Miller Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Miller Creek Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	7% 4 1 4 3 3 3 3 5% 6% 60%	131838
	Night	6% 4 1 4 3 3 3 3 5% 6% 61%	130683
Transparency	Day	25% 4 8% 22% 27% 13%	135643
	Night	23% 4 8% 22% 28% 16%	134942
Seeing	Day	9% 6% 24% 32% 22% 7%	17014
	Night	8% 7% 34% 38% 13% 1	27067

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	6% 5% 2 5% 3 3 4 2 4 5% 59%	5289
	11th to 20th	Day	4 5% 1 3 2 2 2 2 4 5% 70%	5310
	21st to end	Day	14% 6% 2 4 3 2 3 3 3 5% 53%	6188
February	1st to 10th	Day	10% 5% 1 4 2 3 3 3 5% 6% 58%	2100
	11th to 20th	Day	9% 5% 3 5% 3 3 4 4 4 4 56%	2049
	21st to end	Day	11% 6% 2 4 2 2 2 2 3 4 61%	1730
March	1st to 10th	Day	11% 4 1 4 3 3 3 2 5% 6% 58%	4723
	11th to 20th	Day	8% 4 2 5% 3 3 3 2 4 5% 61%	5090
	21st to end	Day	5% 6% 1 4 3 3 4 3 6% 8% 57%	5583
April	1st to 10th	Day	9% 4 1 4 3 3 3 3 5% 6% 60%	2362
	11th to 20th	Day	11% 5% 2 4 2 4 3 2 4 6% 57%	2447
	21st to end	Day	6% 6% 1 4 4 3 3 3 4 6% 60%	2528
May	1st to 10th	Day	12% 5% 2 4 3 4 4 3 5% 7% 52%	5862
	11th to 20th	Day	10% 5% 1 5% 3 3 3 3 4 5% 57%	5862
	21st to end	Day	9% 5% 2 6% 4 5% 4 4 5% 7% 49%	6434
June	1st to 10th	Day	4 4 2 4 3 4 4 4 5% 8% 56%	2600
	11th to 20th	Day	4 2 1 4 4 4 4 4 6% 8% 60%	2600
	21st to end	Day	5% 3 1 3 3 3 4 3 5% 7% 62%	2600
July	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 5% 8% 46%	5900
	11th to 20th	Day	3 2 1 4 4 4 4 4 6% 9% 60%	5900
	21st to end	Day	2 2 1 2 2 3 3 3 5% 9% 68%	6422
August	1st to 10th	Day	2 1 1 2 2 3 3 3 5% 7% 70%	4206
	11th to 20th	Day	3 2 1 3 2 3 4 3 5% 9% 65%	3846
	21st to end	Day	3 2 1 3 3 3 4 4 6% 7% 65%	4312
September	1st to 10th	Day	10% 5% 1 5% 4 3 3 3 4 6% 57%	2231
	11th to 20th	Day	10% 5% 2 6% 4 4 3 4 6% 8% 50%	2200
	21st to end	Day	9% 3 1 4 3 3 3 2 5% 6% 62%	2200
October	1st to 10th	Day	5% 3 1 4 3 3 3 3 4 6% 65%	2200
	11th to 20th	Day	8% 3 1 3 2 3 2 2 3 4 68%	2115
	21st to end	Day	10% 4 1 3 2 2 3 2 3 4 66%	2200
November	1st to 10th	Day	8% 4 1 4 3 2 2 2 3 4 67%	1871
	11th to 20th	Day	5% 3 1 3 2 3 3 2 4 5% 67%	1890
	21st to end	Day	11% 5% 1 4 2 2 2 2 4 5% 60%	1890
December	1st to 10th	Day	7% 5% 2 5% 3 4 4 3 5% 6% 57%	3030
	11th to 20th	Day	7% 3 1 3 4 3 4 3 3 5% 64%	3481
	21st to end	Day	7% 6% 2 5% 3 2 3 2 4 4 62%	4587
January	1st to 10th	Night	7% 5% 2 4 3 3 3 2 4 6% 60%	7634
	11th to 20th	Night	3 3 1 3 2 2 2 2 4 6% 73%	7680
	21st to end	Night	12% 7% 2 5% 3 3 3 2 4 6% 53%	8068
February	1st to 10th	Night	9% 5% 2 4 3 3 3 3 4 4 60%	2520
	11th to 20th	Night	10% 6% 1 5% 3 2 3 3 5% 6% 57%	2458
	21st to end	Night	8% 5% 2 4 4 3 3 2 4 4 61%	2076
March	1st to 10th	Night	7% 6% 2 5% 3 3 3 2 4 6% 59%	5364
	11th to 20th	Night	7% 3 1 3 2 2 2 3 4 6% 65%	5090
	21st to end	Night	4 6% 2 5% 3 3 4 3 5% 6% 60%	5588
April	1st to 10th	Night	7% 5% 2 4 4 3 3 3 4 6% 59%	2252
	11th to 20th	Night	9% 4 1 5% 3 4 3 3 5% 6% 57%	2037
	21st to end	Night	8% 4 1 4 3 3 3 3 4 6% 61%	1870
May	1st to 10th	Night	11% 5% 2 4 3 3 3 2 4 6% 56%	4053
	11th to 20th	Night	6% 3 1 5% 3 4 4 3 6% 7% 58%	4053
	21st to end	Night	9% 4 2 5% 3 4 4 4 5% 8% 54%	4443
June	1st to 10th	Night	4 2 1 3 3 3 4 4 7% 10% 57%	1800
	11th to 20th	Night	2 3 1 3 3 4 4 3 7% 9% 61%	1800
	21st to end	Night	4 3 1 3 2 2 3 4 6% 8% 66%	1800
July	1st to 10th	Night	5% 5% 2 7% 4 5% 4 4 6% 10% 47%	4078
	11th to 20th	Night	3 2 1 3 2 3 3 3 5% 11% 65%	4077
	21st to end	Night	2 1 1 3 3 3 4 3 6% 9% 67%	4449
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 8% 73%	2949
	11th to 20th	Night	3 2 1 3 2 2 3 3 7% 11% 64%	3213
	21st to end	Night	3 2 1 3 3 4 3 4 5% 9% 63%	3597
September	1st to 10th	Night	8% 3 1 4 3 4 4 3 5% 7% 58%	2166
	11th to 20th	Night	9% 4 2 5% 3 3 3 4 5% 7% 54%	2200
	21st to end	Night	8% 4 1 4 3 3 3 3 5% 6% 61%	2200
October	1st to 10th	Night	5% 2 1 3 3 3 4 3 5% 5% 66%	2200
	11th to 20th	Night	7% 4 1 3 2 2 2 2 3 4 71%	2265
	21st to end	Night	8% 3 1 3 2 2 2 2 3 4 68%	2640
November	1st to 10th	Night	8% 3 1 3 2 2 2 3 4 5% 67%	2644
	11th to 20th	Night	4 3 1 3 2 3 3 3 5% 7% 67%	2730
	21st to end	Night	11% 4 1 3 3 3 4 3 5% 5% 59%	2730
December	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 5% 61%	4380
	11th to 20th	Night	5% 4 1 4 3 3 3 3 4 5% 65%	4946
	21st to end	Night	7% 5% 1 5% 3 3 3 2 4 5% 61%	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	28% 3 5% 26% 27% 12%	5290
	11th to 20th	Day	18% 2 5% 20% 40% 15%	5310
	21st to end	Day	34% 3 5% 19% 25% 13%	6188
February	1st to 10th	Day	29% 3 6% 19% 30% 13%	2100
	11th to 20th	Day	32% 4 6% 20% 26% 12%	2059
	21st to end	Day	29% 3 4 15% 29% 20%	1730
March	1st to 10th	Day	27% 4 5% 12% 24% 28%	4723
	11th to 20th	Day	27% 3 7% 13% 25% 24%	5090
	21st to end	Day	24% 5% 9% 23% 27% 12%	5585
April	1st to 10th	Day	25% 5% 7% 22% 25% 16%	2362
	11th to 20th	Day	30% 4 8% 16% 25% 17%	2447
	21st to end	Day	26% 5% 9% 21% 29% 10%	2528
May	1st to 10th	Day	30% 6% 10% 23% 19% 12%	5900
	11th to 20th	Day	29% 4 8% 17% 26% 16%	5866
	21st to end	Day	32% 7% 11% 25% 14% 12%	6447
June	1st to 10th	Day	24% 7% 14% 27% 21% 6%	2600
	11th to 20th	Day	20% 6% 11% 27% 27% 9%	2600
	21st to end	Day	21% 6% 12% 26% 25% 9%	2600
July	1st to 10th	Day	35% 6% 10% 24% 19% 5%	5836
	11th to 20th	Day	20% 6% 13% 29% 27% 6%	5831
	21st to end	Day	12% 4 14% 32% 32% 5%	6240
August	1st to 10th	Day	13% 5% 10% 24% 37% 11%	4192
	11th to 20th	Day	17% 5% 11% 30% 29% 9%	3882
	21st to end	Day	18% 6% 15% 27% 29% 5%	4311
September	1st to 10th	Day	29% 5% 8% 21% 26% 10%	2231
	11th to 20th	Day	32% 6% 9% 19% 25% 9%	2200
	21st to end	Day	24% 5% 7% 21% 29% 15%	2200
October	1st to 10th	Day	12% 3 6% 15% 36% 27%	3590
	11th to 20th	Day	20% 4 6% 20% 30% 19%	3420
	21st to end	Day	16% 2 3 29% 36% 13%	3538
November	1st to 10th	Day	23% 3 7% 22% 30% 16%	1871
	11th to 20th	Day	20% 3 6% 23% 32% 16%	1890
	21st to end	Day	28% 3 4 21% 30% 14%	1890
December	1st to 10th	Day	28% 3 5% 29% 26% 8%	3030
	11th to 20th	Day	24% 4 8% 15% 33% 16%	3479
	21st to end	Day	26% 3 6% 26% 26% 12%	4587
January	1st to 10th	Night	26% 3 5% 26% 32% 9%	7635
	11th to 20th	Night	15% 2 7% 19% 38% 19%	7680
	21st to end	Night	33% 3 6% 18% 27% 12%	8068
February	1st to 10th	Night	27% 4 7% 17% 29% 16%	2520
	11th to 20th	Night	29% 3 8% 19% 24% 17%	2458
	21st to end	Night	27% 3 4 16% 25% 25%	2076
March	1st to 10th	Night	27% 3 4 12% 21% 32%	5364
	11th to 20th	Night	20% 4 7% 17% 18% 34%	5090
	21st to end	Night	25% 4 7% 20% 30% 14%	5588
April	1st to 10th	Night	27% 4 7% 19% 23% 19%	2252
	11th to 20th	Night	27% 5% 10% 16% 23% 19%	2037
	21st to end	Night	24% 5% 10% 21% 28% 11%	1870
May	1st to 10th	Night	30% 5% 9% 23% 19% 14%	4080
	11th to 20th	Night	24% 6% 8% 17% 23% 22%	4054
	21st to end	Night	28% 7% 11% 20% 19% 15%	4452
June	1st to 10th	Night	20% 8% 16% 28% 23% 6%	1800
	11th to 20th	Night	19% 5% 12% 27% 29% 9%	1800
	21st to end	Night	18% 4 11% 27% 27% 13%	1800
July	1st to 10th	Night	32% 7% 11% 28% 19% 3	4035
	11th to 20th	Night	15% 4 9% 32% 32% 8%	4026
	21st to end	Night	14% 5% 12% 33% 31% 5%	4320
August	1st to 10th	Night	10% 4 6% 26% 39% 14%	2939
	11th to 20th	Night	15% 4 12% 29% 30% 11%	3243
	21st to end	Night	17% 6% 15% 29% 27% 6%	3597
September	1st to 10th	Night	26% 5% 8% 21% 25% 14%	2166
	11th to 20th	Night	29% 6% 10% 21% 25% 10%	2200
	21st to end	Night	24% 4 7% 19% 28% 19%	2200
October	1st to 10th	Night	12% 3 6% 13% 35% 32%	3590
	11th to 20th	Night	15% 4 8% 19% 30% 24%	3686
	21st to end	Night	15% 3 3 22% 41% 16%	4252
November	1st to 10th	Night	21% 3 7% 21% 27% 21%	2644
	11th to 20th	Night	18% 3 6% 20% 34% 19%	2730
	21st to end	Night	28% 4 5% 17% 31% 15%	2730
December	1st to 10th	Night	25% 4 6% 25% 28% 12%	4380
	11th to 20th	Night	22% 3 6% 19% 32% 18%	4947
	21st to end	Night	26% 3 6% 24% 28% 13%	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	9% 12% 38% 32% 8% 1	330
	11th to 20th	Day	11% 16% 32% 30% 8% 2	330
	21st to end	Day	12% 11% 39% 29% 8% 1	363
February	1st to 10th	Day	15% 11% 38% 26% 9%	330
	11th to 20th	Day	15% 12% 44% 23% 5% 1	330
	21st to end	Day	18% 11% 44% 22% 4 1	273
March	1st to 10th	Day	15% 12% 36% 30% 5%	330
	11th to 20th	Day	18% 12% 40% 23% 7% 1	330
	21st to end	Day	9% 13% 38% 31% 9% 1	363
April	1st to 10th	Day	11% 11% 41% 26% 10% 1	382
	11th to 20th	Day	15% 15% 44% 22% 4	517
	21st to end	Day	10% 13% 40% 27% 8% 2	628
May	1st to 10th	Day	9% 8% 38% 36% 9%	703
	11th to 20th	Day	12% 6% 33% 31% 13% 5%	725
	21st to end	Day	9% 6% 32% 33% 16% 3	789
June	1st to 10th	Day	6% 3 17% 44% 24% 6%	730
	11th to 20th	Day	4 1 21% 47% 23% 4	730
	21st to end	Day	5% 1 11% 42% 33% 9%	730
July	1st to 10th	Day	6% 1 7% 33% 36% 16%	721
	11th to 20th	Day	3 1 6% 36% 41% 13%	718
	21st to end	Day	2 1 5% 29% 45% 19%	794
August	1st to 10th	Day	3 1 4 31% 47% 14%	711
	11th to 20th	Day	5% 1 6% 32% 40% 15%	517
	21st to end	Day	5% 1 6% 27% 43% 18%	583
September	1st to 10th	Day	12% 2 6% 25% 35% 21%	361
	11th to 20th	Day	5% 1 6% 27% 41% 20%	330
	21st to end	Day	12% 2 7% 28% 37% 14%	330
October	1st to 10th	Day	4 2 14% 38% 30% 12%	330
	11th to 20th	Day	8% 5% 28% 36% 18% 5%	330
	21st to end	Day	16% 7% 24% 33% 17% 2	363
November	1st to 10th	Day	14% 7% 28% 34% 15% 2	330
	11th to 20th	Day	11% 11% 25% 33% 16% 5%	330
	21st to end	Day	15% 10% 36% 26% 9% 4	330
December	1st to 10th	Day	11% 11% 38% 30% 8% 2	330
	11th to 20th	Day	6% 10% 36% 35% 12% 1	330
	21st to end	Day	14% 10% 39% 28% 8%	363
January	1st to 10th	Night	9% 9% 40% 34% 8%	1050
	11th to 20th	Night	9% 10% 37% 37% 7%	1050
	21st to end	Night	11% 9% 40% 33% 7%	1039
February	1st to 10th	Night	12% 10% 43% 31% 5%	840
	11th to 20th	Night	12% 12% 42% 30% 4	824
	21st to end	Night	10% 14% 51% 23% 3	692
March	1st to 10th	Night	12% 13% 43% 28% 5%	788
	11th to 20th	Night	14% 11% 44% 24% 7%	630
	21st to end	Night	7% 13% 49% 28% 3	693
April	1st to 10th	Night	7% 12% 52% 27% 3	630
	11th to 20th	Night	10% 12% 54% 21% 3	611
	21st to end	Night	8% 11% 46% 31% 4	600
May	1st to 10th	Night	9% 8% 48% 30% 5%	600
	11th to 20th	Night	8% 6% 41% 34% 11% 1	591
	21st to end	Night	6% 5% 43% 36% 9% 1	650
June	1st to 10th	Night	5% 2 32% 45% 16% 1	600
	11th to 20th	Night	2 2 38% 51% 7%	600
	21st to end	Night	5% 2 21% 53% 19% 1	600
July	1st to 10th	Night	5% 1 12% 50% 30% 3	585
	11th to 20th	Night	4 9% 55% 30% 1	582
	21st to end	Night	1 1 5% 57% 34% 2	657
August	1st to 10th	Night	1 8% 58% 31% 2	597
	11th to 20th	Night	6% 1 10% 49% 33% 2	594
	21st to end	Night	3 8% 46% 39% 4	660
September	1st to 10th	Night	9% 1 14% 47% 27% 3	600
	11th to 20th	Night	11% 1 14% 47% 24% 2	600
	21st to end	Night	9% 2 19% 43% 25% 2	600
October	1st to 10th	Night	6% 3 26% 43% 21% 1	600
	11th to 20th	Night	8% 5% 36% 38% 12%	665
	21st to end	Night	9% 7% 35% 35% 13% 1	880
November	1st to 10th	Night	9% 5% 35% 39% 12% 1	1004
	11th to 20th	Night	6% 7% 37% 40% 10% 1	1050
	21st to end	Night	12% 8% 37% 32% 11% 1	1050
December	1st to 10th	Night	10% 8% 35% 39% 7%	1050
	11th to 20th	Night	12% 8% 38% 35% 7%	1050
	21st to end	Night	10% 8% 40% 35% 6%	1155

Page generated at 2026-04-14 01:44:31. Version ac07

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