MCCC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MCCC Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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% 7% 2 6% 4 4 4 4 5% 7% 41%	113407
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	114853
Transparency	Day	40% 6% 12% 23% 17% 3	116984
	Night	40% 5% 11% 23% 18% 4	119439
Seeing	Day	22% 14% 28% 22% 11% 3	15457
	Night	20% 11% 31% 28% 10% 1	24869

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	13% 6% 2 5% 4 4 4 3 5% 5% 49%	3597
	11th to 20th	Day	13% 9% 3 9% 5% 4 4 3 4 5% 40%	3676
	21st to end	Day	18% 7% 3 7% 5% 4 4 3 4 6% 40%	4466
February	1st to 10th	Day	16% 7% 3 6% 5% 4 4 4 6% 7% 38%	1800
	11th to 20th	Day	16% 9% 2 7% 5% 3 5% 3 4 6% 39%	1897
	21st to end	Day	18% 8% 3 6% 5% 3 4 4 4 6% 39%	1650
March	1st to 10th	Day	21% 4 2 5% 4 4 4 3 4 6% 44%	4476
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 5% 7% 44%	4980
	21st to end	Day	18% 6% 2 6% 4 4 4 3 4 7% 43%	5785
April	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 5% 7% 39%	2400
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 5% 7% 42%	2254
	21st to end	Day	16% 4 3 6% 5% 4 5% 4 6% 7% 40%	2280
May	1st to 10th	Day	28% 8% 3 6% 4 4 4 4 5% 7% 28%	4079
	11th to 20th	Day	24% 8% 3 8% 5% 5% 4 4 5% 6% 27%	4398
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 7% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 4 8% 9% 37%	2660
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 6% 10% 42%	2660
July	1st to 10th	Day	7% 5% 2 8% 5% 5% 7% 5% 9% 11% 37%	4440
	11th to 20th	Day	9% 7% 2 6% 4 5% 5% 5% 8% 10% 39%	4440
	21st to end	Day	11% 8% 3 7% 6% 6% 6% 5% 7% 8% 34%	4735
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 4 7% 8% 34%	4104
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 7% 8% 44%	3739
	21st to end	Day	11% 5% 2 5% 4 3 4 4 5% 8% 48%	4180
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 4 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 2 3 4 3 5% 7% 52%	2252
	21st to end	Day	18% 7% 3 7% 5% 4 4 3 5% 6% 37%	2089
October	1st to 10th	Day	18% 6% 3 4 3 3 4 4 5% 6% 43%	1900
	11th to 20th	Day	15% 5% 2 5% 3 4 4 3 5% 6% 48%	1900
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 5% 46%	2023
November	1st to 10th	Day	12% 7% 2 4 4 3 3 3 4 5% 53%	1761
	11th to 20th	Day	15% 5% 2 6% 3 3 4 3 6% 6% 48%	1800
	21st to end	Day	16% 6% 2 4 3 4 4 4 6% 7% 44%	1663
December	1st to 10th	Day	14% 8% 2 5% 4 3 4 5% 6% 6% 43%	2600
	11th to 20th	Day	21% 10% 2 4 3 3 3 3 4 6% 42%	2986
	21st to end	Day	17% 6% 2 6% 4 5% 6% 4 6% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 5% 49%	6349
	11th to 20th	Night	14% 7% 2 5% 3 3 4 3 4 6% 47%	6205
	21st to end	Night	18% 8% 2 5% 4 4 4 3 5% 6% 42%	6490
February	1st to 10th	Night	17% 9% 2 6% 3 4 4 4 5% 5% 40%	2600
	11th to 20th	Night	19% 8% 2 6% 5% 4 4 3 4 6% 40%	2388
	21st to end	Night	17% 8% 3 6% 3 3 4 3 5% 5% 42%	1980
March	1st to 10th	Night	18% 8% 2 6% 3 3 3 3 4 5% 44%	5346
	11th to 20th	Night	11% 7% 2 6% 3 4 4 3 4 5% 51%	4979
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 5% 43%	5146
April	1st to 10th	Night	18% 6% 2 6% 5% 4 4 4 5% 7% 39%	2000
	11th to 20th	Night	15% 7% 3 6% 5% 4 4 3 5% 7% 41%	1882
	21st to end	Night	19% 5% 2 5% 3 3 4 3 6% 9% 42%	1900
May	1st to 10th	Night	23% 8% 3 7% 4 4 4 4 5% 8% 29%	2890
	11th to 20th	Night	23% 10% 3 8% 4 4 4 4 5% 6% 29%	2495
	21st to end	Night	13% 6% 2 6% 4 3 4 4 6% 9% 42%	2738
June	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 43%	1520
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 7% 9% 41%	1520
	21st to end	Night	11% 4 2 5% 4 4 5% 5% 8% 10% 42%	1520
July	1st to 10th	Night	8% 6% 2 8% 6% 6% 5% 5% 8% 10% 37%	2520
	11th to 20th	Night	11% 6% 3 6% 5% 5% 5% 5% 8% 10% 37%	2520
	21st to end	Night	14% 7% 3 6% 5% 4 4 5% 7% 9% 36%	2801
August	1st to 10th	Night	20% 7% 3 6% 4 4 5% 4 7% 8% 32%	2825
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 3 6% 7% 42%	3058
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	3476
September	1st to 10th	Night	13% 7% 3 5% 4 4 4 4 4 6% 47%	1900
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 5% 6% 53%	1928
	21st to end	Night	18% 7% 2 5% 4 4 4 3 5% 7% 40%	2090
October	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 4 5% 43%	2277
	11th to 20th	Night	15% 5% 1 5% 3 4 4 3 5% 6% 51%	2280
	21st to end	Night	17% 7% 2 4 3 3 3 3 4 5% 48%	2574
November	1st to 10th	Night	12% 5% 2 5% 3 2 3 3 5% 6% 54%	2537
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 4 5% 48%	2600
	21st to end	Night	14% 5% 2 5% 4 4 4 4 5% 7% 44%	2735
December	1st to 10th	Night	13% 8% 3 5% 4 3 4 3 5% 6% 46%	4580
	11th to 20th	Night	22% 7% 2 4 3 3 4 3 4 6% 41%	5208
	21st to end	Night	19% 8% 2 6% 4 4 4 3 5% 5% 40%	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	35% 3 10% 26% 23% 3	3598
	11th to 20th	Day	43% 4 11% 21% 16% 5%	3706
	21st to end	Day	45% 5% 11% 23% 15% 1	4466
February	1st to 10th	Day	44% 6% 14% 24% 11% 2	1800
	11th to 20th	Day	45% 4 10% 23% 16% 2	1906
	21st to end	Day	45% 5% 11% 23% 13% 4	1650
March	1st to 10th	Day	41% 3 10% 23% 16% 7%	4479
	11th to 20th	Day	37% 4 11% 28% 15% 5%	4980
	21st to end	Day	43% 3 10% 16% 23% 5%	5785
April	1st to 10th	Day	43% 5% 11% 22% 15% 5%	2400
	11th to 20th	Day	40% 5% 13% 21% 17% 4	2254
	21st to end	Day	42% 6% 12% 20% 17% 4	2280
May	1st to 10th	Day	54% 7% 11% 17% 10% 2	4092
	11th to 20th	Day	55% 7% 12% 18% 8% 1	4402
	21st to end	Day	34% 8% 12% 26% 17% 2	4837
June	1st to 10th	Day	39% 8% 13% 24% 15% 1	2660
	11th to 20th	Day	41% 8% 13% 24% 11% 2	2660
	21st to end	Day	37% 9% 12% 25% 16% 1	2660
July	1st to 10th	Day	39% 11% 14% 19% 14% 3	4370
	11th to 20th	Day	38% 8% 15% 23% 14% 1	4365
	21st to end	Day	46% 7% 11% 19% 16% 1	4584
August	1st to 10th	Day	47% 8% 13% 22% 10%	4091
	11th to 20th	Day	35% 7% 12% 30% 15% 1	3775
	21st to end	Day	33% 6% 10% 27% 22% 2	4180
September	1st to 10th	Day	39% 6% 11% 19% 22% 3	2280
	11th to 20th	Day	32% 5% 9% 25% 24% 4	2252
	21st to end	Day	47% 6% 7% 18% 20% 2	2089
October	1st to 10th	Day	36% 4 9% 18% 28% 5%	3160
	11th to 20th	Day	24% 4 11% 25% 29% 7%	3140
	21st to end	Day	25% 3 13% 26% 29% 5%	3313
November	1st to 10th	Day	34% 4 13% 25% 22% 3	1761
	11th to 20th	Day	37% 5% 10% 26% 20% 3	1800
	21st to end	Day	38% 4 14% 30% 12% 2	1663
December	1st to 10th	Day	40% 4 16% 26% 12% 2	2600
	11th to 20th	Day	44% 3 12% 29% 11% 2	2986
	21st to end	Day	42% 5% 13% 21% 17% 1	3960
January	1st to 10th	Night	36% 3 9% 25% 21% 6%	6349
	11th to 20th	Night	39% 3 11% 23% 17% 7%	6250
	21st to end	Night	44% 3 9% 23% 19% 3	6490
February	1st to 10th	Night	44% 5% 12% 25% 12% 2	2600
	11th to 20th	Night	45% 4 9% 20% 18% 4	2390
	21st to end	Night	43% 5% 7% 22% 18% 4	1980
March	1st to 10th	Night	42% 4 9% 23% 13% 9%	5356
	11th to 20th	Night	36% 5% 14% 26% 15% 5%	4980
	21st to end	Night	43% 4 7% 16% 25% 5%	5146
April	1st to 10th	Night	43% 6% 10% 20% 17% 4	2000
	11th to 20th	Night	42% 5% 10% 19% 20% 3	1882
	21st to end	Night	40% 5% 10% 21% 19% 5%	1900
May	1st to 10th	Night	52% 7% 9% 19% 11% 1	2896
	11th to 20th	Night	53% 7% 14% 14% 10% 2	2496
	21st to end	Night	37% 8% 16% 21% 16% 3	2739
June	1st to 10th	Night	39% 6% 12% 22% 20% 2	1520
	11th to 20th	Night	39% 8% 12% 26% 14% 1	1520
	21st to end	Night	34% 10% 15% 26% 15% 2	1520
July	1st to 10th	Night	39% 10% 19% 16% 13% 2	2480
	11th to 20th	Night	39% 9% 16% 23% 11% 1	2472
	21st to end	Night	43% 7% 15% 18% 15% 1	2711
August	1st to 10th	Night	47% 8% 17% 17% 9% 1	2814
	11th to 20th	Night	41% 7% 11% 26% 15% 1	3088
	21st to end	Night	33% 6% 10% 29% 20% 2	3476
September	1st to 10th	Night	38% 7% 11% 20% 22% 3	1900
	11th to 20th	Night	32% 5% 8% 23% 27% 4	1928
	21st to end	Night	44% 6% 7% 20% 21% 2	2090
October	1st to 10th	Night	42% 5% 10% 20% 21% 2	3797
	11th to 20th	Night	24% 4 10% 29% 29% 5%	3779
	21st to end	Night	26% 3 11% 32% 24% 4	4234
November	1st to 10th	Night	32% 5% 12% 24% 21% 5%	2537
	11th to 20th	Night	40% 4 8% 22% 20% 5%	2600
	21st to end	Night	38% 5% 13% 27% 16% 2	2735
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4580
	11th to 20th	Night	45% 3 10% 24% 13% 4	5208
	21st to end	Night	45% 5% 11% 20% 18% 2	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	28% 30% 28% 10% 3 2	200
	11th to 20th	Day	37% 24% 27% 9% 2	221
	21st to end	Day	27% 27% 29% 12% 5% 1	330
February	1st to 10th	Day	29% 26% 31% 9% 4	300
	11th to 20th	Day	33% 22% 33% 10% 1	445
	21st to end	Day	30% 22% 32% 14% 3	414
March	1st to 10th	Day	28% 20% 31% 18% 3	500
	11th to 20th	Day	25% 22% 33% 15% 3 1	500
	21st to end	Day	28% 23% 29% 13% 5% 1	550
April	1st to 10th	Day	29% 22% 30% 13% 5% 1	500
	11th to 20th	Day	22% 24% 29% 16% 6% 2	452
	21st to end	Day	26% 18% 31% 19% 4 2	460
May	1st to 10th	Day	30% 17% 28% 16% 7% 2	463
	11th to 20th	Day	24% 11% 30% 24% 9% 2	487
	21st to end	Day	21% 7% 26% 24% 17% 4	529
June	1st to 10th	Day	14% 13% 28% 27% 13% 3	490
	11th to 20th	Day	15% 10% 27% 30% 15% 3	490
	21st to end	Day	14% 10% 30% 29% 13% 4	490
July	1st to 10th	Day	12% 4 22% 32% 23% 7%	485
	11th to 20th	Day	8% 3 26% 32% 26% 5%	484
	21st to end	Day	12% 7% 26% 33% 17% 5%	532
August	1st to 10th	Day	12% 5% 25% 32% 21% 6%	486
	11th to 20th	Day	14% 5% 18% 36% 21% 6%	479
	21st to end	Day	13% 8% 24% 29% 17% 9%	539
September	1st to 10th	Day	18% 11% 21% 26% 17% 8%	490
	11th to 20th	Day	16% 6% 24% 29% 16% 10%	490
	21st to end	Day	27% 12% 20% 25% 11% 5%	490
October	1st to 10th	Day	25% 7% 27% 20% 14% 7%	490
	11th to 20th	Day	18% 14% 29% 25% 11% 2	490
	21st to end	Day	22% 14% 32% 19% 12% 3	503
November	1st to 10th	Day	20% 12% 30% 24% 12% 2	395
	11th to 20th	Day	22% 18% 33% 23% 5% 1	400
	21st to end	Day	24% 18% 35% 17% 5% 1	263
December	1st to 10th	Day	29% 23% 34% 9% 6%	200
	11th to 20th	Day	36% 19% 31% 11% 5%	200
	21st to end	Day	27% 18% 34% 13% 8%	220
January	1st to 10th	Night	22% 18% 37% 20% 3	800
	11th to 20th	Night	26% 17% 30% 24% 3	800
	21st to end	Night	19% 18% 38% 23% 2	880
February	1st to 10th	Night	23% 15% 38% 24% 1	800
	11th to 20th	Night	27% 19% 36% 17% 1	783
	21st to end	Night	24% 18% 39% 18% 3	660
March	1st to 10th	Night	23% 18% 33% 23% 3	800
	11th to 20th	Night	23% 17% 36% 19% 5% 1	800
	21st to end	Night	21% 13% 41% 20% 5%	880
April	1st to 10th	Night	22% 19% 37% 18% 5%	800
	11th to 20th	Night	19% 13% 38% 25% 6%	754
	21st to end	Night	21% 11% 33% 28% 6%	760
May	1st to 10th	Night	24% 11% 32% 26% 7%	613
	11th to 20th	Night	22% 7% 36% 29% 6%	374
	21st to end	Night	16% 7% 23% 39% 14% 1	411
June	1st to 10th	Night	18% 4 28% 38% 11% 1	380
	11th to 20th	Night	11% 5% 32% 40% 11% 1	380
	21st to end	Night	11% 2 27% 48% 11%	380
July	1st to 10th	Night	13% 4 22% 42% 18% 2	370
	11th to 20th	Night	8% 4 22% 43% 22% 2	368
	21st to end	Night	10% 4 24% 40% 20% 1	454
August	1st to 10th	Night	13% 2 17% 44% 23% 2	567
	11th to 20th	Night	14% 3 18% 38% 24% 2	723
	21st to end	Night	14% 4 18% 39% 23% 2	836
September	1st to 10th	Night	17% 3 26% 34% 17% 3	760
	11th to 20th	Night	16% 4 21% 34% 22% 4	760
	21st to end	Night	23% 4 21% 32% 18% 1	760
October	1st to 10th	Night	23% 7% 24% 25% 18% 4	760
	11th to 20th	Night	16% 11% 27% 32% 13% 1	760
	21st to end	Night	21% 11% 31% 27% 9%	836
November	1st to 10th	Night	15% 8% 36% 28% 12% 1	780
	11th to 20th	Night	21% 15% 30% 26% 8% 1	800
	21st to end	Night	17% 15% 35% 26% 7% 1	800
December	1st to 10th	Night	25% 15% 32% 22% 6% 1	800
	11th to 20th	Night	23% 13% 35% 23% 5%	800
	21st to end	Night	23% 14% 33% 27% 3	880

Page generated at 2025-04-08 09:17:27. Version ac07

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