6% 36%	2600
May	1st to 10th	Day	31% 9% 3 6% 4 4 4 3 5% 7% 25%	5886
	11th to 20th	Day	21% 7% 3 8% 5% 5% 5% 3 5% 7% 32%	5993
	21st to end	Day	20% 5% 2 6% 4 5% 4 4 6% 7% 36%	6949
June	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 7% 8% 34%	2800
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 7% 39%	2800
	21st to end	Day	17% 6% 3 7% 5% 4 5% 3 6% 8% 36%	2800
July	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 7% 9% 34%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 5% 6% 4 7% 11% 39%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 8% 43%	6926
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 4 7% 9% 34%	4302
	11th to 20th	Day	14% 7% 2 7% 4 5% 5% 4 6% 7% 38%	4125
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 6% 6% 47%	2397
	11th to 20th	Day	15% 6% 2 5% 3 3 2 2 5% 5% 52%	2200
	21st to end	Day	17% 6% 2 6% 4 4 3 3 5% 7% 43%	2200
October	1st to 10th	Day	21% 6% 2 6% 3 4 3 3 5% 6% 41%	2200
	11th to 20th	Day	16% 4 2 5% 4 3 4 3 4 7% 46%	1945
	21st to end	Day	22% 6% 3 5% 4 3 3 3 4 6% 41%	1980
November	1st to 10th	Day	15% 6% 2 5% 4 4 3 2 4 5% 49%	1851
	11th to 20th	Day	17% 6% 2 5% 3 3 3 3 4 6% 48%	1890
	21st to end	Day	19% 7% 2 5% 4 3 3 4 5% 5% 43%	1890
December	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 4 6% 47%	2714
	11th to 20th	Day	20% 6% 3 5% 3 3 3 3 4 5% 47%	3066
	21st to end	Day	20% 10% 3 6% 4 3 4 3 4 6% 37%	4048
January	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 45%	8234
	11th to 20th	Night	15% 9% 3 9% 5% 5% 4 3 4 5% 38%	8235
	21st to end	Night	14% 9% 2 6% 4 5% 5% 4 5% 5% 41%	9057
February	1st to 10th	Night	16% 9% 2 6% 5% 4 5% 4 5% 6% 39%	2673
	11th to 20th	Night	14% 9% 3 6% 4 4 5% 3 5% 7% 40%	2458
	21st to end	Night	18% 9% 2 6% 4 3 4 3 5% 6% 40%	2076
March	1st to 10th	Night	23% 9% 2 5% 4 4 3 3 4 5% 37%	5389
	11th to 20th	Night	16% 9% 3 7% 5% 4 5% 3 5% 5% 37%	5177
	21st to end	Night	27% 8% 2 5% 4 4 4 3 5% 5% 33%	5151
April	1st to 10th	Night	20% 7% 2 7% 5% 5% 4 3 5% 7% 35%	2000
	11th to 20th	Night	21% 8% 2 5% 4 4 3 4 4 5% 39%	1955
	21st to end	Night	21% 7% 2 6% 5% 3 4 3 4 6% 39%	1800
May	1st to 10th	Night	33% 9% 2 6% 4 4 3 3 4 6% 25%	4054
	11th to 20th	Night	18% 8% 3 8% 4 5% 6% 5% 6% 8% 29%	3942
	21st to end	Night	21% 7% 3 8% 5% 4 5% 3 5% 6% 33%	3926
June	1st to 10th	Night	18% 8% 3 7% 5% 4 4 3 5% 6% 38%	1600
	11th to 20th	Night	19% 7% 2 5% 5% 4 4 4 5% 8% 37%	1600
	21st to end	Night	16% 6% 3 6% 5% 4 5% 4 6% 9% 38%	1600
July	1st to 10th	Night	12% 6% 3 6% 5% 5% 5% 4 6% 10% 38%	3600
	11th to 20th	Night	12% 6% 2 7% 4 5% 6% 3 7% 8% 40%	3600
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 6% 47%	3926
August	1st to 10th	Night	19% 6% 3 7% 4 5% 5% 5% 5% 6% 34%	2813
	11th to 20th	Night	11% 6% 3 8% 5% 5% 6% 5% 7% 8% 37%	2905
	21st to end	Night	10% 3 1 4 4 3 4 3 6% 8% 55%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 49%	2002
	11th to 20th	Night	13% 5% 2 4 4 3 3 2 5% 6% 52%	2200
	21st to end	Night	18% 5% 3 6% 3 3 4 4 5% 6% 44%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 4 4 2 5% 5% 43%	2200
	11th to 20th	Night	17% 5% 1 5% 3 4 4 3 6% 6% 46%	2451
	21st to end	Night	22% 6% 1 6% 3 4 4 3 4 6% 40%	2860
November	1st to 10th	Night	16% 5% 1 5% 4 3 4 4 5% 5% 49%	2667
	11th to 20th	Night	18% 6% 2 4 4 3 3 3 4 6% 47%	2730
	21st to end	Night	16% 8% 2 6% 4 3 4 2 5% 6% 44%	2730
December	1st to 10th	Night	17% 9% 2 6% 4 4 4 4 5% 5% 41%	4683
	11th to 20th	Night	22% 7% 2 6% 4 3 3 3 4 5% 41%	5348
	21st to end	Night	18% 10% 3 7% 5% 4 3 3 5% 5% 39%	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	44% 4 11% 25% 14% 1	4662
	11th to 20th	Day	41% 5% 10% 22% 19% 4	4680
	21st to end	Day	38% 4 10% 25% 21% 2	5124
February	1st to 10th	Day	46% 6% 12% 19% 14% 3	1941
	11th to 20th	Day	43% 7% 12% 21% 14% 3	2057
	21st to end	Day	48% 5% 10% 20% 14% 3	1730
March	1st to 10th	Day	51% 4 9% 15% 11% 10%	4499
	11th to 20th	Day	42% 7% 14% 24% 11% 2	4768
	21st to end	Day	53% 4 7% 15% 15% 7%	5777
April	1st to 10th	Day	49% 5% 9% 17% 16% 4	2400
	11th to 20th	Day	44% 7% 11% 19% 17% 3	2394
	21st to end	Day	47% 6% 8% 18% 17% 4	2600
May	1st to 10th	Day	59% 7% 12% 13% 8% 1	5897
	11th to 20th	Day	51% 7% 14% 17% 10% 1	5997
	21st to end	Day	45% 7% 12% 21% 14% 1	6949
June	1st to 10th	Day	47% 8% 12% 21% 12%	2800
	11th to 20th	Day	42% 8% 14% 18% 16% 3	2800
	21st to end	Day	44% 8% 12% 21% 13% 1	2800
July	1st to 10th	Day	46% 8% 12% 20% 14% 1	6290
	11th to 20th	Day	37% 8% 16% 25% 13% 1	6285
	21st to end	Day	39% 5% 12% 26% 16% 1	6728
August	1st to 10th	Day	45% 8% 15% 23% 9% 1	4288
	11th to 20th	Day	42% 7% 14% 26% 10% 1	4164
	21st to end	Day	32% 7% 12% 29% 18% 2	4312
September	1st to 10th	Day	37% 6% 11% 24% 21% 2	2397
	11th to 20th	Day	35% 4 9% 26% 23% 2	2200
	21st to end	Day	40% 6% 9% 22% 21% 2	2200
October	1st to 10th	Day	43% 5% 9% 22% 19% 3	3590
	11th to 20th	Day	31% 5% 8% 29% 24% 3	3148
	21st to end	Day	34% 4 8% 31% 20% 3	3180
November	1st to 10th	Day	38% 5% 12% 26% 17% 2	1851
	11th to 20th	Day	39% 4 9% 23% 22% 3	1890
	21st to end	Day	43% 5% 14% 23% 13% 2	1890
December	1st to 10th	Day	40% 4 14% 28% 13% 1	2714
	11th to 20th	Day	40% 3 14% 19% 19% 5%	3066
	21st to end	Day	48% 5% 8% 20% 14% 5%	4048
January	1st to 10th	Night	40% 3 13% 28% 14% 2	8234
	11th to 20th	Night	48% 4 11% 18% 14% 5%	8280
	21st to end	Night	44% 5% 9% 18% 19% 4	9057
February	1st to 10th	Night	45% 6% 12% 21% 14% 2	2673
	11th to 20th	Night	43% 5% 11% 22% 15% 3	2460
	21st to end	Night	45% 5% 9% 23% 15% 3	2076
March	1st to 10th	Night	48% 5% 9% 16% 15% 7%	5399
	11th to 20th	Night	46% 7% 16% 16% 12% 2	5178
	21st to end	Night	52% 4 6% 12% 20% 6%	5151
April	1st to 10th	Night	47% 6% 11% 17% 15% 3	2000
	11th to 20th	Night	46% 5% 9% 19% 18% 3	1955
	21st to end	Night	46% 5% 12% 16% 17% 4	1800
May	1st to 10th	Night	60% 6% 12% 12% 8% 2	4060
	11th to 20th	Night	51% 8% 16% 15% 9% 2	3943
	21st to end	Night	51% 7% 13% 18% 10% 1	3927
June	1st to 10th	Night	47% 6% 12% 22% 12% 1	1600
	11th to 20th	Night	44% 8% 14% 19% 14% 2	1600
	21st to end	Night	42% 9% 14% 20% 14% 1	1600
July	1st to 10th	Night	42% 7% 12% 22% 16%	3560
	11th to 20th	Night	41% 7% 19% 19% 14% 1	3552
	21st to end	Night	37% 5% 11% 30% 16% 1	3809
August	1st to 10th	Night	49% 8% 12% 19% 11% 1	2803
	11th to 20th	Night	42% 10% 14% 23% 10% 1	2932
	21st to end	Night	27% 6% 15% 29% 22% 1	3586
September	1st to 10th	Night	35% 6% 12% 26% 20% 1	2002
	11th to 20th	Night	33% 5% 11% 23% 26% 2	2200
	21st to end	Night	39% 6% 10% 24% 18% 2	2200
October	1st to 10th	Night	39% 5% 15% 23% 16% 2	3590
	11th to 20th	Night	27% 5% 11% 27% 27% 3	3987
	21st to end	Night	35% 4 11% 31% 17% 2	4610
November	1st to 10th	Night	37% 6% 11% 23% 20% 3	2667
	11th to 20th	Night	39% 4 10% 23% 20% 3	2730
	21st to end	Night	41% 5% 14% 24% 14% 2	2730
December	1st to 10th	Night	45% 4 11% 27% 12% 1	4683
	11th to 20th	Night	46% 5% 14% 14% 20% 2	5348
	21st to end	Night	46% 5% 8% 22% 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	28% 26% 30% 14% 3	220
	11th to 20th	Day	34% 23% 31% 10% 2	220
	21st to end	Day	26% 26% 35% 10% 3	242
February	1st to 10th	Day	31% 20% 34% 14% 1	423
	11th to 20th	Day	29% 26% 32% 11% 2	536
	21st to end	Day	27% 21% 37% 13% 3	446
March	1st to 10th	Day	29% 22% 35% 11% 3	508
	11th to 20th	Day	28% 25% 31% 11% 3 1	500
	21st to end	Day	32% 23% 29% 10% 5% 1	550
April	1st to 10th	Day	31% 26% 25% 12% 5% 1	500
	11th to 20th	Day	31% 25% 28% 10% 5%	492
	21st to end	Day	27% 19% 34% 15% 4 1	500
May	1st to 10th	Day	32% 21% 29% 11% 4 2	503
	11th to 20th	Day	23% 17% 38% 16% 4 1	527
	21st to end	Day	20% 14% 38% 21% 6% 1	573
June	1st to 10th	Day	21% 15% 37% 18% 8% 2	530
	11th to 20th	Day	17% 18% 36% 22% 6% 1	530
	21st to end	Day	18% 16% 35% 20% 8% 2	530
July	1st to 10th	Day	18% 10% 31% 29% 10% 2	525
	11th to 20th	Day	14% 8% 33% 29% 14% 2	524
	21st to end	Day	13% 9% 37% 31% 9% 2	576
August	1st to 10th	Day	11% 10% 34% 29% 13% 2	526
	11th to 20th	Day	17% 9% 26% 29% 14% 4	519
	21st to end	Day	11% 10% 35% 26% 14% 4	583
September	1st to 10th	Day	18% 11% 30% 22% 15% 5%	530
	11th to 20th	Day	18% 13% 26% 20% 13% 9%	530
	21st to end	Day	19% 12% 33% 21% 9% 5%	530
October	1st to 10th	Day	25% 14% 24% 22% 11% 5%	530
	11th to 20th	Day	21% 18% 27% 22% 10% 2	467
	21st to end	Day	28% 15% 32% 18% 7% 1	462
November	1st to 10th	Day	22% 16% 33% 19% 8% 1	425
	11th to 20th	Day	29% 20% 30% 15% 5% 1	430
	21st to end	Day	30% 19% 30% 15% 5% 1	430
December	1st to 10th	Day	24% 24% 32% 15% 4	241
	11th to 20th	Day	35% 25% 30% 9% 2	220
	21st to end	Day	25% 26% 30% 14% 4 2	242
January	1st to 10th	Night	22% 20% 39% 17% 2	840
	11th to 20th	Night	27% 19% 34% 17% 2	840
	21st to end	Night	20% 20% 38% 19% 3	924
February	1st to 10th	Night	22% 17% 38% 21% 2	840
	11th to 20th	Night	22% 19% 40% 17% 2	823
	21st to end	Night	25% 18% 35% 19% 3	692
March	1st to 10th	Night	25% 19% 34% 19% 3	809
	11th to 20th	Night	22% 20% 37% 17% 4	800
	21st to end	Night	27% 18% 32% 19% 4	880
April	1st to 10th	Night	25% 19% 33% 20% 4	800
	11th to 20th	Night	26% 20% 33% 17% 5%	771
	21st to end	Night	23% 16% 37% 21% 2 1	600
May	1st to 10th	Night	28% 14% 34% 20% 5%	600
	11th to 20th	Night	21% 14% 37% 22% 5%	553
	21st to end	Night	19% 10% 34% 30% 6%	433
June	1st to 10th	Night	20% 8% 34% 30% 7% 1	400
	11th to 20th	Night	18% 11% 34% 30% 7% 1	400
	21st to end	Night	19% 5% 36% 33% 8%	400
July	1st to 10th	Night	15% 5% 29% 42% 8% 1	390
	11th to 20th	Night	13% 4 27% 43% 12% 1	388
	21st to end	Night	12% 5% 31% 43% 9%	438
August	1st to 10th	Night	12% 5% 28% 42% 12% 1	534
	11th to 20th	Night	13% 6% 29% 39% 13% 1	612
	21st to end	Night	14% 5% 26% 38% 15% 2	880
September	1st to 10th	Night	15% 5% 34% 32% 14% 1	800
	11th to 20th	Night	16% 8% 26% 28% 19% 3	800
	21st to end	Night	20% 7% 27% 31% 13% 2	800
October	1st to 10th	Night	23% 10% 26% 26% 13% 2	800
	11th to 20th	Night	20% 12% 31% 26% 11% 1	800
	21st to end	Night	25% 13% 33% 24% 5%	880
November	1st to 10th	Night	19% 14% 33% 26% 8% 1	820
	11th to 20th	Night	22% 18% 33% 20% 6% 1	840
	21st to end	Night	24% 19% 33% 20% 4 1	840
December	1st to 10th	Night	25% 18% 36% 18% 4	840
	11th to 20th	Night	25% 17% 32% 22% 3 1	840
	21st to end	Night	25% 18% 35% 19% 3	924

Page generated at 2026-02-28 01:40:25. Version ac07

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