Stardom Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stardom Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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% 6% 2 6% 4 4 4 4 6% 8% 40%	149100
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	141974
Transparency	Day	42% 5% 11% 23% 17% 3	152642
	Night	41% 4 11% 23% 17% 3	146520
Seeing	Day	22% 15% 29% 20% 10% 3	17435
	Night	20% 14% 31% 26% 9% 1	26611

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	13% 7% 2 6% 4 4 3 3 5% 6% 48%	4661
	11th to 20th	Day	16% 7% 2 7% 4 3 3 3 4 5% 47%	4692
	21st to end	Day	16% 7% 2 6% 3 4 5% 3 5% 6% 42%	5780
February	1st to 10th	Day	15% 8% 2 7% 5% 5% 4 3 6% 6% 41%	1890
	11th to 20th	Day	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2034
	21st to end	Day	17% 9% 3 6% 5% 4 3 3 5% 5% 40%	1730
March	1st to 10th	Day	22% 6% 2 4 3 3 3 3 4 5% 45%	4576
	11th to 20th	Day	11% 7% 2 6% 4 5% 4 4 6% 6% 45%	5014
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 6% 43%	5977
April	1st to 10th	Day	17% 7% 3 5% 4 4 5% 3 6% 6% 40%	2520
	11th to 20th	Day	14% 6% 2 5% 4 5% 4 4 5% 8% 42%	2494
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 7% 40%	2520
May	1st to 10th	Day	22% 8% 3 6% 4 5% 4 3 5% 8% 34%	7511
	11th to 20th	Day	19% 7% 3 6% 4 4 4 3 5% 7% 39%	8217
	21st to end	Day	24% 7% 2 6% 3 3 4 3 5% 7% 35%	9060
June	1st to 10th	Day	13% 5% 2 5% 5% 5% 5% 4 7% 9% 41%	2940
	11th to 20th	Day	12% 7% 2 6% 6% 4 5% 4 7% 8% 40%	2940
	21st to end	Day	10% 6% 2 7% 4 5% 5% 4 6% 9% 41%	2940
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	8280
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 11% 34%	8280
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 4 7% 10% 37%	8958
August	1st to 10th	Day	14% 6% 3 8% 6% 6% 6% 5% 8% 9% 30%	5946
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 10% 39%	5488
	21st to end	Day	11% 6% 2 6% 5% 4 5% 4 7% 9% 41%	6080
September	1st to 10th	Day	12% 6% 2 6% 4 4 5% 3 5% 8% 45%	2520
	11th to 20th	Day	12% 6% 1 5% 4 4 4 4 5% 7% 49%	2381
	21st to end	Day	18% 7% 3 7% 4 4 4 5% 5% 6% 38%	2200
October	1st to 10th	Day	17% 6% 2 6% 3 4 3 3 5% 6% 46%	2040
	11th to 20th	Day	16% 5% 2 5% 3 3 4 3 4 6% 50%	2000
	21st to end	Day	16% 5% 3 6% 3 2 4 2 5% 6% 47%	2101
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 53%	1851
	11th to 20th	Day	15% 6% 2 6% 3 3 4 3 5% 5% 47%	1890
	21st to end	Day	16% 5% 2 5% 5% 4 4 2 5% 7% 46%	1795
December	1st to 10th	Day	13% 7% 2 5% 4 3 3 3 6% 8% 45%	2680
	11th to 20th	Day	20% 9% 3 4 3 2 3 2 3 5% 45%	3066
	21st to end	Day	16% 6% 2 5% 4 4 5% 5% 6% 7% 41%	4048
January	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 4 6% 47%	8234
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 5% 48%	8175
	21st to end	Night	16% 8% 3 5% 4 3 4 3 4 6% 45%	8401
February	1st to 10th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 41%	2730
	11th to 20th	Night	19% 8% 2 6% 4 4 3 3 5% 6% 41%	2470
	21st to end	Night	18% 8% 2 7% 4 3 3 4 5% 5% 42%	2076
March	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 45%	5484
	11th to 20th	Night	12% 9% 3 5% 3 3 3 3 4 6% 49%	5149
	21st to end	Night	16% 8% 3 6% 4 4 4 3 4 5% 42%	5196
April	1st to 10th	Night	17% 8% 2 7% 3 4 4 3 5% 6% 41%	2100
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 4 7% 42%	2075
	21st to end	Night	17% 6% 2 5% 3 4 4 3 5% 6% 45%	2100
May	1st to 10th	Night	19% 7% 3 7% 4 4 4 4 7% 8% 32%	5385
	11th to 20th	Night	20% 7% 3 7% 4 4 4 4 5% 8% 36%	4698
	21st to end	Night	25% 7% 3 7% 4 4 3 3 5% 8% 32%	5114
June	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 6% 8% 44%	1680
	11th to 20th	Night	12% 7% 3 6% 6% 5% 4 4 5% 6% 42%	1680
	21st to end	Night	9% 5% 2 4 5% 6% 5% 5% 8% 10% 40%	1680
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 5% 4 7% 10% 36%	4680
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 5% 7% 12% 33%	4680
	21st to end	Night	12% 7% 2 7% 4 4 5% 4 6% 9% 39%	5115
August	1st to 10th	Night	18% 7% 2 7% 5% 5% 5% 4 7% 9% 31%	4155
	11th to 20th	Night	13% 6% 3 7% 4 5% 4 3 6% 8% 40%	4557
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 5% 7% 47%	5045
September	1st to 10th	Night	14% 6% 2 5% 5% 4 4 4 4 7% 45%	2100
	11th to 20th	Night	11% 6% 2 4 3 3 4 3 5% 8% 52%	2086
	21st to end	Night	18% 7% 2 6% 4 3 3 3 6% 7% 40%	2200
October	1st to 10th	Night	18% 6% 2 5% 4 4 3 3 5% 6% 46%	2355
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 4 6% 51%	2400
	21st to end	Night	17% 6% 2 5% 4 3 3 3 4 5% 47%	2736
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 5% 7% 53%	2667
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 4 5% 48%	2730
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 7% 45%	2823
December	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 5% 6% 48%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 5% 6% 41%	5348
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 6% 41%	7150

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	39% 3 9% 28% 20% 3	4662
	11th to 20th	Day	42% 3 6% 23% 21% 6%	4722
	21st to end	Day	43% 3 12% 28% 13% 2	5780
February	1st to 10th	Day	44% 4 11% 26% 13% 2	1890
	11th to 20th	Day	46% 4 9% 24% 15% 2	2043
	21st to end	Day	47% 3 8% 24% 14% 4	1730
March	1st to 10th	Day	43% 3 7% 24% 16% 7%	4579
	11th to 20th	Day	38% 3 8% 29% 16% 5%	5014
	21st to end	Day	42% 4 9% 17% 23% 6%	5977
April	1st to 10th	Day	43% 6% 10% 21% 16% 5%	2520
	11th to 20th	Day	40% 5% 12% 23% 17% 3	2494
	21st to end	Day	42% 5% 9% 20% 19% 4	2520
May	1st to 10th	Day	50% 5% 12% 18% 15% 1	7522
	11th to 20th	Day	46% 4 9% 25% 15% 1	8221
	21st to end	Day	49% 5% 8% 22% 14% 2	9061
June	1st to 10th	Day	39% 7% 11% 23% 18% 2	2940
	11th to 20th	Day	40% 7% 12% 24% 14% 3	2940
	21st to end	Day	39% 7% 12% 22% 17% 2	2940
July	1st to 10th	Day	44% 7% 13% 18% 17% 1	8210
	11th to 20th	Day	42% 5% 15% 24% 13% 1	8205
	21st to end	Day	41% 6% 10% 21% 21% 1	8760
August	1st to 10th	Day	49% 7% 17% 19% 8%	5933
	11th to 20th	Day	40% 6% 15% 26% 12% 1	5524
	21st to end	Day	38% 6% 11% 27% 17% 2	6080
September	1st to 10th	Day	39% 5% 12% 20% 21% 3	2520
	11th to 20th	Day	35% 5% 9% 24% 24% 3	2381
	21st to end	Day	48% 5% 8% 18% 20% 2	2200
October	1st to 10th	Day	37% 3 9% 18% 29% 5%	3326
	11th to 20th	Day	26% 3 9% 24% 31% 8%	3240
	21st to end	Day	26% 3 11% 27% 28% 6%	3378
November	1st to 10th	Day	34% 3 12% 26% 22% 3	1851
	11th to 20th	Day	39% 3 8% 25% 22% 3	1890
	21st to end	Day	39% 4 11% 30% 13% 2	1795
December	1st to 10th	Day	39% 3 16% 26% 14% 2	2680
	11th to 20th	Day	44% 2 9% 30% 11% 3	3066
	21st to end	Day	41% 5% 11% 23% 18% 2	4048
January	1st to 10th	Night	38% 3 12% 25% 17% 5%	8234
	11th to 20th	Night	40% 2 8% 23% 19% 7%	8220
	21st to end	Night	42% 2 8% 25% 20% 3	8401
February	1st to 10th	Night	45% 3 12% 24% 13% 3	2730
	11th to 20th	Night	44% 5% 8% 20% 18% 5%	2472
	21st to end	Night	44% 4 7% 23% 17% 5%	2076
March	1st to 10th	Night	43% 3 7% 24% 13% 10%	5494
	11th to 20th	Night	38% 4 12% 26% 15% 5%	5150
	21st to end	Night	44% 3 7% 17% 24% 5%	5196
April	1st to 10th	Night	44% 5% 9% 19% 18% 5%	2100
	11th to 20th	Night	42% 4 9% 21% 20% 4	2075
	21st to end	Night	39% 6% 9% 21% 21% 5%	2100
May	1st to 10th	Night	49% 5% 11% 20% 13% 2	5391
	11th to 20th	Night	48% 4 12% 22% 13% 1	4699
	21st to end	Night	52% 5% 9% 20% 13% 1	5115
June	1st to 10th	Night	38% 6% 11% 23% 20% 2	1680
	11th to 20th	Night	41% 7% 12% 25% 13% 2	1680
	21st to end	Night	37% 7% 15% 24% 15% 2	1680
July	1st to 10th	Night	45% 7% 14% 22% 12% 1	4640
	11th to 20th	Night	42% 6% 18% 23% 10% 1	4632
	21st to end	Night	42% 5% 13% 21% 18% 1	4998
August	1st to 10th	Night	49% 7% 18% 17% 8% 1	4144
	11th to 20th	Night	42% 5% 14% 29% 10% 1	4587
	21st to end	Night	37% 5% 11% 27% 19% 2	5045
September	1st to 10th	Night	39% 6% 11% 20% 20% 3	2100
	11th to 20th	Night	32% 4 11% 24% 25% 4	2086
	21st to end	Night	43% 6% 9% 21% 20% 2	2200
October	1st to 10th	Night	37% 4 10% 24% 23% 3	3849
	11th to 20th	Night	25% 3 9% 27% 30% 6%	3899
	21st to end	Night	27% 3 10% 31% 25% 4	4409
November	1st to 10th	Night	33% 3 11% 25% 22% 5%	2667
	11th to 20th	Night	40% 4 8% 22% 21% 6%	2730
	21st to end	Night	38% 5% 12% 26% 18% 2	2823
December	1st to 10th	Night	38% 3 12% 30% 15% 3	4720
	11th to 20th	Night	44% 3 10% 24% 14% 4	5348
	21st to end	Night	45% 4 10% 20% 18% 3	7150

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	27% 27% 30% 13% 3 1	220
	11th to 20th	Day	39% 25% 24% 11% 1	228
	21st to end	Day	31% 28% 25% 13% 2 1	363
February	1st to 10th	Day	28% 29% 28% 12% 2	330
	11th to 20th	Day	35% 23% 32% 9% 1	522
	21st to end	Day	31% 23% 32% 12% 2	446
March	1st to 10th	Day	31% 21% 29% 15% 3	540
	11th to 20th	Day	29% 22% 33% 12% 3 1	540
	21st to end	Day	28% 26% 29% 13% 4 1	594
April	1st to 10th	Day	30% 24% 29% 11% 4 1	540
	11th to 20th	Day	24% 21% 33% 14% 6% 1	532
	21st to end	Day	29% 18% 32% 15% 5% 1	540
May	1st to 10th	Day	32% 20% 27% 14% 6% 1	543
	11th to 20th	Day	23% 14% 35% 19% 7% 2	567
	21st to end	Day	24% 12% 24% 25% 12% 3	617
June	1st to 10th	Day	16% 12% 31% 26% 12% 3	570
	11th to 20th	Day	19% 14% 30% 25% 10% 3	570
	21st to end	Day	13% 9% 32% 29% 13% 3	570
July	1st to 10th	Day	13% 6% 26% 29% 20% 6%	565
	11th to 20th	Day	7% 6% 30% 31% 21% 5%	564
	21st to end	Day	12% 7% 30% 31% 15% 5%	620
August	1st to 10th	Day	13% 6% 30% 30% 15% 6%	566
	11th to 20th	Day	14% 4 25% 32% 19% 6%	559
	21st to end	Day	12% 6% 30% 27% 16% 8%	627
September	1st to 10th	Day	16% 10% 28% 25% 15% 7%	570
	11th to 20th	Day	15% 7% 26% 31% 14% 8%	542
	21st to end	Day	26% 12% 25% 22% 11% 5%	530
October	1st to 10th	Day	24% 10% 27% 20% 13% 7%	530
	11th to 20th	Day	22% 16% 28% 20% 11% 3	530
	21st to end	Day	19% 18% 34% 18% 10% 2	528
November	1st to 10th	Day	19% 15% 34% 19% 12% 1	425
	11th to 20th	Day	25% 20% 30% 19% 7%	430
	21st to end	Day	23% 20% 31% 19% 6% 1	335
December	1st to 10th	Day	25% 31% 30% 9% 5%	220
	11th to 20th	Day	36% 17% 33% 11% 2	220
	21st to end	Day	32% 17% 29% 16% 5% 1	242
January	1st to 10th	Night	21% 22% 34% 19% 3	840
	11th to 20th	Night	26% 20% 30% 20% 3	840
	21st to end	Night	19% 21% 37% 21% 2	924
February	1st to 10th	Night	22% 18% 36% 22% 2	840
	11th to 20th	Night	26% 25% 34% 14% 1	823
	21st to end	Night	25% 19% 35% 19% 2	692
March	1st to 10th	Night	24% 20% 32% 20% 4	840
	11th to 20th	Night	22% 23% 33% 17% 6%	840
	21st to end	Night	22% 16% 39% 17% 4	924
April	1st to 10th	Night	22% 23% 35% 16% 5%	840
	11th to 20th	Night	21% 16% 38% 21% 5%	831
	21st to end	Night	21% 14% 35% 25% 5%	840
May	1st to 10th	Night	21% 14% 36% 23% 6%	678
	11th to 20th	Night	17% 14% 38% 26% 5%	435
	21st to end	Night	20% 8% 24% 37% 9% 2	455
June	1st to 10th	Night	16% 6% 32% 36% 9%	420
	11th to 20th	Night	14% 5% 30% 40% 11%	420
	21st to end	Night	11% 5% 29% 44% 10% 1	420
July	1st to 10th	Night	11% 2 26% 43% 16% 2	410
	11th to 20th	Night	8% 3 23% 48% 16% 2	408
	21st to end	Night	11% 5% 27% 39% 17%	466
August	1st to 10th	Night	12% 3 18% 42% 22% 2	634
	11th to 20th	Night	14% 4 20% 40% 20% 2	830
	21st to end	Night	14% 3 21% 39% 21% 2	924
September	1st to 10th	Night	16% 5% 27% 34% 16% 2	840
	11th to 20th	Night	14% 4 22% 34% 22% 3	813
	21st to end	Night	23% 7% 24% 29% 16% 1	800
October	1st to 10th	Night	21% 7% 26% 27% 17% 3	800
	11th to 20th	Night	19% 12% 28% 30% 12% 1	800
	21st to end	Night	20% 13% 35% 23% 9%	880
November	1st to 10th	Night	16% 12% 35% 27% 10% 1	820
	11th to 20th	Night	20% 18% 28% 25% 8%	840
	21st to end	Night	19% 17% 35% 22% 7% 1	840
December	1st to 10th	Night	25% 17% 32% 19% 6% 1	840
	11th to 20th	Night	22% 18% 32% 22% 5%	840
	21st to end	Night	23% 17% 33% 25% 3	924

Page generated at 2026-09-11 04:11:55. Version ac07

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