CMiller Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CMiller Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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	15% 9% 4 7% 6% 4 5% 5% 6% 7% 32%	75098
	Night	16% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	75011
Transparency	Day	50% 8% 13% 19% 10% 1	74815
	Night	49% 7% 12% 19% 11% 1	74829
Seeing	Day	20% 11% 27% 23% 13% 5%	13674
	Night	20% 10% 29% 29% 12% 1	23028

By Month

Percentage Cloud Cover NE (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	16% 14% 6% 9% 8% 4 5% 6% 4 6% 22%	1558
	11th to 20th	Day	16% 17% 8% 11% 8% 4 5% 5% 4 6% 17%	1710
	21st to end	Day	15% 16% 7% 13% 8% 4 5% 5% 4 5% 17%	1872
February	1st to 10th	Day	17% 15% 5% 10% 9% 5% 5% 5% 5% 7% 17%	1620
	11th to 20th	Day	14% 13% 8% 12% 8% 4 4 5% 4 5% 23%	1573
	21st to end	Day	18% 13% 4 10% 8% 4 6% 4 4 5% 24%	1423
March	1st to 10th	Day	14% 12% 5% 8% 7% 4 5% 4 6% 7% 28%	1963
	11th to 20th	Day	14% 12% 6% 9% 6% 3 4 4 5% 5% 31%	1980
	21st to end	Day	19% 11% 4 8% 6% 3 5% 4 4 6% 30%	2178
April	1st to 10th	Day	16% 11% 4 8% 6% 4 5% 4 5% 7% 29%	2085
	11th to 20th	Day	18% 8% 3 7% 6% 4 4 4 5% 7% 34%	2242
	21st to end	Day	20% 9% 5% 6% 5% 3 5% 4 4 5% 34%	2340
May	1st to 10th	Day	17% 8% 2 7% 7% 3 5% 5% 5% 8% 33%	2340
	11th to 20th	Day	16% 8% 3 6% 5% 3 5% 3 5% 7% 37%	2379
	21st to end	Day	11% 5% 3 5% 6% 3 5% 5% 6% 8% 43%	2725
June	1st to 10th	Day	13% 6% 2 5% 6% 3 5% 6% 6% 8% 40%	2520
	11th to 20th	Day	11% 6% 3 5% 6% 4 5% 6% 6% 9% 39%	2545
	21st to end	Day	12% 5% 2 5% 5% 3 6% 5% 7% 10% 41%	2700
July	1st to 10th	Day	8% 4 2 5% 6% 4 5% 5% 7% 10% 45%	2615
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 7% 7% 10% 37%	2443
	21st to end	Day	9% 4 3 6% 6% 4 6% 7% 7% 9% 39%	2547
August	1st to 10th	Day	9% 5% 2 6% 6% 4 6% 7% 8% 9% 39%	2327
	11th to 20th	Day	10% 5% 3 6% 7% 5% 5% 7% 7% 9% 37%	2303
	21st to end	Day	12% 6% 2 6% 5% 3 6% 5% 6% 9% 40%	2507
September	1st to 10th	Day	13% 6% 3 6% 5% 4 5% 5% 6% 9% 38%	2160
	11th to 20th	Day	10% 5% 3 5% 4 4 5% 5% 6% 8% 45%	1994
	21st to end	Day	14% 6% 3 6% 6% 4 5% 6% 7% 8% 36%	1980
October	1st to 10th	Day	18% 7% 4 7% 6% 3 5% 5% 5% 7% 34%	1800
	11th to 20th	Day	20% 9% 3 6% 6% 4 5% 5% 6% 8% 29%	1800
	21st to end	Day	24% 10% 3 8% 7% 3 5% 5% 6% 6% 23%	1980
November	1st to 10th	Day	13% 10% 3 7% 7% 4 5% 5% 5% 8% 33%	1651
	11th to 20th	Day	14% 13% 4 6% 7% 3 6% 6% 7% 8% 26%	1520
	21st to end	Day	20% 14% 5% 8% 6% 3 4 5% 5% 8% 22%	1520
December	1st to 10th	Day	15% 17% 5% 10% 8% 4 6% 5% 4 6% 20%	1520
	11th to 20th	Day	19% 15% 7% 9% 6% 3 6% 5% 5% 6% 19%	1906
	21st to end	Day	29% 18% 7% 10% 6% 3 4 4 4 4 10%	2772
January	1st to 10th	Night	17% 12% 6% 10% 7% 5% 5% 5% 5% 6% 22%	2607
	11th to 20th	Night	19% 15% 7% 10% 7% 3 5% 5% 5% 6% 17%	2470
	21st to end	Night	14% 13% 7% 10% 8% 3 5% 4 5% 6% 24%	2711
February	1st to 10th	Night	17% 14% 5% 9% 8% 4 5% 5% 5% 5% 21%	2340
	11th to 20th	Night	15% 12% 5% 9% 7% 3 6% 5% 5% 7% 25%	2273
	21st to end	Night	16% 12% 6% 10% 7% 4 5% 5% 4 6% 25%	1830
March	1st to 10th	Night	15% 8% 3 8% 7% 4 5% 4 6% 7% 33%	1984
	11th to 20th	Night	16% 11% 5% 8% 4 3 4 4 5% 7% 33%	1979
	21st to end	Night	17% 10% 4 7% 5% 3 4 4 4 5% 37%	2178
April	1st to 10th	Night	18% 8% 4 6% 5% 3 4 5% 5% 7% 34%	1873
	11th to 20th	Night	17% 8% 4 5% 5% 3 5% 5% 5% 6% 38%	1683
	21st to end	Night	15% 7% 3 6% 6% 3 5% 5% 5% 9% 36%	1620
May	1st to 10th	Night	16% 8% 3 6% 5% 3 4 3 6% 8% 38%	1620
	11th to 20th	Night	14% 5% 3 6% 5% 3 5% 5% 5% 8% 40%	1514
	21st to end	Night	10% 4 2 4 4 3 4 4 6% 8% 51%	1550
June	1st to 10th	Night	10% 5% 1 5% 5% 3 6% 6% 7% 8% 44%	1440
	11th to 20th	Night	11% 5% 2 5% 4 3 6% 6% 6% 8% 44%	1405
	21st to end	Night	11% 4 2 4 5% 3 5% 6% 6% 8% 47%	1260
July	1st to 10th	Night	6% 3 2 4 4 2 4 4 5% 9% 57%	1336
	11th to 20th	Night	8% 5% 2 5% 3 3 5% 4 5% 8% 51%	1514
	21st to end	Night	8% 3 2 5% 6% 3 5% 5% 7% 8% 49%	1771
August	1st to 10th	Night	6% 4 2 6% 5% 4 5% 5% 7% 9% 47%	1611
	11th to 20th	Night	7% 3 3 5% 4 3 4 4 7% 9% 51%	1598
	21st to end	Night	9% 4 2 6% 4 3 6% 4 5% 8% 48%	1831
September	1st to 10th	Night	11% 6% 2 6% 7% 3 4 4 4 7% 46%	1800
	11th to 20th	Night	10% 4 2 4 5% 3 4 5% 5% 6% 53%	1963
	21st to end	Night	14% 6% 3 6% 5% 4 4 4 5% 6% 42%	1980
October	1st to 10th	Night	16% 7% 2 5% 5% 3 4 4 6% 6% 42%	2160
	11th to 20th	Night	18% 8% 3 6% 6% 3 4 5% 5% 7% 34%	2160
	21st to end	Night	22% 8% 4 7% 7% 3 5% 4 5% 6% 29%	2376
November	1st to 10th	Night	15% 8% 3 4 5% 3 5% 5% 6% 8% 39%	2422
	11th to 20th	Night	16% 10% 3 8% 6% 4 5% 5% 6% 8% 29%	2660
	21st to end	Night	20% 10% 4 7% 6% 3 5% 6% 6% 8% 26%	2660
December	1st to 10th	Night	19% 15% 5% 9% 7% 3 5% 6% 6% 6% 20%	2660
	11th to 20th	Night	15% 17% 5% 8% 6% 4 5% 4 5% 7% 24%	3288
	21st to end	Night	29% 18% 6% 9% 7% 4 4 4 4 4 11%	4884

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	62% 7% 10% 13% 8%	1559
	11th to 20th	Day	68% 6% 11% 10% 5%	1710
	21st to end	Day	69% 7% 8% 10% 6% 1	1872
February	1st to 10th	Day	68% 7% 11% 11% 3	1620
	11th to 20th	Day	63% 5% 10% 14% 7% 1	1582
	21st to end	Day	63% 6% 10% 14% 6% 1	1423
March	1st to 10th	Day	56% 7% 14% 17% 7% 1	1966
	11th to 20th	Day	55% 6% 10% 18% 9% 3	1980
	21st to end	Day	55% 5% 9% 17% 11% 2	2178
April	1st to 10th	Day	55% 6% 11% 15% 12%	2085
	11th to 20th	Day	49% 6% 11% 20% 12% 1	2242
	21st to end	Day	52% 6% 11% 18% 10% 3	2340
May	1st to 10th	Day	48% 7% 12% 21% 12% 1	2340
	11th to 20th	Day	46% 7% 11% 22% 12% 2	2383
	21st to end	Day	38% 8% 15% 23% 14% 2	2725
June	1st to 10th	Day	40% 8% 13% 23% 14% 2	2520
	11th to 20th	Day	39% 9% 14% 22% 15% 1	2545
	21st to end	Day	38% 8% 14% 24% 16%	2700
July	1st to 10th	Day	32% 9% 14% 24% 17% 3	2545
	11th to 20th	Day	38% 10% 16% 24% 12% 1	2368
	21st to end	Day	37% 10% 15% 23% 13% 2	2405
August	1st to 10th	Day	37% 10% 15% 25% 11% 1	2314
	11th to 20th	Day	39% 10% 17% 24% 9% 1	2303
	21st to end	Day	38% 9% 14% 25% 12% 1	2507
September	1st to 10th	Day	41% 8% 15% 23% 11% 1	2160
	11th to 20th	Day	35% 8% 14% 29% 15%	1994
	21st to end	Day	43% 9% 15% 23% 9% 1	1980
October	1st to 10th	Day	49% 8% 14% 18% 11% 1	1800
	11th to 20th	Day	52% 7% 15% 17% 8% 1	1800
	21st to end	Day	60% 7% 12% 15% 6% 1	1980
November	1st to 10th	Day	48% 8% 13% 22% 8% 1	1651
	11th to 20th	Day	52% 9% 16% 14% 8%	1520
	21st to end	Day	59% 8% 16% 13% 4	1520
December	1st to 10th	Day	64% 8% 13% 11% 4	1520
	11th to 20th	Day	64% 9% 13% 10% 4	1906
	21st to end	Day	77% 7% 8% 6% 2	2772
January	1st to 10th	Night	63% 6% 11% 12% 8%	2607
	11th to 20th	Night	67% 7% 11% 9% 5% 1	2470
	21st to end	Night	60% 7% 10% 15% 6% 1	2711
February	1st to 10th	Night	63% 7% 11% 14% 5% 1	2340
	11th to 20th	Night	57% 7% 13% 15% 8%	2275
	21st to end	Night	59% 6% 11% 16% 8%	1830
March	1st to 10th	Night	50% 8% 13% 20% 9% 2	1994
	11th to 20th	Night	50% 6% 10% 21% 11% 2	1980
	21st to end	Night	50% 6% 8% 20% 14% 3	2178
April	1st to 10th	Night	49% 7% 12% 19% 13% 1	1873
	11th to 20th	Night	46% 6% 9% 20% 16% 3	1683
	21st to end	Night	45% 7% 11% 19% 15% 3	1620
May	1st to 10th	Night	44% 7% 10% 22% 15% 3	1620
	11th to 20th	Night	41% 6% 13% 24% 12% 4	1515
	21st to end	Night	31% 7% 15% 25% 19% 3	1551
June	1st to 10th	Night	37% 8% 11% 25% 17% 2	1440
	11th to 20th	Night	35% 8% 13% 25% 18% 2	1405
	21st to end	Night	35% 7% 15% 24% 18% 1	1260
July	1st to 10th	Night	27% 6% 11% 27% 26% 3	1297
	11th to 20th	Night	30% 7% 14% 29% 17% 3	1466
	21st to end	Night	31% 8% 16% 26% 16% 3	1672
August	1st to 10th	Night	30% 11% 15% 30% 12% 2	1600
	11th to 20th	Night	28% 9% 19% 30% 13% 2	1598
	21st to end	Night	32% 9% 13% 29% 15% 2	1831
September	1st to 10th	Night	38% 7% 14% 25% 15% 2	1800
	11th to 20th	Night	30% 7% 13% 31% 18% 1	1963
	21st to end	Night	41% 7% 13% 26% 12% 1	1980
October	1st to 10th	Night	42% 7% 12% 24% 15% 1	2160
	11th to 20th	Night	47% 7% 13% 23% 10%	2160
	21st to end	Night	54% 8% 10% 17% 11% 1	2376
November	1st to 10th	Night	41% 8% 14% 23% 13% 1	2422
	11th to 20th	Night	51% 8% 14% 17% 9% 1	2660
	21st to end	Night	55% 8% 17% 15% 5%	2660
December	1st to 10th	Night	63% 9% 13% 12% 4	2660
	11th to 20th	Night	59% 7% 14% 15% 5%	3288
	21st to end	Night	75% 8% 9% 6% 2	4884

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	26% 22% 28% 17% 8% 1	200
	11th to 20th	Day	27% 19% 37% 12% 4 1	270
	21st to end	Day	28% 16% 34% 18% 4	294
February	1st to 10th	Day	29% 20% 40% 10% 2	240
	11th to 20th	Day	26% 22% 38% 14%	240
	21st to end	Day	37% 23% 30% 9% 1	198
March	1st to 10th	Day	25% 15% 34% 20% 6% 1	406
	11th to 20th	Day	24% 14% 35% 20% 5% 1	420
	21st to end	Day	32% 11% 30% 20% 7% 1	462
April	1st to 10th	Day	21% 20% 30% 21% 7% 2	420
	11th to 20th	Day	27% 13% 25% 20% 11% 3	412
	21st to end	Day	25% 10% 32% 21% 8% 3	420
May	1st to 10th	Day	24% 13% 25% 23% 9% 6%	423
	11th to 20th	Day	17% 11% 27% 24% 15% 5%	447
	21st to end	Day	13% 6% 25% 31% 18% 9%	485
June	1st to 10th	Day	14% 12% 30% 27% 14% 4	450
	11th to 20th	Day	14% 8% 29% 28% 16% 6%	475
	21st to end	Day	13% 8% 29% 33% 14% 4	630
July	1st to 10th	Day	9% 6% 20% 29% 26% 10%	540
	11th to 20th	Day	14% 5% 21% 29% 20% 11%	444
	21st to end	Day	10% 2 26% 35% 20% 6%	488
August	1st to 10th	Day	9% 4 20% 27% 26% 14%	446
	11th to 20th	Day	12% 5% 19% 26% 24% 15%	439
	21st to end	Day	13% 7% 21% 26% 20% 13%	495
September	1st to 10th	Day	13% 8% 20% 28% 19% 13%	450
	11th to 20th	Day	10% 4 18% 26% 28% 14%	450
	21st to end	Day	16% 11% 24% 25% 16% 8%	450
October	1st to 10th	Day	22% 10% 24% 23% 14% 7%	450
	11th to 20th	Day	21% 16% 32% 22% 7% 2	450
	21st to end	Day	30% 12% 30% 20% 7% 1	495
November	1st to 10th	Day	25% 18% 30% 17% 8% 2	267
	11th to 20th	Day	27% 19% 31% 13% 8% 2	180
	21st to end	Day	29% 16% 33% 18% 3 1	180
December	1st to 10th	Day	29% 20% 33% 13% 4 1	180
	11th to 20th	Day	26% 27% 30% 14% 3	180
	21st to end	Day	31% 18% 37% 13% 2	198
January	1st to 10th	Night	23% 18% 35% 18% 6% 1	760
	11th to 20th	Night	30% 14% 31% 22% 3	760
	21st to end	Night	22% 12% 34% 26% 6%	835
February	1st to 10th	Night	26% 16% 36% 21% 2	720
	11th to 20th	Night	24% 14% 35% 22% 5%	703
	21st to end	Night	25% 17% 34% 21% 3	592
March	1st to 10th	Night	19% 13% 35% 25% 7%	720
	11th to 20th	Night	22% 14% 33% 24% 6% 1	720
	21st to end	Night	22% 10% 32% 26% 9% 1	792
April	1st to 10th	Night	23% 12% 31% 29% 5%	720
	11th to 20th	Night	21% 11% 32% 25% 10% 1	619
	21st to end	Night	18% 8% 32% 32% 10%	540
May	1st to 10th	Night	18% 6% 30% 31% 13% 2	540
	11th to 20th	Night	16% 4 28% 39% 11% 1	453
	21st to end	Night	12% 5% 26% 37% 19% 1	389
June	1st to 10th	Night	11% 6% 30% 39% 13% 1	360
	11th to 20th	Night	13% 6% 27% 41% 14% 1	360
	21st to end	Night	11% 4 27% 41% 16%	360
July	1st to 10th	Night	6% 1 21% 47% 21% 3	350
	11th to 20th	Night	10% 2 23% 41% 21% 3	422
	21st to end	Night	10% 2 17% 48% 20% 2	591
August	1st to 10th	Night	7% 2 16% 45% 26% 3	537
	11th to 20th	Night	8% 1 16% 39% 32% 3	534
	21st to end	Night	10% 3 20% 37% 28% 3	643
September	1st to 10th	Night	13% 5% 20% 33% 24% 5%	720
	11th to 20th	Night	12% 5% 17% 34% 28% 4	720
	21st to end	Night	19% 5% 20% 30% 21% 6%	720
October	1st to 10th	Night	18% 8% 25% 28% 17% 3	720
	11th to 20th	Night	22% 10% 29% 29% 9% 1	720
	21st to end	Night	26% 11% 29% 29% 4	792
November	1st to 10th	Night	19% 12% 27% 29% 11% 2	740
	11th to 20th	Night	20% 16% 33% 22% 7% 2	760
	21st to end	Night	26% 12% 33% 22% 7%	760
December	1st to 10th	Night	29% 16% 30% 20% 6%	760
	11th to 20th	Night	22% 17% 37% 19% 5%	760
	21st to end	Night	33% 12% 32% 21% 3	836

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