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 2026-04-02.

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	3 2 1 3 3 3 3 3 5% 7% 66%	139860
	Night	3 3 1 3 3 3 4 3 5% 8% 65%	139447
Transparency	Day	17% 5% 8% 19% 26% 25%	134974
	Night	18% 5% 9% 19% 24% 25%	135338
Seeing	Day	5% 7% 22% 31% 26% 9%	16284
	Night	5% 4 21% 44% 23% 2	26551

By Month

Percentage Cloud Cover SW (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	2 2 1 3 2 2 2 2 4 6% 74%	5885
	11th to 20th	Day	3 3 1 3 2 2 3 2 4 5% 73%	5910
	21st to end	Day	4 4 2 5% 4 3 4 3 4 6% 63%	6468
February	1st to 10th	Day	3 4 2 3 3 4 3 3 5% 5% 65%	2100
	11th to 20th	Day	4 4 1 4 3 3 4 3 4 7% 64%	2049
	21st to end	Day	3 3 1 3 2 2 3 3 6% 7% 67%	1730
March	1st to 10th	Day	3 4 2 4 3 3 3 3 5% 7% 65%	4600
	11th to 20th	Day	5% 3 1 3 2 3 3 3 5% 6% 67%	4861
	21st to end	Day	2 4 1 4 3 3 4 3 5% 7% 65%	6101
April	1st to 10th	Day	2 4 1 3 3 3 3 3 6% 8% 66%	2413
	11th to 20th	Day	2 3 1 3 2 3 3 3 5% 5% 68%	2371
	21st to end	Day	4 2 1 3 2 3 2 2 5% 6% 70%	2400
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	5440
	11th to 20th	Day	3 2 1 3 3 3 4 3 5% 7% 66%	5405
	21st to end	Day	3 2 1 2 2 2 4 4 5% 7% 69%	6346
June	1st to 10th	Day	1 1 1 3 2 2 4 4 6% 9% 67%	2600
	11th to 20th	Day	3 2 1 3 3 2 3 3 5% 8% 67%	2600
	21st to end	Day	3 3 1 4 4 4 4 4 7% 9% 58%	2591
July	1st to 10th	Day	4 2 1 4 3 4 5% 5% 6% 9% 57%	5439
	11th to 20th	Day	2 2 1 2 3 4 5% 5% 8% 11% 59%	5440
	21st to end	Day	2 2 1 4 3 4 4 4 7% 11% 57%	5922
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 10% 70%	5348
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 8% 11% 52%	5366
	21st to end	Day	2 2 1 4 3 4 4 4 6% 9% 60%	5984
September	1st to 10th	Day	6% 5% 2 7% 4 5% 5% 4 7% 10% 46%	2400
	11th to 20th	Day	7% 5% 1 5% 4 4 6% 4 6% 8% 51%	2255
	21st to end	Day	5% 3 1 4 4 4 4 4 5% 7% 59%	2200
October	1st to 10th	Day	1 1 1 2 1 2 2 2 4 5% 80%	3590
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 5% 73%	3447
	21st to end	Day	3 1 1 1 1 1 1 2 4 83%	3548
November	1st to 10th	Day	5% 2 1 2 2 1 1 2 3 5% 76%	1985
	11th to 20th	Day	4 3 1 2 3 2 3 2 4 5% 70%	1890
	21st to end	Day	5% 4 2 4 3 3 4 2 4 6% 63%	1890
December	1st to 10th	Day	4 3 2 6% 6% 5% 4 3 5% 7% 56%	3030
	11th to 20th	Day	1 2 2 4 3 3 4 3 4 5% 70%	3481
	21st to end	Day	2 2 1 2 2 2 2 3 4 5% 75%	4775
January	1st to 10th	Night	2 2 1 3 2 2 3 3 4 6% 73%	7038
	11th to 20th	Night	3 2 1 3 2 2 2 2 4 5% 73%	7080
	21st to end	Night	5% 4 2 5% 4 4 4 3 5% 6% 57%	7746
February	1st to 10th	Night	3 4 1 4 3 3 4 3 4 7% 64%	2520
	11th to 20th	Night	4 4 1 4 3 3 4 4 5% 7% 59%	2458
	21st to end	Night	4 4 1 3 2 3 4 3 5% 6% 65%	2076
March	1st to 10th	Night	3 4 1 4 2 3 4 3 5% 7% 64%	5494
	11th to 20th	Night	4 3 1 3 2 3 3 3 5% 7% 68%	5343
	21st to end	Night	3 3 1 3 2 3 4 3 5% 6% 66%	5094
April	1st to 10th	Night	3 4 1 3 3 3 4 4 5% 7% 63%	2010
	11th to 20th	Night	3 2 1 3 2 4 4 3 5% 6% 67%	1977
	21st to end	Night	3 2 1 2 3 3 3 3 4 6% 70%	2000
May	1st to 10th	Night	2 1 1 1 2 2 3 2 3 6% 77%	4540
	11th to 20th	Night	4 4 2 4 3 4 4 4 5% 6% 60%	4510
	21st to end	Night	3 2 1 3 3 4 4 4 7% 8% 61%	4543
June	1st to 10th	Night	2 2 1 3 4 3 5% 4 8% 11% 56%	1800
	11th to 20th	Night	2 2 1 4 4 4 5% 5% 7% 10% 56%	1800
	21st to end	Night	3 4 2 4 3 4 4 4 7% 9% 57%	1801
July	1st to 10th	Night	2 2 1 4 4 4 5% 5% 8% 12% 53%	4540
	11th to 20th	Night	1 1 1 3 3 3 4 4 7% 10% 63%	4540
	21st to end	Night	2 2 1 4 4 5% 6% 5% 8% 12% 52%	4951
August	1st to 10th	Night	1 1 1 2 2 3 3 6% 11% 68%	4464
	11th to 20th	Night	1 1 1 3 2 3 4 5% 9% 15% 55%	4483
	21st to end	Night	2 2 1 2 3 3 4 4 8% 11% 60%	4993
September	1st to 10th	Night	5% 4 2 6% 5% 4 5% 3 7% 9% 50%	2000
	11th to 20th	Night	5% 3 2 6% 4 4 6% 5% 8% 10% 48%	2145
	21st to end	Night	4 3 1 4 4 5% 5% 5% 6% 10% 53%	2200
October	1st to 10th	Night	2 1 2 2 2 3 2 3 6% 78%	3590
	11th to 20th	Night	4 2 1 3 2 3 3 2 4 6% 71%	3690
	21st to end	Night	2 2 1 2 1 2 2 2 2 4 81%	4252
November	1st to 10th	Night	4 2 1 2 1 2 2 2 4 5% 75%	2509
	11th to 20th	Night	4 3 1 3 2 2 2 2 5% 6% 69%	2730
	21st to end	Night	5% 3 1 4 3 3 3 3 5% 7% 64%	2730
December	1st to 10th	Night	3 3 2 8% 6% 6% 5% 3 6% 7% 51%	4380
	11th to 20th	Night	2 3 1 4 3 3 3 3 4 6% 70%	4946
	21st to end	Night	2 2 1 3 2 2 2 2 4 6% 72%	6474

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	13% 3 4 24% 30% 27%	5888
	11th to 20th	Day	16% 2 4 16% 35% 26%	5910
	21st to end	Day	24% 3 4 18% 30% 20%	6468
February	1st to 10th	Day	21% 3 5% 13% 25% 33%	2100
	11th to 20th	Day	22% 4 6% 14% 25% 29%	2059
	21st to end	Day	16% 4 4 12% 29% 35%	1730
March	1st to 10th	Day	19% 4 3 8% 22% 44%	4600
	11th to 20th	Day	18% 4 4 9% 20% 45%	4861
	21st to end	Day	17% 4 4 9% 26% 40%	6101
April	1st to 10th	Day	17% 4 6% 12% 24% 38%	2413
	11th to 20th	Day	16% 4 6% 13% 22% 38%	2374
	21st to end	Day	16% 3 5% 13% 25% 38%	2400
May	1st to 10th	Day	11% 3 4 14% 24% 44%	5440
	11th to 20th	Day	16% 4 7% 12% 25% 36%	5408
	21st to end	Day	13% 6% 8% 18% 20% 35%	6347
June	1st to 10th	Day	12% 6% 12% 25% 28% 19%	2600
	11th to 20th	Day	16% 5% 7% 18% 32% 22%	2600
	21st to end	Day	21% 6% 14% 23% 22% 14%	2591
July	1st to 10th	Day	22% 8% 18% 30% 20% 3	5381
	11th to 20th	Day	16% 8% 17% 38% 16% 4	5377
	21st to end	Day	19% 8% 18% 27% 22% 6%	5752
August	1st to 10th	Day	14% 6% 11% 30% 33% 7%	3892
	11th to 20th	Day	21% 7% 14% 34% 21% 3	3882
	21st to end	Day	20% 9% 16% 29% 21% 5%	4311
September	1st to 10th	Day	31% 9% 16% 19% 14% 10%	2400
	11th to 20th	Day	29% 8% 14% 19% 23% 8%	2255
	21st to end	Day	24% 6% 11% 20% 25% 14%	2200
October	1st to 10th	Day	8% 4 8% 15% 25% 40%	3590
	11th to 20th	Day	15% 5% 9% 20% 17% 34%	3447
	21st to end	Day	9% 2 3 11% 44% 32%	3548
November	1st to 10th	Day	14% 3 7% 18% 30% 29%	1985
	11th to 20th	Day	17% 3 8% 19% 28% 24%	1890
	21st to end	Day	23% 3 6% 18% 27% 22%	1890
December	1st to 10th	Day	26% 6% 8% 18% 26% 17%	3030
	11th to 20th	Day	18% 3 4 22% 31% 22%	3479
	21st to end	Day	12% 3 4 14% 39% 29%	4775
January	1st to 10th	Night	14% 3 3 20% 34% 26%	7038
	11th to 20th	Night	15% 2 4 14% 31% 34%	7080
	21st to end	Night	26% 4 5% 17% 26% 22%	7746
February	1st to 10th	Night	21% 4 5% 12% 25% 33%	2520
	11th to 20th	Night	24% 4 7% 14% 22% 29%	2458
	21st to end	Night	19% 5% 4 9% 27% 36%	2076
March	1st to 10th	Night	20% 5% 3 9% 21% 42%	5494
	11th to 20th	Night	19% 6% 5% 9% 19% 43%	5343
	21st to end	Night	17% 5% 6% 10% 22% 40%	5094
April	1st to 10th	Night	21% 4 5% 13% 20% 36%	2010
	11th to 20th	Night	17% 5% 8% 15% 19% 35%	1977
	21st to end	Night	16% 5% 8% 16% 22% 33%	2000
May	1st to 10th	Night	11% 4 7% 20% 20% 38%	4537
	11th to 20th	Night	23% 6% 8% 13% 19% 31%	4511
	21st to end	Night	18% 7% 11% 19% 17% 29%	4544
June	1st to 10th	Night	18% 7% 14% 28% 22% 11%	1800
	11th to 20th	Night	20% 7% 11% 19% 27% 16%	1800
	21st to end	Night	22% 7% 15% 23% 20% 13%	1801
July	1st to 10th	Night	22% 9% 21% 34% 14% 1	4490
	11th to 20th	Night	15% 7% 19% 39% 17% 3	4480
	21st to end	Night	22% 9% 19% 27% 20% 4	4808
August	1st to 10th	Night	12% 5% 11% 36% 32% 4	3244
	11th to 20th	Night	14% 7% 21% 41% 15% 2	3243
	21st to end	Night	18% 10% 18% 33% 18% 4	3597
September	1st to 10th	Night	29% 8% 17% 23% 15% 9%	2000
	11th to 20th	Night	29% 9% 18% 20% 17% 7%	2145
	21st to end	Night	25% 8% 12% 24% 17% 14%	2200
October	1st to 10th	Night	10% 4 9% 14% 25% 37%	3590
	11th to 20th	Night	16% 4 11% 19% 15% 35%	3690
	21st to end	Night	10% 2 4 17% 37% 30%	4252
November	1st to 10th	Night	14% 4 6% 16% 31% 29%	2509
	11th to 20th	Night	17% 4 7% 19% 26% 27%	2730
	21st to end	Night	21% 4 8% 16% 25% 26%	2730
December	1st to 10th	Night	30% 6% 7% 18% 19% 20%	4380
	11th to 20th	Night	16% 4 4 20% 32% 25%	4947
	21st to end	Night	15% 3 4 12% 39% 28%	6474

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% 8% 35% 30% 16% 2	330
	11th to 20th	Day	6% 10% 33% 34% 17% 1	330
	21st to end	Day	7% 15% 31% 30% 17% 1	363
February	1st to 10th	Day	8% 10% 23% 31% 20% 7%	330
	11th to 20th	Day	10% 12% 31% 33% 12% 2	330
	21st to end	Day	8% 11% 35% 31% 13% 1	273
March	1st to 10th	Day	8% 11% 32% 31% 14% 3	330
	11th to 20th	Day	9% 17% 35% 23% 12% 4	421
	21st to end	Day	7% 17% 33% 29% 10% 3	594
April	1st to 10th	Day	6% 16% 41% 26% 10% 2	505
	11th to 20th	Day	5% 16% 43% 23% 13%	492
	21st to end	Day	5% 14% 40% 32% 8% 1	500
May	1st to 10th	Day	3 11% 43% 27% 12% 3	503
	11th to 20th	Day	3 11% 34% 36% 14% 2	528
	21st to end	Day	2 6% 26% 40% 24% 1	741
June	1st to 10th	Day	2 2 18% 35% 35% 7%	730
	11th to 20th	Day	3 1 14% 37% 40% 6%	730
	21st to end	Day	3 2 12% 30% 43% 10%	721
July	1st to 10th	Day	3 2 10% 28% 36% 21%	526
	11th to 20th	Day	2 7% 34% 39% 19%	524
	21st to end	Day	2 2 8% 35% 37% 16%	575
August	1st to 10th	Day	3 1 6% 29% 37% 24%	527
	11th to 20th	Day	4 2 8% 31% 32% 22%	517
	21st to end	Day	4 1 7% 31% 37% 21%	583
September	1st to 10th	Day	6% 1 10% 36% 31% 16%	530
	11th to 20th	Day	6% 1 3 27% 39% 23%	385
	21st to end	Day	6% 2 9% 24% 38% 22%	330
October	1st to 10th	Day	3 2 10% 30% 37% 17%	330
	11th to 20th	Day	8% 3 15% 29% 33% 12%	330
	21st to end	Day	7% 4 18% 31% 28% 12%	363
November	1st to 10th	Day	6% 4 18% 32% 30% 10%	330
	11th to 20th	Day	8% 8% 16% 31% 29% 8%	330
	21st to end	Day	8% 11% 24% 33% 20% 4	330
December	1st to 10th	Day	6% 7% 24% 34% 24% 5%	330
	11th to 20th	Day	6% 6% 30% 32% 23% 4	330
	21st to end	Day	4 7% 31% 36% 17% 5%	363
January	1st to 10th	Night	6% 5% 27% 43% 17% 1	840
	11th to 20th	Night	8% 5% 23% 45% 18% 1	840
	21st to end	Night	6% 8% 27% 45% 13%	924
February	1st to 10th	Night	6% 7% 23% 44% 18% 1	840
	11th to 20th	Night	7% 5% 31% 45% 11%	824
	21st to end	Night	6% 10% 32% 44% 8%	692
March	1st to 10th	Night	7% 8% 31% 42% 11%	840
	11th to 20th	Night	8% 10% 31% 38% 13% 1	840
	21st to end	Night	5% 9% 33% 40% 12% 1	693
April	1st to 10th	Night	5% 9% 35% 41% 9%	603
	11th to 20th	Night	2 8% 39% 43% 7%	593
	21st to end	Night	3 9% 35% 44% 8%	600
May	1st to 10th	Night	4 6% 30% 49% 10% 1	600
	11th to 20th	Night	6% 5% 32% 47% 9% 1	591
	21st to end	Night	3 4 30% 49% 15%	650
June	1st to 10th	Night	3 2 23% 50% 21% 1	600
	11th to 20th	Night	3 17% 51% 28% 1	600
	21st to end	Night	4 1 15% 46% 33% 1	600
July	1st to 10th	Night	4 1 10% 43% 40% 2	585
	11th to 20th	Night	2 8% 41% 48% 1	582
	21st to end	Night	3 1 9% 43% 42% 3	657
August	1st to 10th	Night	2 1 5% 40% 50% 2	597
	11th to 20th	Night	3 1 7% 33% 51% 6%	594
	21st to end	Night	5% 6% 37% 49% 2	660
September	1st to 10th	Night	6% 2 7% 41% 42% 3	600
	11th to 20th	Night	6% 1 8% 43% 41% 2	600
	21st to end	Night	5% 3 13% 42% 36% 2	600
October	1st to 10th	Night	3 1 13% 48% 34% 2	600
	11th to 20th	Night	7% 1 15% 50% 25% 2	670
	21st to end	Night	5% 3 14% 43% 30% 5%	880
November	1st to 10th	Night	5% 2 14% 44% 30% 4	869
	11th to 20th	Night	4 4 19% 44% 26% 2	1050
	21st to end	Night	6% 4 26% 43% 19% 2	1050
December	1st to 10th	Night	6% 4 21% 48% 19% 2	1050
	11th to 20th	Night	4 4 26% 48% 18% 1	1050
	21st to end	Night	3 6% 22% 49% 18% 1	1087

Page generated at 2026-03-30 01:18:42. Version ac07

Copyright © 2026 Attilla Danko.