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 2025-12-18.

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%	126453
	Night	17% 8% 3 7% 5% 4 5% 4 5% 7% 36%	124555
Transparency	Day	46% 6% 11% 21% 13% 2	129961
	Night	46% 6% 12% 21% 13% 2	129135
Seeing	Day	25% 16% 29% 19% 8% 2	16285
	Night	23% 13% 28% 27% 9% 1	25267

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	12% 7% 3 7% 6% 7% 7% 5% 7% 6% 33%	3597
	11th to 20th	Day	12% 8% 3 6% 5% 4 5% 4 6% 8% 39%	3570
	21st to end	Day	11% 10% 3 7% 5% 5% 4 4 5% 6% 42%	3960
February	1st to 10th	Day	15% 11% 3 7% 5% 5% 6% 4 5% 6% 34%	1740
	11th to 20th	Day	11% 10% 3 7% 6% 5% 5% 4 7% 6% 36%	1948
	21st to end	Day	14% 10% 4 8% 6% 5% 6% 4 5% 5% 34%	1650
March	1st to 10th	Day	21% 12% 3 8% 5% 5% 4 3 4 5% 29%	4476
	11th to 20th	Day	18% 11% 3 8% 5% 4 4 3 4 5% 35%	4839
	21st to end	Day	25% 9% 2 6% 4 3 4 3 5% 7% 31%	5680
April	1st to 10th	Day	19% 12% 3 7% 4 4 3 3 4 7% 34%	2400
	11th to 20th	Day	19% 6% 2 7% 4 6% 4 4 5% 6% 37%	2462
	21st to end	Day	24% 8% 3 7% 5% 3 4 3 4 5% 34%	2600
May	1st to 10th	Day	33% 10% 3 7% 5% 5% 4 3 5% 5% 21%	5886
	11th to 20th	Day	19% 9% 3 9% 6% 6% 5% 4 5% 7% 28%	6060
	21st to end	Day	20% 7% 3 6% 5% 5% 5% 4 7% 8% 32%	6949
June	1st to 10th	Day	24% 7% 2 6% 5% 5% 5% 4 6% 7% 28%	2800
	11th to 20th	Day	17% 7% 2 6% 4 4 4 5% 5% 7% 38%	2800
	21st to end	Day	22% 7% 2 7% 5% 5% 5% 4 6% 7% 30%	2800
July	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 7% 10% 29%	6360
	11th to 20th	Day	15% 6% 2 5% 4 5% 5% 5% 7% 10% 36%	6360
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	15% 6% 2 7% 6% 5% 5% 5% 7% 8% 34%	4316
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 6% 7% 33%	4175
	21st to end	Day	11% 4 2 5% 4 4 5% 4 7% 9% 44%	4449
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 4 6% 8% 47%	2372
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 5% 7% 52%	2200
	21st to end	Day	16% 6% 2 4 4 3 4 3 5% 8% 44%	2200
October	1st to 10th	Day	19% 6% 2 7% 4 3 4 3 5% 6% 41%	2200
	11th to 20th	Day	18% 7% 2 5% 4 4 4 3 5% 7% 40%	1910
	21st to end	Day	23% 9% 3 6% 4 5% 3 3 4 7% 33%	1980
November	1st to 10th	Day	16% 7% 2 6% 4 3 4 4 5% 6% 43%	1851
	11th to 20th	Day	19% 4 2 5% 4 4 5% 4 7% 6% 40%	1890
	21st to end	Day	20% 9% 2 6% 4 4 4 3 5% 6% 38%	1870
December	1st to 10th	Day	17% 8% 3 8% 5% 4 4 4 6% 8% 33%	2391
	11th to 20th	Day	18% 9% 2 5% 3 4 4 3 6% 7% 38%	2826
	21st to end	Day	19% 10% 3 7% 4 4 5% 4 5% 5% 34%	3960
January	1st to 10th	Night	12% 8% 3 9% 5% 5% 5% 4 5% 6% 38%	6349
	11th to 20th	Night	12% 10% 4 11% 5% 6% 6% 5% 7% 8% 26%	6315
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 4 6% 6% 37%	6996
February	1st to 10th	Night	13% 11% 4 8% 6% 6% 5% 4 6% 6% 33%	2653
	11th to 20th	Night	10% 10% 3 8% 6% 4 5% 4 6% 6% 39%	2338
	21st to end	Night	13% 10% 3 7% 5% 4 5% 4 5% 6% 38%	1980
March	1st to 10th	Night	22% 11% 3 7% 4 4 5% 4 5% 6% 29%	5364
	11th to 20th	Night	15% 10% 4 9% 5% 5% 5% 4 5% 6% 32%	5119
	21st to end	Night	23% 9% 3 6% 4 4 4 4 6% 7% 29%	5248
April	1st to 10th	Night	20% 12% 2 6% 5% 3 5% 3 4 6% 34%	2000
	11th to 20th	Night	18% 7% 2 7% 5% 5% 4 5% 4 7% 35%	1887
	21st to end	Night	23% 7% 3 7% 4 4 5% 3 4 5% 34%	1800
May	1st to 10th	Night	33% 10% 4 6% 4 4 4 3 4 5% 23%	4054
	11th to 20th	Night	17% 10% 4 9% 5% 5% 5% 4 5% 8% 28%	3862
	21st to end	Night	18% 8% 3 7% 5% 4 4 4 6% 7% 34%	3926
June	1st to 10th	Night	22% 9% 3 5% 4 3 5% 4 5% 7% 34%	1600
	11th to 20th	Night	18% 7% 1 6% 4 3 4 3 5% 10% 38%	1600
	21st to end	Night	21% 7% 2 6% 4 4 4 4 6% 8% 34%	1600
July	1st to 10th	Night	13% 6% 2 6% 5% 5% 6% 4 7% 9% 37%	3600
	11th to 20th	Night	16% 8% 2 7% 4 4 4 4 5% 8% 38%	3600
	21st to end	Night	11% 4 1 4 4 4 4 4 6% 9% 48%	3926
August	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 3 5% 6% 34%	2800
	11th to 20th	Night	16% 7% 2 7% 5% 4 5% 4 6% 7% 36%	2872
	21st to end	Night	10% 6% 1 6% 4 4 4 4 6% 8% 45%	3445
September	1st to 10th	Night	13% 5% 2 5% 3 4 3 3 5% 7% 50%	2027
	11th to 20th	Night	15% 5% 2 4 2 3 3 3 5% 6% 53%	2200
	21st to end	Night	16% 6% 1 6% 3 3 3 3 5% 7% 46%	2200
October	1st to 10th	Night	18% 7% 2 5% 4 4 4 4 5% 6% 40%	2200
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 6% 7% 39%	2487
	21st to end	Night	24% 8% 2 5% 4 4 4 3 5% 6% 35%	2860
November	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 5% 6% 41%	2667
	11th to 20th	Night	16% 7% 2 5% 5% 4 4 3 5% 6% 42%	2730
	21st to end	Night	17% 8% 2 6% 5% 4 4 4 6% 7% 38%	2746
December	1st to 10th	Night	17% 10% 3 7% 5% 4 4 4 7% 7% 32%	5009
	11th to 20th	Night	23% 8% 2 5% 4 4 4 3 4 5% 37%	5499
	21st to end	Night	20% 11% 3 7% 4 4 4 3 4 6% 36%	6996

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	46% 5% 11% 25% 12% 1	3598
	11th to 20th	Day	42% 6% 12% 26% 12% 3	3600
	21st to end	Day	42% 5% 9% 20% 18% 5%	3960
February	1st to 10th	Day	49% 6% 10% 20% 13% 3	1740
	11th to 20th	Day	47% 4 13% 19% 13% 3	1957
	21st to end	Day	50% 5% 10% 17% 12% 5%	1650
March	1st to 10th	Day	57% 5% 7% 11% 13% 7%	4479
	11th to 20th	Day	50% 5% 13% 17% 12% 3	4839
	21st to end	Day	53% 3 6% 17% 17% 4	5680
April	1st to 10th	Day	51% 4 8% 17% 15% 4	2400
	11th to 20th	Day	45% 7% 8% 22% 14% 3	2462
	21st to end	Day	53% 4 8% 18% 13% 4	2600
May	1st to 10th	Day	63% 7% 9% 14% 6% 1	5897
	11th to 20th	Day	53% 7% 12% 17% 8% 2	6064
	21st to end	Day	47% 6% 12% 21% 13% 1	6949
June	1st to 10th	Day	52% 7% 12% 20% 9%	2800
	11th to 20th	Day	43% 7% 12% 20% 15% 2	2800
	21st to end	Day	52% 8% 10% 18% 11% 1	2800
July	1st to 10th	Day	47% 7% 15% 21% 10%	6290
	11th to 20th	Day	41% 8% 14% 24% 13% 1	6285
	21st to end	Day	38% 6% 12% 27% 17%	6728
August	1st to 10th	Day	45% 8% 14% 25% 8%	4302
	11th to 20th	Day	49% 7% 14% 23% 7%	4214
	21st to end	Day	33% 7% 15% 25% 18% 2	4449
September	1st to 10th	Day	34% 7% 13% 27% 19% 1	2372
	11th to 20th	Day	32% 5% 10% 31% 20% 2	2200
	21st to end	Day	38% 5% 11% 27% 18% 1	2200
October	1st to 10th	Day	45% 6% 11% 23% 14% 1	3590
	11th to 20th	Day	37% 4 13% 27% 18% 1	3087
	21st to end	Day	38% 3 12% 23% 19% 4	3180
November	1st to 10th	Day	40% 5% 14% 24% 15% 3	1851
	11th to 20th	Day	41% 6% 13% 19% 18% 3	1890
	21st to end	Day	46% 4 17% 22% 11% 1	1870
December	1st to 10th	Day	48% 6% 15% 22% 10% 1	2391
	11th to 20th	Day	44% 5% 10% 18% 18% 6%	2827
	21st to end	Day	51% 5% 9% 17% 12% 6%	3960
January	1st to 10th	Night	46% 5% 12% 26% 10% 1	6349
	11th to 20th	Night	53% 6% 14% 16% 9% 2	6360
	21st to end	Night	45% 5% 8% 19% 17% 5%	6996
February	1st to 10th	Night	49% 6% 12% 17% 13% 3	2653
	11th to 20th	Night	44% 6% 11% 20% 16% 3	2340
	21st to end	Night	44% 6% 10% 22% 14% 4	1980
March	1st to 10th	Night	55% 4 9% 14% 13% 5%	5374
	11th to 20th	Night	51% 6% 16% 16% 9% 2	5120
	21st to end	Night	52% 4 6% 16% 19% 3	5248
April	1st to 10th	Night	51% 4 9% 18% 16% 3	2000
	11th to 20th	Night	47% 6% 10% 21% 16% 1	1887
	21st to end	Night	50% 5% 11% 16% 14% 3	1800
May	1st to 10th	Night	62% 7% 9% 13% 8% 1	4060
	11th to 20th	Night	52% 9% 10% 19% 8% 1	3863
	21st to end	Night	47% 7% 15% 21% 10% 1	3927
June	1st to 10th	Night	50% 7% 12% 20% 11% 1	1600
	11th to 20th	Night	42% 7% 13% 23% 13% 2	1600
	21st to end	Night	47% 8% 14% 20% 10% 1	1600
July	1st to 10th	Night	41% 7% 18% 23% 10%	3560
	11th to 20th	Night	43% 7% 14% 25% 12%	3552
	21st to end	Night	33% 7% 16% 33% 11%	3809
August	1st to 10th	Night	51% 6% 13% 21% 7%	2790
	11th to 20th	Night	45% 8% 15% 21% 11%	2899
	21st to end	Night	35% 7% 15% 27% 15% 1	3445
September	1st to 10th	Night	34% 6% 12% 31% 16%	2027
	11th to 20th	Night	32% 5% 11% 32% 19% 1	2200
	21st to end	Night	38% 6% 12% 28% 15% 2	2200
October	1st to 10th	Night	45% 6% 12% 21% 14% 1	3590
	11th to 20th	Night	35% 5% 10% 28% 20% 3	4049
	21st to end	Night	40% 5% 10% 28% 15% 2	4610
November	1st to 10th	Night	42% 5% 11% 26% 14% 1	2667
	11th to 20th	Night	42% 5% 12% 22% 16% 3	2730
	21st to end	Night	44% 6% 15% 22% 12% 1	2746
December	1st to 10th	Night	49% 5% 11% 27% 8% 1	5009
	11th to 20th	Night	49% 4 9% 17% 19% 3	5499
	21st to end	Night	50% 5% 9% 19% 16% 2	6996

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	36% 17% 29% 16% 4	200
	11th to 20th	Day	28% 19% 33% 18% 2 2	200
	21st to end	Day	20% 25% 32% 18% 5%	220
February	1st to 10th	Day	29% 20% 31% 17% 3	332
	11th to 20th	Day	29% 27% 30% 13% 2	496
	21st to end	Day	24% 23% 32% 18% 4	414
March	1st to 10th	Day	28% 21% 33% 14% 4	500
	11th to 20th	Day	28% 23% 30% 14% 4 1	500
	21st to end	Day	29% 23% 26% 13% 6% 2	550
April	1st to 10th	Day	34% 22% 24% 14% 5% 2	500
	11th to 20th	Day	31% 23% 26% 13% 5% 2	492
	21st to end	Day	34% 20% 28% 10% 5% 2	500
May	1st to 10th	Day	37% 16% 25% 17% 3 2	503
	11th to 20th	Day	23% 19% 32% 17% 7% 2	527
	21st to end	Day	24% 14% 34% 21% 6% 1	573
June	1st to 10th	Day	23% 16% 33% 18% 7% 2	530
	11th to 20th	Day	21% 15% 32% 20% 9% 2	530
	21st to end	Day	23% 13% 35% 18% 9% 2	530
July	1st to 10th	Day	19% 12% 32% 25% 10% 2	525
	11th to 20th	Day	18% 8% 32% 27% 11% 4	524
	21st to end	Day	17% 9% 33% 26% 11% 2	576
August	1st to 10th	Day	13% 8% 32% 28% 13% 5%	526
	11th to 20th	Day	17% 10% 27% 25% 15% 6%	519
	21st to end	Day	14% 11% 30% 27% 13% 5%	583
September	1st to 10th	Day	17% 11% 24% 26% 16% 6%	530
	11th to 20th	Day	18% 10% 26% 20% 16% 10%	530
	21st to end	Day	18% 14% 26% 25% 12% 6%	530
October	1st to 10th	Day	21% 15% 23% 21% 17% 3	530
	11th to 20th	Day	27% 14% 25% 21% 11% 2	434
	21st to end	Day	35% 13% 25% 20% 6%	462
November	1st to 10th	Day	28% 16% 26% 20% 10% 1	425
	11th to 20th	Day	27% 17% 29% 19% 7% 1	430
	21st to end	Day	34% 19% 23% 18% 6%	410
December	1st to 10th	Day	27% 21% 30% 15% 8%	220
	11th to 20th	Day	34% 21% 31% 11% 3	214
	21st to end	Day	32% 19% 25% 18% 6%	220
January	1st to 10th	Night	22% 18% 34% 23% 4	800
	11th to 20th	Night	25% 18% 27% 27% 4	800
	21st to end	Night	18% 19% 35% 23% 4	880
February	1st to 10th	Night	21% 19% 33% 25% 3	800
	11th to 20th	Night	20% 20% 37% 20% 3	783
	21st to end	Night	22% 16% 32% 26% 4	660
March	1st to 10th	Night	27% 17% 33% 21% 2	800
	11th to 20th	Night	23% 18% 30% 22% 7%	800
	21st to end	Night	26% 16% 30% 22% 5% 1	880
April	1st to 10th	Night	30% 15% 29% 23% 3	800
	11th to 20th	Night	22% 16% 32% 25% 5%	703
	21st to end	Night	29% 14% 30% 21% 5%	600
May	1st to 10th	Night	28% 12% 27% 28% 5% 1	600
	11th to 20th	Night	24% 12% 28% 28% 8%	525
	21st to end	Night	21% 7% 26% 37% 8%	433
June	1st to 10th	Night	25% 6% 30% 32% 8%	400
	11th to 20th	Night	20% 6% 27% 33% 14%	400
	21st to end	Night	21% 4 26% 38% 10%	400
July	1st to 10th	Night	17% 4 24% 40% 16%	390
	11th to 20th	Night	16% 3 22% 40% 18% 1	388
	21st to end	Night	16% 3 22% 41% 18% 1	438
August	1st to 10th	Night	17% 4 22% 37% 19% 1	521
	11th to 20th	Night	17% 4 28% 34% 17% 1	592
	21st to end	Night	15% 6% 23% 35% 19% 2	791
September	1st to 10th	Night	16% 5% 27% 32% 19% 1	800
	11th to 20th	Night	18% 6% 23% 27% 22% 4	800
	21st to end	Night	18% 9% 26% 29% 18% 1	800
October	1st to 10th	Night	23% 8% 20% 32% 15% 1	800
	11th to 20th	Night	22% 12% 28% 25% 12% 1	800
	21st to end	Night	29% 11% 28% 27% 5%	880
November	1st to 10th	Night	23% 13% 29% 27% 7% 1	820
	11th to 20th	Night	23% 15% 29% 25% 7% 1	840
	21st to end	Night	25% 15% 27% 27% 6% 1	840
December	1st to 10th	Night	27% 13% 32% 21% 6%	931
	11th to 20th	Night	25% 20% 29% 23% 4	892
	21st to end	Night	28% 16% 27% 25% 4	880

Page generated at 2025-12-15 01:13:56. Version ac07

Copyright © 2025 Attilla Danko.