West Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Sky 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% 8% 3 6% 5% 4 5% 4 6% 7% 35%	101100
	Night	18% 9% 3 7% 5% 4 4 4 5% 7% 36%	98073
Transparency	Day	46% 6% 12% 21% 13% 2	104626
	Night	46% 6% 12% 21% 13% 2	102637
Seeing	Day	25% 16% 29% 20% 9% 2	14922
	Night	23% 12% 28% 27% 9% 1	23846

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% 5% 5% 5% 4 6% 6% 37%	2517
	11th to 20th	Day	15% 9% 2 6% 5% 5% 6% 4 6% 7% 35%	2520
	21st to end	Day	15% 11% 3 7% 5% 4 4 4 5% 6% 34%	2772
February	1st to 10th	Day	15% 11% 3 6% 5% 5% 5% 4 5% 6% 34%	1652
	11th to 20th	Day	11% 10% 3 8% 6% 5% 5% 4 6% 6% 36%	1848
	21st to end	Day	15% 10% 4 8% 6% 5% 6% 4 5% 5% 33%	1570
March	1st to 10th	Day	23% 14% 3 7% 5% 4 4 3 4 4 28%	3116
	11th to 20th	Day	16% 9% 3 8% 6% 5% 5% 3 5% 6% 34%	3381
	21st to end	Day	22% 9% 3 7% 5% 3 3 3 5% 6% 33%	3960
April	1st to 10th	Day	19% 12% 4 6% 4 4 3 3 4 7% 34%	2280
	11th to 20th	Day	18% 6% 2 7% 4 6% 5% 4 5% 6% 37%	2337
	21st to end	Day	24% 8% 3 7% 5% 3 4 3 4 5% 33%	2470
May	1st to 10th	Day	24% 10% 3 7% 5% 5% 5% 3 6% 6% 26%	4106
	11th to 20th	Day	18% 8% 3 9% 6% 6% 5% 4 6% 7% 29%	4224
	21st to end	Day	15% 6% 2 5% 5% 5% 5% 4 6% 9% 37%	4837
June	1st to 10th	Day	24% 7% 3 6% 5% 5% 4 4 6% 7% 29%	2660
	11th to 20th	Day	17% 6% 2 6% 4 4 4 5% 5% 7% 38%	2660
	21st to end	Day	23% 7% 3 7% 6% 5% 5% 5% 5% 7% 29%	2660
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 5% 7% 10% 33%	4440
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 8% 10% 36%	4440
	21st to end	Day	18% 6% 2 7% 5% 4 4 4 6% 7% 36%	4814
August	1st to 10th	Day	15% 7% 3 7% 6% 5% 5% 5% 7% 8% 33%	4183
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 6% 7% 32%	4045
	21st to end	Day	11% 4 2 5% 4 4 5% 4 7% 9% 44%	4313
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 8% 47%	2254
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 7% 52%	2090
	21st to end	Day	16% 6% 2 4 4 3 4 3 5% 9% 43%	2090
October	1st to 10th	Day	20% 7% 2 7% 4 3 4 3 5% 6% 39%	2090
	11th to 20th	Day	18% 7% 2 5% 4 4 4 3 5% 7% 40%	1815
	21st to end	Day	24% 9% 3 6% 4 5% 3 3 4 6% 34%	1881
November	1st to 10th	Day	16% 7% 2 5% 4 3 4 4 5% 6% 44%	1761
	11th to 20th	Day	21% 4 2 5% 4 4 5% 4 7% 6% 38%	1719
	21st to end	Day	20% 8% 2 5% 3 3 4 3 5% 7% 39%	1692
December	1st to 10th	Day	21% 11% 3 7% 5% 3 4 4 6% 5% 31%	1357
	11th to 20th	Day	22% 10% 3 6% 4 4 4 3 4 5% 36%	1774
	21st to end	Day	22% 12% 3 7% 4 4 4 5% 5% 5% 29%	2772
January	1st to 10th	Night	15% 10% 3 7% 4 3 4 4 4 6% 38%	4429
	11th to 20th	Night	13% 9% 3 10% 6% 6% 6% 5% 7% 8% 28%	4440
	21st to end	Night	18% 10% 4 9% 6% 5% 5% 4 5% 5% 29%	4884
February	1st to 10th	Night	13% 10% 4 8% 6% 6% 5% 4 6% 7% 33%	2521
	11th to 20th	Night	10% 10% 3 8% 6% 4 5% 4 5% 5% 39%	2218
	21st to end	Night	13% 10% 3 7% 5% 4 5% 4 5% 6% 38%	1884
March	1st to 10th	Night	25% 12% 2 5% 4 4 4 4 4 6% 31%	3724
	11th to 20th	Night	14% 10% 4 10% 7% 6% 6% 4 5% 5% 31%	3577
	21st to end	Night	24% 10% 3 7% 4 4 4 4 5% 7% 29%	3668
April	1st to 10th	Night	20% 12% 2 6% 4 3 5% 3 4 6% 35%	1900
	11th to 20th	Night	18% 7% 2 7% 5% 5% 4 5% 4 7% 36%	1792
	21st to end	Night	24% 7% 3 6% 4 4 5% 3 4 6% 34%	1710
May	1st to 10th	Night	23% 11% 3 6% 4 5% 4 4 5% 6% 29%	2834
	11th to 20th	Night	19% 10% 3 8% 4 5% 4 5% 6% 8% 27%	2698
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 7% 40%	2738
June	1st to 10th	Night	22% 8% 3 5% 4 3 5% 4 5% 7% 34%	1520
	11th to 20th	Night	18% 8% 1 6% 4 3 4 3 5% 10% 38%	1520
	21st to end	Night	22% 7% 2 6% 4 4 4 4 6% 8% 33%	1520
July	1st to 10th	Night	12% 5% 2 6% 5% 6% 6% 4 7% 10% 37%	2520
	11th to 20th	Night	15% 6% 2 7% 4 4 4 3 5% 8% 41%	2520
	21st to end	Night	16% 6% 2 5% 4 4 4 4 6% 7% 41%	2739
August	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 3 5% 6% 33%	2714
	11th to 20th	Night	16% 7% 2 7% 5% 4 5% 4 6% 7% 36%	2782
	21st to end	Night	10% 6% 1 6% 4 4 4 4 6% 8% 45%	3339
September	1st to 10th	Night	13% 5% 2 5% 3 4 3 3 5% 6% 50%	1925
	11th to 20th	Night	16% 5% 2 4 2 3 3 2 5% 6% 52%	2090
	21st to end	Night	16% 6% 2 6% 3 3 3 3 5% 6% 45%	2090
October	1st to 10th	Night	19% 7% 2 6% 5% 4 4 4 5% 6% 38%	2090
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 6% 7% 39%	2362
	21st to end	Night	24% 8% 2 5% 4 4 4 3 5% 6% 35%	2717
November	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 42%	2537
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 5% 6% 41%	2484
	21st to end	Night	16% 8% 2 6% 4 4 4 4 6% 7% 40%	2484
December	1st to 10th	Night	20% 11% 3 7% 5% 4 5% 3 5% 5% 31%	2815
	11th to 20th	Night	21% 9% 3 5% 4 4 4 3 5% 5% 36%	3404
	21st to end	Night	22% 11% 3 7% 4 4 4 3 4 5% 33%	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	44% 5% 13% 25% 12% 1	2518
	11th to 20th	Day	46% 7% 13% 22% 10% 2	2520
	21st to end	Day	49% 6% 11% 21% 12% 1	2772
February	1st to 10th	Day	49% 6% 10% 20% 12% 3	1652
	11th to 20th	Day	48% 4 12% 19% 13% 3	1857
	21st to end	Day	50% 5% 11% 17% 12% 5%	1570
March	1st to 10th	Day	60% 5% 8% 11% 14% 3	3119
	11th to 20th	Day	48% 7% 10% 17% 14% 4	3381
	21st to end	Day	51% 3 6% 15% 20% 5%	3960
April	1st to 10th	Day	51% 4 9% 18% 14% 4	2280
	11th to 20th	Day	45% 7% 9% 23% 14% 3	2337
	21st to end	Day	54% 5% 8% 17% 13% 4	2470
May	1st to 10th	Day	56% 8% 10% 16% 8% 1	4120
	11th to 20th	Day	52% 8% 13% 17% 9% 2	4228
	21st to end	Day	42% 7% 12% 22% 15% 1	4837
June	1st to 10th	Day	52% 7% 12% 20% 8%	2660
	11th to 20th	Day	43% 8% 12% 19% 15% 2	2660
	21st to end	Day	53% 8% 11% 17% 11% 1	2660
July	1st to 10th	Day	42% 8% 14% 23% 12%	4370
	11th to 20th	Day	39% 9% 15% 28% 9%	4365
	21st to end	Day	45% 7% 11% 22% 14%	4661
August	1st to 10th	Day	46% 8% 14% 24% 8%	4169
	11th to 20th	Day	50% 7% 14% 23% 7%	4084
	21st to end	Day	33% 7% 15% 25% 17% 2	4313
September	1st to 10th	Day	34% 7% 14% 27% 18% 1	2254
	11th to 20th	Day	33% 5% 10% 31% 19% 3	2090
	21st to end	Day	38% 5% 12% 27% 17% 1	2090
October	1st to 10th	Day	46% 6% 11% 22% 13% 1	3480
	11th to 20th	Day	37% 4 13% 27% 18% 1	2992
	21st to end	Day	38% 3 12% 23% 19% 4	3081
November	1st to 10th	Day	39% 5% 14% 24% 15% 3	1761
	11th to 20th	Day	42% 6% 13% 19% 17% 3	1719
	21st to end	Day	45% 4 18% 21% 11% 1	1692
December	1st to 10th	Day	53% 5% 11% 20% 10% 1	1357
	11th to 20th	Day	50% 4 9% 18% 17% 3	1775
	21st to end	Day	56% 6% 10% 17% 9% 2	2772
January	1st to 10th	Night	45% 6% 14% 24% 9% 2	4429
	11th to 20th	Night	51% 7% 15% 15% 10% 3	4440
	21st to end	Night	55% 6% 8% 18% 11% 1	4884
February	1st to 10th	Night	48% 6% 12% 17% 13% 3	2521
	11th to 20th	Night	45% 5% 10% 20% 16% 3	2220
	21st to end	Night	45% 6% 11% 21% 13% 4	1884
March	1st to 10th	Night	55% 4 11% 17% 9% 3	3734
	11th to 20th	Night	52% 7% 11% 17% 11% 2	3578
	21st to end	Night	54% 4 6% 15% 17% 4	3668
April	1st to 10th	Night	50% 4 8% 18% 17% 3	1900
	11th to 20th	Night	46% 6% 10% 21% 16% 1	1792
	21st to end	Night	50% 5% 12% 16% 13% 4	1710
May	1st to 10th	Night	53% 9% 11% 17% 9% 1	2840
	11th to 20th	Night	52% 10% 11% 17% 7% 2	2699
	21st to end	Night	41% 7% 17% 21% 14% 1	2739
June	1st to 10th	Night	50% 7% 12% 20% 11%	1520
	11th to 20th	Night	42% 7% 13% 23% 13% 2	1520
	21st to end	Night	48% 8% 15% 19% 10% 1	1520
July	1st to 10th	Night	40% 8% 19% 21% 11% 1	2480
	11th to 20th	Night	40% 7% 14% 28% 11%	2472
	21st to end	Night	40% 8% 13% 26% 12%	2651
August	1st to 10th	Night	52% 6% 13% 21% 8%	2704
	11th to 20th	Night	45% 8% 15% 20% 11%	2809
	21st to end	Night	35% 7% 15% 27% 15% 1	3339
September	1st to 10th	Night	34% 6% 13% 31% 16%	1925
	11th to 20th	Night	33% 5% 11% 32% 18% 1	2090
	21st to end	Night	38% 5% 11% 28% 15% 2	2090
October	1st to 10th	Night	46% 6% 13% 21% 13% 1	3480
	11th to 20th	Night	35% 5% 10% 28% 20% 3	3924
	21st to end	Night	39% 5% 10% 27% 16% 2	4467
November	1st to 10th	Night	42% 5% 11% 27% 14% 1	2537
	11th to 20th	Night	43% 5% 12% 22% 16% 2	2484
	21st to end	Night	43% 6% 14% 23% 12% 1	2484
December	1st to 10th	Night	53% 5% 10% 20% 10% 1	2815
	11th to 20th	Night	49% 5% 10% 18% 17% 1	3404
	21st to end	Night	53% 6% 10% 21% 10% 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	32% 16% 30% 17% 4	180
	11th to 20th	Day	30% 17% 34% 16% 2 2	180
	21st to end	Day	20% 24% 32% 19% 6%	198
February	1st to 10th	Day	29% 18% 31% 18% 3	304
	11th to 20th	Day	28% 27% 30% 14% 2	456
	21st to end	Day	24% 23% 30% 19% 4	382
March	1st to 10th	Day	27% 20% 34% 15% 4	460
	11th to 20th	Day	27% 23% 30% 15% 4 1	460
	21st to end	Day	29% 24% 25% 13% 7% 2	506
April	1st to 10th	Day	34% 20% 24% 14% 5% 2	460
	11th to 20th	Day	29% 22% 26% 14% 6% 2	452
	21st to end	Day	35% 20% 27% 10% 6% 2	460
May	1st to 10th	Day	36% 16% 26% 17% 3 2	463
	11th to 20th	Day	23% 19% 33% 17% 7% 2	487
	21st to end	Day	22% 14% 33% 22% 6% 2	529
June	1st to 10th	Day	24% 16% 32% 18% 8% 2	490
	11th to 20th	Day	20% 14% 33% 21% 10% 2	490
	21st to end	Day	24% 13% 34% 19% 9% 2	490
July	1st to 10th	Day	19% 11% 32% 26% 10% 2	485
	11th to 20th	Day	17% 7% 33% 27% 12% 4	484
	21st to end	Day	18% 9% 32% 27% 12% 3	532
August	1st to 10th	Day	14% 9% 30% 28% 14% 5%	486
	11th to 20th	Day	18% 10% 27% 26% 14% 5%	479
	21st to end	Day	14% 11% 29% 27% 14% 5%	539
September	1st to 10th	Day	17% 11% 25% 26% 16% 5%	490
	11th to 20th	Day	18% 10% 26% 20% 15% 10%	490
	21st to end	Day	18% 14% 26% 25% 12% 6%	490
October	1st to 10th	Day	22% 15% 23% 20% 17% 3	490
	11th to 20th	Day	27% 13% 26% 21% 11% 2	403
	21st to end	Day	36% 13% 24% 20% 7%	429
November	1st to 10th	Day	27% 15% 27% 20% 11% 1	395
	11th to 20th	Day	30% 14% 27% 21% 7% 1	373
	21st to end	Day	33% 18% 24% 18% 7%	352
December	1st to 10th	Day	27% 22% 29% 14% 8%	180
	11th to 20th	Day	33% 20% 33% 10% 4	180
	21st to end	Day	34% 19% 27% 16% 4 1	198
January	1st to 10th	Night	22% 17% 33% 24% 4	760
	11th to 20th	Night	25% 18% 28% 26% 3	760
	21st to end	Night	19% 19% 35% 23% 4	836
February	1st to 10th	Night	20% 18% 33% 25% 3	760
	11th to 20th	Night	19% 20% 37% 20% 3	743
	21st to end	Night	23% 16% 31% 26% 4	628
March	1st to 10th	Night	26% 17% 33% 21% 2	760
	11th to 20th	Night	23% 18% 30% 22% 6%	760
	21st to end	Night	26% 17% 31% 21% 5% 1	836
April	1st to 10th	Night	29% 15% 29% 24% 4	760
	11th to 20th	Night	23% 15% 32% 25% 5%	668
	21st to end	Night	29% 14% 31% 21% 5%	570
May	1st to 10th	Night	26% 12% 27% 29% 6% 1	570
	11th to 20th	Night	24% 11% 28% 28% 8%	499
	21st to end	Night	20% 8% 27% 36% 9%	411
June	1st to 10th	Night	25% 6% 29% 32% 8%	380
	11th to 20th	Night	20% 6% 26% 33% 14%	380
	21st to end	Night	22% 4 26% 38% 11%	380
July	1st to 10th	Night	17% 4 22% 40% 16%	370
	11th to 20th	Night	16% 3 22% 40% 18% 1	368
	21st to end	Night	16% 2 22% 40% 19% 1	416
August	1st to 10th	Night	17% 4 22% 37% 19% 1	495
	11th to 20th	Night	17% 4 27% 34% 17% 1	562
	21st to end	Night	15% 6% 22% 35% 19% 2	751
September	1st to 10th	Night	15% 5% 27% 32% 19% 1	760
	11th to 20th	Night	19% 6% 23% 26% 21% 4	760
	21st to end	Night	18% 9% 26% 29% 18% 1	760
October	1st to 10th	Night	24% 8% 20% 32% 15% 1	760
	11th to 20th	Night	22% 11% 28% 26% 12% 1	760
	21st to end	Night	30% 12% 28% 26% 5%	836
November	1st to 10th	Night	22% 12% 29% 28% 8% 1	780
	11th to 20th	Night	22% 15% 29% 25% 8% 1	765
	21st to end	Night	24% 15% 26% 28% 6% 1	760
December	1st to 10th	Night	28% 13% 34% 19% 6%	834
	11th to 20th	Night	24% 20% 28% 23% 4	812
	21st to end	Night	29% 17% 27% 24% 3	836

Page generated at 2024-11-08 22:45:33. Version ac07

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