Wonder Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wonder Hill Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	127357
	Night	14% 6% 2 5% 3 3 4 3 5% 7% 47%	124603
Transparency	Day	37% 7% 11% 20% 17% 8%	131061
	Night	35% 6% 11% 20% 18% 10%	128962
Seeing	Day	12% 6% 18% 26% 26% 11%	16407
	Night	15% 7% 23% 34% 19% 2	26363

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	18% 10% 3 6% 3 3 3 3 4 5% 42%	4167
	11th to 20th	Day	14% 7% 2 5% 3 3 4 3 4 6% 49%	4134
	21st to end	Day	30% 8% 3 6% 2 2 2 2 3 5% 36%	4488
February	1st to 10th	Day	22% 9% 4 6% 3 3 3 2 4 5% 39%	1975
	11th to 20th	Day	21% 8% 3 7% 4 3 4 3 3 5% 40%	1949
	21st to end	Day	18% 9% 2 6% 4 3 3 3 5% 5% 42%	1755
March	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 4 5% 7% 41%	4936
	11th to 20th	Day	13% 6% 3 6% 4 4 3 3 4 6% 48%	4980
	21st to end	Day	17% 8% 2 7% 4 4 4 4 4 6% 40%	5462
April	1st to 10th	Day	13% 10% 3 7% 5% 5% 4 3 5% 6% 39%	2200
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 4 5% 6% 39%	2264
	21st to end	Day	13% 8% 3 7% 4 4 4 4 5% 7% 42%	2400
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	5808
	11th to 20th	Day	10% 5% 3 6% 5% 4 5% 4 6% 8% 45%	5862
	21st to end	Day	10% 6% 3 7% 6% 6% 6% 5% 8% 10% 34%	6434
June	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 7% 10% 45%	2600
	11th to 20th	Day	6% 4 1 5% 5% 6% 6% 5% 7% 11% 45%	2600
	21st to end	Day	5% 6% 2 7% 7% 7% 7% 5% 9% 11% 34%	2600
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 8% 11% 37%	5900
	11th to 20th	Day	6% 5% 2 6% 5% 6% 7% 6% 8% 13% 36%	5900
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 11% 36%	6422
August	1st to 10th	Day	1 1 1 3 3 3 4 4 6% 11% 61%	4234
	11th to 20th	Day	3 2 1 4 3 4 5% 5% 10% 14% 48%	4138
	21st to end	Day	5% 4 2 7% 5% 6% 6% 5% 7% 10% 42%	4021
September	1st to 10th	Day	6% 5% 3 6% 5% 6% 6% 6% 7% 9% 42%	2200
	11th to 20th	Day	10% 6% 3 8% 6% 5% 5% 5% 6% 8% 39%	2200
	21st to end	Day	6% 4 2 6% 6% 5% 6% 4 5% 8% 48%	2200
October	1st to 10th	Day	5% 4 1 3 4 5% 4 3 4 8% 59%	2140
	11th to 20th	Day	11% 5% 1 5% 4 4 4 2 4 5% 54%	2000
	21st to end	Day	11% 6% 2 4 3 3 3 3 4 6% 55%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 5% 7% 48%	2058
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 5% 6% 52%	2100
	21st to end	Day	15% 7% 3 7% 4 4 4 3 5% 5% 42%	1932
December	1st to 10th	Day	25% 7% 2 6% 4 3 3 2 4 5% 38%	3030
	11th to 20th	Day	18% 7% 2 6% 4 3 3 3 5% 6% 42%	3481
	21st to end	Day	15% 7% 2 6% 4 4 3 3 4 5% 48%	4587
January	1st to 10th	Night	19% 7% 2 5% 3 3 3 2 4 5% 47%	6018
	11th to 20th	Night	14% 6% 2 4 2 3 3 2 4 5% 55%	5979
	21st to end	Night	30% 7% 2 5% 3 2 3 2 4 5% 37%	6490
February	1st to 10th	Night	20% 8% 3 7% 3 3 3 2 4 6% 41%	2440
	11th to 20th	Night	22% 8% 2 5% 4 3 2 2 3 5% 44%	2338
	21st to end	Night	21% 8% 2 5% 4 3 3 3 5% 5% 40%	1873
March	1st to 10th	Night	18% 10% 3 5% 3 4 3 3 5% 7% 39%	4919
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 5% 46%	4980
	21st to end	Night	19% 10% 3 6% 4 3 3 3 3 5% 41%	5467
April	1st to 10th	Night	13% 6% 3 5% 3 4 5% 4 5% 7% 45%	2200
	11th to 20th	Night	14% 7% 2 7% 3 4 3 3 5% 7% 45%	2084
	21st to end	Night	14% 7% 2 6% 4 4 4 3 4 6% 45%	2000
May	1st to 10th	Night	22% 9% 3 8% 4 4 4 3 5% 7% 30%	4100
	11th to 20th	Night	11% 6% 2 6% 4 4 4 5% 7% 8% 44%	4053
	21st to end	Night	11% 8% 3 7% 5% 5% 5% 5% 7% 10% 35%	4443
June	1st to 10th	Night	5% 3 1 5% 4 3 4 3 7% 10% 54%	1800
	11th to 20th	Night	4 4 1 3 3 3 4 4 7% 10% 56%	1800
	21st to end	Night	4 4 2 5% 5% 4 5% 5% 9% 11% 47%	1800
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 5% 8% 11% 41%	4078
	11th to 20th	Night	4 4 2 6% 4 4 4 4 7% 11% 51%	4077
	21st to end	Night	7% 6% 3 6% 4 4 5% 4 6% 10% 45%	4449
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 11% 71%	2926
	11th to 20th	Night	2 2 1 2 3 3 4 4 7% 13% 60%	2917
	21st to end	Night	3 3 2 5% 4 5% 5% 4 8% 12% 49%	3872
September	1st to 10th	Night	5% 4 2 6% 4 4 5% 5% 6% 8% 50%	2200
	11th to 20th	Night	9% 6% 3 6% 5% 4 5% 4 5% 8% 46%	2200
	21st to end	Night	8% 4 1 4 3 3 4 4 6% 8% 55%	2200
October	1st to 10th	Night	4 4 1 4 2 3 3 3 6% 7% 64%	2238
	11th to 20th	Night	10% 4 1 3 3 3 3 3 4 6% 61%	2400
	21st to end	Night	11% 5% 2 3 3 3 3 2 3 5% 60%	2640
November	1st to 10th	Night	10% 7% 2 6% 4 3 3 3 4 6% 54%	2460
	11th to 20th	Night	10% 6% 2 5% 4 3 3 2 5% 6% 53%	2520
	21st to end	Night	16% 6% 2 6% 3 3 3 2 4 7% 46%	2683
December	1st to 10th	Night	22% 10% 3 7% 3 3 3 3 4 5% 37%	4380
	11th to 20th	Night	17% 9% 3 6% 3 3 3 3 4 5% 45%	4946
	21st to end	Night	16% 6% 2 5% 3 3 3 2 4 4 53%	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	45% 4 5% 21% 17% 7%	4168
	11th to 20th	Day	37% 4 4 19% 25% 12%	4134
	21st to end	Day	52% 2 3 10% 17% 15%	4488
February	1st to 10th	Day	49% 3 5% 12% 19% 13%	1975
	11th to 20th	Day	47% 5% 7% 15% 16% 10%	1959
	21st to end	Day	45% 4 8% 15% 17% 12%	1755
March	1st to 10th	Day	41% 6% 7% 16% 17% 13%	4936
	11th to 20th	Day	39% 4 6% 10% 18% 23%	4980
	21st to end	Day	44% 6% 7% 19% 18% 6%	5464
April	1st to 10th	Day	46% 6% 7% 17% 14% 11%	2200
	11th to 20th	Day	43% 7% 8% 14% 19% 10%	2264
	21st to end	Day	40% 6% 7% 20% 20% 7%	2400
May	1st to 10th	Day	47% 7% 10% 21% 11% 3	5846
	11th to 20th	Day	36% 7% 12% 20% 21% 3	5866
	21st to end	Day	40% 9% 11% 27% 12% 2	6447
June	1st to 10th	Day	32% 8% 14% 27% 16% 2	2600
	11th to 20th	Day	30% 9% 14% 26% 18% 3	2600
	21st to end	Day	40% 10% 15% 24% 10% 1	2600
July	1st to 10th	Day	38% 9% 26% 24% 3	5836
	11th to 20th	Day	33% 11% 22% 25% 8%	5831
	21st to end	Day	37% 10% 18% 25% 10% 1	6240
August	1st to 10th	Day	16% 7% 13% 40% 23%	4220
	11th to 20th	Day	22% 8% 31% 31% 8% 1	4177
	21st to end	Day	33% 12% 18% 23% 13% 1	4020
September	1st to 10th	Day	34% 12% 13% 20% 15% 6%	2200
	11th to 20th	Day	39% 9% 12% 18% 18% 4	2200
	21st to end	Day	33% 7% 9% 21% 17% 13%	2200
October	1st to 10th	Day	17% 3 6% 14% 22% 39%	3491
	11th to 20th	Day	21% 3 7% 16% 35% 17%	3240
	21st to end	Day	26% 4 4 20% 31% 15%	3538
November	1st to 10th	Day	35% 6% 9% 18% 22% 11%	2058
	11th to 20th	Day	33% 6% 9% 17% 22% 14%	2100
	21st to end	Day	43% 5% 8% 15% 18% 11%	1932
December	1st to 10th	Day	49% 4 6% 15% 20% 6%	3030
	11th to 20th	Day	42% 5% 9% 11% 23% 10%	3479
	21st to end	Day	39% 4 5% 15% 26% 12%	4587
January	1st to 10th	Night	40% 4 6% 20% 20% 10%	6018
	11th to 20th	Night	33% 4 3 20% 27% 13%	5979
	21st to end	Night	51% 3 3 12% 18% 13%	6490
February	1st to 10th	Night	46% 4 6% 11% 18% 15%	2440
	11th to 20th	Night	44% 4 6% 16% 17% 13%	2338
	21st to end	Night	47% 5% 7% 13% 16% 13%	1873
March	1st to 10th	Night	44% 5% 7% 15% 18% 11%	4919
	11th to 20th	Night	41% 5% 6% 14% 14% 21%	4980
	21st to end	Night	46% 5% 7% 15% 19% 9%	5467
April	1st to 10th	Night	38% 6% 9% 19% 16% 12%	2200
	11th to 20th	Night	39% 5% 10% 16% 16% 14%	2084
	21st to end	Night	39% 6% 10% 20% 18% 7%	2000
May	1st to 10th	Night	51% 7% 8% 20% 9% 4	4127
	11th to 20th	Night	35% 7% 12% 23% 18% 5%	4054
	21st to end	Night	41% 9% 13% 28% 9% 1	4452
June	1st to 10th	Night	23% 8% 16% 32% 19% 2	1800
	11th to 20th	Night	22% 8% 18% 30% 18% 5%	1800
	21st to end	Night	28% 9% 21% 27% 14% 1	1800
July	1st to 10th	Night	34% 10% 29% 23% 3	4035
	11th to 20th	Night	26% 8% 25% 32% 9%	4026
	21st to end	Night	33% 9% 20% 27% 11% 1	4320
August	1st to 10th	Night	9% 5% 16% 45% 24% 1	2916
	11th to 20th	Night	16% 7% 33% 34% 8% 1	2944
	21st to end	Night	25% 12% 21% 29% 12% 1	3872
September	1st to 10th	Night	29% 10% 19% 21% 14% 7%	2200
	11th to 20th	Night	35% 8% 13% 23% 15% 6%	2200
	21st to end	Night	26% 6% 15% 22% 18% 13%	2200
October	1st to 10th	Night	14% 4 7% 17% 19% 40%	3654
	11th to 20th	Night	16% 3 6% 21% 33% 21%	3899
	21st to end	Night	22% 3 4 22% 31% 18%	4252
November	1st to 10th	Night	32% 6% 9% 19% 22% 12%	2460
	11th to 20th	Night	33% 4 9% 18% 21% 15%	2520
	21st to end	Night	39% 4 8% 17% 19% 12%	2683
December	1st to 10th	Night	51% 4 5% 15% 19% 8%	4380
	11th to 20th	Night	42% 4 9% 9% 21% 15%	4947
	21st to end	Night	36% 4 4 16% 29% 11%	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	22% 9% 37% 22% 8% 2	330
	11th to 20th	Day	24% 13% 29% 26% 8% 1	318
	21st to end	Day	26% 13% 31% 20% 9% 1	330
February	1st to 10th	Day	27% 15% 24% 25% 8%	300
	11th to 20th	Day	30% 18% 28% 17% 7%	300
	21st to end	Day	28% 15% 35% 18% 4	249
March	1st to 10th	Day	19% 16% 37% 19% 9%	300
	11th to 20th	Day	16% 17% 41% 20% 4 1	300
	21st to end	Day	19% 13% 34% 23% 9% 1	330
April	1st to 10th	Day	17% 17% 37% 22% 7% 1	300
	11th to 20th	Day	22% 14% 31% 23% 10% 1	382
	21st to end	Day	18% 11% 28% 29% 14% 1	500
May	1st to 10th	Day	14% 10% 36% 27% 12% 1	675
	11th to 20th	Day	14% 6% 28% 30% 18% 3	725
	21st to end	Day	12% 6% 24% 34% 20% 3	789
June	1st to 10th	Day	6% 2 12% 37% 31% 12%	730
	11th to 20th	Day	8% 2 10% 38% 32% 10%	730
	21st to end	Day	7% 1 8% 28% 38% 17%	730
July	1st to 10th	Day	7% 3 6% 26% 40% 17%	721
	11th to 20th	Day	4 1 3 26% 48% 18%	718
	21st to end	Day	6% 1 4 22% 46% 21%	794
August	1st to 10th	Day	2 1 2 20% 49% 26%	726
	11th to 20th	Day	5% 1 3 18% 46% 28%	695
	21st to end	Day	6% 2 4 16% 32% 42%	409
September	1st to 10th	Day	6% 1 5% 23% 33% 32%	330
	11th to 20th	Day	7% 2 5% 17% 42% 28%	330
	21st to end	Day	6% 2 7% 25% 37% 23%	330
October	1st to 10th	Day	4 3 6% 32% 37% 17%	330
	11th to 20th	Day	9% 3 16% 31% 27% 14%	330
	21st to end	Day	19% 10% 18% 24% 21% 8%	363
November	1st to 10th	Day	13% 5% 20% 32% 24% 7%	330
	11th to 20th	Day	13% 8% 22% 32% 21% 4	330
	21st to end	Day	18% 12% 33% 21% 11% 4	330
December	1st to 10th	Day	20% 7% 31% 24% 14% 4	330
	11th to 20th	Day	23% 15% 26% 24% 11% 2	330
	21st to end	Day	19% 9% 33% 25% 12% 3	363
January	1st to 10th	Night	21% 10% 31% 30% 8%	1050
	11th to 20th	Night	21% 11% 30% 30% 8%	1031
	21st to end	Night	23% 9% 30% 32% 6%	1100
February	1st to 10th	Night	24% 13% 31% 28% 4	840
	11th to 20th	Night	27% 10% 31% 26% 5%	784
	21st to end	Night	26% 14% 37% 20% 3	553
March	1st to 10th	Night	23% 10% 36% 27% 4	600
	11th to 20th	Night	18% 10% 37% 29% 6%	600
	21st to end	Night	17% 10% 39% 28% 5%	660
April	1st to 10th	Night	15% 11% 40% 31% 4	600
	11th to 20th	Night	16% 11% 34% 32% 7%	593
	21st to end	Night	17% 10% 36% 28% 9% 1	600
May	1st to 10th	Night	15% 10% 38% 32% 6%	600
	11th to 20th	Night	12% 5% 31% 38% 14% 1	591
	21st to end	Night	8% 5% 30% 40% 17% 1	650
June	1st to 10th	Night	6% 2 14% 51% 26% 2	600
	11th to 20th	Night	5% 2 16% 53% 22% 2	600
	21st to end	Night	5% 1 9% 44% 37% 4	600
July	1st to 10th	Night	7% 3 5% 40% 39% 6%	585
	11th to 20th	Night	4 3 4 41% 43% 4	582
	21st to end	Night	4 1 4 44% 42% 5%	657
August	1st to 10th	Night	2 1 3 46% 45% 4	597
	11th to 20th	Night	4 3 40% 46% 6%	594
	21st to end	Night	4 1 4 31% 52% 8%	660
September	1st to 10th	Night	7% 1 6% 33% 49% 6%	600
	11th to 20th	Night	10% 2 5% 35% 44% 5%	600
	21st to end	Night	9% 1 10% 32% 42% 6%	600
October	1st to 10th	Night	5% 1 8% 43% 36% 6%	638
	11th to 20th	Night	11% 3 19% 41% 23% 3	800
	21st to end	Night	12% 6% 26% 33% 21% 2	880
November	1st to 10th	Night	11% 6% 21% 39% 21% 1	820
	11th to 20th	Night	12% 7% 26% 37% 17% 1	840
	21st to end	Night	18% 8% 28% 31% 14% 1	1003
December	1st to 10th	Night	22% 9% 25% 32% 12% 1	1050
	11th to 20th	Night	24% 10% 27% 31% 8%	1050
	21st to end	Night	21% 9% 29% 32% 9%	1155

Page generated at 2026-01-14 00:14:27. Version ac07

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