Mabee Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mabee Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	9% 6% 2 6% 4 3 4 3 5% 7% 52%	138176
	Night	9% 5% 2 5% 3 3 4 3 5% 7% 54%	134851
Transparency	Day	31% 5% 11% 27% 21% 5%	141867
	Night	29% 4 11% 27% 23% 6%	139227
Seeing	Day	11% 9% 29% 31% 17% 4	17908
	Night	11% 8% 34% 35% 11% 1	27757

By Month

Percentage Cloud Cover SE (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% 7% 2 7% 4 4 4 3 5% 7% 46%	5289
	11th to 20th	Day	5% 7% 3 8% 5% 4 3 3 5% 7% 52%	5310
	21st to end	Day	9% 10% 3 8% 3 3 3 2 4 5% 50%	5812
February	1st to 10th	Day	8% 10% 3 7% 4 3 4 3 5% 6% 48%	1890
	11th to 20th	Day	7% 8% 3 8% 5% 4 4 3 5% 6% 48%	2044
	21st to end	Day	8% 5% 2 5% 3 4 4 3 6% 7% 53%	1730
March	1st to 10th	Day	7% 7% 2 5% 4 3 4 3 4 6% 56%	4910
	11th to 20th	Day	8% 5% 1 4 3 3 3 3 4 7% 60%	5090
	21st to end	Day	9% 6% 3 7% 4 4 4 4 6% 8% 46%	5583
April	1st to 10th	Day	10% 7% 2 5% 3 3 3 3 5% 6% 52%	2310
	11th to 20th	Day	11% 7% 3 5% 3 4 4 3 5% 7% 47%	2593
	21st to end	Day	15% 8% 2 6% 4 3 4 3 5% 6% 42%	2730
May	1st to 10th	Day	17% 8% 2 6% 3 3 4 3 4 5% 44%	7617
	11th to 20th	Day	8% 4 2 5% 3 3 3 3 5% 6% 57%	7627
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 7% 39%	8392
June	1st to 10th	Day	6% 4 2 4 3 4 4 5% 6% 10% 52%	2730
	11th to 20th	Day	6% 3 2 4 4 4 4 4 6% 9% 53%	2730
	21st to end	Day	6% 3 2 4 4 4 4 4 6% 9% 54%	2730
July	1st to 10th	Day	7% 4 1 5% 4 4 4 4 6% 9% 52%	6030
	11th to 20th	Day	8% 4 1 5% 4 3 5% 4 6% 9% 52%	5966
	21st to end	Day	5% 3 2 5% 4 4 4 4 6% 10% 51%	6422
August	1st to 10th	Day	6% 4 2 6% 4 4 5% 4 5% 8% 52%	4234
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 5% 8% 53%	4171
	21st to end	Day	4 3 1 3 3 3 4 4 5% 8% 62%	4537
September	1st to 10th	Day	10% 4 2 5% 3 3 3 3 5% 6% 55%	2200
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 5% 7% 53%	2200
	21st to end	Day	8% 5% 2 5% 3 3 5% 4 5% 6% 55%	2200
October	1st to 10th	Day	10% 4 2 4 3 3 4 3 5% 7% 56%	2130
	11th to 20th	Day	9% 4 2 3 2 2 3 2 3 5% 64%	2000
	21st to end	Day	12% 6% 2 4 3 3 3 2 4 5% 56%	2200
November	1st to 10th	Day	7% 5% 2 3 3 2 2 3 5% 6% 62%	1891
	11th to 20th	Day	8% 6% 1 5% 3 3 3 3 4 6% 59%	1890
	21st to end	Day	12% 5% 1 4 3 2 2 2 3 6% 59%	1890
December	1st to 10th	Day	5% 4 2 4 3 3 3 3 3 6% 64%	3030
	11th to 20th	Day	7% 5% 2 5% 3 3 3 2 3 6% 60%	3481
	21st to end	Day	14% 10% 4 11% 4 4 4 3 4 6% 37%	4587
January	1st to 10th	Night	11% 8% 3 8% 4 4 4 3 4 6% 45%	7634
	11th to 20th	Night	4 5% 2 5% 4 4 4 4 4 6% 58%	7680
	21st to end	Night	13% 8% 3 6% 3 3 3 3 4 5% 49%	8402
February	1st to 10th	Night	9% 9% 3 7% 4 4 4 3 4 6% 48%	2730
	11th to 20th	Night	9% 8% 3 6% 3 4 3 4 5% 7% 48%	2480
	21st to end	Night	7% 6% 3 5% 3 3 3 4 5% 6% 55%	2076
March	1st to 10th	Night	7% 6% 2 4 3 3 3 3 4 6% 61%	5168
	11th to 20th	Night	5% 4 1 4 3 4 4 3 5% 7% 60%	5090
	21st to end	Night	9% 7% 2 5% 4 4 4 4 5% 6% 50%	5588
April	1st to 10th	Night	10% 5% 2 4 3 3 4 3 5% 8% 53%	2310
	11th to 20th	Night	12% 6% 2 5% 3 4 3 4 5% 6% 51%	1972
	21st to end	Night	15% 6% 2 5% 5% 4 4 3 5% 6% 46%	1890
May	1st to 10th	Night	14% 7% 2 4 3 3 4 3 5% 6% 49%	5266
	11th to 20th	Night	9% 4 2 4 3 3 3 3 5% 8% 56%	5265
	21st to end	Night	18% 6% 2 5% 3 3 3 3 5% 6% 47%	5793
June	1st to 10th	Night	6% 3 2 4 3 3 5% 4 7% 11% 53%	1890
	11th to 20th	Night	4 3 1 3 4 5% 4 4 7% 9% 54%	1890
	21st to end	Night	5% 3 1 4 4 3 4 3 5% 8% 59%	1890
July	1st to 10th	Night	5% 3 1 5% 3 4 4 4 6% 9% 56%	4168
	11th to 20th	Night	5% 3 1 5% 3 4 4 4 6% 10% 53%	4122
	21st to end	Night	6% 4 2 5% 3 4 4 4 6% 10% 51%	4449
August	1st to 10th	Night	4 3 2 4 4 5% 4 3 6% 9% 56%	2926
	11th to 20th	Night	6% 4 2 6% 5% 5% 5% 3 6% 8% 52%	2888
	21st to end	Night	4 3 1 4 3 2 3 3 5% 8% 64%	3352
September	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 6% 61%	2200
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 5% 6% 59%	2200
	21st to end	Night	7% 4 2 4 3 3 3 3 6% 6% 60%	2200
October	1st to 10th	Night	10% 4 2 5% 3 3 2 3 4 5% 58%	2250
	11th to 20th	Night	9% 4 1 3 2 2 3 3 3 6% 63%	2400
	21st to end	Night	11% 5% 2 4 3 3 2 2 4 5% 59%	2640
November	1st to 10th	Night	8% 4 1 4 2 2 2 2 3 5% 65%	2623
	11th to 20th	Night	8% 4 2 3 2 2 3 3 5% 6% 61%	2730
	21st to end	Night	12% 6% 2 4 2 2 3 2 4 5% 57%	2730
December	1st to 10th	Night	4 5% 2 5% 3 2 4 3 4 5% 65%	4380
	11th to 20th	Night	8% 5% 2 4 3 3 3 2 4 7% 59%	4946
	21st to end	Night	14% 8% 3 7% 5% 4 3 3 4 6% 43%	6633

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	37% 3 10% 28% 18% 3	5290
	11th to 20th	Day	32% 3 11% 26% 21% 5%	5310
	21st to end	Day	38% 3 9% 25% 22% 3	5812
February	1st to 10th	Day	38% 6% 11% 26% 15% 4	1890
	11th to 20th	Day	36% 5% 13% 25% 15% 5%	2054
	21st to end	Day	30% 4 10% 24% 26% 6%	1730
March	1st to 10th	Day	31% 3 7% 19% 26% 14%	4910
	11th to 20th	Day	25% 3 10% 20% 28% 13%	5090
	21st to end	Day	33% 6% 13% 35% 11% 2	5585
April	1st to 10th	Day	32% 4 12% 25% 21% 5%	2310
	11th to 20th	Day	37% 6% 11% 25% 17% 5%	2593
	21st to end	Day	41% 6% 11% 27% 13% 1	2730
May	1st to 10th	Day	42% 5% 9% 21% 21% 3	7655
	11th to 20th	Day	27% 5% 9% 25% 28% 6%	7631
	21st to end	Day	44% 6% 11% 21% 16% 3	8405
June	1st to 10th	Day	27% 7% 18% 29% 17% 3	2730
	11th to 20th	Day	26% 6% 15% 28% 20% 4	2730
	21st to end	Day	26% 6% 15% 26% 21% 6%	2730
July	1st to 10th	Day	28% 6% 13% 26% 23% 4	5966
	11th to 20th	Day	28% 6% 12% 30% 21% 3	5897
	21st to end	Day	26% 7% 14% 35% 17% 1	6240
August	1st to 10th	Day	29% 6% 13% 25% 22% 4	4220
	11th to 20th	Day	30% 5% 17% 27% 18% 3	4210
	21st to end	Day	20% 6% 16% 35% 19% 5%	4536
September	1st to 10th	Day	30% 4 12% 24% 22% 8%	2200
	11th to 20th	Day	29% 6% 11% 29% 21% 4	2200
	21st to end	Day	29% 6% 8% 26% 24% 8%	2200
October	1st to 10th	Day	21% 3 7% 19% 42% 8%	3468
	11th to 20th	Day	17% 3 8% 29% 34% 9%	3240
	21st to end	Day	27% 3 8% 26% 29% 7%	3538
November	1st to 10th	Day	24% 4 10% 30% 25% 8%	1891
	11th to 20th	Day	28% 4 9% 28% 27% 6%	1890
	21st to end	Day	29% 2 11% 31% 23% 4	1890
December	1st to 10th	Day	23% 3 10% 40% 23% 1	3030
	11th to 20th	Day	28% 2 9% 36% 22% 2	3479
	21st to end	Day	48% 4 8% 28% 11% 1	4587
January	1st to 10th	Night	39% 3 11% 24% 21% 2	7635
	11th to 20th	Night	27% 3 11% 30% 23% 6%	7680
	21st to end	Night	38% 3 9% 22% 24% 4	8402
February	1st to 10th	Night	38% 4 12% 25% 16% 5%	2730
	11th to 20th	Night	35% 5% 12% 26% 17% 5%	2480
	21st to end	Night	29% 4 10% 22% 26% 10%	2076
March	1st to 10th	Night	25% 4 7% 24% 30% 11%	5168
	11th to 20th	Night	24% 4 7% 18% 32% 16%	5090
	21st to end	Night	33% 5% 15% 32% 12% 2	5588
April	1st to 10th	Night	30% 5% 13% 22% 24% 6%	2310
	11th to 20th	Night	33% 5% 12% 24% 21% 5%	1972
	21st to end	Night	39% 5% 12% 22% 18% 4	1890
May	1st to 10th	Night	35% 5% 12% 23% 22% 4	5293
	11th to 20th	Night	26% 6% 13% 26% 21% 9%	5266
	21st to end	Night	38% 5% 14% 24% 16% 2	5805
June	1st to 10th	Night	24% 8% 21% 28% 16% 3	1890
	11th to 20th	Night	24% 6% 16% 29% 21% 4	1890
	21st to end	Night	23% 6% 14% 29% 21% 7%	1890
July	1st to 10th	Night	24% 7% 15% 28% 23% 4	4125
	11th to 20th	Night	25% 5% 12% 31% 24% 3	4071
	21st to end	Night	27% 7% 14% 37% 15% 1	4320
August	1st to 10th	Night	25% 6% 13% 28% 21% 7%	2916
	11th to 20th	Night	30% 6% 14% 27% 19% 3	2915
	21st to end	Night	19% 5% 15% 38% 18% 5%	3352
September	1st to 10th	Night	26% 4 11% 25% 23% 10%	2200
	11th to 20th	Night	25% 6% 12% 26% 26% 5%	2200
	21st to end	Night	24% 5% 9% 25% 29% 8%	2200
October	1st to 10th	Night	21% 3 6% 15% 46% 10%	3679
	11th to 20th	Night	16% 4 6% 27% 33% 14%	3899
	21st to end	Night	24% 3 7% 23% 37% 6%	4252
November	1st to 10th	Night	23% 4 9% 28% 28% 8%	2623
	11th to 20th	Night	24% 3 9% 29% 28% 6%	2730
	21st to end	Night	31% 4 10% 27% 22% 7%	2730
December	1st to 10th	Night	23% 3 11% 40% 21% 2	4380
	11th to 20th	Night	27% 3 7% 33% 24% 5%	4947
	21st to end	Night	42% 4 10% 28% 13% 3	6633

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	12% 12% 37% 26% 11% 1	330
	11th to 20th	Day	13% 20% 40% 23% 4	330
	21st to end	Day	10% 10% 48% 29% 3	363
February	1st to 10th	Day	14% 18% 42% 21% 4 1	330
	11th to 20th	Day	13% 16% 44% 21% 7%	330
	21st to end	Day	8% 15% 52% 20% 4 1	273
March	1st to 10th	Day	12% 17% 40% 27% 4	330
	11th to 20th	Day	18% 13% 38% 23% 7% 1	330
	21st to end	Day	16% 15% 46% 18% 4 1	363
April	1st to 10th	Day	16% 18% 35% 25% 5%	330
	11th to 20th	Day	13% 16% 44% 23% 5%	631
	21st to end	Day	19% 16% 37% 18% 8% 2	750
May	1st to 10th	Day	17% 12% 35% 26% 9% 1	753
	11th to 20th	Day	14% 10% 31% 26% 15% 4	775
	21st to end	Day	18% 7% 29% 27% 17% 3	844
June	1st to 10th	Day	7% 4 23% 36% 23% 7%	780
	11th to 20th	Day	7% 5% 29% 38% 19% 3	780
	21st to end	Day	5% 5% 29% 43% 16% 1	780
July	1st to 10th	Day	7% 2 16% 35% 31% 10%	771
	11th to 20th	Day	4 2 15% 41% 32% 7%	744
	21st to end	Day	6% 2 13% 38% 34% 8%	794
August	1st to 10th	Day	7% 3 17% 38% 31% 4	726
	11th to 20th	Day	7% 1 13% 41% 32% 6%	715
	21st to end	Day	6% 2 14% 36% 33% 9%	730
September	1st to 10th	Day	13% 2 8% 38% 31% 8%	330
	11th to 20th	Day	5% 3 17% 43% 27% 5%	330
	21st to end	Day	14% 3 26% 28% 22% 8%	330
October	1st to 10th	Day	12% 6% 30% 26% 18% 8%	330
	11th to 20th	Day	8% 10% 42% 27% 12% 1	330
	21st to end	Day	18% 15% 30% 25% 10% 2	363
November	1st to 10th	Day	11% 12% 33% 32% 10% 1	330
	11th to 20th	Day	13% 17% 33% 24% 11% 2	330
	21st to end	Day	19% 11% 35% 24% 9% 2	330
December	1st to 10th	Day	9% 10% 39% 32% 7% 3	330
	11th to 20th	Day	10% 14% 38% 29% 9% 1	330
	21st to end	Day	18% 14% 40% 22% 6% 1	363
January	1st to 10th	Night	10% 10% 40% 35% 6%	1050
	11th to 20th	Night	10% 13% 41% 30% 6%	1050
	21st to end	Night	12% 10% 40% 33% 4	1155
February	1st to 10th	Night	14% 10% 44% 29% 4	1050
	11th to 20th	Night	14% 15% 40% 28% 4	846
	21st to end	Night	11% 13% 43% 30% 3	692
March	1st to 10th	Night	11% 13% 43% 30% 3	696
	11th to 20th	Night	15% 12% 41% 27% 5%	630
	21st to end	Night	17% 12% 40% 27% 4	693
April	1st to 10th	Night	13% 12% 43% 25% 7%	630
	11th to 20th	Night	16% 12% 41% 26% 4	623
	21st to end	Night	17% 12% 37% 27% 6%	630
May	1st to 10th	Night	18% 8% 35% 31% 8%	630
	11th to 20th	Night	13% 8% 31% 35% 12% 1	621
	21st to end	Night	14% 5% 28% 36% 14% 2	683
June	1st to 10th	Night	5% 5% 26% 42% 21% 1	630
	11th to 20th	Night	5% 4 34% 44% 14%	630
	21st to end	Night	5% 3 33% 48% 10%	630
July	1st to 10th	Night	6% 2 14% 49% 27% 1	615
	11th to 20th	Night	3 2 17% 54% 22% 2	597
	21st to end	Night	7% 2 14% 46% 30% 2	657
August	1st to 10th	Night	5% 3 18% 49% 25% 1	597
	11th to 20th	Night	7% 3 13% 52% 24% 1	594
	21st to end	Night	4 1 14% 45% 33% 2	660
September	1st to 10th	Night	10% 2 21% 43% 22% 2	600
	11th to 20th	Night	7% 3 27% 43% 20% 1	600
	21st to end	Night	9% 4 27% 41% 19% 1	600
October	1st to 10th	Night	11% 5% 29% 34% 20% 2	650
	11th to 20th	Night	12% 7% 39% 34% 8%	800
	21st to end	Night	14% 10% 34% 33% 9%	880
November	1st to 10th	Night	10% 8% 37% 34% 11% 1	983
	11th to 20th	Night	9% 11% 40% 33% 7%	1050
	21st to end	Night	16% 10% 30% 36% 8%	1050
December	1st to 10th	Night	11% 9% 38% 36% 7%	1050
	11th to 20th	Night	14% 10% 40% 32% 4	1050
	21st to end	Night	12% 11% 39% 32% 5%	1155

Page generated at 2026-07-13 02:14:52. Version ac07

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