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 2024-11-12.

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%	101279
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	97906
Transparency	Day	23% 47% 7% 13% 9% 1	104828
	Night	24% 47% 6% 12% 9% 1	102446
Seeing	Day	20% 14% 32% 22% 9% 2	15084
	Night	21% 13% 31% 25% 9% 1	24107

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	16% 7% 2 5% 4 4 3 3 5% 5% 46%	2517
	11th to 20th	Day	17% 9% 2 7% 4 4 4 3 5% 6% 39%	2520
	21st to end	Day	23% 10% 3 7% 4 3 3 3 4 6% 33%	2836
February	1st to 10th	Day	18% 8% 2 6% 4 5% 5% 3 5% 7% 38%	1798
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 5% 6% 43%	1848
	21st to end	Day	21% 9% 3 4 5% 4 4 4 5% 6% 36%	1570
March	1st to 10th	Day	27% 9% 3 5% 4 3 3 2 3 5% 35%	3116
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 45%	3244
	21st to end	Day	21% 7% 3 6% 4 4 4 4 5% 7% 37%	4166
April	1st to 10th	Day	19% 7% 2 5% 4 4 4 3 5% 7% 39%	2280
	11th to 20th	Day	21% 6% 2 5% 4 3 3 3 5% 6% 41%	2254
	21st to end	Day	21% 6% 2 4 4 5% 4 3 5% 6% 41%	2394
May	1st to 10th	Day	28% 9% 3 5% 3 3 3 3 5% 7% 31%	4106
	11th to 20th	Day	23% 7% 2 6% 4 4 4 3 5% 7% 33%	4225
	21st to end	Day	16% 5% 2 5% 3 3 4 3 5% 8% 45%	4837
June	1st to 10th	Day	18% 6% 2 4 5% 4 5% 4 7% 9% 36%	2660
	11th to 20th	Day	16% 6% 2 5% 4 4 4 4 6% 9% 40%	2660
	21st to end	Day	13% 6% 2 7% 5% 4 5% 5% 6% 8% 41%	2660
July	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	4440
	11th to 20th	Day	15% 5% 2 5% 5% 4 5% 4 7% 9% 40%	4440
	21st to end	Day	18% 8% 3 6% 4 5% 4 4 6% 7% 34%	4814
August	1st to 10th	Day	19% 7% 2 7% 4 4 4 4 6% 8% 33%	4215
	11th to 20th	Day	12% 6% 2 6% 3 4 4 4 6% 8% 44%	3768
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 9% 44%	4180
September	1st to 10th	Day	13% 6% 3 6% 4 4 4 3 5% 7% 46%	2280
	11th to 20th	Day	13% 5% 2 4 4 3 3 3 5% 7% 52%	2137
	21st to end	Day	20% 6% 3 5% 3 4 4 3 5% 6% 40%	2090
October	1st to 10th	Day	22% 6% 3 5% 4 3 4 3 4 5% 42%	2081
	11th to 20th	Day	14% 6% 2 5% 3 3 4 2 5% 8% 49%	1874
	21st to end	Day	18% 7% 2 5% 3 4 4 4 5% 6% 42%	1881
November	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 4 5% 50%	1761
	11th to 20th	Day	19% 6% 1 4 4 3 3 3 4 6% 47%	1719
	21st to end	Day	17% 6% 2 4 4 3 4 3 4 7% 45%	1710
December	1st to 10th	Day	19% 6% 2 7% 5% 5% 5% 3 5% 6% 38%	1520
	11th to 20th	Day	23% 6% 2 5% 3 2 3 2 4 5% 45%	1906
	21st to end	Day	21% 11% 3 7% 4 4 3 3 5% 6% 34%	2772
January	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 46%	4429
	11th to 20th	Night	18% 9% 3 7% 3 4 3 2 4 4 44%	4440
	21st to end	Night	25% 11% 3 7% 4 3 3 3 4 4 33%	4816
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 4 6% 41%	2380
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 5% 6% 41%	2218
	21st to end	Night	21% 8% 2 6% 4 4 4 2 4 5% 40%	1884
March	1st to 10th	Night	26% 12% 3 5% 3 2 3 2 4 5% 34%	3724
	11th to 20th	Night	11% 7% 2 7% 4 5% 4 4 5% 7% 43%	3696
	21st to end	Night	22% 8% 2 5% 3 3 4 3 5% 4 39%	3466
April	1st to 10th	Night	20% 8% 2 6% 5% 3 4 3 5% 7% 37%	1900
	11th to 20th	Night	20% 6% 2 6% 4 4 3 4 4 5% 41%	1875
	21st to end	Night	21% 6% 2 6% 4 3 4 3 5% 4 41%	1786
May	1st to 10th	Night	24% 11% 4 8% 4 4 4 3 4 7% 27%	2834
	11th to 20th	Night	24% 9% 3 7% 4 4 4 4 5% 7% 28%	2698
	21st to end	Night	19% 8% 2 4 4 3 3 3 5% 8% 40%	2738
June	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 39%	1520
	11th to 20th	Night	17% 6% 2 5% 4 4 3 4 6% 9% 40%	1520
	21st to end	Night	15% 6% 3 5% 4 4 4 5% 5% 9% 41%	1520
July	1st to 10th	Night	16% 8% 3 8% 5% 4 5% 5% 5% 9% 32%	2520
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 6% 10% 35%	2520
	21st to end	Night	20% 9% 2 6% 4 4 4 3 6% 7% 33%	2739
August	1st to 10th	Night	24% 7% 3 7% 4 4 5% 5% 6% 7% 28%	2684
	11th to 20th	Night	12% 7% 3 7% 4 5% 5% 4 6% 8% 39%	3057
	21st to end	Night	9% 5% 2 4 3 3 4 3 5% 8% 55%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 5% 4 4 6% 6% 42%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 6% 53%	2042
	21st to end	Night	19% 6% 3 6% 4 4 5% 3 4 6% 41%	2090
October	1st to 10th	Night	20% 8% 2 6% 4 4 3 3 3 6% 41%	2095
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 6% 49%	2304
	21st to end	Night	21% 7% 2 6% 3 3 3 3 5% 6% 41%	2717
November	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 6% 49%	2537
	11th to 20th	Night	20% 7% 2 4 3 2 3 3 4 5% 47%	2484
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 7% 46%	2470
December	1st to 10th	Night	21% 7% 2 5% 4 4 4 3 5% 6% 39%	2655
	11th to 20th	Night	21% 7% 2 5% 4 4 4 3 4 5% 42%	3288
	21st to end	Night	22% 11% 3 6% 4 4 3 3 4 5% 37%	4884

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	25% 38% 4 22% 8% 2	2518
	11th to 20th	Day	27% 40% 7% 19% 7% 1	2520
	21st to end	Day	37% 41% 5% 10% 5% 1	2836
February	1st to 10th	Day	18% 65% 6% 8% 3 1	1798
	11th to 20th	Day	16% 63% 2 9% 9% 1	1857
	21st to end	Day	17% 65% 5% 8% 4 2	1570
March	1st to 10th	Day	34% 42% 7% 10% 6% 1	3119
	11th to 20th	Day	13% 44% 10% 24% 8% 1	3244
	21st to end	Day	27% 41% 5% 11% 12% 3	4166
April	1st to 10th	Day	19% 65% 4 6% 5% 1	2280
	11th to 20th	Day	15% 66% 3 8% 8%	2254
	21st to end	Day	17% 65% 3 8% 6% 1	2394
May	1st to 10th	Day	35% 42% 11% 6% 5% 1	4120
	11th to 20th	Day	31% 43% 11% 11% 4	4229
	21st to end	Day	21% 43% 6% 18% 13%	4837
June	1st to 10th	Day	16% 65% 3 10% 6%	2660
	11th to 20th	Day	14% 65% 5% 9% 6% 1	2660
	21st to end	Day	13% 64% 5% 10% 8%	2660
July	1st to 10th	Day	32% 39% 6% 12% 10% 1	4370
	11th to 20th	Day	28% 37% 12% 13% 9%	4365
	21st to end	Day	35% 36% 8% 11% 9% 1	4661
August	1st to 10th	Day	35% 37% 11% 14% 4	4201
	11th to 20th	Day	23% 37% 11% 23% 5%	3805
	21st to end	Day	20% 38% 10% 22% 10% 1	4180
September	1st to 10th	Day	15% 61% 6% 10% 8%	2280
	11th to 20th	Day	15% 60% 4 9% 11% 1	2137
	21st to end	Day	20% 60% 4 8% 7% 1	2090
October	1st to 10th	Day	21% 37% 7% 12% 17% 5%	3458
	11th to 20th	Day	14% 36% 4 14% 28% 3	3090
	21st to end	Day	17% 37% 11% 18% 16% 2	3081
November	1st to 10th	Day	17% 56% 7% 9% 10% 1	1761
	11th to 20th	Day	15% 59% 3 9% 12% 2	1719
	21st to end	Day	14% 60% 6% 12% 6% 2	1710
December	1st to 10th	Day	20% 60% 6% 6% 7%	1520
	11th to 20th	Day	21% 48% 6% 10% 13% 2	1906
	21st to end	Day	34% 39% 5% 15% 6%	2772
January	1st to 10th	Night	25% 37% 7% 21% 7% 4	4429
	11th to 20th	Night	27% 37% 8% 16% 10% 2	4440
	21st to end	Night	40% 41% 4 9% 4 1	4816
February	1st to 10th	Night	16% 65% 5% 9% 4 1	2380
	11th to 20th	Night	16% 63% 2 7% 10% 2	2220
	21st to end	Night	17% 64% 3 8% 6% 2	1884
March	1st to 10th	Night	38% 41% 4 10% 6% 1	3734
	11th to 20th	Night	20% 43% 10% 16% 10% 2	3697
	21st to end	Night	30% 42% 4 5% 16% 4	3466
April	1st to 10th	Night	18% 64% 4 8% 5% 1	1900
	11th to 20th	Night	15% 65% 5% 8% 7% 1	1875
	21st to end	Night	16% 65% 3 9% 6% 1	1786
May	1st to 10th	Night	37% 43% 9% 8% 4	2840
	11th to 20th	Night	37% 45% 9% 7% 3	2699
	21st to end	Night	25% 42% 9% 17% 7% 1	2739
June	1st to 10th	Night	16% 65% 2 8% 8% 1	1520
	11th to 20th	Night	13% 65% 5% 11% 6%	1520
	21st to end	Night	14% 64% 4 11% 7%	1520
July	1st to 10th	Night	35% 38% 7% 11% 8% 1	2480
	11th to 20th	Night	31% 36% 12% 14% 7%	2472
	21st to end	Night	35% 36% 9% 8% 12% 1	2651
August	1st to 10th	Night	41% 38% 8% 8% 4 1	2674
	11th to 20th	Night	28% 38% 10% 21% 4	3086
	21st to end	Night	13% 37% 11% 24% 14% 1	3476
September	1st to 10th	Night	15% 61% 4 12% 7% 1	1900
	11th to 20th	Night	15% 59% 3 9% 12% 2	2042
	21st to end	Night	22% 59% 4 8% 7% 1	2090
October	1st to 10th	Night	24% 37% 7% 16% 14% 2	3498
	11th to 20th	Night	11% 37% 5% 17% 27% 4	3827
	21st to end	Night	16% 36% 9% 20% 17% 2	4467
November	1st to 10th	Night	18% 56% 5% 10% 10% 2	2537
	11th to 20th	Night	16% 59% 4 9% 9% 3	2484
	21st to end	Night	16% 59% 5% 13% 6% 2	2470
December	1st to 10th	Night	21% 60% 4 9% 6%	2655
	11th to 20th	Night	23% 49% 6% 9% 11% 2	3288
	21st to end	Night	32% 39% 7% 14% 7%	4884

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% 26% 33% 16% 4	180
	11th to 20th	Day	22% 34% 29% 12% 2 1	180
	21st to end	Day	22% 22% 37% 13% 5% 1	216
February	1st to 10th	Day	24% 20% 38% 17% 1	358
	11th to 20th	Day	23% 25% 38% 12% 2	456
	21st to end	Day	28% 23% 32% 13% 4	382
March	1st to 10th	Day	22% 24% 35% 17% 3	460
	11th to 20th	Day	21% 23% 39% 14% 3 1	460
	21st to end	Day	26% 21% 35% 13% 4 1	506
April	1st to 10th	Day	24% 24% 32% 14% 6%	460
	11th to 20th	Day	25% 19% 33% 16% 6% 1	452
	21st to end	Day	26% 14% 35% 17% 5% 2	460
May	1st to 10th	Day	30% 15% 32% 15% 6% 2	463
	11th to 20th	Day	21% 13% 38% 20% 7% 1	487
	21st to end	Day	22% 8% 26% 27% 14% 2	529
June	1st to 10th	Day	19% 9% 33% 24% 14% 2	490
	11th to 20th	Day	17% 12% 32% 27% 10% 2	490
	21st to end	Day	16% 8% 32% 29% 13% 2	490
July	1st to 10th	Day	17% 6% 27% 29% 17% 4	485
	11th to 20th	Day	14% 4 29% 33% 15% 5%	484
	21st to end	Day	15% 7% 31% 34% 10% 2	532
August	1st to 10th	Day	15% 6% 27% 35% 13% 4	486
	11th to 20th	Day	15% 5% 27% 31% 16% 5%	479
	21st to end	Day	10% 7% 33% 29% 14% 6%	539
September	1st to 10th	Day	18% 9% 27% 25% 16% 5%	490
	11th to 20th	Day	14% 7% 33% 25% 13% 7%	490
	21st to end	Day	23% 9% 34% 22% 10% 2	490
October	1st to 10th	Day	27% 10% 26% 20% 13% 3	490
	11th to 20th	Day	15% 18% 33% 24% 9% 2	475
	21st to end	Day	24% 15% 34% 18% 7% 2	429
November	1st to 10th	Day	16% 13% 36% 22% 9% 3	395
	11th to 20th	Day	23% 21% 31% 19% 6%	373
	21st to end	Day	18% 21% 31% 23% 7%	370
December	1st to 10th	Day	22% 23% 32% 17% 6% 1	180
	11th to 20th	Day	25% 27% 34% 12% 2	180
	21st to end	Day	25% 21% 31% 14% 7% 2	198
January	1st to 10th	Night	20% 22% 36% 17% 4	760
	11th to 20th	Night	28% 20% 32% 18% 2	760
	21st to end	Night	20% 24% 33% 20% 3	836
February	1st to 10th	Night	20% 19% 38% 21% 2	760
	11th to 20th	Night	23% 21% 40% 15% 2	743
	21st to end	Night	25% 21% 34% 17% 4	628
March	1st to 10th	Night	24% 16% 35% 20% 4	760
	11th to 20th	Night	22% 13% 38% 21% 5% 1	760
	21st to end	Night	22% 17% 36% 19% 5%	836
April	1st to 10th	Night	25% 14% 33% 23% 4	760
	11th to 20th	Night	22% 13% 33% 27% 5%	751
	21st to end	Night	22% 12% 35% 24% 7% 1	646
May	1st to 10th	Night	26% 11% 28% 24% 10% 1	570
	11th to 20th	Night	23% 9% 36% 23% 8%	499
	21st to end	Night	23% 5% 22% 34% 15% 1	411
June	1st to 10th	Night	23% 6% 23% 33% 14% 1	380
	11th to 20th	Night	21% 3 22% 39% 14% 1	380
	21st to end	Night	17% 4 23% 42% 13% 1	380
July	1st to 10th	Night	15% 5% 18% 40% 20% 1	370
	11th to 20th	Night	14% 5% 16% 45% 18% 2	368
	21st to end	Night	17% 4 22% 40% 15% 1	416
August	1st to 10th	Night	11% 4 23% 43% 17% 1	478
	11th to 20th	Night	15% 3 18% 41% 20% 2	722
	21st to end	Night	15% 4 20% 37% 20% 5%	836
September	1st to 10th	Night	16% 5% 28% 32% 16% 3	760
	11th to 20th	Night	17% 4 26% 29% 19% 5%	760
	21st to end	Night	23% 7% 26% 27% 15% 2	760
October	1st to 10th	Night	23% 9% 28% 24% 13% 3	760
	11th to 20th	Night	19% 14% 32% 25% 11% 1	760
	21st to end	Night	26% 11% 36% 19% 8%	836
November	1st to 10th	Night	16% 14% 34% 23% 11% 2	780
	11th to 20th	Night	22% 19% 32% 21% 6% 1	765
	21st to end	Night	18% 21% 33% 21% 5% 1	760
December	1st to 10th	Night	26% 18% 29% 21% 5% 1	760
	11th to 20th	Night	24% 21% 30% 21% 4	760
	21st to end	Night	26% 16% 39% 18% 2	836

Page generated at 2024-11-08 22:45:39. Version ac07

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