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 2024-01-22.

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% 4 2 4 4 3 4 4 5% 7% 54%	74944
	Night	10% 4 2 3 3 2 3 4 4 7% 57%	75018
Transparency	Day	30% 4 10% 22% 22% 11%	74669
	Night	27% 4 11% 22% 23% 13%	74813
Seeing	Day	11% 5% 21% 29% 25% 9%	13788
	Night	11% 5% 26% 40% 17% 1	23875

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	11% 6% 2 3 3 2 3 3 3 5% 60%	1707
	11th to 20th	Day	14% 5% 2 4 3 2 4 3 5% 5% 54%	1710
	21st to end	Day	12% 5% 2 3 3 2 3 3 3 5% 58%	1980
February	1st to 10th	Day	13% 6% 2 4 3 1 3 3 5% 6% 54%	1800
	11th to 20th	Day	14% 5% 2 4 4 2 3 3 5% 6% 50%	1749
	21st to end	Day	14% 4 2 4 3 2 3 3 3 6% 55%	1480
March	1st to 10th	Day	15% 7% 2 5% 4 3 3 4 5% 7% 46%	1877
	11th to 20th	Day	14% 5% 2 5% 4 2 3 3 4 6% 52%	1980
	21st to end	Day	12% 4 3 4 3 2 3 3 4 5% 57%	2178
April	1st to 10th	Day	10% 6% 2 4 4 3 4 4 5% 6% 52%	1980
	11th to 20th	Day	15% 5% 2 5% 4 2 3 4 4 5% 51%	2093
	21st to end	Day	11% 5% 2 4 4 3 5% 4 3 5% 54%	2233
May	1st to 10th	Day	14% 4 2 5% 4 3 4 4 5% 8% 47%	2340
	11th to 20th	Day	14% 6% 3 6% 5% 3 5% 4 5% 7% 43%	2302
	21st to end	Day	13% 5% 2 5% 5% 4 5% 5% 5% 8% 44%	2531
June	1st to 10th	Day	6% 3 1 3 4 3 4 5% 6% 9% 57%	2340
	11th to 20th	Day	4 3 1 3 4 2 4 4 6% 9% 59%	2340
	21st to end	Day	7% 3 2 4 4 3 5% 5% 7% 9% 50%	2340
July	1st to 10th	Day	8% 4 3 5% 6% 4 6% 5% 6% 9% 44%	2340
	11th to 20th	Day	3 2 1 3 4 3 4 5% 6% 9% 60%	2340
	21st to end	Day	4 2 1 3 4 3 5% 5% 6% 10% 56%	2546
August	1st to 10th	Day	2 1 1 3 4 3 3 4 5% 8% 66%	2229
	11th to 20th	Day	6% 4 2 5% 5% 3 5% 5% 6% 8% 50%	2123
	21st to end	Day	5% 4 2 4 6% 4 6% 6% 6% 9% 49%	2358
September	1st to 10th	Day	10% 4 2 4 6% 3 4 5% 5% 7% 51%	1980
	11th to 20th	Day	12% 5% 2 5% 5% 3 4 5% 6% 7% 47%	1980
	21st to end	Day	10% 3 1 3 4 2 5% 5% 6% 6% 54%	1980
October	1st to 10th	Day	5% 3 2 4 4 2 4 4 6% 7% 59%	1980
	11th to 20th	Day	11% 4 2 4 3 2 3 4 4 5% 59%	1922
	21st to end	Day	13% 4 1 3 3 2 3 2 3 3 63%	1980
November	1st to 10th	Day	13% 5% 1 4 3 2 2 3 4 6% 56%	1800
	11th to 20th	Day	8% 4 2 4 4 2 4 3 5% 6% 59%	1710
	21st to end	Day	14% 5% 3 5% 4 2 2 3 4 5% 53%	1710
December	1st to 10th	Day	13% 5% 2 4 4 2 3 3 4 6% 55%	1710
	11th to 20th	Day	13% 5% 2 4 5% 3 3 4 4 6% 51%	2161
	21st to end	Day	13% 5% 2 4 3 1 3 2 3 5% 61%	3135
January	1st to 10th	Night	13% 4 2 3 3 2 3 3 3 6% 59%	2458
	11th to 20th	Night	12% 4 2 3 3 2 3 3 4 5% 59%	2470
	21st to end	Night	14% 5% 2 4 3 2 3 3 4 6% 54%	2424
February	1st to 10th	Night	12% 6% 2 4 3 1 2 3 3 6% 56%	2160
	11th to 20th	Night	15% 5% 2 3 3 2 2 3 4 7% 54%	2098
	21st to end	Night	14% 5% 2 3 3 1 4 4 4 6% 54%	1776
March	1st to 10th	Night	14% 6% 2 4 3 2 4 3 5% 7% 51%	2080
	11th to 20th	Night	13% 5% 2 4 4 3 4 4 5% 5% 51%	1980
	21st to end	Night	9% 4 1 3 4 2 3 4 5% 7% 59%	2178
April	1st to 10th	Night	9% 4 2 3 3 1 4 4 5% 8% 55%	1980
	11th to 20th	Night	14% 5% 1 4 4 2 4 4 4 6% 53%	1812
	21st to end	Night	10% 4 1 3 4 2 4 3 4 6% 60%	1724
May	1st to 10th	Night	11% 3 2 3 3 2 3 4 5% 9% 54%	1620
	11th to 20th	Night	12% 4 1 4 4 3 4 5% 5% 8% 50%	1593
	21st to end	Night	11% 4 2 4 4 2 4 5% 5% 10% 50%	1745
June	1st to 10th	Night	2 2 2 2 4 2 4 4 6% 10% 62%	1620
	11th to 20th	Night	3 1 1 2 2 2 2 4 5% 9% 66%	1620
	21st to end	Night	6% 2 2 2 4 3 5% 4 5% 10% 60%	1620
July	1st to 10th	Night	6% 4 2 4 3 2 4 5% 7% 11% 52%	1620
	11th to 20th	Night	3 1 2 3 2 2 4 5% 9% 69%	1620
	21st to end	Night	2 2 1 2 3 1 3 4 6% 12% 64%	1772
August	1st to 10th	Night	1 1 2 3 2 3 3 4 8% 74%	1705
	11th to 20th	Night	5% 4 2 4 4 2 4 4 6% 10% 54%	1778
	21st to end	Night	5% 2 1 3 3 3 4 5% 6% 10% 57%	1998
September	1st to 10th	Night	8% 4 2 4 5% 2 4 5% 4 9% 54%	1980
	11th to 20th	Night	9% 5% 2 4 3 3 5% 5% 7% 9% 49%	1980
	21st to end	Night	8% 3 2 4 4 2 4 4 5% 7% 58%	1980
October	1st to 10th	Night	6% 3 1 2 3 2 3 4 4 7% 65%	1980
	11th to 20th	Night	12% 3 1 2 3 2 4 3 4 6% 60%	2018
	21st to end	Night	13% 4 1 2 2 2 3 3 3 4 62%	2376
November	1st to 10th	Night	12% 4 2 3 3 2 3 3 3 5% 60%	2275
	11th to 20th	Night	8% 3 2 3 3 2 3 4 4 6% 62%	2470
	21st to end	Night	16% 5% 2 3 3 2 3 4 4 6% 52%	2470
December	1st to 10th	Night	13% 4 1 3 3 2 3 3 4 6% 57%	2470
	11th to 20th	Night	13% 7% 3 3 4 2 4 4 4 5% 51%	3036
	21st to end	Night	15% 5% 2 3 2 2 2 2 3 5% 59%	4532

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	29% 2 5% 19% 29% 15%	1708
	11th to 20th	Day	33% 2 5% 17% 25% 17%	1710
	21st to end	Day	30% 3 7% 14% 28% 18%	1980
February	1st to 10th	Day	33% 3 6% 17% 20% 21%	1800
	11th to 20th	Day	35% 3 6% 20% 20% 16%	1759
	21st to end	Day	33% 4 6% 15% 23% 20%	1480
March	1st to 10th	Day	38% 4 9% 16% 21% 14%	1877
	11th to 20th	Day	35% 4 7% 16% 19% 19%	1980
	21st to end	Day	31% 4 10% 19% 22% 15%	2178
April	1st to 10th	Day	33% 4 9% 18% 20% 15%	1980
	11th to 20th	Day	36% 3 7% 16% 22% 14%	2093
	21st to end	Day	32% 4 6% 23% 23% 12%	2233
May	1st to 10th	Day	35% 6% 11% 17% 22% 10%	2340
	11th to 20th	Day	39% 5% 9% 21% 20% 5%	2306
	21st to end	Day	37% 6% 14% 23% 18% 3	2531
June	1st to 10th	Day	23% 5% 14% 31% 22% 5%	2340
	11th to 20th	Day	21% 4 16% 30% 22% 7%	2340
	21st to end	Day	28% 7% 15% 24% 20% 6%	2340
July	1st to 10th	Day	35% 7% 16% 27% 13% 1	2276
	11th to 20th	Day	19% 6% 17% 28% 25% 5%	2271
	21st to end	Day	19% 6% 21% 35% 18% 1	2405
August	1st to 10th	Day	16% 5% 13% 39% 25% 3	2215
	11th to 20th	Day	29% 6% 15% 31% 14% 4	2123
	21st to end	Day	29% 6% 17% 26% 17% 5%	2358
September	1st to 10th	Day	32% 6% 11% 23% 20% 9%	1980
	11th to 20th	Day	35% 6% 13% 19% 21% 5%	1980
	21st to end	Day	27% 5% 11% 23% 22% 12%	1980
October	1st to 10th	Day	24% 4 11% 24% 26% 12%	1980
	11th to 20th	Day	28% 4 7% 17% 25% 19%	1922
	21st to end	Day	28% 2 5% 17% 27% 21%	1980
November	1st to 10th	Day	31% 3 8% 23% 22% 13%	1800
	11th to 20th	Day	27% 3 8% 21% 23% 17%	1710
	21st to end	Day	36% 2 5% 18% 22% 17%	1710
December	1st to 10th	Day	33% 2 8% 21% 25% 12%	1710
	11th to 20th	Day	35% 4 8% 13% 24% 16%	2159
	21st to end	Day	29% 2 4 17% 28% 19%	3135
January	1st to 10th	Night	29% 3 5% 17% 27% 20%	2458
	11th to 20th	Night	30% 3 5% 19% 25% 18%	2470
	21st to end	Night	33% 3 5% 15% 24% 21%	2424
February	1st to 10th	Night	32% 3 7% 14% 23% 20%	2160
	11th to 20th	Night	32% 3 7% 18% 21% 18%	2098
	21st to end	Night	32% 4 6% 14% 21% 22%	1776
March	1st to 10th	Night	33% 3 10% 17% 25% 12%	2080
	11th to 20th	Night	34% 6% 8% 16% 19% 18%	1980
	21st to end	Night	25% 4 14% 21% 20% 16%	2178
April	1st to 10th	Night	28% 5% 10% 22% 19% 17%	1980
	11th to 20th	Night	33% 3 10% 16% 21% 17%	1812
	21st to end	Night	27% 5% 10% 23% 23% 12%	1724
May	1st to 10th	Night	28% 4 12% 23% 21% 12%	1620
	11th to 20th	Night	32% 5% 14% 25% 19% 6%	1594
	21st to end	Night	29% 6% 16% 29% 18% 2	1746
June	1st to 10th	Night	18% 6% 17% 33% 23% 4	1620
	11th to 20th	Night	15% 5% 17% 33% 23% 8%	1620
	21st to end	Night	22% 6% 15% 29% 22% 6%	1620
July	1st to 10th	Night	25% 9% 20% 31% 13% 2	1575
	11th to 20th	Night	12% 5% 19% 32% 28% 4	1566
	21st to end	Night	12% 5% 25% 39% 17% 1	1673
August	1st to 10th	Night	10% 5% 18% 39% 27% 2	1695
	11th to 20th	Night	24% 7% 18% 30% 16% 5%	1778
	21st to end	Night	22% 6% 22% 28% 17% 5%	1998
September	1st to 10th	Night	26% 6% 13% 26% 19% 9%	1980
	11th to 20th	Night	29% 8% 17% 23% 18% 5%	1980
	21st to end	Night	25% 5% 12% 23% 22% 13%	1980
October	1st to 10th	Night	19% 3 13% 28% 24% 13%	1980
	11th to 20th	Night	27% 3 9% 17% 25% 19%	2018
	21st to end	Night	27% 3 6% 16% 25% 22%	2376
November	1st to 10th	Night	27% 3 9% 20% 26% 15%	2275
	11th to 20th	Night	24% 4 8% 23% 24% 18%	2470
	21st to end	Night	35% 3 7% 16% 23% 16%	2470
December	1st to 10th	Night	29% 3 10% 20% 25% 14%	2470
	11th to 20th	Night	36% 3 7% 14% 22% 19%	3037
	21st to end	Night	30% 3 4 15% 30% 17%	4532

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	14% 9% 34% 32% 10% 1	270
	11th to 20th	Day	17% 11% 29% 30% 12% 2	270
	21st to end	Day	13% 9% 42% 25% 9% 1	267
February	1st to 10th	Day	18% 11% 36% 22% 13%	240
	11th to 20th	Day	20% 10% 38% 26% 6%	240
	21st to end	Day	18% 14% 39% 24% 5%	198
March	1st to 10th	Day	17% 8% 39% 26% 10%	240
	11th to 20th	Day	15% 8% 47% 20% 10% 1	240
	21st to end	Day	12% 13% 45% 22% 7% 1	264
April	1st to 10th	Day	14% 11% 42% 28% 5%	240
	11th to 20th	Day	14% 15% 39% 26% 6%	371
	21st to end	Day	13% 10% 43% 25% 8% 1	493
May	1st to 10th	Day	13% 8% 35% 32% 11%	603
	11th to 20th	Day	19% 4 27% 34% 13% 3	625
	21st to end	Day	14% 5% 23% 36% 19% 3	679
June	1st to 10th	Day	6% 2 14% 40% 33% 6%	630
	11th to 20th	Day	5% 1 13% 49% 26% 6%	630
	21st to end	Day	6% 1 9% 36% 38% 10%	630
July	1st to 10th	Day	8% 1 7% 31% 38% 16%	621
	11th to 20th	Day	2 1 3 28% 51% 14%	618
	21st to end	Day	3 3 29% 45% 20%	684
August	1st to 10th	Day	3 2 28% 46% 20%	527
	11th to 20th	Day	8% 1 6% 26% 37% 22%	437
	21st to end	Day	5% 2 5% 25% 37% 26%	477
September	1st to 10th	Day	14% 3 12% 39% 32%	270
	11th to 20th	Day	8% 1 4 20% 37% 30%	270
	21st to end	Day	14% 2 4 19% 39% 21%	270
October	1st to 10th	Day	4 3 9% 28% 45% 11%	270
	11th to 20th	Day	9% 4 18% 37% 23% 9%	270
	21st to end	Day	21% 7% 24% 25% 18% 6%	297
November	1st to 10th	Day	19% 3 21% 32% 21% 4	270
	11th to 20th	Day	16% 9% 23% 30% 16% 5%	270
	21st to end	Day	21% 11% 32% 21% 12% 3	270
December	1st to 10th	Day	16% 4 29% 30% 19% 3	270
	11th to 20th	Day	12% 13% 33% 28% 13% 1	270
	21st to end	Day	21% 7% 33% 25% 13% 1	297
January	1st to 10th	Night	15% 6% 34% 36% 9%	950
	11th to 20th	Night	14% 7% 32% 39% 8%	950
	21st to end	Night	17% 7% 33% 35% 8%	824
February	1st to 10th	Night	16% 9% 34% 34% 7%	720
	11th to 20th	Night	17% 7% 39% 32% 4	704
	21st to end	Night	18% 10% 42% 28% 2	592
March	1st to 10th	Night	15% 8% 39% 34% 3	640
	11th to 20th	Night	14% 7% 40% 32% 6%	540
	21st to end	Night	8% 8% 47% 32% 4	594
April	1st to 10th	Night	9% 6% 48% 33% 4	540
	11th to 20th	Night	13% 9% 48% 28% 3	533
	21st to end	Night	10% 8% 48% 26% 8%	540
May	1st to 10th	Night	9% 7% 41% 39% 4	540
	11th to 20th	Night	12% 3 33% 42% 9% 1	531
	21st to end	Night	10% 4 32% 40% 13%	584
June	1st to 10th	Night	3 1 17% 55% 24%	540
	11th to 20th	Night	4 19% 60% 16%	540
	21st to end	Night	4 2 11% 49% 32% 1	540
July	1st to 10th	Night	6% 1 6% 49% 35% 3	525
	11th to 20th	Night	3 1 6% 54% 35% 2	522
	21st to end	Night	2 4 50% 40% 3	591
August	1st to 10th	Night	1 3 53% 40% 2	537
	11th to 20th	Night	6% 1 5% 45% 40% 3	534
	21st to end	Night	4 1 6% 40% 44% 6%	594
September	1st to 10th	Night	8% 1 5% 43% 38% 4	540
	11th to 20th	Night	10% 6% 38% 44% 2	540
	21st to end	Night	9% 1 9% 44% 35% 2	540
October	1st to 10th	Night	7% 1 11% 49% 31% 1	540
	11th to 20th	Night	12% 3 17% 46% 20% 1	578
	21st to end	Night	13% 6% 25% 36% 19% 1	792
November	1st to 10th	Night	12% 3 23% 42% 20% 1	795
	11th to 20th	Night	9% 5% 27% 43% 15% 1	950
	21st to end	Night	19% 6% 28% 35% 12% 1	950
December	1st to 10th	Night	14% 5% 26% 44% 11%	950
	11th to 20th	Night	19% 8% 28% 38% 7%	950
	21st to end	Night	15% 6% 34% 37% 8% 1	1045

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