Wilson Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilson Hill Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	9% 5% 1 3 2 2 3 2 4 5% 63%	135402
	Night	12% 6% 2 4 3 2 3 2 4 5% 58%	131387
Transparency	Day	25% 5% 6% 18% 25% 21%	130291
	Night	30% 5% 7% 17% 22% 19%	127486
Seeing	Day	13% 12% 27% 29% 14% 4	16233
	Night	15% 7% 20% 33% 22% 3	25378

By Month

Percentage Cloud Cover SW (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	22% 9% 2 6% 4 3 4 3 4 5% 37%	3639
	11th to 20th	Day	20% 7% 2 4 2 2 2 2 3 3 53%	3934
	21st to end	Day	21% 8% 2 4 2 2 3 2 4 5% 47%	4510
February	1st to 10th	Day	18% 8% 2 6% 4 3 3 3 4 6% 42%	1800
	11th to 20th	Day	18% 10% 3 5% 4 2 4 2 4 5% 44%	1800
	21st to end	Day	17% 10% 2 4 3 3 3 4 4 6% 43%	1650
March	1st to 10th	Day	21% 14% 3 8% 4 4 3 2 3 4 35%	4737
	11th to 20th	Day	18% 10% 2 5% 3 3 3 2 4 6% 43%	5000
	21st to end	Day	19% 12% 3 7% 4 4 4 3 5% 6% 34%	5633
April	1st to 10th	Day	15% 10% 3 5% 3 3 4 4 4 8% 41%	2400
	11th to 20th	Day	15% 6% 2 4 2 3 3 3 5% 5% 50%	2369
	21st to end	Day	9% 6% 2 5% 3 3 3 3 5% 7% 55%	2400
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 5% 69%	5887
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 6% 72%	6277
	21st to end	Day	3 2 1 2 2 2 2 2 4 5% 76%	6902
June	1st to 10th	Day	8% 4 2 4 3 3 3 3 5% 7% 58%	2800
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 76%	2800
	21st to end	Day	4 1 1 2 1 1 2 2 2 3 81%	2800
July	1st to 10th	Day	1 1 1 1 1 2 2 91%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	6320
	21st to end	Day	1 1 1 2 1 2 2 2 2 4 82%	6494
August	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 81%	5777
	11th to 20th	Day	1 1 1 1 1 2 3 90%	5630
	21st to end	Day	2 1 1 1 1 1 2 1 3 4 82%	5984
September	1st to 10th	Day	1 1 1 1 2 2 1 3 4 85%	2400
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 5% 71%	2400
	21st to end	Day	3 3 1 2 2 2 2 2 4 6% 75%	2130
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 5% 76%	3211
	11th to 20th	Day	7% 3 1 4 2 2 3 3 4 6% 66%	3161
	21st to end	Day	9% 5% 2 6% 4 4 4 3 4 7% 53%	3448
November	1st to 10th	Day	15% 5% 2 4 3 3 4 4 5% 6% 48%	1803
	11th to 20th	Day	17% 5% 2 4 4 4 4 4 6% 7% 43%	1735
	21st to end	Day	14% 7% 2 6% 3 3 3 2 4 6% 51%	1600
December	1st to 10th	Day	11% 5% 2 4 3 3 3 2 5% 6% 55%	2620
	11th to 20th	Day	28% 9% 3 6% 4 4 4 4 7% 7% 25%	3028
	21st to end	Day	25% 9% 3 8% 5% 4 4 4 5% 5% 28%	4004
January	1st to 10th	Night	17% 10% 3 7% 4 4 4 3 5% 6% 37%	6306
	11th to 20th	Night	18% 7% 2 4 2 2 3 2 3 4 52%	6037
	21st to end	Night	21% 9% 2 6% 3 3 2 2 3 4 45%	6468
February	1st to 10th	Night	20% 7% 2 4 3 3 3 3 4 6% 45%	2600
	11th to 20th	Night	17% 9% 2 6% 4 3 3 3 4 5% 44%	2485
	21st to end	Night	16% 10% 2 6% 4 3 3 3 4 6% 44%	1980
March	1st to 10th	Night	19% 15% 3 6% 4 3 3 3 4 5% 35%	5142
	11th to 20th	Night	19% 11% 2 5% 2 2 3 3 4 4 46%	5000
	21st to end	Night	18% 13% 3 6% 4 4 4 3 5% 7% 35%	5341
April	1st to 10th	Night	14% 7% 3 5% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Night	13% 5% 2 4 3 2 4 4 5% 8% 50%	1979
	21st to end	Night	11% 5% 1 4 3 2 3 3 4 6% 57%	2000
May	1st to 10th	Night	8% 5% 1 3 2 2 2 2 3 4 67%	4089
	11th to 20th	Night	5% 4 1 3 2 2 3 2 4 5% 69%	3616
	21st to end	Night	4 2 1 2 2 2 2 1 3 5% 74%	3972
June	1st to 10th	Night	7% 5% 1 4 3 3 4 4 5% 9% 56%	1600
	11th to 20th	Night	5% 3 1 2 1 2 2 2 4 5% 74%	1600
	21st to end	Night	3 1 1 2 2 1 2 2 3 4 79%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	3640
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 2 88%	3640
	21st to end	Night	1 1 2 1 2 2 2 3 5% 82%	4371
August	1st to 10th	Night	2 1 2 1 2 2 2 3 4 79%	4035
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 89%	4225
	21st to end	Night	3 2 1 2 1 2 1 1 2 3 81%	5015
September	1st to 10th	Night	1 1 1 2 1 1 2 1 2 4 84%	2000
	11th to 20th	Night	4 3 1 2 2 1 2 2 4 5% 75%	2000
	21st to end	Night	4 1 1 2 2 2 2 3 4 5% 76%	2266
October	1st to 10th	Night	4 1 1 2 2 2 2 2 2 3 79%	3836
	11th to 20th	Night	5% 3 1 3 2 2 3 2 4 5% 70%	3777
	21st to end	Night	9% 3 1 3 2 2 3 3 6% 8% 60%	4121
November	1st to 10th	Night	15% 5% 1 4 3 3 4 4 6% 6% 48%	2495
	11th to 20th	Night	15% 6% 2 5% 3 4 4 4 5% 8% 45%	2663
	21st to end	Night	15% 6% 2 4 3 3 3 3 5% 6% 50%	2800
December	1st to 10th	Night	12% 4 1 3 2 2 3 3 4 6% 59%	4560
	11th to 20th	Night	28% 9% 3 6% 4 5% 4 3 6% 6% 25%	5176
	21st to end	Night	29% 8% 3 5% 4 3 3 3 4 6% 32%	6952

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	48% 9% 8% 16% 15% 4	3639
	11th to 20th	Day	38% 4 6% 17% 23% 12%	3934
	21st to end	Day	41% 4 8% 12% 22% 14%	4510
February	1st to 10th	Day	43% 7% 8% 16% 18% 9%	1800
	11th to 20th	Day	43% 6% 8% 18% 19% 6%	1809
	21st to end	Day	42% 7% 8% 20% 15% 7%	1650
March	1st to 10th	Day	52% 7% 4 17% 15% 5%	4737
	11th to 20th	Day	43% 6% 8% 25% 13% 5%	5000
	21st to end	Day	50% 9% 10% 22% 8% 2	5633
April	1st to 10th	Day	42% 8% 12% 20% 16% 3	2400
	11th to 20th	Day	35% 8% 10% 21% 19% 7%	2372
	21st to end	Day	29% 7% 12% 25% 22% 5%	2400
May	1st to 10th	Day	17% 5% 9% 19% 33% 16%	5887
	11th to 20th	Day	15% 4 6% 22% 41% 11%	6280
	21st to end	Day	13% 4 9% 32% 37% 5%	6904
June	1st to 10th	Day	25% 7% 9% 24% 25% 10%	2800
	11th to 20th	Day	13% 4 8% 20% 35% 20%	2800
	21st to end	Day	11% 2 3 16% 34% 34%	2800
July	1st to 10th	Day	3 2 2 12% 21% 59%	6251
	11th to 20th	Day	5% 1 2 6% 20% 66%	6245
	21st to end	Day	9% 2 5% 17% 30% 37%	6306
August	1st to 10th	Day	12% 2 3 11% 25% 47%	4206
	11th to 20th	Day	4 1 2 16% 43% 34%	4077
	21st to end	Day	8% 2 3 19% 38% 31%	4311
September	1st to 10th	Day	7% 2 3 11% 26% 51%	2400
	11th to 20th	Day	16% 4 5% 21% 35% 19%	2400
	21st to end	Day	13% 4 7% 18% 31% 27%	2130
October	1st to 10th	Day	14% 3 4 14% 23% 42%	3211
	11th to 20th	Day	21% 4 5% 24% 32% 14%	3161
	21st to end	Day	31% 6% 6% 19% 25% 12%	3448
November	1st to 10th	Day	36% 7% 8% 19% 16% 13%	1803
	11th to 20th	Day	39% 6% 10% 20% 16% 10%	1735
	21st to end	Day	36% 5% 8% 17% 21% 13%	1600
December	1st to 10th	Day	31% 4 5% 16% 28% 16%	2620
	11th to 20th	Day	57% 7% 9% 15% 9% 3	3028
	21st to end	Day	55% 8% 9% 14% 11% 3	4004
January	1st to 10th	Night	47% 8% 10% 15% 15% 5%	6306
	11th to 20th	Night	36% 7% 5% 13% 25% 14%	6037
	21st to end	Night	45% 4 4 9% 15% 22%	6468
February	1st to 10th	Night	41% 5% 7% 18% 19% 10%	2600
	11th to 20th	Night	41% 6% 8% 17% 19% 8%	2487
	21st to end	Night	42% 6% 10% 19% 16% 7%	1980
March	1st to 10th	Night	49% 8% 7% 17% 13% 6%	5142
	11th to 20th	Night	42% 5% 6% 21% 20% 6%	5000
	21st to end	Night	47% 10% 11% 20% 11% 1	5341
April	1st to 10th	Night	38% 8% 13% 21% 16% 5%	2000
	11th to 20th	Night	32% 7% 12% 22% 19% 8%	1979
	21st to end	Night	28% 8% 12% 28% 20% 4	2000
May	1st to 10th	Night	22% 5% 6% 21% 32% 15%	4086
	11th to 20th	Night	19% 5% 6% 28% 29% 13%	3616
	21st to end	Night	14% 5% 11% 30% 36% 4	3972
June	1st to 10th	Night	25% 8% 10% 28% 23% 7%	1600
	11th to 20th	Night	14% 5% 8% 22% 33% 18%	1600
	21st to end	Night	11% 4 4 17% 31% 33%	1600
July	1st to 10th	Night	4 1 2 10% 24% 59%	3600
	11th to 20th	Night	6% 1 2 5% 19% 66%	3592
	21st to end	Night	7% 3 6% 19% 30% 36%	4247
August	1st to 10th	Night	14% 2 4 10% 25% 47%	2929
	11th to 20th	Night	4 2 2 15% 39% 38%	3050
	21st to end	Night	9% 1 2 16% 38% 35%	3608
September	1st to 10th	Night	7% 2 3 8% 26% 53%	2000
	11th to 20th	Night	13% 4 7% 20% 33% 23%	2000
	21st to end	Night	12% 4 7% 17% 29% 32%	2266
October	1st to 10th	Night	13% 3 4 11% 24% 45%	3836
	11th to 20th	Night	17% 4 6% 28% 30% 15%	3777
	21st to end	Night	22% 4 6% 26% 26% 16%	4121
November	1st to 10th	Night	33% 10% 8% 20% 16% 12%	2495
	11th to 20th	Night	38% 8% 10% 19% 16% 10%	2663
	21st to end	Night	34% 7% 8% 18% 21% 13%	2800
December	1st to 10th	Night	27% 4 5% 16% 27% 21%	4560
	11th to 20th	Night	58% 5% 5% 13% 14% 4	5176
	21st to end	Night	53% 6% 11% 14% 12% 4	6952

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	39% 12% 24% 20% 6% 1	200
	11th to 20th	Day	26% 15% 22% 22% 12% 3	325
	21st to end	Day	23% 14% 23% 23% 15% 3	440
February	1st to 10th	Day	27% 18% 24% 22% 9% 1	400
	11th to 20th	Day	28% 17% 27% 21% 7%	418
	21st to end	Day	21% 17% 33% 21% 8% 1	414
March	1st to 10th	Day	34% 21% 27% 13% 5%	500
	11th to 20th	Day	29% 17% 23% 18% 11% 1	500
	21st to end	Day	25% 24% 28% 17% 5%	550
April	1st to 10th	Day	24% 18% 30% 20% 7% 1	500
	11th to 20th	Day	21% 18% 27% 22% 9% 2	492
	21st to end	Day	11% 16% 36% 24% 11% 2	500
May	1st to 10th	Day	13% 22% 31% 21% 10% 3	503
	11th to 20th	Day	15% 14% 32% 23% 12% 3	528
	21st to end	Day	9% 14% 28% 31% 13% 5%	573
June	1st to 10th	Day	6% 15% 26% 34% 14% 5%	530
	11th to 20th	Day	7% 17% 30% 33% 10% 4	530
	21st to end	Day	2 10% 30% 39% 14% 6%	530
July	1st to 10th	Day	2 4 37% 36% 15% 7%	526
	11th to 20th	Day	3 28% 44% 18% 8%	524
	21st to end	Day	1 2 25% 45% 19% 8%	575
August	1st to 10th	Day	2 5% 26% 41% 18% 9%	527
	11th to 20th	Day	2 5% 26% 38% 21% 8%	517
	21st to end	Day	2 5% 25% 43% 19% 7%	583
September	1st to 10th	Day	2 6% 33% 38% 19% 3	530
	11th to 20th	Day	7% 9% 24% 37% 20% 3	530
	21st to end	Day	5% 12% 30% 36% 15% 3	530
October	1st to 10th	Day	8% 13% 28% 31% 19% 1	503
	11th to 20th	Day	11% 11% 20% 34% 20% 3	490
	21st to end	Day	14% 12% 27% 28% 17% 3	539
November	1st to 10th	Day	20% 11% 21% 25% 18% 5%	339
	11th to 20th	Day	16% 10% 23% 31% 16% 4	267
	21st to end	Day	20% 9% 22% 30% 15% 5%	200
December	1st to 10th	Day	28% 9% 21% 21% 15% 8%	200
	11th to 20th	Day	30% 9% 17% 25% 18% 2	200
	21st to end	Day	26% 12% 22% 27% 12% 1	220
January	1st to 10th	Night	25% 10% 23% 26% 14% 3	800
	11th to 20th	Night	23% 9% 26% 30% 11% 2	800
	21st to end	Night	23% 7% 25% 27% 16% 2	880
February	1st to 10th	Night	24% 9% 25% 28% 14% 1	800
	11th to 20th	Night	23% 11% 23% 28% 13% 2	784
	21st to end	Night	22% 11% 25% 34% 8% 1	660
March	1st to 10th	Night	30% 14% 21% 22% 11% 3	800
	11th to 20th	Night	24% 11% 18% 26% 20% 2	800
	21st to end	Night	23% 13% 23% 28% 12% 2	880
April	1st to 10th	Night	17% 12% 25% 32% 13% 1	800
	11th to 20th	Night	16% 11% 24% 31% 15% 3	791
	21st to end	Night	13% 8% 23% 35% 18% 3	800
May	1st to 10th	Night	11% 10% 29% 29% 18% 2	602
	11th to 20th	Night	11% 6% 22% 34% 25% 3	394
	21st to end	Night	10% 7% 18% 35% 27% 3	433
June	1st to 10th	Night	9% 4 16% 41% 28% 3	400
	11th to 20th	Night	5% 7% 23% 41% 24% 1	400
	21st to end	Night	3 2 16% 44% 33% 3	400
July	1st to 10th	Night	1 1 10% 41% 45% 3	390
	11th to 20th	Night	2 1 10% 45% 40% 3	388
	21st to end	Night	1 5% 36% 51% 6%	608
August	1st to 10th	Night	2 1 7% 36% 47% 7%	597
	11th to 20th	Night	2 1 10% 39% 42% 6%	663
	21st to end	Night	2 1 9% 40% 43% 5%	880
September	1st to 10th	Night	1 2 15% 43% 35% 5%	800
	11th to 20th	Night	5% 4 12% 35% 35% 9%	800
	21st to end	Night	5% 5% 18% 36% 26% 10%	800
October	1st to 10th	Night	7% 7% 19% 41% 22% 5%	772
	11th to 20th	Night	8% 7% 17% 37% 25% 6%	760
	21st to end	Night	12% 8% 20% 32% 22% 7%	836
November	1st to 10th	Night	18% 7% 18% 29% 22% 5%	780
	11th to 20th	Night	18% 7% 26% 30% 17% 2	800
	21st to end	Night	18% 8% 23% 31% 16% 3	800
December	1st to 10th	Night	23% 8% 19% 33% 14% 2	800
	11th to 20th	Night	30% 7% 23% 26% 14% 1	800
	21st to end	Night	30% 9% 27% 24% 10% 1	880

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