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 2024-12-24.

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%	101442
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	98641
Transparency	Day	38% 7% 12% 19% 18% 5%	105261
	Night	37% 6% 11% 19% 20% 7%	102947
Seeing	Day	16% 11% 23% 23% 18% 9%	15045
	Night	16% 10% 23% 28% 19% 3	25102

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% 10% 4 7% 4 3 4 3 5% 5% 37%	2836
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 4 5% 49%	2840
	21st to end	Day	27% 9% 2 5% 3 3 2 2 4 5% 41%	3221
February	1st to 10th	Day	17% 8% 3 6% 4 5% 4 4 6% 6% 39%	1900
	11th to 20th	Day	22% 7% 2 5% 3 3 3 3 4 5% 42%	1848
	21st to end	Day	23% 7% 2 6% 4 4 4 3 4 5% 39%	1570
March	1st to 10th	Day	24% 10% 3 6% 4 3 3 2 5% 5% 34%	3234
	11th to 20th	Day	14% 6% 2 5% 5% 3 4 3 5% 6% 47%	3480
	21st to end	Day	19% 5% 2 6% 5% 4 4 3 5% 6% 40%	3815
April	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 4 6% 6% 45%	2133
	11th to 20th	Day	14% 5% 2 6% 4 3 4 4 5% 8% 46%	2254
	21st to end	Day	11% 4 2 5% 4 3 4 4 6% 6% 51%	2443
May	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 6% 10% 41%	4082
	11th to 20th	Day	18% 7% 3 6% 4 4 5% 4 6% 7% 36%	4081
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 11% 42%	4476
June	1st to 10th	Day	10% 7% 3 6% 6% 5% 5% 5% 9% 10% 34%	2470
	11th to 20th	Day	5% 5% 2 5% 4 5% 6% 5% 8% 11% 43%	2470
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 9% 11% 37%	2470
July	1st to 10th	Day	9% 5% 3 7% 6% 7% 8% 6% 8% 12% 28%	4120
	11th to 20th	Day	9% 5% 2 7% 6% 6% 7% 5% 8% 12% 33%	4120
	21st to end	Day	12% 8% 4 10% 8% 7% 7% 6% 9% 9% 19%	4466
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 38%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 5% 7% 10% 43%	3824
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	4180
September	1st to 10th	Day	11% 7% 3 7% 6% 4 5% 4 6% 8% 40%	2090
	11th to 20th	Day	15% 7% 2 5% 5% 4 5% 5% 6% 7% 40%	2090
	21st to end	Day	14% 5% 1 7% 4 4 4 3 5% 6% 49%	2090
October	1st to 10th	Day	12% 5% 2 4 3 3 4 3 4 5% 56%	2090
	11th to 20th	Day	11% 5% 2 4 3 2 3 2 5% 6% 57%	1975
	21st to end	Day	14% 5% 2 5% 2 3 3 2 4 5% 55%	2090
November	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 5% 55%	1786
	11th to 20th	Day	15% 5% 1 4 3 3 3 3 5% 5% 52%	1800
	21st to end	Day	14% 5% 2 5% 4 4 3 3 5% 7% 47%	1800
December	1st to 10th	Day	22% 7% 2 6% 4 4 3 3 5% 6% 38%	1800
	11th to 20th	Day	17% 6% 3 7% 4 4 4 3 4 5% 43%	2243
	21st to end	Day	22% 8% 3 5% 3 3 4 3 6% 7% 36%	3151
January	1st to 10th	Night	19% 8% 2 6% 3 3 4 3 5% 7% 41%	4109
	11th to 20th	Night	14% 8% 3 6% 4 3 4 3 4 6% 45%	4120
	21st to end	Night	25% 9% 3 6% 4 3 2 2 3 4 39%	4432
February	1st to 10th	Night	16% 7% 2 5% 4 3 4 3 5% 6% 43%	2280
	11th to 20th	Night	22% 8% 2 6% 3 3 3 3 4 4 43%	2218
	21st to end	Night	21% 6% 2 6% 4 4 3 3 5% 5% 39%	1884
March	1st to 10th	Night	24% 8% 3 5% 4 4 4 3 5% 5% 36%	3619
	11th to 20th	Night	12% 6% 1 4 3 3 4 4 6% 8% 49%	3480
	21st to end	Night	17% 7% 3 6% 4 4 4 3 5% 6% 42%	3817
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 48%	2062
	11th to 20th	Night	15% 5% 2 4 3 3 4 4 6% 5% 50%	1875
	21st to end	Night	9% 5% 2 5% 4 4 4 3 6% 8% 51%	1734
May	1st to 10th	Night	10% 7% 2 7% 5% 5% 4 5% 6% 8% 40%	2813
	11th to 20th	Night	16% 8% 3 6% 4 3 5% 3 5% 7% 39%	2812
	21st to end	Night	7% 5% 2 5% 4 4 5% 5% 7% 10% 46%	3079
June	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 6% 8% 10% 36%	1710
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 5% 8% 12% 43%	1710
	21st to end	Night	6% 5% 2 5% 5% 6% 5% 5% 8% 11% 43%	1710
July	1st to 10th	Night	12% 6% 3 9% 6% 5% 6% 4 8% 9% 31%	2840
	11th to 20th	Night	7% 4 2 8% 6% 5% 6% 7% 9% 12% 34%	2840
	21st to end	Night	12% 9% 4 10% 6% 6% 8% 6% 9% 9% 22%	3087
August	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 8% 12% 38%	2825
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 3 5% 9% 46%	3000
	21st to end	Night	7% 5% 2 5% 4 4 4 3 5% 8% 52%	3476
September	1st to 10th	Night	10% 5% 3 7% 5% 5% 6% 4 6% 8% 42%	2070
	11th to 20th	Night	14% 6% 2 5% 5% 4 5% 4 6% 9% 41%	2090
	21st to end	Night	13% 5% 2 5% 4 4 4 4 5% 7% 47%	2090
October	1st to 10th	Night	12% 4 2 4 3 3 3 3 5% 6% 55%	2090
	11th to 20th	Night	11% 4 2 5% 3 3 3 2 4 5% 58%	2204
	21st to end	Night	13% 4 2 3 3 2 3 3 4 6% 57%	2508
November	1st to 10th	Night	13% 5% 2 4 3 3 3 3 4 6% 55%	2507
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 5% 6% 53%	2600
	21st to end	Night	16% 5% 1 4 4 3 4 3 5% 6% 48%	2600
December	1st to 10th	Night	18% 7% 2 7% 4 4 3 3 5% 6% 41%	2600
	11th to 20th	Night	18% 6% 2 4 3 3 3 2 5% 5% 48%	3172
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 7% 34%	4578

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	47% 5% 7% 15% 20% 6%	2837
	11th to 20th	Day	36% 4 8% 21% 24% 7%	2840
	21st to end	Day	47% 2 6% 17% 21% 7%	3221
February	1st to 10th	Day	46% 4 7% 18% 20% 5%	1900
	11th to 20th	Day	44% 4 7% 19% 20% 6%	1858
	21st to end	Day	47% 5% 8% 14% 19% 8%	1570
March	1st to 10th	Day	51% 4 10% 13% 12% 10%	3234
	11th to 20th	Day	37% 4 8% 17% 23% 10%	3480
	21st to end	Day	44% 5% 9% 20% 20% 4	3815
April	1st to 10th	Day	38% 5% 10% 21% 21% 5%	2133
	11th to 20th	Day	36% 6% 9% 17% 24% 8%	2254
	21st to end	Day	31% 6% 11% 28% 20% 3	2443
May	1st to 10th	Day	37% 7% 14% 21% 18% 2	4120
	11th to 20th	Day	44% 8% 12% 24% 12% 1	4085
	21st to end	Day	32% 9% 14% 23% 20% 1	4489
June	1st to 10th	Day	40% 11% 18% 21% 9% 2	2470
	11th to 20th	Day	29% 11% 21% 26% 12% 1	2470
	21st to end	Day	36% 10% 16% 21% 12% 3	2470
July	1st to 10th	Day	42% 14% 20% 19% 5% 1	4055
	11th to 20th	Day	39% 12% 22% 19% 6% 2	4051
	21st to end	Day	56% 13% 14% 12% 5%	4324
August	1st to 10th	Day	36% 9% 14% 32% 8%	4091
	11th to 20th	Day	33% 9% 19% 23% 14% 2	3863
	21st to end	Day	31% 8% 14% 23% 21% 3	4179
September	1st to 10th	Day	40% 8% 15% 15% 18% 4	2090
	11th to 20th	Day	41% 8% 12% 17% 20% 2	2090
	21st to end	Day	38% 4 11% 18% 22% 7%	2090
October	1st to 10th	Day	25% 3 7% 13% 39% 13%	3480
	11th to 20th	Day	20% 2 6% 15% 29% 28%	3254
	21st to end	Day	28% 3 6% 22% 29% 13%	3428
November	1st to 10th	Day	33% 4 7% 15% 31% 10%	1786
	11th to 20th	Day	34% 3 7% 19% 27% 10%	1800
	21st to end	Day	37% 4 8% 18% 27% 6%	1800
December	1st to 10th	Day	46% 5% 6% 18% 18% 6%	1800
	11th to 20th	Day	43% 4 7% 16% 23% 7%	2240
	21st to end	Day	45% 4 11% 18% 18% 3	3151
January	1st to 10th	Night	41% 5% 8% 16% 25% 5%	4109
	11th to 20th	Night	40% 4 7% 22% 19% 9%	4120
	21st to end	Night	49% 2 6% 13% 24% 6%	4432
February	1st to 10th	Night	40% 5% 9% 18% 20% 8%	2280
	11th to 20th	Night	46% 4 6% 17% 20% 8%	2219
	21st to end	Night	45% 4 8% 15% 18% 9%	1884
March	1st to 10th	Night	48% 4 10% 15% 15% 7%	3619
	11th to 20th	Night	31% 6% 9% 16% 27% 11%	3480
	21st to end	Night	42% 5% 7% 19% 18% 8%	3817
April	1st to 10th	Night	34% 5% 10% 19% 25% 6%	2062
	11th to 20th	Night	33% 5% 8% 21% 23% 10%	1875
	21st to end	Night	31% 6% 10% 28% 21% 5%	1734
May	1st to 10th	Night	37% 10% 14% 20% 16% 2	2840
	11th to 20th	Night	41% 8% 14% 24% 12% 1	2813
	21st to end	Night	29% 9% 16% 26% 18% 2	3087
June	1st to 10th	Night	38% 11% 17% 19% 12% 2	1710
	11th to 20th	Night	29% 10% 20% 26% 14% 1	1710
	21st to end	Night	32% 11% 18% 21% 14% 4	1710
July	1st to 10th	Night	46% 11% 18% 18% 6% 1	2795
	11th to 20th	Night	36% 15% 22% 19% 6% 1	2786
	21st to end	Night	54% 14% 15% 13% 4 1	2988
August	1st to 10th	Night	36% 9% 18% 29% 7%	2814
	11th to 20th	Night	36% 8% 17% 23% 15% 1	3027
	21st to end	Night	30% 7% 14% 22% 25% 2	3476
September	1st to 10th	Night	38% 8% 14% 18% 18% 4	2070
	11th to 20th	Night	39% 8% 11% 19% 19% 3	2090
	21st to end	Night	35% 7% 12% 17% 20% 8%	2090
October	1st to 10th	Night	25% 4 10% 13% 37% 11%	3480
	11th to 20th	Night	18% 3 5% 17% 32% 25%	3651
	21st to end	Night	24% 3 4 23% 29% 18%	4120
November	1st to 10th	Night	32% 3 8% 19% 26% 12%	2507
	11th to 20th	Night	33% 3 6% 17% 29% 12%	2600
	21st to end	Night	36% 5% 7% 20% 25% 7%	2600
December	1st to 10th	Night	44% 4 8% 19% 19% 5%	2600
	11th to 20th	Night	37% 4 9% 17% 27% 6%	3174
	21st to end	Night	48% 4 9% 21% 16% 3	4578

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	25% 26% 36% 10% 2	270
	11th to 20th	Day	27% 22% 28% 17% 5%	270
	21st to end	Day	25% 24% 38% 11% 2 1	297
February	1st to 10th	Day	25% 21% 33% 17% 3	270
	11th to 20th	Day	37% 23% 27% 13% 1	270
	21st to end	Day	33% 24% 33% 8% 2	225
March	1st to 10th	Day	24% 17% 33% 20% 5% 1	332
	11th to 20th	Day	20% 21% 32% 20% 5% 1	460
	21st to end	Day	20% 17% 36% 20% 6% 1	506
April	1st to 10th	Day	19% 17% 30% 23% 9% 2	460
	11th to 20th	Day	14% 17% 33% 22% 13% 1	452
	21st to end	Day	16% 10% 28% 27% 15% 3	460
May	1st to 10th	Day	13% 14% 29% 26% 13% 5%	463
	11th to 20th	Day	15% 6% 23% 30% 17% 10%	487
	21st to end	Day	13% 6% 17% 29% 23% 11%	529
June	1st to 10th	Day	13% 3 17% 30% 25% 12%	490
	11th to 20th	Day	6% 4 15% 30% 26% 19%	490
	21st to end	Day	10% 4 17% 28% 27% 16%	490
July	1st to 10th	Day	11% 2 9% 25% 31% 22%	485
	11th to 20th	Day	8% 2 11% 20% 33% 25%	484
	21st to end	Day	7% 1 10% 25% 36% 21%	532
August	1st to 10th	Day	10% 4 15% 24% 26% 21%	486
	11th to 20th	Day	12% 2 13% 25% 33% 15%	479
	21st to end	Day	14% 2 11% 25% 28% 20%	539
September	1st to 10th	Day	11% 5% 13% 29% 28% 15%	490
	11th to 20th	Day	16% 5% 14% 23% 31% 10%	490
	21st to end	Day	15% 4 20% 24% 26% 11%	490
October	1st to 10th	Day	15% 4 22% 30% 19% 9%	490
	11th to 20th	Day	13% 12% 23% 26% 22% 4	490
	21st to end	Day	16% 10% 25% 28% 18% 3	539
November	1st to 10th	Day	22% 8% 27% 25% 15% 2	324
	11th to 20th	Day	18% 19% 34% 21% 7% 1	300
	21st to end	Day	13% 19% 36% 21% 9% 1	300
December	1st to 10th	Day	30% 20% 31% 15% 4 1	300
	11th to 20th	Day	22% 19% 33% 21% 5% 1	300
	21st to end	Day	31% 17% 30% 18% 4 1	306
January	1st to 10th	Night	24% 19% 32% 20% 5%	760
	11th to 20th	Night	22% 18% 31% 23% 6%	760
	21st to end	Night	22% 19% 34% 23% 3	836
February	1st to 10th	Night	19% 15% 34% 27% 5%	760
	11th to 20th	Night	25% 18% 33% 22% 2	743
	21st to end	Night	23% 18% 35% 21% 3	628
March	1st to 10th	Night	19% 16% 33% 27% 4	760
	11th to 20th	Night	18% 13% 35% 24% 9%	760
	21st to end	Night	17% 16% 29% 30% 8% 1	836
April	1st to 10th	Night	14% 16% 30% 30% 10%	732
	11th to 20th	Night	16% 10% 29% 32% 13% 1	563
	21st to end	Night	11% 8% 27% 34% 18% 1	570
May	1st to 10th	Night	11% 8% 25% 32% 23% 1	570
	11th to 20th	Night	13% 4 23% 32% 22% 5%	561
	21st to end	Night	13% 3 14% 32% 32% 6%	617
June	1st to 10th	Night	9% 1 13% 40% 32% 6%	570
	11th to 20th	Night	5% 3 14% 32% 34% 13%	570
	21st to end	Night	6% 1 11% 39% 34% 9%	570
July	1st to 10th	Night	10% 3 8% 25% 42% 12%	555
	11th to 20th	Night	9% 1 9% 20% 48% 13%	552
	21st to end	Night	7% 3 8% 27% 44% 11%	624
August	1st to 10th	Night	10% 2 9% 28% 41% 10%	567
	11th to 20th	Night	10% 3 10% 29% 39% 8%	564
	21st to end	Night	15% 3 10% 26% 37% 10%	627
September	1st to 10th	Night	11% 3 12% 30% 34% 10%	740
	11th to 20th	Night	16% 3 13% 30% 35% 3	760
	21st to end	Night	15% 3 17% 33% 30% 2	760
October	1st to 10th	Night	13% 3 20% 33% 24% 7%	760
	11th to 20th	Night	12% 7% 26% 31% 20% 3	760
	21st to end	Night	15% 9% 22% 34% 18% 2	836
November	1st to 10th	Night	16% 8% 25% 32% 19% 1	780
	11th to 20th	Night	17% 12% 30% 27% 13% 1	800
	21st to end	Night	18% 14% 27% 28% 13% 1	800
December	1st to 10th	Night	21% 15% 32% 25% 7%	800
	11th to 20th	Night	24% 12% 30% 25% 9%	800
	21st to end	Night	27% 15% 27% 26% 5%	851

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