Warren Rupp Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Warren Rupp Observatory 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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	126660
	Night	15% 12% 4 9% 5% 4 4 3 5% 6% 32%	123340
Transparency	Day	52% 3 10% 21% 12% 1	130295
	Night	52% 3 10% 21% 12% 2	127775
Seeing	Day	21% 15% 28% 20% 11% 4	16346
	Night	20% 13% 28% 26% 12% 2	26244

By Month

Percentage Cloud Cover SE (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	14% 19% 8% 19% 7% 5% 4 3 4 5% 12%	4056
	11th to 20th	Day	12% 22% 8% 13% 7% 6% 6% 3 4 4 15%	4060
	21st to end	Day	21% 19% 9% 13% 7% 5% 5% 3 3 3 13%	4466
February	1st to 10th	Day	18% 17% 6% 10% 7% 6% 3 2 4 4 23%	1800
	11th to 20th	Day	15% 14% 6% 13% 7% 5% 5% 3 5% 5% 23%	1880
	21st to end	Day	16% 15% 5% 11% 5% 4 4 3 5% 5% 27%	1650
March	1st to 10th	Day	17% 13% 8% 12% 5% 4 4 3 4 5% 26%	4742
	11th to 20th	Day	11% 11% 4 9% 4 3 4 3 5% 6% 38%	4980
	21st to end	Day	20% 12% 4 9% 5% 4 4 3 4 6% 28%	5465
April	1st to 10th	Day	15% 15% 4 8% 5% 5% 4 3 5% 6% 30%	2222
	11th to 20th	Day	16% 12% 4 8% 5% 4 4 3 5% 6% 34%	2558
	21st to end	Day	19% 10% 2 7% 5% 4 5% 3 4 7% 33%	2600
May	1st to 10th	Day	22% 10% 3 8% 6% 5% 4 4 5% 5% 28%	5862
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 6% 8% 32%	5861
	21st to end	Day	17% 6% 2 7% 5% 5% 5% 5% 6% 8% 33%	6433
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	2600
	11th to 20th	Day	11% 9% 3 7% 6% 5% 5% 4 6% 9% 34%	2607
	21st to end	Day	11% 5% 2 6% 6% 6% 5% 5% 8% 9% 38%	2800
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 4 6% 9% 40%	6140
	11th to 20th	Day	10% 6% 2 8% 5% 5% 6% 6% 7% 11% 34%	5900
	21st to end	Day	11% 6% 2 7% 6% 5% 6% 5% 7% 9% 37%	6424
August	1st to 10th	Day	11% 7% 3 7% 5% 6% 5% 4 6% 9% 37%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 7% 6% 8% 10% 35%	4174
	21st to end	Day	7% 6% 2 7% 5% 5% 7% 4 7% 10% 40%	4652
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 5% 7% 46%	2210
	11th to 20th	Day	8% 6% 3 6% 5% 4 4 5% 6% 8% 44%	2200
	21st to end	Day	20% 8% 3 6% 4 4 4 3 4 6% 40%	2200
October	1st to 10th	Day	14% 7% 2 6% 4 3 4 4 5% 8% 43%	2114
	11th to 20th	Day	15% 9% 3 7% 4 4 4 4 5% 7% 39%	2000
	21st to end	Day	19% 12% 3 7% 5% 4 3 3 6% 7% 31%	2154
November	1st to 10th	Day	11% 9% 3 6% 4 4 4 3 5% 7% 42%	1780
	11th to 20th	Day	17% 14% 5% 8% 5% 4 4 3 4 6% 31%	1800
	21st to end	Day	18% 17% 5% 9% 4 4 4 3 4 5% 26%	1778
December	1st to 10th	Day	18% 14% 5% 16% 5% 4 4 3 4 5% 22%	2600
	11th to 20th	Day	21% 18% 7% 11% 4 4 4 3 4 5% 19%	3192
	21st to end	Day	24% 24% 8% 11% 5% 3 4 2 3 3 14%	4466
January	1st to 10th	Night	14% 20% 7% 21% 6% 4 3 2 3 4 14%	5889
	11th to 20th	Night	11% 19% 10% 16% 8% 8% 6% 4 3 4 12%	5900
	21st to end	Night	20% 17% 6% 13% 6% 4 4 2 4 4 21%	6490
February	1st to 10th	Night	18% 14% 5% 12% 6% 5% 4 3 5% 5% 22%	2600
	11th to 20th	Night	16% 14% 5% 11% 7% 5% 5% 3 5% 5% 24%	2403
	21st to end	Night	18% 15% 5% 11% 5% 5% 4 3 4 5% 25%	1980
March	1st to 10th	Night	19% 13% 5% 8% 4 4 3 3 4 4 33%	5110
	11th to 20th	Night	11% 8% 4 7% 4 4 4 4 6% 8% 40%	4980
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 5% 5% 36%	5467
April	1st to 10th	Night	17% 12% 3 7% 5% 4 4 4 5% 8% 32%	2195
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 5% 7% 37%	1789
	21st to end	Night	14% 10% 3 6% 5% 4 5% 3 6% 8% 36%	1800
May	1st to 10th	Night	23% 12% 3 7% 5% 5% 4 3 5% 6% 26%	4033
	11th to 20th	Night	17% 9% 3 7% 3 3 4 3 6% 8% 37%	4032
	21st to end	Night	14% 7% 2 7% 4 4 5% 4 7% 8% 39%	4421
June	1st to 10th	Night	9% 7% 2 6% 6% 4 4 4 5% 8% 45%	1800
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 8% 9% 41%	1782
	21st to end	Night	9% 5% 2 7% 4 4 4 5% 6% 10% 44%	1600
July	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 7% 9% 44%	3794
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 42%	4057
	21st to end	Night	8% 6% 3 7% 4 4 5% 4 5% 9% 44%	4427
August	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 5% 6% 9% 38%	2915
	11th to 20th	Night	10% 9% 3 7% 4 4 4 4 5% 7% 44%	2875
	21st to end	Night	6% 4 2 6% 4 4 5% 4 7% 9% 50%	3223
September	1st to 10th	Night	11% 4 2 5% 3 4 4 3 4 6% 53%	2175
	11th to 20th	Night	9% 4 1 4 3 3 4 4 5% 7% 56%	2200
	21st to end	Night	17% 7% 2 6% 4 4 3 3 6% 7% 43%	2200
October	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2285
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 7% 41%	2400
	21st to end	Night	20% 9% 2 6% 4 4 4 3 4 6% 36%	2684
November	1st to 10th	Night	13% 8% 2 6% 5% 4 4 3 5% 7% 44%	2563
	11th to 20th	Night	16% 13% 3 8% 5% 4 4 3 5% 6% 33%	2600
	21st to end	Night	19% 14% 5% 9% 4 4 4 3 6% 6% 27%	2613
December	1st to 10th	Night	16% 15% 6% 13% 5% 5% 4 4 5% 5% 22%	4580
	11th to 20th	Night	18% 19% 7% 12% 5% 3 4 3 4 5% 20%	4988
	21st to end	Night	23% 20% 8% 10% 5% 3 3 3 4 5% 15%	6490

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	75% 2 7% 12% 4	4057
	11th to 20th	Day	75% 2 8% 9% 7%	4060
	21st to end	Day	78% 2 5% 10% 5% 1	4466
February	1st to 10th	Day	67% 2 8% 15% 7% 1	1800
	11th to 20th	Day	64% 4 7% 16% 8% 1	1889
	21st to end	Day	60% 1 12% 18% 8% 2	1650
March	1st to 10th	Day	63% 2 6% 23% 7%	4742
	11th to 20th	Day	48% 2 9% 29% 9% 2	4980
	21st to end	Day	59% 2 8% 21% 10% 1	5465
April	1st to 10th	Day	56% 3 11% 17% 12% 1	2222
	11th to 20th	Day	52% 3 12% 18% 14% 2	2558
	21st to end	Day	52% 3 10% 22% 12% 2	2600
May	1st to 10th	Day	58% 3 9% 17% 11% 2	5897
	11th to 20th	Day	51% 4 12% 23% 10% 1	5865
	21st to end	Day	47% 4 11% 23% 11% 3	6447
June	1st to 10th	Day	44% 4 11% 26% 15% 1	2600
	11th to 20th	Day	46% 4 13% 22% 13% 1	2607
	21st to end	Day	42% 5% 14% 23% 15% 2	2800
July	1st to 10th	Day	40% 5% 10% 29% 14% 2	6075
	11th to 20th	Day	42% 6% 14% 23% 15% 1	5831
	21st to end	Day	41% 5% 12% 20% 19% 2	6240
August	1st to 10th	Day	43% 5% 14% 24% 14%	4221
	11th to 20th	Day	40% 6% 11% 30% 12% 1	4213
	21st to end	Day	38% 5% 13% 29% 12% 3	4652
September	1st to 10th	Day	38% 5% 11% 23% 19% 4	2210
	11th to 20th	Day	37% 3 11% 28% 18% 3	2200
	21st to end	Day	46% 3 9% 21% 20% 1	2200
October	1st to 10th	Day	28% 3 11% 33% 23% 1	3439
	11th to 20th	Day	41% 4 8% 25% 19% 3	3240
	21st to end	Day	41% 2 12% 25% 18% 1	3456
November	1st to 10th	Day	42% 3 11% 26% 16% 2	1780
	11th to 20th	Day	56% 3 8% 21% 10% 2	1800
	21st to end	Day	62% 3 9% 17% 8% 1	1778
December	1st to 10th	Day	66% 2 14% 14% 4	2600
	11th to 20th	Day	69% 2 9% 15% 5%	3189
	21st to end	Day	78% 2 7% 8% 5%	4466
January	1st to 10th	Night	76% 1 5% 13% 4	5889
	11th to 20th	Night	79% 1 7% 8% 5%	5900
	21st to end	Night	69% 2 10% 12% 5% 2	6490
February	1st to 10th	Night	65% 3 9% 16% 7% 1	2600
	11th to 20th	Night	62% 3 8% 17% 8% 1	2405
	21st to end	Night	63% 3 8% 15% 9% 2	1980
March	1st to 10th	Night	57% 2 8% 23% 10% 1	5110
	11th to 20th	Night	43% 3 12% 31% 10% 2	4980
	21st to end	Night	51% 2 8% 23% 15% 2	5467
April	1st to 10th	Night	51% 4 11% 19% 14% 2	2195
	11th to 20th	Night	48% 3 11% 18% 17% 3	1789
	21st to end	Night	46% 4 10% 23% 15% 2	1800
May	1st to 10th	Night	60% 3 8% 16% 10% 3	4060
	11th to 20th	Night	47% 3 11% 24% 14% 1	4033
	21st to end	Night	43% 4 13% 27% 12% 2	4429
June	1st to 10th	Night	38% 5% 12% 24% 19% 2	1800
	11th to 20th	Night	37% 5% 15% 24% 17% 2	1782
	21st to end	Night	35% 6% 14% 26% 17% 3	1600
July	1st to 10th	Night	36% 5% 12% 26% 17% 4	3752
	11th to 20th	Night	37% 5% 15% 28% 13% 1	4006
	21st to end	Night	37% 5% 13% 22% 22% 2	4297
August	1st to 10th	Night	42% 6% 13% 24% 14% 1	2904
	11th to 20th	Night	40% 4 9% 29% 17% 1	2902
	21st to end	Night	31% 5% 15% 32% 15% 2	3223
September	1st to 10th	Night	33% 4 11% 28% 18% 5%	2175
	11th to 20th	Night	28% 4 10% 31% 25% 2	2200
	21st to end	Night	41% 3 10% 22% 22% 2	2200
October	1st to 10th	Night	29% 3 7% 30% 30% 2	3740
	11th to 20th	Night	36% 3 10% 26% 21% 4	3899
	21st to end	Night	37% 3 8% 30% 18% 3	4332
November	1st to 10th	Night	41% 3 11% 25% 17% 3	2563
	11th to 20th	Night	53% 3 8% 21% 12% 3	2600
	21st to end	Night	59% 2 12% 19% 7% 1	2613
December	1st to 10th	Night	64% 2 11% 17% 5%	4580
	11th to 20th	Night	68% 2 12% 14% 4	4990
	21st to end	Night	74% 2 9% 12% 4	6490

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	31% 27% 30% 8% 4	300
	11th to 20th	Day	33% 28% 31% 6% 2	300
	21st to end	Day	31% 30% 28% 9% 1	330
February	1st to 10th	Day	35% 26% 26% 11% 2	300
	11th to 20th	Day	34% 30% 29% 7%	300
	21st to end	Day	30% 37% 27% 6%	249
March	1st to 10th	Day	29% 25% 31% 13% 2	406
	11th to 20th	Day	30% 22% 31% 12% 5% 1	500
	21st to end	Day	34% 18% 28% 16% 4	550
April	1st to 10th	Day	27% 20% 31% 14% 6% 1	500
	11th to 20th	Day	26% 24% 26% 14% 7% 3	492
	21st to end	Day	27% 17% 33% 15% 5% 2	500
May	1st to 10th	Day	27% 17% 30% 16% 6% 3	503
	11th to 20th	Day	24% 11% 32% 19% 10% 4	527
	21st to end	Day	16% 12% 26% 25% 14% 7%	573
June	1st to 10th	Day	16% 8% 33% 27% 10% 5%	530
	11th to 20th	Day	17% 9% 28% 28% 15% 3	537
	21st to end	Day	14% 11% 31% 27% 13% 4	730
July	1st to 10th	Day	14% 5% 20% 28% 20% 12%	635
	11th to 20th	Day	13% 5% 28% 27% 18% 10%	524
	21st to end	Day	11% 5% 23% 32% 20% 9%	576
August	1st to 10th	Day	11% 6% 21% 31% 20% 11%	526
	11th to 20th	Day	14% 5% 26% 27% 19% 9%	519
	21st to end	Day	9% 6% 23% 31% 21% 10%	583
September	1st to 10th	Day	13% 6% 24% 24% 22% 10%	530
	11th to 20th	Day	9% 4 16% 31% 28% 12%	530
	21st to end	Day	25% 9% 22% 24% 14% 7%	530
October	1st to 10th	Day	13% 12% 28% 24% 18% 5%	530
	11th to 20th	Day	20% 17% 30% 20% 10% 2	530
	21st to end	Day	28% 18% 32% 17% 4 1	537
November	1st to 10th	Day	17% 24% 36% 14% 9% 1	306
	11th to 20th	Day	30% 19% 29% 15% 5% 1	300
	21st to end	Day	30% 25% 34% 10% 1	288
December	1st to 10th	Day	26% 31% 29% 9% 4 2	200
	11th to 20th	Day	29% 26% 33% 9% 3	245
	21st to end	Day	35% 21% 32% 10% 2	330
January	1st to 10th	Night	27% 24% 32% 15% 2	800
	11th to 20th	Night	29% 24% 31% 13% 3	800
	21st to end	Night	25% 24% 31% 17% 3	880
February	1st to 10th	Night	28% 20% 35% 16% 2	800
	11th to 20th	Night	26% 23% 31% 17% 2	783
	21st to end	Night	28% 20% 35% 15% 2	660
March	1st to 10th	Night	25% 19% 32% 21% 4	800
	11th to 20th	Night	23% 19% 33% 19% 6% 1	800
	21st to end	Night	26% 15% 33% 20% 6%	880
April	1st to 10th	Night	24% 16% 33% 22% 5%	795
	11th to 20th	Night	21% 14% 34% 21% 8% 2	593
	21st to end	Night	19% 11% 39% 24% 6% 1	600
May	1st to 10th	Night	22% 10% 31% 28% 9% 1	600
	11th to 20th	Night	23% 7% 29% 27% 13% 2	591
	21st to end	Night	12% 4 26% 34% 22% 2	650
June	1st to 10th	Night	13% 4 28% 39% 16% 1	600
	11th to 20th	Night	13% 4 25% 40% 17% 2	600
	21st to end	Night	11% 5% 27% 36% 20% 2	600
July	1st to 10th	Night	11% 2 18% 39% 24% 5%	585
	11th to 20th	Night	8% 3 21% 38% 26% 4	582
	21st to end	Night	9% 4 16% 41% 28% 2	657
August	1st to 10th	Night	9% 4 13% 44% 25% 4	597
	11th to 20th	Night	11% 2 16% 39% 28% 4	594
	21st to end	Night	8% 3 18% 36% 29% 5%	660
September	1st to 10th	Night	14% 5% 20% 31% 26% 4	788
	11th to 20th	Night	10% 3 14% 38% 30% 5%	800
	21st to end	Night	20% 8% 21% 30% 19% 4	800
October	1st to 10th	Night	15% 6% 26% 31% 19% 3	800
	11th to 20th	Night	20% 14% 30% 25% 10% 2	800
	21st to end	Night	24% 15% 31% 23% 7%	880
November	1st to 10th	Night	18% 14% 30% 25% 12% 1	789
	11th to 20th	Night	25% 17% 30% 21% 6% 1	800
	21st to end	Night	27% 18% 29% 20% 5% 1	800
December	1st to 10th	Night	31% 17% 32% 17% 3	800
	11th to 20th	Night	27% 19% 32% 18% 3	800
	21st to end	Night	29% 18% 32% 18% 3	880

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