Miller Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Miller Observatory clear sky chart from 2005-11-06 to 2024-08-30.

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	16% 11% 4 8% 5% 4 5% 4 5% 7% 32%	95291
	Night	16% 11% 4 8% 5% 4 4 3 5% 6% 34%	93600
Transparency	Day	51% 6% 11% 19% 11% 1	95065
	Night	50% 6% 11% 20% 12% 2	93430
Seeing	Day	21% 16% 28% 20% 12% 4	14561
	Night	20% 13% 28% 26% 12% 1	24683

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	19% 20% 6% 12% 5% 4 3 3 5% 6% 16%	2836
	11th to 20th	Day	15% 23% 7% 12% 5% 7% 6% 2 3 4 15%	2840
	21st to end	Day	27% 21% 10% 10% 6% 4 4 2 2 4 11%	3124
February	1st to 10th	Day	19% 18% 5% 10% 6% 4 3 3 4 4 24%	1710
	11th to 20th	Day	16% 14% 6% 13% 6% 4 4 3 4 6% 24%	1787
	21st to end	Day	18% 15% 6% 9% 5% 3 3 3 4 5% 29%	1570
March	1st to 10th	Day	21% 15% 6% 9% 5% 4 4 3 4 5% 23%	3302
	11th to 20th	Day	13% 10% 4 9% 5% 4 4 4 4 6% 36%	3480
	21st to end	Day	22% 10% 4 8% 4 4 4 3 4 6% 32%	3815
April	1st to 10th	Day	16% 14% 3 7% 5% 4 4 3 4 7% 32%	2104
	11th to 20th	Day	18% 11% 3 7% 4 4 4 3 4 6% 35%	2428
	21st to end	Day	20% 8% 3 7% 6% 4 4 3 5% 7% 32%	2470
May	1st to 10th	Day	22% 10% 3 8% 6% 4 5% 4 4 6% 29%	4082
	11th to 20th	Day	21% 9% 3 6% 4 4 3 3 5% 7% 34%	4081
	21st to end	Day	12% 7% 3 7% 5% 5% 7% 5% 7% 9% 35%	4476
June	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 5% 6% 9% 36%	2470
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 10% 36%	2470
	21st to end	Day	12% 6% 2 6% 5% 6% 6% 5% 8% 10% 36%	2516
July	1st to 10th	Day	11% 7% 3 7% 6% 5% 6% 4 7% 8% 37%	4120
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 8% 11% 41%	4120
	21st to end	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 9% 36%	4466
August	1st to 10th	Day	10% 5% 2 6% 6% 6% 6% 5% 7% 10% 36%	2457
	11th to 20th	Day	11% 7% 2 7% 4 6% 6% 5% 8% 10% 35%	2433
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 4 7% 9% 41%	2680
September	1st to 10th	Day	14% 6% 2 6% 4 4 5% 3 5% 7% 44%	1987
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 44%	1980
	21st to end	Day	20% 5% 2 6% 4 4 4 4 4 6% 43%	1980
October	1st to 10th	Day	16% 6% 3 7% 5% 4 5% 3 4 7% 41%	1904
	11th to 20th	Day	16% 9% 2 6% 4 4 4 4 5% 6% 39%	1800
	21st to end	Day	22% 10% 2 7% 4 4 4 3 4 6% 33%	1949
November	1st to 10th	Day	11% 8% 2 6% 5% 4 4 4 5% 7% 45%	1672
	11th to 20th	Day	18% 12% 4 8% 4 4 4 3 4 5% 33%	1710
	21st to end	Day	19% 14% 4 9% 5% 4 5% 3 4 5% 28%	1710
December	1st to 10th	Day	21% 19% 7% 10% 6% 4 3 2 5% 5% 19%	1542
	11th to 20th	Day	19% 16% 7% 12% 6% 3 4 3 5% 6% 20%	2096
	21st to end	Day	25% 25% 6% 10% 5% 3 4 3 3 4 12%	3124
January	1st to 10th	Night	16% 21% 8% 12% 5% 5% 4 3 4 4 19%	4109
	11th to 20th	Night	15% 19% 7% 16% 5% 5% 7% 4 3 4 14%	4120
	21st to end	Night	26% 20% 8% 13% 5% 3 3 2 3 4 15%	4532
February	1st to 10th	Night	20% 15% 5% 10% 5% 4 4 4 4 5% 25%	2470
	11th to 20th	Night	17% 14% 4 9% 7% 4 4 3 5% 6% 26%	2277
	21st to end	Night	20% 15% 5% 10% 6% 4 3 2 4 5% 25%	1884
March	1st to 10th	Night	22% 14% 4 7% 4 4 3 3 4 5% 31%	3550
	11th to 20th	Night	14% 10% 3 6% 5% 4 4 4 5% 6% 39%	3480
	21st to end	Night	18% 10% 3 7% 4 4 4 3 4 6% 38%	3817
April	1st to 10th	Night	17% 10% 3 6% 5% 4 4 4 5% 7% 36%	2090
	11th to 20th	Night	15% 8% 2 7% 4 5% 4 4 6% 7% 37%	1703
	21st to end	Night	14% 9% 2 7% 5% 5% 4 4 5% 7% 37%	1710
May	1st to 10th	Night	26% 10% 3 7% 4 5% 4 3 4 5% 27%	2813
	11th to 20th	Night	18% 8% 2 7% 4 4 3 3 4 8% 39%	2812
	21st to end	Night	9% 6% 2 5% 4 4 4 4 7% 11% 42%	3079
June	1st to 10th	Night	9% 7% 2 7% 4 4 5% 4 5% 9% 45%	1710
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 5% 7% 10% 43%	1710
	21st to end	Night	10% 5% 1 6% 5% 5% 4 5% 7% 10% 42%	1604
July	1st to 10th	Night	11% 6% 3 6% 4 4 4 3 6% 8% 45%	2810
	11th to 20th	Night	9% 3 2 5% 4 4 5% 4 8% 9% 47%	2840
	21st to end	Night	8% 6% 3 7% 5% 4 5% 5% 6% 9% 43%	3087
August	1st to 10th	Night	6% 5% 2 5% 5% 5% 5% 5% 9% 9% 43%	1701
	11th to 20th	Night	9% 7% 2 7% 4 5% 4 3 6% 9% 45%	1688
	21st to end	Night	7% 5% 2 5% 3 4 4 4 6% 8% 52%	1872
September	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 4 7% 50%	1963
	11th to 20th	Night	9% 4 1 5% 3 3 3 3 4 8% 55%	1980
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 6% 46%	1980
October	1st to 10th	Night	12% 6% 2 5% 4 3 5% 3 5% 7% 48%	2056
	11th to 20th	Night	16% 8% 3 6% 5% 4 4 3 4 6% 40%	2160
	21st to end	Night	22% 9% 2 6% 4 4 4 3 4 6% 35%	2405
November	1st to 10th	Night	14% 8% 2 5% 4 4 4 3 5% 6% 45%	2406
	11th to 20th	Night	18% 11% 3 7% 4 4 3 4 5% 6% 34%	2470
	21st to end	Night	20% 13% 4 8% 5% 4 4 4 5% 5% 28%	2470
December	1st to 10th	Night	22% 16% 6% 9% 5% 4 4 3 4 5% 22%	2625
	11th to 20th	Night	17% 15% 6% 9% 5% 5% 4 3 5% 6% 25%	3085
	21st to end	Night	24% 20% 7% 10% 5% 4 4 3 4 4 15%	4532

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	69% 4 8% 13% 5%	2837
	11th to 20th	Day	74% 3 9% 10% 4	2840
	21st to end	Day	79% 4 6% 7% 2	3124
February	1st to 10th	Day	65% 4 10% 14% 8% 1	1710
	11th to 20th	Day	62% 5% 8% 15% 8% 1	1797
	21st to end	Day	59% 3 11% 18% 7% 2	1570
March	1st to 10th	Day	62% 5% 8% 19% 5%	3302
	11th to 20th	Day	48% 6% 10% 26% 10% 1	3480
	21st to end	Day	53% 5% 10% 21% 10%	3815
April	1st to 10th	Day	53% 5% 12% 16% 12% 2	2104
	11th to 20th	Day	49% 6% 12% 17% 14% 3	2428
	21st to end	Day	51% 6% 10% 20% 11% 2	2470
May	1st to 10th	Day	55% 6% 12% 19% 7% 1	4120
	11th to 20th	Day	49% 6% 13% 21% 9% 1	4085
	21st to end	Day	40% 10% 13% 21% 12% 4	4489
June	1st to 10th	Day	43% 7% 12% 23% 14% 1	2470
	11th to 20th	Day	43% 8% 14% 21% 13% 1	2470
	21st to end	Day	40% 9% 15% 20% 14% 2	2516
July	1st to 10th	Day	42% 8% 11% 22% 15% 2	4055
	11th to 20th	Day	35% 7% 12% 26% 19% 1	4051
	21st to end	Day	41% 8% 13% 17% 18% 3	4324
August	1st to 10th	Day	38% 9% 16% 24% 13%	2444
	11th to 20th	Day	41% 8% 15% 24% 11% 1	2433
	21st to end	Day	36% 9% 14% 23% 15% 4	2680
September	1st to 10th	Day	38% 8% 12% 21% 18% 3	1987
	11th to 20th	Day	36% 6% 14% 24% 17% 3	1980
	21st to end	Day	43% 4 11% 22% 19% 1	1980
October	1st to 10th	Day	43% 6% 11% 22% 17% 1	1904
	11th to 20th	Day	45% 5% 13% 22% 14% 1	1800
	21st to end	Day	52% 5% 13% 18% 10% 1	1949
November	1st to 10th	Day	39% 5% 12% 25% 17% 2	1672
	11th to 20th	Day	53% 5% 8% 20% 10% 3	1710
	21st to end	Day	58% 5% 10% 18% 9% 1	1710
December	1st to 10th	Day	68% 5% 9% 14% 3	1542
	11th to 20th	Day	65% 4 10% 16% 5%	2093
	21st to end	Day	77% 4 9% 6% 3	3124
January	1st to 10th	Night	69% 4 8% 13% 5%	4109
	11th to 20th	Night	74% 5% 9% 10% 3	4120
	21st to end	Night	76% 3 6% 11% 3	4532
February	1st to 10th	Night	62% 4 10% 16% 8% 1	2470
	11th to 20th	Night	60% 4 10% 16% 9% 1	2278
	21st to end	Night	61% 5% 9% 14% 9% 2	1884
March	1st to 10th	Night	55% 5% 10% 21% 9% 1	3550
	11th to 20th	Night	44% 5% 12% 29% 9% 2	3480
	21st to end	Night	48% 4 10% 26% 11% 1	3817
April	1st to 10th	Night	47% 5% 12% 20% 14% 2	2090
	11th to 20th	Night	46% 5% 12% 16% 18% 3	1703
	21st to end	Night	45% 6% 13% 18% 15% 2	1710
May	1st to 10th	Night	57% 7% 8% 18% 8% 2	2840
	11th to 20th	Night	44% 6% 12% 25% 11% 1	2813
	21st to end	Night	33% 9% 15% 27% 14% 3	3087
June	1st to 10th	Night	35% 9% 12% 23% 19% 2	1710
	11th to 20th	Night	33% 7% 16% 24% 16% 2	1710
	21st to end	Night	35% 9% 13% 24% 17% 2	1604
July	1st to 10th	Night	37% 6% 12% 22% 17% 5%	2765
	11th to 20th	Night	31% 7% 14% 31% 17% 1	2786
	21st to end	Night	36% 9% 13% 21% 18% 3	2988
August	1st to 10th	Night	32% 9% 15% 26% 17% 1	1690
	11th to 20th	Night	35% 9% 14% 24% 17% 1	1688
	21st to end	Night	27% 8% 17% 26% 19% 4	1872
September	1st to 10th	Night	34% 6% 13% 23% 18% 6%	1963
	11th to 20th	Night	29% 5% 12% 28% 25% 2	1980
	21st to end	Night	38% 5% 11% 21% 24% 2	1980
October	1st to 10th	Night	34% 7% 11% 25% 21% 2	2056
	11th to 20th	Night	44% 6% 10% 24% 14% 1	2160
	21st to end	Night	51% 4 9% 21% 12% 2	2405
November	1st to 10th	Night	40% 4 13% 23% 18% 3	2406
	11th to 20th	Night	50% 5% 8% 21% 13% 2	2470
	21st to end	Night	56% 4 12% 19% 7% 1	2470
December	1st to 10th	Night	65% 5% 9% 16% 5%	2625
	11th to 20th	Night	59% 6% 11% 19% 5%	3087
	21st to end	Night	73% 4 10% 10% 3	4532

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	29% 27% 30% 9% 5%	270
	11th to 20th	Day	33% 27% 33% 6% 1	270
	21st to end	Day	31% 31% 28% 9% 1	297
February	1st to 10th	Day	35% 25% 27% 11% 2	270
	11th to 20th	Day	31% 31% 30% 7% 1	270
	21st to end	Day	30% 36% 28% 5%	225
March	1st to 10th	Day	29% 25% 29% 16% 2	374
	11th to 20th	Day	29% 22% 32% 12% 5%	460
	21st to end	Day	31% 20% 29% 16% 3	506
April	1st to 10th	Day	25% 21% 29% 16% 7% 1	460
	11th to 20th	Day	27% 21% 27% 14% 7% 3	452
	21st to end	Day	27% 19% 32% 15% 5% 2	460
May	1st to 10th	Day	27% 20% 29% 14% 7% 3	463
	11th to 20th	Day	22% 14% 30% 19% 11% 4	487
	21st to end	Day	13% 12% 26% 27% 18% 5%	529
June	1st to 10th	Day	13% 10% 31% 28% 13% 5%	490
	11th to 20th	Day	16% 9% 30% 28% 14% 4	490
	21st to end	Day	14% 11% 31% 26% 12% 6%	536
July	1st to 10th	Day	14% 5% 21% 26% 20% 14%	485
	11th to 20th	Day	11% 7% 25% 29% 19% 8%	484
	21st to end	Day	11% 5% 26% 30% 21% 8%	532
August	1st to 10th	Day	12% 6% 21% 29% 22% 11%	486
	11th to 20th	Day	13% 7% 27% 27% 20% 8%	479
	21st to end	Day	8% 5% 22% 30% 25% 10%	532
September	1st to 10th	Day	13% 6% 24% 25% 22% 10%	450
	11th to 20th	Day	11% 4 17% 28% 28% 12%	450
	21st to end	Day	22% 11% 20% 26% 16% 6%	450
October	1st to 10th	Day	12% 11% 31% 22% 18% 5%	450
	11th to 20th	Day	18% 19% 31% 21% 9% 1	450
	21st to end	Day	28% 18% 31% 16% 6% 1	464
November	1st to 10th	Day	16% 22% 34% 16% 10% 2	270
	11th to 20th	Day	30% 17% 30% 16% 6% 1	270
	21st to end	Day	29% 24% 37% 9% 1	270
December	1st to 10th	Day	31% 26% 30% 7% 4 2	190
	11th to 20th	Day	25% 31% 32% 9% 3	243
	21st to end	Day	34% 25% 29% 11% 2	297
January	1st to 10th	Night	27% 23% 32% 15% 3	760
	11th to 20th	Night	29% 22% 32% 15% 2	760
	21st to end	Night	25% 23% 30% 19% 2	836
February	1st to 10th	Night	28% 18% 35% 17% 2	760
	11th to 20th	Night	27% 21% 33% 17% 2	743
	21st to end	Night	29% 19% 34% 16% 2	628
March	1st to 10th	Night	23% 20% 30% 23% 4	760
	11th to 20th	Night	23% 20% 30% 20% 6%	760
	21st to end	Night	26% 15% 32% 21% 6% 1	836
April	1st to 10th	Night	21% 17% 35% 21% 5%	760
	11th to 20th	Night	20% 14% 33% 22% 9% 2	565
	21st to end	Night	19% 11% 39% 24% 6% 1	570
May	1st to 10th	Night	22% 10% 30% 27% 10% 1	570
	11th to 20th	Night	19% 8% 25% 33% 14% 1	561
	21st to end	Night	9% 5% 25% 35% 23% 2	617
June	1st to 10th	Night	9% 5% 29% 38% 18% 1	570
	11th to 20th	Night	10% 5% 24% 41% 18% 2	570
	21st to end	Night	10% 6% 26% 36% 21% 1	570
July	1st to 10th	Night	10% 4 19% 36% 25% 6%	555
	11th to 20th	Night	7% 3 21% 37% 28% 5%	552
	21st to end	Night	9% 3 18% 39% 29% 2	624
August	1st to 10th	Night	8% 2 16% 41% 28% 4	567
	11th to 20th	Night	10% 4 15% 40% 26% 4	564
	21st to end	Night	7% 3 20% 33% 32% 6%	624
September	1st to 10th	Night	14% 4 20% 30% 27% 5%	713
	11th to 20th	Night	11% 3 16% 35% 31% 5%	720
	21st to end	Night	17% 8% 24% 28% 20% 3	720
October	1st to 10th	Night	14% 7% 26% 31% 20% 3	720
	11th to 20th	Night	21% 12% 32% 25% 10%	720
	21st to end	Night	27% 14% 28% 24% 7%	792
November	1st to 10th	Night	18% 13% 30% 24% 13% 1	740
	11th to 20th	Night	25% 16% 30% 22% 8% 1	760
	21st to end	Night	27% 16% 29% 21% 7%	760
December	1st to 10th	Night	32% 17% 31% 16% 4	760
	11th to 20th	Night	25% 19% 34% 18% 4	760
	21st to end	Night	29% 19% 30% 19% 3	836

Page generated at 2024-08-27 09:49:44. Version ac07

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