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 2025-02-06.

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% 31%	108728
	Night	16% 11% 4 9% 5% 4 4 3 5% 6% 33%	109418
Transparency	Day	51% 6% 11% 20% 11% 1	112420
	Night	50% 5% 11% 20% 12% 2	113865
Seeing	Day	21% 16% 28% 20% 12% 4	15089
	Night	20% 13% 28% 26% 12% 1	25319

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	15% 20% 7% 18% 7% 4 3 3 4 5% 13%	4056
	11th to 20th	Day	14% 23% 6% 14% 6% 7% 6% 2 3 4 16%	4060
	21st to end	Day	21% 19% 9% 11% 8% 6% 5% 3 3 4 13%	4466
February	1st to 10th	Day	20% 18% 5% 9% 6% 4 3 3 4 4 24%	1755
	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	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 36%	4104
	11th to 20th	Day	10% 6% 2 7% 4 6% 6% 5% 8% 9% 36%	4044
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 9% 42%	4505
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	2097
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	2090
	21st to end	Day	21% 6% 2 6% 4 3 3 4 4 6% 41%	2090
October	1st to 10th	Day	15% 6% 3 6% 5% 4 5% 3 4 7% 44%	2009
	11th to 20th	Day	15% 9% 2 6% 4 4 4 4 5% 6% 40%	1900
	21st to end	Day	21% 10% 2 7% 4 4 4 3 4 6% 34%	2057
November	1st to 10th	Day	11% 8% 2 6% 5% 4 4 4 5% 7% 45%	1762
	11th to 20th	Day	19% 12% 4 7% 5% 4 4 4 4 5% 32%	1800
	21st to end	Day	19% 16% 4 9% 5% 3 4 3 4 5% 27%	1800
December	1st to 10th	Day	21% 13% 6% 15% 6% 4 3 2 4 4 23%	2636
	11th to 20th	Day	24% 16% 7% 10% 6% 3 3 2 4 5% 20%	3274
	21st to end	Day	25% 24% 6% 11% 5% 4 4 3 3 4 14%	4466
January	1st to 10th	Night	15% 20% 8% 20% 5% 4 4 3 3 3 15%	5889
	11th to 20th	Night	13% 20% 8% 15% 7% 8% 6% 3 3 4 13%	5900
	21st to end	Night	21% 17% 6% 12% 6% 3 4 2 4 4 22%	6490
February	1st to 10th	Night	20% 15% 5% 10% 5% 5% 4 4 4 5% 24%	2536
	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	10% 7% 2 7% 6% 5% 5% 5% 8% 8% 36%	2825
	11th to 20th	Night	12% 8% 2 8% 4 4 4 3 5% 7% 44%	2785
	21st to end	Night	5% 4 2 5% 4 5% 5% 5% 7% 8% 50%	3124
September	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 4 7% 52%	2073
	11th to 20th	Night	9% 4 2 5% 3 3 3 3 4 8% 56%	2090
	21st to end	Night	16% 7% 2 5% 4 4 3 3 6% 6% 44%	2090
October	1st to 10th	Night	11% 6% 2 5% 4 3 5% 3 5% 7% 49%	2171
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 4 6% 41%	2280
	21st to end	Night	21% 9% 2 6% 4 4 4 3 4 6% 36%	2539
November	1st to 10th	Night	14% 8% 2 5% 4 4 4 3 5% 6% 45%	2536
	11th to 20th	Night	18% 12% 3 7% 4 4 4 4 5% 6% 34%	2600
	21st to end	Night	20% 14% 4 8% 4 4 4 4 5% 5% 28%	2600
December	1st to 10th	Night	18% 14% 6% 12% 6% 5% 4 3 5% 5% 23%	4531
	11th to 20th	Night	19% 19% 7% 11% 4 4 4 3 4 5% 21%	4893
	21st to end	Night	25% 20% 6% 10% 5% 4 4 3 3 5% 16%	6490

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	74% 4 7% 11% 4	4057
	11th to 20th	Day	75% 3 8% 7% 7%	4060
	21st to end	Day	76% 4 6% 8% 5%	4466
February	1st to 10th	Day	64% 4 10% 14% 7% 1	1755
	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	41% 8% 15% 24% 12%	4091
	11th to 20th	Day	40% 7% 12% 27% 14% 1	4083
	21st to end	Day	35% 9% 13% 29% 11% 3	4505
September	1st to 10th	Day	36% 7% 12% 21% 20% 3	2097
	11th to 20th	Day	35% 6% 14% 24% 18% 3	2090
	21st to end	Day	45% 4 11% 21% 18% 1	2090
October	1st to 10th	Day	28% 4 11% 33% 23% 1	3334
	11th to 20th	Day	38% 5% 10% 25% 18% 4	3140
	21st to end	Day	39% 4 13% 25% 18% 1	3371
November	1st to 10th	Day	39% 5% 12% 26% 17% 2	1762
	11th to 20th	Day	54% 6% 8% 20% 10% 2	1800
	21st to end	Day	60% 5% 9% 17% 8% 1	1800
December	1st to 10th	Day	66% 4 13% 14% 3	2636
	11th to 20th	Day	68% 3 11% 13% 5%	3271
	21st to end	Day	76% 4 8% 8% 4	4466
January	1st to 10th	Night	74% 4 6% 12% 4	5889
	11th to 20th	Night	75% 4 8% 8% 5%	5900
	21st to end	Night	67% 3 10% 13% 5% 2	6490
February	1st to 10th	Night	62% 4 10% 16% 7% 1	2536
	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	41% 8% 14% 22% 13% 1	2814
	11th to 20th	Night	39% 7% 10% 25% 19% 1	2812
	21st to end	Night	28% 8% 15% 30% 16% 2	3124
September	1st to 10th	Night	33% 6% 12% 24% 20% 6%	2073
	11th to 20th	Night	28% 5% 11% 27% 25% 3	2090
	21st to end	Night	40% 5% 11% 20% 22% 1	2090
October	1st to 10th	Night	28% 5% 7% 30% 30% 2	3626
	11th to 20th	Night	32% 4 12% 27% 20% 4	3779
	21st to end	Night	36% 4 8% 28% 21% 3	4175
November	1st to 10th	Night	39% 4 12% 23% 18% 3	2536
	11th to 20th	Night	50% 5% 8% 21% 13% 3	2600
	21st to end	Night	57% 4 12% 19% 7% 1	2600
December	1st to 10th	Night	63% 4 11% 17% 5%	4531
	11th to 20th	Night	65% 4 13% 13% 4	4895
	21st to end	Night	73% 4 9% 11% 4	6490

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	30% 27% 30% 8% 4	300
	11th to 20th	Day	34% 28% 31% 6% 1	300
	21st to end	Day	31% 30% 28% 9% 1	330
February	1st to 10th	Day	35% 25% 27% 11% 2	285
	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% 26% 10%	539
September	1st to 10th	Day	12% 6% 24% 26% 22% 9%	490
	11th to 20th	Day	10% 4 17% 28% 29% 12%	490
	21st to end	Day	24% 12% 19% 24% 15% 6%	490
October	1st to 10th	Day	11% 11% 31% 23% 18% 5%	490
	11th to 20th	Day	18% 18% 31% 21% 10% 3	490
	21st to end	Day	26% 18% 31% 17% 7% 1	506
November	1st to 10th	Day	16% 22% 35% 16% 9% 2	300
	11th to 20th	Day	30% 17% 30% 16% 6% 1	300
	21st to end	Day	30% 24% 36% 8% 1	300
December	1st to 10th	Day	31% 27% 29% 8% 4 1	211
	11th to 20th	Day	28% 29% 31% 9% 3	270
	21st to end	Day	35% 23% 29% 11% 2	330
January	1st to 10th	Night	27% 24% 32% 14% 3	800
	11th to 20th	Night	29% 23% 32% 14% 2	800
	21st to end	Night	26% 23% 30% 18% 2	880
February	1st to 10th	Night	28% 18% 35% 17% 2	781
	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 19% 33% 32% 6%	627
September	1st to 10th	Night	13% 5% 20% 31% 26% 5%	753
	11th to 20th	Night	10% 3 15% 37% 31% 4	760
	21st to end	Night	19% 8% 24% 28% 19% 3	760
October	1st to 10th	Night	14% 7% 26% 31% 20% 3	760
	11th to 20th	Night	21% 12% 31% 25% 11% 1	760
	21st to end	Night	25% 14% 29% 24% 8%	836
November	1st to 10th	Night	18% 13% 30% 25% 13% 1	780
	11th to 20th	Night	25% 16% 30% 22% 7% 1	800
	21st to end	Night	28% 16% 29% 21% 6%	800
December	1st to 10th	Night	31% 17% 31% 17% 4	800
	11th to 20th	Night	27% 19% 33% 18% 3	800
	21st to end	Night	30% 18% 30% 20% 3	880

Page generated at 2025-02-03 03:05:54. Version ac07

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