Stillwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stillwell clear sky chart from 2005-11-06 to 2025-11-18.

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% 11% 4 9% 5% 5% 5% 4 5% 7% 32%	126212
	Night	16% 11% 4 9% 5% 4 4 3 5% 6% 33%	124146
Transparency	Day	51% 4 10% 21% 12% 1	129857
	Night	51% 3 10% 21% 12% 2	128557
Seeing	Day	22% 14% 28% 20% 11% 4	16076
	Night	22% 11% 27% 26% 13% 2	26354

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	15% 21% 5% 20% 6% 3 4 3 4 5% 15%	4056
	11th to 20th	Day	15% 22% 8% 14% 6% 5% 4 2 4 4 17%	4060
	21st to end	Day	22% 20% 8% 10% 8% 6% 4 3 4 3 13%	4466
February	1st to 10th	Day	19% 17% 5% 11% 5% 5% 4 3 3 4 24%	1800
	11th to 20th	Day	17% 13% 6% 13% 6% 4 5% 2 4 5% 24%	1862
	21st to end	Day	16% 16% 5% 10% 5% 4 4 2 3 5% 30%	1650
March	1st to 10th	Day	18% 15% 5% 11% 5% 4 4 3 4 5% 27%	4774
	11th to 20th	Day	11% 10% 4 8% 5% 4 4 3 4 6% 40%	4980
	21st to end	Day	20% 11% 4 8% 5% 5% 4 3 4 6% 31%	5465
April	1st to 10th	Day	16% 15% 3 8% 5% 4 4 3 5% 7% 31%	2201
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 3 5% 6% 34%	2564
	21st to end	Day	18% 9% 2 8% 6% 4 4 4 5% 5% 33%	2600
May	1st to 10th	Day	22% 10% 3 8% 6% 5% 5% 4 5% 6% 27%	5862
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 3 5% 7% 32%	5861
	21st to end	Day	18% 6% 2 6% 5% 5% 5% 4 7% 8% 33%	6433
June	1st to 10th	Day	12% 6% 3 7% 6% 5% 5% 5% 7% 7% 36%	2600
	11th to 20th	Day	12% 7% 3 8% 6% 6% 5% 5% 6% 8% 35%	2600
	21st to end	Day	11% 6% 2 5% 5% 6% 5% 5% 9% 10% 37%	2600
July	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 4 6% 9% 39%	5900
	11th to 20th	Day	11% 6% 3 7% 6% 6% 6% 5% 7% 10% 34%	5900
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	6424
August	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 10% 35%	4234
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 10% 35%	4174
	21st to end	Day	7% 4 2 6% 5% 6% 6% 4 8% 10% 42%	4620
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 8% 46%	2234
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 7% 8% 45%	2200
	21st to end	Day	20% 7% 3 6% 4 4 4 2 4 6% 39%	2200
October	1st to 10th	Day	13% 7% 2 7% 5% 4 4 3 5% 7% 43%	2099
	11th to 20th	Day	15% 9% 2 7% 4 4 5% 3 6% 6% 40%	2000
	21st to end	Day	20% 12% 3 6% 4 4 4 4 5% 5% 34%	2182
November	1st to 10th	Day	12% 8% 3 8% 4 3 4 4 5% 6% 43%	1852
	11th to 20th	Day	18% 14% 3 7% 5% 4 4 3 5% 5% 32%	1863
	21st to end	Day	18% 16% 4 10% 4 4 3 4 4 6% 27%	1730
December	1st to 10th	Day	20% 14% 5% 14% 5% 4 4 3 4 4 24%	2600
	11th to 20th	Day	23% 17% 6% 10% 5% 3 3 3 4 5% 21%	3100
	21st to end	Day	25% 22% 5% 11% 5% 4 4 3 3 4 14%	4466
January	1st to 10th	Night	16% 19% 8% 20% 5% 4 3 2 3 3 15%	5889
	11th to 20th	Night	14% 21% 8% 14% 8% 6% 5% 3 4 4 13%	5900
	21st to end	Night	22% 17% 6% 12% 5% 4 4 3 3 4 21%	6490
February	1st to 10th	Night	19% 16% 5% 11% 6% 4 4 3 5% 4 24%	2600
	11th to 20th	Night	18% 14% 4 11% 7% 5% 4 3 4 6% 23%	2420
	21st to end	Night	20% 15% 5% 10% 6% 4 4 3 4 4 25%	1980
March	1st to 10th	Night	21% 13% 4 7% 4 3 3 2 4 5% 32%	5065
	11th to 20th	Night	13% 10% 3 6% 4 3 4 4 6% 8% 39%	4980
	21st to end	Night	18% 10% 3 8% 4 4 4 3 4 5% 36%	5467
April	1st to 10th	Night	18% 12% 3 6% 5% 4 4 4 4 7% 34%	2197
	11th to 20th	Night	16% 8% 2 7% 5% 4 5% 4 6% 6% 37%	1802
	21st to end	Night	15% 10% 2 7% 5% 5% 4 3 6% 7% 38%	1800
May	1st to 10th	Night	27% 10% 3 7% 5% 4 4 3 5% 6% 26%	4033
	11th to 20th	Night	18% 8% 3 7% 4 4 4 3 5% 9% 37%	4032
	21st to end	Night	15% 6% 2 6% 5% 5% 5% 4 6% 9% 37%	4421
June	1st to 10th	Night	9% 6% 2 7% 5% 4 5% 4 5% 8% 45%	1800
	11th to 20th	Night	9% 7% 3 5% 4 5% 4 4 7% 10% 41%	1800
	21st to end	Night	10% 5% 2 5% 5% 4 5% 4 6% 9% 44%	1800
July	1st to 10th	Night	10% 6% 2 7% 4 4 4 4 7% 8% 43%	4058
	11th to 20th	Night	10% 5% 2 7% 5% 5% 5% 4 8% 9% 40%	4057
	21st to end	Night	7% 6% 3 7% 4 5% 5% 4 6% 9% 44%	4427
August	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 5% 8% 8% 37%	2915
	11th to 20th	Night	11% 8% 2 7% 4 4 4 3 5% 7% 44%	2875
	21st to end	Night	6% 4 2 6% 4 5% 6% 4 7% 9% 48%	3244
September	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 7% 54%	2162
	11th to 20th	Night	9% 4 2 4 3 3 4 3 5% 8% 57%	2200
	21st to end	Night	18% 7% 3 5% 4 3 4 3 5% 6% 44%	2200
October	1st to 10th	Night	12% 5% 2 5% 3 4 5% 3 6% 7% 48%	2300
	11th to 20th	Night	15% 8% 2 7% 5% 4 4 3 5% 6% 41%	2400
	21st to end	Night	22% 8% 3 6% 4 4 4 3 4 6% 36%	2655
November	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 45%	2666
	11th to 20th	Night	18% 11% 4 7% 4 4 4 3 5% 6% 34%	2692
	21st to end	Night	19% 13% 5% 8% 5% 4 5% 3 5% 5% 28%	2664
December	1st to 10th	Night	18% 15% 6% 12% 5% 4 4 4 5% 5% 23%	4580
	11th to 20th	Night	20% 19% 8% 11% 4 4 3 2 4 5% 21%	5085
	21st to end	Night	24% 21% 6% 10% 5% 4 4 2 4 5% 16%	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	74% 2 8% 12% 4	4057
	11th to 20th	Day	73% 2 8% 9% 9%	4060
	21st to end	Day	77% 2 6% 10% 5%	4466
February	1st to 10th	Day	66% 2 8% 16% 8% 1	1800
	11th to 20th	Day	63% 2 7% 17% 9% 1	1871
	21st to end	Day	60% 1 10% 19% 8% 2	1650
March	1st to 10th	Day	61% 2 7% 24% 6% 1	4774
	11th to 20th	Day	47% 2 9% 29% 11% 2	4980
	21st to end	Day	57% 2 8% 21% 12% 1	5465
April	1st to 10th	Day	55% 2 11% 18% 12% 2	2201
	11th to 20th	Day	51% 2 12% 18% 15% 2	2564
	21st to end	Day	53% 3 8% 21% 13% 2	2600
May	1st to 10th	Day	58% 4 9% 17% 10% 3	5897
	11th to 20th	Day	52% 3 10% 21% 12% 1	5865
	21st to end	Day	48% 4 11% 23% 12% 3	6447
June	1st to 10th	Day	45% 3 11% 25% 15% 1	2600
	11th to 20th	Day	47% 4 12% 23% 13% 1	2600
	21st to end	Day	41% 5% 14% 23% 15% 3	2600
July	1st to 10th	Day	40% 6% 11% 29% 14% 1	5835
	11th to 20th	Day	41% 8% 13% 23% 15% 1	5831
	21st to end	Day	40% 6% 11% 21% 19% 3	6240
August	1st to 10th	Day	43% 7% 14% 24% 13%	4221
	11th to 20th	Day	40% 6% 11% 30% 12% 1	4213
	21st to end	Day	35% 6% 13% 31% 12% 3	4620
September	1st to 10th	Day	37% 4 12% 24% 21% 3	2234
	11th to 20th	Day	36% 4 12% 28% 18% 3	2200
	21st to end	Day	48% 3 8% 21% 19% 1	2200
October	1st to 10th	Day	30% 4 10% 32% 23% 1	3411
	11th to 20th	Day	40% 5% 10% 24% 17% 4	3240
	21st to end	Day	40% 3 11% 26% 17% 2	3508
November	1st to 10th	Day	42% 3 10% 26% 17% 2	1852
	11th to 20th	Day	55% 2 8% 22% 10% 3	1863
	21st to end	Day	60% 3 10% 18% 8% 1	1730
December	1st to 10th	Day	64% 2 14% 15% 4	2600
	11th to 20th	Day	67% 2 10% 15% 6%	3096
	21st to end	Day	76% 2 8% 8% 4	4466
January	1st to 10th	Night	77% 1 6% 13% 4	5889
	11th to 20th	Night	77% 2 8% 9% 4	5900
	21st to end	Night	69% 2 10% 12% 5% 2	6490
February	1st to 10th	Night	64% 2 9% 17% 7% 1	2600
	11th to 20th	Night	64% 2 8% 17% 8% 1	2422
	21st to end	Night	64% 3 8% 15% 10% 2	1980
March	1st to 10th	Night	56% 2 8% 24% 10% 1	5065
	11th to 20th	Night	43% 3 11% 31% 10% 2	4980
	21st to end	Night	51% 2 8% 22% 15% 1	5467
April	1st to 10th	Night	51% 2 10% 20% 14% 2	2197
	11th to 20th	Night	48% 2 10% 18% 18% 3	1802
	21st to end	Night	47% 2 10% 22% 16% 2	1800
May	1st to 10th	Night	60% 4 8% 16% 8% 4	4060
	11th to 20th	Night	47% 3 10% 24% 14% 1	4033
	21st to end	Night	44% 4 13% 27% 11% 2	4429
June	1st to 10th	Night	38% 4 11% 25% 19% 3	1800
	11th to 20th	Night	38% 3 15% 25% 17% 3	1800
	21st to end	Night	36% 6% 13% 25% 18% 2	1800
July	1st to 10th	Night	36% 6% 12% 24% 18% 4	4015
	11th to 20th	Night	37% 8% 15% 26% 12% 1	4006
	21st to end	Night	36% 6% 12% 22% 21% 2	4297
August	1st to 10th	Night	41% 6% 14% 24% 14% 1	2904
	11th to 20th	Night	40% 5% 9% 27% 18% 1	2902
	21st to end	Night	31% 6% 14% 32% 15% 2	3244
September	1st to 10th	Night	33% 3 11% 28% 20% 6%	2162
	11th to 20th	Night	27% 4 11% 31% 26% 2	2200
	21st to end	Night	43% 3 9% 22% 22% 1	2200
October	1st to 10th	Night	29% 3 7% 30% 29% 1	3755
	11th to 20th	Night	35% 4 12% 26% 19% 5%	3899
	21st to end	Night	35% 3 9% 30% 21% 2	4279
November	1st to 10th	Night	41% 2 11% 26% 18% 3	2666
	11th to 20th	Night	51% 3 8% 23% 13% 3	2692
	21st to end	Night	59% 2 11% 22% 6% 1	2664
December	1st to 10th	Night	62% 4 10% 18% 5%	4580
	11th to 20th	Night	67% 2 12% 14% 3	5088
	21st to end	Night	73% 2 9% 13% 3	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	32% 26% 29% 8% 4 1	300
	11th to 20th	Day	39% 22% 32% 6% 1	300
	21st to end	Day	31% 29% 29% 10% 1	330
February	1st to 10th	Day	31% 29% 29% 7% 4	300
	11th to 20th	Day	32% 32% 30% 5% 1	300
	21st to end	Day	34% 31% 31% 4	249
March	1st to 10th	Day	28% 24% 32% 14% 2	425
	11th to 20th	Day	32% 20% 30% 12% 5% 1	500
	21st to end	Day	36% 17% 29% 15% 3	550
April	1st to 10th	Day	29% 19% 30% 14% 6% 2	500
	11th to 20th	Day	29% 20% 26% 15% 7% 3	492
	21st to end	Day	27% 17% 32% 15% 7% 2	500
May	1st to 10th	Day	30% 18% 27% 16% 6% 3	503
	11th to 20th	Day	25% 11% 31% 20% 9% 4	527
	21st to end	Day	15% 13% 27% 23% 16% 6%	573
June	1st to 10th	Day	17% 10% 32% 25% 12% 4	530
	11th to 20th	Day	17% 11% 26% 28% 13% 5%	530
	21st to end	Day	13% 10% 31% 27% 13% 6%	530
July	1st to 10th	Day	16% 6% 20% 27% 22% 11%	525
	11th to 20th	Day	14% 5% 26% 28% 18% 9%	524
	21st to end	Day	11% 6% 25% 30% 20% 9%	576
August	1st to 10th	Day	13% 5% 21% 31% 21% 10%	526
	11th to 20th	Day	13% 7% 24% 29% 18% 8%	519
	21st to end	Day	8% 5% 22% 31% 24% 9%	583
September	1st to 10th	Day	13% 6% 22% 26% 23% 9%	530
	11th to 20th	Day	9% 3 16% 30% 29% 12%	530
	21st to end	Day	24% 9% 22% 23% 16% 5%	530
October	1st to 10th	Day	13% 9% 28% 27% 19% 5%	530
	11th to 20th	Day	19% 17% 33% 20% 9% 2	530
	21st to end	Day	30% 14% 32% 17% 7% 1	565
November	1st to 10th	Day	18% 19% 36% 16% 8% 2	330
	11th to 20th	Day	33% 16% 31% 14% 5% 1	321
	21st to end	Day	31% 23% 35% 9% 2	265
December	1st to 10th	Day	27% 27% 32% 10% 3 2	200
	11th to 20th	Day	29% 25% 32% 11% 2	223
	21st to end	Day	35% 20% 30% 12% 2	330
January	1st to 10th	Night	29% 20% 32% 16% 3	800
	11th to 20th	Night	31% 20% 32% 16% 3	800
	21st to end	Night	30% 20% 30% 18% 3	880
February	1st to 10th	Night	30% 18% 32% 18% 2	800
	11th to 20th	Night	31% 19% 32% 17% 2	783
	21st to end	Night	31% 18% 32% 16% 2	660
March	1st to 10th	Night	26% 18% 31% 22% 4	800
	11th to 20th	Night	26% 18% 30% 20% 6%	800
	21st to end	Night	28% 14% 32% 19% 6%	880
April	1st to 10th	Night	26% 14% 34% 20% 5% 1	800
	11th to 20th	Night	23% 12% 31% 22% 10% 1	618
	21st to end	Night	21% 11% 36% 25% 6% 1	600
May	1st to 10th	Night	24% 9% 28% 29% 10% 1	600
	11th to 20th	Night	22% 7% 25% 31% 15% 1	591
	21st to end	Night	11% 5% 23% 37% 22% 2	650
June	1st to 10th	Night	12% 4 27% 38% 19% 1	600
	11th to 20th	Night	13% 7% 21% 38% 20% 2	600
	21st to end	Night	12% 4 22% 37% 24% 1	600
July	1st to 10th	Night	10% 3 15% 38% 28% 6%	585
	11th to 20th	Night	9% 3 20% 34% 29% 5%	582
	21st to end	Night	11% 3 16% 37% 31% 3	657
August	1st to 10th	Night	8% 2 13% 43% 30% 4	597
	11th to 20th	Night	12% 2 15% 40% 27% 5%	594
	21st to end	Night	9% 3 18% 32% 32% 6%	668
September	1st to 10th	Night	15% 4 18% 31% 27% 5%	800
	11th to 20th	Night	11% 3 13% 36% 32% 5%	800
	21st to end	Night	20% 7% 23% 29% 19% 3	800
October	1st to 10th	Night	15% 6% 23% 33% 21% 3	800
	11th to 20th	Night	20% 10% 32% 26% 12% 1	800
	21st to end	Night	27% 11% 29% 25% 8%	880
November	1st to 10th	Night	19% 10% 30% 28% 13% 1	820
	11th to 20th	Night	27% 15% 28% 20% 8% 1	829
	21st to end	Night	28% 15% 29% 22% 6% 1	800
December	1st to 10th	Night	34% 13% 32% 17% 4	800
	11th to 20th	Night	28% 17% 31% 21% 4	800
	21st to end	Night	34% 14% 29% 20% 3	880

Page generated at 2025-11-15 00:55:57. Version ac07

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