Wes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wes Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	11% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	127423
	Night	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	124605
Transparency	Day	37% 8% 13% 19% 17% 7%	131147
	Night	36% 7% 12% 17% 19% 9%	128950
Seeing	Day	12% 7% 20% 24% 22% 14%	16858
	Night	13% 6% 22% 33% 22% 3	26951

By Month

Percentage Cloud Cover SE (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	11% 6% 2 5% 3 3 4 3 3 5% 54%	4146
	11th to 20th	Day	16% 5% 1 5% 2 3 3 2 5% 7% 50%	4150
	21st to end	Day	13% 8% 3 6% 4 3 4 3 5% 7% 46%	4565
February	1st to 10th	Day	15% 8% 2 6% 5% 4 4 4 5% 8% 40%	1991
	11th to 20th	Day	18% 6% 2 4 4 3 3 3 5% 6% 46%	1948
	21st to end	Day	14% 7% 3 5% 4 3 4 2 4 7% 46%	1650
March	1st to 10th	Day	19% 7% 3 7% 4 3 4 3 5% 7% 38%	4930
	11th to 20th	Day	7% 4 1 4 4 4 4 4 6% 7% 55%	4980
	21st to end	Day	14% 6% 3 6% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 3 5% 7% 49%	2200
	11th to 20th	Day	12% 4 2 4 4 3 4 4 5% 7% 50%	2227
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 7% 55%	2600
May	1st to 10th	Day	9% 6% 3 6% 5% 4 5% 4 6% 9% 43%	5862
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 9% 40%	5861
	21st to end	Day	6% 3 1 5% 5% 5% 6% 5% 7% 10% 46%	6433
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 5% 8% 11% 34%	2600
	11th to 20th	Day	6% 4 1 5% 5% 6% 6% 7% 8% 11% 42%	2600
	21st to end	Day	6% 5% 2 7% 6% 7% 7% 7% 10% 11% 33%	2600
July	1st to 10th	Day	9% 7% 3 9% 8% 8% 9% 7% 9% 11% 21%	5900
	11th to 20th	Day	3 3 1 6% 5% 6% 7% 7% 10% 16% 37%	5900
	21st to end	Day	6% 5% 3 8% 6% 8% 8% 6% 10% 14% 27%	6424
August	1st to 10th	Day	14% 5% 3 8% 6% 7% 7% 6% 8% 10% 25%	4234
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 39%	4149
	21st to end	Day	8% 6% 3 8% 6% 5% 6% 5% 8% 11% 34%	4081
September	1st to 10th	Day	10% 7% 3 8% 5% 6% 7% 5% 6% 8% 36%	2200
	11th to 20th	Day	13% 6% 2 7% 6% 4 5% 4 6% 9% 38%	2200
	21st to end	Day	13% 6% 3 6% 4 4 6% 4 5% 8% 41%	2200
October	1st to 10th	Day	13% 6% 3 6% 3 4 4 3 5% 6% 48%	2190
	11th to 20th	Day	10% 3 2 4 3 2 3 3 5% 6% 59%	2000
	21st to end	Day	9% 5% 2 5% 3 4 3 3 4 6% 57%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 4 3 3 5% 5% 53%	2057
	11th to 20th	Day	11% 6% 2 5% 3 3 3 2 4 7% 56%	1942
	21st to end	Day	13% 6% 2 5% 4 4 3 3 4 7% 48%	1890
December	1st to 10th	Day	11% 5% 2 6% 4 3 4 3 4 5% 52%	3020
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 4 7% 48%	3463
	21st to end	Day	18% 9% 3 6% 4 4 3 2 5% 6% 41%	4565
January	1st to 10th	Night	11% 4 2 4 3 3 4 3 5% 6% 54%	6019
	11th to 20th	Night	11% 7% 2 7% 4 4 4 3 5% 6% 46%	6030
	21st to end	Night	15% 7% 3 6% 4 4 3 3 5% 7% 43%	6633
February	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 4 7% 45%	2406
	11th to 20th	Night	16% 7% 2 5% 3 4 3 3 4 5% 46%	2338
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 6% 49%	1980
March	1st to 10th	Night	18% 8% 3 5% 4 3 3 2 4 6% 42%	4921
	11th to 20th	Night	8% 4 2 5% 3 4 5% 4 6% 8% 52%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	5467
April	1st to 10th	Night	11% 5% 2 5% 5% 3 4 4 5% 7% 50%	2200
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 6% 7% 53%	2120
	21st to end	Night	6% 4 2 5% 3 5% 4 4 6% 7% 55%	1814
May	1st to 10th	Night	9% 6% 2 7% 4 4 5% 4 6% 10% 43%	4033
	11th to 20th	Night	15% 6% 2 5% 4 4 4 4 7% 10% 40%	4032
	21st to end	Night	7% 4 2 5% 4 4 5% 5% 8% 10% 46%	4421
June	1st to 10th	Night	9% 5% 2 7% 5% 6% 6% 6% 8% 12% 34%	1800
	11th to 20th	Night	6% 5% 2 5% 5% 5% 6% 6% 8% 11% 42%	1800
	21st to end	Night	6% 4 1 6% 7% 7% 7% 7% 11% 10% 34%	1800
July	1st to 10th	Night	11% 8% 4 9% 7% 7% 7% 6% 8% 11% 24%	4058
	11th to 20th	Night	4 4 3 7% 7% 6% 7% 6% 10% 14% 32%	4057
	21st to end	Night	7% 6% 3 9% 7% 7% 7% 5% 10% 11% 28%	4427
August	1st to 10th	Night	14% 6% 2 8% 5% 6% 7% 5% 8% 12% 26%	2915
	11th to 20th	Night	6% 5% 2 7% 4 5% 6% 4 8% 12% 41%	2896
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 7% 11% 43%	3782
September	1st to 10th	Night	11% 5% 2 6% 4 5% 5% 5% 7% 11% 38%	2200
	11th to 20th	Night	10% 6% 3 8% 5% 4 5% 4 6% 8% 41%	2200
	21st to end	Night	13% 6% 3 6% 4 5% 5% 4 6% 8% 41%	2200
October	1st to 10th	Night	11% 7% 2 5% 3 4 4 3 5% 6% 50%	2208
	11th to 20th	Night	11% 5% 2 3 3 3 3 2 4 4 59%	2400
	21st to end	Night	10% 4 1 5% 3 3 4 3 4 6% 56%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 54%	2461
	11th to 20th	Night	13% 5% 2 5% 3 3 4 2 4 6% 53%	2672
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 7% 49%	2730
December	1st to 10th	Night	11% 6% 2 6% 3 3 3 2 4 5% 55%	4380
	11th to 20th	Night	14% 7% 3 6% 3 3 3 2 4 5% 48%	4952
	21st to end	Night	18% 8% 2 6% 4 3 4 3 4 5% 43%	6633

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	33% 4 5% 14% 33% 10%	4147
	11th to 20th	Day	34% 4 6% 16% 25% 16%	4150
	21st to end	Day	39% 4 5% 18% 20% 13%	4565
February	1st to 10th	Day	42% 4 9% 16% 20% 8%	1991
	11th to 20th	Day	39% 4 8% 14% 27% 9%	1958
	21st to end	Day	39% 4 7% 18% 19% 13%	1650
March	1st to 10th	Day	44% 5% 8% 13% 21% 10%	4930
	11th to 20th	Day	27% 5% 10% 15% 20% 23%	4980
	21st to end	Day	40% 5% 7% 18% 18% 12%	5465
April	1st to 10th	Day	34% 6% 10% 21% 18% 11%	2200
	11th to 20th	Day	34% 5% 8% 19% 22% 12%	2227
	21st to end	Day	27% 6% 11% 25% 25% 6%	2600
May	1st to 10th	Day	37% 8% 14% 25% 14% 2	5897
	11th to 20th	Day	41% 6% 17% 24% 12% 1	5865
	21st to end	Day	30% 9% 17% 23% 19% 1	6447
June	1st to 10th	Day	40% 12% 16% 18% 13% 1	2600
	11th to 20th	Day	31% 12% 19% 23% 14% 1	2600
	21st to end	Day	38% 14% 20% 17% 9% 2	2600
July	1st to 10th	Day	50% 15% 20% 11% 2 1	5835
	11th to 20th	Day	31% 16% 32% 18% 2	5831
	21st to end	Day	39% 16% 24% 18% 3	6240
August	1st to 10th	Day	49% 12% 21% 16% 2	4221
	11th to 20th	Day	33% 12% 18% 26% 10%	4188
	21st to end	Day	41% 11% 16% 26% 5% 1	4081
September	1st to 10th	Day	44% 10% 15% 15% 12% 4	2200
	11th to 20th	Day	44% 7% 12% 17% 16% 5%	2200
	21st to end	Day	41% 6% 10% 19% 17% 5%	2200
October	1st to 10th	Day	40% 4 8% 12% 22% 15%	3567
	11th to 20th	Day	17% 3 5% 14% 27% 35%	3240
	21st to end	Day	23% 3 5% 18% 36% 16%	3538
November	1st to 10th	Day	32% 5% 8% 16% 23% 15%	2057
	11th to 20th	Day	30% 4 8% 18% 26% 14%	1942
	21st to end	Day	36% 4 9% 19% 21% 11%	1890
December	1st to 10th	Day	35% 5% 7% 16% 27% 10%	3020
	11th to 20th	Day	37% 3 6% 24% 23% 7%	3460
	21st to end	Day	45% 4 6% 19% 23% 4	4565
January	1st to 10th	Night	30% 4 6% 15% 33% 12%	6019
	11th to 20th	Night	39% 4 5% 15% 26% 11%	6030
	21st to end	Night	41% 4 7% 14% 24% 11%	6633
February	1st to 10th	Night	39% 5% 8% 17% 21% 10%	2406
	11th to 20th	Night	41% 4 7% 15% 21% 12%	2339
	21st to end	Night	35% 6% 8% 17% 18% 15%	1980
March	1st to 10th	Night	43% 4 9% 15% 17% 12%	4921
	11th to 20th	Night	29% 4 9% 14% 23% 21%	4980
	21st to end	Night	35% 5% 9% 16% 22% 14%	5467
April	1st to 10th	Night	32% 7% 12% 19% 19% 13%	2200
	11th to 20th	Night	28% 6% 8% 22% 23% 13%	2120
	21st to end	Night	26% 7% 12% 26% 21% 8%	1814
May	1st to 10th	Night	35% 7% 16% 25% 14% 3	4060
	11th to 20th	Night	37% 8% 20% 23% 11% 2	4033
	21st to end	Night	30% 9% 16% 26% 17% 2	4429
June	1st to 10th	Night	38% 12% 19% 19% 11% 1	1800
	11th to 20th	Night	33% 12% 21% 24% 10%	1800
	21st to end	Night	36% 15% 21% 17% 10% 1	1800
July	1st to 10th	Night	51% 13% 22% 11% 3	4015
	11th to 20th	Night	37% 14% 34% 13% 2	4006
	21st to end	Night	43% 16% 20% 16% 3	4297
August	1st to 10th	Night	46% 13% 27% 11% 2	2904
	11th to 20th	Night	33% 10% 23% 25% 7% 1	2923
	21st to end	Night	32% 11% 22% 30% 5% 1	3782
September	1st to 10th	Night	38% 11% 19% 17% 12% 3	2200
	11th to 20th	Night	38% 12% 16% 17% 15% 3	2200
	21st to end	Night	40% 8% 15% 16% 18% 4	2200
October	1st to 10th	Night	37% 5% 11% 13% 21% 12%	3611
	11th to 20th	Night	19% 2 4 12% 28% 36%	3899
	21st to end	Night	24% 3 5% 16% 33% 20%	4252
November	1st to 10th	Night	33% 4 10% 13% 24% 16%	2461
	11th to 20th	Night	34% 4 6% 16% 25% 14%	2672
	21st to end	Night	34% 5% 9% 19% 20% 13%	2730
December	1st to 10th	Night	32% 4 8% 17% 30% 9%	4380
	11th to 20th	Night	40% 4 9% 17% 21% 9%	4954
	21st to end	Night	43% 4 6% 18% 25% 3	6633

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	11% 15% 50% 19% 5% 1	330
	11th to 20th	Day	15% 16% 34% 22% 10% 2	330
	21st to end	Day	22% 18% 40% 16% 3 1	363
February	1st to 10th	Day	17% 17% 35% 23% 8%	300
	11th to 20th	Day	26% 12% 32% 26% 4	300
	21st to end	Day	15% 18% 39% 23% 3 2	249
March	1st to 10th	Day	14% 16% 28% 28% 12% 1	495
	11th to 20th	Day	12% 11% 36% 29% 11% 1	500
	21st to end	Day	15% 11% 34% 30% 9% 2	550
April	1st to 10th	Day	14% 13% 28% 27% 16% 2	500
	11th to 20th	Day	12% 8% 28% 27% 20% 7%	492
	21st to end	Day	8% 7% 20% 34% 22% 10%	500
May	1st to 10th	Day	6% 9% 24% 27% 23% 11%	503
	11th to 20th	Day	13% 2 16% 27% 26% 17%	527
	21st to end	Day	9% 3 14% 22% 29% 23%	573
June	1st to 10th	Day	10% 2 13% 24% 29% 22%	530
	11th to 20th	Day	7% 3 11% 23% 27% 30%	530
	21st to end	Day	6% 2 8% 23% 32% 29%	530
July	1st to 10th	Day	5% 2 5% 17% 33% 38%	525
	11th to 20th	Day	6% 1 3 13% 33% 45%	524
	21st to end	Day	5% 2 6% 16% 39% 32%	576
August	1st to 10th	Day	12% 2 8% 18% 33% 27%	526
	11th to 20th	Day	8% 2 4 13% 34% 39%	519
	21st to end	Day	13% 2 10% 21% 29% 25%	583
September	1st to 10th	Day	12% 3 9% 24% 30% 22%	530
	11th to 20th	Day	14% 2 12% 29% 30% 13%	530
	21st to end	Day	14% 3 10% 26% 32% 15%	530
October	1st to 10th	Day	14% 5% 23% 28% 23% 8%	530
	11th to 20th	Day	9% 5% 20% 31% 24% 10%	530
	21st to end	Day	11% 5% 22% 32% 24% 5%	583
November	1st to 10th	Day	17% 6% 23% 26% 23% 5%	535
	11th to 20th	Day	13% 9% 36% 26% 14% 2	382
	21st to end	Day	10% 16% 27% 31% 15% 1	330
December	1st to 10th	Day	20% 14% 34% 19% 12% 2	330
	11th to 20th	Day	17% 14% 32% 21% 14% 1	330
	21st to end	Day	23% 9% 31% 24% 12% 1	363
January	1st to 10th	Night	15% 11% 36% 31% 5% 1	840
	11th to 20th	Night	14% 10% 34% 31% 10%	840
	21st to end	Night	20% 10% 39% 26% 5%	924
February	1st to 10th	Night	15% 9% 33% 35% 7%	801
	11th to 20th	Night	20% 11% 34% 30% 4	783
	21st to end	Night	13% 13% 35% 36% 3	660
March	1st to 10th	Night	12% 11% 32% 39% 6%	800
	11th to 20th	Night	13% 10% 34% 33% 10% 1	800
	21st to end	Night	13% 10% 36% 32% 10%	880
April	1st to 10th	Night	12% 8% 34% 34% 12% 1	800
	11th to 20th	Night	11% 9% 30% 35% 15% 1	791
	21st to end	Night	7% 5% 25% 44% 18%	614
May	1st to 10th	Night	8% 5% 26% 40% 20% 1	600
	11th to 20th	Night	8% 4 20% 42% 23% 3	591
	21st to end	Night	8% 1 12% 41% 34% 4	650
June	1st to 10th	Night	11% 2 11% 38% 34% 5%	600
	11th to 20th	Night	8% 1 11% 35% 36% 10%	600
	21st to end	Night	6% 2 6% 38% 38% 10%	600
July	1st to 10th	Night	8% 1 5% 27% 47% 12%	585
	11th to 20th	Night	9% 1 5% 21% 52% 13%	582
	21st to end	Night	6% 1 4 27% 54% 7%	657
August	1st to 10th	Night	11% 2 4 27% 45% 11%	597
	11th to 20th	Night	10% 1 4 23% 51% 11%	594
	21st to end	Night	12% 2 8% 26% 40% 11%	778
September	1st to 10th	Night	12% 2 8% 30% 40% 10%	800
	11th to 20th	Night	11% 3 9% 35% 39% 4	800
	21st to end	Night	14% 3 12% 31% 36% 5%	800
October	1st to 10th	Night	13% 3 18% 35% 26% 5%	800
	11th to 20th	Night	14% 5% 19% 34% 26% 3	800
	21st to end	Night	11% 4 22% 37% 24% 2	880
November	1st to 10th	Night	13% 7% 20% 37% 21% 2	820
	11th to 20th	Night	14% 8% 26% 35% 15% 1	840
	21st to end	Night	14% 7% 28% 35% 15% 1	840
December	1st to 10th	Night	18% 7% 29% 33% 12% 1	840
	11th to 20th	Night	17% 8% 27% 34% 13% 1	840
	21st to end	Night	20% 9% 27% 34% 10%	924

Page generated at 2026-01-29 01:21:40. Version ac07

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