C.E.K. Mees Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C.E.K. Mees 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	14% 11% 5% 9% 7% 4 5% 5% 5% 7% 28%	74854
	Night	14% 11% 5% 9% 7% 4 5% 5% 5% 6% 29%	75266
Transparency	Day	54% 8% 12% 17% 9% 1	74564
	Night	53% 8% 12% 17% 8% 1	75090
Seeing	Day	21% 19% 30% 18% 8% 3	13537
	Night	20% 18% 29% 23% 9% 1	22992

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	9% 17% 9% 14% 11% 5% 7% 5% 4 5% 14%	1517
	11th to 20th	Day	13% 19% 8% 14% 11% 4 5% 4 3 4 14%	1582
	21st to end	Day	9% 14% 10% 16% 14% 6% 7% 5% 4 4 11%	1872
February	1st to 10th	Day	15% 15% 9% 14% 11% 5% 4 4 4 4 15%	1620
	11th to 20th	Day	11% 15% 10% 17% 9% 4 5% 5% 4 4 16%	1573
	21st to end	Day	17% 16% 5% 14% 10% 4 5% 4 4 4 19%	1455
March	1st to 10th	Day	13% 13% 7% 15% 8% 3 5% 4 5% 5% 22%	1909
	11th to 20th	Day	12% 17% 7% 9% 7% 3 5% 4 5% 5% 26%	1980
	21st to end	Day	17% 13% 6% 10% 7% 3 4 3 5% 5% 26%	2220
April	1st to 10th	Day	14% 14% 6% 9% 6% 2 5% 4 5% 6% 28%	2160
	11th to 20th	Day	17% 12% 4 6% 6% 3 4 4 4 5% 34%	2134
	21st to end	Day	18% 13% 4 8% 6% 3 5% 4 4 5% 30%	2254
May	1st to 10th	Day	16% 11% 4 7% 7% 4 5% 5% 4 6% 31%	2407
	11th to 20th	Day	16% 9% 3 6% 6% 3 6% 4 5% 8% 34%	2478
	21st to end	Day	13% 6% 3 6% 6% 4 5% 5% 6% 8% 38%	2725
June	1st to 10th	Day	17% 7% 3 6% 6% 3 5% 5% 6% 7% 35%	2520
	11th to 20th	Day	14% 5% 2 6% 7% 4 6% 6% 8% 8% 35%	2520
	21st to end	Day	13% 6% 2 6% 6% 4 4 5% 6% 8% 38%	2520
July	1st to 10th	Day	10% 6% 2 6% 6% 4 5% 7% 7% 9% 39%	2520
	11th to 20th	Day	11% 5% 3 6% 7% 5% 6% 6% 8% 9% 34%	2520
	21st to end	Day	11% 6% 2 7% 7% 4 7% 6% 8% 9% 32%	2667
August	1st to 10th	Day	11% 5% 3 6% 6% 5% 6% 6% 6% 10% 36%	2327
	11th to 20th	Day	12% 6% 3 7% 7% 5% 7% 6% 7% 9% 30%	2297
	21st to end	Day	14% 7% 3 6% 7% 4 5% 6% 7% 8% 33%	2376
September	1st to 10th	Day	16% 6% 3 5% 6% 4 5% 5% 5% 8% 36%	2160
	11th to 20th	Day	15% 5% 2 5% 5% 3 6% 4 6% 6% 43%	2160
	21st to end	Day	16% 7% 3 7% 6% 4 6% 6% 5% 7% 34%	1888
October	1st to 10th	Day	17% 9% 3 7% 7% 3 5% 4 5% 6% 33%	1800
	11th to 20th	Day	20% 11% 4 8% 7% 4 4 5% 5% 6% 28%	1800
	21st to end	Day	23% 16% 4 7% 6% 4 4 4 3 5% 23%	1972
November	1st to 10th	Day	12% 14% 4 8% 7% 4 4 4 4 5% 32%	1671
	11th to 20th	Day	13% 17% 6% 10% 7% 4 4 4 5% 7% 23%	1532
	21st to end	Day	14% 18% 7% 10% 6% 5% 5% 5% 4 5% 21%	1520
December	1st to 10th	Day	14% 22% 9% 14% 7% 3 4 4 4 4 16%	1520
	11th to 20th	Day	16% 17% 8% 15% 7% 4 4 4 5% 5% 16%	1906
	21st to end	Day	18% 22% 8% 13% 8% 4 4 4 3 4 11%	2772
January	1st to 10th	Night	12% 16% 8% 14% 11% 5% 7% 5% 5% 5% 14%	2649
	11th to 20th	Night	12% 16% 8% 14% 12% 5% 5% 5% 5% 5% 14%	2594
	21st to end	Night	9% 14% 8% 13% 13% 7% 7% 6% 5% 4 14%	2711
February	1st to 10th	Night	12% 16% 7% 14% 10% 4 5% 5% 5% 4 18%	2340
	11th to 20th	Night	11% 12% 8% 15% 10% 3 7% 5% 5% 5% 18%	2273
	21st to end	Night	15% 12% 6% 14% 11% 3 5% 4 4 6% 19%	1799
March	1st to 10th	Night	11% 13% 8% 13% 9% 4 5% 4 5% 6% 23%	2036
	11th to 20th	Night	16% 13% 6% 9% 7% 3 5% 4 4 6% 28%	1979
	21st to end	Night	14% 13% 5% 10% 8% 3 4 4 4 5% 30%	2134
April	1st to 10th	Night	14% 14% 5% 7% 6% 4 5% 5% 5% 6% 29%	1800
	11th to 20th	Night	16% 11% 4 7% 5% 4 4 4 5% 6% 33%	1775
	21st to end	Night	15% 9% 4 9% 7% 3 5% 4 5% 6% 33%	1720
May	1st to 10th	Night	17% 10% 3 5% 6% 4 6% 5% 5% 7% 30%	1552
	11th to 20th	Night	11% 7% 3 7% 6% 3 5% 6% 6% 8% 39%	1415
	21st to end	Night	11% 6% 2 7% 4 3 4 5% 6% 7% 45%	1550
June	1st to 10th	Night	14% 6% 3 6% 5% 3 6% 5% 6% 8% 37%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 6% 5% 8% 43%	1440
	21st to end	Night	10% 5% 2 6% 5% 3 6% 6% 6% 9% 42%	1440
July	1st to 10th	Night	6% 6% 3 6% 6% 2 5% 6% 5% 10% 45%	1440
	11th to 20th	Night	10% 4 2 4 6% 2 5% 6% 6% 9% 46%	1440
	21st to end	Night	11% 6% 3 5% 4 3 5% 5% 7% 8% 43%	1648
August	1st to 10th	Night	9% 5% 2 5% 6% 4 5% 5% 6% 8% 45%	1611
	11th to 20th	Night	11% 7% 2 6% 6% 3 4 7% 5% 8% 42%	1598
	21st to end	Night	10% 6% 2 6% 4 3 4 4 6% 9% 45%	1967
September	1st to 10th	Night	14% 5% 2 5% 5% 3 4 5% 4 7% 45%	1800
	11th to 20th	Night	15% 6% 3 4 4 2 5% 5% 6% 7% 44%	1800
	21st to end	Night	15% 6% 3 5% 5% 3 5% 6% 6% 7% 38%	2068
October	1st to 10th	Night	17% 8% 3 7% 6% 3 5% 4 5% 6% 35%	2160
	11th to 20th	Night	17% 10% 3 7% 6% 3 5% 5% 6% 6% 32%	2160
	21st to end	Night	25% 13% 4 7% 5% 3 3 4 4 5% 26%	2383
November	1st to 10th	Night	13% 14% 3 6% 6% 3 5% 4 5% 6% 35%	2407
	11th to 20th	Night	15% 13% 5% 9% 7% 4 5% 4 5% 6% 27%	2645
	21st to end	Night	13% 15% 7% 10% 7% 5% 5% 5% 6% 6% 22%	2660
December	1st to 10th	Night	15% 18% 8% 11% 7% 4 6% 5% 5% 5% 15%	2660
	11th to 20th	Night	15% 17% 9% 13% 7% 4 5% 4 4 5% 19%	3288
	21st to end	Night	20% 17% 12% 10% 8% 4 4 4 5% 5% 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	69% 8% 8% 11% 3	1518
	11th to 20th	Day	73% 8% 8% 8% 3	1582
	21st to end	Day	73% 7% 9% 7% 3	1872
February	1st to 10th	Day	72% 6% 9% 8% 3	1620
	11th to 20th	Day	70% 5% 8% 12% 4	1582
	21st to end	Day	67% 5% 9% 13% 5% 1	1455
March	1st to 10th	Day	63% 6% 12% 14% 5% 1	1912
	11th to 20th	Day	58% 6% 11% 14% 10% 2	1980
	21st to end	Day	59% 6% 7% 16% 10% 2	2220
April	1st to 10th	Day	56% 6% 10% 17% 9% 2	2160
	11th to 20th	Day	51% 6% 10% 18% 14% 1	2134
	21st to end	Day	54% 6% 9% 17% 12% 2	2254
May	1st to 10th	Day	53% 6% 10% 18% 11% 2	2407
	11th to 20th	Day	47% 9% 11% 19% 12% 2	2482
	21st to end	Day	43% 9% 13% 21% 12% 1	2725
June	1st to 10th	Day	45% 8% 12% 18% 15% 1	2520
	11th to 20th	Day	43% 9% 14% 21% 12% 1	2520
	21st to end	Day	43% 9% 14% 22% 13% 1	2520
July	1st to 10th	Day	40% 10% 12% 25% 11% 2	2450
	11th to 20th	Day	41% 10% 16% 21% 11%	2445
	21st to end	Day	44% 10% 14% 20% 10% 1	2518
August	1st to 10th	Day	41% 10% 15% 22% 11% 1	2314
	11th to 20th	Day	46% 11% 13% 21% 8% 1	2297
	21st to end	Day	46% 8% 13% 22% 9% 1	2376
September	1st to 10th	Day	45% 8% 11% 24% 10% 1	2160
	11th to 20th	Day	40% 8% 13% 24% 15% 1	2160
	21st to end	Day	47% 8% 12% 20% 11% 1	1888
October	1st to 10th	Day	50% 7% 13% 17% 12% 1	1800
	11th to 20th	Day	56% 7% 13% 16% 7% 1	1800
	21st to end	Day	63% 6% 11% 13% 6% 2	1972
November	1st to 10th	Day	53% 7% 12% 16% 10% 2	1671
	11th to 20th	Day	59% 8% 11% 15% 6% 1	1532
	21st to end	Day	63% 6% 14% 13% 3	1520
December	1st to 10th	Day	71% 7% 11% 9% 2	1520
	11th to 20th	Day	69% 7% 13% 7% 2 1	1906
	21st to end	Day	76% 7% 8% 8% 1	2772
January	1st to 10th	Night	68% 8% 10% 10% 4	2649
	11th to 20th	Night	69% 8% 11% 8% 3	2594
	21st to end	Night	68% 9% 10% 9% 4	2711
February	1st to 10th	Night	66% 7% 11% 11% 5%	2340
	11th to 20th	Night	65% 8% 11% 11% 5%	2275
	21st to end	Night	63% 7% 11% 12% 7% 1	1799
March	1st to 10th	Night	60% 7% 11% 16% 5% 1	2046
	11th to 20th	Night	57% 7% 10% 17% 9% 1	1980
	21st to end	Night	56% 6% 9% 15% 12% 2	2134
April	1st to 10th	Night	52% 8% 11% 18% 9% 1	1800
	11th to 20th	Night	50% 6% 12% 17% 14% 1	1775
	21st to end	Night	48% 7% 12% 18% 12% 2	1720
May	1st to 10th	Night	51% 8% 11% 16% 12% 2	1552
	11th to 20th	Night	41% 9% 13% 23% 13% 1	1416
	21st to end	Night	35% 11% 16% 22% 15% 2	1551
June	1st to 10th	Night	41% 9% 13% 21% 15% 1	1440
	11th to 20th	Night	36% 10% 15% 23% 13% 3	1440
	21st to end	Night	35% 11% 13% 25% 14% 1	1440
July	1st to 10th	Night	33% 10% 15% 23% 17% 3	1400
	11th to 20th	Night	33% 9% 14% 28% 16% 1	1392
	21st to end	Night	36% 10% 15% 24% 14% 2	1556
August	1st to 10th	Night	34% 9% 16% 28% 12% 1	1600
	11th to 20th	Night	39% 8% 16% 29% 8%	1598
	21st to end	Night	35% 9% 15% 27% 12% 2	1967
September	1st to 10th	Night	37% 7% 17% 25% 12% 3	1800
	11th to 20th	Night	37% 6% 14% 25% 16% 1	1800
	21st to end	Night	42% 9% 14% 22% 13% 1	2068
October	1st to 10th	Night	47% 9% 13% 18% 13% 1	2160
	11th to 20th	Night	49% 8% 16% 18% 9% 1	2160
	21st to end	Night	59% 8% 13% 13% 6% 2	2383
November	1st to 10th	Night	48% 7% 12% 20% 10% 3	2407
	11th to 20th	Night	54% 9% 13% 16% 7% 2	2645
	21st to end	Night	59% 9% 14% 14% 3	2660
December	1st to 10th	Night	68% 7% 14% 9% 2	2660
	11th to 20th	Night	67% 7% 11% 12% 3	3288
	21st to end	Night	74% 7% 9% 9% 1	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	24% 28% 33% 9% 4 1	180
	11th to 20th	Day	28% 35% 30% 7% 1	210
	21st to end	Day	19% 33% 34% 11% 2	294
February	1st to 10th	Day	32% 36% 26% 5%	240
	11th to 20th	Day	34% 33% 32% 2	240
	21st to end	Day	36% 24% 30% 9% 1	321
March	1st to 10th	Day	28% 25% 30% 13% 3	420
	11th to 20th	Day	28% 27% 30% 12% 3	420
	21st to end	Day	30% 23% 30% 13% 5% 1	462
April	1st to 10th	Day	27% 25% 30% 12% 4 1	420
	11th to 20th	Day	27% 25% 24% 15% 7% 2	412
	21st to end	Day	27% 23% 29% 16% 4 1	420
May	1st to 10th	Day	25% 21% 30% 13% 9% 3	423
	11th to 20th	Day	19% 17% 33% 21% 7% 3	447
	21st to end	Day	15% 15% 31% 22% 13% 4	485
June	1st to 10th	Day	18% 15% 34% 23% 8% 2	450
	11th to 20th	Day	14% 14% 33% 25% 11% 3	450
	21st to end	Day	14% 14% 36% 26% 7% 4	450
July	1st to 10th	Day	10% 7% 29% 34% 16% 4	445
	11th to 20th	Day	12% 7% 28% 32% 15% 6%	444
	21st to end	Day	13% 9% 38% 27% 11% 3	488
August	1st to 10th	Day	9% 9% 29% 30% 16% 7%	446
	11th to 20th	Day	14% 8% 30% 27% 13% 9%	439
	21st to end	Day	11% 13% 30% 23% 15% 8%	495
September	1st to 10th	Day	18% 12% 24% 22% 15% 8%	450
	11th to 20th	Day	13% 10% 26% 23% 18% 10%	450
	21st to end	Day	17% 17% 28% 19% 14% 5%	450
October	1st to 10th	Day	21% 17% 27% 18% 14% 4	450
	11th to 20th	Day	23% 23% 30% 17% 6% 1	450
	21st to end	Day	31% 22% 29% 15% 2	491
November	1st to 10th	Day	20% 24% 31% 15% 10% 1	365
	11th to 20th	Day	28% 27% 30% 13% 3	192
	21st to end	Day	24% 34% 33% 9%	180
December	1st to 10th	Day	28% 29% 29% 9% 3 1	180
	11th to 20th	Day	33% 29% 26% 11% 1	180
	21st to end	Day	26% 33% 30% 9% 2	198
January	1st to 10th	Night	24% 29% 31% 15% 2	760
	11th to 20th	Night	21% 30% 30% 15% 3 1	760
	21st to end	Night	18% 28% 33% 18% 3	835
February	1st to 10th	Night	24% 27% 34% 14% 1	720
	11th to 20th	Night	23% 30% 32% 13% 2	703
	21st to end	Night	26% 26% 31% 15% 2	592
March	1st to 10th	Night	22% 27% 32% 17% 3	720
	11th to 20th	Night	27% 24% 28% 19% 4	720
	21st to end	Night	24% 18% 32% 20% 5% 1	792
April	1st to 10th	Night	23% 21% 33% 21% 3	720
	11th to 20th	Night	21% 20% 32% 20% 6% 2	711
	21st to end	Night	20% 13% 33% 26% 8%	640
May	1st to 10th	Night	22% 11% 24% 30% 12% 1	472
	11th to 20th	Night	15% 8% 35% 33% 8% 1	354
	21st to end	Night	11% 8% 31% 33% 17%	389
June	1st to 10th	Night	15% 7% 29% 32% 16% 1	360
	11th to 20th	Night	11% 9% 28% 36% 15%	360
	21st to end	Night	10% 8% 28% 43% 10% 1	360
July	1st to 10th	Night	7% 4 23% 45% 19% 2	350
	11th to 20th	Night	10% 5% 23% 39% 22% 1	348
	21st to end	Night	11% 4 28% 38% 17% 2	468
August	1st to 10th	Night	11% 4 23% 37% 22% 3	537
	11th to 20th	Night	10% 7% 21% 34% 25% 4	534
	21st to end	Night	12% 5% 25% 35% 22% 2	779
September	1st to 10th	Night	16% 7% 25% 30% 19% 3	720
	11th to 20th	Night	17% 7% 20% 28% 24% 4	720
	21st to end	Night	16% 12% 25% 26% 17% 3	720
October	1st to 10th	Night	20% 11% 29% 20% 18% 2	720
	11th to 20th	Night	23% 17% 29% 24% 6% 1	720
	21st to end	Night	32% 16% 31% 18% 3	792
November	1st to 10th	Night	22% 16% 32% 22% 7% 1	740
	11th to 20th	Night	22% 24% 32% 16% 5% 2	760
	21st to end	Night	23% 24% 29% 19% 4	760
December	1st to 10th	Night	28% 25% 31% 13% 3	760
	11th to 20th	Night	25% 25% 30% 16% 3	760
	21st to end	Night	27% 26% 33% 12% 1	836

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