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 2024-05-13.

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 4 3 3 3 3 4 5% 57%	83694
	Night	14% 6% 2 4 3 3 3 3 4 5% 53%	85896
Transparency	Day	30% 6% 7% 18% 22% 17%	83406
	Night	33% 6% 8% 17% 20% 15%	85705
Seeing	Day	14% 13% 27% 29% 14% 3	14362
	Night	15% 7% 19% 32% 23% 4	23692

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	23% 12% 2 7% 3 3 3 2 4 5% 36%	2539
	11th to 20th	Day	28% 10% 3 6% 3 3 3 3 4 4 33%	2736
	21st to end	Day	26% 11% 2 5% 2 3 3 2 5% 7% 33%	3146
February	1st to 10th	Day	17% 8% 3 6% 4 3 3 4 4 6% 43%	1710
	11th to 20th	Day	17% 10% 2 5% 4 2 3 2 4 5% 45%	1707
	21st to end	Day	17% 11% 2 5% 3 3 3 4 4 6% 42%	1570
March	1st to 10th	Day	26% 16% 3 8% 4 3 3 3 3 4 28%	3292
	11th to 20th	Day	14% 9% 2 4 2 2 2 2 3 6% 54%	3490
	21st to end	Day	19% 11% 3 7% 5% 4 4 3 5% 6% 32%	3933
April	1st to 10th	Day	16% 10% 3 5% 3 3 4 4 4 8% 40%	2280
	11th to 20th	Day	16% 6% 2 4 2 3 3 3 5% 6% 49%	2249
	21st to end	Day	9% 5% 2 4 3 3 3 3 5% 7% 55%	2280
May	1st to 10th	Day	8% 5% 1 4 3 2 3 3 4 6% 60%	2465
	11th to 20th	Day	7% 5% 1 4 3 3 3 4 5% 7% 57%	2508
	21st to end	Day	8% 4 2 4 3 3 4 3 5% 6% 57%	2723
June	1st to 10th	Day	8% 4 2 4 3 3 3 3 6% 7% 56%	2520
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 76%	2520
	21st to end	Day	4 1 1 2 1 1 2 2 2 3 81%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 2 3 85%	2520
	11th to 20th	Day	1 1 2 1 1 1 1 2 2 87%	2520
	21st to end	Day	1 1 1 2 1 2 1 1 2 3 84%	2586
August	1st to 10th	Day	2 1 1 2 1 1 2 2 3 4 81%	2328
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2236
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2376
September	1st to 10th	Day	1 1 1 1 1 2 1 3 3 86%	2160
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 5% 72%	2160
	21st to end	Day	3 3 1 2 2 2 2 2 4 6% 74%	1918
October	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 5% 71%	1800
	11th to 20th	Day	9% 4 1 4 2 2 3 3 4 5% 65%	1800
	21st to end	Day	10% 4 2 4 3 2 3 2 4 6% 61%	1980
November	1st to 10th	Day	16% 5% 2 4 3 3 5% 4 5% 7% 47%	1711
	11th to 20th	Day	16% 5% 2 4 4 4 4 4 6% 7% 44%	1649
	21st to end	Day	13% 7% 2 6% 3 3 3 2 4 5% 51%	1520
December	1st to 10th	Day	18% 7% 2 5% 3 3 4 3 5% 6% 46%	1520
	11th to 20th	Day	29% 9% 3 7% 4 4 3 3 5% 5% 28%	1928
	21st to end	Day	21% 9% 4 6% 4 4 4 4 5% 5% 34%	2794
January	1st to 10th	Night	17% 10% 2 6% 4 4 4 4 5% 7% 37%	4406
	11th to 20th	Night	26% 10% 3 6% 3 3 4 3 4 5% 34%	4228
	21st to end	Night	30% 11% 3 7% 3 3 3 3 3 4 30%	4521
February	1st to 10th	Night	18% 7% 2 4 3 3 3 3 4 6% 45%	2470
	11th to 20th	Night	16% 9% 2 6% 4 3 3 2 4 6% 45%	2358
	21st to end	Night	16% 10% 2 6% 4 3 3 3 4 7% 43%	1884
March	1st to 10th	Night	23% 17% 4 6% 3 3 2 2 3 4 31%	3567
	11th to 20th	Night	16% 7% 1 3 2 2 2 3 4 5% 55%	3490
	21st to end	Night	17% 12% 3 6% 4 4 5% 4 5% 6% 34%	3719
April	1st to 10th	Night	14% 7% 2 5% 5% 3 3 3 5% 7% 45%	1900
	11th to 20th	Night	14% 5% 2 4 4 2 4 4 5% 8% 49%	1879
	21st to end	Night	11% 4 1 3 3 3 3 3 5% 6% 57%	1900
May	1st to 10th	Night	10% 4 1 3 3 3 4 3 4 5% 61%	1712
	11th to 20th	Night	8% 5% 1 4 3 4 4 5% 6% 8% 53%	1440
	21st to end	Night	8% 5% 2 4 3 3 4 2 5% 6% 56%	1552
June	1st to 10th	Night	8% 5% 1 4 3 3 4 4 5% 9% 54%	1440
	11th to 20th	Night	5% 4 1 2 1 2 2 2 4 6% 72%	1440
	21st to end	Night	3 1 1 2 2 1 1 2 3 4 78%	1440
July	1st to 10th	Night	1 2 2 1 1 2 1 3 3 85%	1440
	11th to 20th	Night	1 2 2 1 2 1 2 2 2 85%	1440
	21st to end	Night	1 1 1 2 2 1 1 2 3 3 84%	1728
August	1st to 10th	Night	2 1 1 2 1 2 2 2 2 4 81%	1611
	11th to 20th	Night	1 1 1 1 1 3 1 3 3 84%	1663
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	1980
September	1st to 10th	Night	1 1 1 2 1 1 1 1 2 4 86%	1800
	11th to 20th	Night	4 2 1 2 2 1 2 2 4 5% 76%	1800
	21st to end	Night	4 1 1 2 2 2 2 3 4 5% 75%	2038
October	1st to 10th	Night	6% 2 1 2 2 2 2 2 3 4 73%	2160
	11th to 20th	Night	7% 3 1 3 2 2 3 2 3 5% 69%	2160
	21st to end	Night	10% 3 1 2 2 2 3 2 5% 6% 63%	2376
November	1st to 10th	Night	15% 6% 1 4 3 3 4 4 5% 6% 48%	2367
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 5% 8% 46%	2529
	21st to end	Night	13% 5% 2 4 3 3 3 4 6% 7% 51%	2660
December	1st to 10th	Night	21% 6% 2 4 3 3 3 3 4 7% 46%	2660
	11th to 20th	Night	29% 8% 3 6% 3 4 3 3 6% 6% 31%	3276
	21st to end	Night	23% 8% 3 6% 4 3 4 3 4 6% 35%	4862

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% 9% 9% 19% 12% 3	2539
	11th to 20th	Day	55% 6% 8% 13% 11% 7%	2736
	21st to end	Day	52% 6% 11% 14% 12% 4	3146
February	1st to 10th	Day	42% 7% 8% 17% 18% 8%	1710
	11th to 20th	Day	42% 6% 8% 19% 20% 6%	1716
	21st to end	Day	43% 7% 8% 20% 15% 6%	1570
March	1st to 10th	Day	59% 8% 5% 11% 14% 3	3292
	11th to 20th	Day	33% 6% 6% 30% 18% 8%	3490
	21st to end	Day	50% 11% 10% 18% 9% 2	3933
April	1st to 10th	Day	42% 8% 12% 19% 17% 3	2280
	11th to 20th	Day	35% 8% 10% 21% 18% 7%	2252
	21st to end	Day	28% 7% 13% 26% 22% 5%	2280
May	1st to 10th	Day	25% 6% 11% 22% 28% 9%	2465
	11th to 20th	Day	26% 8% 9% 24% 25% 8%	2511
	21st to end	Day	28% 8% 13% 26% 21% 5%	2724
June	1st to 10th	Day	26% 7% 10% 23% 23% 11%	2520
	11th to 20th	Day	13% 5% 9% 20% 34% 20%	2520
	21st to end	Day	11% 3 3 16% 33% 33%	2520
July	1st to 10th	Day	7% 3 3 13% 30% 44%	2451
	11th to 20th	Day	7% 2 3 13% 30% 45%	2445
	21st to end	Day	8% 2 3 9% 25% 52%	2442
August	1st to 10th	Day	9% 2 4 16% 30% 38%	2312
	11th to 20th	Day	6% 1 2 12% 39% 39%	2236
	21st to end	Day	6% 2 2 14% 36% 41%	2376
September	1st to 10th	Day	7% 2 3 10% 26% 53%	2160
	11th to 20th	Day	15% 4 5% 20% 37% 20%	2160
	21st to end	Day	13% 4 7% 20% 30% 26%	1918
October	1st to 10th	Day	18% 4 6% 20% 29% 23%	1800
	11th to 20th	Day	23% 5% 8% 14% 31% 20%	1800
	21st to end	Day	25% 6% 8% 17% 24% 20%	1980
November	1st to 10th	Day	37% 7% 8% 19% 16% 14%	1711
	11th to 20th	Day	38% 6% 10% 20% 16% 10%	1649
	21st to end	Day	36% 5% 8% 17% 22% 12%	1520
December	1st to 10th	Day	39% 6% 8% 15% 20% 12%	1520
	11th to 20th	Day	57% 7% 6% 13% 12% 5%	1928
	21st to end	Day	49% 8% 10% 16% 13% 4	2794
January	1st to 10th	Night	46% 9% 13% 19% 11% 3	4406
	11th to 20th	Night	51% 10% 7% 12% 13% 8%	4228
	21st to end	Night	58% 6% 5% 12% 13% 6%	4521
February	1st to 10th	Night	40% 5% 7% 19% 19% 9%	2470
	11th to 20th	Night	41% 6% 8% 18% 20% 8%	2360
	21st to end	Night	42% 6% 10% 19% 16% 7%	1884
March	1st to 10th	Night	55% 9% 9% 12% 11% 5%	3567
	11th to 20th	Night	33% 6% 6% 23% 24% 8%	3490
	21st to end	Night	46% 12% 11% 19% 10% 2	3719
April	1st to 10th	Night	38% 8% 14% 20% 16% 5%	1900
	11th to 20th	Night	33% 7% 12% 22% 18% 7%	1879
	21st to end	Night	27% 8% 12% 28% 21% 4	1900
May	1st to 10th	Night	26% 6% 9% 21% 29% 9%	1712
	11th to 20th	Night	28% 8% 12% 25% 19% 8%	1440
	21st to end	Night	30% 8% 11% 29% 18% 4	1552
June	1st to 10th	Night	27% 8% 10% 26% 22% 7%	1440
	11th to 20th	Night	15% 5% 9% 22% 32% 17%	1440
	21st to end	Night	12% 5% 5% 17% 30% 32%	1440
July	1st to 10th	Night	8% 2 3 13% 29% 45%	1400
	11th to 20th	Night	8% 2 4 11% 24% 51%	1392
	21st to end	Night	7% 2 3 9% 23% 55%	1632
August	1st to 10th	Night	10% 2 5% 14% 33% 36%	1602
	11th to 20th	Night	7% 3 3 14% 32% 41%	1663
	21st to end	Night	5% 2 2 15% 33% 43%	1980
September	1st to 10th	Night	6% 2 3 8% 26% 55%	1800
	11th to 20th	Night	12% 4 7% 19% 33% 25%	1800
	21st to end	Night	12% 4 7% 18% 28% 31%	2038
October	1st to 10th	Night	18% 4 6% 18% 27% 27%	2160
	11th to 20th	Night	19% 5% 6% 19% 27% 24%	2160
	21st to end	Night	22% 5% 8% 19% 24% 21%	2376
November	1st to 10th	Night	34% 11% 8% 19% 16% 13%	2367
	11th to 20th	Night	36% 8% 9% 20% 16% 10%	2529
	21st to end	Night	33% 7% 8% 19% 21% 12%	2660
December	1st to 10th	Night	39% 6% 8% 15% 21% 11%	2660
	11th to 20th	Night	55% 5% 5% 12% 17% 6%	3276
	21st to end	Night	49% 7% 10% 15% 14% 5%	4862

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	41% 13% 22% 18% 6% 1	180
	11th to 20th	Day	28% 15% 22% 21% 11% 2	298
	21st to end	Day	24% 15% 23% 21% 14% 3	407
February	1st to 10th	Day	24% 19% 25% 22% 10% 1	370
	11th to 20th	Day	28% 17% 26% 21% 8%	385
	21st to end	Day	21% 19% 33% 19% 8%	382
March	1st to 10th	Day	36% 21% 26% 13% 4	460
	11th to 20th	Day	27% 18% 23% 19% 12% 2	460
	21st to end	Day	24% 25% 29% 17% 5%	506
April	1st to 10th	Day	25% 19% 29% 20% 6% 1	460
	11th to 20th	Day	22% 19% 26% 22% 9% 2	452
	21st to end	Day	10% 17% 35% 24% 11% 2	460
May	1st to 10th	Day	14% 22% 30% 21% 10% 3	463
	11th to 20th	Day	15% 14% 31% 24% 12% 4	457
	21st to end	Day	10% 15% 27% 29% 14% 5%	485
June	1st to 10th	Day	6% 17% 26% 32% 13% 5%	450
	11th to 20th	Day	7% 16% 30% 34% 10% 3	450
	21st to end	Day	2 10% 29% 41% 14% 5%	450
July	1st to 10th	Day	2 3 38% 37% 14% 6%	446
	11th to 20th	Day	3 29% 43% 18% 7%	444
	21st to end	Day	1 1 25% 46% 19% 7%	487
August	1st to 10th	Day	2 5% 27% 40% 18% 9%	447
	11th to 20th	Day	2 4 25% 39% 22% 8%	437
	21st to end	Day	2 5% 27% 42% 19% 6%	495
September	1st to 10th	Day	1 5% 31% 40% 20% 3	450
	11th to 20th	Day	6% 9% 24% 38% 20% 2	450
	21st to end	Day	4 13% 32% 33% 15% 3	450
October	1st to 10th	Day	7% 14% 28% 30% 20% 1	450
	11th to 20th	Day	11% 12% 20% 34% 20% 3	450
	21st to end	Day	14% 12% 25% 27% 18% 3	495
November	1st to 10th	Day	20% 11% 20% 25% 19% 5%	307
	11th to 20th	Day	13% 9% 22% 34% 17% 5%	241
	21st to end	Day	18% 9% 23% 33% 14% 3	180
December	1st to 10th	Day	31% 9% 21% 19% 13% 6%	180
	11th to 20th	Day	28% 10% 18% 25% 17% 2	180
	21st to end	Day	25% 11% 23% 27% 13% 2	198
January	1st to 10th	Night	25% 9% 23% 27% 15% 3	760
	11th to 20th	Night	24% 8% 25% 31% 10% 2	760
	21st to end	Night	24% 7% 25% 26% 16% 2	836
February	1st to 10th	Night	22% 9% 24% 29% 14% 1	760
	11th to 20th	Night	23% 11% 21% 29% 13% 2	744
	21st to end	Night	22% 11% 24% 34% 8% 1	628
March	1st to 10th	Night	31% 13% 21% 20% 11% 3	760
	11th to 20th	Night	22% 11% 18% 26% 21% 2	760
	21st to end	Night	23% 13% 23% 27% 12% 2	836
April	1st to 10th	Night	17% 12% 25% 31% 14% 1	760
	11th to 20th	Night	16% 11% 24% 30% 15% 3	751
	21st to end	Night	13% 8% 23% 34% 18% 3	760
May	1st to 10th	Night	11% 10% 29% 28% 19% 2	572
	11th to 20th	Night	11% 6% 20% 34% 27% 3	360
	21st to end	Night	11% 7% 16% 33% 29% 3	389
June	1st to 10th	Night	9% 5% 16% 38% 29% 3	360
	11th to 20th	Night	6% 8% 21% 39% 26% 1	360
	21st to end	Night	3 2 15% 41% 36% 3	360
July	1st to 10th	Night	1 8% 40% 48% 3	350
	11th to 20th	Night	2 9% 43% 43% 3	348
	21st to end	Night	1 1 5% 33% 54% 7%	546
August	1st to 10th	Night	2 1 7% 32% 50% 8%	537
	11th to 20th	Night	2 1 9% 37% 44% 7%	595
	21st to end	Night	2 1 9% 39% 45% 6%	792
September	1st to 10th	Night	1 2 13% 42% 37% 5%	720
	11th to 20th	Night	4 4 10% 35% 38% 9%	720
	21st to end	Night	5% 6% 18% 34% 27% 11%	720
October	1st to 10th	Night	7% 8% 19% 38% 23% 5%	720
	11th to 20th	Night	9% 7% 17% 37% 25% 6%	720
	21st to end	Night	12% 8% 18% 32% 23% 7%	792
November	1st to 10th	Night	18% 7% 17% 29% 23% 6%	740
	11th to 20th	Night	17% 7% 25% 31% 18% 2	760
	21st to end	Night	16% 9% 23% 31% 17% 4	760
December	1st to 10th	Night	25% 8% 19% 32% 14% 2	760
	11th to 20th	Night	29% 6% 22% 26% 14% 1	760
	21st to end	Night	29% 9% 26% 25% 10% 1	836

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