May River Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the May River Road Observatory clear sky chart from 2005-11-06 to 2024-10-20.

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	11% 6% 2 6% 4 5% 5% 4 7% 9% 42%	100945
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 42%	97663
Transparency	Day	36% 10% 16% 20% 14% 5%	100758
	Night	38% 8% 14% 18% 16% 6%	97520
Seeing	Day	12% 5% 18% 29% 27% 9%	15397
	Night	13% 6% 20% 31% 26% 6%	25461

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% 5% 2 5% 4 5% 5% 4 7% 8% 45%	2836
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 5% 8% 46%	2840
	21st to end	Day	14% 7% 3 6% 3 4 5% 4 5% 5% 44%	3124
February	1st to 10th	Day	14% 8% 2 6% 5% 3 5% 4 6% 6% 41%	1767
	11th to 20th	Day	18% 6% 1 4 3 3 3 3 5% 5% 47%	1848
	21st to end	Day	13% 7% 2 5% 5% 3 4 3 4 7% 47%	1666
March	1st to 10th	Day	20% 10% 3 6% 3 4 3 3 4 6% 37%	3435
	11th to 20th	Day	8% 4 2 5% 3 4 4 4 6% 7% 55%	3480
	21st to end	Day	16% 7% 2 5% 4 4 4 3 5% 6% 44%	3815
April	1st to 10th	Day	9% 4 1 4 3 4 4 4 6% 8% 51%	2090
	11th to 20th	Day	11% 5% 3 6% 4 4 3 4 4 6% 51%	2211
	21st to end	Day	4 4 1 5% 3 4 4 4 6% 8% 58%	2309
May	1st to 10th	Day	6% 4 1 5% 4 4 5% 5% 7% 11% 48%	4082
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 4 5% 8% 41%	4081
	21st to end	Day	6% 4 2 5% 4 4 5% 4 7% 10% 51%	4476
June	1st to 10th	Day	8% 5% 2 7% 5% 6% 7% 6% 9% 10% 34%	2470
	11th to 20th	Day	6% 4 2 5% 5% 6% 6% 5% 8% 11% 40%	2470
	21st to end	Day	8% 4 2 7% 5% 7% 7% 6% 9% 11% 33%	2470
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 7% 6% 10% 12% 27%	4120
	11th to 20th	Day	5% 4 2 5% 5% 6% 8% 7% 12% 15% 31%	4120
	21st to end	Day	5% 5% 2 7% 7% 7% 8% 7% 10% 16% 27%	4466
August	1st to 10th	Day	17% 6% 3 7% 5% 5% 6% 5% 8% 11% 24%	4104
	11th to 20th	Day	6% 5% 2 6% 4 5% 7% 6% 10% 13% 38%	3995
	21st to end	Day	11% 7% 3 7% 4 4 6% 5% 8% 11% 34%	4102
September	1st to 10th	Day	12% 6% 3 8% 6% 6% 6% 6% 8% 8% 32%	2090
	11th to 20th	Day	14% 7% 2 6% 5% 5% 7% 4 6% 8% 35%	2090
	21st to end	Day	12% 6% 2 7% 4 5% 6% 4 6% 8% 39%	2090
October	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 5% 5% 47%	1990
	11th to 20th	Day	9% 4 1 4 2 2 3 3 4 7% 60%	1890
	21st to end	Day	11% 6% 2 5% 3 3 3 3 4 7% 54%	1980
November	1st to 10th	Day	10% 4 2 5% 3 4 4 3 5% 5% 54%	1857
	11th to 20th	Day	12% 6% 3 5% 4 3 4 3 4 6% 51%	1884
	21st to end	Day	14% 5% 2 4 3 3 3 4 5% 6% 51%	1710
December	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 7% 46%	1710
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 4 6% 51%	2153
	21st to end	Day	17% 9% 3 5% 4 3 3 3 4 6% 42%	3124
January	1st to 10th	Night	14% 6% 2 6% 4 5% 4 4 6% 6% 43%	4109
	11th to 20th	Night	10% 7% 2 7% 4 4 5% 4 6% 8% 44%	4120
	21st to end	Night	14% 9% 4 8% 4 3 3 3 4 5% 42%	4532
February	1st to 10th	Night	15% 8% 2 6% 5% 4 4 4 5% 5% 43%	2409
	11th to 20th	Night	15% 7% 3 6% 4 4 3 3 6% 5% 45%	2218
	21st to end	Night	12% 6% 2 7% 5% 5% 4 3 5% 7% 44%	1786
March	1st to 10th	Night	19% 13% 4 7% 4 3 3 3 4 5% 36%	3418
	11th to 20th	Night	9% 5% 2 6% 4 3 4 4 6% 8% 49%	3480
	21st to end	Night	15% 5% 2 6% 4 4 4 3 5% 7% 45%	3817
April	1st to 10th	Night	10% 4 2 4 4 4 4 4 5% 7% 50%	2090
	11th to 20th	Night	11% 5% 2 5% 3 3 3 4 4 7% 53%	1916
	21st to end	Night	7% 4 2 4 3 3 4 3 6% 8% 55%	1884
May	1st to 10th	Night	5% 4 1 5% 4 4 4 4 7% 11% 50%	2813
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 44%	2812
	21st to end	Night	7% 4 2 4 3 4 5% 4 7% 9% 52%	3079
June	1st to 10th	Night	9% 6% 2 6% 5% 6% 5% 6% 9% 13% 34%	1710
	11th to 20th	Night	6% 5% 2 6% 5% 5% 5% 5% 10% 13% 39%	1710
	21st to end	Night	7% 4 2 6% 6% 6% 7% 7% 10% 14% 32%	1710
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 7% 6% 9% 12% 29%	2840
	11th to 20th	Night	5% 4 2 8% 6% 6% 7% 7% 10% 15% 29%	2840
	21st to end	Night	7% 6% 3 8% 7% 7% 7% 7% 10% 14% 25%	3087
August	1st to 10th	Night	16% 7% 3 7% 4 6% 6% 5% 9% 13% 25%	2825
	11th to 20th	Night	6% 4 1 6% 5% 6% 7% 6% 9% 11% 37%	2785
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 4 5% 8% 43%	3535
September	1st to 10th	Night	12% 6% 3 7% 5% 5% 6% 5% 8% 11% 33%	2090
	11th to 20th	Night	11% 7% 4 8% 4 5% 5% 4 8% 9% 36%	2090
	21st to end	Night	12% 7% 3 9% 5% 6% 5% 4 6% 7% 37%	2090
October	1st to 10th	Night	13% 6% 3 7% 3 4 3 3 5% 6% 47%	2190
	11th to 20th	Night	9% 3 1 4 3 3 3 3 5% 6% 57%	2275
	21st to end	Night	9% 4 3 5% 4 4 4 3 4 6% 55%	2376
November	1st to 10th	Night	10% 5% 2 5% 4 2 3 3 5% 5% 55%	2221
	11th to 20th	Night	13% 6% 2 4 4 4 4 3 4 6% 49%	2292
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 6% 47%	2470
December	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 8% 42%	2470
	11th to 20th	Night	15% 7% 2 6% 4 3 3 2 4 5% 48%	3042
	21st to end	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	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	35% 5% 11% 18% 21% 10%	2837
	11th to 20th	Day	36% 7% 7% 18% 25% 8%	2840
	21st to end	Day	40% 8% 7% 14% 22% 9%	3124
February	1st to 10th	Day	42% 7% 9% 17% 18% 7%	1767
	11th to 20th	Day	40% 5% 9% 15% 24% 8%	1858
	21st to end	Day	38% 6% 8% 17% 19% 11%	1666
March	1st to 10th	Day	47% 7% 9% 16% 15% 6%	3435
	11th to 20th	Day	27% 8% 14% 17% 21% 14%	3480
	21st to end	Day	40% 6% 10% 19% 17% 9%	3815
April	1st to 10th	Day	30% 7% 14% 22% 19% 9%	2090
	11th to 20th	Day	33% 5% 12% 22% 19% 10%	2211
	21st to end	Day	22% 7% 15% 29% 21% 6%	2309
May	1st to 10th	Day	26% 11% 18% 30% 12% 3	4120
	11th to 20th	Day	39% 9% 14% 25% 12% 1	4085
	21st to end	Day	24% 11% 19% 25% 20%	4489
June	1st to 10th	Day	36% 16% 18% 19% 11%	2470
	11th to 20th	Day	33% 13% 18% 26% 10%	2470
	21st to end	Day	36% 17% 22% 17% 7% 1	2470
July	1st to 10th	Day	41% 15% 22% 18% 3 1	4055
	11th to 20th	Day	32% 19% 36% 10% 3	4051
	21st to end	Day	37% 19% 29% 13% 2	4324
August	1st to 10th	Day	48% 13% 25% 11% 3	4091
	11th to 20th	Day	30% 15% 24% 24% 8%	4034
	21st to end	Day	39% 12% 18% 29% 3	4102
September	1st to 10th	Day	44% 13% 16% 15% 10% 2	2090
	11th to 20th	Day	44% 12% 15% 17% 11% 2	2090
	21st to end	Day	40% 9% 15% 18% 15% 3	2090
October	1st to 10th	Day	37% 7% 11% 16% 21% 8%	1990
	11th to 20th	Day	23% 7% 11% 22% 27% 9%	1890
	21st to end	Day	31% 4 10% 17% 24% 13%	1980
November	1st to 10th	Day	30% 7% 10% 17% 23% 12%	1857
	11th to 20th	Day	36% 6% 8% 17% 23% 10%	1884
	21st to end	Day	34% 6% 9% 24% 19% 8%	1710
December	1st to 10th	Day	37% 8% 13% 20% 15% 7%	1710
	11th to 20th	Day	35% 6% 8% 22% 23% 6%	2150
	21st to end	Day	44% 5% 7% 20% 20% 4	3124
January	1st to 10th	Night	39% 5% 8% 14% 22% 11%	4109
	11th to 20th	Night	37% 6% 8% 16% 27% 6%	4120
	21st to end	Night	42% 7% 7% 13% 22% 10%	4532
February	1st to 10th	Night	42% 5% 9% 18% 18% 8%	2409
	11th to 20th	Night	40% 5% 9% 14% 20% 12%	2219
	21st to end	Night	39% 5% 10% 19% 15% 12%	1786
March	1st to 10th	Night	51% 6% 10% 13% 13% 7%	3418
	11th to 20th	Night	33% 5% 12% 16% 21% 12%	3480
	21st to end	Night	38% 5% 11% 18% 20% 8%	3817
April	1st to 10th	Night	32% 7% 11% 20% 19% 9%	2090
	11th to 20th	Night	31% 6% 9% 23% 19% 11%	1916
	21st to end	Night	26% 6% 14% 25% 24% 6%	1884
May	1st to 10th	Night	25% 10% 18% 29% 15% 3	2840
	11th to 20th	Night	36% 10% 15% 24% 14% 1	2813
	21st to end	Night	27% 9% 16% 30% 18% 2	3087
June	1st to 10th	Night	37% 12% 21% 20% 9%	1710
	11th to 20th	Night	33% 13% 22% 25% 7%	1710
	21st to end	Night	36% 15% 25% 18% 5%	1710
July	1st to 10th	Night	42% 13% 21% 21% 2 1	2795
	11th to 20th	Night	36% 18% 34% 9% 3	2786
	21st to end	Night	43% 17% 26% 13% 1	2988
August	1st to 10th	Night	47% 14% 24% 12% 2	2814
	11th to 20th	Night	34% 14% 23% 22% 7%	2812
	21st to end	Night	39% 9% 18% 31% 3	3535
September	1st to 10th	Night	42% 12% 19% 14% 10% 2	2090
	11th to 20th	Night	43% 11% 16% 18% 11% 1	2090
	21st to end	Night	45% 8% 13% 17% 15% 2	2090
October	1st to 10th	Night	38% 7% 11% 18% 19% 7%	2190
	11th to 20th	Night	27% 6% 9% 22% 24% 12%	2275
	21st to end	Night	31% 5% 8% 16% 25% 14%	2376
November	1st to 10th	Night	31% 5% 11% 17% 24% 13%	2221
	11th to 20th	Night	37% 5% 8% 18% 22% 11%	2292
	21st to end	Night	38% 4 11% 18% 21% 8%	2470
December	1st to 10th	Night	39% 8% 14% 17% 18% 5%	2470
	11th to 20th	Night	38% 5% 9% 16% 26% 6%	3044
	21st to end	Night	45% 5% 8% 20% 18% 4	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	15% 9% 39% 33% 5%	270
	11th to 20th	Day	7% 10% 33% 36% 11% 3	270
	21st to end	Day	20% 14% 38% 24% 3	297
February	1st to 10th	Day	17% 11% 29% 36% 8%	270
	11th to 20th	Day	23% 10% 31% 31% 4	270
	21st to end	Day	14% 11% 35% 33% 7%	321
March	1st to 10th	Day	15% 8% 30% 32% 13% 2	460
	11th to 20th	Day	10% 9% 33% 32% 13% 2	460
	21st to end	Day	13% 9% 30% 35% 11% 2	506
April	1st to 10th	Day	13% 9% 31% 31% 15% 1	460
	11th to 20th	Day	9% 8% 29% 30% 19% 5%	452
	21st to end	Day	5% 3 22% 38% 25% 6%	460
May	1st to 10th	Day	4 7% 25% 35% 23% 6%	463
	11th to 20th	Day	9% 2 17% 33% 30% 9%	487
	21st to end	Day	12% 2 9% 30% 36% 12%	529
June	1st to 10th	Day	10% 1 9% 30% 37% 13%	490
	11th to 20th	Day	8% 2 11% 28% 32% 19%	490
	21st to end	Day	8% 2 6% 29% 38% 17%	490
July	1st to 10th	Day	7% 2 5% 16% 47% 22%	485
	11th to 20th	Day	7% 1 4 16% 49% 24%	484
	21st to end	Day	6% 1 5% 22% 50% 16%	532
August	1st to 10th	Day	12% 2 5% 19% 44% 18%	486
	11th to 20th	Day	6% 7% 17% 46% 23%	479
	21st to end	Day	16% 2 6% 18% 37% 21%	539
September	1st to 10th	Day	13% 2 7% 21% 41% 16%	490
	11th to 20th	Day	17% 3 11% 23% 38% 8%	490
	21st to end	Day	13% 2 9% 24% 40% 11%	490
October	1st to 10th	Day	16% 5% 15% 32% 23% 8%	490
	11th to 20th	Day	11% 4 19% 32% 26% 8%	486
	21st to end	Day	13% 6% 18% 29% 27% 6%	495
November	1st to 10th	Day	14% 6% 20% 35% 19% 7%	455
	11th to 20th	Day	17% 8% 27% 33% 14% 2	444
	21st to end	Day	11% 9% 24% 40% 14% 2	270
December	1st to 10th	Day	17% 6% 27% 35% 13% 2	270
	11th to 20th	Day	18% 8% 29% 31% 13% 2	270
	21st to end	Day	21% 7% 24% 32% 15% 1	297
January	1st to 10th	Night	13% 13% 34% 31% 9% 1	760
	11th to 20th	Night	14% 11% 29% 32% 13% 1	760
	21st to end	Night	18% 11% 36% 30% 6%	836
February	1st to 10th	Night	18% 8% 31% 33% 10%	760
	11th to 20th	Night	18% 12% 30% 35% 6%	743
	21st to end	Night	14% 7% 33% 39% 7%	628
March	1st to 10th	Night	13% 8% 28% 39% 12% 1	760
	11th to 20th	Night	13% 8% 25% 38% 16% 1	760
	21st to end	Night	11% 9% 28% 35% 16% 1	836
April	1st to 10th	Night	11% 7% 31% 34% 16% 1	760
	11th to 20th	Night	13% 6% 22% 35% 21% 3	751
	21st to end	Night	8% 5% 23% 38% 23% 3	744
May	1st to 10th	Night	7% 4 21% 36% 27% 5%	570
	11th to 20th	Night	9% 3 16% 37% 28% 7%	561
	21st to end	Night	11% 2 8% 28% 39% 12%	617
June	1st to 10th	Night	10% 2 8% 35% 37% 8%	570
	11th to 20th	Night	7% 2 6% 30% 39% 16%	570
	21st to end	Night	8% 2 5% 31% 41% 13%	570
July	1st to 10th	Night	8% 2 9% 19% 46% 16%	555
	11th to 20th	Night	8% 1 3 19% 52% 17%	552
	21st to end	Night	7% 5% 26% 52% 9%	624
August	1st to 10th	Night	10% 1 7% 23% 44% 14%	567
	11th to 20th	Night	7% 1 6% 23% 48% 16%	564
	21st to end	Night	13% 2 8% 18% 42% 16%	836
September	1st to 10th	Night	13% 3 6% 21% 46% 11%	760
	11th to 20th	Night	14% 2 10% 26% 41% 7%	760
	21st to end	Night	16% 2 10% 26% 38% 8%	760
October	1st to 10th	Night	16% 3 16% 29% 28% 8%	760
	11th to 20th	Night	10% 6% 17% 33% 26% 7%	759
	21st to end	Night	10% 6% 21% 29% 29% 5%	792
November	1st to 10th	Night	12% 6% 21% 33% 24% 4	740
	11th to 20th	Night	16% 10% 24% 32% 16% 2	760
	21st to end	Night	14% 8% 24% 33% 19% 2	760
December	1st to 10th	Night	15% 9% 25% 33% 16% 2	760
	11th to 20th	Night	17% 9% 25% 30% 16% 2	760
	21st to end	Night	21% 8% 27% 29% 14%	836

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