STAR School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the STAR School clear sky chart from 2005-11-06 to 2025-05-26.

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	2 3 1 4 3 3 4 3 5% 7% 64%	118801
	Night	2 3 1 3 3 3 3 3 5% 6% 68%	119435
Transparency	Day	18% 5% 8% 19% 28% 23%	118568
	Night	16% 4 7% 19% 30% 23%	119258
Seeing	Day	5% 11% 22% 28% 24% 10%	15552
	Night	3 6% 25% 38% 25% 4	25740

By Month

Percentage Cloud Cover SW (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	1 4 2 5% 3 3 4 3 4 6% 65%	4098
	11th to 20th	Day	1 3 1 3 2 2 3 3 4 4 73%	4100
	21st to end	Day	3 6% 2 5% 3 4 4 3 4 5% 62%	4510
February	1st to 10th	Day	1 3 1 5% 3 3 3 4 4 8% 66%	1800
	11th to 20th	Day	2 4 2 4 2 3 3 4 6% 7% 64%	1928
	21st to end	Day	2 2 1 3 3 3 3 3 4 6% 69%	1680
March	1st to 10th	Day	1 4 2 3 2 2 2 2 4 7% 70%	4954
	11th to 20th	Day	4 8% 3 6% 4 3 4 3 5% 7% 56%	5000
	21st to end	Day	2 3 1 3 3 3 3 3 5% 8% 66%	5486
April	1st to 10th	Day	1 2 1 3 3 3 4 3 5% 8% 67%	2200
	11th to 20th	Day	1 3 1 4 2 2 2 2 4 6% 71%	2348
	21st to end	Day	2 2 1 4 2 3 3 3 4 6% 70%	2570
May	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 77%	4240
	11th to 20th	Day	2 3 1 5% 4 4 4 3 6% 6% 63%	4201
	21st to end	Day	1 1 1 3 2 2 2 2 3 4 79%	4544
June	1st to 10th	Day	1 2 1 2 3 2 3 2 5% 6% 74%	2470
	11th to 20th	Day	2 2 1 2 1 1 3 1 3 4 81%	2470
	21st to end	Day	1 2 1 3 3 4 4 3 5% 6% 68%	2470
July	1st to 10th	Day	2 2 1 3 3 4 5% 3 5% 6% 66%	4110
	11th to 20th	Day	3 3 1 5% 6% 7% 9% 7% 10% 11% 37%	4110
	21st to end	Day	3 4 2 6% 6% 7% 8% 6% 9% 10% 39%	4454
August	1st to 10th	Day	4 4 2 7% 7% 7% 9% 7% 8% 10% 36%	4092
	11th to 20th	Day	2 3 1 6% 5% 6% 8% 6% 9% 9% 45%	4028
	21st to end	Day	4 3 1 5% 4 5% 6% 5% 7% 8% 52%	4291
September	1st to 10th	Day	3 4 1 5% 5% 5% 5% 4 6% 8% 54%	2090
	11th to 20th	Day	2 2 1 3 3 3 4 3 5% 6% 69%	2090
	21st to end	Day	2 2 1 3 3 3 2 3 4 6% 72%	2090
October	1st to 10th	Day	2 2 1 2 2 3 3 2 4 4 75%	3343
	11th to 20th	Day	4 2 1 3 2 2 3 2 3 5% 73%	3150
	21st to end	Day	1 2 2 1 2 2 2 3 5% 81%	3438
November	1st to 10th	Day	1 2 1 2 2 2 3 2 3 6% 77%	1959
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 5% 78%	1818
	21st to end	Day	3 4 1 3 3 2 2 2 5% 6% 70%	1800
December	1st to 10th	Day	1 2 1 2 2 2 2 2 4 7% 74%	2950
	11th to 20th	Day	1 4 2 5% 3 3 3 3 6% 10% 61%	3409
	21st to end	Day	3 4 1 5% 3 3 3 3 6% 8% 61%	4510
January	1st to 10th	Night	1 5% 2 4 2 3 3 3 5% 6% 65%	5867
	11th to 20th	Night	1 2 1 2 2 2 2 2 4 6% 77%	5880
	21st to end	Night	3 6% 2 4 3 3 4 3 4 6% 63%	6468
February	1st to 10th	Night	1 3 1 3 3 3 3 3 5% 5% 71%	2600
	11th to 20th	Night	2 3 1 4 2 3 3 3 4 6% 68%	2357
	21st to end	Night	1 3 1 3 2 2 3 3 5% 7% 70%	1949
March	1st to 10th	Night	1 4 1 3 2 2 2 2 4 5% 73%	4940
	11th to 20th	Night	5% 8% 2 5% 4 3 4 3 6% 6% 54%	5000
	21st to end	Night	1 3 2 4 3 3 3 3 5% 7% 67%	5489
April	1st to 10th	Night	1 2 1 4 2 2 3 2 5% 7% 70%	2200
	11th to 20th	Night	1 3 1 3 2 3 3 3 4 6% 71%	1998
	21st to end	Night	1 2 1 2 2 2 2 2 4 4 77%	1826
May	1st to 10th	Night	1 2 1 3 3 2 2 2 3 5% 74%	2950
	11th to 20th	Night	1 2 1 4 3 4 3 2 4 5% 71%	2924
	21st to end	Night	1 1 1 1 1 1 1 2 4 85%	3159
June	1st to 10th	Night	2 2 2 2 2 3 2 3 6% 75%	1710
	11th to 20th	Night	2 2 1 2 1 2 1 1 2 3 81%	1710
	21st to end	Night	1 2 1 3 2 2 2 3 4 6% 73%	1710
July	1st to 10th	Night	1 1 1 2 3 3 3 3 5% 7% 71%	2860
	11th to 20th	Night	3 3 2 7% 6% 7% 8% 6% 9% 10% 39%	2860
	21st to end	Night	3 4 1 4 4 6% 7% 5% 9% 10% 48%	3109
August	1st to 10th	Night	4 4 3 9% 6% 7% 7% 5% 9% 10% 36%	2847
	11th to 20th	Night	2 2 1 3 3 4 4 5% 8% 11% 59%	2811
	21st to end	Night	3 3 1 3 4 4 5% 4 8% 10% 55%	3373
September	1st to 10th	Night	3 3 1 4 3 3 4 5% 6% 8% 60%	2090
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 6% 75%	2090
	21st to end	Night	2 1 1 2 2 2 2 2 4 5% 76%	2090
October	1st to 10th	Night	2 2 1 3 2 2 3 3 4 4 75%	3631
	11th to 20th	Night	4 3 1 4 2 2 2 2 3 4 72%	3778
	21st to end	Night	2 2 1 2 1 1 2 2 2 5% 79%	4120
November	1st to 10th	Night	1 2 1 3 3 2 2 2 4 6% 75%	2340
	11th to 20th	Night	1 2 1 2 1 2 2 2 4 4 79%	2580
	21st to end	Night	3 2 1 3 2 3 4 3 4 7% 70%	2600
December	1st to 10th	Night	2 3 1 2 1 1 2 2 4 8% 73%	4240
	11th to 20th	Night	1 2 1 2 2 3 3 3 5% 8% 68%	4811
	21st to end	Night	3 4 1 4 3 3 3 2 4 5% 68%	6468

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	19% 5% 8% 29% 32% 7%	4099
	11th to 20th	Day	13% 5% 9% 24% 25% 24%	4100
	21st to end	Day	22% 6% 8% 19% 27% 17%	4510
February	1st to 10th	Day	17% 4 12% 19% 33% 16%	1800
	11th to 20th	Day	17% 5% 9% 24% 27% 17%	1938
	21st to end	Day	16% 5% 9% 20% 32% 18%	1680
March	1st to 10th	Day	14% 3 5% 17% 37% 23%	4954
	11th to 20th	Day	25% 6% 7% 24% 28% 11%	5000
	21st to end	Day	15% 4 6% 30% 33% 11%	5486
April	1st to 10th	Day	16% 4 7% 18% 36% 20%	2200
	11th to 20th	Day	13% 3 7% 15% 35% 27%	2351
	21st to end	Day	14% 4 7% 16% 37% 23%	2570
May	1st to 10th	Day	10% 2 6% 15% 36% 31%	4240
	11th to 20th	Day	13% 8% 9% 13% 31% 25%	4204
	21st to end	Day	8% 3 5% 10% 26% 48%	4544
June	1st to 10th	Day	10% 3 5% 10% 29% 43%	2470
	11th to 20th	Day	9% 2 4 6% 21% 59%	2470
	21st to end	Day	15% 4 6% 10% 22% 43%	2470
July	1st to 10th	Day	17% 4 7% 9% 10% 53%	4046
	11th to 20th	Day	31% 10% 9% 20% 17% 13%	4041
	21st to end	Day	31% 10% 12% 18% 18% 10%	4314
August	1st to 10th	Day	39% 7% 15% 20% 9% 11%	4078
	11th to 20th	Day	29% 7% 11% 19% 16% 16%	4067
	21st to end	Day	26% 8% 11% 17% 25% 14%	4290
September	1st to 10th	Day	26% 7% 11% 13% 16% 27%	2090
	11th to 20th	Day	13% 6% 8% 15% 27% 31%	2090
	21st to end	Day	14% 3 6% 15% 33% 30%	2090
October	1st to 10th	Day	13% 4 4 9% 23% 47%	3343
	11th to 20th	Day	15% 3 2 9% 48% 23%	3150
	21st to end	Day	8% 2 3 16% 41% 29%	3438
November	1st to 10th	Day	11% 2 5% 26% 36% 19%	1959
	11th to 20th	Day	10% 3 6% 22% 40% 19%	1818
	21st to end	Day	16% 3 6% 24% 35% 16%	1800
December	1st to 10th	Day	12% 3 7% 30% 34% 14%	2950
	11th to 20th	Day	20% 5% 9% 30% 32% 4	3408
	21st to end	Day	20% 5% 10% 28% 32% 5%	4510
January	1st to 10th	Night	17% 6% 7% 28% 34% 7%	5867
	11th to 20th	Night	10% 4 9% 25% 29% 24%	5880
	21st to end	Night	22% 5% 7% 18% 27% 21%	6468
February	1st to 10th	Night	15% 4 8% 23% 32% 18%	2600
	11th to 20th	Night	16% 4 8% 23% 31% 18%	2358
	21st to end	Night	13% 4 8% 20% 33% 22%	1949
March	1st to 10th	Night	12% 3 6% 21% 38% 20%	4940
	11th to 20th	Night	26% 6% 8% 20% 27% 13%	5000
	21st to end	Night	14% 6% 7% 26% 35% 13%	5489
April	1st to 10th	Night	14% 4 7% 18% 35% 23%	2200
	11th to 20th	Night	14% 3 4 16% 34% 29%	1998
	21st to end	Night	11% 2 6% 16% 40% 25%	1826
May	1st to 10th	Night	10% 4 7% 11% 35% 32%	2950
	11th to 20th	Night	12% 4 7% 14% 31% 32%	2925
	21st to end	Night	6% 1 4 8% 30% 51%	3159
June	1st to 10th	Night	10% 4 5% 8% 24% 49%	1710
	11th to 20th	Night	10% 1 3 6% 22% 59%	1710
	21st to end	Night	12% 2 4 10% 21% 50%	1710
July	1st to 10th	Night	13% 4 7% 11% 11% 54%	2815
	11th to 20th	Night	36% 6% 9% 22% 15% 12%	2806
	21st to end	Night	28% 7% 17% 18% 18% 14%	3011
August	1st to 10th	Night	41% 4 13% 19% 10% 13%	2838
	11th to 20th	Night	17% 5% 15% 23% 21% 18%	2838
	21st to end	Night	21% 5% 10% 19% 31% 14%	3373
September	1st to 10th	Night	19% 6% 11% 17% 20% 27%	2090
	11th to 20th	Night	11% 3 6% 17% 28% 35%	2090
	21st to end	Night	11% 3 6% 12% 31% 37%	2090
October	1st to 10th	Night	12% 5% 4 11% 19% 49%	3631
	11th to 20th	Night	17% 3 2 13% 42% 24%	3778
	21st to end	Night	10% 2 3 16% 35% 33%	4120
November	1st to 10th	Night	12% 3 5% 20% 38% 22%	2340
	11th to 20th	Night	9% 3 6% 19% 41% 22%	2580
	21st to end	Night	16% 4 7% 19% 33% 21%	2600
December	1st to 10th	Night	12% 3 6% 25% 35% 18%	4240
	11th to 20th	Night	14% 5% 7% 24% 40% 10%	4811
	21st to end	Night	19% 4 8% 26% 32% 11%	6468

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	6% 11% 27% 36% 18% 3	300
	11th to 20th	Day	5% 11% 27% 32% 21% 5%	300
	21st to end	Day	4 11% 22% 34% 23% 7%	330
February	1st to 10th	Day	5% 15% 25% 31% 17% 7%	300
	11th to 20th	Day	8% 16% 28% 34% 12% 1	473
	21st to end	Day	7% 23% 27% 31% 11% 2	414
March	1st to 10th	Day	9% 21% 28% 26% 14% 2	500
	11th to 20th	Day	7% 19% 28% 26% 16% 3	500
	21st to end	Day	9% 21% 31% 25% 12% 2	550
April	1st to 10th	Day	5% 17% 29% 24% 19% 6%	500
	11th to 20th	Day	9% 19% 31% 24% 12% 4	492
	21st to end	Day	5% 19% 30% 24% 17% 5%	500
May	1st to 10th	Day	4 15% 30% 29% 17% 4	503
	11th to 20th	Day	5% 14% 27% 27% 20% 8%	528
	21st to end	Day	5% 15% 29% 27% 19% 6%	550
June	1st to 10th	Day	2 6% 26% 29% 27% 10%	490
	11th to 20th	Day	3 13% 26% 29% 19% 10%	490
	21st to end	Day	2 6% 21% 29% 33% 9%	490
July	1st to 10th	Day	3 1 13% 33% 34% 16%	486
	11th to 20th	Day	5% 1 10% 27% 33% 24%	484
	21st to end	Day	4 2 7% 22% 35% 29%	531
August	1st to 10th	Day	3 1 8% 26% 39% 23%	487
	11th to 20th	Day	4 2 12% 24% 38% 19%	477
	21st to end	Day	3 2 10% 27% 35% 23%	539
September	1st to 10th	Day	5% 2 11% 25% 38% 18%	490
	11th to 20th	Day	4 5% 20% 32% 29% 10%	490
	21st to end	Day	4 9% 15% 29% 32% 11%	490
October	1st to 10th	Day	3 9% 21% 27% 26% 13%	408
	11th to 20th	Day	5% 6% 17% 31% 30% 12%	300
	21st to end	Day	6% 7% 18% 30% 30% 8%	330
November	1st to 10th	Day	7% 11% 17% 27% 31% 6%	300
	11th to 20th	Day	3 7% 18% 33% 30% 9%	300
	21st to end	Day	5% 13% 22% 28% 25% 8%	300
December	1st to 10th	Day	5% 5% 25% 32% 29% 4	300
	11th to 20th	Day	3 8% 26% 34% 24% 6%	300
	21st to end	Day	7% 12% 26% 39% 14% 3	330
January	1st to 10th	Night	4 6% 28% 34% 23% 5%	800
	11th to 20th	Night	4 5% 25% 40% 22% 4	800
	21st to end	Night	5% 7% 28% 37% 20% 2	880
February	1st to 10th	Night	3 7% 34% 37% 17% 3	800
	11th to 20th	Night	3 8% 40% 33% 15% 2	784
	21st to end	Night	4 12% 37% 33% 13% 1	660
March	1st to 10th	Night	4 14% 34% 33% 14% 1	800
	11th to 20th	Night	4 9% 31% 34% 18% 4	800
	21st to end	Night	3 11% 35% 36% 13% 2	880
April	1st to 10th	Night	3 14% 36% 33% 13% 2	800
	11th to 20th	Night	4 13% 40% 30% 11% 2	614
	21st to end	Night	3 8% 40% 35% 11% 3	600
May	1st to 10th	Night	3 6% 35% 44% 12% 1	600
	11th to 20th	Night	2 6% 33% 37% 20% 2	591
	21st to end	Night	3 6% 35% 37% 17% 2	634
June	1st to 10th	Night	1 4 29% 50% 16% 1	570
	11th to 20th	Night	2 3 32% 45% 16% 2	570
	21st to end	Night	2 2 18% 52% 24% 1	570
July	1st to 10th	Night	2 9% 50% 36% 4	555
	11th to 20th	Night	3 1 5% 43% 45% 4	552
	21st to end	Night	6% 1 6% 32% 48% 7%	624
August	1st to 10th	Night	4 6% 36% 46% 8%	567
	11th to 20th	Night	4 1 8% 36% 46% 6%	564
	21st to end	Night	4 1 7% 33% 49% 7%	629
September	1st to 10th	Night	3 1 9% 35% 47% 6%	760
	11th to 20th	Night	2 2 14% 42% 33% 7%	760
	21st to end	Night	2 4 16% 36% 34% 7%	760
October	1st to 10th	Night	3 6% 23% 39% 27% 3	760
	11th to 20th	Night	5% 3 19% 40% 27% 6%	760
	21st to end	Night	3 4 22% 37% 28% 7%	836
November	1st to 10th	Night	3 4 25% 38% 25% 5%	780
	11th to 20th	Night	2 4 23% 39% 27% 6%	800
	21st to end	Night	4 8% 24% 36% 24% 4	800
December	1st to 10th	Night	4 6% 24% 37% 26% 4	800
	11th to 20th	Night	4 7% 25% 37% 25% 2	800
	21st to end	Night	6% 7% 28% 40% 17% 3	880

Page generated at 2025-05-23 06:21:01. Version ac07

Copyright © 2025 Attilla Danko.