Mariposa Fairgrounds
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mariposa Fairgrounds clear sky chart from 2005-11-06 to 2025-09-19.

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	7% 3 1 3 2 2 2 2 3 5% 70%	134656
	Night	8% 4 1 3 2 2 3 2 4 5% 66%	131811
Transparency	Day	19% 3 7% 18% 27% 27%	129670
	Night	22% 3 9% 19% 25% 22%	127787
Seeing	Day	10% 10% 28% 33% 14% 6%	16388
	Night	10% 2 12% 37% 30% 8%	25809

By Month

Percentage Cloud Cover SW (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 4 3 3 3 2 5% 8% 53%	3639
	11th to 20th	Day	12% 5% 2 6% 4 4 4 2 3 4 55%	3939
	21st to end	Day	14% 5% 2 5% 4 4 4 3 4 7% 47%	4510
February	1st to 10th	Day	18% 6% 2 4 3 3 3 3 4 6% 48%	1855
	11th to 20th	Day	14% 6% 1 4 4 3 3 4 5% 7% 49%	1950
	21st to end	Day	17% 7% 2 6% 3 3 4 3 4 6% 46%	1650
March	1st to 10th	Day	15% 10% 2 6% 4 4 4 4 7% 8% 36%	4518
	11th to 20th	Day	15% 6% 1 3 3 3 3 3 4 6% 52%	4955
	21st to end	Day	15% 7% 2 5% 3 4 4 3 5% 8% 44%	5827
April	1st to 10th	Day	11% 5% 2 4 3 3 3 3 5% 5% 55%	2400
	11th to 20th	Day	10% 5% 1 5% 3 3 4 2 5% 6% 56%	2369
	21st to end	Day	8% 4 1 3 4 3 3 3 5% 6% 61%	2400
May	1st to 10th	Day	4 4 1 2 1 1 2 2 3 5% 76%	5561
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 78%	5986
	21st to end	Day	3 1 1 2 1 2 2 2 2 4 81%	6902
June	1st to 10th	Day	3 3 1 2 2 2 3 2 4 6% 72%	2800
	11th to 20th	Day	1 1 1 1 1 2 2 2 4 85%	2800
	21st to end	Day	2 2 1 1 1 1 1 1 2 3 86%	2800
July	1st to 10th	Day	1 1 1 1 1 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	6320
	21st to end	Day	1 1 1 1 1 2 3 3 87%	6495
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 2 89%	5577
	11th to 20th	Day	1 1 1 2 93%	5364
	21st to end	Day	1 1 1 2 1 2 2 1 3 3 84%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 2 88%	2400
	11th to 20th	Day	2 2 1 2 1 1 2 2 3 3 81%	2344
	21st to end	Day	2 2 1 2 1 1 1 2 3 4 82%	2090
October	1st to 10th	Day	2 2 1 2 2 1 2 2 3 4 78%	3249
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 4 79%	3161
	21st to end	Day	4 2 1 3 3 3 3 3 4 5% 68%	3433
November	1st to 10th	Day	8% 4 1 3 4 3 4 3 4 7% 59%	1764
	11th to 20th	Day	8% 3 1 4 3 3 4 3 5% 6% 60%	1800
	21st to end	Day	12% 4 1 4 3 4 3 3 4 8% 56%	1800
December	1st to 10th	Day	9% 3 1 3 3 4 3 3 4 7% 60%	2663
	11th to 20th	Day	22% 7% 2 5% 4 3 3 3 4 6% 40%	3028
	21st to end	Day	17% 6% 3 6% 4 4 5% 4 6% 8% 37%	4004
January	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 6% 8% 51%	6306
	11th to 20th	Night	14% 5% 2 4 2 3 3 3 4 5% 55%	6029
	21st to end	Night	15% 6% 2 6% 4 3 3 2 4 6% 50%	6468
February	1st to 10th	Night	17% 6% 2 4 4 2 3 3 4 6% 50%	2543
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 4 7% 54%	2338
	21st to end	Night	16% 7% 2 4 2 2 3 3 4 5% 52%	1980
March	1st to 10th	Night	17% 8% 2 5% 4 3 4 4 5% 6% 42%	5376
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 5% 51%	5029
	21st to end	Night	20% 6% 2 4 2 2 3 3 4 6% 48%	5147
April	1st to 10th	Night	11% 5% 2 3 3 3 3 4 4 7% 57%	2000
	11th to 20th	Night	7% 3 1 3 4 3 4 3 3 5% 62%	1979
	21st to end	Night	6% 4 1 3 3 2 3 3 4 7% 66%	2000
May	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 77%	4432
	11th to 20th	Night	2 1 2 1 1 2 2 3 5% 79%	3903
	21st to end	Night	2 1 2 1 1 1 1 2 4 85%	3972
June	1st to 10th	Night	3 2 1 2 2 2 2 3 3 6% 73%	1600
	11th to 20th	Night	1 1 1 1 2 1 2 4 88%	1600
	21st to end	Night	1 1 1 1 2 2 1 2 2 86%	1600
July	1st to 10th	Night	1 1 96%	3640
	11th to 20th	Night	2 1 2 1 2 1 1 2 2 85%	3640
	21st to end	Night	1 1 1 1 1 1 2 1 2 3 86%	4370
August	1st to 10th	Night	1 1 1 1 1 1 1 2 2 87%	4232
	11th to 20th	Night	1 1 1 1 1 1 1 93%	4506
	21st to end	Night	2 1 2 1 1 2 1 2 2 84%	5015
September	1st to 10th	Night	1 1 1 2 1 1 2 2 89%	2000
	11th to 20th	Night	2 1 1 2 1 2 2 2 2 3 82%	2013
	21st to end	Night	2 1 1 2 1 1 2 1 3 4 83%	2090
October	1st to 10th	Night	2 1 1 2 1 1 2 1 2 4 82%	3728
	11th to 20th	Night	2 1 1 2 1 2 2 2 2 4 81%	3777
	21st to end	Night	5% 3 1 3 1 2 1 2 3 5% 74%	4121
November	1st to 10th	Night	10% 4 1 2 2 2 2 2 4 7% 64%	2535
	11th to 20th	Night	8% 3 1 4 2 2 3 3 4 7% 62%	2600
	21st to end	Night	11% 3 2 3 3 3 3 3 5% 6% 59%	2600
December	1st to 10th	Night	9% 3 1 3 3 3 3 3 5% 6% 62%	4514
	11th to 20th	Night	21% 6% 2 4 3 3 3 3 7% 9% 39%	5176
	21st to end	Night	15% 7% 3 8% 6% 6% 6% 5% 6% 8% 32%	6952

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	31% 7% 19% 24% 15% 4	3639
	11th to 20th	Day	34% 4 7% 16% 22% 16%	3939
	21st to end	Day	38% 5% 9% 17% 24% 6%	4510
February	1st to 10th	Day	38% 4 8% 25% 17% 8%	1855
	11th to 20th	Day	32% 3 12% 25% 21% 6%	1960
	21st to end	Day	38% 5% 10% 25% 20% 3	1650
March	1st to 10th	Day	42% 7% 10% 19% 19% 3	4518
	11th to 20th	Day	33% 3 16% 28% 12% 7%	4955
	21st to end	Day	35% 5% 16% 27% 15% 3	5827
April	1st to 10th	Day	29% 4 12% 28% 19% 7%	2400
	11th to 20th	Day	28% 4 11% 28% 23% 7%	2372
	21st to end	Day	24% 4 10% 31% 26% 6%	2400
May	1st to 10th	Day	14% 2 6% 28% 38% 13%	5561
	11th to 20th	Day	10% 4 4 28% 42% 12%	5989
	21st to end	Day	10% 2 5% 22% 50% 12%	6904
June	1st to 10th	Day	14% 3 9% 24% 32% 19%	2800
	11th to 20th	Day	7% 1 5% 16% 38% 33%	2800
	21st to end	Day	7% 1 2 7% 31% 52%	2800
July	1st to 10th	Day	2 1 3 15% 79%	6251
	11th to 20th	Day	5% 1 1 3 13% 76%	6245
	21st to end	Day	6% 1 1 8% 36% 49%	6307
August	1st to 10th	Day	8% 1 1 5% 25% 59%	4061
	11th to 20th	Day	4 1 1 4 24% 66%	3880
	21st to end	Day	6% 1 2 6% 29% 55%	4311
September	1st to 10th	Day	7% 1 2 8% 28% 54%	2400
	11th to 20th	Day	9% 1 3 21% 37% 28%	2344
	21st to end	Day	9% 1 3 17% 37% 33%	2090
October	1st to 10th	Day	12% 2 4 10% 20% 52%	3249
	11th to 20th	Day	11% 2 4 20% 43% 20%	3161
	21st to end	Day	18% 2 4 15% 43% 17%	3433
November	1st to 10th	Day	24% 3 12% 22% 24% 14%	1764
	11th to 20th	Day	24% 4 12% 25% 27% 8%	1800
	21st to end	Day	28% 5% 8% 19% 29% 11%	1800
December	1st to 10th	Day	26% 3 8% 27% 27% 8%	2663
	11th to 20th	Day	46% 3 10% 17% 19% 6%	3028
	21st to end	Day	43% 6% 16% 24% 7% 3	4004
January	1st to 10th	Night	29% 6% 21% 23% 17% 4	6306
	11th to 20th	Night	32% 4 12% 14% 28% 9%	6029
	21st to end	Night	36% 4 9% 21% 21% 9%	6468
February	1st to 10th	Night	36% 3 10% 22% 20% 9%	2543
	11th to 20th	Night	30% 4 11% 27% 21% 7%	2339
	21st to end	Night	36% 2 12% 27% 19% 5%	1980
March	1st to 10th	Night	40% 4 10% 20% 21% 4	5376
	11th to 20th	Night	35% 3 12% 29% 14% 6%	5029
	21st to end	Night	37% 3 19% 22% 15% 4	5147
April	1st to 10th	Night	27% 4 16% 27% 19% 7%	2000
	11th to 20th	Night	23% 3 13% 29% 23% 8%	1979
	21st to end	Night	20% 3 12% 32% 28% 5%	2000
May	1st to 10th	Night	11% 3 7% 32% 33% 13%	4429
	11th to 20th	Night	9% 2 5% 31% 41% 12%	3904
	21st to end	Night	8% 1 5% 23% 53% 10%	3972
June	1st to 10th	Night	13% 2 12% 23% 32% 18%	1600
	11th to 20th	Night	5% 1 5% 18% 39% 33%	1600
	21st to end	Night	7% 1 2 8% 30% 52%	1600
July	1st to 10th	Night	1 2 14% 82%	3600
	11th to 20th	Night	8% 1 1 3 12% 76%	3592
	21st to end	Night	6% 1 1 6% 35% 51%	4246
August	1st to 10th	Night	9% 1 1 6% 20% 62%	3078
	11th to 20th	Night	5% 1 1 4 19% 69%	3256
	21st to end	Night	7% 1 1 6% 27% 59%	3608
September	1st to 10th	Night	5% 1 2 7% 27% 57%	2000
	11th to 20th	Night	9% 2 4 16% 40% 30%	2013
	21st to end	Night	9% 1 3 14% 39% 34%	2090
October	1st to 10th	Night	10% 1 3 9% 22% 55%	3728
	11th to 20th	Night	10% 1 4 23% 40% 21%	3777
	21st to end	Night	16% 1 3 20% 37% 23%	4121
November	1st to 10th	Night	22% 3 9% 25% 22% 18%	2535
	11th to 20th	Night	21% 3 12% 26% 25% 13%	2600
	21st to end	Night	26% 3 8% 22% 30% 10%	2600
December	1st to 10th	Night	25% 3 9% 29% 23% 11%	4514
	11th to 20th	Night	41% 4 14% 18% 20% 4	5176
	21st to end	Night	46% 7% 15% 22% 7% 3	6952

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	30% 10% 17% 28% 11% 5%	200
	11th to 20th	Day	20% 6% 24% 31% 15% 4	330
	21st to end	Day	18% 5% 26% 30% 15% 5%	440
February	1st to 10th	Day	26% 12% 23% 22% 13% 5%	427
	11th to 20th	Day	19% 11% 34% 24% 11% 2	495
	21st to end	Day	17% 12% 36% 24% 10% 1	414
March	1st to 10th	Day	23% 13% 34% 22% 7% 1	500
	11th to 20th	Day	22% 15% 27% 24% 10% 2	500
	21st to end	Day	22% 14% 33% 22% 7% 1	550
April	1st to 10th	Day	16% 19% 30% 23% 11% 1	500
	11th to 20th	Day	15% 18% 29% 26% 9% 3	492
	21st to end	Day	8% 18% 30% 27% 12% 5%	500
May	1st to 10th	Day	9% 19% 32% 26% 9% 5%	503
	11th to 20th	Day	9% 16% 30% 30% 9% 6%	528
	21st to end	Day	9% 11% 27% 33% 13% 7%	573
June	1st to 10th	Day	4 12% 28% 35% 13% 7%	530
	11th to 20th	Day	3 13% 32% 33% 12% 8%	530
	21st to end	Day	4 8% 31% 33% 15% 10%	530
July	1st to 10th	Day	1 3 28% 43% 15% 10%	526
	11th to 20th	Day	1 2 17% 54% 14% 12%	524
	21st to end	Day	1 2 24% 46% 16% 11%	575
August	1st to 10th	Day	4 3 30% 39% 14% 9%	527
	11th to 20th	Day	1 4 28% 43% 15% 9%	517
	21st to end	Day	1 3 23% 46% 17% 9%	583
September	1st to 10th	Day	2 4 26% 44% 18% 5%	530
	11th to 20th	Day	5% 8% 32% 34% 18% 4	523
	21st to end	Day	3 7% 26% 45% 17% 2	490
October	1st to 10th	Day	3 11% 27% 37% 20% 2	490
	11th to 20th	Day	6% 8% 27% 35% 20% 4	490
	21st to end	Day	6% 8% 27% 35% 20% 3	536
November	1st to 10th	Day	9% 8% 24% 27% 24% 7%	300
	11th to 20th	Day	7% 7% 24% 37% 17% 8%	300
	21st to end	Day	13% 8% 23% 31% 19% 6%	300
December	1st to 10th	Day	16% 3 26% 26% 20% 8%	215
	11th to 20th	Day	16% 7% 25% 28% 19% 6%	200
	21st to end	Day	22% 7% 22% 26% 17% 5%	220
January	1st to 10th	Night	19% 2 18% 32% 21% 9%	800
	11th to 20th	Night	15% 3 15% 40% 21% 7%	800
	21st to end	Night	19% 2 14% 36% 22% 7%	880
February	1st to 10th	Night	21% 3 16% 35% 20% 6%	800
	11th to 20th	Night	17% 4 17% 33% 22% 8%	784
	21st to end	Night	20% 4 16% 40% 18% 3	660
March	1st to 10th	Night	22% 4 16% 35% 18% 4	800
	11th to 20th	Night	18% 4 16% 31% 25% 6%	800
	21st to end	Night	19% 3 17% 35% 22% 3	880
April	1st to 10th	Night	13% 4 18% 37% 24% 4	800
	11th to 20th	Night	9% 5% 20% 37% 22% 6%	791
	21st to end	Night	7% 5% 16% 39% 25% 7%	800
May	1st to 10th	Night	7% 5% 18% 39% 24% 7%	750
	11th to 20th	Night	6% 3 17% 38% 33% 2	525
	21st to end	Night	5% 3 12% 40% 34% 6%	433
June	1st to 10th	Night	3 4 8% 36% 43% 6%	400
	11th to 20th	Night	1 2 10% 40% 43% 5%	400
	21st to end	Night	1 7% 38% 47% 7%	400
July	1st to 10th	Night	4 38% 51% 7%	390
	11th to 20th	Night	3 1 34% 53% 9%	388
	21st to end	Night	2 2 29% 52% 15%	607
August	1st to 10th	Night	2 3 39% 43% 12%	681
	11th to 20th	Night	1 6% 39% 43% 11%	792
	21st to end	Night	1 4 33% 49% 12%	880
September	1st to 10th	Night	1 1 5% 36% 47% 11%	800
	11th to 20th	Night	2 1 6% 39% 40% 12%	792
	21st to end	Night	2 1 9% 36% 40% 12%	760
October	1st to 10th	Night	5% 1 8% 46% 31% 10%	760
	11th to 20th	Night	4 2 11% 39% 33% 11%	760
	21st to end	Night	6% 1 12% 35% 31% 16%	836
November	1st to 10th	Night	11% 3 10% 34% 31% 11%	780
	11th to 20th	Night	10% 2 16% 38% 26% 8%	800
	21st to end	Night	12% 2 17% 37% 25% 8%	800
December	1st to 10th	Night	16% 2 13% 41% 21% 7%	800
	11th to 20th	Night	22% 1 14% 39% 19% 5%	800
	21st to end	Night	21% 4 18% 36% 17% 4	880

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