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 2026-06-01.

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 9% 5% 5% 5% 4 5% 7% 31%	132037
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 32%	130987
Transparency	Day	51% 6% 11% 20% 11% 1	135685
	Night	51% 5% 11% 20% 12% 2	135409
Seeing	Day	22% 16% 28% 19% 11% 4	16846
	Night	20% 13% 28% 26% 12% 1	27044

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	17% 19% 6% 16% 6% 4 3 3 4 6% 15%	5258
	11th to 20th	Day	12% 20% 7% 15% 8% 6% 5% 2 3 4 16%	5280
	21st to end	Day	19% 16% 7% 12% 9% 6% 5% 3 4 5% 15%	5780
February	1st to 10th	Day	18% 18% 5% 10% 6% 4 3 3 4 5% 24%	1890
	11th to 20th	Day	17% 14% 6% 12% 6% 4 4 3 4 6% 24%	1981
	21st to end	Day	17% 14% 6% 10% 5% 3 3 2 4 6% 30%	1730
March	1st to 10th	Day	18% 14% 6% 11% 5% 4 4 3 4 4 27%	4851
	11th to 20th	Day	12% 9% 4 9% 5% 4 4 3 4 6% 40%	5090
	21st to end	Day	20% 10% 4 9% 5% 4 4 3 5% 5% 30%	5586
April	1st to 10th	Day	16% 14% 3 8% 5% 5% 4 3 4 7% 31%	2326
	11th to 20th	Day	17% 11% 3 7% 4 4 4 3 4 7% 35%	2686
	21st to end	Day	19% 8% 3 7% 6% 4 4 3 5% 7% 34%	2730
May	1st to 10th	Day	22% 10% 3 8% 6% 5% 5% 4 5% 6% 28%	5992
	11th to 20th	Day	19% 9% 3 7% 5% 5% 4 3 5% 8% 32%	5991
	21st to end	Day	18% 7% 2 6% 4 4 6% 4 6% 8% 33%	6576
June	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 5% 6% 9% 37%	2600
	11th to 20th	Day	13% 7% 3 7% 6% 5% 5% 4 6% 10% 35%	2600
	21st to end	Day	12% 6% 2 6% 5% 6% 5% 5% 8% 10% 36%	2649
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	5900
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 11% 33%	5900
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 5% 7% 9% 37%	6424
August	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 36%	4234
	11th to 20th	Day	10% 6% 2 7% 4 6% 6% 5% 8% 10% 36%	4174
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 43%	4647
September	1st to 10th	Day	13% 6% 2 5% 4 3 4 3 5% 7% 47%	2207
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	2200
	21st to end	Day	21% 6% 2 6% 4 3 4 4 4 6% 41%	2200
October	1st to 10th	Day	15% 6% 3 6% 4 3 5% 3 4 7% 45%	2115
	11th to 20th	Day	15% 9% 2 6% 4 4 4 4 5% 6% 41%	2000
	21st to end	Day	22% 10% 2 7% 4 4 4 3 4 7% 34%	2165
November	1st to 10th	Day	12% 8% 2 6% 5% 4 4 4 5% 7% 44%	1852
	11th to 20th	Day	19% 12% 4 7% 4 4 4 4 4 5% 32%	1890
	21st to end	Day	19% 16% 5% 10% 5% 3 4 3 4 5% 27%	1890
December	1st to 10th	Day	21% 14% 6% 15% 6% 4 3 2 4 4 22%	2717
	11th to 20th	Day	23% 16% 7% 11% 6% 3 3 2 4 5% 20%	3361
	21st to end	Day	25% 23% 6% 11% 5% 4 4 3 3 4 14%	4565
January	1st to 10th	Night	15% 19% 7% 18% 6% 5% 5% 3 4 3 15%	7635
	11th to 20th	Night	11% 18% 9% 16% 7% 8% 6% 3 3 4 15%	7680
	21st to end	Night	17% 16% 5% 11% 6% 6% 5% 3 4 5% 23%	8401
February	1st to 10th	Night	19% 14% 5% 10% 6% 5% 4 4 4 5% 25%	2730
	11th to 20th	Night	17% 14% 5% 11% 7% 4 4 3 5% 5% 25%	2523
	21st to end	Night	19% 15% 5% 11% 6% 4 3 2 4 5% 26%	2076
March	1st to 10th	Night	21% 13% 4 8% 4 3 3 3 4 5% 32%	5220
	11th to 20th	Night	13% 9% 3 6% 4 4 4 4 6% 7% 40%	5090
	21st to end	Night	18% 10% 3 8% 5% 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	18% 10% 3 6% 5% 4 4 4 5% 7% 35%	2310
	11th to 20th	Night	14% 8% 2 7% 4 5% 5% 4 6% 7% 36%	1885
	21st to end	Night	15% 9% 2 7% 5% 5% 4 4 5% 7% 37%	1890
May	1st to 10th	Night	26% 10% 3 7% 4 5% 4 3 5% 5% 27%	4123
	11th to 20th	Night	17% 9% 3 7% 4 4 3 3 5% 9% 36%	4122
	21st to end	Night	15% 7% 2 6% 5% 4 4 4 6% 9% 37%	4520
June	1st to 10th	Night	9% 7% 2 7% 4 4 5% 4 5% 8% 45%	1800
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 5% 7% 10% 42%	1800
	21st to end	Night	9% 5% 1 6% 5% 5% 4 5% 7% 11% 42%	1688
July	1st to 10th	Night	10% 6% 3 6% 4 4 4 3 6% 8% 44%	4015
	11th to 20th	Night	11% 5% 2 7% 5% 5% 5% 4 8% 9% 40%	4057
	21st to end	Night	8% 7% 3 7% 5% 4 5% 4 6% 8% 43%	4427
August	1st to 10th	Night	10% 7% 2 7% 6% 5% 5% 5% 8% 8% 37%	2915
	11th to 20th	Night	12% 8% 2 7% 4 4 4 3 5% 7% 44%	2875
	21st to end	Night	5% 4 2 5% 4 5% 5% 5% 7% 8% 50%	3223
September	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 7% 53%	2183
	11th to 20th	Night	8% 4 1 5% 3 3 3 3 4 8% 56%	2200
	21st to end	Night	16% 7% 2 5% 4 4 4 3 6% 6% 44%	2200
October	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 5% 7% 50%	2285
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 3 4 6% 42%	2400
	21st to end	Night	21% 8% 2 6% 3 4 4 3 5% 6% 37%	2673
November	1st to 10th	Night	14% 8% 2 5% 4 4 4 3 5% 6% 45%	2666
	11th to 20th	Night	18% 11% 3 7% 4 4 4 3 5% 6% 34%	2730
	21st to end	Night	20% 14% 5% 8% 4 4 4 3 5% 5% 27%	2730
December	1st to 10th	Night	18% 14% 6% 12% 6% 5% 4 3 5% 5% 23%	4669
	11th to 20th	Night	18% 19% 7% 11% 4 4 4 3 4 5% 21%	5025
	21st to end	Night	25% 20% 6% 10% 5% 4 4 3 3 5% 16%	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	72% 3 11% 11% 3	5259
	11th to 20th	Day	73% 2 8% 7% 9%	5280
	21st to end	Day	73% 4 7% 11% 4	5780
February	1st to 10th	Day	64% 3 10% 15% 7% 1	1890
	11th to 20th	Day	63% 5% 8% 16% 8% 1	1991
	21st to end	Day	58% 3 11% 19% 6% 2	1730
March	1st to 10th	Day	60% 4 7% 22% 7% 1	4851
	11th to 20th	Day	45% 5% 11% 27% 10% 2	5090
	21st to end	Day	55% 4 9% 20% 10% 1	5586
April	1st to 10th	Day	53% 5% 12% 16% 12% 2	2326
	11th to 20th	Day	49% 6% 12% 17% 13% 2	2686
	21st to end	Day	50% 6% 10% 22% 11% 2	2730
May	1st to 10th	Day	56% 6% 10% 16% 9% 2	6027
	11th to 20th	Day	50% 6% 12% 21% 10% 1	5995
	21st to end	Day	45% 8% 11% 22% 11% 3	6590
June	1st to 10th	Day	43% 7% 11% 23% 15% 1	2600
	11th to 20th	Day	44% 8% 14% 20% 13% 1	2600
	21st to end	Day	40% 9% 15% 20% 14% 2	2649
July	1st to 10th	Day	39% 7% 10% 28% 14% 2	5835
	11th to 20th	Day	42% 8% 13% 23% 14% 1	5831
	21st to end	Day	40% 8% 12% 19% 18% 3	6240
August	1st to 10th	Day	40% 8% 15% 24% 12%	4221
	11th to 20th	Day	39% 7% 12% 27% 13% 1	4213
	21st to end	Day	35% 9% 13% 30% 11% 3	4647
September	1st to 10th	Day	35% 7% 12% 22% 20% 3	2207
	11th to 20th	Day	34% 6% 13% 25% 19% 3	2200
	21st to end	Day	45% 4 11% 20% 19% 1	2200
October	1st to 10th	Day	28% 4 10% 33% 24% 1	3440
	11th to 20th	Day	38% 5% 10% 25% 18% 4	3240
	21st to end	Day	39% 4 12% 25% 18% 2	3479
November	1st to 10th	Day	39% 5% 12% 25% 17% 2	1852
	11th to 20th	Day	53% 6% 8% 20% 10% 3	1890
	21st to end	Day	60% 4 9% 17% 8% 1	1890
December	1st to 10th	Day	67% 3 13% 13% 3	2717
	11th to 20th	Day	68% 3 10% 13% 5%	3358
	21st to end	Day	76% 4 7% 8% 4	4565
January	1st to 10th	Night	74% 4 7% 11% 3	7636
	11th to 20th	Night	74% 4 7% 9% 6% 1	7680
	21st to end	Night	64% 3 10% 15% 6% 1	8401
February	1st to 10th	Night	61% 4 10% 17% 7% 1	2730
	11th to 20th	Night	61% 4 10% 16% 8% 1	2524
	21st to end	Night	61% 5% 9% 14% 9% 2	2076
March	1st to 10th	Night	54% 4 9% 22% 9% 1	5220
	11th to 20th	Night	41% 5% 12% 31% 10% 2	5090
	21st to end	Night	50% 4 9% 21% 14% 2	5588
April	1st to 10th	Night	48% 5% 12% 20% 13% 2	2310
	11th to 20th	Night	46% 5% 13% 16% 17% 3	1885
	21st to end	Night	45% 6% 12% 20% 15% 2	1890
May	1st to 10th	Night	58% 7% 8% 15% 9% 3	4150
	11th to 20th	Night	45% 6% 11% 24% 13% 1	4123
	21st to end	Night	41% 7% 13% 26% 10% 2	4528
June	1st to 10th	Night	35% 8% 12% 23% 19% 2	1800
	11th to 20th	Night	35% 7% 16% 24% 16% 2	1800
	21st to end	Night	34% 9% 13% 24% 18% 2	1688
July	1st to 10th	Night	36% 6% 12% 24% 18% 4	3972
	11th to 20th	Night	37% 9% 15% 26% 13% 1	4006
	21st to end	Night	36% 8% 13% 20% 20% 2	4297
August	1st to 10th	Night	40% 8% 14% 23% 13% 1	2904
	11th to 20th	Night	39% 7% 10% 26% 19% 1	2902
	21st to end	Night	29% 8% 15% 31% 16% 2	3223
September	1st to 10th	Night	32% 6% 12% 25% 21% 6%	2183
	11th to 20th	Night	27% 5% 11% 29% 25% 3	2200
	21st to end	Night	41% 5% 11% 20% 23% 1	2200
October	1st to 10th	Night	27% 4 7% 30% 30% 2	3740
	11th to 20th	Night	32% 4 12% 26% 21% 4	3899
	21st to end	Night	35% 4 8% 28% 21% 3	4309
November	1st to 10th	Night	39% 4 12% 24% 18% 3	2666
	11th to 20th	Night	50% 5% 8% 21% 13% 3	2730
	21st to end	Night	58% 4 12% 19% 6% 1	2730
December	1st to 10th	Night	63% 4 10% 16% 6%	4669
	11th to 20th	Night	65% 4 13% 13% 4	5027
	21st to end	Night	72% 4 9% 11% 4	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	30% 27% 30% 8% 4	330
	11th to 20th	Day	37% 28% 30% 5% 1	330
	21st to end	Day	33% 29% 28% 9% 1	363
February	1st to 10th	Day	32% 27% 27% 12% 2	330
	11th to 20th	Day	35% 30% 28% 6% 1	330
	21st to end	Day	29% 34% 32% 5%	273
March	1st to 10th	Day	29% 26% 29% 14% 1	445
	11th to 20th	Day	31% 22% 30% 11% 6%	540
	21st to end	Day	31% 22% 29% 15% 3	594
April	1st to 10th	Day	26% 22% 30% 14% 6% 1	540
	11th to 20th	Day	27% 23% 27% 14% 7% 3	532
	21st to end	Day	25% 19% 31% 16% 6% 2	540
May	1st to 10th	Day	29% 20% 28% 15% 6% 3	543
	11th to 20th	Day	23% 13% 32% 19% 10% 4	567
	21st to end	Day	17% 13% 26% 25% 16% 4	617
June	1st to 10th	Day	14% 11% 31% 28% 12% 5%	530
	11th to 20th	Day	16% 9% 30% 28% 14% 4	530
	21st to end	Day	13% 11% 31% 27% 13% 6%	579
July	1st to 10th	Day	14% 6% 21% 26% 21% 13%	525
	11th to 20th	Day	12% 6% 25% 29% 19% 8%	524
	21st to end	Day	10% 5% 25% 30% 21% 9%	576
August	1st to 10th	Day	11% 6% 22% 28% 23% 11%	526
	11th to 20th	Day	12% 7% 25% 27% 20% 8%	519
	21st to end	Day	8% 5% 23% 30% 25% 9%	583
September	1st to 10th	Day	12% 7% 23% 25% 22% 10%	530
	11th to 20th	Day	10% 4 17% 28% 30% 12%	530
	21st to end	Day	23% 12% 20% 24% 16% 6%	530
October	1st to 10th	Day	11% 11% 30% 23% 19% 5%	530
	11th to 20th	Day	17% 18% 31% 22% 9% 3	530
	21st to end	Day	26% 19% 32% 17% 6% 1	548
November	1st to 10th	Day	19% 21% 35% 15% 8% 2	330
	11th to 20th	Day	32% 17% 30% 15% 5% 1	330
	21st to end	Day	32% 24% 35% 8% 1	330
December	1st to 10th	Day	31% 27% 29% 8% 3 1	232
	11th to 20th	Day	29% 30% 30% 9% 3	297
	21st to end	Day	37% 23% 29% 10% 2	363
January	1st to 10th	Night	27% 23% 33% 15% 3	840
	11th to 20th	Night	29% 24% 31% 14% 2	840
	21st to end	Night	26% 24% 31% 18% 2	924
February	1st to 10th	Night	27% 19% 35% 18% 2	840
	11th to 20th	Night	28% 21% 32% 17% 1	823
	21st to end	Night	28% 19% 35% 16% 2	692
March	1st to 10th	Night	24% 20% 31% 21% 4	840
	11th to 20th	Night	24% 20% 30% 19% 6%	840
	21st to end	Night	26% 16% 32% 20% 5% 1	924
April	1st to 10th	Night	23% 17% 35% 20% 5%	840
	11th to 20th	Night	19% 16% 33% 22% 8% 2	625
	21st to end	Night	19% 11% 38% 24% 7% 1	630
May	1st to 10th	Night	23% 10% 30% 27% 10% 1	630
	11th to 20th	Night	19% 9% 25% 33% 13% 1	621
	21st to end	Night	11% 5% 26% 34% 21% 2	683
June	1st to 10th	Night	10% 5% 29% 38% 18% 1	600
	11th to 20th	Night	12% 6% 23% 41% 17% 2	600
	21st to end	Night	10% 6% 26% 36% 22% 2	600
July	1st to 10th	Night	10% 4 19% 36% 26% 6%	585
	11th to 20th	Night	7% 3 21% 36% 29% 5%	582
	21st to end	Night	9% 3 18% 39% 30% 2	657
August	1st to 10th	Night	8% 3 16% 42% 28% 4	597
	11th to 20th	Night	10% 4 15% 40% 27% 4	594
	21st to end	Night	7% 3 20% 33% 32% 6%	660
September	1st to 10th	Night	13% 4 20% 32% 27% 5%	793
	11th to 20th	Night	10% 3 14% 37% 32% 4	800
	21st to end	Night	18% 8% 23% 29% 19% 3	800
October	1st to 10th	Night	13% 7% 26% 30% 22% 3	800
	11th to 20th	Night	20% 12% 31% 26% 11% 1	800
	21st to end	Night	25% 14% 30% 24% 8%	880
November	1st to 10th	Night	18% 13% 31% 25% 12% 1	820
	11th to 20th	Night	26% 16% 30% 21% 7% 1	840
	21st to end	Night	29% 17% 28% 20% 6%	840
December	1st to 10th	Night	31% 18% 32% 17% 4	840
	11th to 20th	Night	26% 19% 33% 18% 3	840
	21st to end	Night	30% 18% 30% 19% 3	924

Page generated at 2026-05-29 01:53:30. Version ac07

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