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 2026-01-02.

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	18% 7% 2 6% 4 4 4 3 5% 7% 40%	127026
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	124323
Transparency	Day	28% 38% 8% 14% 10% 2	130557
	Night	28% 37% 7% 14% 11% 2	128879
Seeing	Day	21% 15% 32% 21% 9% 2	16496
	Night	22% 13% 30% 25% 9% 1	25581

By Month

Percentage Cloud Cover NE (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	13% 8% 2 6% 4 4 4 3 5% 6% 46%	3605
	11th to 20th	Day	14% 9% 2 6% 3 4 4 3 5% 5% 45%	3570
	21st to end	Day	19% 7% 3 7% 4 3 3 3 4 5% 40%	4066
February	1st to 10th	Day	18% 7% 2 6% 4 5% 5% 3 5% 7% 38%	1893
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 4 5% 6% 42%	1948
	21st to end	Day	21% 9% 3 4 4 3 4 4 5% 6% 37%	1650
March	1st to 10th	Day	23% 9% 3 5% 4 3 3 2 4 5% 41%	4476
	11th to 20th	Day	13% 8% 2 6% 5% 4 4 3 5% 7% 41%	4646
	21st to end	Day	20% 8% 3 6% 4 4 4 4 5% 7% 36%	5970
April	1st to 10th	Day	20% 7% 2 5% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	20% 6% 2 5% 4 3 3 3 5% 6% 41%	2374
	21st to end	Day	20% 6% 2 4 4 5% 4 3 5% 6% 41%	2520
May	1st to 10th	Day	30% 9% 2 5% 3 3 4 3 5% 8% 28%	5886
	11th to 20th	Day	22% 8% 3 6% 4 4 4 3 5% 7% 35%	6061
	21st to end	Day	21% 7% 3 6% 4 4 5% 4 5% 8% 35%	6949
June	1st to 10th	Day	18% 6% 2 4 5% 4 5% 4 7% 9% 36%	2800
	11th to 20th	Day	17% 7% 2 5% 4 4 4 4 5% 9% 39%	2800
	21st to end	Day	13% 6% 2 7% 5% 4 5% 5% 6% 8% 41%	2800
July	1st to 10th	Day	16% 8% 2 7% 4 4 4 4 6% 9% 35%	6360
	11th to 20th	Day	14% 7% 2 6% 5% 4 5% 4 7% 10% 36%	6360
	21st to end	Day	16% 7% 3 5% 3 4 4 4 5% 7% 42%	6926
August	1st to 10th	Day	19% 7% 2 7% 4 4 4 4 6% 8% 34%	4349
	11th to 20th	Day	12% 6% 2 6% 3 4 4 4 6% 8% 44%	3889
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 9% 45%	4312
September	1st to 10th	Day	13% 6% 3 6% 4 4 4 3 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 2 4 4 3 3 3 5% 7% 53%	2249
	21st to end	Day	20% 7% 3 5% 3 4 4 3 5% 6% 39%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 3 4 3 4 5% 44%	2190
	11th to 20th	Day	14% 6% 2 5% 3 4 4 2 5% 8% 48%	1972
	21st to end	Day	18% 7% 2 5% 3 4 4 4 5% 6% 42%	1980
November	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 4 5% 49%	1851
	11th to 20th	Day	18% 6% 1 4 4 3 3 3 4 6% 48%	1890
	21st to end	Day	18% 7% 2 4 4 3 4 3 4 7% 44%	1890
December	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 4 6% 45%	2680
	11th to 20th	Day	22% 7% 2 5% 2 2 3 2 4 5% 46%	3066
	21st to end	Day	21% 9% 3 7% 4 3 3 2 4 5% 40%	4048
January	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 5% 6% 49%	6364
	11th to 20th	Night	15% 7% 3 7% 4 3 3 3 4 5% 47%	6315
	21st to end	Night	18% 9% 3 7% 4 4 4 4 4 5% 38%	6886
February	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 5% 6% 40%	2505
	11th to 20th	Night	18% 8% 2 6% 4 3 3 3 5% 7% 41%	2338
	21st to end	Night	21% 7% 2 6% 4 4 4 2 4 5% 40%	1980
March	1st to 10th	Night	22% 9% 2 5% 3 3 3 3 4 6% 40%	5364
	11th to 20th	Night	17% 10% 3 6% 3 4 3 3 5% 6% 41%	5294
	21st to end	Night	21% 9% 3 6% 4 3 4 3 4 5% 37%	4962
April	1st to 10th	Night	21% 8% 3 6% 5% 3 4 3 5% 7% 37%	2000
	11th to 20th	Night	20% 6% 2 5% 4 4 3 3 4 5% 41%	1975
	21st to end	Night	21% 6% 2 6% 4 4 4 3 4 4 41%	1880
May	1st to 10th	Night	28% 11% 4 7% 3 4 3 3 4 7% 26%	4054
	11th to 20th	Night	24% 10% 3 6% 3 4 4 3 5% 8% 31%	3862
	21st to end	Night	24% 9% 3 6% 4 4 4 3 5% 7% 33%	3926
June	1st to 10th	Night	19% 7% 2 6% 5% 5% 4 3 5% 6% 39%	1600
	11th to 20th	Night	18% 6% 2 5% 4 4 3 4 6% 9% 39%	1600
	21st to end	Night	14% 6% 3 5% 4 4 4 5% 5% 9% 41%	1600
July	1st to 10th	Night	18% 9% 3 7% 5% 4 4 4 5% 8% 33%	3600
	11th to 20th	Night	21% 8% 3 6% 4 4 4 4 6% 9% 31%	3600
	21st to end	Night	15% 7% 2 5% 4 4 4 3 6% 7% 42%	3926
August	1st to 10th	Night	24% 7% 3 7% 4 4 5% 5% 6% 7% 29%	2769
	11th to 20th	Night	12% 7% 3 7% 4 5% 5% 4 6% 8% 39%	3156
	21st to end	Night	9% 4 2 4 3 3 4 3 5% 8% 55%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 5% 5% 4 6% 6% 43%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 3 2 4 6% 53%	2150
	21st to end	Night	19% 6% 3 6% 4 4 5% 3 4 6% 41%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 4 3 3 3 6% 43%	2205
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 4 6% 48%	2426
	21st to end	Night	21% 7% 2 5% 3 4 3 3 5% 6% 41%	2860
November	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 49%	2667
	11th to 20th	Night	18% 6% 2 4 3 3 3 3 4 5% 48%	2730
	21st to end	Night	16% 6% 2 6% 4 3 4 3 4 7% 46%	2730
December	1st to 10th	Night	17% 7% 2 5% 3 3 4 3 4 5% 48%	4715
	11th to 20th	Night	26% 7% 2 4 3 3 3 3 3 4 42%	5348
	21st to end	Night	20% 9% 3 6% 3 3 3 2 4 5% 42%	7150

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	28% 27% 3 23% 15% 3	3606
	11th to 20th	Day	27% 28% 8% 18% 13% 7%	3600
	21st to end	Day	36% 29% 7% 15% 12% 1	4066
February	1st to 10th	Day	19% 62% 6% 8% 4 1	1893
	11th to 20th	Day	18% 60% 2 9% 9% 1	1957
	21st to end	Day	17% 62% 5% 8% 5% 2	1650
March	1st to 10th	Day	34% 30% 7% 11% 9% 9%	4479
	11th to 20th	Day	26% 31% 11% 23% 8% 1	4646
	21st to end	Day	34% 29% 7% 10% 14% 5%	5970
April	1st to 10th	Day	21% 62% 4 6% 6% 1	2400
	11th to 20th	Day	16% 62% 3 8% 9% 1	2374
	21st to end	Day	18% 62% 3 9% 7% 1	2520
May	1st to 10th	Day	42% 31% 14% 8% 5%	5897
	11th to 20th	Day	37% 30% 11% 13% 8% 1	6065
	21st to end	Day	37% 30% 7% 17% 9%	6949
June	1st to 10th	Day	17% 62% 3 11% 7%	2800
	11th to 20th	Day	16% 61% 5% 10% 6% 1	2800
	21st to end	Day	15% 61% 5% 11% 8% 1	2800
July	1st to 10th	Day	37% 29% 8% 15% 10% 1	6290
	11th to 20th	Day	35% 26% 15% 16% 7%	6285
	21st to end	Day	32% 25% 8% 19% 15% 1	6728
August	1st to 10th	Day	34% 36% 10% 14% 5% 1	4335
	11th to 20th	Day	24% 36% 11% 24% 5%	3926
	21st to end	Day	20% 37% 10% 22% 10% 1	4312
September	1st to 10th	Day	16% 58% 6% 12% 8%	2400
	11th to 20th	Day	15% 57% 4 10% 12% 1	2249
	21st to end	Day	22% 57% 5% 8% 7% 1	2200
October	1st to 10th	Day	21% 36% 7% 13% 18% 5%	3567
	11th to 20th	Day	15% 35% 5% 14% 27% 3	3188
	21st to end	Day	18% 36% 10% 18% 16% 2	3180
November	1st to 10th	Day	19% 54% 7% 9% 10% 1	1851
	11th to 20th	Day	16% 53% 3 11% 14% 3	1890
	21st to end	Day	18% 54% 6% 14% 6% 2	1890
December	1st to 10th	Day	25% 35% 10% 21% 9%	2680
	11th to 20th	Day	29% 30% 9% 13% 16% 3	3066
	21st to end	Day	37% 27% 4 15% 13% 5%	4048
January	1st to 10th	Night	26% 26% 6% 22% 17% 3	6364
	11th to 20th	Night	29% 26% 9% 16% 15% 5%	6360
	21st to end	Night	38% 29% 6% 12% 11% 4	6886
February	1st to 10th	Night	18% 62% 5% 10% 4 1	2505
	11th to 20th	Night	17% 60% 2 8% 10% 2	2340
	21st to end	Night	19% 61% 3 8% 7% 2	1980
March	1st to 10th	Night	36% 29% 7% 11% 10% 7%	5374
	11th to 20th	Night	32% 30% 12% 16% 7% 3	5295
	21st to end	Night	38% 30% 4 7% 18% 4	4962
April	1st to 10th	Night	20% 61% 4 8% 6% 2	2000
	11th to 20th	Night	17% 62% 5% 8% 8% 1	1975
	21st to end	Night	17% 62% 4 9% 8% 1	1880
May	1st to 10th	Night	45% 30% 11% 8% 5%	4060
	11th to 20th	Night	40% 31% 10% 10% 6% 2	3863
	21st to end	Night	39% 30% 9% 16% 5%	3927
June	1st to 10th	Night	18% 62% 2 8% 9% 1	1600
	11th to 20th	Night	16% 62% 5% 11% 6%	1600
	21st to end	Night	15% 61% 4 11% 8% 1	1600
July	1st to 10th	Night	40% 27% 8% 15% 8% 1	3560
	11th to 20th	Night	41% 26% 12% 14% 7%	3552
	21st to end	Night	32% 25% 10% 19% 14% 1	3809
August	1st to 10th	Night	41% 37% 8% 9% 5% 1	2759
	11th to 20th	Night	28% 37% 10% 21% 4	3185
	21st to end	Night	14% 36% 11% 24% 15% 1	3586
September	1st to 10th	Night	17% 58% 4 13% 8% 1	2000
	11th to 20th	Night	16% 57% 4 10% 13% 1	2150
	21st to end	Night	23% 56% 4 8% 7% 1	2200
October	1st to 10th	Night	24% 36% 7% 16% 14% 2	3608
	11th to 20th	Night	12% 35% 5% 17% 27% 4	3949
	21st to end	Night	17% 35% 9% 20% 17% 2	4610
November	1st to 10th	Night	18% 54% 5% 11% 10% 1	2667
	11th to 20th	Night	17% 54% 3 10% 12% 4	2730
	21st to end	Night	18% 54% 5% 15% 7% 1	2730
December	1st to 10th	Night	26% 34% 7% 22% 10% 1	4715
	11th to 20th	Night	34% 30% 6% 12% 15% 2	5348
	21st to end	Night	34% 27% 7% 14% 14% 4	7150

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	25% 26% 31% 14% 3	202
	11th to 20th	Day	25% 34% 28% 12% 2 1	200
	21st to end	Day	24% 23% 35% 12% 4 1	241
February	1st to 10th	Day	26% 20% 38% 16% 1	393
	11th to 20th	Day	26% 26% 36% 11% 1	496
	21st to end	Day	26% 25% 32% 12% 4	414
March	1st to 10th	Day	23% 25% 33% 16% 3	500
	11th to 20th	Day	22% 22% 39% 14% 3 1	500
	21st to end	Day	28% 21% 35% 12% 3 1	550
April	1st to 10th	Day	27% 24% 31% 13% 5%	500
	11th to 20th	Day	25% 20% 33% 15% 5% 1	492
	21st to end	Day	26% 15% 35% 17% 5% 2	500
May	1st to 10th	Day	30% 16% 32% 15% 6% 2	503
	11th to 20th	Day	21% 15% 38% 19% 6% 1	527
	21st to end	Day	24% 8% 27% 26% 13% 2	573
June	1st to 10th	Day	19% 9% 34% 23% 13% 2	530
	11th to 20th	Day	19% 12% 32% 26% 10% 2	530
	21st to end	Day	16% 8% 31% 29% 13% 2	530
July	1st to 10th	Day	17% 6% 28% 29% 16% 4	525
	11th to 20th	Day	14% 5% 29% 34% 15% 4	524
	21st to end	Day	15% 7% 33% 34% 10% 2	576
August	1st to 10th	Day	14% 7% 28% 33% 13% 4	526
	11th to 20th	Day	15% 5% 27% 32% 16% 5%	519
	21st to end	Day	10% 9% 34% 28% 13% 6%	583
September	1st to 10th	Day	18% 10% 28% 24% 15% 5%	530
	11th to 20th	Day	13% 7% 35% 25% 13% 7%	530
	21st to end	Day	23% 9% 34% 23% 9% 2	530
October	1st to 10th	Day	26% 10% 26% 21% 14% 4	530
	11th to 20th	Day	17% 18% 32% 23% 9% 2	513
	21st to end	Day	24% 15% 34% 17% 7% 2	462
November	1st to 10th	Day	18% 14% 35% 22% 9% 2	425
	11th to 20th	Day	23% 23% 31% 18% 5%	430
	21st to end	Day	22% 23% 28% 21% 6%	430
December	1st to 10th	Day	25% 22% 32% 15% 5%	220
	11th to 20th	Day	29% 26% 32% 11% 2	220
	21st to end	Day	29% 20% 29% 14% 7% 1	242
January	1st to 10th	Night	21% 23% 36% 16% 3	805
	11th to 20th	Night	28% 20% 32% 19% 2	800
	21st to end	Night	20% 24% 32% 20% 3	880
February	1st to 10th	Night	21% 20% 37% 20% 2	800
	11th to 20th	Night	25% 20% 39% 14% 2	783
	21st to end	Night	24% 21% 35% 16% 3	660
March	1st to 10th	Night	25% 17% 34% 20% 4	800
	11th to 20th	Night	23% 13% 38% 21% 5% 1	800
	21st to end	Night	22% 17% 37% 19% 5%	880
April	1st to 10th	Night	26% 15% 33% 22% 4	800
	11th to 20th	Night	23% 14% 33% 26% 5%	791
	21st to end	Night	22% 12% 35% 23% 6% 1	680
May	1st to 10th	Night	27% 10% 29% 24% 9% 1	600
	11th to 20th	Night	24% 9% 37% 23% 8%	525
	21st to end	Night	23% 5% 23% 33% 14% 1	433
June	1st to 10th	Night	23% 6% 24% 32% 14% 1	400
	11th to 20th	Night	22% 4 22% 38% 14% 1	400
	21st to end	Night	17% 5% 23% 42% 14% 1	400
July	1st to 10th	Night	16% 5% 18% 40% 19% 1	390
	11th to 20th	Night	15% 5% 16% 44% 18% 2	388
	21st to end	Night	16% 4 23% 41% 15% 1	438
August	1st to 10th	Night	12% 4 23% 43% 17% 1	503
	11th to 20th	Night	15% 3 19% 42% 19% 2	761
	21st to end	Night	15% 5% 21% 36% 19% 4	880
September	1st to 10th	Night	15% 5% 28% 32% 17% 3	800
	11th to 20th	Night	16% 4 26% 31% 18% 5%	800
	21st to end	Night	23% 7% 25% 28% 15% 2	800
October	1st to 10th	Night	22% 10% 28% 24% 13% 4	800
	11th to 20th	Night	20% 14% 31% 24% 10% 1	800
	21st to end	Night	26% 12% 36% 19% 7%	880
November	1st to 10th	Night	18% 14% 33% 23% 11% 2	820
	11th to 20th	Night	21% 21% 31% 21% 5% 1	840
	21st to end	Night	19% 22% 32% 22% 5% 1	840
December	1st to 10th	Night	26% 19% 29% 20% 4 1	840
	11th to 20th	Night	27% 21% 30% 20% 3	840
	21st to end	Night	27% 17% 36% 18% 2	924

Page generated at 2025-12-30 01:14:09. Version ac07

Copyright © 2025 Attilla Danko.