Mason
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mason clear sky chart from 2005-11-06 to 2026-05-17.

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	10% 5% 2 5% 3 3 4 3 5% 7% 54%	132335
	Night	11% 4 2 4 3 3 3 3 4 7% 57%	130850
Transparency	Day	30% 4 9% 22% 22% 13%	136153
	Night	28% 4 10% 21% 23% 15%	135096
Seeing	Day	11% 6% 21% 30% 24% 8%	16862
	Night	11% 5% 26% 40% 17% 1	26881

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	9% 6% 2 4 3 2 3 2 3 5% 61%	5289
	11th to 20th	Day	11% 5% 1 3 2 2 3 2 4 5% 61%	5310
	21st to end	Day	18% 6% 2 5% 3 3 3 3 3 5% 48%	6445
February	1st to 10th	Day	12% 7% 2 5% 3 3 2 3 5% 6% 53%	2100
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 6% 6% 51%	2049
	21st to end	Day	13% 5% 2 5% 3 2 3 2 3 6% 57%	1730
March	1st to 10th	Day	10% 6% 1 5% 3 3 3 3 4 6% 55%	4811
	11th to 20th	Day	13% 5% 2 4 3 3 2 2 4 5% 56%	5090
	21st to end	Day	9% 5% 2 5% 3 4 4 3 5% 7% 53%	5583
April	1st to 10th	Day	10% 6% 2 5% 4 4 3 3 5% 6% 51%	2310
	11th to 20th	Day	15% 6% 2 6% 4 3 3 3 4 5% 48%	2447
	21st to end	Day	11% 6% 2 5% 4 4 4 3 4 6% 51%	2606
May	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 6% 6% 44%	5992
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 48%	5941
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 6% 9% 42%	6434
June	1st to 10th	Day	6% 3 1 3 4 3 4 4 6% 9% 55%	2600
	11th to 20th	Day	4 3 1 4 4 3 4 3 6% 9% 58%	2600
	21st to end	Day	6% 4 2 4 4 5% 5% 4 8% 9% 51%	2600
July	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 42%	5900
	11th to 20th	Day	4 3 1 5% 5% 4 5% 4 6% 9% 55%	5900
	21st to end	Day	5% 4 2 4 4 4 5% 4 7% 10% 53%	6422
August	1st to 10th	Day	1 1 1 2 2 3 3 3 5% 7% 72%	4045
	11th to 20th	Day	3 2 1 4 3 3 3 3 5% 8% 65%	3846
	21st to end	Day	3 3 2 4 4 5% 5% 4 6% 9% 54%	4279
September	1st to 10th	Day	10% 5% 2 5% 5% 4 4 4 5% 6% 51%	2200
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 7% 49%	2200
	21st to end	Day	9% 3 1 4 3 3 5% 5% 6% 6% 55%	2200
October	1st to 10th	Day	5% 3 1 4 3 3 4 3 6% 6% 62%	2200
	11th to 20th	Day	10% 4 1 4 3 3 3 3 4 5% 61%	2135
	21st to end	Day	12% 4 1 4 2 2 2 2 3 3 65%	2200
November	1st to 10th	Day	13% 6% 1 4 3 2 3 3 4 5% 56%	1993
	11th to 20th	Day	8% 4 1 4 3 2 3 2 5% 6% 61%	1890
	21st to end	Day	14% 6% 2 5% 4 3 2 2 4 6% 53%	1890
December	1st to 10th	Day	19% 7% 2 6% 3 3 3 2 3 5% 47%	3030
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 5% 5% 48%	3481
	21st to end	Day	11% 5% 1 4 2 2 2 2 3 4 64%	4587
January	1st to 10th	Night	12% 5% 2 4 2 2 3 2 3 5% 61%	7634
	11th to 20th	Night	7% 3 1 3 2 2 2 2 3 6% 69%	7680
	21st to end	Night	24% 6% 2 5% 2 3 3 2 3 5% 45%	7824
February	1st to 10th	Night	12% 6% 2 5% 3 2 2 3 4 5% 57%	2520
	11th to 20th	Night	14% 6% 2 4 3 2 3 2 4 7% 54%	2458
	21st to end	Night	14% 5% 1 3 2 2 4 3 4 6% 55%	2076
March	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 8% 56%	5270
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 4 5% 56%	5090
	21st to end	Night	10% 6% 2 4 3 3 3 3 5% 6% 56%	5588
April	1st to 10th	Night	10% 5% 1 4 3 3 4 3 5% 7% 54%	2310
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 5% 6% 50%	2117
	21st to end	Night	13% 4 1 4 4 3 4 3 4 6% 56%	2011
May	1st to 10th	Night	20% 6% 2 4 3 3 3 2 4 6% 48%	4143
	11th to 20th	Night	8% 4 2 6% 4 4 4 4 5% 7% 53%	4107
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 6% 9% 45%	4443
June	1st to 10th	Night	3 2 2 3 3 3 4 3 6% 10% 61%	1800
	11th to 20th	Night	4 2 1 3 2 3 3 4 5% 9% 65%	1800
	21st to end	Night	5% 3 1 2 4 4 4 3 5% 9% 60%	1800
July	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 9% 47%	4078
	11th to 20th	Night	3 1 1 3 3 3 4 3 6% 9% 65%	4077
	21st to end	Night	3 3 2 4 4 3 4 3 7% 10% 56%	4449
August	1st to 10th	Night	1 1 1 3 2 2 2 2 3 7% 75%	3110
	11th to 20th	Night	3 2 1 3 2 2 3 3 6% 11% 63%	3213
	21st to end	Night	4 2 1 3 3 4 4 4 7% 12% 57%	3629
September	1st to 10th	Night	8% 5% 1 5% 4 3 4 4 4 8% 53%	2200
	11th to 20th	Night	9% 4 2 5% 3 4 4 4 7% 8% 50%	2200
	21st to end	Night	8% 3 2 4 3 3 4 3 5% 7% 59%	2200
October	1st to 10th	Night	6% 3 1 3 3 2 3 3 4 6% 67%	2200
	11th to 20th	Night	11% 3 1 3 2 3 3 3 4 5% 63%	2243
	21st to end	Night	13% 4 1 3 2 3 3 3 3 4 63%	2640
November	1st to 10th	Night	12% 4 1 4 3 2 3 2 4 5% 61%	2521
	11th to 20th	Night	8% 3 1 3 2 3 3 3 4 5% 63%	2730
	21st to end	Night	16% 5% 2 4 3 3 3 3 4 6% 52%	2730
December	1st to 10th	Night	19% 6% 2 5% 2 3 3 3 4 5% 48%	4380
	11th to 20th	Night	16% 7% 2 4 3 3 3 3 4 5% 51%	4946
	21st to end	Night	14% 5% 1 3 2 2 2 2 3 5% 62%	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	28% 2 5% 25% 27% 13%	5290
	11th to 20th	Day	28% 1 4 18% 29% 20%	5310
	21st to end	Day	41% 2 5% 14% 26% 13%	6445
February	1st to 10th	Day	34% 2 5% 17% 20% 22%	2100
	11th to 20th	Day	34% 3 7% 21% 20% 15%	2059
	21st to end	Day	31% 3 5% 16% 26% 19%	1730
March	1st to 10th	Day	31% 3 5% 15% 19% 27%	4811
	11th to 20th	Day	31% 3 6% 11% 15% 34%	5090
	21st to end	Day	30% 4 8% 19% 23% 15%	5585
April	1st to 10th	Day	34% 4 9% 18% 18% 16%	2310
	11th to 20th	Day	39% 3 8% 16% 21% 14%	2447
	21st to end	Day	34% 4 7% 23% 21% 11%	2606
May	1st to 10th	Day	39% 6% 9% 17% 20% 9%	6030
	11th to 20th	Day	35% 4 9% 15% 24% 13%	5945
	21st to end	Day	36% 7% 12% 26% 15% 5%	6447
June	1st to 10th	Day	24% 6% 14% 31% 22% 5%	2600
	11th to 20th	Day	21% 4 15% 29% 24% 7%	2600
	21st to end	Day	26% 7% 15% 25% 20% 6%	2600
July	1st to 10th	Day	40% 5% 17% 30% 9% 1	5836
	11th to 20th	Day	26% 4 16% 34% 17% 3	5831
	21st to end	Day	25% 5% 18% 29% 21% 2	6240
August	1st to 10th	Day	13% 3 10% 38% 33% 3	4031
	11th to 20th	Day	18% 4 18% 42% 16% 2	3882
	21st to end	Day	26% 5% 20% 25% 22% 3	4278
September	1st to 10th	Day	32% 6% 10% 22% 20% 10%	2200
	11th to 20th	Day	33% 6% 13% 20% 23% 5%	2200
	21st to end	Day	27% 5% 10% 24% 22% 14%	2200
October	1st to 10th	Day	14% 2 6% 14% 28% 36%	3590
	11th to 20th	Day	22% 3 5% 21% 25% 24%	3453
	21st to end	Day	22% 2 4 19% 39% 14%	3538
November	1st to 10th	Day	31% 3 8% 21% 24% 14%	1993
	11th to 20th	Day	26% 3 7% 21% 23% 19%	1890
	21st to end	Day	36% 2 5% 20% 22% 16%	1890
December	1st to 10th	Day	43% 2 6% 18% 21% 11%	3030
	11th to 20th	Day	38% 3 7% 12% 25% 15%	3479
	21st to end	Day	27% 2 4 19% 26% 23%	4587
January	1st to 10th	Night	29% 2 5% 21% 29% 14%	7635
	11th to 20th	Night	20% 1 3 18% 30% 27%	7680
	21st to end	Night	45% 3 3 12% 23% 16%	7824
February	1st to 10th	Night	31% 3 7% 16% 23% 21%	2520
	11th to 20th	Night	32% 3 7% 20% 21% 17%	2458
	21st to end	Night	32% 4 5% 14% 23% 22%	2076
March	1st to 10th	Night	27% 4 8% 20% 23% 18%	5270
	11th to 20th	Night	31% 5% 6% 11% 17% 30%	5090
	21st to end	Night	28% 4 9% 20% 23% 16%	5588
April	1st to 10th	Night	29% 4 10% 20% 17% 19%	2310
	11th to 20th	Night	36% 3 10% 15% 20% 16%	2117
	21st to end	Night	30% 5% 11% 22% 22% 11%	2011
May	1st to 10th	Night	38% 4 9% 21% 16% 11%	4170
	11th to 20th	Night	31% 4 12% 19% 17% 17%	4108
	21st to end	Night	36% 5% 14% 26% 14% 5%	4452
June	1st to 10th	Night	18% 6% 17% 34% 23% 3	1800
	11th to 20th	Night	16% 5% 16% 32% 23% 8%	1800
	21st to end	Night	22% 5% 15% 31% 22% 6%	1800
July	1st to 10th	Night	34% 6% 21% 31% 8% 1	4035
	11th to 20th	Night	16% 5% 20% 39% 18% 2	4026
	21st to end	Night	23% 5% 20% 31% 19% 2	4320
August	1st to 10th	Night	11% 3 13% 35% 36% 1	3100
	11th to 20th	Night	16% 4 22% 41% 14% 3	3243
	21st to end	Night	20% 8% 23% 25% 21% 3	3629
September	1st to 10th	Night	28% 6% 12% 24% 19% 10%	2200
	11th to 20th	Night	28% 8% 16% 24% 18% 6%	2200
	21st to end	Night	24% 5% 11% 24% 22% 14%	2200
October	1st to 10th	Night	12% 3 7% 16% 24% 38%	3590
	11th to 20th	Night	19% 3 8% 18% 30% 22%	3651
	21st to end	Night	21% 2 4 21% 35% 16%	4252
November	1st to 10th	Night	26% 3 8% 19% 27% 17%	2521
	11th to 20th	Night	23% 3 8% 23% 24% 20%	2730
	21st to end	Night	34% 3 8% 17% 23% 15%	2730
December	1st to 10th	Night	40% 2 7% 18% 20% 13%	4380
	11th to 20th	Night	37% 3 6% 13% 24% 17%	4947
	21st to end	Night	28% 2 4 19% 29% 18%	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	13% 10% 35% 31% 11% 1	330
	11th to 20th	Day	15% 11% 29% 32% 11% 2	330
	21st to end	Day	15% 10% 40% 25% 9% 1	363
February	1st to 10th	Day	17% 11% 34% 25% 13% 1	330
	11th to 20th	Day	17% 10% 38% 27% 7% 1	330
	21st to end	Day	15% 13% 38% 28% 6%	273
March	1st to 10th	Day	17% 11% 36% 28% 8%	330
	11th to 20th	Day	16% 9% 44% 21% 8% 1	330
	21st to end	Day	11% 12% 40% 26% 9% 1	363
April	1st to 10th	Day	14% 13% 40% 27% 6%	330
	11th to 20th	Day	15% 16% 40% 24% 5%	485
	21st to end	Day	14% 10% 42% 26% 8%	626
May	1st to 10th	Day	14% 9% 35% 31% 10%	753
	11th to 20th	Day	18% 5% 29% 33% 12% 3	756
	21st to end	Day	14% 5% 24% 36% 18% 3	789
June	1st to 10th	Day	6% 2 14% 42% 31% 5%	730
	11th to 20th	Day	6% 1 14% 48% 26% 5%	730
	21st to end	Day	5% 1 9% 35% 39% 11%	730
July	1st to 10th	Day	9% 1 7% 30% 38% 16%	721
	11th to 20th	Day	3 2 4 29% 49% 13%	718
	21st to end	Day	4 1 4 29% 44% 19%	794
August	1st to 10th	Day	2 3 27% 46% 22%	615
	11th to 20th	Day	7% 1 5% 26% 38% 22%	517
	21st to end	Day	4 1 6% 24% 39% 25%	563
September	1st to 10th	Day	14% 1 3 16% 38% 27%	330
	11th to 20th	Day	7% 2 4 20% 39% 28%	330
	21st to end	Day	13% 2 4 24% 37% 20%	330
October	1st to 10th	Day	4 2 7% 28% 45% 14%	330
	11th to 20th	Day	8% 4 17% 38% 25% 8%	330
	21st to end	Day	17% 8% 24% 26% 18% 7%	363
November	1st to 10th	Day	18% 5% 22% 31% 22% 4	330
	11th to 20th	Day	14% 9% 22% 30% 19% 5%	330
	21st to end	Day	20% 11% 33% 19% 14% 3	330
December	1st to 10th	Day	16% 6% 31% 28% 16% 2	330
	11th to 20th	Day	12% 12% 31% 29% 14% 3	330
	21st to end	Day	18% 9% 31% 26% 15% 1	363
January	1st to 10th	Night	14% 6% 34% 36% 9%	1050
	11th to 20th	Night	13% 7% 34% 38% 8%	1050
	21st to end	Night	19% 8% 32% 34% 7%	950
February	1st to 10th	Night	15% 9% 34% 36% 7%	840
	11th to 20th	Night	17% 9% 38% 32% 4	824
	21st to end	Night	17% 10% 41% 30% 2	692
March	1st to 10th	Night	16% 9% 38% 34% 3	746
	11th to 20th	Night	14% 8% 41% 31% 6%	630
	21st to end	Night	9% 8% 45% 35% 4	693
April	1st to 10th	Night	10% 7% 46% 33% 4	630
	11th to 20th	Night	15% 9% 48% 25% 3	623
	21st to end	Night	12% 8% 46% 26% 7%	630
May	1st to 10th	Night	11% 7% 40% 38% 4	630
	11th to 20th	Night	12% 3 35% 41% 9%	609
	21st to end	Night	10% 5% 33% 40% 12%	650
June	1st to 10th	Night	3 1 19% 54% 23% 1	600
	11th to 20th	Night	4 1 20% 59% 16%	600
	21st to end	Night	4 2 12% 49% 32% 1	600
July	1st to 10th	Night	6% 1 6% 48% 35% 3	585
	11th to 20th	Night	3 1 7% 52% 36% 2	582
	21st to end	Night	2 5% 49% 41% 2	657
August	1st to 10th	Night	1 3 51% 42% 3	597
	11th to 20th	Night	5% 1 5% 45% 42% 3	594
	21st to end	Night	4 1 6% 40% 44% 6%	660
September	1st to 10th	Night	10% 2 6% 42% 38% 4	600
	11th to 20th	Night	9% 6% 39% 45% 2	600
	21st to end	Night	8% 1 9% 45% 35% 2	600
October	1st to 10th	Night	7% 1 10% 48% 34% 1	600
	11th to 20th	Night	11% 3 17% 47% 22% 1	643
	21st to end	Night	13% 6% 25% 37% 18% 1	880
November	1st to 10th	Night	12% 3 23% 42% 19%	881
	11th to 20th	Night	9% 5% 26% 44% 15% 1	1050
	21st to end	Night	18% 6% 28% 34% 12% 1	1050
December	1st to 10th	Night	15% 5% 27% 43% 10%	1050
	11th to 20th	Night	18% 7% 27% 38% 9%	1050
	21st to end	Night	15% 6% 33% 37% 9% 1	1155

Page generated at 2026-05-14 00:58:57. Version ac07

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