Stull Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stull Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	14% 12% 5% 10% 6% 5% 5% 4 5% 6% 28%	126867
	Night	14% 13% 5% 12% 7% 5% 4 3 5% 6% 27%	122803
Transparency	Day	53% 7% 11% 18% 10% 1	130457
	Night	55% 7% 12% 17% 9% 1	127299
Seeing	Day	24% 20% 29% 17% 8% 3	15863
	Night	23% 19% 28% 20% 9% 1	25178

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% 19% 11% 22% 7% 4 3 2 3 3 15%	3597
	11th to 20th	Day	8% 20% 11% 24% 8% 6% 3 2 3 3 12%	3825
	21st to end	Day	15% 14% 7% 14% 8% 5% 6% 4 5% 5% 16%	4466
February	1st to 10th	Day	13% 18% 7% 14% 10% 5% 5% 3 4 4 17%	1800
	11th to 20th	Day	10% 15% 8% 18% 8% 5% 5% 3 4 5% 20%	1753
	21st to end	Day	15% 16% 4 12% 7% 4 3 4 5% 6% 23%	1610
March	1st to 10th	Day	14% 14% 9% 11% 5% 4 3 3 3 5% 30%	4838
	11th to 20th	Day	10% 16% 7% 9% 4 4 4 3 4 5% 33%	4980
	21st to end	Day	12% 14% 5% 14% 6% 5% 5% 3 4 5% 26%	5544
April	1st to 10th	Day	14% 19% 4 10% 4 3 4 3 5% 6% 27%	2400
	11th to 20th	Day	17% 13% 4 6% 5% 5% 5% 4 5% 6% 30%	2374
	21st to end	Day	18% 12% 4 8% 5% 4 4 3 5% 5% 32%	2518
May	1st to 10th	Day	25% 11% 3 8% 4 4 4 3 4 6% 27%	5970
	11th to 20th	Day	19% 12% 4 9% 5% 5% 5% 4 5% 7% 26%	6318
	21st to end	Day	19% 9% 3 8% 5% 4 5% 4 6% 6% 31%	6949
June	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 7% 35%	2800
	11th to 20th	Day	15% 7% 3 8% 6% 5% 5% 4 6% 9% 33%	2800
	21st to end	Day	13% 8% 2 8% 5% 5% 5% 4 6% 9% 36%	2800
July	1st to 10th	Day	9% 6% 3 8% 6% 5% 6% 5% 7% 9% 37%	6360
	11th to 20th	Day	7% 8% 3 8% 6% 6% 6% 5% 8% 10% 32%	6360
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 4 6% 7% 41%	6589
August	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 5% 7% 8% 31%	4234
	11th to 20th	Day	15% 8% 3 9% 6% 7% 6% 6% 7% 9% 25%	4174
	21st to end	Day	12% 9% 3 7% 6% 5% 6% 4 6% 8% 36%	4315
September	1st to 10th	Day	17% 7% 2 6% 4 5% 4 4 6% 7% 38%	2400
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 5% 8% 43%	2397
	21st to end	Day	20% 9% 3 8% 5% 5% 4 4 6% 7% 31%	2088
October	1st to 10th	Day	17% 9% 4 7% 5% 4 4 3 5% 8% 35%	2000
	11th to 20th	Day	18% 14% 3 9% 5% 4 4 4 4 6% 30%	1980
	21st to end	Day	21% 18% 4 7% 4 4 3 3 5% 5% 26%	2090
November	1st to 10th	Day	11% 15% 5% 9% 4 5% 3 3 5% 6% 33%	1787
	11th to 20th	Day	13% 20% 6% 10% 4 5% 4 3 6% 6% 23%	1605
	21st to end	Day	15% 21% 7% 11% 6% 4 5% 3 5% 5% 18%	1600
December	1st to 10th	Day	14% 22% 11% 19% 7% 5% 4 3 3 4 9%	2600
	11th to 20th	Day	16% 17% 7% 16% 9% 4 3 3 4 4 18%	2986
	21st to end	Day	14% 22% 9% 15% 9% 4 3 3 3 5% 14%	3960
January	1st to 10th	Night	11% 16% 9% 22% 12% 4 4 2 3 4 12%	6349
	11th to 20th	Night	8% 17% 8% 24% 11% 6% 5% 3 4 4 11%	6044
	21st to end	Night	13% 15% 6% 17% 12% 8% 6% 3 3 3 14%	6490
February	1st to 10th	Night	12% 17% 6% 15% 11% 6% 5% 3 4 5% 17%	2600
	11th to 20th	Night	10% 14% 7% 17% 9% 6% 6% 4 4 5% 17%	2533
	21st to end	Night	13% 15% 6% 16% 8% 5% 4 3 5% 4 20%	2019
March	1st to 10th	Night	13% 13% 5% 12% 9% 6% 4 3 5% 6% 24%	4998
	11th to 20th	Night	12% 15% 7% 10% 5% 3 4 3 4 6% 32%	4979
	21st to end	Night	13% 13% 5% 14% 5% 4 3 3 4 5% 30%	5386
April	1st to 10th	Night	16% 15% 5% 10% 5% 5% 4 3 4 6% 27%	2000
	11th to 20th	Night	16% 15% 4 7% 5% 4 4 3 4 6% 33%	1975
	21st to end	Night	15% 11% 4 8% 5% 5% 3 3 5% 7% 34%	1898
May	1st to 10th	Night	28% 12% 3 7% 4 4 4 3 4 5% 26%	3968
	11th to 20th	Night	20% 10% 3 6% 5% 5% 4 3 5% 6% 32%	3575
	21st to end	Night	18% 12% 3 8% 4 3 4 3 5% 7% 33%	3926
June	1st to 10th	Night	16% 7% 2 7% 5% 5% 4 4 6% 8% 36%	1600
	11th to 20th	Night	11% 9% 3 7% 5% 4 4 5% 7% 8% 38%	1600
	21st to end	Night	10% 7% 2 6% 5% 4 5% 4 6% 7% 44%	1600
July	1st to 10th	Night	10% 8% 4 8% 5% 4 5% 3 5% 9% 38%	3600
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 4 7% 9% 40%	3600
	21st to end	Night	10% 6% 3 6% 4 4 4 3 6% 8% 46%	4245
August	1st to 10th	Night	16% 9% 3 6% 4 4 5% 4 6% 7% 35%	2915
	11th to 20th	Night	16% 9% 3 7% 3 4 4 3 6% 7% 37%	2875
	21st to end	Night	11% 7% 3 6% 5% 5% 5% 4 6% 8% 40%	3549
September	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 4 8% 46%	2000
	11th to 20th	Night	13% 7% 2 5% 3 4 4 3 5% 7% 47%	2000
	21st to end	Night	18% 9% 2 7% 5% 4 4 4 5% 7% 34%	2311
October	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 5% 7% 39%	2400
	11th to 20th	Night	16% 13% 3 7% 5% 4 4 4 4 7% 33%	2377
	21st to end	Night	24% 14% 3 7% 5% 4 4 3 5% 6% 27%	2508
November	1st to 10th	Night	11% 15% 4 9% 5% 3 4 3 5% 6% 36%	2508
	11th to 20th	Night	14% 18% 4 11% 6% 5% 4 4 5% 6% 24%	2791
	21st to end	Night	13% 19% 6% 12% 7% 4 5% 4 4 5% 20%	2800
December	1st to 10th	Night	10% 17% 9% 21% 10% 7% 5% 4 4 4 10%	4580
	11th to 20th	Night	15% 19% 9% 15% 6% 5% 4 3 4 4 17%	5208
	21st to end	Night	15% 19% 9% 14% 7% 5% 4 3 5% 4 14%	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	74% 3 7% 10% 5% 1	3598
	11th to 20th	Day	77% 4 6% 9% 3	3858
	21st to end	Day	69% 5% 10% 12% 4	4466
February	1st to 10th	Day	70% 6% 9% 10% 4	1800
	11th to 20th	Day	67% 5% 6% 15% 7%	1762
	21st to end	Day	60% 6% 11% 17% 6% 2	1610
March	1st to 10th	Day	56% 5% 11% 18% 9% 1	4841
	11th to 20th	Day	52% 5% 17% 16% 7% 2	4980
	21st to end	Day	57% 7% 6% 18% 11% 2	5544
April	1st to 10th	Day	57% 5% 10% 16% 11% 1	2400
	11th to 20th	Day	53% 6% 10% 17% 13% 1	2374
	21st to end	Day	52% 6% 10% 17% 13% 2	2518
May	1st to 10th	Day	58% 5% 8% 16% 10% 2	5979
	11th to 20th	Day	55% 8% 9% 18% 10%	6322
	21st to end	Day	50% 9% 11% 19% 10% 1	6949
June	1st to 10th	Day	47% 7% 12% 19% 14% 1	2800
	11th to 20th	Day	46% 8% 13% 21% 11% 1	2800
	21st to end	Day	45% 7% 13% 22% 13%	2800
July	1st to 10th	Day	40% 7% 14% 26% 12% 1	6290
	11th to 20th	Day	40% 10% 15% 21% 13%	6285
	21st to end	Day	40% 8% 12% 25% 16%	6401
August	1st to 10th	Day	47% 8% 14% 20% 10%	4221
	11th to 20th	Day	49% 10% 15% 20% 5%	4213
	21st to end	Day	43% 8% 11% 29% 8% 1	4315
September	1st to 10th	Day	42% 9% 10% 26% 11% 2	2400
	11th to 20th	Day	39% 7% 11% 27% 15% 1	2397
	21st to end	Day	52% 6% 11% 19% 12% 1	2088
October	1st to 10th	Day	42% 8% 13% 21% 16% 1	3260
	11th to 20th	Day	48% 5% 13% 16% 14% 4	3220
	21st to end	Day	46% 5% 15% 24% 9% 1	3428
November	1st to 10th	Day	50% 7% 12% 17% 12% 2	1787
	11th to 20th	Day	58% 7% 11% 17% 5% 3	1605
	21st to end	Day	65% 8% 10% 13% 5% 1	1600
December	1st to 10th	Day	75% 7% 7% 9% 2	2600
	11th to 20th	Day	69% 5% 10% 10% 5%	2986
	21st to end	Day	72% 6% 8% 8% 5%	3960
January	1st to 10th	Night	73% 6% 7% 8% 5%	6349
	11th to 20th	Night	72% 7% 9% 8% 3	6086
	21st to end	Night	71% 6% 8% 8% 6% 1	6490
February	1st to 10th	Night	67% 6% 11% 11% 5%	2600
	11th to 20th	Night	66% 7% 11% 11% 5% 1	2535
	21st to end	Night	63% 6% 10% 13% 7% 1	2019
March	1st to 10th	Night	58% 7% 14% 14% 6% 1	5008
	11th to 20th	Night	52% 6% 16% 17% 6% 3	4980
	21st to end	Night	52% 7% 9% 16% 14% 2	5386
April	1st to 10th	Night	57% 6% 11% 16% 9% 1	2000
	11th to 20th	Night	52% 5% 12% 18% 12% 1	1975
	21st to end	Night	49% 6% 12% 18% 13% 2	1898
May	1st to 10th	Night	60% 6% 7% 17% 8% 2	3975
	11th to 20th	Night	51% 8% 14% 18% 9%	3576
	21st to end	Night	49% 8% 12% 18% 11% 2	3927
June	1st to 10th	Night	44% 7% 12% 23% 13% 1	1600
	11th to 20th	Night	40% 9% 15% 23% 12% 2	1600
	21st to end	Night	38% 8% 15% 25% 13% 1	1600
July	1st to 10th	Night	42% 7% 14% 24% 13% 1	3560
	11th to 20th	Night	37% 10% 16% 26% 11% 1	3552
	21st to end	Night	36% 7% 12% 26% 19% 1	4118
August	1st to 10th	Night	45% 8% 16% 22% 8% 1	2904
	11th to 20th	Night	42% 8% 15% 26% 8%	2902
	21st to end	Night	38% 9% 13% 31% 8% 1	3549
September	1st to 10th	Night	37% 6% 15% 27% 12% 2	2000
	11th to 20th	Night	36% 7% 12% 27% 16% 1	2000
	21st to end	Night	47% 7% 14% 19% 12% 1	2311
October	1st to 10th	Night	45% 7% 10% 23% 14% 1	3920
	11th to 20th	Night	44% 6% 12% 18% 13% 6%	3876
	21st to end	Night	44% 6% 16% 24% 9% 1	4120
November	1st to 10th	Night	48% 7% 12% 19% 12% 2	2508
	11th to 20th	Night	58% 6% 12% 15% 7% 2	2791
	21st to end	Night	62% 7% 13% 14% 4	2800
December	1st to 10th	Night	70% 10% 12% 7% 2	4580
	11th to 20th	Night	68% 6% 9% 12% 5%	5208
	21st to end	Night	71% 7% 8% 9% 5%	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	32% 32% 24% 10% 4	200
	11th to 20th	Day	33% 33% 28% 5% 1	257
	21st to end	Day	29% 34% 28% 7% 2	330
February	1st to 10th	Day	38% 32% 24% 6%	300
	11th to 20th	Day	37% 35% 26% 1 1	300
	21st to end	Day	38% 30% 25% 6% 1	374
March	1st to 10th	Day	32% 27% 27% 10% 2	500
	11th to 20th	Day	31% 30% 28% 9% 3	500
	21st to end	Day	36% 24% 26% 10% 4 1	550
April	1st to 10th	Day	35% 23% 26% 10% 4 1	500
	11th to 20th	Day	30% 28% 25% 10% 6% 2	492
	21st to end	Day	29% 22% 32% 12% 4 2	500
May	1st to 10th	Day	30% 21% 28% 13% 5% 3	503
	11th to 20th	Day	24% 20% 30% 17% 6% 3	527
	21st to end	Day	19% 17% 30% 22% 9% 4	573
June	1st to 10th	Day	20% 18% 32% 21% 7% 2	530
	11th to 20th	Day	18% 15% 32% 21% 11% 2	530
	21st to end	Day	13% 17% 38% 22% 6% 3	530
July	1st to 10th	Day	11% 11% 32% 29% 12% 4	525
	11th to 20th	Day	11% 11% 32% 29% 13% 5%	524
	21st to end	Day	12% 10% 38% 27% 10% 3	576
August	1st to 10th	Day	13% 10% 27% 28% 16% 6%	526
	11th to 20th	Day	15% 9% 30% 27% 13% 6%	519
	21st to end	Day	14% 12% 32% 22% 13% 7%	583
September	1st to 10th	Day	18% 13% 24% 23% 15% 7%	530
	11th to 20th	Day	12% 10% 24% 22% 22% 9%	530
	21st to end	Day	24% 15% 30% 16% 11% 4	530
October	1st to 10th	Day	20% 15% 28% 20% 14% 3	530
	11th to 20th	Day	26% 25% 28% 14% 6% 1	522
	21st to end	Day	32% 24% 24% 17% 2	539
November	1st to 10th	Day	27% 21% 29% 14% 7% 2	408
	11th to 20th	Day	35% 23% 28% 12% 2	205
	21st to end	Day	37% 23% 35% 6% 1	200
December	1st to 10th	Day	39% 26% 28% 4 4 1	200
	11th to 20th	Day	42% 24% 27% 7% 1	200
	21st to end	Day	32% 31% 30% 6% 1	220
January	1st to 10th	Night	27% 29% 31% 11% 2	800
	11th to 20th	Night	27% 29% 29% 13% 2	800
	21st to end	Night	25% 32% 28% 14% 1	880
February	1st to 10th	Night	28% 30% 29% 13% 1	800
	11th to 20th	Night	31% 28% 30% 10% 1	783
	21st to end	Night	31% 27% 29% 11% 2	660
March	1st to 10th	Night	27% 28% 30% 13% 2	800
	11th to 20th	Night	29% 25% 27% 15% 3 1	800
	21st to end	Night	25% 24% 31% 14% 5% 1	880
April	1st to 10th	Night	28% 21% 31% 18% 2	800
	11th to 20th	Night	27% 20% 30% 16% 6% 1	791
	21st to end	Night	21% 15% 33% 26% 5%	698
May	1st to 10th	Night	23% 14% 24% 29% 9% 1	567
	11th to 20th	Night	19% 12% 30% 30% 9%	394
	21st to end	Night	15% 11% 27% 31% 15% 1	433
June	1st to 10th	Night	16% 11% 32% 29% 13% 1	400
	11th to 20th	Night	15% 11% 26% 37% 12%	400
	21st to end	Night	10% 10% 32% 37% 11%	400
July	1st to 10th	Night	8% 7% 24% 42% 16% 2	390
	11th to 20th	Night	10% 7% 26% 36% 21% 1	388
	21st to end	Night	12% 5% 25% 36% 20% 2	578
August	1st to 10th	Night	11% 6% 23% 36% 22% 2	597
	11th to 20th	Night	12% 7% 22% 30% 26% 2	594
	21st to end	Night	15% 6% 24% 32% 22% 2	856
September	1st to 10th	Night	17% 8% 25% 30% 17% 3	800
	11th to 20th	Night	16% 7% 20% 26% 28% 3	800
	21st to end	Night	21% 13% 24% 23% 15% 3	800
October	1st to 10th	Night	20% 12% 27% 22% 17% 2	800
	11th to 20th	Night	24% 19% 30% 21% 6% 1	793
	21st to end	Night	32% 19% 30% 16% 3	836
November	1st to 10th	Night	20% 21% 31% 20% 7%	780
	11th to 20th	Night	29% 22% 29% 14% 4 2	800
	21st to end	Night	26% 27% 28% 15% 4	800
December	1st to 10th	Night	31% 28% 27% 10% 4	800
	11th to 20th	Night	29% 28% 28% 12% 2	800
	21st to end	Night	29% 28% 30% 11% 2	880

Page generated at 2025-10-16 01:50:29. Version ac07

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