Changstars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Changstars Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	13% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	101238
	Night	14% 6% 2 5% 4 4 4 3 5% 8% 45%	98425
Transparency	Day	40% 8% 12% 19% 14% 6%	104926
	Night	35% 7% 13% 20% 16% 8%	102851
Seeing	Day	12% 7% 20% 28% 22% 10%	14876
	Night	14% 6% 20% 32% 25% 3	24738

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	19% 10% 3 6% 3 3 4 3 4 5% 41%	2847
	11th to 20th	Day	15% 7% 2 4 3 3 3 2 3 7% 52%	2850
	21st to end	Day	24% 8% 3 6% 4 3 3 4 5% 7% 33%	3135
February	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 3 4 6% 33%	1809
	11th to 20th	Day	22% 9% 3 5% 4 4 4 3 3 5% 37%	1877
	21st to end	Day	21% 8% 2 7% 3 4 4 3 4 6% 39%	1727
March	1st to 10th	Day	19% 8% 3 9% 6% 5% 5% 4 5% 7% 29%	3436
	11th to 20th	Day	18% 7% 3 10% 6% 5% 5% 4 6% 6% 29%	3480
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 43%	3814
April	1st to 10th	Day	15% 8% 3 8% 4 4 5% 4 5% 6% 38%	2090
	11th to 20th	Day	17% 8% 3 6% 4 4 4 4 5% 8% 37%	2208
	21st to end	Day	10% 7% 3 6% 5% 6% 5% 4 6% 6% 41%	2280
May	1st to 10th	Day	17% 8% 3 8% 6% 5% 5% 4 6% 7% 32%	3797
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 6% 6% 40%	4082
	21st to end	Day	9% 6% 2 7% 6% 6% 6% 6% 8% 10% 36%	4477
June	1st to 10th	Day	7% 4 2 7% 6% 6% 6% 6% 7% 10% 41%	2470
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 7% 8% 11% 38%	2470
	21st to end	Day	7% 7% 2 7% 8% 8% 9% 7% 9% 9% 27%	2470
July	1st to 10th	Day	9% 6% 2 8% 6% 7% 8% 6% 9% 10% 30%	4120
	11th to 20th	Day	6% 7% 3 12% 8% 8% 8% 6% 8% 11% 24%	4120
	21st to end	Day	14% 8% 3 8% 6% 7% 6% 5% 7% 10% 25%	4465
August	1st to 10th	Day	2 2 1 3 4 4 5% 4 8% 10% 55%	4104
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 9% 13% 45%	3857
	21st to end	Day	8% 5% 2 8% 6% 7% 7% 5% 8% 10% 34%	4093
September	1st to 10th	Day	7% 7% 3 10% 5% 6% 6% 6% 7% 9% 33%	2090
	11th to 20th	Day	14% 5% 2 8% 5% 5% 7% 6% 7% 8% 34%	2090
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 6% 9% 45%	2090
October	1st to 10th	Day	8% 5% 2 5% 4 4 3 4 6% 7% 54%	1975
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 6% 7% 50%	1900
	21st to end	Day	12% 5% 2 5% 4 3 4 2 4 5% 53%	2090
November	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 5% 7% 44%	1958
	11th to 20th	Day	13% 7% 2 6% 4 4 3 3 6% 6% 45%	2000
	21st to end	Day	16% 7% 3 7% 5% 4 4 3 4 5% 42%	1961
December	1st to 10th	Day	18% 8% 3 6% 5% 4 4 3 5% 7% 39%	1710
	11th to 20th	Day	15% 6% 3 6% 4 5% 4 3 6% 7% 40%	2161
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 43%	3135
January	1st to 10th	Night	21% 8% 2 6% 4 4 4 3 5% 6% 40%	4108
	11th to 20th	Night	18% 6% 2 5% 3 3 2 3 3 5% 50%	4120
	21st to end	Night	24% 9% 3 5% 3 3 3 2 5% 7% 35%	4532
February	1st to 10th	Night	20% 8% 3 6% 4 3 3 3 4 6% 38%	2386
	11th to 20th	Night	21% 8% 3 5% 3 3 3 2 3 5% 43%	2187
	21st to end	Night	22% 9% 2 6% 4 4 2 2 3 5% 40%	1727
March	1st to 10th	Night	25% 12% 2 7% 3 3 3 3 5% 7% 29%	3419
	11th to 20th	Night	23% 9% 4 7% 5% 4 5% 3 5% 6% 29%	3480
	21st to end	Night	14% 6% 2 6% 4 4 4 4 4 6% 46%	3817
April	1st to 10th	Night	15% 7% 2 6% 5% 4 5% 3 5% 8% 39%	2090
	11th to 20th	Night	20% 6% 2 4 3 4 3 4 4 7% 43%	1918
	21st to end	Night	13% 7% 2 6% 5% 4 3 4 6% 7% 43%	1900
May	1st to 10th	Night	25% 10% 3 6% 4 4 4 3 5% 6% 30%	3082
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	2823
	21st to end	Night	13% 7% 2 6% 5% 4 4 4 8% 11% 34%	3090
June	1st to 10th	Night	5% 2 1 3 3 3 4 4 6% 12% 56%	1710
	11th to 20th	Night	5% 3 1 4 3 3 3 4 7% 10% 56%	1710
	21st to end	Night	5% 5% 1 5% 5% 4 6% 5% 7% 11% 46%	1710
July	1st to 10th	Night	6% 4 2 5% 3 4 4 4 7% 12% 48%	2850
	11th to 20th	Night	3 4 2 6% 6% 5% 5% 5% 8% 12% 44%	2850
	21st to end	Night	9% 6% 3 8% 4 5% 5% 4 7% 10% 39%	3098
August	1st to 10th	Night	1 1 1 2 1 2 2 3 4 10% 73%	2836
	11th to 20th	Night	2 2 1 3 2 2 3 4 7% 14% 59%	2973
	21st to end	Night	5% 3 1 5% 4 4 5% 5% 8% 12% 48%	3558
September	1st to 10th	Night	5% 4 2 7% 5% 5% 5% 4 7% 9% 47%	2090
	11th to 20th	Night	11% 6% 3 6% 5% 3 4 4 5% 7% 46%	2090
	21st to end	Night	8% 5% 1 4 4 3 3 4 5% 8% 54%	2090
October	1st to 10th	Night	5% 4 2 4 3 2 3 3 5% 7% 62%	2185
	11th to 20th	Night	9% 3 1 3 3 3 3 3 5% 7% 61%	2280
	21st to end	Night	11% 4 2 4 3 3 3 2 3 5% 61%	2508
November	1st to 10th	Night	11% 5% 1 5% 4 3 4 3 5% 7% 53%	2340
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 5% 53%	2400
	21st to end	Night	15% 7% 2 5% 3 4 3 3 5% 6% 46%	2430
December	1st to 10th	Night	18% 8% 2 5% 4 4 3 3 4 6% 42%	2470
	11th to 20th	Night	15% 8% 2 7% 5% 4 4 4 5% 6% 41%	3036
	21st to end	Night	16% 7% 2 6% 4 3 4 3 4 5% 45%	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	46% 5% 5% 17% 18% 9%	2848
	11th to 20th	Day	32% 6% 7% 25% 19% 11%	2850
	21st to end	Day	50% 3 5% 15% 16% 10%	3135
February	1st to 10th	Day	52% 5% 6% 12% 14% 11%	1809
	11th to 20th	Day	51% 5% 8% 11% 17% 9%	1887
	21st to end	Day	47% 6% 8% 15% 14% 9%	1727
March	1st to 10th	Day	50% 8% 13% 14% 12% 3	3436
	11th to 20th	Day	53% 9% 9% 10% 14% 5%	3480
	21st to end	Day	41% 6% 9% 18% 21% 5%	3814
April	1st to 10th	Day	46% 6% 8% 17% 14% 9%	2090
	11th to 20th	Day	46% 6% 10% 15% 15% 9%	2208
	21st to end	Day	40% 7% 10% 17% 19% 6%	2280
May	1st to 10th	Day	49% 9% 12% 17% 10% 4	3832
	11th to 20th	Day	41% 7% 10% 17% 21% 4	4086
	21st to end	Day	36% 11% 14% 30% 8%	4489
June	1st to 10th	Day	34% 10% 18% 23% 15% 1	2470
	11th to 20th	Day	36% 10% 15% 25% 14% 2	2470
	21st to end	Day	44% 11% 18% 19% 7%	2470
July	1st to 10th	Day	42% 11% 26% 18% 3	4056
	11th to 20th	Day	50% 12% 19% 16% 4	4051
	21st to end	Day	50% 12% 17% 19% 2	4325
August	1st to 10th	Day	20% 8% 16% 41% 14%	4090
	11th to 20th	Day	25% 11% 32% 28% 4 1	3896
	21st to end	Day	41% 11% 17% 19% 12% 1	4092
September	1st to 10th	Day	44% 12% 13% 15% 12% 3	2090
	11th to 20th	Day	44% 10% 13% 17% 13% 3	2090
	21st to end	Day	36% 8% 13% 18% 16% 10%	2090
October	1st to 10th	Day	21% 4 6% 16% 15% 38%	3274
	11th to 20th	Day	23% 4 7% 17% 36% 13%	3140
	21st to end	Day	26% 4 6% 17% 34% 13%	3428
November	1st to 10th	Day	38% 7% 10% 19% 19% 8%	1958
	11th to 20th	Day	39% 6% 10% 13% 20% 13%	2000
	21st to end	Day	44% 4 9% 15% 17% 10%	1961
December	1st to 10th	Day	44% 6% 9% 19% 15% 7%	1710
	11th to 20th	Day	42% 6% 10% 12% 20% 9%	2159
	21st to end	Day	40% 6% 8% 15% 19% 12%	3135
January	1st to 10th	Night	44% 7% 8% 13% 15% 13%	4108
	11th to 20th	Night	37% 5% 6% 16% 25% 11%	4120
	21st to end	Night	49% 4 5% 15% 17% 10%	4532
February	1st to 10th	Night	47% 5% 8% 11% 17% 12%	2386
	11th to 20th	Night	44% 5% 7% 13% 17% 13%	2187
	21st to end	Night	48% 5% 6% 13% 17% 10%	1727
March	1st to 10th	Night	54% 7% 10% 14% 10% 5%	3419
	11th to 20th	Night	54% 8% 9% 13% 11% 5%	3480
	21st to end	Night	37% 6% 10% 14% 24% 7%	3817
April	1st to 10th	Night	42% 7% 11% 17% 14% 9%	2090
	11th to 20th	Night	41% 6% 11% 16% 14% 13%	1918
	21st to end	Night	39% 7% 10% 20% 16% 8%	1900
May	1st to 10th	Night	53% 7% 11% 14% 9% 5%	3112
	11th to 20th	Night	39% 8% 11% 18% 16% 7%	2824
	21st to end	Night	39% 12% 18% 24% 7% 1	3099
June	1st to 10th	Night	22% 8% 22% 30% 19%	1710
	11th to 20th	Night	23% 7% 19% 31% 18% 3	1710
	21st to end	Night	29% 12% 23% 26% 11%	1710
July	1st to 10th	Night	29% 8% 35% 26% 3	2805
	11th to 20th	Night	31% 10% 29% 24% 5%	2796
	21st to end	Night	39% 11% 25% 22% 3	2999
August	1st to 10th	Night	9% 5% 20% 51% 15%	2826
	11th to 20th	Night	15% 8% 36% 36% 4 1	3000
	21st to end	Night	27% 14% 22% 27% 9% 1	3558
September	1st to 10th	Night	31% 12% 22% 19% 13% 4	2090
	11th to 20th	Night	36% 9% 14% 23% 16% 3	2090
	21st to end	Night	28% 8% 14% 22% 19% 9%	2090
October	1st to 10th	Night	17% 4 8% 18% 17% 36%	3640
	11th to 20th	Night	16% 4 7% 23% 34% 17%	3779
	21st to end	Night	20% 4 5% 17% 36% 18%	4120
November	1st to 10th	Night	31% 7% 12% 21% 21% 9%	2340
	11th to 20th	Night	33% 5% 11% 19% 19% 12%	2400
	21st to end	Night	39% 5% 10% 19% 17% 9%	2430
December	1st to 10th	Night	43% 6% 8% 20% 15% 9%	2470
	11th to 20th	Night	42% 7% 8% 11% 17% 14%	3037
	21st to end	Night	42% 5% 7% 13% 25% 8%	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	22% 13% 37% 19% 8% 1	270
	11th to 20th	Day	22% 11% 35% 24% 6% 1	270
	21st to end	Day	20% 18% 32% 23% 7%	297
February	1st to 10th	Day	20% 20% 36% 17% 7%	270
	11th to 20th	Day	27% 17% 31% 20% 5% 1	270
	21st to end	Day	24% 19% 41% 14% 1	225
March	1st to 10th	Day	21% 21% 37% 16% 5%	270
	11th to 20th	Day	17% 18% 43% 16% 4 1	270
	21st to end	Day	22% 18% 35% 19% 7%	297
April	1st to 10th	Day	17% 21% 34% 21% 6%	270
	11th to 20th	Day	21% 18% 27% 22% 10% 1	406
	21st to end	Day	17% 11% 32% 25% 14% 1	460
May	1st to 10th	Day	16% 10% 33% 27% 13%	482
	11th to 20th	Day	13% 5% 31% 32% 16% 3	675
	21st to end	Day	9% 5% 26% 40% 17% 3	734
June	1st to 10th	Day	6% 3 16% 37% 29% 10%	680
	11th to 20th	Day	7% 2 14% 39% 28% 10%	680
	21st to end	Day	10% 2 10% 31% 31% 16%	680
July	1st to 10th	Day	8% 3 7% 28% 33% 21%	671
	11th to 20th	Day	5% 1 3 29% 36% 26%	668
	21st to end	Day	6% 2 4 29% 36% 22%	739
August	1st to 10th	Day	3 3 26% 42% 25%	676
	11th to 20th	Day	5% 4 19% 43% 30%	559
	21st to end	Day	11% 3 5% 17% 33% 31%	491
September	1st to 10th	Day	11% 2 8% 26% 34% 18%	300
	11th to 20th	Day	10% 3 8% 28% 33% 18%	300
	21st to end	Day	9% 1 6% 32% 39% 13%	300
October	1st to 10th	Day	7% 2 10% 44% 29% 7%	300
	11th to 20th	Day	10% 5% 15% 38% 22% 9%	300
	21st to end	Day	18% 9% 25% 25% 19% 5%	330
November	1st to 10th	Day	14% 6% 24% 35% 16% 4	300
	11th to 20th	Day	16% 6% 27% 34% 14% 3	300
	21st to end	Day	17% 14% 35% 21% 9% 4	299
December	1st to 10th	Day	20% 6% 32% 25% 12% 6%	270
	11th to 20th	Day	18% 16% 36% 23% 6% 1	270
	21st to end	Day	18% 11% 39% 20% 10% 2	297
January	1st to 10th	Night	20% 9% 30% 30% 10%	950
	11th to 20th	Night	21% 10% 26% 34% 8% 1	950
	21st to end	Night	21% 10% 28% 34% 7%	1045
February	1st to 10th	Night	24% 10% 31% 30% 6%	866
	11th to 20th	Night	25% 10% 26% 31% 8%	713
	21st to end	Night	25% 12% 35% 25% 4	471
March	1st to 10th	Night	24% 8% 30% 31% 7%	570
	11th to 20th	Night	19% 10% 31% 32% 8%	570
	21st to end	Night	16% 8% 32% 32% 11% 1	627
April	1st to 10th	Night	15% 9% 34% 35% 7%	570
	11th to 20th	Night	23% 9% 25% 32% 11%	563
	21st to end	Night	16% 9% 28% 34% 12%	570
May	1st to 10th	Night	15% 8% 28% 40% 9%	570
	11th to 20th	Night	12% 5% 24% 42% 16% 1	561
	21st to end	Night	7% 4 23% 41% 24% 2	617
June	1st to 10th	Night	5% 2 8% 44% 39% 2	570
	11th to 20th	Night	5% 2 8% 45% 37% 3	570
	21st to end	Night	6% 1 10% 29% 45% 8%	570
July	1st to 10th	Night	6% 1 5% 28% 50% 10%	555
	11th to 20th	Night	3 1 2 23% 59% 12%	552
	21st to end	Night	4 1 2 24% 58% 9%	624
August	1st to 10th	Night	1 2 24% 63% 10%	567
	11th to 20th	Night	4 2 19% 64% 11%	564
	21st to end	Night	5% 1 5% 19% 55% 15%	627
September	1st to 10th	Night	5% 1 5% 22% 55% 12%	570
	11th to 20th	Night	12% 1 5% 24% 50% 8%	570
	21st to end	Night	11% 6% 29% 46% 7%	570
October	1st to 10th	Night	6% 1 9% 35% 43% 6%	665
	11th to 20th	Night	9% 2 18% 40% 28% 3	760
	21st to end	Night	11% 6% 20% 36% 24% 3	836
November	1st to 10th	Night	11% 3 19% 41% 25% 2	780
	11th to 20th	Night	13% 5% 23% 39% 21%	800
	21st to end	Night	17% 7% 27% 32% 15% 2	830
December	1st to 10th	Night	19% 7% 27% 30% 17% 1	950
	11th to 20th	Night	21% 9% 26% 34% 10%	950
	21st to end	Night	20% 8% 32% 30% 9% 1	1045

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