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 2025-02-21.

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	13% 12% 5% 11% 6% 5% 5% 4 6% 7% 28%	108759
	Night	13% 12% 5% 12% 6% 5% 5% 4 5% 6% 27%	109811
Transparency	Day	53% 8% 12% 18% 9% 1	112377
	Night	53% 8% 12% 17% 8% 1	114356
Seeing	Day	21% 19% 30% 18% 8% 3	14882
	Night	21% 18% 29% 23% 9% 1	24340

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	10% 19% 11% 21% 9% 4 4 2 4 3 12%	3597
	11th to 20th	Day	10% 19% 9% 21% 10% 5% 4 3 3 3 11%	3726
	21st to end	Day	13% 15% 9% 16% 9% 7% 6% 4 5% 4 13%	4466
February	1st to 10th	Day	14% 17% 8% 16% 9% 6% 4 3 4 4 16%	1800
	11th to 20th	Day	11% 17% 8% 19% 9% 5% 5% 4 4 3 16%	1753
	21st to end	Day	16% 16% 4 14% 9% 4 4 3 4 4 20%	1543
March	1st to 10th	Day	17% 15% 6% 11% 5% 3 3 3 4 5% 28%	3326
	11th to 20th	Day	8% 18% 6% 11% 6% 4 5% 3 5% 5% 28%	3480
	21st to end	Day	12% 14% 5% 15% 5% 5% 4 3 5% 5% 26%	3898
April	1st to 10th	Day	15% 16% 5% 9% 5% 4 4 4 5% 6% 27%	2280
	11th to 20th	Day	17% 13% 3 8% 6% 4 4 4 4 5% 33%	2254
	21st to end	Day	18% 13% 4 9% 5% 4 4 3 4 5% 30%	2380
May	1st to 10th	Day	18% 10% 3 8% 5% 4 5% 4 4 6% 32%	4231
	11th to 20th	Day	15% 11% 4 9% 5% 6% 6% 4 6% 8% 27%	4398
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	4837
June	1st to 10th	Day	16% 8% 2 7% 5% 5% 5% 4 7% 7% 33%	2660
	11th to 20th	Day	14% 6% 2 7% 6% 5% 6% 4 8% 9% 34%	2660
	21st to end	Day	13% 6% 2 7% 6% 6% 4 4 7% 8% 37%	2660
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 6% 8% 11% 36%	4440
	11th to 20th	Day	8% 5% 2 7% 5% 6% 7% 5% 10% 11% 35%	4440
	21st to end	Day	9% 6% 2 8% 6% 6% 6% 5% 8% 9% 35%	4670
August	1st to 10th	Day	17% 6% 3 7% 4 5% 5% 4 6% 8% 34%	4104
	11th to 20th	Day	13% 6% 3 9% 6% 6% 7% 5% 7% 10% 27%	4024
	21st to end	Day	12% 7% 2 8% 5% 5% 5% 4 6% 8% 37%	4180
September	1st to 10th	Day	15% 7% 3 6% 5% 5% 5% 4 5% 8% 37%	2280
	11th to 20th	Day	14% 5% 2 6% 4 5% 5% 3 6% 6% 44%	2280
	21st to end	Day	17% 8% 3 8% 5% 5% 5% 4 5% 7% 32%	1992
October	1st to 10th	Day	17% 9% 3 8% 5% 4 5% 3 6% 6% 34%	1900
	11th to 20th	Day	19% 11% 3 8% 5% 5% 4 4 5% 6% 29%	1900
	21st to end	Day	21% 17% 4 8% 4 5% 4 4 3 5% 24%	2081
November	1st to 10th	Day	12% 15% 4 9% 6% 5% 4 3 4 5% 32%	1761
	11th to 20th	Day	12% 18% 6% 10% 6% 5% 4 3 5% 7% 23%	1612
	21st to end	Day	14% 19% 6% 11% 5% 6% 4 4 4 5% 20%	1600
December	1st to 10th	Day	12% 24% 9% 17% 7% 4 5% 3 4 4 11%	2600
	11th to 20th	Day	16% 16% 7% 17% 6% 4 4 3 4 4 19%	2986
	21st to end	Day	14% 19% 8% 15% 8% 6% 5% 4 4 5% 12%	3960
January	1st to 10th	Night	12% 16% 8% 24% 11% 5% 4 3 3 4 12%	6349
	11th to 20th	Night	7% 17% 10% 22% 10% 7% 5% 4 4 4 10%	6155
	21st to end	Night	10% 17% 7% 15% 10% 7% 6% 4 5% 4 16%	6490
February	1st to 10th	Night	12% 17% 6% 15% 9% 5% 5% 4 5% 4 18%	2600
	11th to 20th	Night	10% 13% 7% 18% 9% 5% 6% 4 5% 5% 17%	2533
	21st to end	Night	14% 14% 6% 15% 9% 4 5% 3 5% 6% 20%	1910
March	1st to 10th	Night	16% 12% 5% 11% 6% 5% 4 3 4 5% 29%	3512
	11th to 20th	Night	12% 16% 8% 13% 6% 5% 5% 3 5% 5% 22%	3479
	21st to end	Night	13% 11% 4 14% 6% 4 3 3 5% 6% 31%	3732
April	1st to 10th	Night	15% 16% 4 8% 6% 5% 4 3 5% 6% 29%	1900
	11th to 20th	Night	16% 12% 4 8% 4 4 4 3 5% 6% 33%	1875
	21st to end	Night	15% 9% 3 10% 5% 4 4 4 5% 7% 34%	1815
May	1st to 10th	Night	20% 10% 3 7% 5% 5% 6% 4 5% 6% 29%	2709
	11th to 20th	Night	14% 8% 3 8% 5% 4 5% 4 6% 8% 33%	2495
	21st to end	Night	10% 6% 3 7% 4 5% 4 4 6% 8% 43%	2738
June	1st to 10th	Night	14% 7% 3 8% 4 5% 5% 4 6% 9% 36%	1520
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 5% 6% 8% 42%	1520
	21st to end	Night	10% 6% 3 6% 4 5% 6% 5% 6% 9% 41%	1520
July	1st to 10th	Night	8% 7% 3 8% 5% 4 5% 5% 5% 10% 40%	2520
	11th to 20th	Night	8% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	2520
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 6% 8% 43%	2880
August	1st to 10th	Night	16% 7% 2 7% 4 4 4 4 6% 7% 38%	2825
	11th to 20th	Night	13% 7% 2 7% 5% 4 5% 5% 6% 7% 39%	2785
	21st to end	Night	8% 6% 2 6% 4 4 5% 4 7% 9% 45%	3463
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 4 7% 45%	1900
	11th to 20th	Night	14% 6% 2 5% 4 3 5% 3 6% 7% 45%	1900
	21st to end	Night	16% 8% 2 7% 4 4 5% 5% 6% 7% 36%	2184
October	1st to 10th	Night	17% 8% 3 8% 5% 4 4 3 5% 6% 36%	2280
	11th to 20th	Night	17% 10% 3 7% 5% 5% 4 4 6% 6% 33%	2280
	21st to end	Night	24% 13% 4 8% 4 4 4 3 4 5% 28%	2516
November	1st to 10th	Night	12% 15% 3 7% 5% 5% 4 3 5% 6% 36%	2537
	11th to 20th	Night	15% 14% 4 10% 6% 5% 5% 3 6% 6% 27%	2785
	21st to end	Night	13% 16% 7% 11% 6% 6% 5% 4 6% 6% 21%	2800
December	1st to 10th	Night	10% 17% 7% 16% 9% 8% 6% 4 5% 6% 12%	4580
	11th to 20th	Night	15% 18% 8% 13% 6% 5% 4 3 4 4 18%	5208
	21st to end	Night	16% 18% 10% 13% 8% 6% 4 3 5% 5% 14%	6996

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	74% 5% 7% 10% 3	3598
	11th to 20th	Day	74% 7% 6% 9% 3	3757
	21st to end	Day	70% 8% 10% 10% 3	4466
February	1st to 10th	Day	70% 7% 9% 10% 3	1800
	11th to 20th	Day	69% 6% 8% 12% 5%	1762
	21st to end	Day	66% 5% 10% 14% 5% 1	1543
March	1st to 10th	Day	58% 6% 10% 20% 6%	3329
	11th to 20th	Day	53% 9% 15% 14% 8% 1	3480
	21st to end	Day	55% 7% 8% 22% 7% 1	3898
April	1st to 10th	Day	56% 6% 10% 17% 9% 2	2280
	11th to 20th	Day	52% 7% 10% 17% 13% 1	2254
	21st to end	Day	54% 6% 9% 17% 13% 2	2380
May	1st to 10th	Day	51% 7% 11% 18% 12% 2	4243
	11th to 20th	Day	52% 10% 12% 18% 7% 1	4402
	21st to end	Day	39% 9% 11% 24% 14% 2	4837
June	1st to 10th	Day	46% 9% 12% 18% 14% 1	2660
	11th to 20th	Day	42% 9% 15% 21% 12% 1	2660
	21st to end	Day	43% 9% 14% 22% 13% 1	2660
July	1st to 10th	Day	39% 9% 14% 26% 11% 1	4370
	11th to 20th	Day	36% 10% 17% 22% 14%	4365
	21st to end	Day	42% 10% 16% 22% 10% 1	4521
August	1st to 10th	Day	46% 8% 13% 20% 12% 1	4091
	11th to 20th	Day	47% 11% 15% 21% 5%	4063
	21st to end	Day	42% 8% 11% 30% 8% 1	4180
September	1st to 10th	Day	44% 8% 11% 25% 11% 1	2280
	11th to 20th	Day	39% 7% 13% 25% 15% 1	2280
	21st to end	Day	49% 8% 12% 19% 11% 1	1992
October	1st to 10th	Day	41% 8% 14% 23% 14% 1	3160
	11th to 20th	Day	48% 6% 14% 17% 15% 1	3140
	21st to end	Day	45% 6% 14% 26% 8% 1	3407
November	1st to 10th	Day	53% 7% 12% 17% 10% 2	1761
	11th to 20th	Day	59% 7% 11% 15% 6% 2	1612
	21st to end	Day	64% 7% 14% 13% 3	1600
December	1st to 10th	Day	74% 8% 9% 7% 2	2600
	11th to 20th	Day	67% 6% 12% 9% 6%	2986
	21st to end	Day	72% 7% 11% 7% 3	3960
January	1st to 10th	Night	72% 9% 8% 8% 3	6349
	11th to 20th	Night	71% 9% 9% 8% 3	6199
	21st to end	Night	66% 7% 10% 9% 8% 1	6490
February	1st to 10th	Night	65% 7% 11% 11% 5%	2600
	11th to 20th	Night	64% 8% 11% 11% 4	2535
	21st to end	Night	62% 7% 11% 12% 7% 1	1910
March	1st to 10th	Night	55% 8% 13% 18% 6%	3522
	11th to 20th	Night	58% 10% 12% 15% 5%	3480
	21st to end	Night	49% 9% 10% 21% 10% 1	3732
April	1st to 10th	Night	53% 8% 11% 18% 9% 1	1900
	11th to 20th	Night	51% 7% 12% 17% 13% 1	1875
	21st to end	Night	47% 7% 12% 19% 13% 2	1815
May	1st to 10th	Night	52% 9% 12% 16% 9% 3	2716
	11th to 20th	Night	45% 11% 17% 19% 8% 1	2496
	21st to end	Night	34% 11% 16% 20% 17% 2	2739
June	1st to 10th	Night	41% 9% 14% 21% 15% 1	1520
	11th to 20th	Night	36% 9% 15% 24% 13% 3	1520
	21st to end	Night	37% 11% 12% 25% 14% 1	1520
July	1st to 10th	Night	39% 8% 15% 21% 14% 2	2480
	11th to 20th	Night	31% 8% 17% 30% 13% 1	2472
	21st to end	Night	37% 8% 14% 24% 15% 2	2788
August	1st to 10th	Night	42% 8% 15% 25% 10% 1	2814
	11th to 20th	Night	42% 7% 16% 27% 8%	2812
	21st to end	Night	32% 9% 15% 33% 10% 1	3463
September	1st to 10th	Night	37% 7% 16% 25% 12% 2	1900
	11th to 20th	Night	36% 6% 14% 26% 16% 1	1900
	21st to end	Night	45% 8% 14% 21% 12% 1	2184
October	1st to 10th	Night	43% 9% 11% 24% 13%	3800
	11th to 20th	Night	42% 7% 15% 19% 13% 4	3779
	21st to end	Night	44% 6% 15% 26% 8% 1	4140
November	1st to 10th	Night	48% 7% 12% 21% 10% 2	2537
	11th to 20th	Night	54% 8% 13% 16% 7% 2	2785
	21st to end	Night	59% 9% 14% 14% 3	2800
December	1st to 10th	Night	69% 8% 13% 9% 1	4580
	11th to 20th	Night	65% 6% 9% 12% 6%	5208
	21st to end	Night	71% 7% 8% 9% 4	6996

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% 30% 32% 9% 4 1	200
	11th to 20th	Day	29% 35% 29% 6% 1	234
	21st to end	Day	23% 31% 34% 9% 2	330
February	1st to 10th	Day	34% 33% 26% 7%	300
	11th to 20th	Day	36% 31% 31% 2	300
	21st to end	Day	36% 23% 31% 9% 1	355
March	1st to 10th	Day	28% 25% 30% 13% 3 1	460
	11th to 20th	Day	29% 27% 30% 11% 3	460
	21st to end	Day	30% 23% 30% 12% 4 1	506
April	1st to 10th	Day	28% 26% 29% 12% 3 1	460
	11th to 20th	Day	29% 26% 23% 14% 6% 2	452
	21st to end	Day	27% 23% 29% 16% 5% 1	460
May	1st to 10th	Day	25% 20% 30% 14% 8% 3	463
	11th to 20th	Day	18% 16% 34% 22% 8% 3	487
	21st to end	Day	15% 14% 32% 22% 13% 3	529
June	1st to 10th	Day	18% 16% 34% 22% 8% 2	490
	11th to 20th	Day	13% 14% 33% 26% 11% 2	490
	21st to end	Day	15% 15% 36% 24% 6% 3	490
July	1st to 10th	Day	10% 9% 31% 32% 15% 3	485
	11th to 20th	Day	12% 8% 28% 31% 15% 5%	484
	21st to end	Day	12% 8% 39% 27% 11% 3	532
August	1st to 10th	Day	12% 9% 28% 29% 16% 6%	486
	11th to 20th	Day	14% 9% 30% 27% 13% 8%	479
	21st to end	Day	11% 13% 30% 23% 15% 8%	539
September	1st to 10th	Day	18% 13% 24% 22% 16% 7%	490
	11th to 20th	Day	12% 9% 25% 24% 20% 10%	490
	21st to end	Day	20% 16% 28% 18% 13% 4	490
October	1st to 10th	Day	20% 17% 28% 18% 13% 4	490
	11th to 20th	Day	23% 22% 29% 17% 7% 1	490
	21st to end	Day	30% 22% 30% 15% 3	534
November	1st to 10th	Day	22% 24% 30% 14% 9% 1	395
	11th to 20th	Day	27% 26% 31% 13% 3	212
	21st to end	Day	26% 35% 32% 8%	200
December	1st to 10th	Day	29% 29% 31% 9% 3 1	200
	11th to 20th	Day	34% 30% 26% 11% 1	200
	21st to end	Day	25% 32% 30% 10% 2	220
January	1st to 10th	Night	24% 29% 31% 14% 2	800
	11th to 20th	Night	22% 30% 30% 15% 3 1	800
	21st to end	Night	20% 27% 33% 17% 3	880
February	1st to 10th	Night	25% 27% 32% 15% 1	800
	11th to 20th	Night	24% 30% 31% 13% 2	783
	21st to end	Night	27% 26% 30% 15% 2	629
March	1st to 10th	Night	22% 27% 32% 17% 3	760
	11th to 20th	Night	26% 25% 27% 19% 3	760
	21st to end	Night	24% 19% 31% 20% 5%	836
April	1st to 10th	Night	23% 21% 32% 21% 3	760
	11th to 20th	Night	22% 19% 31% 20% 6% 1	751
	21st to end	Night	19% 13% 33% 26% 8%	675
May	1st to 10th	Night	22% 11% 24% 30% 11% 1	498
	11th to 20th	Night	15% 8% 34% 34% 9% 1	374
	21st to end	Night	11% 8% 30% 34% 17%	411
June	1st to 10th	Night	15% 7% 30% 32% 16% 1	380
	11th to 20th	Night	12% 9% 29% 36% 14%	380
	21st to end	Night	11% 8% 29% 42% 10% 1	380
July	1st to 10th	Night	8% 5% 23% 44% 18% 2	370
	11th to 20th	Night	10% 5% 23% 39% 22% 1	368
	21st to end	Night	11% 4 27% 39% 17% 2	495
August	1st to 10th	Night	11% 4 23% 37% 22% 3	567
	11th to 20th	Night	10% 8% 21% 32% 25% 3	564
	21st to end	Night	12% 5% 25% 35% 22% 2	823
September	1st to 10th	Night	16% 8% 25% 29% 19% 3	760
	11th to 20th	Night	16% 7% 19% 28% 26% 5%	760
	21st to end	Night	18% 12% 24% 26% 16% 3	760
October	1st to 10th	Night	20% 11% 30% 20% 17% 2	760
	11th to 20th	Night	23% 17% 28% 24% 7% 1	760
	21st to end	Night	31% 16% 32% 18% 3	836
November	1st to 10th	Night	23% 17% 31% 22% 7% 1	780
	11th to 20th	Night	22% 23% 32% 16% 5% 2	800
	21st to end	Night	24% 25% 28% 18% 4	800
December	1st to 10th	Night	27% 25% 31% 13% 3	800
	11th to 20th	Night	26% 25% 30% 16% 3	800
	21st to end	Night	27% 26% 33% 13% 1	880

Page generated at 2025-02-18 03:26:27. Version ac07

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