Star
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	136871
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	136262
Transparency	Day	39% 7% 11% 18% 18% 6%	140553
	Night	38% 6% 11% 18% 19% 8%	140659
Seeing	Day	15% 9% 22% 25% 21% 9%	17297
	Night	15% 8% 23% 30% 20% 4	26939

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	16% 12% 3 9% 3 3 3 3 4 5% 38%	5289
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 5% 7% 52%	5310
	21st to end	Day	23% 10% 3 7% 4 3 3 3 3 5% 37%	6078
February	1st to 10th	Day	16% 10% 4 5% 4 4 4 3 4 5% 41%	2100
	11th to 20th	Day	21% 8% 3 6% 4 4 3 3 4 5% 39%	2048
	21st to end	Day	16% 8% 3 8% 5% 4 3 3 4 5% 40%	1730
March	1st to 10th	Day	20% 9% 3 9% 5% 5% 5% 3 5% 7% 31%	4721
	11th to 20th	Day	13% 5% 2 6% 5% 4 4 4 5% 6% 44%	5090
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 5% 40%	5898
April	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	2520
	11th to 20th	Day	14% 6% 2 6% 5% 4 4 2 5% 7% 44%	2494
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 6% 8% 44%	2520
May	1st to 10th	Day	19% 8% 3 7% 4 4 5% 3 6% 7% 34%	7021
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 5% 6% 7% 40%	7031
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 7% 10% 34%	8112
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 5% 7% 10% 36%	2850
	11th to 20th	Day	6% 5% 3 6% 7% 6% 6% 6% 7% 10% 37%	2940
	21st to end	Day	6% 4 2 7% 6% 7% 7% 6% 8% 9% 36%	2912
July	1st to 10th	Day	6% 5% 2 8% 7% 7% 7% 6% 9% 10% 32%	6030
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 7% 9% 13% 35%	6030
	21st to end	Day	9% 6% 3 7% 6% 6% 7% 5% 8% 11% 32%	6231
August	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 7% 62%	3905
	11th to 20th	Day	5% 4 2 6% 5% 4 6% 4 7% 11% 46%	3849
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 6% 8% 43%	4312
September	1st to 10th	Day	9% 5% 2 6% 6% 6% 6% 5% 6% 8% 41%	2400
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 4 5% 7% 43%	2337
	21st to end	Day	10% 4 2 5% 3 3 4 3 5% 8% 52%	2200
October	1st to 10th	Day	7% 4 1 5% 4 4 4 3 5% 6% 57%	2101
	11th to 20th	Day	10% 4 2 6% 3 3 3 3 4 6% 57%	2000
	21st to end	Day	12% 5% 2 5% 3 3 3 2 4 5% 57%	2200
November	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 4 6% 54%	2050
	11th to 20th	Day	11% 7% 2 4 4 3 4 3 4 5% 54%	1890
	21st to end	Day	15% 6% 2 6% 3 4 4 3 4 5% 47%	1890
December	1st to 10th	Day	18% 8% 3 5% 3 3 4 3 5% 6% 43%	3020
	11th to 20th	Day	13% 9% 3 8% 5% 4 4 2 3 5% 43%	3463
	21st to end	Day	15% 6% 2 6% 4 3 4 3 4 6% 47%	4299
January	1st to 10th	Night	19% 10% 4 7% 3 3 3 3 4 4 41%	7634
	11th to 20th	Night	16% 6% 3 5% 3 3 3 2 3 5% 50%	7680
	21st to end	Night	25% 10% 3 7% 4 3 3 2 3 4 37%	8104
February	1st to 10th	Night	15% 9% 3 6% 4 3 3 4 4 5% 44%	2520
	11th to 20th	Night	21% 8% 2 5% 4 3 3 3 4 5% 42%	2458
	21st to end	Night	20% 8% 2 6% 4 4 3 2 5% 5% 40%	2076
March	1st to 10th	Night	18% 9% 2 6% 4 4 4 4 5% 6% 38%	5381
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 49%	5090
	21st to end	Night	17% 6% 2 6% 4 4 5% 3 5% 6% 43%	5271
April	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2100
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 6% 8% 47%	2075
	21st to end	Night	9% 5% 2 4 3 4 5% 3 6% 9% 51%	2100
May	1st to 10th	Night	19% 6% 3 6% 4 4 4 3 5% 8% 38%	5832
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 5% 6% 9% 38%	5829
	21st to end	Night	11% 7% 3 8% 5% 5% 6% 5% 7% 10% 32%	6021
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	1763
	11th to 20th	Night	5% 5% 2 4 5% 5% 5% 5% 6% 10% 48%	1680
	21st to end	Night	5% 5% 2 5% 4 5% 5% 6% 9% 11% 44%	1698
July	1st to 10th	Night	5% 5% 3 8% 6% 5% 6% 5% 8% 11% 38%	4168
	11th to 20th	Night	6% 4 2 7% 5% 5% 6% 6% 10% 15% 34%	4167
	21st to end	Night	7% 6% 3 8% 6% 6% 6% 5% 8% 13% 33%	4845
August	1st to 10th	Night	2 2 1 3 3 3 4 4 6% 9% 61%	3244
	11th to 20th	Night	3 3 1 5% 3 5% 5% 5% 9% 13% 47%	3200
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 4 6% 8% 46%	3586
September	1st to 10th	Night	7% 4 2 6% 5% 5% 6% 5% 6% 8% 44%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 48%	2041
	21st to end	Night	8% 4 2 5% 4 4 4 4 5% 8% 54%	2200
October	1st to 10th	Night	6% 3 2 5% 4 3 3 3 5% 6% 61%	2293
	11th to 20th	Night	9% 5% 2 5% 3 3 4 2 4 6% 58%	2400
	21st to end	Night	14% 5% 1 5% 3 3 3 2 3 4 57%	2640
November	1st to 10th	Night	11% 5% 1 4 3 3 2 3 5% 5% 57%	2465
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	2730
	21st to end	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2730
December	1st to 10th	Night	21% 7% 2 5% 3 3 3 3 4 4 44%	4380
	11th to 20th	Night	16% 10% 2 6% 4 4 4 3 4 4 45%	4952
	21st to end	Night	19% 10% 2 5% 3 3 2 2 3 5% 46%	6909

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	47% 4 6% 21% 19% 3	5290
	11th to 20th	Day	31% 4 7% 22% 31% 7%	5310
	21st to end	Day	52% 3 5% 11% 17% 12%	6078
February	1st to 10th	Day	46% 5% 7% 15% 18% 10%	2100
	11th to 20th	Day	47% 5% 6% 16% 19% 7%	2058
	21st to end	Day	45% 6% 8% 13% 18% 10%	1730
March	1st to 10th	Day	52% 5% 7% 14% 17% 6%	4721
	11th to 20th	Day	39% 5% 7% 14% 19% 16%	5090
	21st to end	Day	46% 5% 8% 17% 16% 7%	5900
April	1st to 10th	Day	39% 6% 10% 20% 17% 8%	2520
	11th to 20th	Day	40% 5% 7% 18% 22% 7%	2494
	21st to end	Day	35% 8% 11% 22% 22% 3	2520
May	1st to 10th	Day	47% 8% 11% 18% 13% 3	7056
	11th to 20th	Day	40% 7% 15% 18% 18% 2	7035
	21st to end	Day	42% 10% 16% 20% 12% 1	8126
June	1st to 10th	Day	40% 11% 14% 21% 12% 2	2850
	11th to 20th	Day	38% 10% 17% 20% 12% 3	2940
	21st to end	Day	38% 13% 18% 20% 10% 2	2912
July	1st to 10th	Day	41% 12% 19% 19% 8% 1	5966
	11th to 20th	Day	35% 14% 30% 18% 2 1	5961
	21st to end	Day	42% 11% 17% 25% 4	6057
August	1st to 10th	Day	21% 7% 13% 28% 27% 3	3893
	11th to 20th	Day	29% 11% 18% 28% 12% 2	3885
	21st to end	Day	38% 9% 13% 19% 16% 3	4311
September	1st to 10th	Day	38% 11% 14% 17% 15% 6%	2400
	11th to 20th	Day	39% 9% 10% 17% 18% 7%	2337
	21st to end	Day	30% 6% 13% 24% 19% 9%	2200
October	1st to 10th	Day	21% 4 7% 14% 26% 29%	3426
	11th to 20th	Day	18% 3 5% 13% 31% 30%	3240
	21st to end	Day	23% 3 4 16% 37% 17%	3538
November	1st to 10th	Day	32% 6% 7% 16% 28% 11%	2050
	11th to 20th	Day	32% 4 7% 16% 27% 13%	1890
	21st to end	Day	39% 4 8% 16% 23% 10%	1890
December	1st to 10th	Day	42% 5% 7% 16% 25% 5%	3020
	11th to 20th	Day	45% 4 8% 15% 20% 8%	3460
	21st to end	Day	38% 5% 6% 22% 26% 2	4299
January	1st to 10th	Night	47% 3 7% 18% 21% 5%	7635
	11th to 20th	Night	38% 3 5% 14% 28% 12%	7680
	21st to end	Night	53% 3 4 14% 19% 9%	8104
February	1st to 10th	Night	43% 4 7% 16% 21% 9%	2520
	11th to 20th	Night	45% 4 7% 15% 20% 9%	2459
	21st to end	Night	46% 5% 7% 13% 16% 13%	2076
March	1st to 10th	Night	45% 6% 8% 13% 19% 9%	5381
	11th to 20th	Night	34% 5% 8% 17% 19% 16%	5090
	21st to end	Night	42% 6% 10% 17% 18% 7%	5271
April	1st to 10th	Night	38% 7% 10% 18% 18% 10%	2100
	11th to 20th	Night	33% 5% 10% 22% 21% 9%	2075
	21st to end	Night	29% 6% 12% 23% 24% 6%	2100
May	1st to 10th	Night	44% 7% 12% 19% 15% 4	5862
	11th to 20th	Night	40% 8% 15% 20% 15% 2	5830
	21st to end	Night	43% 11% 16% 18% 11% 1	6032
June	1st to 10th	Night	35% 10% 16% 22% 15% 1	1763
	11th to 20th	Night	28% 10% 20% 25% 13% 3	1680
	21st to end	Night	29% 11% 22% 23% 13% 2	1698
July	1st to 10th	Night	36% 13% 23% 19% 7% 1	4125
	11th to 20th	Night	34% 15% 31% 16% 3 1	4116
	21st to end	Night	41% 13% 19% 22% 5%	4706
August	1st to 10th	Night	18% 9% 17% 31% 23% 2	3232
	11th to 20th	Night	26% 11% 21% 30% 10% 2	3230
	21st to end	Night	36% 10% 13% 20% 18% 4	3586
September	1st to 10th	Night	34% 12% 15% 17% 16% 6%	2000
	11th to 20th	Night	33% 8% 14% 18% 19% 8%	2041
	21st to end	Night	29% 6% 11% 24% 21% 10%	2200
October	1st to 10th	Night	19% 4 7% 13% 24% 32%	3748
	11th to 20th	Night	18% 3 4 18% 24% 33%	3899
	21st to end	Night	22% 3 5% 13% 35% 21%	4252
November	1st to 10th	Night	28% 5% 8% 17% 27% 14%	2465
	11th to 20th	Night	31% 4 7% 15% 28% 15%	2730
	21st to end	Night	36% 4 9% 19% 20% 12%	2730
December	1st to 10th	Night	44% 4 6% 16% 25% 5%	4380
	11th to 20th	Night	44% 4 7% 15% 24% 7%	4954
	21st to end	Night	43% 3 5% 19% 26% 4	6909

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	25% 24% 35% 14% 2 1	330
	11th to 20th	Day	24% 20% 37% 13% 6% 1	330
	21st to end	Day	31% 19% 33% 14% 2	363
February	1st to 10th	Day	24% 22% 38% 11% 5% 1	330
	11th to 20th	Day	38% 16% 32% 12% 2 1	330
	21st to end	Day	21% 29% 38% 10% 2	273
March	1st to 10th	Day	27% 17% 37% 15% 4	330
	11th to 20th	Day	26% 22% 33% 16% 3	330
	21st to end	Day	22% 15% 32% 26% 5%	497
April	1st to 10th	Day	19% 16% 34% 22% 8%	540
	11th to 20th	Day	20% 15% 31% 22% 10% 2	532
	21st to end	Day	13% 11% 33% 27% 15% 1	540
May	1st to 10th	Day	17% 12% 28% 27% 13% 3	543
	11th to 20th	Day	13% 7% 28% 27% 18% 8%	567
	21st to end	Day	8% 4 25% 32% 21% 9%	617
June	1st to 10th	Day	13% 3 15% 35% 24% 9%	690
	11th to 20th	Day	8% 3 14% 36% 30% 9%	780
	21st to end	Day	7% 2 13% 29% 34% 15%	780
July	1st to 10th	Day	5% 2 8% 32% 37% 16%	771
	11th to 20th	Day	7% 1 5% 23% 43% 20%	768
	21st to end	Day	7% 1 9% 26% 40% 17%	701
August	1st to 10th	Day	5% 1 7% 24% 40% 23%	526
	11th to 20th	Day	7% 2 6% 23% 38% 25%	519
	21st to end	Day	13% 2 8% 26% 29% 22%	583
September	1st to 10th	Day	9% 1 12% 29% 34% 15%	530
	11th to 20th	Day	12% 2 10% 21% 36% 18%	530
	21st to end	Day	11% 4 14% 31% 29% 11%	530
October	1st to 10th	Day	8% 6% 23% 38% 17% 9%	431
	11th to 20th	Day	10% 6% 28% 31% 20% 6%	330
	21st to end	Day	20% 8% 26% 29% 12% 5%	363
November	1st to 10th	Day	16% 10% 27% 33% 10% 4	330
	11th to 20th	Day	15% 13% 37% 25% 10% 1	330
	21st to end	Day	17% 20% 35% 15% 12% 2	330
December	1st to 10th	Day	26% 15% 35% 17% 6% 1	330
	11th to 20th	Day	19% 16% 38% 21% 5% 1	330
	21st to end	Day	24% 17% 32% 18% 9%	363
January	1st to 10th	Night	25% 14% 31% 25% 5%	1050
	11th to 20th	Night	23% 15% 31% 25% 6%	1050
	21st to end	Night	25% 15% 32% 24% 3	1051
February	1st to 10th	Night	20% 14% 35% 27% 4	840
	11th to 20th	Night	28% 16% 29% 24% 4	823
	21st to end	Night	26% 19% 35% 19% 2	692
March	1st to 10th	Night	20% 15% 31% 29% 5%	793
	11th to 20th	Night	15% 13% 38% 27% 7%	630
	21st to end	Night	16% 10% 39% 27% 8%	693
April	1st to 10th	Night	18% 11% 33% 27% 10% 1	630
	11th to 20th	Night	14% 11% 36% 27% 11% 1	623
	21st to end	Night	10% 8% 33% 35% 13%	630
May	1st to 10th	Night	12% 5% 30% 36% 15% 1	630
	11th to 20th	Night	11% 4 26% 37% 19% 3	621
	21st to end	Night	8% 3 19% 35% 29% 6%	683
June	1st to 10th	Night	9% 2 13% 41% 30% 5%	630
	11th to 20th	Night	5% 1 9% 38% 39% 8%	630
	21st to end	Night	6% 2 8% 30% 42% 13%	630
July	1st to 10th	Night	6% 1 6% 27% 48% 12%	615
	11th to 20th	Night	6% 1 5% 24% 47% 17%	612
	21st to end	Night	5% 1 6% 28% 47% 13%	687
August	1st to 10th	Night	3 5% 25% 49% 19%	597
	11th to 20th	Night	6% 2 6% 25% 47% 15%	594
	21st to end	Night	11% 2 6% 25% 38% 17%	660
September	1st to 10th	Night	8% 1 10% 30% 44% 7%	600
	11th to 20th	Night	10% 3 10% 28% 42% 7%	641
	21st to end	Night	10% 2 12% 37% 35% 5%	800
October	1st to 10th	Night	7% 2 19% 37% 30% 6%	800
	11th to 20th	Night	12% 5% 22% 35% 23% 3	800
	21st to end	Night	16% 7% 22% 33% 20% 2	880
November	1st to 10th	Night	14% 6% 20% 39% 19% 3	820
	11th to 20th	Night	13% 8% 28% 36% 14% 1	840
	21st to end	Night	17% 9% 31% 29% 13% 1	840
December	1st to 10th	Night	22% 11% 29% 30% 9%	840
	11th to 20th	Night	22% 9% 31% 30% 8%	840
	21st to end	Night	24% 14% 29% 26% 6%	1144

Page generated at 2026-07-28 02:25:26. Version ac07

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