WHS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WHS Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 7% 3 6% 4 4 4 3 5% 7% 45%	137796
	Night	11% 7% 3 6% 4 4 4 3 5% 7% 48%	135206
Transparency	Day	38% 5% 12% 26% 17% 3	141408
	Night	37% 4 11% 26% 19% 3	139674
Seeing	Day	15% 8% 29% 29% 16% 4	18019
	Night	14% 8% 31% 34% 12% 1	27842

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	13% 9% 4 9% 4 4 4 3 4 6% 40%	5289
	11th to 20th	Day	6% 8% 3 9% 7% 7% 6% 3 4 6% 41%	5310
	21st to end	Day	11% 13% 5% 9% 5% 4 4 2 4 5% 38%	5812
February	1st to 10th	Day	13% 11% 4 7% 5% 4 4 3 5% 6% 38%	1890
	11th to 20th	Day	12% 11% 4 7% 5% 4 4 3 5% 7% 41%	1844
	21st to end	Day	11% 9% 3 6% 3 3 3 3 5% 6% 47%	1775
March	1st to 10th	Day	15% 9% 3 5% 3 2 3 3 3 5% 51%	5046
	11th to 20th	Day	10% 6% 2 6% 3 3 3 3 4 6% 54%	5090
	21st to end	Day	12% 11% 5% 7% 5% 4 5% 3 6% 7% 35%	5583
April	1st to 10th	Day	15% 8% 3 5% 3 3 3 3 5% 7% 44%	2425
	11th to 20th	Day	15% 9% 2 7% 4 3 4 4 5% 7% 41%	2494
	21st to end	Day	20% 8% 3 6% 4 5% 4 3 6% 6% 35%	2688
May	1st to 10th	Day	15% 7% 2 6% 4 4 3 3 4 7% 44%	7617
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 7% 47%	7627
	21st to end	Day	18% 8% 3 6% 4 4 4 4 5% 7% 38%	8666
June	1st to 10th	Day	8% 4 2 5% 4 5% 5% 4 6% 9% 48%	2940
	11th to 20th	Day	8% 4 2 6% 4 4 5% 4 6% 9% 49%	2940
	21st to end	Day	7% 4 1 6% 4 5% 5% 4 6% 9% 49%	2940
July	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 6% 9% 45%	6083
	11th to 20th	Day	8% 5% 3 5% 5% 4 5% 4 6% 9% 46%	5966
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 9% 45%	6422
August	1st to 10th	Day	5% 4 2 5% 5% 4 4 4 6% 9% 52%	4234
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 8% 48%	4171
	21st to end	Day	6% 4 2 5% 4 4 5% 3 6% 8% 53%	4421
September	1st to 10th	Day	11% 5% 1 5% 3 3 3 3 5% 7% 53%	2400
	11th to 20th	Day	9% 5% 2 5% 4 5% 4 4 5% 7% 48%	2214
	21st to end	Day	9% 4 2 5% 3 4 3 3 5% 5% 57%	2190
October	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 6% 54%	2000
	11th to 20th	Day	11% 5% 2 5% 3 3 2 3 4 5% 56%	2000
	21st to end	Day	16% 8% 2 6% 3 3 3 2 4 6% 47%	2200
November	1st to 10th	Day	9% 5% 2 5% 3 3 4 2 4 7% 55%	2037
	11th to 20th	Day	14% 6% 2 4 4 3 4 3 4 5% 52%	1795
	21st to end	Day	15% 10% 3 6% 2 3 3 2 4 5% 45%	1680
December	1st to 10th	Day	6% 5% 2 6% 4 4 4 4 5% 6% 54%	2690
	11th to 20th	Day	10% 8% 3 7% 4 3 4 3 4 5% 48%	3088
	21st to end	Day	18% 15% 7% 13% 4 3 3 3 4 5% 25%	4229
January	1st to 10th	Night	12% 10% 3 14% 5% 5% 4 2 4 5% 37%	7634
	11th to 20th	Night	7% 8% 3 8% 4 5% 4 3 4 6% 48%	7680
	21st to end	Night	13% 11% 5% 9% 4 5% 4 3 4 6% 37%	8402
February	1st to 10th	Night	11% 12% 3 8% 5% 4 4 3 5% 6% 39%	2730
	11th to 20th	Night	11% 9% 3 7% 4 4 4 3 5% 7% 42%	2663
	21st to end	Night	11% 7% 3 7% 3 4 4 3 5% 5% 49%	2048
March	1st to 10th	Night	15% 7% 3 5% 3 2 3 3 4 5% 50%	5029
	11th to 20th	Night	9% 6% 2 4 3 3 3 3 5% 7% 56%	5090
	21st to end	Night	14% 9% 4 7% 5% 4 4 4 5% 7% 38%	5588
April	1st to 10th	Night	11% 7% 2 6% 3 3 4 4 5% 8% 47%	2195
	11th to 20th	Night	12% 8% 2 5% 3 4 4 3 4 6% 47%	2075
	21st to end	Night	17% 8% 2 6% 4 4 3 3 6% 7% 41%	1927
May	1st to 10th	Night	14% 7% 2 6% 4 3 4 3 4 7% 46%	5266
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 50%	5265
	21st to end	Night	16% 7% 2 6% 4 3 4 4 5% 6% 45%	5502
June	1st to 10th	Night	6% 2 2 4 3 4 4 5% 7% 9% 54%	1680
	11th to 20th	Night	5% 4 2 4 4 5% 4 4 7% 10% 52%	1680
	21st to end	Night	5% 4 1 4 2 3 3 3 6% 9% 61%	1680
July	1st to 10th	Night	7% 6% 2 6% 4 3 4 4 6% 8% 51%	4109
	11th to 20th	Night	4 4 2 5% 3 4 5% 4 7% 9% 52%	4122
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 7% 11% 45%	4449
August	1st to 10th	Night	2 2 1 3 3 4 4 4 7% 10% 60%	2926
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 6% 52%	2888
	21st to end	Night	5% 4 2 5% 4 4 4 3 6% 8% 57%	3479
September	1st to 10th	Night	10% 5% 2 4 4 3 3 2 4 6% 57%	2000
	11th to 20th	Night	8% 4 2 5% 3 3 4 5% 6% 7% 54%	2182
	21st to end	Night	7% 4 1 5% 4 4 4 3 4 5% 59%	2200
October	1st to 10th	Night	11% 5% 2 3 3 3 3 3 3 6% 57%	2390
	11th to 20th	Night	13% 4 2 4 3 3 2 2 4 6% 56%	2400
	21st to end	Night	15% 8% 2 5% 3 3 3 2 3 4 51%	2640
November	1st to 10th	Night	8% 5% 2 3 3 3 3 2 4 6% 61%	2479
	11th to 20th	Night	10% 6% 1 4 3 3 3 3 4 7% 55%	2823
	21st to end	Night	14% 8% 3 5% 3 2 3 3 3 5% 50%	2940
December	1st to 10th	Night	7% 7% 2 5% 3 3 3 3 5% 6% 55%	4720
	11th to 20th	Night	11% 8% 3 7% 3 3 4 3 5% 7% 46%	5339
	21st to end	Night	19% 16% 5% 9% 4 3 3 3 3 4 30%	6986

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% 3 11% 27% 11% 1	5290
	11th to 20th	Day	44% 4 11% 22% 15% 5%	5310
	21st to end	Day	51% 3 12% 18% 15% 1	5812
February	1st to 10th	Day	48% 3 13% 23% 10% 2	1890
	11th to 20th	Day	45% 4 13% 22% 14% 2	1853
	21st to end	Day	38% 3 12% 25% 20% 3	1775
March	1st to 10th	Day	39% 3 9% 21% 23% 5%	5046
	11th to 20th	Day	32% 3 9% 22% 29% 5%	5090
	21st to end	Day	48% 5% 16% 24% 6% 1	5585
April	1st to 10th	Day	41% 4 13% 23% 17% 2	2425
	11th to 20th	Day	44% 4 12% 22% 16% 1	2494
	21st to end	Day	49% 5% 11% 25% 9% 1	2688
May	1st to 10th	Day	41% 5% 8% 28% 17% 1	7655
	11th to 20th	Day	36% 5% 11% 25% 21% 2	7631
	21st to end	Day	46% 6% 9% 25% 13% 1	8680
June	1st to 10th	Day	30% 9% 14% 29% 15% 2	2940
	11th to 20th	Day	31% 7% 14% 29% 17% 1	2940
	21st to end	Day	30% 6% 16% 30% 15% 3	2940
July	1st to 10th	Day	34% 6% 12% 28% 17% 2	6019
	11th to 20th	Day	34% 6% 13% 28% 18% 2	5897
	21st to end	Day	32% 7% 13% 34% 11% 2	6240
August	1st to 10th	Day	27% 7% 15% 29% 20% 2	4220
	11th to 20th	Day	35% 5% 16% 27% 16% 1	4210
	21st to end	Day	28% 6% 15% 31% 14% 6%	4420
September	1st to 10th	Day	32% 5% 11% 27% 19% 6%	2400
	11th to 20th	Day	34% 6% 12% 26% 20% 3	2214
	21st to end	Day	30% 5% 9% 31% 24% 3	2190
October	1st to 10th	Day	20% 2 7% 22% 43% 6%	3260
	11th to 20th	Day	21% 2 4 26% 37% 9%	3240
	21st to end	Day	34% 3 11% 26% 23% 3	3538
November	1st to 10th	Day	31% 4 13% 30% 19% 4	2037
	11th to 20th	Day	35% 3 10% 29% 20% 3	1795
	21st to end	Day	42% 3 9% 27% 18% 1	1680
December	1st to 10th	Day	31% 4 14% 33% 17% 1	2690
	11th to 20th	Day	40% 3 14% 32% 10% 1	3085
	21st to end	Day	63% 4 11% 15% 6% 1	4229
January	1st to 10th	Night	51% 4 12% 22% 11% 1	7635
	11th to 20th	Night	39% 3 10% 27% 19% 4	7680
	21st to end	Night	50% 3 12% 17% 14% 3	8402
February	1st to 10th	Night	46% 3 12% 22% 14% 3	2730
	11th to 20th	Night	43% 4 12% 23% 16% 3	2664
	21st to end	Night	38% 3 9% 23% 21% 6%	2048
March	1st to 10th	Night	38% 3 8% 23% 25% 4	5029
	11th to 20th	Night	29% 3 6% 25% 29% 8%	5090
	21st to end	Night	45% 5% 15% 24% 9% 2	5588
April	1st to 10th	Night	36% 5% 11% 23% 21% 3	2195
	11th to 20th	Night	38% 6% 9% 26% 17% 4	2075
	21st to end	Night	43% 5% 10% 24% 15% 2	1927
May	1st to 10th	Night	39% 5% 9% 26% 18% 2	5293
	11th to 20th	Night	34% 5% 11% 24% 21% 5%	5266
	21st to end	Night	41% 6% 9% 26% 17% 1	5513
June	1st to 10th	Night	25% 7% 17% 32% 16% 3	1680
	11th to 20th	Night	27% 6% 15% 30% 20% 2	1680
	21st to end	Night	21% 5% 15% 35% 20% 3	1680
July	1st to 10th	Night	31% 5% 12% 25% 24% 3	4066
	11th to 20th	Night	27% 5% 15% 30% 20% 2	4071
	21st to end	Night	31% 6% 15% 36% 11% 1	4320
August	1st to 10th	Night	18% 6% 20% 29% 24% 3	2916
	11th to 20th	Night	33% 4 12% 29% 21% 1	2915
	21st to end	Night	25% 5% 14% 35% 17% 5%	3479
September	1st to 10th	Night	29% 4 10% 27% 22% 7%	2000
	11th to 20th	Night	28% 6% 11% 28% 24% 4	2182
	21st to end	Night	28% 4 8% 28% 29% 4	2200
October	1st to 10th	Night	20% 3 5% 19% 47% 6%	3910
	11th to 20th	Night	21% 2 6% 27% 31% 12%	3899
	21st to end	Night	35% 3 7% 28% 25% 2	4252
November	1st to 10th	Night	26% 4 13% 29% 23% 5%	2479
	11th to 20th	Night	31% 4 11% 29% 22% 4	2823
	21st to end	Night	38% 3 10% 28% 18% 2	2940
December	1st to 10th	Night	30% 4 14% 35% 17% 1	4720
	11th to 20th	Night	39% 3 14% 28% 14% 1	5341
	21st to end	Night	59% 4 10% 19% 6% 1	6986

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	21% 11% 36% 26% 5% 1	330
	11th to 20th	Day	21% 15% 40% 21% 3	330
	21st to end	Day	18% 11% 43% 24% 3	363
February	1st to 10th	Day	20% 16% 40% 20% 3	330
	11th to 20th	Day	19% 17% 43% 13% 8%	330
	21st to end	Day	14% 19% 41% 21% 5%	273
March	1st to 10th	Day	22% 12% 42% 20% 3	330
	11th to 20th	Day	27% 11% 34% 22% 5% 1	330
	21st to end	Day	31% 17% 31% 17% 3 1	363
April	1st to 10th	Day	27% 13% 35% 20% 5%	445
	11th to 20th	Day	20% 14% 38% 20% 7%	532
	21st to end	Day	25% 10% 36% 19% 9% 1	708
May	1st to 10th	Day	20% 10% 34% 25% 9% 2	753
	11th to 20th	Day	20% 7% 29% 28% 13% 3	775
	21st to end	Day	19% 5% 30% 27% 16% 3	844
June	1st to 10th	Day	7% 5% 22% 35% 24% 8%	780
	11th to 20th	Day	9% 4 25% 36% 20% 6%	780
	21st to end	Day	8% 4 34% 37% 15% 2	780
July	1st to 10th	Day	11% 2 15% 33% 29% 11%	771
	11th to 20th	Day	6% 1 19% 40% 26% 8%	744
	21st to end	Day	9% 2 14% 38% 29% 8%	794
August	1st to 10th	Day	8% 1 16% 41% 28% 6%	726
	11th to 20th	Day	10% 2 17% 36% 29% 6%	715
	21st to end	Day	10% 1 14% 32% 31% 11%	653
September	1st to 10th	Day	13% 2 16% 32% 31% 6%	530
	11th to 20th	Day	6% 2 21% 37% 25% 8%	344
	21st to end	Day	15% 4 24% 33% 16% 8%	330
October	1st to 10th	Day	14% 4 28% 29% 19% 5%	330
	11th to 20th	Day	11% 14% 43% 22% 9% 2	330
	21st to end	Day	23% 13% 34% 22% 7% 1	363
November	1st to 10th	Day	17% 8% 38% 28% 7% 2	330
	11th to 20th	Day	18% 14% 35% 23% 9% 2	330
	21st to end	Day	25% 11% 33% 21% 10%	330
December	1st to 10th	Day	9% 15% 40% 27% 8% 2	330
	11th to 20th	Day	20% 12% 38% 23% 7% 1	330
	21st to end	Day	21% 15% 40% 21% 3	363
January	1st to 10th	Night	13% 12% 38% 30% 8%	1050
	11th to 20th	Night	16% 13% 38% 27% 6% 1	1050
	21st to end	Night	16% 9% 39% 32% 4	1155
February	1st to 10th	Night	17% 11% 41% 27% 4	1050
	11th to 20th	Night	16% 13% 38% 29% 5%	1029
	21st to end	Night	17% 10% 45% 23% 5%	664
March	1st to 10th	Night	16% 11% 42% 26% 5%	630
	11th to 20th	Night	19% 8% 39% 27% 5% 1	630
	21st to end	Night	24% 10% 38% 24% 3	693
April	1st to 10th	Night	15% 11% 39% 30% 4	630
	11th to 20th	Night	19% 13% 34% 30% 4	623
	21st to end	Night	21% 7% 37% 28% 7% 1	630
May	1st to 10th	Night	17% 7% 32% 35% 8%	630
	11th to 20th	Night	15% 5% 31% 37% 12% 1	621
	21st to end	Night	13% 4 28% 39% 14% 1	683
June	1st to 10th	Night	7% 3 21% 44% 24% 1	630
	11th to 20th	Night	8% 2 27% 44% 17% 2	630
	21st to end	Night	8% 2 30% 46% 15%	630
July	1st to 10th	Night	8% 1 12% 45% 32% 3	615
	11th to 20th	Night	6% 1 16% 47% 29% 2	597
	21st to end	Night	6% 1 11% 48% 31% 2	657
August	1st to 10th	Night	4 1 15% 49% 31% 1	597
	11th to 20th	Night	8% 1 11% 45% 31% 3	594
	21st to end	Night	8% 1 10% 44% 33% 3	660
September	1st to 10th	Night	13% 2 16% 41% 26% 4	600
	11th to 20th	Night	9% 1 19% 45% 24% 3	600
	21st to end	Night	9% 3 21% 44% 21% 3	600
October	1st to 10th	Night	14% 5% 27% 34% 19% 2	790
	11th to 20th	Night	16% 7% 38% 31% 8%	800
	21st to end	Night	20% 9% 33% 29% 8%	880
November	1st to 10th	Night	10% 10% 35% 34% 10%	839
	11th to 20th	Night	14% 12% 36% 30% 7%	1050
	21st to end	Night	20% 9% 33% 30% 7%	1050
December	1st to 10th	Night	15% 11% 34% 34% 6% 1	1050
	11th to 20th	Night	20% 8% 38% 28% 6%	1050
	21st to end	Night	20% 13% 34% 29% 5%	1155

Page generated at 2026-07-13 02:14:52. Version ac07

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