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 2024-09-14.

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% 10% 3 8% 5% 5% 5% 4 6% 7% 32%	101233
	Night	15% 10% 3 8% 5% 4 4 4 5% 7% 34%	96661
Transparency	Day	48% 8% 13% 19% 10% 1	101001
	Night	48% 8% 13% 19% 11% 1	96513
Seeing	Day	20% 11% 28% 23% 13% 5%	14570
	Night	19% 10% 29% 29% 12% 1	23740

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	17% 16% 6% 10% 6% 5% 5% 3 4 5% 23%	2584
	11th to 20th	Day	12% 20% 8% 17% 7% 4 4 3 4 5% 17%	2840
	21st to end	Day	23% 19% 7% 12% 6% 5% 4 3 3 4 14%	3124
February	1st to 10th	Day	16% 16% 5% 12% 8% 6% 5% 4 5% 6% 18%	1710
	11th to 20th	Day	13% 15% 7% 13% 7% 5% 4 4 4 5% 23%	1663
	21st to end	Day	18% 14% 4 11% 7% 5% 5% 3 4 5% 24%	1510
March	1st to 10th	Day	20% 13% 4 8% 6% 5% 3 3 4 6% 27%	3418
	11th to 20th	Day	11% 14% 5% 10% 6% 5% 5% 4 6% 6% 29%	3480
	21st to end	Day	17% 11% 3 10% 5% 5% 4 3 5% 6% 31%	3815
April	1st to 10th	Day	16% 13% 4 9% 5% 5% 5% 3 5% 7% 29%	2201
	11th to 20th	Day	19% 8% 3 8% 4 5% 4 3 5% 7% 34%	2369
	21st to end	Day	20% 10% 4 7% 5% 4 5% 3 4 5% 34%	2470
May	1st to 10th	Day	18% 7% 2 8% 5% 5% 5% 4 5% 7% 33%	4108
	11th to 20th	Day	16% 10% 3 8% 5% 5% 4 4 6% 8% 31%	4229
	21st to end	Day	12% 5% 2 6% 5% 4 4 4 5% 7% 47%	4837
June	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 5% 6% 8% 38%	2660
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	2687
	21st to end	Day	13% 5% 2 6% 4 5% 6% 4 7% 9% 40%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	4605
	11th to 20th	Day	10% 5% 2 6% 4 4 6% 5% 8% 10% 42%	4293
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 9% 41%	4466
August	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 7% 8% 36%	4104
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	4044
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 4 7% 9% 41%	4409
September	1st to 10th	Day	12% 6% 3 7% 4 5% 5% 4 6% 9% 39%	2280
	11th to 20th	Day	10% 5% 2 6% 3 4 5% 4 6% 8% 46%	2028
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 7% 8% 36%	1980
October	1st to 10th	Day	18% 7% 3 8% 5% 4 5% 3 6% 7% 34%	1800
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 4 6% 8% 29%	1800
	21st to end	Day	24% 11% 3 9% 6% 4 5% 4 6% 6% 23%	1980
November	1st to 10th	Day	13% 10% 3 8% 5% 6% 4 4 6% 7% 33%	1651
	11th to 20th	Day	14% 14% 4 7% 6% 5% 5% 5% 7% 7% 26%	1520
	21st to end	Day	19% 15% 5% 9% 5% 4 4 4 6% 7% 22%	1520
December	1st to 10th	Day	15% 18% 5% 11% 7% 5% 5% 4 5% 5% 20%	1520
	11th to 20th	Day	18% 16% 7% 10% 5% 5% 6% 4 6% 5% 19%	1906
	21st to end	Day	29% 19% 7% 11% 5% 4 4 3 4 3 10%	2772
January	1st to 10th	Night	17% 15% 5% 12% 6% 5% 4 4 5% 6% 20%	4361
	11th to 20th	Night	15% 15% 6% 13% 8% 6% 6% 4 5% 5% 17%	4120
	21st to end	Night	23% 16% 7% 12% 5% 4 4 3 4 4 19%	4532
February	1st to 10th	Night	16% 16% 4 10% 7% 6% 5% 4 5% 5% 22%	2470
	11th to 20th	Night	14% 13% 4 9% 7% 5% 5% 4 6% 7% 25%	2403
	21st to end	Night	16% 13% 5% 11% 6% 5% 5% 4 5% 6% 25%	1941
March	1st to 10th	Night	18% 10% 3 8% 6% 4 4 3 5% 6% 33%	3422
	11th to 20th	Night	12% 10% 4 8% 4 4 4 3 5% 7% 38%	3479
	21st to end	Night	17% 10% 3 7% 4 4 4 3 4 5% 40%	3817
April	1st to 10th	Night	19% 9% 3 7% 5% 5% 4 4 5% 6% 33%	1977
	11th to 20th	Night	17% 9% 3 6% 5% 4 5% 4 5% 6% 37%	1777
	21st to end	Night	14% 8% 3 7% 5% 5% 4 4 5% 9% 36%	1710
May	1st to 10th	Night	19% 8% 2 6% 3 4 4 2 5% 7% 39%	2833
	11th to 20th	Night	13% 9% 3 8% 6% 5% 5% 4 6% 7% 35%	2664
	21st to end	Night	11% 4 2 5% 3 4 4 3 5% 8% 51%	2738
June	1st to 10th	Night	11% 5% 2 6% 5% 4 6% 5% 8% 8% 43%	1520
	11th to 20th	Night	10% 5% 2 5% 4 4 6% 5% 7% 9% 44%	1483
	21st to end	Night	11% 5% 2 5% 5% 4 5% 5% 6% 8% 46%	1330
July	1st to 10th	Night	6% 5% 2 5% 4 4 4 3 5% 9% 53%	2351
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 6% 8% 52%	2664
	21st to end	Night	6% 4 2 5% 5% 4 5% 5% 7% 8% 49%	3087
August	1st to 10th	Night	10% 5% 3 6% 4 5% 5% 3 6% 8% 44%	2825
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 6% 8% 47%	2785
	21st to end	Night	8% 4 2 6% 4 4 5% 3 6% 8% 49%	3215
September	1st to 10th	Night	10% 6% 2 7% 5% 4 4 3 4 7% 48%	1900
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 5% 7% 54%	2007
	21st to end	Night	14% 6% 3 7% 4 5% 3 4 5% 6% 42%	1980
October	1st to 10th	Night	16% 8% 2 6% 4 3 4 3 6% 6% 42%	2160
	11th to 20th	Night	18% 9% 2 7% 5% 4 4 4 6% 7% 34%	2160
	21st to end	Night	22% 9% 3 8% 5% 5% 5% 3 5% 6% 29%	2376
November	1st to 10th	Night	14% 9% 2 5% 5% 4 5% 4 6% 8% 38%	2422
	11th to 20th	Night	16% 11% 2 8% 6% 5% 5% 4 6% 7% 29%	2660
	21st to end	Night	20% 10% 3 8% 5% 4 5% 5% 6% 8% 26%	2660
December	1st to 10th	Night	19% 16% 4 10% 6% 5% 4 5% 6% 6% 20%	2660
	11th to 20th	Night	15% 18% 4 9% 5% 5% 4 4 5% 6% 24%	3288
	21st to end	Night	29% 19% 6% 10% 6% 5% 4 3 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	61% 10% 12% 10% 8%	2585
	11th to 20th	Day	71% 5% 11% 10% 4	2840
	21st to end	Day	74% 8% 7% 7% 3	3124
February	1st to 10th	Day	66% 7% 11% 12% 4	1710
	11th to 20th	Day	62% 5% 11% 15% 7% 1	1672
	21st to end	Day	62% 6% 10% 13% 7% 1	1510
March	1st to 10th	Day	57% 6% 11% 20% 5%	3421
	11th to 20th	Day	52% 8% 15% 18% 6% 2	3480
	21st to end	Day	50% 8% 11% 19% 10% 1	3815
April	1st to 10th	Day	55% 6% 11% 15% 12%	2201
	11th to 20th	Day	50% 6% 11% 20% 11% 1	2369
	21st to end	Day	52% 6% 11% 18% 11% 2	2470
May	1st to 10th	Day	47% 8% 13% 18% 11% 3	4120
	11th to 20th	Day	48% 10% 14% 20% 7% 1	4233
	21st to end	Day	35% 8% 13% 21% 19% 4	4837
June	1st to 10th	Day	40% 8% 13% 23% 14% 2	2660
	11th to 20th	Day	39% 9% 14% 22% 15% 1	2687
	21st to end	Day	39% 8% 14% 24% 15%	2850
July	1st to 10th	Day	35% 8% 16% 25% 14% 2	4535
	11th to 20th	Day	33% 8% 15% 32% 12%	4218
	21st to end	Day	35% 9% 14% 21% 18% 1	4324
August	1st to 10th	Day	43% 9% 13% 22% 13% 1	4091
	11th to 20th	Day	40% 10% 17% 24% 9% 1	4083
	21st to end	Day	35% 9% 13% 31% 11% 1	4409
September	1st to 10th	Day	40% 8% 15% 24% 12% 1	2280
	11th to 20th	Day	34% 8% 14% 29% 15%	2028
	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	62% 10% 12% 11% 5% 1	4361
	11th to 20th	Night	65% 9% 12% 10% 3	4120
	21st to end	Night	68% 8% 9% 11% 4 1	4532
February	1st to 10th	Night	61% 7% 11% 15% 5%	2470
	11th to 20th	Night	56% 7% 14% 15% 8%	2405
	21st to end	Night	58% 6% 12% 16% 8%	1941
March	1st to 10th	Night	51% 7% 13% 22% 6% 1	3432
	11th to 20th	Night	43% 7% 18% 19% 12% 1	3480
	21st to end	Night	44% 7% 10% 22% 16% 2	3817
April	1st to 10th	Night	50% 7% 11% 18% 13% 1	1977
	11th to 20th	Night	47% 7% 9% 20% 16% 2	1777
	21st to end	Night	45% 7% 11% 18% 15% 3	1710
May	1st to 10th	Night	44% 6% 12% 18% 15% 4	2840
	11th to 20th	Night	44% 10% 16% 20% 7% 2	2665
	21st to end	Night	30% 8% 14% 23% 24% 2	2739
June	1st to 10th	Night	37% 8% 11% 25% 17% 2	1520
	11th to 20th	Night	34% 8% 13% 25% 17% 2	1483
	21st to end	Night	36% 7% 15% 23% 18% 1	1330
July	1st to 10th	Night	29% 6% 12% 26% 22% 5%	2312
	11th to 20th	Night	30% 7% 14% 32% 17% 1	2616
	21st to end	Night	30% 9% 16% 25% 18% 2	2988
August	1st to 10th	Night	35% 9% 13% 26% 16% 2	2814
	11th to 20th	Night	34% 8% 17% 28% 12% 1	2812
	21st to end	Night	30% 8% 13% 32% 17% 1	3215
September	1st to 10th	Night	36% 6% 13% 26% 16% 2	1900
	11th to 20th	Night	29% 7% 14% 31% 19% 1	2007
	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	27% 16% 34% 18% 4	297
February	1st to 10th	Day	27% 20% 37% 11% 5%	270
	11th to 20th	Day	25% 22% 37% 16% 1	270
	21st to end	Day	36% 21% 32% 10% 1	225
March	1st to 10th	Day	25% 14% 34% 20% 5% 1	446
	11th to 20th	Day	25% 15% 35% 20% 5% 1	460
	21st to end	Day	31% 12% 30% 20% 6% 1	506
April	1st to 10th	Day	22% 19% 31% 20% 6% 2	460
	11th to 20th	Day	29% 14% 25% 19% 10% 3	452
	21st to end	Day	25% 10% 33% 20% 8% 3	460
May	1st to 10th	Day	24% 13% 25% 24% 10% 5%	463
	11th to 20th	Day	17% 11% 27% 24% 16% 5%	487
	21st to end	Day	13% 6% 26% 30% 17% 8%	529
June	1st to 10th	Day	14% 14% 28% 26% 14% 4	490
	11th to 20th	Day	14% 8% 29% 28% 16% 6%	517
	21st to end	Day	14% 9% 29% 32% 13% 3	680
July	1st to 10th	Day	10% 6% 20% 29% 26% 9%	585
	11th to 20th	Day	13% 6% 22% 28% 20% 11%	484
	21st to end	Day	9% 2 26% 34% 21% 7%	532
August	1st to 10th	Day	11% 5% 20% 26% 24% 13%	486
	11th to 20th	Day	12% 4 20% 25% 24% 14%	479
	21st to end	Day	13% 6% 20% 27% 21% 13%	539
September	1st to 10th	Day	13% 8% 21% 26% 18% 13%	490
	11th to 20th	Day	10% 4 17% 26% 28% 14%	463
	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	21% 12% 35% 25% 6%	836
February	1st to 10th	Night	25% 15% 35% 23% 2	760
	11th to 20th	Night	24% 14% 35% 23% 4	743
	21st to end	Night	24% 17% 33% 22% 3	628
March	1st to 10th	Night	19% 12% 35% 26% 7%	760
	11th to 20th	Night	22% 14% 34% 24% 6% 1	760
	21st to end	Night	21% 11% 33% 26% 8% 1	836
April	1st to 10th	Night	24% 11% 31% 28% 5%	760
	11th to 20th	Night	22% 11% 32% 25% 9% 1	653
	21st to end	Night	18% 8% 32% 33% 9%	570
May	1st to 10th	Night	18% 6% 30% 31% 13% 2	570
	11th to 20th	Night	16% 4 27% 39% 12% 1	478
	21st to end	Night	12% 6% 26% 37% 19% 1	411
June	1st to 10th	Night	12% 6% 30% 38% 13% 1	380
	11th to 20th	Night	12% 6% 26% 42% 13% 1	380
	21st to end	Night	12% 4 27% 41% 15% 1	380
July	1st to 10th	Night	6% 1 22% 48% 20% 3	370
	11th to 20th	Night	10% 2 22% 41% 22% 3	447
	21st to end	Night	9% 2 17% 47% 22% 2	624
August	1st to 10th	Night	7% 2 17% 44% 26% 3	567
	11th to 20th	Night	9% 1 16% 39% 32% 3	564
	21st to end	Night	11% 3 19% 37% 28% 3	679
September	1st to 10th	Night	13% 5% 20% 34% 24% 5%	760
	11th to 20th	Night	12% 4 17% 34% 29% 4	736
	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

Page generated at 2024-09-11 10:00:06. Version ac07

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