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 2024-01-15.

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% 3 6% 6% 4 6% 5% 6% 8% 39%	74971
	Night	13% 5% 2 4 4 3 4 4 5% 8% 48%	74823
Transparency	Day	41% 8% 13% 18% 14% 6%	74696
	Night	35% 7% 14% 20% 17% 8%	74618
Seeing	Day	12% 7% 20% 28% 23% 11%	13679
	Night	14% 5% 20% 32% 25% 3	23659

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	20% 6% 2 5% 4 2 5% 4 3 5% 42%	1707
	11th to 20th	Day	19% 7% 3 4 4 3 4 3 3 7% 44%	1656
	21st to end	Day	18% 7% 3 5% 6% 3 4 4 4 6% 41%	1782
February	1st to 10th	Day	21% 9% 4 6% 5% 3 4 4 4 6% 34%	1714
	11th to 20th	Day	23% 9% 3 4 5% 3 5% 4 3 5% 36%	1776
	21st to end	Day	22% 7% 2 6% 4 3 4 4 4 6% 38%	1628
March	1st to 10th	Day	21% 9% 3 6% 6% 3 4 4 4 6% 34%	1980
	11th to 20th	Day	18% 7% 3 7% 5% 3 4 4 5% 5% 39%	1980
	21st to end	Day	17% 6% 3 5% 4 3 4 3 5% 7% 43%	2178
April	1st to 10th	Day	16% 7% 3 8% 4 4 5% 5% 5% 6% 38%	1980
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 4 4 8% 38%	2090
	21st to end	Day	11% 6% 3 5% 6% 4 6% 5% 5% 7% 42%	2160
May	1st to 10th	Day	12% 6% 3 6% 6% 3 5% 6% 5% 7% 40%	2178
	11th to 20th	Day	10% 6% 3 6% 6% 3 5% 6% 7% 8% 40%	2302
	21st to end	Day	10% 6% 2 7% 8% 5% 8% 7% 8% 11% 29%	2531
June	1st to 10th	Day	7% 4 2 6% 7% 4 6% 7% 7% 10% 40%	2340
	11th to 20th	Day	6% 4 2 5% 6% 4 7% 8% 8% 11% 38%	2340
	21st to end	Day	8% 6% 3 7% 9% 6% 9% 9% 8% 9% 27%	2340
July	1st to 10th	Day	11% 6% 3 6% 7% 4 8% 7% 8% 11% 29%	2340
	11th to 20th	Day	5% 5% 2 7% 7% 5% 6% 7% 9% 13% 33%	2340
	21st to end	Day	6% 4 2 6% 7% 5% 8% 7% 9% 13% 32%	2546
August	1st to 10th	Day	4 3 2 4 6% 5% 8% 7% 9% 11% 41%	2328
	11th to 20th	Day	5% 4 2 5% 7% 5% 8% 7% 8% 11% 37%	2201
	21st to end	Day	9% 5% 3 7% 7% 5% 7% 7% 8% 11% 32%	2331
September	1st to 10th	Day	6% 7% 3 8% 7% 5% 7% 7% 7% 9% 34%	1980
	11th to 20th	Day	15% 5% 2 7% 6% 4 7% 7% 7% 8% 32%	1980
	21st to end	Day	8% 6% 3 5% 6% 3 6% 5% 6% 9% 44%	1980
October	1st to 10th	Day	8% 5% 2 4 4 3 4 4 6% 7% 53%	1872
	11th to 20th	Day	11% 5% 2 5% 4 3 5% 4 6% 7% 49%	1800
	21st to end	Day	12% 5% 2 4 4 2 4 3 4 5% 54%	1980
November	1st to 10th	Day	11% 5% 2 5% 4 3 5% 5% 5% 7% 46%	1858
	11th to 20th	Day	13% 6% 3 6% 5% 3 4 4 6% 6% 45%	1900
	21st to end	Day	17% 7% 4 6% 5% 3 5% 4 4 5% 41%	1867
December	1st to 10th	Day	18% 7% 3 5% 5% 3 4 4 5% 7% 39%	1710
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 5% 7% 40%	2161
	21st to end	Day	17% 6% 2 5% 5% 3 4 4 5% 6% 44%	3135
January	1st to 10th	Night	19% 6% 3 4 4 2 4 3 5% 6% 45%	2458
	11th to 20th	Night	20% 6% 2 5% 4 2 3 3 4 5% 47%	2393
	21st to end	Night	19% 6% 3 5% 5% 3 3 4 4 6% 43%	2574
February	1st to 10th	Night	20% 8% 3 5% 4 3 3 4 4 6% 39%	2260
	11th to 20th	Night	21% 7% 3 5% 4 2 3 3 3 5% 43%	2068
	21st to end	Night	23% 8% 2 6% 5% 2 3 3 3 5% 39%	1628
March	1st to 10th	Night	20% 8% 2 6% 3 2 3 4 5% 6% 41%	1980
	11th to 20th	Night	17% 8% 3 4 5% 3 5% 4 5% 7% 40%	1980
	21st to end	Night	16% 5% 2 4 3 2 3 4 4 7% 49%	2178
April	1st to 10th	Night	15% 6% 3 5% 5% 3 6% 4 5% 8% 40%	1980
	11th to 20th	Night	20% 5% 2 4 3 2 4 4 4 7% 44%	1816
	21st to end	Night	13% 6% 3 5% 6% 3 4 4 5% 7% 43%	1800
May	1st to 10th	Night	11% 6% 3 5% 5% 3 5% 5% 6% 8% 42%	1778
	11th to 20th	Night	10% 5% 2 4 4 2 5% 5% 5% 9% 48%	1593
	21st to end	Night	6% 5% 2 5% 4 4 5% 5% 8% 11% 44%	1745
June	1st to 10th	Night	5% 2 2 3 4 2 4 4 6% 12% 56%	1620
	11th to 20th	Night	5% 2 1 3 4 2 3 5% 7% 11% 56%	1620
	21st to end	Night	5% 4 2 5% 5% 3 6% 6% 7% 11% 45%	1620
July	1st to 10th	Night	7% 4 3 5% 4 3 5% 6% 7% 12% 44%	1620
	11th to 20th	Night	3 2 1 3 4 2 4 5% 6% 11% 59%	1620
	21st to end	Night	4 3 2 3 3 3 4 5% 6% 12% 54%	1772
August	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 11% 69%	1611
	11th to 20th	Night	4 3 1 3 4 2 4 5% 7% 15% 53%	1697
	21st to end	Night	5% 3 2 5% 6% 3 5% 6% 6% 11% 47%	2020
September	1st to 10th	Night	5% 3 2 5% 6% 3 5% 5% 7% 10% 48%	1980
	11th to 20th	Night	11% 6% 3 5% 6% 2 4 5% 6% 7% 45%	1980
	21st to end	Night	9% 5% 2 3 5% 2 4 4 5% 8% 53%	1980
October	1st to 10th	Night	6% 4 2 3 3 2 3 4 5% 7% 61%	2070
	11th to 20th	Night	10% 3 2 3 3 2 3 3 5% 7% 59%	2160
	21st to end	Night	11% 4 2 4 3 2 3 3 3 5% 60%	2376
November	1st to 10th	Night	10% 4 1 4 4 3 3 4 5% 7% 55%	2220
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 5% 52%	2280
	21st to end	Night	16% 7% 2 5% 4 3 4 4 5% 6% 45%	2308
December	1st to 10th	Night	18% 7% 2 5% 5% 3 4 4 4 6% 42%	2470
	11th to 20th	Night	15% 8% 3 5% 5% 3 4 4 5% 6% 42%	3036
	21st to end	Night	16% 7% 2 5% 5% 3 4 4 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	45% 5% 7% 16% 20% 8%	1708
	11th to 20th	Day	42% 5% 6% 17% 18% 12%	1656
	21st to end	Day	45% 3 7% 16% 16% 12%	1782
February	1st to 10th	Day	52% 5% 6% 13% 13% 11%	1714
	11th to 20th	Day	52% 5% 8% 11% 17% 8%	1786
	21st to end	Day	48% 6% 8% 14% 14% 9%	1628
March	1st to 10th	Day	50% 6% 10% 13% 16% 5%	1980
	11th to 20th	Day	47% 6% 8% 12% 18% 9%	1980
	21st to end	Day	41% 6% 10% 15% 21% 7%	2178
April	1st to 10th	Day	46% 6% 8% 17% 14% 8%	1980
	11th to 20th	Day	46% 5% 10% 15% 15% 9%	2090
	21st to end	Day	40% 7% 10% 17% 19% 6%	2160
May	1st to 10th	Day	42% 7% 10% 17% 16% 7%	2178
	11th to 20th	Day	39% 8% 12% 20% 17% 3	2306
	21st to end	Day	44% 11% 15% 18% 11% 1	2531
June	1st to 10th	Day	35% 9% 17% 23% 14% 1	2340
	11th to 20th	Day	36% 9% 15% 25% 14% 1	2340
	21st to end	Day	45% 12% 18% 18% 7%	2340
July	1st to 10th	Day	42% 11% 22% 20% 5%	2276
	11th to 20th	Day	37% 13% 24% 21% 5%	2271
	21st to end	Day	35% 14% 22% 23% 4 1	2405
August	1st to 10th	Day	31% 11% 23% 26% 8%	2314
	11th to 20th	Day	36% 14% 21% 23% 6% 1	2201
	21st to end	Day	42% 12% 19% 18% 7% 2	2331
September	1st to 10th	Day	42% 13% 14% 15% 12% 3	1980
	11th to 20th	Day	46% 10% 13% 16% 12% 3	1980
	21st to end	Day	37% 8% 13% 17% 16% 8%	1980
October	1st to 10th	Day	30% 6% 9% 25% 18% 12%	1872
	11th to 20th	Day	34% 5% 9% 19% 20% 14%	1800
	21st to end	Day	33% 5% 7% 16% 23% 17%	1980
November	1st to 10th	Day	36% 6% 10% 19% 20% 8%	1858
	11th to 20th	Day	39% 6% 10% 13% 20% 12%	1900
	21st to end	Day	45% 4 8% 15% 16% 10%	1867
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	41% 5% 8% 14% 20% 12%	2458
	11th to 20th	Night	41% 4 6% 16% 20% 13%	2393
	21st to end	Night	43% 4 6% 15% 18% 13%	2574
February	1st to 10th	Night	48% 5% 8% 11% 17% 12%	2260
	11th to 20th	Night	45% 5% 8% 13% 17% 12%	2068
	21st to end	Night	49% 5% 7% 13% 16% 11%	1628
March	1st to 10th	Night	43% 5% 11% 16% 15% 9%	1980
	11th to 20th	Night	44% 6% 11% 13% 17% 9%	1980
	21st to end	Night	37% 5% 12% 17% 20% 10%	2178
April	1st to 10th	Night	42% 7% 11% 17% 14% 8%	1980
	11th to 20th	Night	40% 6% 11% 16% 15% 13%	1816
	21st to end	Night	40% 6% 10% 19% 17% 8%	1800
May	1st to 10th	Night	38% 7% 12% 18% 16% 8%	1778
	11th to 20th	Night	32% 8% 13% 23% 19% 4	1594
	21st to end	Night	31% 10% 20% 27% 12% 1	1746
June	1st to 10th	Night	22% 8% 22% 29% 19%	1620
	11th to 20th	Night	23% 7% 19% 31% 18% 2	1620
	21st to end	Night	31% 12% 22% 24% 11%	1620
July	1st to 10th	Night	32% 10% 27% 26% 5%	1575
	11th to 20th	Night	20% 10% 33% 30% 7%	1566
	21st to end	Night	21% 11% 31% 32% 5%	1673
August	1st to 10th	Night	12% 7% 32% 39% 10%	1601
	11th to 20th	Night	21% 13% 29% 31% 6% 1	1697
	21st to end	Night	29% 12% 22% 28% 7% 2	2020
September	1st to 10th	Night	29% 12% 23% 19% 13% 4	1980
	11th to 20th	Night	36% 9% 14% 22% 15% 3	1980
	21st to end	Night	29% 8% 14% 22% 19% 8%	1980
October	1st to 10th	Night	23% 6% 12% 28% 20% 12%	2070
	11th to 20th	Night	24% 6% 10% 23% 23% 13%	2160
	21st to end	Night	28% 4 7% 17% 23% 20%	2376
November	1st to 10th	Night	29% 6% 12% 22% 22% 10%	2220
	11th to 20th	Night	33% 6% 11% 20% 18% 11%	2280
	21st to end	Night	41% 5% 9% 19% 17% 9%	2308
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	23% 10% 34% 26% 7% 1	252
	21st to end	Day	19% 17% 33% 24% 6%	264
February	1st to 10th	Day	21% 17% 37% 18% 7%	240
	11th to 20th	Day	27% 18% 30% 19% 6% 1	240
	21st to end	Day	26% 17% 43% 13% 1	198
March	1st to 10th	Day	23% 20% 36% 16% 5%	240
	11th to 20th	Day	18% 18% 42% 17% 5% 2	240
	21st to end	Day	20% 19% 34% 19% 8%	264
April	1st to 10th	Day	17% 20% 35% 23% 6%	240
	11th to 20th	Day	22% 17% 25% 24% 11% 1	368
	21st to end	Day	17% 10% 32% 27% 13% 1	420
May	1st to 10th	Day	15% 9% 32% 29% 15%	441
	11th to 20th	Day	12% 4 31% 33% 17% 3	625
	21st to end	Day	9% 4 24% 41% 18% 3	679
June	1st to 10th	Day	6% 2 15% 37% 29% 11%	630
	11th to 20th	Day	6% 1 14% 40% 28% 11%	630
	21st to end	Day	10% 2 10% 30% 30% 17%	630
July	1st to 10th	Day	8% 2 7% 29% 34% 20%	621
	11th to 20th	Day	4 1 3 29% 36% 27%	618
	21st to end	Day	4 1 4 30% 37% 24%	684
August	1st to 10th	Day	3 4 26% 43% 24%	626
	11th to 20th	Day	5% 4 19% 43% 30%	515
	21st to end	Day	12% 2 6% 15% 33% 32%	450
September	1st to 10th	Day	10% 1 7% 25% 37% 20%	270
	11th to 20th	Day	11% 3 7% 28% 35% 17%	270
	21st to end	Day	10% 1 6% 31% 40% 13%	270
October	1st to 10th	Day	8% 2 10% 41% 31% 8%	270
	11th to 20th	Day	11% 6% 16% 36% 21% 10%	270
	21st to end	Day	19% 10% 27% 23% 19% 3	297
November	1st to 10th	Day	13% 4 24% 36% 18% 5%	270
	11th to 20th	Day	17% 6% 27% 33% 13% 3	270
	21st to end	Day	19% 14% 35% 21% 8% 3	270
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% 9% 26% 35% 8% 1	921
	21st to end	Night	21% 9% 29% 34% 7%	990
February	1st to 10th	Night	24% 9% 31% 30% 6%	820
	11th to 20th	Night	25% 9% 26% 31% 8%	674
	21st to end	Night	26% 11% 33% 25% 4	444
March	1st to 10th	Night	23% 8% 30% 31% 8%	540
	11th to 20th	Night	19% 9% 31% 33% 8%	540
	21st to end	Night	17% 8% 31% 33% 11% 1	594
April	1st to 10th	Night	14% 8% 35% 35% 8%	540
	11th to 20th	Night	23% 8% 26% 33% 11%	533
	21st to end	Night	16% 8% 29% 34% 13%	540
May	1st to 10th	Night	14% 7% 27% 42% 10%	540
	11th to 20th	Night	11% 5% 24% 42% 17% 1	531
	21st to end	Night	6% 3 22% 42% 25% 2	584
June	1st to 10th	Night	5% 2 7% 44% 40% 2	540
	11th to 20th	Night	5% 2 8% 45% 38% 3	540
	21st to end	Night	7% 1 10% 29% 46% 8%	540
July	1st to 10th	Night	5% 1 6% 29% 50% 9%	525
	11th to 20th	Night	2 1 2 24% 58% 12%	522
	21st to end	Night	4 1 2 25% 58% 10%	591
August	1st to 10th	Night	1 2 24% 63% 10%	537
	11th to 20th	Night	5% 2 20% 63% 10%	534
	21st to end	Night	5% 1 5% 19% 54% 16%	594
September	1st to 10th	Night	4 1 5% 21% 57% 13%	540
	11th to 20th	Night	12% 1 6% 23% 51% 8%	540
	21st to end	Night	11% 7% 28% 46% 7%	540
October	1st to 10th	Night	7% 1 9% 33% 43% 6%	630
	11th to 20th	Night	10% 2 19% 38% 28% 3	720
	21st to end	Night	12% 6% 20% 36% 24% 3	792
November	1st to 10th	Night	10% 2 18% 41% 26% 2	740
	11th to 20th	Night	13% 5% 22% 39% 20%	760
	21st to end	Night	18% 7% 27% 31% 15% 2	788
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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