Stellafane
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stellafane 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	16% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	126671
	Night	16% 9% 3 8% 5% 5% 4 4 5% 6% 35%	123376
Transparency	Day	46% 7% 12% 21% 13% 2	130165
	Night	45% 7% 12% 21% 13% 2	127969
Seeing	Day	23% 18% 28% 19% 9% 3	16132
	Night	21% 14% 28% 27% 9% 1	25047

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	10% 8% 5% 16% 8% 5% 5% 4 5% 5% 28%	3597
	11th to 20th	Day	11% 10% 5% 14% 7% 5% 5% 4 4 5% 31%	3570
	21st to end	Day	12% 10% 5% 9% 5% 5% 5% 3 5% 5% 35%	3960
February	1st to 10th	Day	13% 11% 4 10% 5% 6% 5% 4 6% 7% 30%	1799
	11th to 20th	Day	10% 10% 4 11% 6% 6% 5% 5% 5% 6% 34%	1948
	21st to end	Day	16% 11% 3 10% 6% 5% 4 4 5% 5% 32%	1650
March	1st to 10th	Day	18% 11% 4 12% 6% 4 4 4 4 4 30%	4476
	11th to 20th	Day	15% 10% 4 7% 4 5% 5% 4 5% 6% 37%	4648
	21st to end	Day	22% 10% 2 7% 5% 4 3 3 5% 6% 33%	5965
April	1st to 10th	Day	18% 10% 3 8% 5% 5% 3 3 4 6% 35%	2400
	11th to 20th	Day	17% 9% 3 6% 5% 4 4 5% 5% 7% 36%	2374
	21st to end	Day	20% 8% 4 8% 5% 4 4 4 5% 6% 33%	2570
May	1st to 10th	Day	31% 9% 4 7% 5% 4 4 3 5% 5% 23%	5886
	11th to 20th	Day	18% 10% 4 9% 5% 5% 4 4 5% 7% 29%	6271
	21st to end	Day	18% 7% 3 6% 4 5% 5% 4 6% 8% 33%	6949
June	1st to 10th	Day	18% 7% 2 6% 6% 6% 6% 4 6% 8% 31%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 5% 6% 8% 36%	2800
	21st to end	Day	17% 7% 3 7% 6% 5% 5% 5% 5% 7% 33%	2800
July	1st to 10th	Day	11% 6% 3 7% 5% 6% 6% 5% 7% 9% 34%	6360
	11th to 20th	Day	8% 5% 3 6% 5% 6% 6% 5% 8% 11% 37%	6360
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 42%	6926
August	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 35%	4466
	11th to 20th	Day	13% 7% 3 8% 5% 5% 6% 5% 6% 9% 35%	4020
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 11% 42%	4312
September	1st to 10th	Day	12% 4 2 6% 4 5% 4 3 7% 8% 46%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 6% 49%	2254
	21st to end	Day	16% 5% 2 6% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Day	20% 7% 4 7% 4 4 4 4 4 5% 38%	2170
	11th to 20th	Day	15% 8% 2 7% 3 5% 4 4 5% 9% 39%	1935
	21st to end	Day	21% 9% 3 8% 4 4 4 3 4 6% 33%	1980
November	1st to 10th	Day	12% 7% 2 7% 4 4 4 3 5% 8% 43%	1779
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 5% 5% 40%	1800
	21st to end	Day	18% 9% 3 7% 4 4 5% 4 6% 7% 34%	1700
December	1st to 10th	Day	14% 11% 5% 10% 6% 4 5% 3 5% 6% 29%	2600
	11th to 20th	Day	20% 9% 3 8% 5% 4 3 3 4 5% 34%	2986
	21st to end	Day	18% 12% 4 9% 4 5% 4 3 4 5% 32%	3960
January	1st to 10th	Night	12% 7% 5% 15% 7% 5% 5% 4 5% 5% 31%	6349
	11th to 20th	Night	10% 12% 6% 12% 7% 6% 5% 4 5% 6% 28%	6315
	21st to end	Night	13% 9% 4 9% 5% 5% 6% 5% 6% 6% 32%	6996
February	1st to 10th	Night	12% 12% 3 9% 6% 5% 5% 5% 6% 6% 31%	2598
	11th to 20th	Night	10% 9% 4 8% 7% 6% 5% 4 7% 8% 33%	2338
	21st to end	Night	14% 9% 3 8% 4 5% 5% 5% 6% 7% 34%	1980
March	1st to 10th	Night	20% 10% 3 7% 5% 4 4 4 5% 6% 31%	5364
	11th to 20th	Night	13% 11% 3 8% 5% 5% 5% 3 5% 6% 36%	5311
	21st to end	Night	21% 10% 3 8% 5% 3 4 3 5% 5% 32%	4963
April	1st to 10th	Night	18% 8% 3 6% 4 5% 5% 4 5% 7% 35%	2000
	11th to 20th	Night	18% 9% 3 7% 4 3 4 4 5% 5% 38%	1975
	21st to end	Night	19% 8% 3 6% 5% 4 3 3 5% 6% 38%	1828
May	1st to 10th	Night	32% 9% 2 6% 4 4 4 3 5% 6% 24%	4054
	11th to 20th	Night	16% 10% 4 10% 6% 5% 5% 3 5% 7% 30%	3651
	21st to end	Night	20% 7% 2 8% 5% 5% 4 3 6% 7% 33%	3926
June	1st to 10th	Night	19% 7% 2 6% 4 4 5% 3 7% 6% 38%	1600
	11th to 20th	Night	18% 6% 2 7% 3 4 3 4 6% 7% 40%	1600
	21st to end	Night	13% 7% 2 7% 5% 5% 6% 5% 7% 5% 39%	1600
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 5% 5% 6% 9% 42%	3600
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 5% 7% 8% 38%	3600
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 6% 49%	3926
August	1st to 10th	Night	18% 7% 3 8% 4 5% 4 4 5% 6% 35%	2652
	11th to 20th	Night	13% 8% 2 8% 4 5% 4 3 5% 7% 40%	3023
	21st to end	Night	9% 4 2 4 4 4 5% 4 5% 9% 50%	3586
September	1st to 10th	Night	11% 6% 2 5% 4 4 4 2 6% 7% 51%	2000
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 5% 6% 53%	2145
	21st to end	Night	16% 5% 2 5% 3 4 4 3 5% 7% 46%	2200
October	1st to 10th	Night	19% 7% 2 7% 4 4 4 3 5% 5% 40%	2228
	11th to 20th	Night	15% 6% 2 6% 4 4 5% 4 6% 6% 41%	2465
	21st to end	Night	21% 8% 3 7% 5% 5% 5% 4 5% 5% 34%	2860
November	1st to 10th	Night	13% 7% 2 6% 5% 4 4 3 5% 6% 44%	2564
	11th to 20th	Night	15% 7% 3 6% 5% 5% 4 3 5% 6% 41%	2600
	21st to end	Night	14% 9% 3 7% 5% 4 5% 4 5% 6% 38%	2695
December	1st to 10th	Night	14% 12% 4 9% 7% 6% 5% 3 6% 7% 28%	4580
	11th to 20th	Night	22% 10% 3 8% 5% 4 3 3 4 6% 32%	5208
	21st to end	Night	20% 13% 5% 9% 4 4 3 3 5% 6% 28%	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	51% 8% 9% 23% 8%	3598
	11th to 20th	Day	51% 8% 9% 17% 13% 2	3600
	21st to end	Day	47% 7% 8% 21% 15% 3	3960
February	1st to 10th	Day	52% 7% 13% 17% 10% 2	1799
	11th to 20th	Day	49% 7% 11% 18% 12% 3	1957
	21st to end	Day	53% 5% 10% 17% 13% 2	1650
March	1st to 10th	Day	56% 5% 9% 12% 10% 8%	4479
	11th to 20th	Day	45% 8% 15% 19% 10% 3	4648
	21st to end	Day	51% 4 6% 15% 18% 5%	5965
April	1st to 10th	Day	51% 5% 9% 20% 12% 3	2400
	11th to 20th	Day	46% 7% 10% 18% 16% 2	2374
	21st to end	Day	51% 6% 8% 17% 14% 3	2570
May	1st to 10th	Day	62% 8% 9% 14% 7% 1	5897
	11th to 20th	Day	54% 8% 13% 16% 9% 1	6275
	21st to end	Day	46% 8% 13% 22% 11% 1	6949
June	1st to 10th	Day	49% 7% 12% 20% 11% 1	2800
	11th to 20th	Day	45% 8% 13% 18% 15% 2	2800
	21st to end	Day	47% 9% 13% 20% 11% 1	2800
July	1st to 10th	Day	43% 8% 15% 20% 14%	6290
	11th to 20th	Day	37% 9% 17% 23% 13% 1	6285
	21st to end	Day	39% 6% 14% 27% 14% 1	6728
August	1st to 10th	Day	43% 8% 16% 21% 12% 1	4452
	11th to 20th	Day	42% 9% 15% 26% 7% 1	4059
	21st to end	Day	32% 9% 18% 27% 14% 1	4312
September	1st to 10th	Day	35% 8% 12% 27% 17% 1	2400
	11th to 20th	Day	35% 6% 11% 26% 21% 2	2254
	21st to end	Day	41% 6% 12% 24% 15% 2	2200
October	1st to 10th	Day	44% 8% 8% 22% 16% 2	3534
	11th to 20th	Day	38% 6% 11% 27% 16% 1	3125
	21st to end	Day	39% 5% 10% 31% 13% 3	3180
November	1st to 10th	Day	39% 7% 14% 22% 16% 1	1779
	11th to 20th	Day	44% 7% 10% 18% 18% 2	1800
	21st to end	Day	48% 6% 16% 22% 8% 1	1700
December	1st to 10th	Day	53% 7% 12% 22% 6% 1	2600
	11th to 20th	Day	50% 6% 10% 18% 14% 2	2986
	21st to end	Day	53% 5% 8% 17% 14% 3	3960
January	1st to 10th	Night	48% 10% 12% 19% 10% 1	6349
	11th to 20th	Night	53% 8% 11% 16% 11% 2	6360
	21st to end	Night	48% 7% 10% 15% 16% 4	6996
February	1st to 10th	Night	49% 8% 13% 19% 11% 1	2598
	11th to 20th	Night	45% 7% 12% 21% 13% 2	2340
	21st to end	Night	46% 7% 11% 23% 9% 4	1980
March	1st to 10th	Night	50% 6% 10% 16% 14% 4	5374
	11th to 20th	Night	46% 8% 16% 18% 11% 2	5312
	21st to end	Night	51% 6% 6% 13% 20% 3	4963
April	1st to 10th	Night	46% 6% 11% 20% 15% 2	2000
	11th to 20th	Night	46% 7% 9% 21% 16% 1	1975
	21st to end	Night	45% 7% 9% 19% 16% 3	1828
May	1st to 10th	Night	60% 8% 10% 14% 8% 1	4060
	11th to 20th	Night	52% 10% 12% 16% 8% 1	3652
	21st to end	Night	49% 9% 13% 17% 11% 1	3927
June	1st to 10th	Night	44% 7% 13% 23% 12% 1	1600
	11th to 20th	Night	43% 9% 15% 20% 12% 1	1600
	21st to end	Night	42% 9% 13% 23% 11% 1	1600
July	1st to 10th	Night	37% 8% 17% 25% 12% 1	3560
	11th to 20th	Night	40% 10% 14% 20% 14%	3552
	21st to end	Night	34% 6% 13% 34% 13% 1	3809
August	1st to 10th	Night	46% 9% 14% 21% 10%	2642
	11th to 20th	Night	42% 9% 17% 22% 10%	3050
	21st to end	Night	26% 8% 17% 35% 14% 1	3586
September	1st to 10th	Night	32% 8% 13% 28% 18% 1	2000
	11th to 20th	Night	32% 7% 10% 29% 21% 1	2145
	21st to end	Night	37% 8% 10% 28% 16% 2	2200
October	1st to 10th	Night	41% 8% 14% 23% 13% 1	3644
	11th to 20th	Night	31% 8% 12% 26% 21% 2	4014
	21st to end	Night	40% 6% 10% 33% 11% 1	4610
November	1st to 10th	Night	40% 7% 13% 22% 16% 2	2564
	11th to 20th	Night	42% 7% 10% 22% 16% 3	2600
	21st to end	Night	43% 8% 16% 22% 10% 1	2695
December	1st to 10th	Night	51% 8% 13% 22% 7%	4580
	11th to 20th	Night	52% 6% 11% 14% 16% 1	5208
	21st to end	Night	54% 5% 9% 17% 12% 3	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	26% 28% 25% 17% 5%	200
	11th to 20th	Day	30% 24% 30% 14% 2 1	200
	21st to end	Day	24% 30% 28% 13% 5%	220
February	1st to 10th	Day	30% 34% 24% 11% 2	317
	11th to 20th	Day	31% 24% 30% 14% 2	496
	21st to end	Day	29% 24% 29% 16% 2	414
March	1st to 10th	Day	29% 21% 32% 15% 3	500
	11th to 20th	Day	29% 26% 28% 13% 4 1	500
	21st to end	Day	31% 24% 27% 10% 6% 2	550
April	1st to 10th	Day	37% 22% 23% 10% 7% 1	500
	11th to 20th	Day	34% 23% 23% 12% 6% 2	492
	21st to end	Day	29% 21% 30% 13% 5% 2	500
May	1st to 10th	Day	31% 20% 26% 14% 7% 2	503
	11th to 20th	Day	23% 19% 31% 20% 5% 2	527
	21st to end	Day	19% 16% 31% 24% 9% 2	573
June	1st to 10th	Day	19% 17% 33% 19% 10% 1	530
	11th to 20th	Day	19% 16% 33% 20% 10% 2	530
	21st to end	Day	20% 16% 32% 21% 8% 3	530
July	1st to 10th	Day	15% 11% 31% 26% 13% 3	525
	11th to 20th	Day	14% 7% 35% 29% 12% 3	524
	21st to end	Day	14% 11% 35% 26% 11% 2	576
August	1st to 10th	Day	11% 12% 27% 29% 16% 5%	526
	11th to 20th	Day	14% 10% 28% 25% 15% 9%	519
	21st to end	Day	12% 13% 28% 26% 15% 7%	583
September	1st to 10th	Day	16% 11% 26% 23% 16% 8%	530
	11th to 20th	Day	14% 12% 25% 21% 17% 11%	530
	21st to end	Day	17% 13% 28% 25% 11% 6%	530
October	1st to 10th	Day	25% 14% 22% 21% 12% 5%	530
	11th to 20th	Day	23% 18% 26% 22% 9% 2	494
	21st to end	Day	31% 14% 30% 17% 7% 1	462
November	1st to 10th	Day	23% 18% 25% 23% 10% 1	401
	11th to 20th	Day	30% 19% 26% 19% 5% 1	400
	21st to end	Day	33% 19% 25% 17% 5% 1	300
December	1st to 10th	Day	25% 30% 23% 17% 6%	200
	11th to 20th	Day	38% 23% 26% 12% 2	200
	21st to end	Day	30% 19% 31% 16% 5%	220
January	1st to 10th	Night	22% 21% 33% 22% 3	800
	11th to 20th	Night	25% 20% 31% 22% 2	800
	21st to end	Night	19% 18% 36% 23% 4	880
February	1st to 10th	Night	22% 18% 36% 21% 2	800
	11th to 20th	Night	22% 22% 36% 19% 2	783
	21st to end	Night	24% 22% 32% 21% 2	660
March	1st to 10th	Night	27% 18% 31% 22% 2	800
	11th to 20th	Night	24% 21% 29% 22% 4	800
	21st to end	Night	26% 18% 29% 21% 6%	880
April	1st to 10th	Night	27% 19% 29% 22% 4	800
	11th to 20th	Night	24% 20% 28% 21% 7%	791
	21st to end	Night	25% 17% 30% 23% 4	628
May	1st to 10th	Night	28% 13% 27% 25% 6%	600
	11th to 20th	Night	20% 11% 32% 29% 7%	431
	21st to end	Night	19% 7% 27% 33% 13% 1	433
June	1st to 10th	Night	21% 7% 27% 34% 11%	400
	11th to 20th	Night	18% 9% 29% 33% 12%	400
	21st to end	Night	13% 7% 27% 41% 12% 1	400
July	1st to 10th	Night	14% 7% 22% 44% 13% 1	390
	11th to 20th	Night	13% 4 22% 41% 18% 2	388
	21st to end	Night	14% 3 25% 40% 18% 1	438
August	1st to 10th	Night	12% 4 19% 45% 19% 1	438
	11th to 20th	Night	14% 7% 24% 33% 19% 3	678
	21st to end	Night	15% 5% 21% 37% 20% 2	880
September	1st to 10th	Night	13% 7% 27% 33% 19% 1	800
	11th to 20th	Night	15% 7% 19% 33% 20% 5%	800
	21st to end	Night	18% 9% 23% 32% 17% 2	800
October	1st to 10th	Night	23% 12% 18% 30% 15% 2	800
	11th to 20th	Night	20% 12% 27% 28% 12% 1	800
	21st to end	Night	26% 15% 27% 25% 6%	880
November	1st to 10th	Night	20% 15% 28% 28% 9%	789
	11th to 20th	Night	20% 18% 29% 25% 7% 1	800
	21st to end	Night	22% 17% 30% 24% 6% 1	800
December	1st to 10th	Night	23% 20% 32% 21% 4	800
	11th to 20th	Night	26% 17% 31% 23% 3	800
	21st to end	Night	27% 17% 33% 20% 2	880

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