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 2025-03-12.

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	11% 7% 3 6% 4 4 4 3 5% 7% 45%	108925
	Night	11% 7% 3 6% 4 4 4 3 5% 6% 48%	110299
Transparency	Day	38% 5% 12% 26% 17% 3	112576
	Night	36% 4 12% 26% 19% 3	114788
Seeing	Day	15% 7% 28% 29% 16% 4	16078
	Night	14% 7% 31% 34% 12% 1	26118

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	15% 8% 4 10% 4 4 4 3 4 7% 36%	4077
	11th to 20th	Day	7% 9% 4 10% 6% 7% 6% 3 3 5% 41%	4080
	21st to end	Day	12% 13% 5% 10% 5% 5% 4 2 4 5% 36%	4488
February	1st to 10th	Day	13% 11% 4 7% 5% 4 4 3 5% 6% 37%	1800
	11th to 20th	Day	12% 11% 4 7% 5% 3 4 3 4 6% 41%	1754
	21st to end	Day	11% 10% 3 6% 3 3 3 3 5% 6% 46%	1693
March	1st to 10th	Day	14% 10% 2 6% 3 3 3 3 4 5% 47%	3546
	11th to 20th	Day	12% 7% 2 6% 3 3 3 3 5% 7% 51%	3491
	21st to end	Day	16% 13% 5% 7% 5% 4 4 3 5% 5% 33%	3814
April	1st to 10th	Day	15% 8% 3 5% 3 3 3 3 5% 7% 45%	2194
	11th to 20th	Day	15% 10% 2 6% 5% 3 4 4 5% 7% 40%	2254
	21st to end	Day	20% 7% 3 6% 4 5% 4 3 6% 6% 36%	2432
May	1st to 10th	Day	16% 9% 3 6% 5% 4 4 4 5% 7% 39%	4082
	11th to 20th	Day	16% 7% 3 7% 4 4 5% 3 5% 7% 39%	4082
	21st to end	Day	13% 6% 2 6% 4 4 4 4 5% 8% 45%	4625
June	1st to 10th	Day	7% 4 1 5% 4 4 5% 4 7% 10% 49%	2660
	11th to 20th	Day	7% 4 2 6% 4 4 6% 4 6% 9% 49%	2660
	21st to end	Day	7% 3 1 5% 4 5% 5% 4 6% 9% 50%	2660
July	1st to 10th	Day	11% 6% 2 7% 4 4 6% 4 6% 9% 40%	4158
	11th to 20th	Day	5% 4 2 4 4 3 4 4 6% 9% 53%	4120
	21st to end	Day	6% 4 1 5% 4 5% 5% 5% 8% 10% 47%	4465
August	1st to 10th	Day	5% 4 2 5% 5% 4 4 4 6% 9% 52%	4104
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 47%	4041
	21st to end	Day	6% 4 2 5% 4 4 5% 3 6% 8% 54%	4286
September	1st to 10th	Day	11% 5% 1 5% 3 3 3 3 5% 7% 53%	2280
	11th to 20th	Day	9% 5% 2 5% 4 5% 4 4 5% 7% 49%	2104
	21st to end	Day	9% 4 2 5% 3 4 3 3 5% 5% 57%	2081
October	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 6% 54%	1900
	11th to 20th	Day	12% 5% 2 5% 3 3 2 3 4 5% 56%	1900
	21st to end	Day	14% 7% 2 6% 4 3 3 2 5% 7% 48%	2090
November	1st to 10th	Day	9% 5% 2 5% 3 3 4 2 5% 7% 54%	1938
	11th to 20th	Day	14% 6% 2 4 4 3 4 3 4 5% 51%	1710
	21st to end	Day	15% 10% 3 5% 2 3 3 2 5% 5% 47%	1600
December	1st to 10th	Day	6% 4 2 5% 4 4 4 4 5% 6% 54%	2610
	11th to 20th	Day	11% 8% 3 7% 4 4 4 3 4 5% 48%	3008
	21st to end	Day	18% 15% 7% 13% 4 3 3 3 4 5% 25%	4138
January	1st to 10th	Night	12% 9% 4 15% 5% 5% 4 3 4 5% 34%	5888
	11th to 20th	Night	8% 9% 3 8% 4 5% 5% 4 4 5% 43%	5900
	21st to end	Night	14% 10% 5% 10% 5% 4 4 3 4 6% 38%	6490
February	1st to 10th	Night	12% 12% 3 8% 5% 4 4 3 5% 6% 39%	2600
	11th to 20th	Night	11% 9% 3 7% 4 4 4 3 5% 7% 42%	2533
	21st to end	Night	11% 7% 3 7% 3 4 4 3 5% 5% 48%	1953
March	1st to 10th	Night	13% 7% 3 6% 3 3 3 3 4 5% 50%	3529
	11th to 20th	Night	11% 6% 2 4 3 3 3 2 5% 6% 56%	3497
	21st to end	Night	16% 9% 4 8% 5% 4 4 3 5% 6% 35%	3817
April	1st to 10th	Night	10% 7% 2 5% 3 3 3 4 5% 8% 49%	1986
	11th to 20th	Night	13% 8% 2 5% 3 4 4 3 4 6% 47%	1875
	21st to end	Night	17% 7% 2 6% 4 4 3 3 6% 7% 42%	1743
May	1st to 10th	Night	15% 7% 2 6% 5% 4 4 4 5% 8% 41%	2823
	11th to 20th	Night	15% 6% 2 6% 4 3 4 4 5% 8% 43%	2823
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 6% 52%	2939
June	1st to 10th	Night	6% 2 1 4 3 4 4 5% 6% 9% 56%	1520
	11th to 20th	Night	5% 4 1 4 3 5% 4 4 7% 10% 51%	1520
	21st to end	Night	4 3 1 3 2 3 3 3 6% 9% 62%	1520
July	1st to 10th	Night	10% 7% 2 6% 4 3 3 4 6% 7% 48%	2807
	11th to 20th	Night	2 2 1 4 3 3 4 3 7% 9% 61%	2850
	21st to end	Night	5% 4 2 5% 4 4 5% 4 8% 11% 49%	3098
August	1st to 10th	Night	2 2 1 3 3 4 4 4 7% 10% 60%	2836
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 7% 52%	2798
	21st to end	Night	4 4 2 5% 4 4 4 3 6% 8% 57%	3372
September	1st to 10th	Night	9% 5% 1 4 3 4 3 2 4 6% 57%	1900
	11th to 20th	Night	9% 4 2 5% 2 3 3 4 6% 7% 55%	2073
	21st to end	Night	7% 4 1 5% 4 4 4 3 4 5% 59%	2090
October	1st to 10th	Night	12% 5% 1 3 3 3 3 3 3 5% 58%	2270
	11th to 20th	Night	13% 4 2 4 3 2 2 2 4 6% 57%	2280
	21st to end	Night	15% 8% 2 5% 3 3 3 2 3 4 52%	2508
November	1st to 10th	Night	8% 5% 2 3 3 3 3 2 4 7% 61%	2358
	11th to 20th	Night	10% 7% 2 4 3 3 3 3 4 6% 54%	2688
	21st to end	Night	14% 8% 3 4 3 2 3 3 4 5% 52%	2800
December	1st to 10th	Night	7% 6% 2 5% 3 3 3 3 5% 6% 56%	4580
	11th to 20th	Night	11% 8% 3 7% 3 4 4 3 5% 7% 45%	5199
	21st to end	Night	19% 16% 5% 9% 4 3 3 3 3 4 29%	6836

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	49% 4 12% 25% 10% 1	4078
	11th to 20th	Day	48% 3 10% 20% 14% 4	4080
	21st to end	Day	53% 3 11% 16% 16% 1	4488
February	1st to 10th	Day	49% 3 13% 21% 10% 3	1800
	11th to 20th	Day	45% 4 12% 22% 14% 2	1763
	21st to end	Day	39% 3 12% 25% 19% 3	1693
March	1st to 10th	Day	40% 4 10% 21% 22% 2	3546
	11th to 20th	Day	34% 4 11% 28% 20% 2	3491
	21st to end	Day	51% 6% 15% 22% 5% 1	3814
April	1st to 10th	Day	40% 5% 13% 23% 17% 2	2194
	11th to 20th	Day	45% 4 12% 22% 16% 1	2254
	21st to end	Day	47% 6% 11% 25% 10% 1	2432
May	1st to 10th	Day	44% 7% 10% 23% 14% 2	4120
	11th to 20th	Day	44% 7% 12% 22% 15%	4086
	21st to end	Day	36% 7% 10% 30% 14% 2	4638
June	1st to 10th	Day	29% 9% 15% 30% 15% 2	2660
	11th to 20th	Day	31% 8% 14% 29% 17% 1	2660
	21st to end	Day	29% 6% 16% 30% 15% 3	2660
July	1st to 10th	Day	39% 7% 13% 24% 14% 3	4094
	11th to 20th	Day	27% 6% 11% 35% 19% 2	4051
	21st to end	Day	28% 8% 15% 32% 14% 2	4325
August	1st to 10th	Day	27% 7% 15% 29% 21% 2	4090
	11th to 20th	Day	36% 5% 16% 26% 16% 1	4080
	21st to end	Day	28% 6% 15% 31% 14% 6%	4285
September	1st to 10th	Day	31% 5% 11% 27% 20% 6%	2280
	11th to 20th	Day	33% 6% 11% 26% 21% 3	2104
	21st to end	Day	29% 5% 9% 31% 24% 3	2081
October	1st to 10th	Day	19% 2 7% 22% 43% 6%	3160
	11th to 20th	Day	21% 2 4 26% 37% 9%	3140
	21st to end	Day	33% 3 11% 26% 24% 3	3428
November	1st to 10th	Day	32% 4 13% 28% 19% 4	1938
	11th to 20th	Day	35% 3 11% 28% 20% 3	1710
	21st to end	Day	40% 3 10% 27% 18% 1	1600
December	1st to 10th	Day	31% 4 14% 33% 17% 1	2610
	11th to 20th	Day	40% 3 14% 32% 10% 1	3005
	21st to end	Day	64% 4 11% 14% 6% 1	4138
January	1st to 10th	Night	52% 4 11% 21% 10% 1	5888
	11th to 20th	Night	44% 3 11% 23% 15% 4	5900
	21st to end	Night	50% 3 11% 17% 15% 4	6490
February	1st to 10th	Night	47% 3 12% 21% 14% 3	2600
	11th to 20th	Night	43% 4 11% 22% 16% 3	2534
	21st to end	Night	39% 3 9% 23% 20% 6%	1953
March	1st to 10th	Night	37% 4 11% 24% 22% 3	3529
	11th to 20th	Night	30% 4 8% 29% 24% 5%	3497
	21st to end	Night	49% 6% 16% 22% 6% 1	3817
April	1st to 10th	Night	34% 6% 12% 24% 21% 4	1986
	11th to 20th	Night	38% 6% 9% 27% 16% 3	1875
	21st to end	Night	42% 5% 10% 25% 16% 1	1743
May	1st to 10th	Night	40% 8% 13% 22% 16% 2	2850
	11th to 20th	Night	39% 6% 11% 27% 16% 1	2824
	21st to end	Night	32% 6% 11% 30% 19% 1	2947
June	1st to 10th	Night	23% 7% 17% 32% 17% 3	1520
	11th to 20th	Night	26% 7% 15% 30% 20% 2	1520
	21st to end	Night	19% 5% 15% 36% 21% 3	1520
July	1st to 10th	Night	36% 5% 12% 24% 20% 4	2762
	11th to 20th	Night	18% 5% 15% 37% 23% 2	2796
	21st to end	Night	26% 7% 19% 33% 15% 1	2999
August	1st to 10th	Night	18% 6% 20% 28% 25% 4	2826
	11th to 20th	Night	33% 4 12% 28% 21% 1	2825
	21st to end	Night	25% 5% 14% 35% 17% 5%	3372
September	1st to 10th	Night	29% 4 10% 27% 23% 8%	1900
	11th to 20th	Night	27% 6% 11% 28% 25% 4	2073
	21st to end	Night	28% 4 7% 28% 29% 4	2090
October	1st to 10th	Night	19% 3 6% 20% 47% 6%	3790
	11th to 20th	Night	21% 3 5% 28% 32% 12%	3779
	21st to end	Night	35% 3 7% 28% 26% 2	4120
November	1st to 10th	Night	26% 4 14% 28% 23% 6%	2358
	11th to 20th	Night	31% 4 10% 28% 22% 5%	2688
	21st to end	Night	36% 3 10% 29% 19% 3	2800
December	1st to 10th	Night	30% 4 14% 35% 17% 1	4580
	11th to 20th	Night	39% 3 14% 28% 14% 1	5201
	21st to end	Night	60% 4 10% 19% 6% 1	6836

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	300
	11th to 20th	Day	22% 14% 39% 21% 4	300
	21st to end	Day	18% 11% 43% 25% 3	330
February	1st to 10th	Day	22% 17% 39% 20% 2	300
	11th to 20th	Day	19% 18% 43% 12% 8%	300
	21st to end	Day	15% 19% 41% 19% 5%	249
March	1st to 10th	Day	21% 12% 43% 21% 3	300
	11th to 20th	Day	26% 10% 36% 22% 5% 1	273
	21st to end	Day	34% 16% 27% 19% 3 1	297
April	1st to 10th	Day	24% 13% 36% 21% 5% 1	374
	11th to 20th	Day	20% 12% 37% 22% 8%	452
	21st to end	Day	25% 11% 35% 20% 9% 1	612
May	1st to 10th	Day	21% 10% 35% 24% 9% 2	653
	11th to 20th	Day	21% 6% 28% 28% 14% 3	675
	21st to end	Day	18% 4 30% 28% 17% 3	734
June	1st to 10th	Day	6% 5% 20% 35% 25% 9%	680
	11th to 20th	Day	8% 4 24% 37% 21% 6%	680
	21st to end	Day	8% 4 33% 38% 15% 2	680
July	1st to 10th	Day	11% 2 14% 33% 29% 11%	671
	11th to 20th	Day	6% 1 19% 38% 27% 8%	668
	21st to end	Day	9% 1 14% 38% 29% 8%	739
August	1st to 10th	Day	8% 1 15% 41% 29% 6%	676
	11th to 20th	Day	10% 2 18% 36% 28% 6%	665
	21st to end	Day	10% 1 14% 33% 32% 11%	606
September	1st to 10th	Day	13% 2 16% 31% 31% 7%	490
	11th to 20th	Day	6% 2 22% 36% 26% 8%	314
	21st to end	Day	15% 4 25% 35% 14% 6%	300
October	1st to 10th	Day	15% 5% 27% 27% 21% 6%	300
	11th to 20th	Day	11% 15% 44% 21% 9% 2	300
	21st to end	Day	20% 13% 35% 22% 7% 1	330
November	1st to 10th	Day	18% 8% 35% 29% 8% 2	300
	11th to 20th	Day	19% 15% 34% 23% 8% 2	300
	21st to end	Day	23% 11% 34% 22% 10%	300
December	1st to 10th	Day	8% 15% 39% 28% 8% 2	300
	11th to 20th	Day	21% 11% 36% 24% 7% 1	300
	21st to end	Day	22% 15% 41% 20% 2	330
January	1st to 10th	Night	13% 12% 37% 30% 8%	1000
	11th to 20th	Night	16% 12% 37% 27% 6% 1	1000
	21st to end	Night	16% 9% 39% 32% 4	1100
February	1st to 10th	Night	17% 12% 41% 26% 4	1000
	11th to 20th	Night	16% 13% 37% 29% 5%	979
	21st to end	Night	17% 10% 44% 23% 5%	633
March	1st to 10th	Night	15% 11% 42% 26% 5%	600
	11th to 20th	Night	20% 7% 37% 29% 6% 1	575
	21st to end	Night	25% 10% 37% 25% 4	627
April	1st to 10th	Night	14% 11% 40% 30% 5%	570
	11th to 20th	Night	19% 11% 34% 31% 5%	563
	21st to end	Night	21% 6% 36% 28% 7% 1	570
May	1st to 10th	Night	17% 7% 32% 35% 9% 1	570
	11th to 20th	Night	15% 4 30% 38% 12% 1	561
	21st to end	Night	12% 5% 29% 39% 15% 2	617
June	1st to 10th	Night	7% 2 19% 45% 25% 1	570
	11th to 20th	Night	7% 2 25% 45% 18% 2	570
	21st to end	Night	7% 2 29% 47% 15%	570
July	1st to 10th	Night	8% 1 11% 45% 32% 3	555
	11th to 20th	Night	5% 1 15% 47% 30% 2	552
	21st to end	Night	6% 1 11% 48% 32% 3	624
August	1st to 10th	Night	4 1 14% 48% 32% 1	567
	11th to 20th	Night	9% 1 12% 46% 29% 3	564
	21st to end	Night	8% 1 11% 43% 34% 4	627
September	1st to 10th	Night	13% 1 15% 41% 26% 4	570
	11th to 20th	Night	9% 1 20% 44% 23% 3	570
	21st to end	Night	9% 4 22% 44% 20% 2	570
October	1st to 10th	Night	14% 5% 27% 33% 19% 2	750
	11th to 20th	Night	16% 6% 38% 31% 8%	760
	21st to end	Night	19% 9% 33% 30% 8%	836
November	1st to 10th	Night	11% 10% 34% 35% 10% 1	798
	11th to 20th	Night	15% 12% 36% 29% 8%	1000
	21st to end	Night	19% 9% 34% 30% 8%	1000
December	1st to 10th	Night	15% 11% 34% 33% 6% 1	1000
	11th to 20th	Night	20% 8% 37% 28% 6%	1000
	21st to end	Night	20% 13% 34% 28% 4	1100

Page generated at 2025-03-09 07:08:44. Version ac07

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