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 2026-08-30.

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%	143989
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 42%	138225
Transparency	Day	42% 6% 12% 22% 16% 2	147505
	Night	42% 5% 11% 22% 16% 3	142798
Seeing	Day	22% 18% 31% 19% 8% 2	17318
	Night	21% 16% 31% 24% 8% 1	26358

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	13% 7% 2 6% 5% 4 4 4 4 6% 44%	4661
	11th to 20th	Day	16% 9% 2 6% 4 4 3 3 4 5% 44%	4650
	21st to end	Day	16% 7% 3 6% 4 4 4 4 5% 6% 41%	5464
February	1st to 10th	Day	16% 9% 3 6% 4 5% 5% 3 5% 6% 39%	1914
	11th to 20th	Day	15% 8% 3 6% 6% 4 4 3 5% 6% 41%	2048
	21st to end	Day	19% 10% 3 5% 4 3 4 4 4 5% 40%	1730
March	1st to 10th	Day	23% 7% 2 5% 4 3 4 3 4 5% 40%	4576
	11th to 20th	Day	13% 6% 3 7% 4 4 5% 4 6% 7% 42%	4808
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 6% 39%	6097
April	1st to 10th	Day	19% 9% 3 6% 4 3 4 3 5% 7% 38%	2520
	11th to 20th	Day	18% 6% 2 6% 3 4 4 4 5% 7% 41%	2494
	21st to end	Day	19% 7% 2 6% 4 4 4 4 6% 6% 40%	2576
May	1st to 10th	Day	26% 8% 2 6% 3 4 4 3 5% 7% 32%	7639
	11th to 20th	Day	19% 9% 3 6% 4 3 4 3 4 6% 38%	8134
	21st to end	Day	24% 7% 2 6% 4 4 5% 4 6% 7% 32%	9060
June	1st to 10th	Day	17% 6% 2 5% 4 5% 4 4 7% 8% 38%	2940
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 5% 6% 8% 37%	2940
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 6% 8% 38%	2940
July	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 35%	8280
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 5% 7% 11% 36%	8280
	21st to end	Day	14% 6% 2 7% 4 5% 5% 4 6% 7% 39%	8993
August	1st to 10th	Day	20% 7% 2 7% 5% 5% 5% 5% 6% 7% 31%	4483
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 4 6% 9% 40%	3970
	21st to end	Day	10% 5% 2 6% 4 5% 5% 3 6% 10% 45%	4420
September	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 3 6% 6% 44%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 4 5% 7% 50%	2299
	21st to end	Day	17% 6% 3 8% 4 4 4 3 5% 7% 39%	2200
October	1st to 10th	Day	21% 6% 2 4 3 4 3 2 4 6% 43%	2110
	11th to 20th	Day	16% 5% 1 5% 4 4 3 3 4 7% 47%	2000
	21st to end	Day	21% 5% 2 5% 4 4 4 3 5% 6% 42%	2015
November	1st to 10th	Day	14% 5% 2 5% 4 2 3 3 4 6% 51%	1851
	11th to 20th	Day	18% 6% 2 5% 3 3 4 3 4 6% 46%	1890
	21st to end	Day	17% 7% 2 5% 4 4 4 4 5% 7% 42%	1813
December	1st to 10th	Day	16% 7% 2 7% 4 4 4 3 5% 6% 42%	2680
	11th to 20th	Day	22% 8% 2 4 3 3 2 3 4 5% 45%	3066
	21st to end	Day	19% 9% 3 7% 4 4 4 3 5% 6% 37%	4048
January	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	8234
	11th to 20th	Night	16% 9% 2 7% 4 3 3 3 5% 5% 43%	8235
	21st to end	Night	15% 9% 2 6% 4 3 4 3 4 6% 44%	8699
February	1st to 10th	Night	16% 9% 3 6% 4 3 4 3 4 6% 42%	2704
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 3 5% 6% 40%	2458
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 5% 41%	2076
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 6% 42%	5484
	11th to 20th	Night	16% 8% 3 6% 4 3 3 3 4 6% 44%	5353
	21st to end	Night	19% 9% 3 7% 4 3 3 3 5% 5% 39%	5077
April	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 5% 7% 36%	2100
	11th to 20th	Night	20% 7% 2 6% 3 5% 3 3 5% 7% 39%	2075
	21st to end	Night	20% 6% 2 6% 3 3 4 3 5% 5% 42%	2042
May	1st to 10th	Night	25% 9% 3 7% 4 4 4 3 5% 7% 31%	5259
	11th to 20th	Night	19% 9% 2 7% 4 3 4 4 5% 7% 35%	4781
	21st to end	Night	25% 9% 2 6% 4 4 4 3 4 6% 34%	5114
June	1st to 10th	Night	17% 6% 1 5% 4 4 4 3 5% 8% 43%	1680
	11th to 20th	Night	14% 8% 1 5% 4 5% 4 4 6% 8% 40%	1680
	21st to end	Night	12% 6% 2 5% 5% 4 4 4 7% 8% 42%	1680
July	1st to 10th	Night	15% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	4680
	11th to 20th	Night	13% 8% 3 7% 4 4 5% 4 7% 9% 37%	4680
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 8% 43%	5084
August	1st to 10th	Night	23% 7% 2 7% 5% 5% 5% 4 5% 7% 32%	2847
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 3 6% 7% 39%	3297
	21st to end	Night	11% 5% 2 4 3 3 4 4 5% 8% 51%	3677
September	1st to 10th	Night	15% 5% 2 6% 5% 4 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 5% 2 3 3 3 3 3 4 7% 52%	2100
	21st to end	Night	18% 7% 3 6% 3 3 4 2 4 6% 44%	2200
October	1st to 10th	Night	19% 7% 3 6% 4 3 3 2 3 6% 44%	2285
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 49%	2400
	21st to end	Night	22% 6% 2 5% 3 3 4 3 4 6% 43%	2825
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 6% 6% 50%	2667
	11th to 20th	Night	18% 7% 2 5% 3 2 3 3 5% 5% 47%	2730
	21st to end	Night	15% 7% 2 4 4 4 4 3 5% 6% 46%	2804
December	1st to 10th	Night	16% 7% 2 5% 4 4 3 3 4 5% 47%	4720
	11th to 20th	Night	25% 8% 2 4 3 3 3 3 4 4 41%	5348
	21st to end	Night	19% 9% 2 6% 4 4 4 3 4 5% 40%	7150

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	38% 6% 10% 28% 16% 2	4662
	11th to 20th	Day	42% 5% 9% 21% 15% 7%	4680
	21st to end	Day	41% 6% 10% 25% 17% 1	5464
February	1st to 10th	Day	45% 5% 12% 22% 14% 2	1914
	11th to 20th	Day	44% 4 10% 23% 17% 3	2057
	21st to end	Day	46% 5% 10% 22% 14% 2	1730
March	1st to 10th	Day	44% 5% 12% 17% 13% 9%	4579
	11th to 20th	Day	37% 6% 16% 27% 10% 3	4808
	21st to end	Day	45% 5% 9% 13% 22% 6%	6097
April	1st to 10th	Day	46% 5% 9% 20% 15% 4	2520
	11th to 20th	Day	42% 6% 12% 19% 19% 2	2494
	21st to end	Day	43% 6% 10% 21% 16% 3	2576
May	1st to 10th	Day	51% 7% 12% 16% 13% 1	7651
	11th to 20th	Day	45% 7% 11% 22% 14% 1	8138
	21st to end	Day	48% 8% 12% 19% 12% 1	9061
June	1st to 10th	Day	41% 9% 12% 21% 16% 1	2940
	11th to 20th	Day	43% 7% 14% 23% 12% 1	2940
	21st to end	Day	41% 9% 13% 22% 14% 1	2940
July	1st to 10th	Day	43% 9% 12% 19% 15% 1	8210
	11th to 20th	Day	37% 8% 17% 25% 12% 1	8205
	21st to end	Day	41% 8% 11% 21% 18% 1	8795
August	1st to 10th	Day	48% 8% 13% 21% 9% 1	4469
	11th to 20th	Day	37% 7% 16% 29% 10%	4006
	21st to end	Day	33% 7% 12% 28% 18% 1	4420
September	1st to 10th	Day	40% 7% 10% 23% 18% 2	2400
	11th to 20th	Day	33% 5% 11% 24% 26% 1	2299
	21st to end	Day	45% 6% 10% 21% 16% 3	2200
October	1st to 10th	Day	35% 6% 10% 19% 24% 5%	3435
	11th to 20th	Day	27% 5% 10% 25% 28% 5%	3240
	21st to end	Day	29% 4 15% 24% 24% 4	3227
November	1st to 10th	Day	34% 5% 13% 25% 21% 2	1851
	11th to 20th	Day	38% 4 10% 23% 21% 4	1890
	21st to end	Day	42% 4 13% 27% 12% 2	1813
December	1st to 10th	Day	41% 5% 15% 27% 11% 1	2680
	11th to 20th	Day	42% 4 13% 21% 17% 3	3066
	21st to end	Day	48% 6% 8% 20% 16% 3	4048
January	1st to 10th	Night	38% 5% 12% 25% 18% 3	8234
	11th to 20th	Night	42% 5% 12% 20% 16% 6%	8280
	21st to end	Night	41% 4 9% 21% 20% 4	8699
February	1st to 10th	Night	44% 5% 11% 24% 13% 2	2704
	11th to 20th	Night	45% 5% 10% 20% 17% 4	2460
	21st to end	Night	45% 4 8% 23% 16% 4	2076
March	1st to 10th	Night	44% 4 11% 19% 15% 7%	5494
	11th to 20th	Night	40% 6% 14% 23% 12% 4	5354
	21st to end	Night	47% 5% 7% 13% 24% 5%	5077
April	1st to 10th	Night	47% 5% 11% 19% 15% 4	2100
	11th to 20th	Night	45% 5% 11% 18% 20% 1	2075
	21st to end	Night	44% 6% 9% 20% 18% 4	2042
May	1st to 10th	Night	53% 6% 11% 16% 12% 1	5265
	11th to 20th	Night	46% 8% 12% 19% 12% 2	4782
	21st to end	Night	51% 5% 12% 21% 10% 1	5115
June	1st to 10th	Night	41% 7% 9% 23% 18% 2	1680
	11th to 20th	Night	40% 8% 16% 24% 11% 1	1680
	21st to end	Night	36% 10% 14% 26% 14% 2	1680
July	1st to 10th	Night	44% 8% 13% 20% 14% 1	4640
	11th to 20th	Night	41% 8% 14% 24% 12% 1	4632
	21st to end	Night	37% 7% 13% 24% 18% 1	4967
August	1st to 10th	Night	50% 9% 13% 17% 10% 1	2837
	11th to 20th	Night	43% 7% 13% 28% 8% 1	3327
	21st to end	Night	30% 7% 13% 33% 17% 1	3677
September	1st to 10th	Night	39% 6% 9% 26% 19% 1	2000
	11th to 20th	Night	33% 5% 10% 24% 25% 3	2100
	21st to end	Night	42% 7% 7% 22% 20% 2	2200
October	1st to 10th	Night	39% 6% 11% 23% 20% 2	3740
	11th to 20th	Night	26% 4 10% 28% 29% 3	3899
	21st to end	Night	31% 5% 13% 29% 19% 3	4563
November	1st to 10th	Night	34% 5% 12% 24% 21% 4	2667
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2730
	21st to end	Night	38% 4 13% 30% 13% 1	2804
December	1st to 10th	Night	40% 3 12% 31% 13% 1	4720
	11th to 20th	Night	46% 3 11% 20% 17% 3	5348
	21st to end	Night	45% 4 10% 19% 17% 4	7150

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	28% 31% 28% 10% 2	220
	11th to 20th	Day	32% 29% 29% 9% 1	220
	21st to end	Day	24% 31% 31% 10% 3	305
February	1st to 10th	Day	29% 29% 32% 9% 1	354
	11th to 20th	Day	29% 28% 34% 9% 1	536
	21st to end	Day	31% 25% 31% 11% 2	446
March	1st to 10th	Day	29% 23% 32% 13% 3	540
	11th to 20th	Day	25% 27% 32% 13% 2 1	540
	21st to end	Day	32% 26% 29% 10% 3 1	594
April	1st to 10th	Day	33% 25% 24% 12% 4 1	540
	11th to 20th	Day	26% 27% 29% 11% 7% 1	532
	21st to end	Day	28% 20% 34% 14% 4 1	540
May	1st to 10th	Day	32% 21% 29% 12% 4 1	543
	11th to 20th	Day	22% 18% 36% 19% 4 1	567
	21st to end	Day	22% 15% 28% 24% 8% 2	617
June	1st to 10th	Day	19% 12% 32% 26% 9% 2	570
	11th to 20th	Day	19% 15% 33% 24% 8% 1	570
	21st to end	Day	15% 15% 31% 26% 12% 1	570
July	1st to 10th	Day	15% 7% 31% 29% 14% 3	565
	11th to 20th	Day	10% 8% 32% 31% 16% 2	564
	21st to end	Day	13% 10% 32% 31% 12% 1	620
August	1st to 10th	Day	15% 8% 28% 29% 14% 5%	566
	11th to 20th	Day	15% 6% 30% 31% 14% 4	559
	21st to end	Day	11% 12% 30% 28% 13% 6%	619
September	1st to 10th	Day	18% 12% 28% 22% 15% 4	530
	11th to 20th	Day	14% 8% 31% 26% 11% 9%	530
	21st to end	Day	21% 11% 32% 21% 11% 4	530
October	1st to 10th	Day	25% 12% 25% 20% 13% 4	530
	11th to 20th	Day	20% 18% 33% 19% 9% 2	530
	21st to end	Day	26% 17% 30% 19% 8% 1	481
November	1st to 10th	Day	16% 20% 32% 21% 8% 2	425
	11th to 20th	Day	26% 21% 33% 15% 5%	430
	21st to end	Day	26% 24% 28% 17% 4 1	353
December	1st to 10th	Day	25% 27% 30% 13% 5%	220
	11th to 20th	Day	36% 25% 29% 8% 3	220
	21st to end	Day	29% 21% 34% 14% 3	242
January	1st to 10th	Night	22% 23% 35% 17% 3	840
	11th to 20th	Night	25% 23% 32% 18% 2	840
	21st to end	Night	21% 23% 37% 17% 2	924
February	1st to 10th	Night	23% 20% 36% 19% 2	840
	11th to 20th	Night	26% 24% 34% 14% 1	823
	21st to end	Night	25% 22% 34% 16% 3	692
March	1st to 10th	Night	24% 22% 33% 18% 3	840
	11th to 20th	Night	23% 21% 36% 17% 3	840
	21st to end	Night	23% 23% 35% 15% 4	924
April	1st to 10th	Night	25% 21% 33% 16% 4	840
	11th to 20th	Night	23% 18% 35% 19% 4	831
	21st to end	Night	24% 17% 34% 19% 4	782
May	1st to 10th	Night	26% 17% 33% 19% 6%	630
	11th to 20th	Night	24% 15% 31% 25% 5%	467
	21st to end	Night	22% 10% 25% 34% 8% 1	455
June	1st to 10th	Night	18% 8% 30% 34% 10%	420
	11th to 20th	Night	15% 8% 30% 36% 11%	420
	21st to end	Night	13% 4 29% 41% 12%	420
July	1st to 10th	Night	14% 6% 23% 39% 17% 1	410
	11th to 20th	Night	12% 5% 26% 40% 16% 1	408
	21st to end	Night	12% 5% 23% 42% 17% 1	460
August	1st to 10th	Night	14% 4 23% 40% 17% 1	521
	11th to 20th	Night	13% 6% 23% 36% 20% 1	830
	21st to end	Night	16% 7% 22% 34% 18% 3	917
September	1st to 10th	Night	18% 6% 28% 32% 15% 2	800
	11th to 20th	Night	15% 6% 25% 32% 18% 4	800
	21st to end	Night	22% 6% 26% 31% 14% 2	800
October	1st to 10th	Night	22% 12% 28% 23% 13% 2	800
	11th to 20th	Night	20% 14% 31% 25% 10%	800
	21st to end	Night	25% 16% 32% 20% 7%	880
November	1st to 10th	Night	16% 15% 36% 24% 9% 1	820
	11th to 20th	Night	24% 18% 33% 19% 6% 1	840
	21st to end	Night	21% 20% 31% 23% 6% 1	840
December	1st to 10th	Night	26% 19% 33% 17% 5% 1	840
	11th to 20th	Night	25% 20% 32% 20% 3	840
	21st to end	Night	26% 20% 34% 17% 3	924

Page generated at 2026-08-27 04:07:10. Version ac07

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