Star Watcher Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Watcher Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	16% 7% 2 6% 4 4 4 4 5% 7% 42%	101265
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	97851
Transparency	Day	41% 6% 12% 23% 16% 2	104812
	Night	43% 5% 11% 23% 16% 2	102381
Seeing	Day	18% 9% 27% 28% 15% 3	15086
	Night	20% 9% 30% 29% 11% 1	24136

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	14% 7% 3 6% 4 4 4 3 4 6% 45%	2517
	11th to 20th	Day	17% 8% 3 6% 5% 4 5% 4 5% 6% 37%	2520
	21st to end	Day	23% 10% 3 7% 4 4 3 3 4 5% 32%	2897
February	1st to 10th	Day	17% 8% 3 5% 4 4 4 3 5% 7% 39%	1786
	11th to 20th	Day	14% 6% 2 7% 4 3 4 3 5% 7% 45%	1848
	21st to end	Day	18% 9% 2 6% 5% 4 4 3 5% 6% 39%	1570
March	1st to 10th	Day	26% 8% 3 5% 3 3 3 2 4 5% 37%	3116
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 8% 51%	3264
	21st to end	Day	19% 7% 2 5% 3 3 4 3 5% 6% 42%	4166
April	1st to 10th	Day	17% 8% 2 5% 3 3 4 4 5% 7% 41%	2280
	11th to 20th	Day	16% 7% 2 6% 4 3 4 4 5% 6% 44%	2254
	21st to end	Day	18% 6% 2 4 4 3 3 4 6% 7% 43%	2365
May	1st to 10th	Day	25% 9% 2 5% 3 2 3 4 5% 7% 34%	4106
	11th to 20th	Day	23% 8% 2 6% 4 3 3 3 5% 7% 36%	4234
	21st to end	Day	13% 5% 2 5% 4 3 4 4 6% 8% 47%	4837
June	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 7% 9% 42%	2660
	11th to 20th	Day	12% 6% 3 5% 4 4 5% 4 6% 8% 43%	2660
	21st to end	Day	10% 6% 2 6% 5% 4 4 3 6% 10% 45%	2660
July	1st to 10th	Day	8% 6% 3 8% 5% 6% 5% 5% 8% 9% 37%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 3 5% 5% 7% 9% 41%	4440
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 5% 6% 7% 36%	4814
August	1st to 10th	Day	16% 6% 3 6% 5% 5% 4 4 6% 9% 35%	4212
	11th to 20th	Day	10% 6% 3 6% 4 4 4 3 6% 8% 47%	3730
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 9% 49%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 6% 7% 47%	2280
	11th to 20th	Day	12% 6% 2 5% 2 3 3 3 5% 6% 53%	2150
	21st to end	Day	18% 6% 2 7% 4 3 4 3 5% 6% 41%	2090
October	1st to 10th	Day	21% 6% 2 4 3 2 4 3 4 8% 44%	2062
	11th to 20th	Day	14% 5% 2 6% 4 3 3 4 6% 7% 47%	1898
	21st to end	Day	18% 6% 2 5% 4 4 4 4 5% 7% 41%	1881
November	1st to 10th	Day	14% 7% 2 5% 3 4 4 3 4 5% 49%	1734
	11th to 20th	Day	17% 6% 2 5% 3 2 3 3 4 8% 46%	1710
	21st to end	Day	15% 6% 2 6% 4 3 4 4 5% 7% 44%	1706
December	1st to 10th	Day	18% 6% 3 7% 4 4 4 4 6% 8% 36%	1520
	11th to 20th	Day	20% 8% 3 4 3 2 2 2 5% 7% 44%	1906
	21st to end	Day	18% 11% 3 8% 5% 4 4 3 5% 6% 32%	2772
January	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 5% 7% 44%	4429
	11th to 20th	Night	17% 10% 3 7% 3 3 3 3 3 5% 43%	4440
	21st to end	Night	24% 11% 3 7% 4 3 3 3 5% 5% 31%	4756
February	1st to 10th	Night	16% 9% 3 6% 5% 4 4 3 5% 6% 39%	2393
	11th to 20th	Night	17% 8% 2 6% 5% 3 4 3 6% 6% 40%	2218
	21st to end	Night	20% 7% 1 5% 4 4 4 2 5% 7% 40%	1884
March	1st to 10th	Night	24% 11% 3 7% 3 3 3 3 4 5% 35%	3724
	11th to 20th	Night	13% 6% 2 7% 4 5% 4 4 5% 6% 45%	3690
	21st to end	Night	23% 9% 2 5% 3 3 3 3 5% 5% 39%	3466
April	1st to 10th	Night	19% 6% 2 7% 5% 4 5% 3 6% 6% 38%	1900
	11th to 20th	Night	17% 7% 2 7% 5% 3 3 3 4 7% 41%	1875
	21st to end	Night	18% 8% 2 7% 4 3 4 3 4 6% 42%	1814
May	1st to 10th	Night	22% 11% 3 7% 5% 4 4 3 5% 7% 28%	2834
	11th to 20th	Night	25% 9% 3 7% 5% 3 4 3 6% 7% 28%	2688
	21st to end	Night	16% 8% 3 6% 4 3 4 4 6% 8% 40%	2738
June	1st to 10th	Night	18% 7% 2 4 5% 4 4 3 5% 8% 40%	1520
	11th to 20th	Night	15% 5% 2 5% 4 4 4 4 7% 9% 41%	1520
	21st to end	Night	11% 6% 2 6% 5% 6% 5% 4 6% 10% 40%	1520
July	1st to 10th	Night	9% 10% 3 8% 6% 5% 5% 5% 7% 9% 34%	2520
	11th to 20th	Night	10% 8% 3 8% 5% 5% 5% 5% 6% 9% 35%	2520
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 6% 8% 32%	2739
August	1st to 10th	Night	20% 7% 3 6% 4 5% 6% 4 6% 9% 30%	2681
	11th to 20th	Night	12% 7% 3 6% 4 4 6% 4 7% 7% 40%	3097
	21st to end	Night	8% 4 2 5% 3 4 4 4 6% 7% 54%	3476
September	1st to 10th	Night	14% 4 2 7% 4 5% 4 4 5% 5% 45%	1900
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 5% 6% 51%	2028
	21st to end	Night	17% 6% 3 6% 4 4 4 3 5% 7% 41%	2090
October	1st to 10th	Night	21% 8% 2 6% 4 4 4 3 4 6% 39%	2115
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 4 5% 49%	2280
	21st to end	Night	21% 7% 2 5% 3 4 3 3 5% 6% 40%	2717
November	1st to 10th	Night	13% 6% 2 6% 5% 4 4 3 5% 6% 47%	2505
	11th to 20th	Night	19% 7% 2 4 4 3 3 3 4 6% 45%	2470
	21st to end	Night	14% 6% 2 5% 4 3 4 3 6% 7% 45%	2472
December	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 38%	2660
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 5% 40%	3288
	21st to end	Night	20% 10% 3 6% 4 4 4 3 4 6% 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	40% 5% 9% 29% 13% 2	2518
	11th to 20th	Day	45% 6% 10% 25% 13% 1	2520
	21st to end	Day	53% 7% 8% 21% 10% 1	2897
February	1st to 10th	Day	44% 5% 13% 23% 14% 1	1786
	11th to 20th	Day	40% 5% 10% 25% 17% 3	1857
	21st to end	Day	47% 4 10% 21% 15% 4	1570
March	1st to 10th	Day	50% 5% 11% 20% 12% 2	3119
	11th to 20th	Day	30% 7% 13% 33% 16% 1	3264
	21st to end	Day	43% 6% 7% 18% 21% 5%	4166
April	1st to 10th	Day	44% 6% 8% 21% 17% 4	2280
	11th to 20th	Day	41% 6% 11% 21% 19% 3	2254
	21st to end	Day	41% 6% 10% 22% 17% 4	2365
May	1st to 10th	Day	50% 8% 15% 15% 11% 1	4120
	11th to 20th	Day	47% 9% 13% 19% 10% 2	4238
	21st to end	Day	33% 9% 12% 25% 19% 2	4837
June	1st to 10th	Day	39% 8% 12% 23% 16% 2	2660
	11th to 20th	Day	38% 8% 13% 26% 13% 2	2660
	21st to end	Day	35% 8% 17% 23% 15% 1	2660
July	1st to 10th	Day	43% 9% 12% 20% 16% 1	4370
	11th to 20th	Day	39% 5% 18% 24% 13% 1	4365
	21st to end	Day	46% 6% 13% 18% 15% 2	4661
August	1st to 10th	Day	46% 5% 17% 22% 9%	4198
	11th to 20th	Day	37% 5% 13% 33% 10% 1	3766
	21st to end	Day	31% 5% 14% 28% 20% 2	4180
September	1st to 10th	Day	37% 7% 13% 22% 20% 2	2280
	11th to 20th	Day	32% 6% 11% 25% 25% 2	2150
	21st to end	Day	44% 7% 8% 19% 20% 2	2090
October	1st to 10th	Day	37% 4 9% 22% 23% 5%	3426
	11th to 20th	Day	26% 4 10% 25% 32% 3	3126
	21st to end	Day	31% 3 15% 27% 21% 2	3081
November	1st to 10th	Day	38% 5% 13% 22% 19% 2	1734
	11th to 20th	Day	39% 4 11% 22% 21% 3	1710
	21st to end	Day	39% 5% 14% 27% 12% 2	1706
December	1st to 10th	Day	44% 5% 16% 19% 14% 1	1520
	11th to 20th	Day	42% 4 15% 18% 19% 2	1906
	21st to end	Day	51% 6% 10% 23% 9% 1	2772
January	1st to 10th	Night	40% 3 11% 30% 12% 3	4429
	11th to 20th	Night	45% 4 11% 24% 13% 2	4440
	21st to end	Night	54% 4 8% 21% 11% 1	4756
February	1st to 10th	Night	44% 6% 12% 23% 13% 2	2393
	11th to 20th	Night	44% 5% 9% 21% 17% 4	2220
	21st to end	Night	44% 4 7% 24% 16% 4	1884
March	1st to 10th	Night	53% 4 9% 21% 11% 2	3734
	11th to 20th	Night	38% 6% 12% 26% 16% 3	3691
	21st to end	Night	46% 5% 6% 13% 22% 6%	3466
April	1st to 10th	Night	47% 4 9% 19% 17% 3	1900
	11th to 20th	Night	44% 5% 11% 18% 21% 1	1875
	21st to end	Night	44% 6% 8% 21% 16% 5%	1814
May	1st to 10th	Night	55% 8% 10% 16% 10% 1	2840
	11th to 20th	Night	54% 8% 13% 15% 9% 2	2689
	21st to end	Night	41% 9% 13% 21% 14% 2	2739
June	1st to 10th	Night	43% 7% 10% 20% 18% 2	1520
	11th to 20th	Night	38% 8% 13% 28% 13% 1	1520
	21st to end	Night	39% 10% 12% 25% 13% 1	1520
July	1st to 10th	Night	46% 8% 13% 20% 12% 1	2480
	11th to 20th	Night	45% 5% 14% 24% 11% 1	2472
	21st to end	Night	49% 6% 13% 16% 13% 1	2651
August	1st to 10th	Night	51% 6% 16% 17% 9% 1	2671
	11th to 20th	Night	41% 6% 14% 30% 9% 1	3127
	21st to end	Night	29% 5% 12% 35% 19% 1	3476
September	1st to 10th	Night	40% 8% 9% 24% 18% 2	1900
	11th to 20th	Night	33% 5% 10% 24% 24% 3	2028
	21st to end	Night	43% 7% 8% 20% 19% 3	2090
October	1st to 10th	Night	41% 3 11% 25% 18% 3	3531
	11th to 20th	Night	26% 4 9% 27% 31% 4	3779
	21st to end	Night	32% 3 13% 28% 21% 3	4467
November	1st to 10th	Night	38% 5% 12% 23% 20% 3	2505
	11th to 20th	Night	42% 4 9% 24% 18% 4	2470
	21st to end	Night	38% 4 13% 29% 14% 2	2472
December	1st to 10th	Night	46% 5% 13% 21% 14% 1	2660
	11th to 20th	Night	44% 4 12% 20% 18% 2	3288
	21st to end	Night	47% 5% 10% 24% 12% 1	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% 16% 35% 21% 7% 1	180
	11th to 20th	Day	15% 28% 34% 18% 3 1	180
	21st to end	Day	21% 12% 40% 19% 8% 1	233
February	1st to 10th	Day	24% 12% 41% 20% 3	346
	11th to 20th	Day	19% 16% 41% 21% 2	456
	21st to end	Day	24% 15% 34% 21% 6% 1	382
March	1st to 10th	Day	17% 16% 35% 26% 5%	460
	11th to 20th	Day	17% 12% 39% 26% 5% 1	460
	21st to end	Day	22% 13% 38% 20% 7% 1	506
April	1st to 10th	Day	21% 14% 34% 22% 9% 1	460
	11th to 20th	Day	21% 9% 33% 25% 11% 1	452
	21st to end	Day	24% 9% 31% 28% 7% 2	460
May	1st to 10th	Day	27% 10% 28% 25% 9% 1	463
	11th to 20th	Day	20% 7% 27% 34% 12% 1	487
	21st to end	Day	19% 4 18% 31% 25% 4	529
June	1st to 10th	Day	17% 5% 20% 37% 19% 3	490
	11th to 20th	Day	14% 6% 22% 38% 17% 3	490
	21st to end	Day	12% 6% 21% 35% 21% 4	490
July	1st to 10th	Day	13% 3 17% 35% 26% 6%	485
	11th to 20th	Day	9% 4 17% 36% 26% 8%	484
	21st to end	Day	12% 5% 18% 40% 23% 3	532
August	1st to 10th	Day	12% 3 17% 36% 26% 7%	486
	11th to 20th	Day	14% 2 14% 38% 25% 7%	479
	21st to end	Day	10% 5% 19% 34% 23% 8%	539
September	1st to 10th	Day	17% 5% 20% 27% 24% 8%	490
	11th to 20th	Day	14% 3 21% 31% 21% 10%	490
	21st to end	Day	22% 5% 25% 27% 16% 4	490
October	1st to 10th	Day	27% 5% 23% 20% 18% 7%	490
	11th to 20th	Day	14% 10% 32% 25% 15% 4	488
	21st to end	Day	21% 10% 32% 23% 11% 2	429
November	1st to 10th	Day	16% 12% 26% 32% 12% 3	386
	11th to 20th	Day	21% 15% 31% 24% 9% 1	370
	21st to end	Day	16% 14% 33% 27% 10% 1	366
December	1st to 10th	Day	19% 15% 37% 22% 6% 1	180
	11th to 20th	Day	24% 22% 29% 20% 4 1	180
	21st to end	Day	24% 14% 39% 15% 7% 1	198
January	1st to 10th	Night	19% 13% 40% 22% 6%	760
	11th to 20th	Night	26% 13% 34% 23% 4	760
	21st to end	Night	19% 13% 41% 24% 3	836
February	1st to 10th	Night	20% 12% 38% 28% 2	760
	11th to 20th	Night	23% 14% 41% 20% 2	743
	21st to end	Night	25% 14% 35% 23% 4	628
March	1st to 10th	Night	22% 11% 39% 23% 5%	760
	11th to 20th	Night	20% 11% 41% 24% 4 1	760
	21st to end	Night	21% 11% 39% 21% 7%	836
April	1st to 10th	Night	21% 8% 39% 26% 5%	760
	11th to 20th	Night	21% 9% 33% 30% 8%	751
	21st to end	Night	21% 7% 33% 31% 7% 1	674
May	1st to 10th	Night	25% 6% 30% 28% 10% 1	570
	11th to 20th	Night	22% 7% 34% 29% 8%	489
	21st to end	Night	20% 3 18% 39% 18% 1	411
June	1st to 10th	Night	20% 4 22% 35% 18% 1	380
	11th to 20th	Night	18% 2 17% 43% 18% 1	380
	21st to end	Night	12% 4 21% 45% 17% 1	380
July	1st to 10th	Night	12% 3 21% 41% 21% 2	370
	11th to 20th	Night	13% 4 16% 42% 24% 2	368
	21st to end	Night	15% 3 16% 46% 18% 1	416
August	1st to 10th	Night	9% 2 17% 45% 25% 1	475
	11th to 20th	Night	15% 3 15% 39% 26% 2	750
	21st to end	Night	13% 4 15% 39% 26% 4	836
September	1st to 10th	Night	15% 4 22% 34% 20% 4	760
	11th to 20th	Night	16% 3 20% 33% 21% 7%	760
	21st to end	Night	21% 4 24% 31% 16% 3	760
October	1st to 10th	Night	24% 7% 26% 23% 16% 3	760
	11th to 20th	Night	19% 10% 28% 31% 12% 1	760
	21st to end	Night	24% 8% 32% 26% 9%	836
November	1st to 10th	Night	15% 9% 34% 27% 13% 2	771
	11th to 20th	Night	21% 12% 33% 24% 9% 1	760
	21st to end	Night	18% 13% 33% 26% 8% 1	760
December	1st to 10th	Night	24% 14% 33% 22% 7% 1	760
	11th to 20th	Night	21% 14% 35% 25% 4	760
	21st to end	Night	23% 11% 38% 24% 3	836

Page generated at 2024-11-05 08:15:52. Version ac07

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