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 2025-03-12.

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 4 5% 4 7% 9% 43%	109317
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	109672
Transparency	Day	35% 9% 15% 19% 16% 6%	113020
	Night	36% 7% 13% 18% 19% 7%	114108
Seeing	Day	12% 5% 19% 29% 26% 9%	15873
	Night	13% 6% 20% 31% 25% 5%	26033

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% 5% 2 5% 3 4 4 3 5% 6% 55%	4056
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 5% 6% 48%	4060
	21st to end	Day	14% 7% 3 6% 3 4 4 3 5% 7% 45%	4466
February	1st to 10th	Day	13% 7% 2 6% 5% 3 5% 5% 7% 6% 42%	1860
	11th to 20th	Day	19% 6% 1 4 3 3 3 3 5% 5% 46%	1948
	21st to end	Day	13% 7% 2 6% 5% 3 3 3 4 7% 48%	1751
March	1st to 10th	Day	20% 10% 3 6% 3 4 3 3 4 6% 37%	3545
	11th to 20th	Day	8% 4 2 5% 3 4 4 4 6% 7% 55%	3491
	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%	1900
	21st to end	Day	10% 5% 2 5% 3 3 3 3 4 7% 55%	2090
November	1st to 10th	Day	11% 5% 2 5% 3 4 4 3 5% 6% 52%	1957
	11th to 20th	Day	11% 6% 3 5% 4 4 4 3 4 6% 51%	1983
	21st to end	Day	13% 5% 2 5% 3 3 3 4 5% 6% 51%	1800
December	1st to 10th	Day	7% 4 1 6% 4 4 4 3 5% 6% 55%	2930
	11th to 20th	Day	13% 5% 3 6% 4 3 3 3 4 6% 49%	3373
	21st to end	Day	18% 9% 3 6% 4 3 3 3 4 6% 40%	4466
January	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 4 5% 56%	5889
	11th to 20th	Night	13% 7% 2 7% 4 4 4 4 6% 7% 43%	5900
	21st to end	Night	13% 7% 3 7% 4 4 3 3 4 7% 44%	6490
February	1st to 10th	Night	14% 7% 2 7% 5% 4 4 4 5% 5% 44%	2535
	11th to 20th	Night	16% 8% 3 6% 4 3 3 3 5% 5% 44%	2338
	21st to end	Night	12% 6% 2 7% 5% 5% 4 3 5% 6% 45%	1877
March	1st to 10th	Night	19% 12% 4 7% 4 3 3 3 4 5% 36%	3528
	11th to 20th	Night	9% 5% 2 6% 4 3 4 4 6% 7% 49%	3498
	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% 58%	2280
	21st to end	Night	9% 4 3 5% 4 4 4 3 4 6% 56%	2508
November	1st to 10th	Night	11% 5% 2 6% 4 3 4 3 5% 5% 54%	2341
	11th to 20th	Night	14% 6% 2 4 4 4 4 3 4 7% 49%	2413
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 6% 48%	2600
December	1st to 10th	Night	10% 5% 2 4 3 3 3 3 4 6% 56%	4250
	11th to 20th	Night	13% 8% 3 9% 5% 3 3 2 4 5% 44%	4822
	21st to end	Night	17% 8% 2 6% 3 4 4 3 5% 6% 42%	6490

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	30% 4 8% 16% 33% 9%	4057
	11th to 20th	Day	37% 5% 6% 17% 26% 9%	4060
	21st to end	Day	39% 6% 7% 18% 20% 10%	4466
February	1st to 10th	Day	41% 7% 9% 18% 19% 6%	1860
	11th to 20th	Day	41% 5% 8% 15% 23% 7%	1958
	21st to end	Day	37% 6% 8% 18% 19% 11%	1751
March	1st to 10th	Day	46% 7% 9% 16% 15% 6%	3545
	11th to 20th	Day	26% 7% 14% 17% 21% 14%	3491
	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	42% 5% 8% 11% 24% 10%	3302
	11th to 20th	Day	15% 5% 7% 15% 31% 28%	3140
	21st to end	Day	23% 4 9% 19% 35% 10%	3428
November	1st to 10th	Day	32% 7% 10% 17% 22% 12%	1957
	11th to 20th	Day	35% 6% 8% 17% 23% 10%	1983
	21st to end	Day	34% 6% 9% 23% 20% 8%	1800
December	1st to 10th	Day	30% 6% 10% 18% 26% 10%	2930
	11th to 20th	Day	35% 6% 9% 25% 21% 5%	3370
	21st to end	Day	45% 4 7% 21% 19% 3	4466
January	1st to 10th	Night	30% 4 7% 13% 36% 10%	5889
	11th to 20th	Night	40% 5% 6% 16% 27% 6%	5900
	21st to end	Night	39% 5% 8% 14% 23% 11%	6490
February	1st to 10th	Night	41% 5% 9% 18% 19% 8%	2535
	11th to 20th	Night	41% 6% 9% 15% 20% 11%	2339
	21st to end	Night	39% 5% 10% 19% 15% 12%	1877
March	1st to 10th	Night	50% 6% 10% 13% 14% 7%	3528
	11th to 20th	Night	33% 5% 12% 16% 21% 12%	3498
	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	39% 5% 11% 13% 23% 9%	3658
	11th to 20th	Night	17% 4 6% 14% 29% 30%	3779
	21st to end	Night	23% 4 7% 18% 34% 14%	4120
November	1st to 10th	Night	32% 5% 12% 17% 22% 12%	2341
	11th to 20th	Night	37% 5% 8% 17% 23% 10%	2413
	21st to end	Night	37% 4 10% 18% 22% 9%	2600
December	1st to 10th	Night	30% 5% 9% 18% 29% 10%	4250
	11th to 20th	Night	43% 6% 10% 15% 19% 7%	4824
	21st to end	Night	42% 5% 8% 20% 21% 3	6490

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% 40% 32% 4	300
	11th to 20th	Day	9% 9% 33% 35% 10% 3	300
	21st to end	Day	19% 14% 39% 24% 3	330
February	1st to 10th	Day	15% 10% 30% 36% 8% 1	300
	11th to 20th	Day	25% 11% 31% 29% 4	300
	21st to end	Day	13% 11% 35% 34% 8%	350
March	1st to 10th	Day	16% 9% 31% 30% 12% 2	500
	11th to 20th	Day	10% 9% 34% 33% 13% 2	464
	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 20% 32% 26% 8%	490
	21st to end	Day	12% 6% 17% 30% 29% 6%	539
November	1st to 10th	Day	14% 6% 21% 34% 19% 6%	495
	11th to 20th	Day	16% 8% 28% 32% 14% 2	483
	21st to end	Day	11% 9% 25% 39% 14% 2	300
December	1st to 10th	Day	16% 6% 29% 35% 11% 2	300
	11th to 20th	Day	17% 9% 29% 31% 12% 2	300
	21st to end	Day	23% 7% 24% 31% 14% 1	330
January	1st to 10th	Night	12% 14% 34% 32% 8% 1	800
	11th to 20th	Night	14% 11% 29% 32% 12% 1	800
	21st to end	Night	18% 11% 36% 30% 5%	880
February	1st to 10th	Night	17% 8% 30% 34% 11%	800
	11th to 20th	Night	18% 13% 30% 33% 6%	783
	21st to end	Night	13% 7% 33% 40% 7%	660
March	1st to 10th	Night	13% 8% 29% 38% 11% 1	800
	11th to 20th	Night	13% 8% 25% 38% 15% 1	767
	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%	760
	21st to end	Night	10% 6% 20% 30% 30% 5%	836
November	1st to 10th	Night	13% 6% 21% 32% 25% 4	780
	11th to 20th	Night	16% 10% 24% 31% 17% 2	800
	21st to end	Night	14% 9% 24% 33% 19% 2	800
December	1st to 10th	Night	14% 9% 26% 33% 15% 2	800
	11th to 20th	Night	17% 9% 26% 30% 16% 2	800
	21st to end	Night	21% 8% 28% 30% 14%	880

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