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 2025-01-07.

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	7% 5% 2 7% 6% 6% 6% 5% 7% 10% 39%	104974
	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	104007
Transparency	Day	37% 10% 16% 19% 14% 5%	108765
	Night	36% 9% 16% 19% 14% 5%	108355
Seeing	Day	8% 6% 19% 26% 23% 17%	15702
	Night	9% 4 21% 33% 25% 8%	25786

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	10% 7% 3 7% 5% 5% 5% 3 6% 8% 41%	2890
	11th to 20th	Day	21% 8% 3 7% 4 3 3 3 5% 8% 34%	2840
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 8% 45%	3357
February	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 46%	1900
	11th to 20th	Day	8% 4 2 6% 4 4 4 3 6% 6% 55%	1848
	21st to end	Day	7% 6% 2 5% 3 4 4 4 6% 9% 50%	1632
March	1st to 10th	Day	7% 6% 3 8% 6% 6% 6% 5% 7% 9% 38%	3435
	11th to 20th	Day	6% 4 2 5% 4 3 4 3 7% 11% 50%	3480
	21st to end	Day	9% 5% 2 4 4 3 3 4 6% 8% 52%	3815
April	1st to 10th	Day	6% 4 2 5% 5% 4 5% 4 7% 10% 48%	2090
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 5% 6% 9% 45%	2066
	21st to end	Day	4 4 1 5% 4 5% 5% 4 7% 9% 52%	2263
May	1st to 10th	Day	3 2 1 3 3 3 4 5% 7% 9% 59%	4082
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 6% 8% 11% 40%	4081
	21st to end	Day	4 3 2 5% 5% 5% 5% 5% 7% 9% 52%	4476
June	1st to 10th	Day	12% 7% 3 9% 7% 6% 7% 6% 9% 10% 23%	2470
	11th to 20th	Day	6% 6% 3 9% 8% 8% 8% 6% 10% 11% 26%	2470
	21st to end	Day	7% 6% 3 8% 8% 8% 7% 7% 9% 10% 25%	2470
July	1st to 10th	Day	6% 6% 2 10% 9% 8% 9% 7% 9% 12% 22%	4120
	11th to 20th	Day	4 5% 3 10% 9% 8% 10% 7% 9% 13% 22%	4120
	21st to end	Day	4 5% 3 10% 9% 9% 9% 7% 10% 11% 23%	4466
August	1st to 10th	Day	6% 6% 3 10% 8% 7% 7% 6% 9% 12% 25%	4104
	11th to 20th	Day	4 5% 2 7% 7% 6% 7% 6% 8% 12% 35%	3712
	21st to end	Day	7% 6% 3 9% 7% 8% 7% 6% 10% 14% 24%	3828
September	1st to 10th	Day	7% 7% 3 10% 8% 8% 10% 7% 8% 11% 22%	2090
	11th to 20th	Day	10% 7% 4 9% 7% 8% 7% 7% 8% 8% 25%	2090
	21st to end	Day	8% 7% 3 9% 6% 6% 7% 5% 8% 11% 30%	2090
October	1st to 10th	Day	9% 7% 2 8% 7% 6% 6% 5% 7% 8% 33%	2090
	11th to 20th	Day	6% 5% 2 5% 5% 5% 5% 5% 5% 9% 47%	1929
	21st to end	Day	7% 5% 2 6% 4 4 4 4 6% 8% 49%	2090
November	1st to 10th	Day	8% 4 2 6% 4 4 5% 4 7% 8% 50%	1957
	11th to 20th	Day	8% 6% 3 6% 5% 4 5% 5% 5% 7% 48%	2000
	21st to end	Day	9% 6% 2 6% 4 5% 5% 4 6% 8% 46%	1854
December	1st to 10th	Day	4 4 1 4 3 4 3 4 5% 8% 59%	2930
	11th to 20th	Day	8% 6% 3 7% 4 4 4 4 5% 8% 47%	3373
	21st to end	Day	11% 7% 3 8% 5% 5% 6% 4 6% 8% 38%	4466
January	1st to 10th	Night	11% 7% 4 8% 5% 5% 4 4 5% 8% 39%	4188
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 3 5% 7% 32%	4120
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	4294
February	1st to 10th	Night	11% 7% 2 5% 3 4 4 4 6% 7% 46%	2280
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 6% 8% 51%	2218
	21st to end	Night	7% 5% 2 7% 4 4 4 4 5% 7% 51%	1820
March	1st to 10th	Night	14% 8% 3 8% 5% 5% 4 3 5% 6% 37%	3418
	11th to 20th	Night	4 3 2 5% 4 4 4 5% 9% 12% 48%	3480
	21st to end	Night	10% 5% 2 5% 4 3 4 4 5% 7% 52%	3817
April	1st to 10th	Night	6% 3 2 5% 4 4 4 3 7% 10% 53%	2090
	11th to 20th	Night	7% 4 2 6% 4 4 5% 4 6% 10% 49%	2063
	21st to end	Night	5% 5% 2 4 3 4 4 3 4 8% 57%	1924
May	1st to 10th	Night	5% 3 1 4 4 3 4 4 6% 8% 58%	2815
	11th to 20th	Night	7% 5% 2 5% 4 5% 4 5% 7% 9% 48%	2812
	21st to end	Night	5% 3 1 4 3 4 4 3 5% 8% 60%	3079
June	1st to 10th	Night	10% 7% 3 6% 5% 5% 6% 5% 8% 9% 36%	1710
	11th to 20th	Night	5% 7% 2 6% 6% 5% 6% 6% 9% 14% 35%	1710
	21st to end	Night	6% 6% 2 8% 5% 6% 7% 7% 9% 13% 32%	1710
July	1st to 10th	Night	6% 6% 3 11% 8% 7% 9% 6% 8% 11% 26%	2840
	11th to 20th	Night	3 5% 3 9% 7% 7% 9% 7% 11% 11% 26%	2840
	21st to end	Night	5% 8% 4 13% 9% 8% 8% 6% 9% 10% 19%	3087
August	1st to 10th	Night	5% 5% 3 8% 6% 5% 7% 6% 9% 13% 33%	2825
	11th to 20th	Night	3 5% 1 6% 5% 6% 7% 6% 10% 16% 36%	3084
	21st to end	Night	6% 5% 3 9% 7% 7% 8% 6% 9% 13% 28%	3821
September	1st to 10th	Night	7% 7% 2 9% 6% 6% 7% 7% 10% 12% 27%	2090
	11th to 20th	Night	9% 7% 3 9% 7% 6% 6% 6% 8% 11% 29%	2090
	21st to end	Night	9% 8% 3 8% 5% 6% 5% 5% 8% 8% 34%	2090
October	1st to 10th	Night	9% 7% 2 8% 4 4 5% 5% 8% 9% 39%	2090
	11th to 20th	Night	7% 4 1 6% 5% 4 5% 4 6% 8% 50%	2248
	21st to end	Night	7% 6% 3 7% 4 3 3 3 5% 8% 51%	2508
November	1st to 10th	Night	7% 5% 1 5% 4 4 5% 4 5% 7% 53%	2341
	11th to 20th	Night	8% 6% 2 6% 5% 4 5% 4 5% 7% 49%	2400
	21st to end	Night	7% 5% 2 8% 5% 4 5% 3 6% 8% 47%	2543
December	1st to 10th	Night	5% 3 1 4 3 3 3 3 6% 9% 60%	4250
	11th to 20th	Night	10% 6% 2 6% 4 3 3 3 5% 7% 51%	4822
	21st to end	Night	12% 9% 3 8% 5% 5% 5% 4 5% 7% 36%	6490

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	40% 6% 8% 19% 19% 8%	2891
	11th to 20th	Day	49% 4 7% 16% 17% 8%	2840
	21st to end	Day	36% 6% 8% 19% 23% 7%	3357
February	1st to 10th	Day	35% 7% 9% 19% 19% 11%	1900
	11th to 20th	Day	28% 5% 9% 19% 26% 12%	1858
	21st to end	Day	32% 5% 11% 16% 24% 11%	1632
March	1st to 10th	Day	39% 10% 13% 18% 14% 6%	3435
	11th to 20th	Day	27% 6% 13% 29% 20% 6%	3480
	21st to end	Day	29% 5% 9% 30% 17% 9%	3815
April	1st to 10th	Day	29% 8% 15% 22% 18% 8%	2090
	11th to 20th	Day	33% 9% 12% 17% 21% 8%	2066
	21st to end	Day	27% 6% 12% 23% 25% 8%	2263
May	1st to 10th	Day	17% 8% 15% 34% 20% 6%	4120
	11th to 20th	Day	34% 10% 17% 24% 13% 2	4085
	21st to end	Day	25% 9% 12% 32% 20% 2	4489
June	1st to 10th	Day	50% 12% 18% 15% 5%	2470
	11th to 20th	Day	45% 15% 19% 18% 3	2470
	21st to end	Day	47% 15% 22% 13% 2 1	2470
July	1st to 10th	Day	47% 15% 30% 7% 1	4055
	11th to 20th	Day	47% 17% 29% 6%	4051
	21st to end	Day	48% 18% 24% 10% 1	4324
August	1st to 10th	Day	46% 14% 27% 11% 2	4091
	11th to 20th	Day	37% 15% 25% 19% 5%	3748
	21st to end	Day	45% 16% 25% 13% 1	3828
September	1st to 10th	Day	49% 19% 19% 12% 2	2090
	11th to 20th	Day	50% 15% 19% 11% 5%	2090
	21st to end	Day	44% 12% 14% 16% 10% 3	2090
October	1st to 10th	Day	56% 12% 12% 9% 6% 5%	3480
	11th to 20th	Day	27% 5% 9% 21% 33% 5%	3182
	21st to end	Day	28% 6% 9% 22% 28% 7%	3428
November	1st to 10th	Day	30% 7% 10% 22% 20% 11%	1957
	11th to 20th	Day	35% 7% 9% 20% 20% 10%	2000
	21st to end	Day	35% 6% 10% 19% 20% 10%	1854
December	1st to 10th	Day	24% 5% 13% 30% 19% 9%	2930
	11th to 20th	Day	34% 5% 11% 19% 20% 10%	3370
	21st to end	Day	42% 6% 12% 20% 15% 5%	4466
January	1st to 10th	Night	43% 6% 10% 17% 19% 6%	4188
	11th to 20th	Night	52% 4 8% 19% 13% 5%	4120
	21st to end	Night	40% 7% 12% 15% 19% 7%	4294
February	1st to 10th	Night	36% 6% 11% 18% 20% 10%	2280
	11th to 20th	Night	31% 6% 12% 18% 23% 10%	2219
	21st to end	Night	31% 5% 14% 18% 23% 8%	1820
March	1st to 10th	Night	45% 9% 11% 16% 14% 5%	3418
	11th to 20th	Night	24% 9% 22% 25% 15% 5%	3480
	21st to end	Night	31% 5% 10% 26% 22% 6%	3817
April	1st to 10th	Night	26% 6% 19% 25% 16% 7%	2090
	11th to 20th	Night	29% 8% 14% 23% 21% 6%	2063
	21st to end	Night	25% 6% 13% 26% 24% 7%	1924
May	1st to 10th	Night	20% 11% 18% 28% 17% 5%	2842
	11th to 20th	Night	29% 11% 21% 26% 13% 1	2813
	21st to end	Night	21% 8% 16% 36% 19% 1	3087
June	1st to 10th	Night	39% 12% 22% 21% 6%	1710
	11th to 20th	Night	37% 12% 27% 21% 4	1710
	21st to end	Night	39% 17% 25% 14% 5%	1710
July	1st to 10th	Night	48% 17% 28% 7% 1	2795
	11th to 20th	Night	45% 20% 30% 5%	2786
	21st to end	Night	56% 17% 20% 7%	2988
August	1st to 10th	Night	38% 15% 31% 11% 4	2814
	11th to 20th	Night	31% 16% 32% 18% 3	3114
	21st to end	Night	43% 18% 27% 10% 2	3821
September	1st to 10th	Night	43% 20% 23% 12% 2	2090
	11th to 20th	Night	46% 15% 22% 15% 2	2090
	21st to end	Night	43% 12% 17% 18% 8% 2	2090
October	1st to 10th	Night	57% 11% 10% 11% 8% 4	3480
	11th to 20th	Night	26% 5% 10% 29% 25% 5%	3734
	21st to end	Night	30% 5% 9% 22% 28% 6%	4120
November	1st to 10th	Night	29% 6% 13% 21% 21% 10%	2341
	11th to 20th	Night	34% 7% 10% 20% 20% 9%	2400
	21st to end	Night	34% 6% 9% 22% 20% 8%	2543
December	1st to 10th	Night	21% 5% 18% 29% 15% 12%	4250
	11th to 20th	Night	33% 6% 12% 22% 17% 10%	4824
	21st to end	Night	44% 7% 13% 18% 14% 4	6490

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	10% 14% 44% 25% 7% 1	288
	11th to 20th	Day	9% 15% 34% 31% 10% 2	270
	21st to end	Day	16% 16% 45% 16% 6%	297
February	1st to 10th	Day	11% 11% 37% 27% 11% 1	270
	11th to 20th	Day	10% 10% 35% 30% 11% 3	270
	21st to end	Day	8% 13% 31% 31% 15% 1	287
March	1st to 10th	Day	4 13% 40% 31% 11% 1	460
	11th to 20th	Day	9% 9% 30% 31% 19% 2	460
	21st to end	Day	6% 11% 30% 33% 19% 2	506
April	1st to 10th	Day	7% 11% 33% 28% 17% 3	460
	11th to 20th	Day	9% 10% 28% 32% 18% 4	452
	21st to end	Day	5% 6% 23% 35% 25% 6%	460
May	1st to 10th	Day	4 5% 20% 37% 25% 10%	463
	11th to 20th	Day	9% 4 19% 29% 26% 14%	487
	21st to end	Day	7% 2 9% 24% 34% 24%	529
June	1st to 10th	Day	14% 2 10% 23% 27% 23%	490
	11th to 20th	Day	7% 2 10% 24% 29% 28%	490
	21st to end	Day	5% 1 4 16% 29% 45%	490
July	1st to 10th	Day	7% 1 2 12% 28% 49%	485
	11th to 20th	Day	5% 1 3 15% 29% 47%	484
	21st to end	Day	7% 1 4 17% 29% 41%	532
August	1st to 10th	Day	7% 2 4 14% 30% 41%	486
	11th to 20th	Day	7% 2 4 13% 27% 46%	479
	21st to end	Day	9% 3 6% 18% 32% 32%	539
September	1st to 10th	Day	8% 3 6% 16% 30% 37%	490
	11th to 20th	Day	9% 3 9% 24% 29% 26%	490
	21st to end	Day	7% 3 8% 26% 33% 23%	490
October	1st to 10th	Day	11% 3 19% 33% 26% 8%	490
	11th to 20th	Day	5% 4 20% 35% 28% 8%	490
	21st to end	Day	7% 6% 21% 39% 24% 4	539
November	1st to 10th	Day	9% 5% 22% 34% 24% 5%	495
	11th to 20th	Day	11% 7% 23% 37% 18% 4	500
	21st to end	Day	8% 9% 33% 31% 16% 4	354
December	1st to 10th	Day	7% 11% 38% 25% 14% 5%	300
	11th to 20th	Day	9% 11% 36% 30% 12% 2	300
	21st to end	Day	8% 13% 32% 34% 12% 2	330
January	1st to 10th	Night	11% 9% 31% 37% 11%	785
	11th to 20th	Night	12% 9% 25% 41% 12% 1	760
	21st to end	Night	14% 9% 34% 32% 9%	836
February	1st to 10th	Night	16% 6% 31% 32% 14%	760
	11th to 20th	Night	11% 8% 29% 38% 13% 1	743
	21st to end	Night	9% 6% 31% 44% 10%	628
March	1st to 10th	Night	7% 7% 36% 40% 9% 1	760
	11th to 20th	Night	7% 7% 32% 39% 14% 1	760
	21st to end	Night	7% 7% 31% 39% 16%	836
April	1st to 10th	Night	7% 6% 31% 41% 15%	760
	11th to 20th	Night	7% 5% 36% 38% 13% 1	751
	21st to end	Night	8% 5% 26% 44% 16% 1	760
May	1st to 10th	Night	4 1 16% 48% 28% 3	572
	11th to 20th	Night	7% 3 17% 40% 28% 4	561
	21st to end	Night	6% 1 13% 38% 37% 5%	617
June	1st to 10th	Night	10% 2 9% 26% 42% 10%	570
	11th to 20th	Night	7% 2 4 21% 43% 23%	570
	21st to end	Night	6% 2 4 18% 42% 28%	570
July	1st to 10th	Night	7% 1 2 11% 48% 31%	555
	11th to 20th	Night	5% 3 9% 45% 37%	552
	21st to end	Night	5% 1 4 12% 46% 32%	624
August	1st to 10th	Night	5% 1 2 11% 48% 32%	567
	11th to 20th	Night	5% 2 2 10% 40% 42%	564
	21st to end	Night	11% 2 8% 21% 36% 23%	829
September	1st to 10th	Night	9% 2 7% 22% 42% 18%	760
	11th to 20th	Night	10% 3 8% 29% 36% 14%	760
	21st to end	Night	12% 3 10% 30% 34% 11%	760
October	1st to 10th	Night	12% 3 21% 37% 23% 4	760
	11th to 20th	Night	6% 4 18% 39% 30% 2	760
	21st to end	Night	9% 3 23% 41% 22% 2	836
November	1st to 10th	Night	8% 3 30% 36% 19% 3	780
	11th to 20th	Night	11% 4 24% 41% 20% 1	800
	21st to end	Night	9% 6% 29% 42% 15% 1	800
December	1st to 10th	Night	8% 4 29% 38% 21% 1	800
	11th to 20th	Night	10% 7% 24% 41% 17% 1	800
	21st to end	Night	11% 6% 29% 38% 15% 1	880

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