D. S. Millas Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the D. S. Millas Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	131638
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	130535
Transparency	Day	39% 6% 12% 20% 18% 6%	135413
	Night	38% 6% 11% 19% 20% 7%	134816
Seeing	Day	17% 11% 23% 24% 18% 8%	16761
	Night	16% 10% 24% 29% 19% 3	26862

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	20% 10% 3 6% 3 3 3 2 3 4 42%	5258
	11th to 20th	Day	10% 7% 2 4 3 3 3 2 4 4 58%	5280
	21st to end	Day	25% 9% 3 7% 3 3 3 2 4 5% 37%	5975
February	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 6% 6% 41%	2100
	11th to 20th	Day	23% 7% 2 5% 3 3 3 2 4 5% 42%	2048
	21st to end	Day	21% 7% 2 6% 4 4 4 3 4 6% 41%	1730
March	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 5% 5% 34%	4739
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 50%	5090
	21st to end	Day	17% 5% 2 6% 5% 4 4 3 4 5% 45%	5586
April	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 4 7% 7% 45%	2357
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 5% 7% 48%	2446
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 7% 49%	2572
May	1st to 10th	Day	15% 7% 2 7% 5% 4 5% 4 6% 9% 36%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 5% 6% 5% 7% 8% 29%	5861
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 39%	6433
June	1st to 10th	Day	10% 6% 3 6% 6% 5% 6% 5% 9% 10% 34%	2600
	11th to 20th	Day	5% 5% 2 5% 5% 5% 6% 5% 9% 11% 42%	2600
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 9% 11% 38%	2600
July	1st to 10th	Day	8% 4 2 6% 6% 6% 7% 6% 8% 12% 35%	5900
	11th to 20th	Day	7% 4 2 6% 5% 6% 7% 6% 8% 13% 35%	5900
	21st to end	Day	11% 7% 3 9% 7% 6% 7% 6% 8% 10% 26%	6424
August	1st to 10th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 37%	4234
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 5% 7% 10% 43%	3947
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 47%	4312
September	1st to 10th	Day	11% 7% 3 7% 6% 4 5% 4 7% 8% 40%	2200
	11th to 20th	Day	14% 7% 2 5% 5% 4 5% 5% 6% 7% 41%	2200
	21st to end	Day	13% 6% 1 7% 4 4 4 3 5% 6% 48%	2200
October	1st to 10th	Day	12% 5% 2 4 3 3 4 3 4 6% 55%	2200
	11th to 20th	Day	10% 5% 2 4 3 2 3 2 5% 6% 58%	2079
	21st to end	Day	15% 5% 2 5% 2 3 3 2 4 5% 54%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 6% 55%	1877
	11th to 20th	Day	14% 5% 1 4 3 3 3 2 5% 5% 53%	1890
	21st to end	Day	14% 5% 2 5% 4 4 3 3 5% 6% 48%	1890
December	1st to 10th	Day	21% 6% 1 5% 3 3 3 3 4 5% 45%	3020
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 4 5% 42%	3463
	21st to end	Day	23% 7% 2 5% 3 3 4 3 5% 6% 40%	4565
January	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 4 6% 45%	7635
	11th to 20th	Night	14% 7% 3 5% 3 3 3 2 4 5% 50%	7680
	21st to end	Night	22% 9% 3 6% 4 3 3 2 3 4 42%	8203
February	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 6% 44%	2520
	11th to 20th	Night	22% 7% 2 6% 3 3 3 3 4 4 42%	2458
	21st to end	Night	20% 6% 2 6% 4 3 3 3 5% 5% 40%	2076
March	1st to 10th	Night	22% 9% 3 5% 4 4 4 3 5% 6% 36%	5320
	11th to 20th	Night	11% 5% 1 4 3 3 4 4 6% 8% 52%	5090
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 7% 45%	5588
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 48%	2279
	11th to 20th	Night	14% 4 2 4 2 3 3 4 6% 5% 52%	2035
	21st to end	Night	10% 6% 2 6% 4 4 4 3 6% 7% 50%	1825
May	1st to 10th	Night	11% 8% 3 7% 5% 5% 4 5% 7% 9% 35%	4033
	11th to 20th	Night	20% 8% 3 6% 5% 4 6% 4 6% 7% 31%	4032
	21st to end	Night	13% 6% 2 5% 4 4 5% 4 7% 9% 42%	4421
June	1st to 10th	Night	8% 5% 2 7% 6% 6% 6% 6% 8% 10% 36%	1800
	11th to 20th	Night	5% 4 3 6% 5% 5% 5% 5% 8% 12% 43%	1800
	21st to end	Night	6% 4 2 5% 5% 6% 5% 5% 8% 11% 44%	1800
July	1st to 10th	Night	10% 6% 3 9% 5% 5% 6% 4 8% 9% 35%	4058
	11th to 20th	Night	7% 4 2 7% 6% 5% 6% 6% 9% 13% 35%	4057
	21st to end	Night	11% 8% 3 8% 5% 5% 7% 5% 8% 9% 29%	4427
August	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 5% 8% 12% 38%	2915
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 3 5% 8% 45%	3097
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	10% 5% 3 7% 5% 5% 6% 5% 6% 8% 41%	2179
	11th to 20th	Night	13% 6% 2 5% 5% 4 5% 4 5% 9% 42%	2200
	21st to end	Night	13% 5% 2 6% 4 4 4 4 5% 8% 46%	2200
October	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 54%	2200
	11th to 20th	Night	10% 4 2 5% 3 3 3 2 4 5% 59%	2320
	21st to end	Night	13% 4 2 3 3 2 3 3 4 5% 56%	2640
November	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 6% 55%	2636
	11th to 20th	Night	14% 5% 2 4 3 3 2 2 4 6% 54%	2730
	21st to end	Night	16% 5% 1 5% 4 3 4 3 5% 6% 48%	2730
December	1st to 10th	Night	17% 7% 2 6% 3 3 2 2 4 5% 49%	4380
	11th to 20th	Night	19% 6% 2 5% 3 3 4 3 5% 5% 46%	4952
	21st to end	Night	22% 7% 2 6% 4 3 3 3 4 6% 41%	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	46% 3 6% 21% 18% 5%	5259
	11th to 20th	Day	31% 3 5% 17% 31% 13%	5280
	21st to end	Day	51% 2 5% 15% 18% 9%	5975
February	1st to 10th	Day	44% 4 7% 20% 21% 5%	2100
	11th to 20th	Day	45% 4 7% 19% 19% 6%	2058
	21st to end	Day	45% 5% 7% 14% 21% 8%	1730
March	1st to 10th	Day	51% 3 9% 11% 18% 8%	4739
	11th to 20th	Day	34% 3 7% 15% 23% 17%	5090
	21st to end	Day	42% 4 7% 22% 20% 5%	5586
April	1st to 10th	Day	37% 5% 10% 22% 20% 5%	2357
	11th to 20th	Day	35% 5% 8% 19% 24% 8%	2446
	21st to end	Day	32% 6% 12% 27% 19% 3	2572
May	1st to 10th	Day	42% 7% 15% 19% 14% 1	5897
	11th to 20th	Day	49% 9% 14% 18% 9% 1	5865
	21st to end	Day	40% 8% 11% 22% 18% 1	6447
June	1st to 10th	Day	40% 10% 18% 21% 9% 2	2600
	11th to 20th	Day	30% 11% 21% 25% 12% 1	2600
	21st to end	Day	35% 10% 17% 22% 12% 3	2600
July	1st to 10th	Day	36% 13% 19% 25% 7% 1	5835
	11th to 20th	Day	34% 14% 28% 18% 4 1	5831
	21st to end	Day	48% 12% 18% 19% 4	6240
August	1st to 10th	Day	37% 9% 14% 32% 8%	4221
	11th to 20th	Day	33% 9% 19% 23% 14% 2	3986
	21st to end	Day	31% 8% 14% 23% 21% 3	4311
September	1st to 10th	Day	39% 8% 15% 16% 18% 4	2200
	11th to 20th	Day	40% 8% 12% 19% 20% 2	2200
	21st to end	Day	38% 5% 12% 18% 21% 7%	2200
October	1st to 10th	Day	25% 3 7% 13% 39% 13%	3590
	11th to 20th	Day	20% 2 6% 15% 29% 28%	3358
	21st to end	Day	28% 3 6% 22% 29% 13%	3538
November	1st to 10th	Day	32% 4 8% 16% 31% 10%	1877
	11th to 20th	Day	33% 3 7% 20% 27% 10%	1890
	21st to end	Day	37% 4 8% 19% 26% 6%	1890
December	1st to 10th	Day	41% 4 5% 17% 26% 7%	3020
	11th to 20th	Day	46% 4 8% 17% 20% 7%	3460
	21st to end	Day	44% 3 10% 25% 16% 2	4565
January	1st to 10th	Night	39% 4 9% 20% 22% 5%	7636
	11th to 20th	Night	37% 3 5% 18% 27% 10%	7680
	21st to end	Night	47% 2 5% 15% 23% 8%	8203
February	1st to 10th	Night	39% 5% 9% 19% 21% 7%	2520
	11th to 20th	Night	46% 4 6% 18% 20% 7%	2459
	21st to end	Night	44% 4 7% 16% 19% 10%	2076
March	1st to 10th	Night	48% 4 9% 14% 19% 6%	5320
	11th to 20th	Night	29% 4 8% 15% 27% 16%	5090
	21st to end	Night	39% 4 7% 22% 20% 8%	5588
April	1st to 10th	Night	34% 6% 11% 20% 24% 6%	2279
	11th to 20th	Night	31% 5% 8% 22% 24% 10%	2035
	21st to end	Night	32% 6% 11% 26% 20% 4	1825
May	1st to 10th	Night	42% 9% 16% 20% 12% 1	4060
	11th to 20th	Night	49% 7% 15% 18% 9% 1	4033
	21st to end	Night	36% 8% 14% 24% 16% 2	4429
June	1st to 10th	Night	38% 11% 17% 20% 12% 2	1800
	11th to 20th	Night	30% 10% 20% 25% 14% 1	1800
	21st to end	Night	31% 12% 19% 22% 14% 4	1800
July	1st to 10th	Night	42% 11% 17% 23% 6% 1	4015
	11th to 20th	Night	36% 16% 26% 17% 4 1	4006
	21st to end	Night	48% 12% 18% 16% 4 1	4297
August	1st to 10th	Night	37% 9% 18% 28% 7%	2904
	11th to 20th	Night	36% 8% 17% 22% 15% 1	3124
	21st to end	Night	31% 7% 14% 22% 24% 2	3586
September	1st to 10th	Night	38% 8% 14% 18% 17% 4	2179
	11th to 20th	Night	39% 8% 11% 20% 19% 3	2200
	21st to end	Night	36% 7% 13% 17% 20% 8%	2200
October	1st to 10th	Night	26% 4 10% 13% 36% 11%	3590
	11th to 20th	Night	18% 3 5% 18% 32% 25%	3767
	21st to end	Night	24% 3 4 23% 29% 17%	4252
November	1st to 10th	Night	32% 3 8% 19% 26% 12%	2636
	11th to 20th	Night	32% 3 6% 18% 29% 11%	2730
	21st to end	Night	36% 5% 7% 20% 25% 7%	2730
December	1st to 10th	Night	39% 3 6% 18% 28% 7%	4380
	11th to 20th	Night	40% 4 10% 16% 23% 6%	4954
	21st to end	Night	44% 3 7% 27% 16% 2	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	28% 25% 35% 10% 2	330
	11th to 20th	Day	28% 25% 27% 16% 4 1	330
	21st to end	Day	27% 24% 37% 10% 2 1	363
February	1st to 10th	Day	22% 22% 35% 18% 3	330
	11th to 20th	Day	38% 23% 26% 12% 1	330
	21st to end	Day	30% 22% 34% 11% 3	273
March	1st to 10th	Day	25% 17% 33% 20% 5% 1	398
	11th to 20th	Day	21% 21% 33% 20% 5% 1	540
	21st to end	Day	19% 17% 36% 21% 6% 1	594
April	1st to 10th	Day	18% 16% 30% 25% 9% 1	540
	11th to 20th	Day	13% 17% 34% 23% 12% 2	516
	21st to end	Day	17% 10% 27% 27% 15% 4	500
May	1st to 10th	Day	14% 14% 30% 25% 13% 5%	503
	11th to 20th	Day	16% 6% 24% 29% 16% 9%	527
	21st to end	Day	15% 7% 18% 27% 22% 10%	573
June	1st to 10th	Day	13% 3 18% 30% 25% 11%	530
	11th to 20th	Day	6% 4 15% 32% 25% 18%	530
	21st to end	Day	9% 3 15% 28% 28% 16%	530
July	1st to 10th	Day	10% 2 9% 26% 31% 22%	525
	11th to 20th	Day	7% 2 10% 21% 35% 24%	524
	21st to end	Day	6% 1 9% 26% 37% 21%	576
August	1st to 10th	Day	13% 5% 15% 24% 25% 19%	526
	11th to 20th	Day	12% 2 12% 24% 33% 16%	519
	21st to end	Day	14% 2 11% 26% 27% 20%	583
September	1st to 10th	Day	11% 5% 13% 31% 27% 14%	530
	11th to 20th	Day	15% 5% 13% 24% 32% 11%	530
	21st to end	Day	14% 5% 20% 25% 26% 10%	530
October	1st to 10th	Day	15% 5% 24% 31% 18% 8%	530
	11th to 20th	Day	13% 11% 23% 26% 24% 4	530
	21st to end	Day	17% 10% 26% 27% 17% 3	583
November	1st to 10th	Day	22% 8% 28% 24% 15% 2	355
	11th to 20th	Day	16% 18% 35% 22% 7% 1	330
	21st to end	Day	14% 19% 36% 21% 9% 1	330
December	1st to 10th	Day	32% 21% 30% 14% 4 1	330
	11th to 20th	Day	22% 19% 31% 22% 5% 1	330
	21st to end	Day	31% 16% 32% 17% 4 1	363
January	1st to 10th	Night	25% 19% 32% 20% 5%	840
	11th to 20th	Night	22% 18% 31% 22% 6%	840
	21st to end	Night	22% 19% 34% 23% 2	924
February	1st to 10th	Night	19% 16% 33% 28% 5%	840
	11th to 20th	Night	26% 18% 32% 21% 2	823
	21st to end	Night	23% 18% 35% 22% 3	692
March	1st to 10th	Night	20% 15% 33% 27% 5%	840
	11th to 20th	Night	18% 13% 35% 26% 8%	840
	21st to end	Night	17% 15% 31% 30% 7% 1	924
April	1st to 10th	Night	13% 15% 31% 30% 10%	809
	11th to 20th	Night	15% 10% 29% 32% 14% 1	611
	21st to end	Night	11% 8% 27% 35% 18% 1	600
May	1st to 10th	Night	11% 8% 25% 34% 22% 1	600
	11th to 20th	Night	14% 5% 24% 31% 21% 5%	591
	21st to end	Night	14% 3 15% 31% 30% 6%	650
June	1st to 10th	Night	9% 1 14% 40% 31% 5%	600
	11th to 20th	Night	6% 3 13% 31% 35% 13%	600
	21st to end	Night	6% 1 11% 38% 36% 9%	600
July	1st to 10th	Night	10% 3 8% 25% 43% 12%	585
	11th to 20th	Night	9% 1 9% 20% 48% 13%	582
	21st to end	Night	7% 3 8% 26% 45% 10%	657
August	1st to 10th	Night	11% 3 9% 29% 39% 9%	597
	11th to 20th	Night	11% 3 10% 28% 39% 9%	594
	21st to end	Night	15% 3 10% 27% 36% 9%	660
September	1st to 10th	Night	11% 3 13% 30% 33% 9%	779
	11th to 20th	Night	15% 3 13% 30% 36% 3	800
	21st to end	Night	15% 4 16% 34% 29% 2	800
October	1st to 10th	Night	13% 4 20% 34% 23% 6%	800
	11th to 20th	Night	12% 7% 26% 32% 20% 3	800
	21st to end	Night	15% 9% 22% 34% 18% 2	880
November	1st to 10th	Night	16% 8% 24% 33% 18% 1	820
	11th to 20th	Night	16% 12% 30% 28% 13% 1	840
	21st to end	Night	18% 14% 27% 27% 12% 1	840
December	1st to 10th	Night	22% 15% 32% 25% 7%	840
	11th to 20th	Night	24% 12% 29% 26% 9%	840
	21st to end	Night	27% 14% 28% 27% 5%	924

Page generated at 2026-04-14 02:44:28. Version ac07

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