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 2024-11-05.

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	10% 5% 1 4 2 2 3 2 4 5% 62%	105296
	Night	12% 6% 2 4 3 3 3 3 4 5% 57%	102466
Transparency	Day	26% 5% 7% 17% 24% 21%	105048
	Night	30% 6% 7% 17% 22% 19%	102303
Seeing	Day	13% 13% 27% 29% 14% 3	15063
	Night	15% 7% 19% 32% 23% 4	24231

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% 4 1 3 2 2 3 2 4 5% 65%	4117
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 5% 74%	4377
	21st to end	Day	5% 3 1 3 2 2 3 3 4 6% 70%	4813
June	1st to 10th	Day	8% 4 2 4 3 3 3 3 6% 7% 57%	2660
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 76%	2660
	21st to end	Day	4 1 1 2 1 1 2 2 2 3 81%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 1 2 92%	4420
	11th to 20th	Day	1 1 2 2 1 1 1 2 2 86%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	4521
August	1st to 10th	Day	2 2 1 3 1 2 2 3 4 4 76%	4091
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	3912
	21st to end	Day	2 2 1 1 1 1 1 2 3 85%	4180
September	1st to 10th	Day	1 1 1 1 2 1 3 3 86%	2280
	11th to 20th	Day	4 3 1 3 2 2 3 2 4 5% 71%	2280
	21st to end	Day	3 3 2 2 2 2 2 4 6% 75%	2024
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 5% 76%	3180
	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	16% 5% 2 5% 3 3 5% 4 5% 7% 47%	1749
	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% 6% 1 3 3 2 3 2 3 3 64%	2849
	11th to 20th	Night	4 3 1 2 2 2 3 3 4 5% 72%	2516
	21st to end	Night	5% 4 2 3 2 3 3 2 4 5% 68%	2762
June	1st to 10th	Night	8% 5% 1 4 3 3 4 4 5% 9% 54%	1520
	11th to 20th	Night	5% 3 1 2 1 2 2 2 4 5% 73%	1520
	21st to end	Night	3 1 1 2 2 1 1 2 3 4 78%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	2540
	11th to 20th	Night	1 2 1 2 2 2 2 1 2 2 83%	2540
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	3036
August	1st to 10th	Night	3 2 1 2 2 2 3 3 3 4 75%	2848
	11th to 20th	Night	1 1 1 1 1 2 1 2 2 90%	2929
	21st to end	Night	4 2 1 2 1 1 1 1 1 2 86%	3498
September	1st to 10th	Night	1 1 1 2 1 1 1 1 2 4 86%	1900
	11th to 20th	Night	4 2 1 2 2 1 2 2 4 5% 76%	1900
	21st to end	Night	4 1 1 2 2 2 2 3 4 5% 76%	2152
October	1st to 10th	Night	3 1 1 2 2 2 2 2 2 3 80%	3800
	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% 6% 1 4 3 3 4 4 6% 6% 48%	2429
	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	20% 7% 12% 20% 27% 14%	4117
	11th to 20th	Day	15% 5% 5% 16% 45% 14%	4380
	21st to end	Day	17% 5% 13% 34% 27% 3	4814
June	1st to 10th	Day	26% 7% 10% 24% 24% 10%	2660
	11th to 20th	Day	13% 4 8% 20% 34% 20%	2660
	21st to end	Day	11% 2 3 16% 33% 35%	2660
July	1st to 10th	Day	4 2 2 7% 17% 68%	4351
	11th to 20th	Day	8% 2 3 7% 22% 59%	4345
	21st to end	Day	6% 1 2 8% 30% 53%	4378
August	1st to 10th	Day	12% 2 3 10% 24% 49%	4076
	11th to 20th	Day	4 1 2 16% 43% 34%	3951
	21st to end	Day	8% 2 3 19% 38% 31%	4179
September	1st to 10th	Day	6% 2 3 9% 27% 53%	2280
	11th to 20th	Day	16% 4 5% 21% 36% 19%	2280
	21st to end	Day	13% 4 6% 19% 31% 27%	2024
October	1st to 10th	Day	13% 3 4 14% 23% 42%	3180
	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	37% 7% 8% 19% 16% 13%	1749
	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% 7% 22% 28% 11%	2849
	11th to 20th	Night	16% 5% 7% 21% 32% 19%	2516
	21st to end	Night	19% 7% 14% 31% 26% 4	2762
June	1st to 10th	Night	26% 8% 10% 27% 21% 7%	1520
	11th to 20th	Night	15% 5% 8% 22% 33% 17%	1520
	21st to end	Night	11% 4 4 16% 30% 34%	1520
July	1st to 10th	Night	4 1 2 7% 16% 69%	2500
	11th to 20th	Night	9% 2 3 7% 18% 61%	2492
	21st to end	Night	5% 2 2 8% 32% 51%	2941
August	1st to 10th	Night	14% 2 4 9% 24% 48%	2839
	11th to 20th	Night	4 2 2 15% 39% 39%	2956
	21st to end	Night	9% 1 1 16% 37% 35%	3498
September	1st to 10th	Night	6% 2 3 7% 26% 56%	1900
	11th to 20th	Night	13% 4 7% 20% 33% 24%	1900
	21st to end	Night	11% 4 7% 17% 29% 32%	2152
October	1st to 10th	Night	13% 3 4 11% 24% 45%	3800
	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	34% 11% 8% 20% 16% 12%	2429
	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	14% 14% 32% 24% 13% 3	488
	21st to end	Day	10% 15% 27% 30% 13% 5%	529
June	1st to 10th	Day	6% 16% 27% 33% 13% 5%	490
	11th to 20th	Day	7% 17% 30% 33% 10% 3	490
	21st to end	Day	2 10% 30% 40% 14% 5%	490
July	1st to 10th	Day	2 3 37% 36% 15% 7%	486
	11th to 20th	Day	3 29% 43% 18% 7%	484
	21st to end	Day	1 1 26% 46% 19% 8%	531
August	1st to 10th	Day	2 5% 27% 41% 18% 8%	487
	11th to 20th	Day	2 5% 26% 38% 21% 7%	477
	21st to end	Day	2 5% 26% 42% 19% 6%	539
September	1st to 10th	Day	1 6% 32% 39% 19% 3	490
	11th to 20th	Day	7% 9% 25% 37% 19% 2	490
	21st to end	Day	4 12% 31% 35% 15% 3	490
October	1st to 10th	Day	7% 13% 27% 32% 20% 1	490
	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% 12% 21% 25% 18% 4	321
	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% 5% 20% 35% 26% 3	374
	21st to end	Night	11% 8% 17% 34% 28% 3	411
June	1st to 10th	Night	9% 4 16% 40% 28% 3	380
	11th to 20th	Night	5% 7% 22% 40% 24% 1	380
	21st to end	Night	3 2 16% 42% 34% 3	380
July	1st to 10th	Night	1 1 9% 40% 46% 3	370
	11th to 20th	Night	2 1 10% 44% 41% 3	368
	21st to end	Night	1 1 5% 35% 52% 6%	577
August	1st to 10th	Night	2 1 8% 34% 48% 7%	567
	11th to 20th	Night	2 1 10% 39% 42% 7%	629
	21st to end	Night	2 1 9% 39% 44% 5%	836
September	1st to 10th	Night	1 2 15% 42% 35% 5%	760
	11th to 20th	Night	5% 4 12% 35% 36% 9%	760
	21st to end	Night	5% 5% 18% 36% 26% 11%	760
October	1st to 10th	Night	6% 7% 19% 41% 22% 5%	760
	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% 28% 22% 6%	760
	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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