Star Watcher Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Watcher Observatory 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	15% 7% 2 6% 4 4 4 4 5% 7% 42%	126714
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	123672
Transparency	Day	41% 5% 12% 23% 16% 2	130243
	Night	43% 4 11% 23% 16% 3	128218
Seeing	Day	19% 10% 27% 27% 14% 3	16408
	Night	20% 9% 30% 28% 11% 1	25447

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	12% 6% 2 5% 4 3 4 3 5% 7% 49%	3597
	11th to 20th	Day	14% 7% 3 6% 4 3 4 4 5% 6% 44%	3570
	21st to end	Day	19% 8% 3 8% 4 4 3 3 4 5% 40%	4141
February	1st to 10th	Day	17% 8% 3 6% 4 4 4 4 5% 7% 40%	1880
	11th to 20th	Day	14% 7% 2 7% 4 3 4 3 5% 7% 44%	1948
	21st to end	Day	18% 8% 2 6% 5% 3 3 3 5% 6% 40%	1650
March	1st to 10th	Day	20% 7% 2 5% 4 3 3 3 4 6% 44%	4476
	11th to 20th	Day	10% 8% 3 5% 4 3 4 4 6% 8% 45%	4680
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 6% 41%	5970
April	1st to 10th	Day	17% 8% 2 5% 3 3 4 4 5% 7% 40%	2400
	11th to 20th	Day	16% 7% 2 6% 4 3 4 3 5% 6% 44%	2374
	21st to end	Day	17% 6% 2 4 4 3 3 4 6% 7% 43%	2490
May	1st to 10th	Day	26% 9% 2 5% 3 3 4 4 5% 8% 31%	5886
	11th to 20th	Day	20% 9% 3 6% 4 3 3 3 4 7% 39%	6084
	21st to end	Day	18% 7% 2 5% 4 3 4 4 6% 8% 39%	6949
June	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 7% 9% 42%	2800
	11th to 20th	Day	13% 6% 3 5% 4 4 4 4 6% 8% 42%	2800
	21st to end	Day	10% 6% 2 6% 4 4 4 3 6% 10% 45%	2800
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 11% 36%	6360
	11th to 20th	Day	9% 7% 3 8% 5% 4 5% 6% 8% 11% 36%	6360
	21st to end	Day	13% 6% 2 6% 4 4 4 4 5% 8% 43%	6926
August	1st to 10th	Day	16% 6% 3 6% 5% 5% 4 4 6% 9% 36%	4345
	11th to 20th	Day	10% 6% 3 6% 4 4 4 3 6% 8% 47%	3850
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 9% 50%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 6% 7% 48%	2400
	11th to 20th	Day	11% 6% 2 5% 2 3 3 3 5% 6% 53%	2263
	21st to end	Day	18% 6% 2 7% 5% 4 4 3 5% 6% 40%	2200
October	1st to 10th	Day	20% 6% 2 4 3 2 4 2 4 8% 45%	2170
	11th to 20th	Day	15% 5% 2 5% 4 3 4 4 6% 7% 46%	1998
	21st to end	Day	18% 6% 2 5% 4 3 4 4 5% 7% 42%	1980
November	1st to 10th	Day	14% 6% 2 5% 3 4 4 3 4 5% 50%	1851
	11th to 20th	Day	16% 6% 2 5% 3 2 3 3 4 8% 47%	1863
	21st to end	Day	15% 6% 2 6% 4 3 4 4 5% 7% 44%	1795
December	1st to 10th	Day	14% 7% 3 7% 4 3 3 3 5% 7% 43%	2600
	11th to 20th	Day	20% 9% 2 4 2 2 2 2 4 6% 46%	2986
	21st to end	Day	17% 10% 3 7% 4 4 4 3 4 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 49%	6349
	11th to 20th	Night	15% 9% 3 7% 4 3 3 3 4 5% 46%	6315
	21st to end	Night	17% 9% 3 7% 4 4 4 4 5% 6% 37%	6812
February	1st to 10th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 38%	2518
	11th to 20th	Night	18% 8% 2 6% 5% 3 3 3 6% 6% 40%	2338
	21st to end	Night	20% 7% 2 6% 4 4 4 2 5% 6% 40%	1980
March	1st to 10th	Night	19% 9% 3 7% 3 3 3 2 4 6% 41%	5364
	11th to 20th	Night	14% 10% 3 7% 4 4 4 3 4 6% 41%	5274
	21st to end	Night	20% 9% 3 6% 4 4 4 3 5% 5% 38%	4962
April	1st to 10th	Night	19% 6% 2 7% 5% 4 5% 3 5% 6% 37%	2000
	11th to 20th	Night	18% 7% 2 7% 4 3 3 3 4 7% 41%	1975
	21st to end	Night	18% 8% 2 7% 4 3 4 3 4 6% 42%	1909
May	1st to 10th	Night	24% 10% 3 7% 5% 4 4 3 5% 7% 28%	4054
	11th to 20th	Night	23% 10% 2 6% 5% 3 4 3 5% 7% 32%	3852
	21st to end	Night	21% 9% 3 6% 4 4 4 3 5% 7% 33%	3926
June	1st to 10th	Night	18% 7% 2 5% 5% 4 4 3 5% 7% 40%	1600
	11th to 20th	Night	16% 6% 2 5% 4 4 4 4 7% 9% 40%	1600
	21st to end	Night	11% 6% 2 6% 5% 5% 4 4 6% 9% 40%	1600
July	1st to 10th	Night	9% 10% 4 8% 5% 5% 5% 4 7% 9% 34%	3600
	11th to 20th	Night	11% 10% 4 9% 6% 5% 5% 5% 6% 9% 31%	3600
	21st to end	Night	13% 7% 3 7% 4 5% 4 4 5% 8% 41%	3926
August	1st to 10th	Night	20% 7% 3 6% 4 5% 5% 4 6% 9% 31%	2767
	11th to 20th	Night	11% 7% 3 6% 4 4 6% 4 7% 7% 40%	3197
	21st to end	Night	8% 4 2 5% 3 4 4 4 6% 7% 54%	3586
September	1st to 10th	Night	14% 4 2 7% 4 5% 4 4 5% 5% 46%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 6% 51%	2135
	21st to end	Night	17% 7% 3 6% 4 4 4 3 5% 7% 41%	2200
October	1st to 10th	Night	20% 8% 2 6% 3 4 4 3 4 6% 41%	2227
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 4 5% 48%	2400
	21st to end	Night	20% 7% 2 5% 4 4 3 3 5% 6% 40%	2860
November	1st to 10th	Night	13% 6% 2 5% 4 4 4 2 5% 6% 48%	2667
	11th to 20th	Night	18% 7% 2 4 4 3 3 3 4 5% 46%	2692
	21st to end	Night	14% 6% 2 5% 4 3 4 3 6% 7% 45%	2603
December	1st to 10th	Night	15% 8% 2 5% 4 4 3 3 4 5% 46%	4580
	11th to 20th	Night	23% 8% 2 5% 4 3 3 2 4 4 41%	5208
	21st to end	Night	17% 9% 3 7% 4 4 4 3 4 5% 41%	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	36% 4 7% 31% 20% 2	3598
	11th to 20th	Day	40% 5% 10% 23% 16% 7%	3600
	21st to end	Day	48% 5% 9% 22% 15% 1	4141
February	1st to 10th	Day	44% 5% 13% 22% 14% 1	1880
	11th to 20th	Day	41% 4 10% 25% 17% 3	1957
	21st to end	Day	45% 4 11% 21% 16% 3	1650
March	1st to 10th	Day	43% 4 10% 17% 15% 9%	4479
	11th to 20th	Day	37% 5% 13% 29% 14% 1	4680
	21st to end	Day	44% 5% 9% 15% 23% 5%	5970
April	1st to 10th	Day	44% 6% 9% 20% 17% 4	2400
	11th to 20th	Day	41% 6% 11% 21% 19% 3	2374
	21st to end	Day	40% 5% 10% 23% 18% 4	2490
May	1st to 10th	Day	52% 6% 17% 15% 9% 1	5897
	11th to 20th	Day	46% 7% 11% 21% 13% 2	6088
	21st to end	Day	43% 7% 10% 25% 14% 1	6949
June	1st to 10th	Day	39% 8% 12% 23% 16% 2	2800
	11th to 20th	Day	40% 8% 13% 25% 13% 2	2800
	21st to end	Day	36% 8% 17% 23% 15% 1	2800
July	1st to 10th	Day	43% 8% 13% 21% 14% 1	6290
	11th to 20th	Day	43% 4 19% 23% 10% 1	6285
	21st to end	Day	40% 4 11% 24% 19% 1	6728
August	1st to 10th	Day	46% 5% 16% 23% 9% 1	4331
	11th to 20th	Day	37% 5% 13% 33% 10% 1	3886
	21st to end	Day	31% 5% 14% 28% 21% 2	4312
September	1st to 10th	Day	37% 7% 13% 23% 20% 2	2400
	11th to 20th	Day	32% 5% 11% 25% 25% 2	2263
	21st to end	Day	45% 6% 9% 19% 19% 2	2200
October	1st to 10th	Day	36% 4 9% 22% 24% 5%	3534
	11th to 20th	Day	27% 4 10% 25% 31% 3	3226
	21st to end	Day	31% 3 14% 27% 22% 2	3180
November	1st to 10th	Day	38% 4 13% 22% 20% 2	1851
	11th to 20th	Day	37% 4 10% 22% 23% 3	1863
	21st to end	Day	40% 5% 14% 27% 12% 2	1795
December	1st to 10th	Day	40% 3 16% 28% 12% 1	2600
	11th to 20th	Day	42% 3 14% 20% 19% 3	2986
	21st to end	Day	48% 5% 8% 22% 15% 3	3960
January	1st to 10th	Night	36% 2 9% 27% 23% 3	6349
	11th to 20th	Night	42% 3 11% 22% 16% 5%	6360
	21st to end	Night	48% 3 9% 21% 16% 4	6812
February	1st to 10th	Night	45% 6% 12% 24% 12% 2	2518
	11th to 20th	Night	44% 5% 9% 21% 17% 4	2340
	21st to end	Night	45% 4 7% 23% 17% 4	1980
March	1st to 10th	Night	46% 3 10% 20% 14% 8%	5374
	11th to 20th	Night	44% 4 12% 24% 12% 4	5275
	21st to end	Night	48% 4 6% 13% 23% 6%	4962
April	1st to 10th	Night	48% 4 9% 19% 16% 4	2000
	11th to 20th	Night	44% 5% 11% 18% 21% 2	1975
	21st to end	Night	44% 5% 8% 21% 17% 5%	1909
May	1st to 10th	Night	57% 6% 12% 14% 10% 1	4060
	11th to 20th	Night	52% 6% 12% 16% 11% 3	3853
	21st to end	Night	50% 6% 12% 21% 10% 1	3927
June	1st to 10th	Night	43% 6% 10% 20% 19% 2	1600
	11th to 20th	Night	40% 7% 13% 27% 13% 1	1600
	21st to end	Night	39% 10% 12% 25% 13% 2	1600
July	1st to 10th	Night	47% 7% 12% 23% 11% 1	3560
	11th to 20th	Night	50% 4 15% 21% 10%	3552
	21st to end	Night	41% 5% 13% 25% 15% 1	3809
August	1st to 10th	Night	50% 6% 16% 18% 10% 1	2757
	11th to 20th	Night	40% 5% 14% 30% 9% 1	3227
	21st to end	Night	29% 5% 11% 34% 19% 1	3586
September	1st to 10th	Night	40% 7% 9% 25% 18% 2	2000
	11th to 20th	Night	34% 5% 10% 25% 24% 3	2135
	21st to end	Night	44% 7% 8% 20% 19% 3	2200
October	1st to 10th	Night	40% 3 10% 25% 18% 3	3643
	11th to 20th	Night	27% 3 9% 27% 30% 4	3899
	21st to end	Night	33% 3 13% 28% 21% 2	4610
November	1st to 10th	Night	37% 4 11% 24% 20% 3	2667
	11th to 20th	Night	41% 3 8% 24% 19% 4	2692
	21st to end	Night	38% 4 13% 29% 14% 2	2603
December	1st to 10th	Night	41% 3 12% 30% 12% 2	4580
	11th to 20th	Night	48% 3 9% 24% 15% 2	5208
	21st to end	Night	45% 4 9% 21% 17% 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% 17% 33% 19% 6% 1	200
	11th to 20th	Day	17% 30% 34% 17% 3 1	200
	21st to end	Day	24% 14% 37% 18% 7% 1	259
February	1st to 10th	Day	25% 13% 41% 18% 3	380
	11th to 20th	Day	23% 17% 38% 20% 2	496
	21st to end	Day	23% 17% 34% 20% 5%	414
March	1st to 10th	Day	20% 17% 34% 24% 5%	500
	11th to 20th	Day	18% 12% 39% 25% 5% 1	500
	21st to end	Day	22% 14% 38% 19% 6% 1	550
April	1st to 10th	Day	23% 15% 33% 20% 8% 1	500
	11th to 20th	Day	22% 11% 33% 23% 10% 1	492
	21st to end	Day	24% 10% 32% 27% 6% 2	500
May	1st to 10th	Day	28% 10% 28% 24% 8% 1	503
	11th to 20th	Day	20% 8% 28% 31% 11% 1	527
	21st to end	Day	21% 5% 19% 29% 23% 4	573
June	1st to 10th	Day	17% 5% 22% 35% 19% 3	530
	11th to 20th	Day	16% 6% 23% 36% 15% 3	530
	21st to end	Day	13% 7% 20% 35% 21% 4	530
July	1st to 10th	Day	13% 3 18% 35% 24% 6%	525
	11th to 20th	Day	10% 4 18% 36% 25% 7%	524
	21st to end	Day	11% 5% 20% 39% 22% 3	576
August	1st to 10th	Day	11% 4 19% 35% 24% 7%	526
	11th to 20th	Day	14% 2 14% 39% 25% 7%	519
	21st to end	Day	10% 5% 22% 33% 22% 8%	583
September	1st to 10th	Day	17% 6% 21% 26% 23% 7%	530
	11th to 20th	Day	14% 3 23% 32% 19% 10%	530
	21st to end	Day	22% 5% 26% 27% 15% 4	530
October	1st to 10th	Day	26% 5% 23% 20% 18% 7%	530
	11th to 20th	Day	16% 11% 31% 23% 15% 3	528
	21st to end	Day	23% 11% 32% 22% 11% 2	462
November	1st to 10th	Day	18% 13% 25% 31% 11% 2	425
	11th to 20th	Day	22% 16% 32% 22% 8%	421
	21st to end	Day	18% 16% 32% 25% 9% 1	395
December	1st to 10th	Day	20% 16% 37% 22% 6% 1	200
	11th to 20th	Day	28% 21% 29% 18% 4 1	200
	21st to end	Day	23% 15% 39% 15% 8%	220
January	1st to 10th	Night	20% 15% 39% 21% 5%	800
	11th to 20th	Night	26% 14% 34% 23% 4	800
	21st to end	Night	19% 14% 40% 24% 3	880
February	1st to 10th	Night	20% 14% 37% 27% 2	800
	11th to 20th	Night	25% 14% 39% 20% 2	783
	21st to end	Night	24% 15% 35% 22% 4	660
March	1st to 10th	Night	23% 12% 38% 22% 5%	800
	11th to 20th	Night	20% 10% 41% 24% 5% 1	800
	21st to end	Night	21% 12% 40% 20% 7%	880
April	1st to 10th	Night	22% 9% 39% 25% 5%	800
	11th to 20th	Night	22% 9% 33% 29% 7%	791
	21st to end	Night	21% 7% 34% 30% 7% 1	709
May	1st to 10th	Night	25% 6% 31% 27% 10% 1	600
	11th to 20th	Night	22% 7% 34% 28% 8%	515
	21st to end	Night	22% 4 19% 37% 17% 1	433
June	1st to 10th	Night	20% 5% 23% 35% 17% 1	400
	11th to 20th	Night	20% 2 18% 42% 18% 1	400
	21st to end	Night	12% 5% 22% 45% 17% 1	400
July	1st to 10th	Night	13% 3 22% 41% 20% 2	390
	11th to 20th	Night	14% 4 17% 40% 24% 2	388
	21st to end	Night	15% 3 17% 47% 18% 1	438
August	1st to 10th	Night	10% 2 17% 46% 25% 1	501
	11th to 20th	Night	14% 3 15% 39% 25% 2	790
	21st to end	Night	13% 4 16% 38% 25% 4	880
September	1st to 10th	Night	15% 4 22% 34% 20% 4	800
	11th to 20th	Night	16% 3 20% 34% 20% 7%	800
	21st to end	Night	22% 4 24% 31% 16% 3	800
October	1st to 10th	Night	23% 7% 27% 24% 16% 4	800
	11th to 20th	Night	20% 11% 27% 30% 11% 1	800
	21st to end	Night	24% 9% 32% 26% 9%	880
November	1st to 10th	Night	16% 10% 33% 27% 12% 2	820
	11th to 20th	Night	22% 14% 32% 22% 9%	829
	21st to end	Night	19% 14% 32% 26% 8% 1	800
December	1st to 10th	Night	24% 15% 33% 21% 7% 1	800
	11th to 20th	Night	22% 14% 34% 24% 4	800
	21st to end	Night	23% 12% 37% 24% 3	880

Page generated at 2025-11-15 00:56:00. Version ac07

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