The Star Deck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Star Deck Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	14% 9% 4 8% 5% 5% 5% 4 6% 7% 35%	131430
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	131538
Transparency	Day	47% 4 11% 23% 13% 2	135042
	Night	48% 3 10% 23% 14% 2	135999
Seeing	Day	17% 9% 25% 25% 17% 7%	17571
	Night	18% 8% 28% 30% 14% 2	27458

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% 14% 7% 13% 6% 4 4 3 5% 6% 21%	4691
	11th to 20th	Day	10% 13% 6% 15% 9% 8% 6% 4 4 4 21%	4710
	21st to end	Day	17% 16% 7% 13% 5% 5% 5% 3 4 5% 19%	5478
February	1st to 10th	Day	18% 14% 6% 11% 5% 4 4 3 4 5% 26%	2079
	11th to 20th	Day	15% 13% 4 12% 6% 6% 4 3 4 6% 28%	2048
	21st to end	Day	16% 14% 5% 10% 4 3 3 3 4 5% 32%	1730
March	1st to 10th	Day	19% 15% 7% 8% 4 3 3 2 3 5% 31%	4580
	11th to 20th	Day	12% 10% 3 6% 4 4 4 3 5% 8% 41%	4948
	21st to end	Day	16% 12% 4 8% 5% 5% 4 3 5% 6% 31%	5768
April	1st to 10th	Day	16% 11% 3 8% 5% 5% 4 4 5% 7% 33%	2520
	11th to 20th	Day	17% 9% 3 6% 4 5% 4 4 5% 7% 36%	2494
	21st to end	Day	18% 8% 3 7% 4 4 4 4 5% 7% 35%	2718
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 28%	5992
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 3 5% 7% 35%	5939
	21st to end	Day	12% 7% 3 7% 5% 5% 6% 4 7% 9% 33%	6496
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 5% 8% 42%	2800
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 43%	2800
	21st to end	Day	9% 6% 2 6% 5% 4 6% 4 7% 10% 41%	2800
July	1st to 10th	Day	11% 5% 2 7% 4 4 5% 4 6% 10% 41%	6360
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 11% 38%	6360
	21st to end	Day	10% 8% 2 7% 5% 5% 6% 5% 7% 8% 35%	6657
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 46%	4234
	11th to 20th	Day	9% 5% 2 6% 4 4 6% 4 8% 12% 40%	4027
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 9% 52%	4312
September	1st to 10th	Day	11% 6% 2 6% 3 4 4 4 6% 7% 48%	2350
	11th to 20th	Day	10% 5% 2 5% 3 4 4 5% 5% 8% 49%	2200
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 7% 47%	2200
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 4 7% 49%	2200
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 6% 45%	2050
	21st to end	Day	19% 9% 3 7% 4 5% 3 3 4 6% 35%	1980
November	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 5% 7% 50%	1852
	11th to 20th	Day	16% 12% 4 7% 4 3 5% 3 5% 6% 35%	1890
	21st to end	Day	20% 11% 4 8% 5% 4 3 3 4 5% 33%	1890
December	1st to 10th	Day	15% 12% 4 7% 5% 3 3 2 4 5% 39%	3020
	11th to 20th	Day	17% 15% 6% 10% 5% 5% 5% 3 5% 5% 25%	3187
	21st to end	Day	25% 19% 8% 10% 4 5% 3 3 4 5% 14%	4070
January	1st to 10th	Night	15% 14% 6% 13% 6% 4 4 3 6% 6% 22%	8232
	11th to 20th	Night	10% 17% 6% 16% 7% 6% 5% 3 4 5% 21%	8280
	21st to end	Night	18% 13% 5% 8% 5% 5% 4 4 5% 7% 28%	8703
February	1st to 10th	Night	19% 12% 5% 9% 6% 4 3 3 5% 5% 27%	2536
	11th to 20th	Night	15% 12% 5% 10% 5% 3 4 3 4 6% 32%	2458
	21st to end	Night	18% 13% 4 9% 4 4 4 3 4 5% 31%	2076
March	1st to 10th	Night	19% 12% 3 6% 3 4 3 2 4 5% 38%	5493
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 5% 7% 44%	5231
	21st to end	Night	18% 9% 3 8% 5% 4 4 3 6% 7% 33%	5433
April	1st to 10th	Night	15% 10% 2 7% 4 4 5% 3 5% 6% 38%	2100
	11th to 20th	Night	15% 8% 3 6% 5% 4 3 3 5% 8% 39%	2075
	21st to end	Night	17% 7% 3 7% 4 5% 4 3 7% 7% 36%	1901
May	1st to 10th	Night	19% 10% 4 8% 5% 5% 5% 4 6% 7% 28%	4123
	11th to 20th	Night	15% 8% 3 7% 4 4 4 3 5% 9% 38%	4086
	21st to end	Night	13% 7% 3 8% 4 4 5% 3 6% 9% 38%	4344
June	1st to 10th	Night	10% 8% 2 5% 4 4 4 3 5% 7% 48%	1600
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 10% 45%	1600
	21st to end	Night	7% 3 2 5% 5% 5% 4 5% 8% 11% 45%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	3618
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 7% 10% 46%	3617
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 5% 6% 10% 39%	4204
August	1st to 10th	Night	8% 6% 2 7% 4 4 4 3 5% 7% 50%	2915
	11th to 20th	Night	6% 6% 3 7% 5% 5% 5% 4 6% 9% 45%	3005
	21st to end	Night	4 4 2 5% 4 4 5% 4 6% 8% 55%	3586
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 5% 8% 50%	2028
	11th to 20th	Night	8% 4 1 5% 3 4 4 4 5% 7% 56%	2200
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 6% 48%	2200
October	1st to 10th	Night	9% 4 2 5% 4 3 4 3 4 7% 56%	2200
	11th to 20th	Night	15% 7% 2 5% 4 3 4 2 5% 6% 46%	2346
	21st to end	Night	18% 8% 3 6% 4 4 4 4 4 6% 40%	2860
November	1st to 10th	Night	12% 8% 3 5% 4 3 4 3 5% 5% 48%	2666
	11th to 20th	Night	14% 10% 3 7% 4 4 4 2 5% 7% 39%	2730
	21st to end	Night	19% 13% 4 9% 5% 4 3 3 4 6% 31%	2730
December	1st to 10th	Night	15% 11% 4 8% 5% 3 3 3 4 5% 38%	4380
	11th to 20th	Night	18% 15% 6% 11% 5% 4 4 3 4 5% 26%	5232
	21st to end	Night	26% 19% 6% 10% 5% 4 4 3 3 4 16%	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	64% 2 10% 18% 5%	4692
	11th to 20th	Day	68% 2 9% 11% 9% 2	4710
	21st to end	Day	68% 2 7% 15% 6% 2	5478
February	1st to 10th	Day	62% 2 9% 17% 10% 1	2079
	11th to 20th	Day	59% 2 9% 20% 10% 1	2058
	21st to end	Day	56% 2 10% 22% 9% 2	1730
March	1st to 10th	Day	59% 2 6% 20% 12% 2	4580
	11th to 20th	Day	43% 2 10% 32% 12% 1	4948
	21st to end	Day	55% 2 13% 19% 9% 1	5770
April	1st to 10th	Day	51% 4 10% 21% 13% 1	2520
	11th to 20th	Day	48% 3 11% 25% 10% 3	2494
	21st to end	Day	49% 3 11% 24% 11% 2	2718
May	1st to 10th	Day	55% 3 11% 20% 10%	6027
	11th to 20th	Day	50% 3 11% 24% 11% 1	5943
	21st to end	Day	46% 4 11% 24% 13% 1	6510
June	1st to 10th	Day	39% 5% 12% 25% 16% 3	2800
	11th to 20th	Day	35% 5% 12% 26% 20% 2	2800
	21st to end	Day	38% 5% 16% 20% 16% 4	2800
July	1st to 10th	Day	38% 5% 10% 27% 19% 1	6291
	11th to 20th	Day	38% 6% 17% 25% 14% 1	6285
	21st to end	Day	43% 7% 14% 21% 13% 2	6467
August	1st to 10th	Day	31% 6% 16% 31% 14% 1	4221
	11th to 20th	Day	34% 6% 13% 32% 14%	4066
	21st to end	Day	29% 4 13% 32% 18% 4	4311
September	1st to 10th	Day	36% 4 10% 27% 18% 5%	2350
	11th to 20th	Day	33% 4 10% 27% 23% 4	2200
	21st to end	Day	37% 3 9% 25% 25% 2	2200
October	1st to 10th	Day	24% 3 8% 31% 32% 2	3590
	11th to 20th	Day	33% 3 9% 28% 25% 2	3318
	21st to end	Day	42% 2 9% 24% 21% 1	3180
November	1st to 10th	Day	35% 3 11% 30% 17% 3	1852
	11th to 20th	Day	50% 2 9% 22% 14% 4	1890
	21st to end	Day	55% 2 8% 23% 10% 1	1890
December	1st to 10th	Day	50% 2 9% 27% 11%	3020
	11th to 20th	Day	62% 3 9% 18% 7%	3184
	21st to end	Day	74% 2 8% 10% 5%	4070
January	1st to 10th	Night	63% 2 10% 18% 7%	8233
	11th to 20th	Night	67% 2 7% 13% 10% 2	8280
	21st to end	Night	57% 2 10% 22% 8% 1	8703
February	1st to 10th	Night	59% 3 10% 18% 8% 1	2536
	11th to 20th	Night	54% 2 10% 20% 12% 1	2459
	21st to end	Night	56% 2 10% 19% 11% 2	2076
March	1st to 10th	Night	51% 2 6% 27% 12% 3	5493
	11th to 20th	Night	41% 2 10% 28% 17% 2	5231
	21st to end	Night	51% 2 9% 28% 9% 1	5433
April	1st to 10th	Night	47% 3 11% 24% 14% 2	2100
	11th to 20th	Night	45% 2 9% 22% 17% 4	2075
	21st to end	Night	47% 3 10% 24% 13% 4	1901
May	1st to 10th	Night	56% 4 9% 19% 10% 2	4150
	11th to 20th	Night	45% 3 12% 27% 12% 1	4087
	21st to end	Night	44% 4 14% 24% 12% 2	4352
June	1st to 10th	Night	36% 3 10% 25% 21% 4	1600
	11th to 20th	Night	36% 3 11% 25% 21% 3	1600
	21st to end	Night	30% 7% 15% 25% 18% 4	1600
July	1st to 10th	Night	33% 6% 11% 26% 19% 4	3580
	11th to 20th	Night	33% 6% 16% 28% 16% 1	3572
	21st to end	Night	39% 5% 14% 25% 13% 3	4081
August	1st to 10th	Night	34% 4 13% 34% 14% 1	2904
	11th to 20th	Night	35% 5% 13% 29% 18% 1	3032
	21st to end	Night	27% 5% 12% 30% 23% 3	3586
September	1st to 10th	Night	33% 3 10% 26% 21% 6%	2028
	11th to 20th	Night	28% 3 9% 29% 28% 4	2200
	21st to end	Night	37% 3 9% 24% 24% 3	2200
October	1st to 10th	Night	21% 3 7% 27% 41% 2	3590
	11th to 20th	Night	29% 3 8% 25% 34% 2	3817
	21st to end	Night	35% 2 7% 27% 24% 4	4610
November	1st to 10th	Night	38% 2 9% 26% 22% 3	2666
	11th to 20th	Night	46% 2 8% 24% 17% 3	2730
	21st to end	Night	56% 2 9% 22% 10%	2730
December	1st to 10th	Night	49% 3 11% 26% 11%	4380
	11th to 20th	Night	62% 2 9% 17% 9% 1	5234
	21st to end	Night	74% 2 8% 11% 4	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	23% 17% 31% 23% 6% 1	330
	11th to 20th	Day	22% 20% 38% 16% 4 1	330
	21st to end	Day	28% 14% 31% 23% 4	363
February	1st to 10th	Day	27% 10% 36% 22% 4	330
	11th to 20th	Day	22% 18% 35% 19% 6%	330
	21st to end	Day	25% 12% 40% 21% 3	273
March	1st to 10th	Day	24% 16% 38% 19% 3	330
	11th to 20th	Day	26% 13% 34% 18% 8%	476
	21st to end	Day	27% 15% 34% 18% 6% 1	594
April	1st to 10th	Day	20% 14% 34% 24% 8% 2	540
	11th to 20th	Day	21% 14% 31% 23% 9% 3	532
	21st to end	Day	23% 11% 35% 21% 8% 2	540
May	1st to 10th	Day	23% 11% 30% 24% 9% 3	543
	11th to 20th	Day	21% 7% 28% 23% 15% 6%	551
	21st to end	Day	14% 5% 25% 25% 20% 11%	597
June	1st to 10th	Day	13% 5% 23% 31% 21% 7%	730
	11th to 20th	Day	13% 4 20% 32% 23% 7%	730
	21st to end	Day	11% 5% 23% 30% 23% 9%	730
July	1st to 10th	Day	12% 2 13% 26% 27% 20%	721
	11th to 20th	Day	8% 3 15% 33% 27% 12%	718
	21st to end	Day	10% 2 13% 29% 32% 14%	679
August	1st to 10th	Day	10% 2 11% 25% 33% 19%	526
	11th to 20th	Day	13% 4 14% 27% 29% 14%	519
	21st to end	Day	8% 3 10% 29% 34% 16%	583
September	1st to 10th	Day	10% 4 15% 26% 32% 13%	530
	11th to 20th	Day	9% 3 11% 26% 32% 19%	530
	21st to end	Day	18% 6% 19% 22% 24% 12%	530
October	1st to 10th	Day	11% 6% 19% 33% 21% 10%	530
	11th to 20th	Day	13% 11% 32% 26% 15% 3	480
	21st to end	Day	27% 12% 36% 20% 5% 1	363
November	1st to 10th	Day	14% 15% 32% 24% 13% 2	330
	11th to 20th	Day	24% 15% 29% 19% 11% 2	330
	21st to end	Day	25% 10% 37% 23% 5% 1	330
December	1st to 10th	Day	19% 14% 42% 18% 5% 1	330
	11th to 20th	Day	22% 15% 35% 23% 4 2	330
	21st to end	Day	29% 16% 33% 17% 4 1	363
January	1st to 10th	Night	23% 13% 34% 25% 5%	1050
	11th to 20th	Night	26% 12% 39% 20% 4	1050
	21st to end	Night	24% 12% 35% 26% 3	1032
February	1st to 10th	Night	26% 10% 35% 26% 3	840
	11th to 20th	Night	22% 13% 36% 26% 2	823
	21st to end	Night	24% 11% 39% 24% 2	692
March	1st to 10th	Night	23% 12% 33% 25% 6%	840
	11th to 20th	Night	20% 12% 36% 25% 6% 1	840
	21st to end	Night	25% 8% 33% 28% 5% 1	859
April	1st to 10th	Night	18% 11% 37% 25% 8%	630
	11th to 20th	Night	17% 11% 35% 29% 8% 1	623
	21st to end	Night	21% 6% 38% 26% 8% 1	630
May	1st to 10th	Night	21% 7% 33% 28% 11%	630
	11th to 20th	Night	20% 4 22% 34% 17% 2	609
	21st to end	Night	11% 3 21% 38% 25% 2	650
June	1st to 10th	Night	12% 4 19% 46% 18% 1	600
	11th to 20th	Night	12% 4 17% 44% 21% 2	600
	21st to end	Night	7% 2 21% 37% 30% 3	600
July	1st to 10th	Night	10% 3 8% 36% 37% 6%	585
	11th to 20th	Night	9% 3 12% 39% 33% 5%	582
	21st to end	Night	10% 3 10% 41% 34% 3	657
August	1st to 10th	Night	7% 3 9% 41% 36% 5%	597
	11th to 20th	Night	7% 2 10% 42% 32% 6%	594
	21st to end	Night	7% 2 11% 37% 38% 5%	660
September	1st to 10th	Night	12% 2 11% 38% 32% 5%	628
	11th to 20th	Night	9% 3 13% 35% 35% 6%	800
	21st to end	Night	15% 4 19% 34% 25% 4	800
October	1st to 10th	Night	11% 4 23% 36% 22% 5%	800
	11th to 20th	Night	18% 8% 29% 30% 15% 1	800
	21st to end	Night	21% 9% 36% 27% 7% 1	880
November	1st to 10th	Night	17% 7% 32% 30% 13%	820
	11th to 20th	Night	18% 13% 33% 28% 7%	840
	21st to end	Night	26% 10% 30% 27% 7%	840
December	1st to 10th	Night	23% 9% 38% 24% 5%	840
	11th to 20th	Night	25% 9% 34% 25% 6% 1	982
	21st to end	Night	32% 12% 30% 22% 4	1155

Page generated at 2026-05-14 01:58:57. Version ac07

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