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 2026-07-16.

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% 5% 2 6% 4 4 5% 4 7% 9% 42%	137990
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	134673
Transparency	Day	35% 9% 16% 19% 15% 6%	141649
	Night	36% 7% 14% 18% 18% 7%	139086
Seeing	Day	12% 5% 19% 29% 26% 9%	17741
	Night	12% 6% 20% 31% 25% 5%	27804

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% 5% 2 6% 4 4 4 3 5% 7% 51%	5258
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 47%	5280
	21st to end	Day	15% 7% 3 6% 3 4 4 3 5% 7% 43%	5780
February	1st to 10th	Day	13% 7% 2 6% 5% 3 5% 4 6% 6% 43%	1953
	11th to 20th	Day	19% 6% 2 4 3 3 3 3 5% 5% 46%	2048
	21st to end	Day	12% 7% 3 6% 5% 3 4 3 4 7% 47%	1836
March	1st to 10th	Day	20% 9% 3 6% 3 3 3 3 5% 6% 39%	5045
	11th to 20th	Day	8% 4 2 4 3 4 4 3 5% 7% 57%	5090
	21st to end	Day	12% 6% 2 6% 4 4 4 3 4 5% 49%	5586
April	1st to 10th	Day	10% 4 1 4 3 3 4 5% 7% 8% 50%	2310
	11th to 20th	Day	10% 5% 2 5% 3 3 3 4 5% 6% 53%	2447
	21st to end	Day	4 4 1 5% 4 4 4 4 6% 8% 56%	2553
May	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 7% 10% 39%	7616
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 43%	7626
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	8391
June	1st to 10th	Day	8% 5% 2 7% 5% 6% 7% 6% 9% 10% 34%	2730
	11th to 20th	Day	6% 4 2 5% 5% 6% 7% 6% 9% 11% 39%	2730
	21st to end	Day	8% 4 2 6% 4 6% 7% 6% 9% 12% 36%	2730
July	1st to 10th	Day	9% 5% 3 7% 6% 6% 8% 7% 10% 12% 26%	6030
	11th to 20th	Day	4 3 2 5% 4 6% 7% 6% 11% 17% 35%	5965
	21st to end	Day	4 4 2 5% 5% 6% 7% 6% 9% 15% 38%	6424
August	1st to 10th	Day	18% 6% 3 7% 5% 5% 6% 5% 8% 11% 24%	4234
	11th to 20th	Day	6% 5% 2 6% 4 5% 7% 6% 10% 13% 38%	4124
	21st to end	Day	12% 7% 3 7% 4 4 6% 5% 8% 11% 34%	4232
September	1st to 10th	Day	11% 6% 2 8% 6% 6% 6% 5% 8% 8% 34%	2200
	11th to 20th	Day	14% 7% 2 6% 5% 5% 7% 4 6% 8% 36%	2200
	21st to end	Day	13% 6% 2 6% 4 5% 6% 4 6% 8% 39%	2200
October	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 5% 6% 46%	2095
	11th to 20th	Day	10% 4 1 4 2 2 3 3 4 7% 61%	2000
	21st to end	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	2200
November	1st to 10th	Day	10% 4 2 5% 3 4 4 3 5% 5% 54%	2057
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 4 6% 53%	2082
	21st to end	Day	13% 5% 2 4 3 3 3 4 5% 6% 52%	1890
December	1st to 10th	Day	8% 5% 1 6% 4 4 4 3 5% 6% 54%	3020
	11th to 20th	Day	13% 5% 3 6% 4 3 3 3 4 6% 49%	3463
	21st to end	Day	18% 9% 3 6% 4 3 3 3 4 6% 41%	4565
January	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 6% 52%	7635
	11th to 20th	Night	11% 6% 2 6% 4 3 4 3 5% 7% 47%	7680
	21st to end	Night	15% 8% 3 7% 4 3 3 3 4 6% 43%	8401
February	1st to 10th	Night	14% 7% 2 7% 5% 4 4 4 5% 5% 44%	2662
	11th to 20th	Night	15% 7% 3 6% 4 4 3 3 5% 5% 44%	2458
	21st to end	Night	11% 7% 2 7% 5% 5% 4 3 5% 6% 44%	1968
March	1st to 10th	Night	17% 10% 3 6% 3 3 3 3 4 6% 42%	5028
	11th to 20th	Night	8% 5% 2 6% 3 3 4 4 6% 9% 51%	5090
	21st to end	Night	12% 5% 2 5% 3 4 3 3 5% 7% 51%	5588
April	1st to 10th	Night	10% 5% 2 4 4 4 5% 4 6% 8% 49%	2310
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 4 7% 55%	2120
	21st to end	Night	7% 4 2 5% 4 3 4 4 6% 8% 53%	2082
May	1st to 10th	Night	10% 6% 3 7% 5% 5% 5% 4 6% 9% 41%	5237
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 7% 10% 42%	5234
	21st to end	Night	9% 5% 2 7% 5% 5% 7% 5% 9% 9% 38%	5761
June	1st to 10th	Night	8% 6% 2 6% 5% 6% 5% 6% 8% 13% 35%	1890
	11th to 20th	Night	6% 5% 2 7% 6% 5% 5% 5% 9% 12% 37%	1890
	21st to end	Night	6% 4 2 6% 6% 6% 7% 7% 10% 13% 33%	1890
July	1st to 10th	Night	8% 6% 3 8% 6% 7% 7% 6% 9% 12% 29%	4148
	11th to 20th	Night	4 4 2 7% 6% 6% 7% 7% 9% 14% 34%	4102
	21st to end	Night	6% 5% 2 7% 6% 6% 6% 7% 9% 13% 32%	4427
August	1st to 10th	Night	17% 7% 3 7% 4 6% 6% 5% 9% 13% 24%	2915
	11th to 20th	Night	6% 4 2 7% 5% 6% 7% 6% 9% 11% 36%	2875
	21st to end	Night	9% 6% 3 7% 5% 5% 4 4 5% 8% 42%	3647
September	1st to 10th	Night	12% 6% 3 7% 5% 5% 6% 5% 8% 11% 34%	2200
	11th to 20th	Night	10% 6% 3 8% 4 5% 5% 4 8% 9% 36%	2200
	21st to end	Night	13% 7% 3 9% 5% 6% 5% 4 6% 8% 37%	2200
October	1st to 10th	Night	13% 7% 3 7% 3 4 3 3 5% 6% 45%	2305
	11th to 20th	Night	10% 3 2 4 3 3 3 3 5% 6% 58%	2400
	21st to end	Night	9% 4 3 5% 4 4 4 3 4 5% 56%	2640
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 5% 55%	2461
	11th to 20th	Night	13% 6% 2 4 4 3 4 3 4 7% 50%	2534
	21st to end	Night	11% 6% 2 5% 4 4 4 4 5% 6% 47%	2730
December	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 55%	4380
	11th to 20th	Night	13% 8% 3 9% 5% 3 3 2 4 5% 44%	4952
	21st to end	Night	17% 7% 2 6% 3 4 4 3 5% 6% 43%	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	31% 4 10% 20% 26% 7%	5259
	11th to 20th	Day	36% 4 5% 17% 26% 12%	5280
	21st to end	Day	39% 5% 9% 19% 18% 10%	5780
February	1st to 10th	Day	40% 6% 8% 19% 20% 7%	1953
	11th to 20th	Day	41% 5% 9% 16% 23% 7%	2058
	21st to end	Day	38% 6% 8% 18% 19% 11%	1836
March	1st to 10th	Day	45% 6% 8% 15% 19% 8%	5045
	11th to 20th	Day	25% 6% 12% 17% 19% 21%	5090
	21st to end	Day	37% 6% 10% 20% 16% 12%	5586
April	1st to 10th	Day	30% 6% 14% 22% 18% 8%	2310
	11th to 20th	Day	31% 5% 11% 23% 20% 10%	2447
	21st to end	Day	24% 7% 14% 29% 21% 5%	2553
May	1st to 10th	Day	38% 9% 17% 23% 12% 2	7651
	11th to 20th	Day	35% 8% 19% 25% 12% 1	7630
	21st to end	Day	34% 11% 20% 18% 15%	8405
June	1st to 10th	Day	36% 15% 18% 18% 11% 1	2730
	11th to 20th	Day	33% 13% 20% 24% 9%	2730
	21st to end	Day	33% 16% 24% 19% 7% 1	2730
July	1st to 10th	Day	43% 15% 25% 14% 2	5965
	11th to 20th	Day	28% 18% 40% 11% 2	5896
	21st to end	Day	29% 16% 29% 24% 2	6240
August	1st to 10th	Day	49% 13% 25% 11% 2	4221
	11th to 20th	Day	30% 15% 24% 23% 8%	4163
	21st to end	Day	39% 12% 18% 29% 3	4232
September	1st to 10th	Day	43% 12% 16% 15% 12% 2	2200
	11th to 20th	Day	42% 11% 15% 19% 11% 2	2200
	21st to end	Day	40% 9% 16% 18% 14% 3	2200
October	1st to 10th	Day	43% 5% 8% 12% 23% 9%	3407
	11th to 20th	Day	16% 5% 7% 15% 31% 28%	3240
	21st to end	Day	23% 4 8% 19% 35% 10%	3538
November	1st to 10th	Day	30% 7% 10% 17% 23% 12%	2057
	11th to 20th	Day	34% 6% 8% 19% 24% 10%	2082
	21st to end	Day	33% 6% 10% 23% 20% 8%	1890
December	1st to 10th	Day	31% 6% 10% 17% 25% 10%	3020
	11th to 20th	Day	35% 6% 9% 25% 21% 5%	3460
	21st to end	Day	44% 4 7% 22% 19% 3	4565
January	1st to 10th	Night	32% 4 9% 17% 30% 7%	7636
	11th to 20th	Night	36% 4 6% 15% 29% 10%	7680
	21st to end	Night	42% 5% 8% 15% 21% 10%	8401
February	1st to 10th	Night	41% 5% 8% 18% 20% 8%	2662
	11th to 20th	Night	41% 6% 9% 15% 19% 11%	2459
	21st to end	Night	40% 5% 9% 19% 15% 13%	1968
March	1st to 10th	Night	44% 4 9% 15% 17% 12%	5028
	11th to 20th	Night	29% 5% 11% 15% 23% 17%	5090
	21st to end	Night	33% 5% 11% 18% 22% 11%	5588
April	1st to 10th	Night	32% 7% 13% 22% 18% 8%	2310
	11th to 20th	Night	28% 6% 9% 25% 21% 11%	2120
	21st to end	Night	27% 7% 13% 24% 23% 6%	2082
May	1st to 10th	Night	38% 8% 17% 23% 12% 2	5264
	11th to 20th	Night	35% 8% 20% 24% 12% 1	5235
	21st to end	Night	38% 10% 17% 23% 11% 1	5771
June	1st to 10th	Night	37% 12% 21% 20% 10% 1	1890
	11th to 20th	Night	36% 13% 22% 24% 6%	1890
	21st to end	Night	35% 15% 25% 19% 6%	1890
July	1st to 10th	Night	42% 14% 26% 16% 2 1	4105
	11th to 20th	Night	34% 16% 38% 9% 2	4051
	21st to end	Night	38% 15% 24% 21% 2	4297
August	1st to 10th	Night	48% 14% 24% 12% 2	2904
	11th to 20th	Night	35% 14% 23% 22% 7%	2902
	21st to end	Night	39% 9% 18% 31% 3	3647
September	1st to 10th	Night	41% 12% 18% 15% 11% 1	2200
	11th to 20th	Night	42% 11% 16% 19% 11% 1	2200
	21st to end	Night	45% 8% 13% 18% 15% 2	2200
October	1st to 10th	Night	40% 5% 11% 13% 22% 9%	3773
	11th to 20th	Night	17% 4 6% 14% 30% 30%	3899
	21st to end	Night	24% 4 7% 17% 34% 15%	4252
November	1st to 10th	Night	31% 5% 11% 17% 23% 13%	2461
	11th to 20th	Night	35% 5% 8% 18% 24% 11%	2534
	21st to end	Night	37% 4 11% 18% 22% 9%	2730
December	1st to 10th	Night	31% 5% 9% 18% 28% 9%	4380
	11th to 20th	Night	42% 5% 10% 15% 20% 7%	4954
	21st to end	Night	42% 5% 8% 21% 21% 3	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	14% 8% 40% 33% 5%	330
	11th to 20th	Day	9% 10% 34% 34% 9% 3	330
	21st to end	Day	19% 14% 40% 23% 3	363
February	1st to 10th	Day	15% 10% 32% 35% 8% 1	330
	11th to 20th	Day	23% 11% 34% 28% 3	330
	21st to end	Day	14% 12% 34% 32% 8%	379
March	1st to 10th	Day	15% 9% 29% 30% 14% 2	540
	11th to 20th	Day	10% 11% 33% 31% 13% 2	540
	21st to end	Day	13% 9% 31% 33% 12% 2	594
April	1st to 10th	Day	14% 9% 30% 32% 14% 2	540
	11th to 20th	Day	8% 7% 28% 30% 21% 5%	532
	21st to end	Day	6% 4 22% 36% 26% 6%	540
May	1st to 10th	Day	7% 7% 27% 33% 21% 6%	543
	11th to 20th	Day	10% 2 17% 34% 28% 9%	567
	21st to end	Day	11% 2 12% 30% 34% 11%	617
June	1st to 10th	Day	9% 3 11% 29% 35% 12%	570
	11th to 20th	Day	7% 2 11% 29% 33% 18%	570
	21st to end	Day	6% 2 6% 29% 39% 17%	570
July	1st to 10th	Day	7% 2 5% 18% 47% 22%	565
	11th to 20th	Day	7% 1 3 16% 48% 24%	544
	21st to end	Day	5% 1 5% 22% 49% 18%	576
August	1st to 10th	Day	13% 3 5% 19% 42% 17%	526
	11th to 20th	Day	6% 1 7% 17% 45% 24%	519
	21st to end	Day	17% 2 6% 20% 35% 20%	583
September	1st to 10th	Day	13% 2 7% 23% 40% 14%	530
	11th to 20th	Day	16% 3 11% 25% 37% 8%	530
	21st to end	Day	15% 2 9% 24% 39% 11%	530
October	1st to 10th	Day	17% 5% 17% 32% 23% 8%	530
	11th to 20th	Day	12% 4 19% 32% 26% 7%	530
	21st to end	Day	14% 6% 18% 30% 27% 5%	583
November	1st to 10th	Day	13% 6% 21% 35% 19% 7%	535
	11th to 20th	Day	14% 8% 29% 33% 14% 2	522
	21st to end	Day	10% 9% 28% 38% 13% 2	330
December	1st to 10th	Day	18% 6% 30% 33% 11% 2	330
	11th to 20th	Day	16% 10% 29% 31% 14% 2	330
	21st to end	Day	21% 7% 25% 33% 14% 1	363
January	1st to 10th	Night	12% 13% 33% 32% 8% 1	840
	11th to 20th	Night	14% 12% 30% 31% 12% 1	840
	21st to end	Night	18% 11% 36% 30% 5%	924
February	1st to 10th	Night	17% 8% 31% 34% 10%	840
	11th to 20th	Night	17% 12% 31% 33% 6%	823
	21st to end	Night	14% 8% 33% 38% 7%	692
March	1st to 10th	Night	13% 8% 28% 37% 13% 1	840
	11th to 20th	Night	12% 9% 26% 38% 15%	840
	21st to end	Night	10% 9% 29% 35% 16% 1	924
April	1st to 10th	Night	11% 7% 30% 34% 16% 1	840
	11th to 20th	Night	12% 6% 23% 35% 21% 3	831
	21st to end	Night	8% 5% 22% 39% 24% 3	822
May	1st to 10th	Night	8% 4 21% 37% 25% 4	630
	11th to 20th	Night	9% 3 16% 37% 28% 6%	621
	21st to end	Night	11% 2 9% 29% 37% 11%	683
June	1st to 10th	Night	10% 3 9% 34% 36% 8%	630
	11th to 20th	Night	7% 2 6% 33% 37% 15%	630
	21st to end	Night	7% 2 5% 31% 42% 13%	630
July	1st to 10th	Night	8% 2 8% 20% 47% 16%	615
	11th to 20th	Night	8% 1 4 19% 52% 17%	597
	21st to end	Night	7% 5% 25% 53% 10%	657
August	1st to 10th	Night	11% 1 7% 23% 44% 14%	597
	11th to 20th	Night	7% 1 6% 22% 48% 16%	594
	21st to end	Night	13% 3 9% 19% 41% 16%	880
September	1st to 10th	Night	13% 3 8% 22% 44% 11%	800
	11th to 20th	Night	13% 2 10% 27% 41% 8%	800
	21st to end	Night	16% 2 10% 26% 38% 8%	800
October	1st to 10th	Night	16% 4 17% 29% 27% 8%	800
	11th to 20th	Night	11% 6% 17% 32% 27% 7%	800
	21st to end	Night	10% 6% 21% 30% 29% 5%	880
November	1st to 10th	Night	12% 6% 20% 34% 24% 4	820
	11th to 20th	Night	16% 9% 24% 33% 17% 2	840
	21st to end	Night	13% 9% 24% 33% 19% 1	840
December	1st to 10th	Night	16% 9% 26% 33% 15% 2	840
	11th to 20th	Night	17% 9% 26% 30% 16% 2	840
	21st to end	Night	20% 8% 27% 31% 14%	924

Page generated at 2026-07-13 03:14:52. Version ac07

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