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 2024-12-24.

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% 59%	101466
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	98778
Transparency	Day	24% 5% 9% 22% 28% 12%	105310
	Night	23% 4 8% 21% 28% 15%	103061
Seeing	Day	9% 6% 24% 32% 23% 7%	15313
	Night	8% 7% 33% 38% 13% 1	25299

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% 6% 2 6% 4 3 3 2 3 5% 59%	2847
	11th to 20th	Day	6% 6% 1 3 2 2 3 2 4 6% 65%	2850
	21st to end	Day	14% 8% 3 5% 3 2 3 3 3 5% 51%	3331
February	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 5% 5% 57%	1900
	11th to 20th	Day	9% 5% 2 5% 3 3 4 4 4 4 57%	1849
	21st to end	Day	12% 7% 2 4 2 2 2 2 3 4 59%	1570
March	1st to 10th	Day	12% 5% 1 4 3 3 3 2 5% 6% 56%	3212
	11th to 20th	Day	11% 6% 2 6% 3 4 3 2 4 5% 54%	3480
	21st to end	Day	6% 8% 1 5% 4 4 4 3 6% 8% 51%	3814
April	1st to 10th	Day	8% 4 1 4 3 3 3 3 5% 6% 60%	2137
	11th to 20th	Day	11% 5% 2 4 2 4 3 2 4 6% 57%	2254
	21st to end	Day	6% 6% 1 4 3 2 3 3 4 5% 61%	2401
May	1st to 10th	Day	11% 5% 2 4 3 4 4 3 5% 7% 54%	4082
	11th to 20th	Day	12% 6% 1 5% 4 3 3 2 5% 6% 54%	4082
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 5% 7% 51%	4477
June	1st to 10th	Day	4 4 1 5% 4 4 4 4 5% 8% 57%	2470
	11th to 20th	Day	4 2 1 4 4 4 3 4 6% 9% 60%	2470
	21st to end	Day	5% 3 1 3 3 3 4 3 5% 7% 62%	2470
July	1st to 10th	Day	7% 4 2 4 3 3 4 3 5% 8% 56%	4120
	11th to 20th	Day	2 2 1 3 3 3 3 4 6% 9% 62%	4120
	21st to end	Day	2 2 1 3 3 3 4 3 5% 9% 65%	4465
August	1st to 10th	Day	2 1 1 2 2 3 3 3 5% 7% 70%	4077
	11th to 20th	Day	3 2 1 3 2 3 3 3 5% 9% 66%	3726
	21st to end	Day	3 2 1 3 3 3 4 4 6% 7% 65%	4180
September	1st to 10th	Day	10% 4 1 5% 4 3 3 2 4 6% 57%	2120
	11th to 20th	Day	10% 5% 2 6% 4 4 3 4 6% 7% 50%	2090
	21st to end	Day	9% 3 1 4 3 3 3 2 5% 5% 62%	2090
October	1st to 10th	Day	5% 3 1 4 3 3 3 3 4 6% 65%	2090
	11th to 20th	Day	8% 4 1 3 2 2 2 2 3 4 68%	2009
	21st to end	Day	10% 3 1 3 2 2 2 2 3 4 66%	2090
November	1st to 10th	Day	8% 4 1 4 3 2 2 2 3 5% 65%	1780
	11th to 20th	Day	5% 4 1 3 2 3 3 2 4 5% 67%	1800
	21st to end	Day	11% 5% 1 4 3 2 2 2 5% 5% 60%	1800
December	1st to 10th	Day	9% 6% 2 4 2 4 4 3 4 6% 57%	1800
	11th to 20th	Day	9% 4 1 4 4 4 5% 3 4 6% 57%	2251
	21st to end	Day	9% 6% 2 5% 3 3 3 3 4 4 59%	3162
January	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 6% 60%	4108
	11th to 20th	Night	6% 5% 1 4 2 2 2 2 4 5% 66%	4120
	21st to end	Night	10% 8% 2 5% 3 3 3 2 4 5% 54%	4363
February	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 4 58%	2280
	11th to 20th	Night	10% 7% 1 5% 3 2 3 3 4 5% 57%	2218
	21st to end	Night	9% 6% 2 5% 4 3 3 2 4 4 60%	1884
March	1st to 10th	Night	8% 7% 2 5% 3 3 4 2 4 5% 58%	3648
	11th to 20th	Night	10% 4 2 4 3 2 2 3 5% 6% 59%	3480
	21st to end	Night	4 8% 2 5% 4 3 4 3 5% 6% 54%	3817
April	1st to 10th	Night	6% 5% 2 4 4 3 4 3 5% 6% 60%	2038
	11th to 20th	Night	9% 4 1 5% 3 4 4 3 5% 6% 58%	1877
	21st to end	Night	8% 4 1 4 3 3 3 3 4 6% 62%	1777
May	1st to 10th	Night	11% 5% 2 4 4 3 3 3 4 6% 55%	2823
	11th to 20th	Night	8% 4 2 5% 3 4 3 3 6% 6% 55%	2823
	21st to end	Night	7% 4 1 3 3 3 4 3 5% 8% 59%	3090
June	1st to 10th	Night	4 2 1 4 3 3 4 4 7% 10% 57%	1710
	11th to 20th	Night	2 3 1 3 3 4 4 3 7% 9% 61%	1710
	21st to end	Night	5% 3 1 3 2 2 3 4 6% 8% 66%	1710
July	1st to 10th	Night	4 4 2 5% 3 4 4 4 6% 10% 55%	2850
	11th to 20th	Night	2 1 1 2 2 2 3 3 5% 12% 65%	2850
	21st to end	Night	2 1 1 3 3 3 4 3 5% 8% 67%	3098
August	1st to 10th	Night	1 2 2 2 2 3 3 5% 8% 72%	2858
	11th to 20th	Night	3 2 1 2 2 2 3 3 7% 11% 65%	3113
	21st to end	Night	3 2 1 3 3 4 3 4 5% 9% 64%	3487
September	1st to 10th	Night	8% 3 1 4 3 4 4 3 4 7% 57%	2057
	11th to 20th	Night	9% 4 2 5% 3 3 3 4 5% 7% 54%	2090
	21st to end	Night	8% 4 1 4 2 3 3 3 5% 6% 62%	2090
October	1st to 10th	Night	6% 2 1 3 3 3 4 3 5% 5% 65%	2090
	11th to 20th	Night	7% 4 1 3 2 2 2 2 3 4 70%	2152
	21st to end	Night	8% 3 1 3 2 2 2 2 3 5% 68%	2508
November	1st to 10th	Night	8% 3 2 3 2 2 2 3 4 5% 67%	2515
	11th to 20th	Night	4 3 1 3 2 3 3 3 4 7% 67%	2600
	21st to end	Night	11% 4 1 3 3 3 4 3 5% 5% 59%	2600
December	1st to 10th	Night	9% 3 1 4 2 3 3 3 5% 6% 61%	2600
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 5% 5% 59%	3166
	21st to end	Night	7% 5% 1 5% 4 3 3 2 4 5% 60%	4578

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% 5% 6% 21% 27% 14%	2848
	11th to 20th	Day	22% 3 7% 21% 36% 12%	2850
	21st to end	Day	37% 4 5% 20% 27% 7%	3331
February	1st to 10th	Day	29% 3 6% 20% 29% 13%	1900
	11th to 20th	Day	31% 4 6% 19% 26% 13%	1859
	21st to end	Day	32% 3 4 14% 28% 19%	1570
March	1st to 10th	Day	28% 4 6% 11% 28% 23%	3212
	11th to 20th	Day	33% 5% 9% 19% 26% 8%	3480
	21st to end	Day	30% 6% 9% 21% 24% 10%	3814
April	1st to 10th	Day	25% 5% 7% 22% 25% 17%	2137
	11th to 20th	Day	30% 4 7% 16% 25% 18%	2254
	21st to end	Day	25% 5% 9% 21% 30% 10%	2401
May	1st to 10th	Day	28% 7% 10% 20% 18% 17%	4120
	11th to 20th	Day	31% 5% 10% 21% 24% 9%	4086
	21st to end	Day	29% 8% 11% 21% 17% 13%	4489
June	1st to 10th	Day	24% 7% 14% 27% 22% 6%	2470
	11th to 20th	Day	20% 6% 11% 26% 27% 9%	2470
	21st to end	Day	22% 6% 12% 26% 26% 10%	2470
July	1st to 10th	Day	25% 5% 10% 31% 22% 7%	4056
	11th to 20th	Day	17% 5% 11% 31% 30% 6%	4051
	21st to end	Day	16% 5% 11% 29% 36% 5%	4325
August	1st to 10th	Day	13% 5% 10% 24% 38% 11%	4063
	11th to 20th	Day	16% 5% 11% 30% 29% 9%	3762
	21st to end	Day	17% 6% 15% 27% 29% 5%	4179
September	1st to 10th	Day	29% 5% 8% 21% 26% 11%	2120
	11th to 20th	Day	33% 7% 9% 18% 25% 9%	2090
	21st to end	Day	24% 5% 7% 21% 29% 15%	2090
October	1st to 10th	Day	13% 3 6% 14% 36% 28%	3480
	11th to 20th	Day	20% 4 6% 19% 31% 20%	3314
	21st to end	Day	16% 2 3 30% 36% 13%	3428
November	1st to 10th	Day	24% 3 7% 22% 29% 15%	1780
	11th to 20th	Day	20% 3 6% 24% 31% 17%	1800
	21st to end	Day	28% 3 4 20% 30% 15%	1800
December	1st to 10th	Day	29% 4 6% 24% 27% 9%	1800
	11th to 20th	Day	29% 5% 10% 15% 29% 12%	2249
	21st to end	Day	29% 4 7% 23% 27% 11%	3162
January	1st to 10th	Night	25% 4 5% 23% 32% 11%	4108
	11th to 20th	Night	21% 3 10% 19% 32% 15%	4120
	21st to end	Night	33% 4 7% 17% 27% 12%	4363
February	1st to 10th	Night	29% 4 7% 17% 27% 16%	2280
	11th to 20th	Night	29% 3 7% 19% 24% 18%	2218
	21st to end	Night	29% 3 4 15% 24% 24%	1884
March	1st to 10th	Night	28% 4 5% 12% 22% 29%	3648
	11th to 20th	Night	25% 5% 9% 24% 23% 13%	3480
	21st to end	Night	28% 5% 9% 22% 26% 10%	3817
April	1st to 10th	Night	26% 5% 7% 19% 23% 21%	2038
	11th to 20th	Night	26% 5% 10% 17% 23% 19%	1877
	21st to end	Night	24% 5% 10% 22% 29% 11%	1777
May	1st to 10th	Night	31% 6% 8% 18% 17% 20%	2850
	11th to 20th	Night	27% 8% 11% 21% 22% 11%	2824
	21st to end	Night	23% 7% 11% 18% 25% 16%	3099
June	1st to 10th	Night	20% 7% 15% 27% 23% 7%	1710
	11th to 20th	Night	19% 6% 12% 27% 29% 9%	1710
	21st to end	Night	17% 4 11% 26% 28% 14%	1710
July	1st to 10th	Night	24% 6% 11% 33% 22% 4	2805
	11th to 20th	Night	13% 4 8% 34% 34% 8%	2796
	21st to end	Night	16% 5% 10% 30% 36% 3	2999
August	1st to 10th	Night	11% 4 7% 26% 39% 14%	2848
	11th to 20th	Night	14% 4 12% 28% 30% 11%	3143
	21st to end	Night	17% 6% 14% 30% 28% 6%	3487
September	1st to 10th	Night	27% 5% 8% 20% 25% 15%	2057
	11th to 20th	Night	29% 6% 10% 20% 25% 10%	2090
	21st to end	Night	24% 3 7% 19% 28% 19%	2090
October	1st to 10th	Night	12% 3 6% 12% 34% 33%	3480
	11th to 20th	Night	16% 4 8% 19% 30% 24%	3573
	21st to end	Night	15% 3 3 22% 41% 16%	4120
November	1st to 10th	Night	22% 3 8% 22% 25% 20%	2515
	11th to 20th	Night	18% 4 6% 21% 32% 19%	2600
	21st to end	Night	28% 4 5% 17% 31% 16%	2600
December	1st to 10th	Night	24% 4 7% 23% 28% 14%	2600
	11th to 20th	Night	28% 4 6% 19% 28% 15%	3167
	21st to end	Night	26% 4 6% 20% 30% 15%	4578

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% 13% 37% 32% 9% 1	270
	11th to 20th	Day	12% 15% 30% 31% 9% 3	270
	21st to end	Day	11% 10% 39% 29% 9% 1	297
February	1st to 10th	Day	16% 13% 36% 26% 9%	270
	11th to 20th	Day	16% 10% 45% 24% 5% 1	270
	21st to end	Day	21% 12% 44% 21% 2	225
March	1st to 10th	Day	17% 11% 37% 29% 6%	270
	11th to 20th	Day	18% 12% 38% 25% 6% 1	270
	21st to end	Day	10% 15% 39% 26% 8% 1	297
April	1st to 10th	Day	10% 11% 45% 23% 10% 1	317
	11th to 20th	Day	15% 15% 43% 23% 5%	452
	21st to end	Day	10% 13% 38% 28% 9% 2	581
May	1st to 10th	Day	9% 8% 39% 36% 9%	653
	11th to 20th	Day	12% 6% 31% 32% 13% 5%	675
	21st to end	Day	8% 7% 32% 33% 17% 3	734
June	1st to 10th	Day	5% 3 16% 43% 26% 6%	680
	11th to 20th	Day	4 1 21% 46% 23% 5%	680
	21st to end	Day	5% 1 11% 41% 33% 9%	680
July	1st to 10th	Day	5% 1 7% 33% 37% 16%	671
	11th to 20th	Day	3 1 6% 35% 41% 14%	668
	21st to end	Day	2 1 4 29% 45% 19%	739
August	1st to 10th	Day	4 1 4 31% 46% 15%	662
	11th to 20th	Day	5% 1 6% 32% 41% 15%	477
	21st to end	Day	4 1 6% 27% 43% 19%	539
September	1st to 10th	Day	12% 2 5% 22% 36% 23%	330
	11th to 20th	Day	6% 1 5% 28% 41% 20%	300
	21st to end	Day	13% 2 6% 28% 36% 15%	300
October	1st to 10th	Day	4 2 15% 38% 30% 12%	300
	11th to 20th	Day	8% 5% 28% 37% 18% 4	300
	21st to end	Day	16% 7% 25% 33% 16% 3	330
November	1st to 10th	Day	15% 6% 30% 32% 15% 3	300
	11th to 20th	Day	11% 12% 25% 31% 17% 4	300
	21st to end	Day	15% 10% 35% 26% 8% 4	300
December	1st to 10th	Day	11% 11% 36% 31% 8% 2	300
	11th to 20th	Day	6% 10% 38% 33% 12% 1	300
	21st to end	Day	15% 10% 41% 26% 8%	306
January	1st to 10th	Night	9% 8% 39% 34% 9%	950
	11th to 20th	Night	10% 10% 35% 37% 8%	950
	21st to end	Night	10% 9% 38% 35% 8%	941
February	1st to 10th	Night	12% 10% 42% 31% 5%	760
	11th to 20th	Night	12% 11% 42% 31% 4	744
	21st to end	Night	11% 15% 52% 20% 2	628
March	1st to 10th	Night	12% 12% 43% 27% 5%	713
	11th to 20th	Night	15% 11% 42% 25% 7%	570
	21st to end	Night	7% 14% 49% 27% 4	627
April	1st to 10th	Night	6% 12% 55% 25% 2	570
	11th to 20th	Night	10% 12% 53% 23% 3	563
	21st to end	Night	8% 10% 46% 31% 4	570
May	1st to 10th	Night	9% 8% 49% 29% 5%	570
	11th to 20th	Night	9% 6% 39% 34% 11% 1	561
	21st to end	Night	6% 5% 42% 36% 10% 1	617
June	1st to 10th	Night	5% 2 31% 45% 16% 1	570
	11th to 20th	Night	2 2 38% 51% 6%	570
	21st to end	Night	5% 2 21% 52% 19% 1	570
July	1st to 10th	Night	4 12% 51% 30% 3	555
	11th to 20th	Night	5% 9% 54% 30% 1	552
	21st to end	Night	1 4 58% 34% 2	624
August	1st to 10th	Night	1 8% 57% 32% 2	567
	11th to 20th	Night	6% 1 9% 49% 33% 2	564
	21st to end	Night	3 8% 47% 38% 4	627
September	1st to 10th	Night	9% 1 14% 47% 27% 3	570
	11th to 20th	Night	12% 1 15% 46% 25% 2	570
	21st to end	Night	9% 1 18% 43% 25% 2	570
October	1st to 10th	Night	6% 3 26% 41% 22% 1	570
	11th to 20th	Night	9% 5% 36% 38% 12%	632
	21st to end	Night	9% 7% 35% 35% 14% 1	836
November	1st to 10th	Night	10% 5% 34% 38% 12% 1	955
	11th to 20th	Night	5% 8% 37% 39% 10% 1	1000
	21st to end	Night	12% 8% 37% 32% 11% 1	1000
December	1st to 10th	Night	10% 9% 34% 40% 8%	1000
	11th to 20th	Night	13% 7% 37% 35% 7%	1000
	21st to end	Night	10% 8% 40% 36% 7%	1063

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