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 2024-07-31.

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	8% 6% 2 7% 6% 5% 6% 5% 7% 9% 39%	90091
	Night	9% 6% 2 6% 5% 5% 5% 4 7% 9% 43%	89686
Transparency	Day	38% 10% 14% 19% 14% 5%	89865
	Night	36% 10% 16% 19% 14% 5%	89516
Seeing	Day	8% 6% 19% 26% 23% 17%	15108
	Night	9% 4 21% 33% 25% 8%	25169

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	11% 7% 3 8% 5% 5% 5% 3 6% 8% 40%	2836
	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	9% 7% 2 10% 8% 9% 9% 7% 8% 10% 22%	2470
	11th to 20th	Day	6% 7% 3 10% 9% 7% 9% 6% 8% 11% 22%	2470
	21st to end	Day	6% 7% 3 11% 9% 8% 8% 7% 8% 10% 22%	2679
August	1st to 10th	Day	6% 6% 4 9% 10% 9% 8% 7% 9% 11% 22%	2327
	11th to 20th	Day	6% 8% 3 10% 10% 8% 9% 7% 8% 9% 22%	2122
	21st to end	Day	10% 7% 3 10% 8% 8% 8% 5% 7% 11% 23%	2178
September	1st to 10th	Day	7% 7% 3 9% 8% 8% 9% 7% 8% 12% 23%	1980
	11th to 20th	Day	10% 8% 4 9% 7% 7% 7% 7% 8% 8% 25%	1980
	21st to end	Day	8% 7% 3 9% 7% 6% 7% 5% 8% 11% 29%	1980
October	1st to 10th	Day	7% 7% 3 8% 7% 7% 6% 5% 7% 8% 35%	1980
	11th to 20th	Day	6% 5% 2 5% 6% 5% 5% 5% 5% 9% 46%	1828
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 48%	1980
November	1st to 10th	Day	8% 4 2 6% 4 4 5% 4 6% 8% 50%	1857
	11th to 20th	Day	8% 5% 3 6% 6% 4 5% 5% 5% 6% 47%	1900
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 6% 8% 45%	1762
December	1st to 10th	Day	7% 5% 2 6% 5% 4 4 5% 7% 9% 47%	1710
	11th to 20th	Day	10% 7% 3 6% 4 4 4 4 5% 8% 45%	2153
	21st to end	Day	12% 8% 4 9% 5% 5% 6% 4 6% 7% 35%	3124
January	1st to 10th	Night	11% 7% 4 8% 5% 5% 4 4 6% 7% 38%	4109
	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% 7% 3 7% 6% 6% 8% 6% 8% 12% 31%	1710
	11th to 20th	Night	4 5% 4 7% 7% 7% 9% 8% 11% 11% 28%	1710
	21st to end	Night	4 6% 3 9% 7% 6% 7% 6% 11% 12% 28%	1870
August	1st to 10th	Night	4 5% 2 6% 6% 6% 8% 7% 11% 13% 33%	1611
	11th to 20th	Night	4 7% 2 8% 7% 7% 7% 6% 8% 15% 31%	1760
	21st to end	Night	9% 7% 3 10% 6% 6% 6% 5% 8% 11% 29%	2171
September	1st to 10th	Night	6% 6% 2 8% 6% 6% 7% 7% 10% 12% 28%	1980
	11th to 20th	Night	9% 7% 3 9% 6% 6% 6% 6% 8% 11% 29%	1980
	21st to end	Night	9% 8% 3 8% 5% 6% 6% 5% 8% 8% 34%	1980
October	1st to 10th	Night	7% 6% 2 8% 4 4 5% 5% 8% 9% 41%	1980
	11th to 20th	Night	7% 5% 1 6% 5% 4 5% 4 6% 8% 50%	2129
	21st to end	Night	8% 6% 3 7% 4 3 3 3 5% 8% 50%	2376
November	1st to 10th	Night	7% 5% 1 4 4 4 5% 4 4 7% 54%	2221
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 5% 7% 49%	2280
	21st to end	Night	7% 5% 2 8% 5% 4 5% 3 7% 8% 45%	2415
December	1st to 10th	Night	8% 5% 2 6% 5% 4 4 3 6% 9% 48%	2470
	11th to 20th	Night	13% 6% 2 6% 4 4 3 3 4 7% 48%	3042
	21st to end	Night	14% 8% 3 8% 5% 5% 5% 5% 6% 7% 35%	4532

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	41% 6% 9% 19% 18% 8%	2837
	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	51% 17% 24% 7% 1	2405
	11th to 20th	Day	51% 17% 23% 8%	2401
	21st to end	Day	50% 17% 22% 11% 1	2537
August	1st to 10th	Day	48% 17% 26% 8%	2314
	11th to 20th	Day	52% 17% 21% 9% 1	2122
	21st to end	Day	52% 16% 20% 11% 2	2178
September	1st to 10th	Day	48% 18% 20% 12% 2	1980
	11th to 20th	Day	50% 15% 19% 11% 5%	1980
	21st to end	Day	45% 13% 14% 15% 10% 3	1980
October	1st to 10th	Day	43% 11% 12% 16% 11% 8%	1980
	11th to 20th	Day	34% 7% 11% 21% 22% 5%	1828
	21st to end	Day	31% 7% 12% 18% 21% 12%	1980
November	1st to 10th	Day	30% 7% 11% 21% 19% 12%	1857
	11th to 20th	Day	36% 7% 9% 20% 20% 9%	1900
	21st to end	Day	36% 6% 10% 20% 20% 8%	1762
December	1st to 10th	Day	33% 7% 12% 26% 16% 6%	1710
	11th to 20th	Day	37% 5% 11% 18% 18% 12%	2150
	21st to end	Day	46% 6% 11% 16% 15% 5%	3124
January	1st to 10th	Night	44% 6% 10% 17% 18% 6%	4109
	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	41% 19% 30% 8% 2	1665
	11th to 20th	Night	43% 21% 28% 8% 1	1656
	21st to end	Night	41% 20% 28% 11%	1771
August	1st to 10th	Night	34% 20% 35% 10% 1	1600
	11th to 20th	Night	38% 17% 32% 12%	1760
	21st to end	Night	46% 19% 20% 13% 2	2171
September	1st to 10th	Night	41% 21% 23% 13% 2	1980
	11th to 20th	Night	46% 15% 22% 15% 2	1980
	21st to end	Night	43% 12% 17% 16% 9% 3	1980
October	1st to 10th	Night	35% 12% 14% 19% 13% 7%	1980
	11th to 20th	Night	30% 7% 12% 24% 22% 5%	2129
	21st to end	Night	32% 6% 13% 18% 22% 10%	2376
November	1st to 10th	Night	28% 6% 14% 20% 21% 11%	2221
	11th to 20th	Night	34% 7% 10% 20% 20% 9%	2280
	21st to end	Night	35% 7% 10% 22% 19% 7%	2415
December	1st to 10th	Night	32% 7% 14% 27% 16% 5%	2470
	11th to 20th	Night	36% 6% 10% 19% 17% 13%	3044
	21st to end	Night	45% 8% 13% 16% 15% 4	4532

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% 42% 26% 7% 1	270
	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% 30% 40%	529
August	1st to 10th	Day	7% 1 4 13% 31% 43%	446
	11th to 20th	Day	8% 2 4 13% 28% 45%	439
	21st to end	Day	10% 2 6% 18% 30% 34%	495
September	1st to 10th	Day	8% 2 6% 15% 30% 38%	450
	11th to 20th	Day	10% 3 9% 22% 29% 27%	450
	21st to end	Day	7% 2 7% 25% 34% 24%	450
October	1st to 10th	Day	8% 4 19% 35% 26% 8%	450
	11th to 20th	Day	5% 4 18% 35% 30% 8%	450
	21st to end	Day	7% 6% 19% 38% 25% 4	495
November	1st to 10th	Day	9% 5% 23% 33% 24% 5%	455
	11th to 20th	Day	11% 7% 25% 36% 17% 4	460
	21st to end	Day	8% 8% 33% 32% 16% 2	322
December	1st to 10th	Day	8% 10% 37% 25% 15% 6%	270
	11th to 20th	Day	10% 11% 36% 29% 13% 2	270
	21st to end	Day	8% 12% 30% 34% 13% 3	297
January	1st to 10th	Night	12% 9% 31% 37% 12%	760
	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	4 1 1 11% 49% 33%	537
	11th to 20th	Night	5% 2 2 10% 40% 42%	534
	21st to end	Night	11% 2 7% 20% 36% 23%	785
September	1st to 10th	Night	8% 2 7% 22% 42% 19%	720
	11th to 20th	Night	11% 3 8% 28% 36% 14%	720
	21st to end	Night	12% 3 10% 30% 34% 11%	720
October	1st to 10th	Night	10% 2 21% 39% 23% 4	720
	11th to 20th	Night	7% 4 17% 39% 31% 2	720
	21st to end	Night	10% 4 21% 41% 23% 2	792
November	1st to 10th	Night	8% 3 30% 37% 19% 3	740
	11th to 20th	Night	11% 4 24% 41% 20% 1	760
	21st to end	Night	9% 6% 28% 42% 14% 1	760
December	1st to 10th	Night	8% 4 28% 37% 22% 1	760
	11th to 20th	Night	10% 7% 24% 40% 18% 1	760
	21st to end	Night	11% 6% 28% 38% 16% 1	836

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