Wright's Mill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wright's Mill Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	113417
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 42%	115166
Transparency	Day	42% 6% 12% 22% 16% 3	116979
	Night	41% 5% 11% 22% 16% 3	119768
Seeing	Day	22% 18% 30% 20% 8% 2	15524
	Night	21% 15% 31% 24% 8% 1	24775

By Month

Percentage Cloud Cover NE (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	14% 6% 2 6% 5% 4 4 3 4 6% 46%	3597
	11th to 20th	Day	15% 9% 3 7% 5% 5% 4 4 4 5% 41%	3570
	21st to end	Day	19% 8% 3 7% 4 4 4 3 4 5% 39%	4218
February	1st to 10th	Day	17% 9% 2 6% 4 5% 5% 3 5% 6% 38%	1823
	11th to 20th	Day	15% 8% 3 6% 6% 4 4 3 4 6% 42%	1948
	21st to end	Day	20% 9% 3 5% 4 3 4 4 4 5% 39%	1650
March	1st to 10th	Day	23% 7% 2 5% 4 3 4 3 4 5% 41%	4476
	11th to 20th	Day	13% 7% 3 7% 4 4 5% 4 6% 7% 42%	4704
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 6% 40%	5965
April	1st to 10th	Day	19% 10% 3 6% 4 3 4 3 5% 7% 38%	2400
	11th to 20th	Day	19% 6% 2 6% 4 4 4 4 5% 7% 40%	2374
	21st to end	Day	19% 6% 2 6% 4 4 4 4 6% 6% 40%	2390
May	1st to 10th	Day	28% 9% 2 5% 3 3 4 3 5% 8% 30%	4106
	11th to 20th	Day	23% 9% 3 7% 4 4 4 4 4 7% 30%	4352
	21st to end	Day	15% 5% 1 5% 4 3 4 4 6% 8% 44%	4837
June	1st to 10th	Day	18% 6% 2 5% 4 5% 4 4 7% 8% 37%	2660
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 5% 6% 8% 37%	2660
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 6% 8% 39%	2660
July	1st to 10th	Day	10% 7% 2 8% 6% 5% 6% 5% 8% 9% 34%	4440
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 5% 7% 10% 37%	4440
	21st to end	Day	15% 8% 3 8% 5% 6% 5% 4 7% 7% 32%	4814
August	1st to 10th	Day	21% 7% 2 7% 4 5% 5% 5% 6% 7% 31%	4216
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 6% 8% 40%	3730
	21st to end	Day	10% 6% 2 6% 4 4 4 3 6% 10% 45%	4180
September	1st to 10th	Day	14% 5% 2 6% 5% 5% 5% 3 6% 6% 44%	2280
	11th to 20th	Day	13% 5% 2 4 3 3 4 4 5% 7% 49%	2184
	21st to end	Day	17% 6% 2 7% 4 4 4 3 4 6% 40%	2090
October	1st to 10th	Day	22% 7% 2 5% 4 4 3 2 4 6% 42%	2005
	11th to 20th	Day	15% 5% 1 5% 4 4 3 4 4 7% 48%	1900
	21st to end	Day	22% 5% 2 5% 4 4 4 3 5% 6% 41%	1914
November	1st to 10th	Day	14% 6% 2 5% 3 2 2 3 4 6% 52%	1761
	11th to 20th	Day	19% 6% 2 4 2 3 4 3 5% 6% 47%	1800
	21st to end	Day	17% 6% 2 5% 4 4 4 4 5% 7% 43%	1727
December	1st to 10th	Day	16% 7% 2 7% 4 4 4 3 5% 6% 42%	2600
	11th to 20th	Day	22% 8% 2 4 3 3 2 3 4 5% 45%	2986
	21st to end	Day	19% 8% 3 7% 4 4 4 3 5% 6% 37%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 6% 47%	6349
	11th to 20th	Night	14% 8% 2 7% 4 4 3 3 5% 6% 46%	6315
	21st to end	Night	17% 10% 2 6% 4 3 4 3 4 5% 39%	6720
February	1st to 10th	Night	17% 9% 3 5% 4 3 4 2 4 6% 41%	2575
	11th to 20th	Night	17% 8% 2 6% 5% 3 4 3 5% 6% 41%	2338
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 5% 42%	1980
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 6% 42%	5364
	11th to 20th	Night	16% 8% 3 6% 4 3 3 3 4 5% 44%	5237
	21st to end	Night	19% 9% 3 7% 4 3 3 3 5% 5% 39%	4967
April	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 5% 7% 36%	2000
	11th to 20th	Night	21% 7% 2 6% 3 4 3 2 5% 7% 40%	1975
	21st to end	Night	20% 7% 3 6% 3 3 4 2 5% 5% 42%	1905
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 5% 7% 29%	2834
	11th to 20th	Night	22% 10% 3 7% 5% 3 4 3 5% 6% 32%	2569
	21st to end	Night	18% 6% 2 5% 3 3 3 3 5% 7% 43%	2738
June	1st to 10th	Night	18% 6% 1 5% 4 4 3 3 5% 8% 42%	1520
	11th to 20th	Night	14% 8% 1 5% 4 5% 4 4 6% 8% 41%	1520
	21st to end	Night	12% 6% 2 4 4 4 4 4 7% 9% 43%	1520
July	1st to 10th	Night	11% 6% 2 7% 6% 6% 6% 5% 7% 9% 36%	2520
	11th to 20th	Night	14% 7% 2 6% 4 5% 5% 3 7% 9% 38%	2520
	21st to end	Night	19% 8% 3 6% 4 4 4 3 6% 8% 36%	2739
August	1st to 10th	Night	23% 7% 2 7% 5% 5% 5% 4 5% 7% 31%	2677
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 3 6% 7% 39%	3097
	21st to end	Night	11% 5% 2 4 3 3 4 4 5% 8% 52%	3476
September	1st to 10th	Night	15% 5% 2 6% 5% 4 4 3 5% 6% 46%	1900
	11th to 20th	Night	14% 6% 2 3 3 3 3 3 4 7% 52%	1995
	21st to end	Night	18% 7% 2 6% 3 3 4 2 4 6% 44%	2090
October	1st to 10th	Night	19% 8% 3 6% 4 3 3 2 3 6% 43%	2171
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	22% 6% 2 5% 3 3 4 3 4 6% 42%	2684
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 5% 6% 50%	2537
	11th to 20th	Night	19% 7% 2 5% 3 2 3 3 5% 5% 47%	2600
	21st to end	Night	15% 7% 2 4 4 3 4 4 5% 7% 46%	2670
December	1st to 10th	Night	16% 7% 2 5% 4 4 3 2 4 5% 47%	4580
	11th to 20th	Night	25% 8% 2 4 3 3 3 3 4 4 41%	5208
	21st to end	Night	20% 9% 2 6% 4 4 3 3 4 5% 40%	6996

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	36% 6% 8% 29% 17% 3	3598
	11th to 20th	Day	44% 5% 10% 22% 13% 6%	3600
	21st to end	Day	46% 6% 9% 23% 15% 1	4218
February	1st to 10th	Day	46% 5% 12% 22% 13% 2	1823
	11th to 20th	Day	43% 4 10% 23% 17% 3	1957
	21st to end	Day	46% 5% 10% 22% 15% 3	1650
March	1st to 10th	Day	44% 5% 12% 17% 13% 9%	4479
	11th to 20th	Day	37% 6% 16% 27% 10% 3	4704
	21st to end	Day	45% 5% 9% 13% 22% 6%	5965
April	1st to 10th	Day	46% 5% 9% 20% 15% 4	2400
	11th to 20th	Day	43% 5% 11% 19% 19% 3	2374
	21st to end	Day	43% 6% 10% 22% 16% 3	2390
May	1st to 10th	Day	52% 7% 13% 15% 12% 1	4120
	11th to 20th	Day	50% 9% 12% 18% 9% 1	4356
	21st to end	Day	35% 9% 12% 25% 17% 2	4837
June	1st to 10th	Day	42% 9% 13% 21% 14% 1	2660
	11th to 20th	Day	43% 7% 14% 23% 11% 2	2660
	21st to end	Day	41% 9% 13% 22% 14% 1	2660
July	1st to 10th	Day	42% 10% 11% 20% 15% 1	4370
	11th to 20th	Day	39% 8% 17% 22% 12% 2	4365
	21st to end	Day	47% 9% 12% 17% 14% 1	4661
August	1st to 10th	Day	49% 8% 13% 21% 8%	4202
	11th to 20th	Day	38% 7% 16% 29% 10% 1	3766
	21st to end	Day	33% 7% 13% 28% 18% 1	4180
September	1st to 10th	Day	40% 7% 10% 23% 18% 2	2280
	11th to 20th	Day	34% 5% 11% 23% 26% 1	2184
	21st to end	Day	45% 5% 9% 21% 17% 3	2090
October	1st to 10th	Day	36% 6% 10% 19% 24% 5%	3330
	11th to 20th	Day	27% 5% 10% 25% 29% 5%	3140
	21st to end	Day	30% 4 15% 24% 24% 4	3126
November	1st to 10th	Day	34% 4 13% 25% 21% 3	1761
	11th to 20th	Day	38% 4 10% 23% 21% 4	1800
	21st to end	Day	41% 4 13% 28% 12% 2	1727
December	1st to 10th	Day	41% 5% 15% 27% 11% 1	2600
	11th to 20th	Day	43% 4 13% 21% 17% 3	2986
	21st to end	Day	47% 6% 8% 20% 16% 3	3960
January	1st to 10th	Night	35% 6% 9% 26% 21% 3	6349
	11th to 20th	Night	39% 5% 14% 21% 15% 6%	6360
	21st to end	Night	45% 4 10% 19% 16% 5%	6720
February	1st to 10th	Night	45% 5% 12% 24% 12% 2	2575
	11th to 20th	Night	44% 5% 10% 19% 18% 5%	2340
	21st to end	Night	45% 4 8% 23% 17% 4	1980
March	1st to 10th	Night	43% 4 11% 19% 15% 8%	5374
	11th to 20th	Night	40% 6% 14% 23% 12% 4	5238
	21st to end	Night	47% 5% 6% 14% 24% 5%	4967
April	1st to 10th	Night	47% 5% 11% 20% 15% 3	2000
	11th to 20th	Night	45% 5% 10% 19% 20% 1	1975
	21st to end	Night	44% 6% 9% 21% 17% 4	1905
May	1st to 10th	Night	55% 7% 10% 17% 9% 1	2840
	11th to 20th	Night	52% 8% 13% 15% 10% 1	2570
	21st to end	Night	39% 7% 13% 25% 13% 3	2739
June	1st to 10th	Night	41% 8% 9% 22% 18% 2	1520
	11th to 20th	Night	39% 8% 17% 25% 11% 1	1520
	21st to end	Night	35% 10% 14% 27% 14% 1	1520
July	1st to 10th	Night	42% 8% 15% 18% 15% 2	2480
	11th to 20th	Night	41% 8% 15% 24% 11% 1	2472
	21st to end	Night	46% 6% 14% 15% 17% 2	2651
August	1st to 10th	Night	51% 9% 14% 16% 9% 1	2667
	11th to 20th	Night	43% 7% 13% 28% 9% 1	3127
	21st to end	Night	29% 7% 13% 33% 17% 1	3476
September	1st to 10th	Night	39% 6% 9% 26% 19% 1	1900
	11th to 20th	Night	33% 5% 10% 25% 25% 3	1995
	21st to end	Night	41% 7% 7% 22% 21% 2	2090
October	1st to 10th	Night	39% 6% 11% 22% 19% 2	3626
	11th to 20th	Night	25% 4 10% 28% 29% 4	3779
	21st to end	Night	32% 5% 13% 29% 19% 3	4422
November	1st to 10th	Night	34% 5% 12% 23% 22% 4	2537
	11th to 20th	Night	40% 3 8% 24% 20% 5%	2600
	21st to end	Night	38% 4 13% 30% 13% 1	2670
December	1st to 10th	Night	40% 3 12% 31% 13% 1	4580
	11th to 20th	Night	47% 3 11% 20% 16% 3	5208
	21st to end	Night	45% 4 10% 19% 17% 4	6996

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	29% 31% 27% 11% 2 1	200
	11th to 20th	Day	29% 30% 31% 10% 1	200
	21st to end	Day	22% 31% 32% 10% 4	277
February	1st to 10th	Day	31% 30% 30% 8% 1	323
	11th to 20th	Day	29% 29% 33% 9% 1	496
	21st to end	Day	31% 24% 31% 11% 2	414
March	1st to 10th	Day	28% 24% 33% 13% 2	500
	11th to 20th	Day	23% 28% 32% 13% 3 1	500
	21st to end	Day	31% 24% 30% 10% 3 1	550
April	1st to 10th	Day	32% 26% 24% 13% 4 1	500
	11th to 20th	Day	26% 26% 28% 12% 8% 1	492
	21st to end	Day	27% 19% 35% 15% 4 1	480
May	1st to 10th	Day	32% 20% 28% 13% 5% 2	463
	11th to 20th	Day	22% 16% 37% 20% 5% 1	487
	21st to end	Day	18% 13% 29% 27% 10% 3	529
June	1st to 10th	Day	20% 12% 33% 24% 10% 2	490
	11th to 20th	Day	16% 15% 33% 26% 9% 1	490
	21st to end	Day	15% 14% 30% 27% 13% 2	490
July	1st to 10th	Day	15% 7% 31% 28% 15% 4	485
	11th to 20th	Day	12% 6% 31% 32% 17% 2	484
	21st to end	Day	13% 10% 32% 32% 12% 1	532
August	1st to 10th	Day	16% 9% 26% 30% 14% 5%	486
	11th to 20th	Day	15% 5% 28% 31% 15% 5%	479
	21st to end	Day	12% 10% 28% 28% 15% 7%	539
September	1st to 10th	Day	18% 12% 27% 23% 16% 4	490
	11th to 20th	Day	14% 9% 30% 25% 12% 10%	490
	21st to end	Day	22% 12% 32% 20% 11% 4	490
October	1st to 10th	Day	27% 13% 24% 20% 12% 4	490
	11th to 20th	Day	18% 18% 34% 20% 9% 2	490
	21st to end	Day	26% 16% 30% 19% 8% 1	446
November	1st to 10th	Day	15% 20% 32% 22% 8% 3	395
	11th to 20th	Day	26% 21% 34% 15% 5%	400
	21st to end	Day	24% 23% 29% 18% 4 1	327
December	1st to 10th	Day	25% 27% 31% 14% 5%	200
	11th to 20th	Day	35% 26% 29% 8% 3	200
	21st to end	Day	26% 21% 35% 15% 4	220
January	1st to 10th	Night	23% 22% 36% 17% 3	800
	11th to 20th	Night	24% 22% 32% 19% 3	800
	21st to end	Night	20% 23% 37% 18% 2	880
February	1st to 10th	Night	23% 21% 37% 18% 2	800
	11th to 20th	Night	26% 25% 34% 14% 1	783
	21st to end	Night	26% 22% 34% 16% 3	660
March	1st to 10th	Night	24% 22% 33% 18% 3	800
	11th to 20th	Night	22% 21% 36% 17% 3	800
	21st to end	Night	23% 22% 35% 16% 4	880
April	1st to 10th	Night	25% 21% 34% 17% 4	800
	11th to 20th	Night	24% 18% 35% 19% 4	791
	21st to end	Night	24% 17% 34% 20% 5%	731
May	1st to 10th	Night	26% 16% 32% 20% 6%	570
	11th to 20th	Night	24% 15% 32% 25% 5%	422
	21st to end	Night	19% 10% 24% 37% 9% 1	411
June	1st to 10th	Night	18% 8% 30% 33% 10% 1	380
	11th to 20th	Night	14% 6% 29% 38% 12%	380
	21st to end	Night	13% 3 30% 42% 11% 1	380
July	1st to 10th	Night	14% 7% 22% 38% 19% 1	370
	11th to 20th	Night	12% 4 24% 41% 17% 1	368
	21st to end	Night	13% 5% 24% 42% 16% 1	416
August	1st to 10th	Night	14% 4 24% 40% 17% 1	471
	11th to 20th	Night	14% 7% 21% 36% 22% 1	750
	21st to end	Night	16% 6% 21% 35% 19% 3	836
September	1st to 10th	Night	18% 6% 28% 32% 14% 2	760
	11th to 20th	Night	16% 6% 25% 31% 18% 4	760
	21st to end	Night	21% 6% 27% 31% 14% 1	760
October	1st to 10th	Night	22% 12% 28% 24% 12% 2	760
	11th to 20th	Night	19% 13% 32% 25% 11%	760
	21st to end	Night	26% 15% 31% 20% 7%	836
November	1st to 10th	Night	15% 14% 37% 24% 9% 1	780
	11th to 20th	Night	24% 17% 33% 20% 6% 1	800
	21st to end	Night	21% 20% 31% 22% 6% 1	800
December	1st to 10th	Night	26% 19% 33% 17% 5% 1	800
	11th to 20th	Night	25% 19% 33% 20% 3	800
	21st to end	Night	25% 20% 35% 17% 3	880

Page generated at 2025-04-23 09:25:08. Version ac07

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