Ernest B Wright Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ernest B Wright Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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% 12% 4 10% 5% 5% 4 4 5% 6% 28%	127426
	Night	17% 13% 5% 11% 5% 4 4 3 4 6% 28%	123625
Transparency	Day	56% 4 11% 19% 10% 1	130917
	Night	58% 4 11% 17% 9% 1	128221
Seeing	Day	29% 16% 27% 17% 8% 3	16156
	Night	26% 16% 25% 23% 9% 1	25342

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	12% 15% 12% 25% 6% 3 3 2 2 4 16%	3597
	11th to 20th	Day	12% 17% 9% 21% 10% 4 3 3 4 3 13%	3570
	21st to end	Day	15% 17% 9% 12% 8% 4 4 3 3 5% 21%	4304
February	1st to 10th	Day	16% 18% 6% 15% 10% 5% 4 3 5% 3 18%	1800
	11th to 20th	Day	11% 16% 6% 18% 8% 6% 4 3 4 5% 20%	1857
	21st to end	Day	18% 16% 4 13% 7% 4 4 3 4 4 23%	1650
March	1st to 10th	Day	21% 15% 7% 11% 6% 4 3 3 3 4 22%	4476
	11th to 20th	Day	13% 17% 5% 9% 4 3 3 3 3 5% 34%	4924
	21st to end	Day	18% 12% 4 11% 5% 4 4 3 4 5% 29%	5827
April	1st to 10th	Day	18% 16% 4 9% 5% 4 3 3 4 5% 29%	2400
	11th to 20th	Day	17% 13% 3 7% 5% 4 4 3 5% 5% 33%	2374
	21st to end	Day	20% 14% 3 7% 4 4 4 3 5% 5% 32%	2410
May	1st to 10th	Day	30% 11% 3 8% 4 4 4 3 4 6% 24%	6196
	11th to 20th	Day	18% 11% 3 8% 5% 5% 5% 4 6% 7% 28%	6318
	21st to end	Day	21% 10% 3 7% 4 4 4 4 6% 8% 28%	6949
June	1st to 10th	Day	19% 7% 2 7% 5% 5% 5% 4 7% 7% 31%	2800
	11th to 20th	Day	18% 8% 2 7% 5% 5% 5% 4 7% 8% 30%	2800
	21st to end	Day	18% 7% 2 8% 5% 5% 5% 4 6% 8% 31%	2800
July	1st to 10th	Day	10% 7% 3 8% 5% 6% 5% 5% 8% 9% 35%	6360
	11th to 20th	Day	9% 6% 3 8% 6% 7% 6% 6% 8% 10% 31%	6360
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 6% 7% 39%	6926
August	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 8% 29%	4372
	11th to 20th	Day	14% 9% 4 8% 5% 6% 6% 5% 8% 10% 27%	3944
	21st to end	Day	15% 9% 2 6% 4 5% 4 4 7% 9% 34%	4312
September	1st to 10th	Day	15% 7% 2 6% 4 5% 5% 4 5% 8% 39%	2400
	11th to 20th	Day	15% 7% 2 6% 3 3 3 3 5% 7% 45%	2365
	21st to end	Day	18% 7% 3 7% 5% 4 5% 5% 6% 7% 35%	2200
October	1st to 10th	Day	19% 9% 3 9% 5% 4 4 3 5% 7% 33%	2000
	11th to 20th	Day	18% 11% 3 7% 4 5% 4 3 5% 7% 34%	2000
	21st to end	Day	23% 15% 3 6% 5% 4 4 4 5% 6% 26%	2062
November	1st to 10th	Day	14% 14% 3 6% 5% 3 4 3 6% 7% 35%	1851
	11th to 20th	Day	16% 18% 5% 10% 5% 4 4 4 4 4 27%	1860
	21st to end	Day	16% 19% 5% 10% 6% 5% 5% 3 4 4 22%	1680
December	1st to 10th	Day	15% 20% 10% 20% 6% 4 4 3 4 4 12%	2680
	11th to 20th	Day	17% 17% 9% 14% 6% 4 3 2 3 4 22%	3042
	21st to end	Day	17% 19% 9% 11% 5% 3 3 3 5% 5% 19%	3960
January	1st to 10th	Night	13% 14% 10% 27% 7% 3 4 2 3 4 13%	6349
	11th to 20th	Night	10% 18% 9% 23% 8% 5% 4 3 3 4 15%	6315
	21st to end	Night	15% 15% 6% 15% 8% 6% 5% 4 4 5% 18%	6650
February	1st to 10th	Night	14% 17% 6% 15% 8% 5% 4 3 4 4 20%	2600
	11th to 20th	Night	12% 14% 6% 16% 10% 6% 5% 4 4 5% 18%	2428
	21st to end	Night	18% 14% 5% 12% 8% 4 4 3 4 6% 22%	1980
March	1st to 10th	Night	17% 13% 6% 13% 8% 4 4 3 4 5% 23%	5364
	11th to 20th	Night	15% 17% 6% 9% 5% 3 3 2 3 5% 32%	5033
	21st to end	Night	19% 14% 5% 12% 5% 4 3 3 4 4 27%	5105
April	1st to 10th	Night	19% 16% 5% 7% 5% 5% 4 3 4 6% 27%	2000
	11th to 20th	Night	19% 14% 3 6% 4 3 3 2 5% 7% 34%	1975
	21st to end	Night	21% 12% 4 6% 4 3 3 3 5% 5% 35%	1988
May	1st to 10th	Night	32% 13% 3 6% 3 3 3 3 4 5% 24%	3788
	11th to 20th	Night	18% 11% 3 8% 5% 4 4 4 4 6% 32%	3575
	21st to end	Night	23% 12% 3 7% 4 4 3 3 5% 6% 29%	3926
June	1st to 10th	Night	17% 7% 3 8% 5% 3 3 4 5% 7% 38%	1600
	11th to 20th	Night	18% 7% 3 6% 4 4 4 4 5% 9% 37%	1600
	21st to end	Night	12% 6% 2 6% 4 6% 5% 3 8% 8% 40%	1600
July	1st to 10th	Night	10% 7% 3 8% 6% 5% 4 4 7% 9% 38%	3600
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 4 5% 8% 37%	3600
	21st to end	Night	12% 7% 3 6% 4 4 4 4 5% 7% 45%	3926
August	1st to 10th	Night	22% 9% 3 6% 3 4 4 3 5% 7% 35%	2773
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 3 4 6% 38%	3070
	21st to end	Night	14% 8% 2 6% 4 3 3 3 5% 8% 42%	3586
September	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	2000
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 5% 7% 49%	2035
	21st to end	Night	18% 9% 2 7% 4 4 4 3 6% 6% 38%	2200
October	1st to 10th	Night	20% 8% 3 7% 4 4 4 3 5% 6% 36%	2397
	11th to 20th	Night	19% 11% 2 7% 5% 4 5% 3 5% 5% 34%	2400
	21st to end	Night	23% 15% 3 8% 4 4 3 3 5% 7% 25%	2776
November	1st to 10th	Night	13% 14% 4 8% 4 4 4 3 5% 6% 35%	2667
	11th to 20th	Night	17% 16% 5% 9% 4 4 4 3 4 5% 28%	2756
	21st to end	Night	14% 17% 6% 10% 6% 4 5% 4 5% 6% 25%	2940
December	1st to 10th	Night	14% 19% 8% 16% 6% 4 4 3 5% 5% 15%	4720
	11th to 20th	Night	21% 17% 7% 12% 5% 3 3 2 3 4 24%	5307
	21st to end	Night	19% 19% 8% 12% 6% 4 3 2 3 4 19%	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	75% 2 6% 12% 3 1	3598
	11th to 20th	Day	77% 3 5% 10% 4 1	3600
	21st to end	Day	66% 3 8% 16% 6% 1	4304
February	1st to 10th	Day	71% 4 8% 12% 5%	1800
	11th to 20th	Day	66% 5% 6% 15% 7% 1	1866
	21st to end	Day	64% 4 8% 16% 8%	1650
March	1st to 10th	Day	67% 3 9% 12% 8% 1	4479
	11th to 20th	Day	55% 2 16% 17% 7% 3	4924
	21st to end	Day	57% 4 7% 13% 17% 2	5827
April	1st to 10th	Day	59% 3 7% 21% 9% 2	2400
	11th to 20th	Day	54% 4 11% 18% 12% 1	2374
	21st to end	Day	55% 3 9% 20% 11% 2	2410
May	1st to 10th	Day	64% 3 9% 14% 9% 1	6207
	11th to 20th	Day	55% 4 11% 20% 9% 1	6322
	21st to end	Day	54% 5% 11% 20% 9% 1	6949
June	1st to 10th	Day	50% 5% 13% 20% 12% 1	2800
	11th to 20th	Day	50% 4 14% 22% 9% 1	2800
	21st to end	Day	50% 5% 14% 22% 10% 1	2800
July	1st to 10th	Day	45% 6% 15% 24% 10%	6290
	11th to 20th	Day	44% 6% 15% 24% 10%	6285
	21st to end	Day	43% 4 12% 28% 12%	6728
August	1st to 10th	Day	53% 4 14% 19% 10% 1	4358
	11th to 20th	Day	50% 6% 16% 22% 5%	3983
	21st to end	Day	45% 5% 12% 30% 7% 1	4312
September	1st to 10th	Day	44% 5% 11% 28% 12% 1	2400
	11th to 20th	Day	40% 3 10% 25% 20% 1	2365
	21st to end	Day	48% 5% 11% 22% 13% 2	2200
October	1st to 10th	Day	50% 4 7% 24% 13% 1	3260
	11th to 20th	Day	46% 3 11% 21% 15% 3	3240
	21st to end	Day	45% 5% 14% 25% 11% 1	3313
November	1st to 10th	Day	49% 6% 12% 20% 12% 1	1851
	11th to 20th	Day	61% 3 9% 16% 9% 2	1860
	21st to end	Day	65% 3 12% 15% 5% 1	1680
December	1st to 10th	Day	77% 3 8% 10% 3	2680
	11th to 20th	Day	68% 2 9% 12% 8%	3042
	21st to end	Day	67% 3 8% 11% 9% 1	3960
January	1st to 10th	Night	76% 3 9% 8% 4	6349
	11th to 20th	Night	75% 2 8% 9% 5% 1	6360
	21st to end	Night	69% 3 7% 9% 10% 1	6650
February	1st to 10th	Night	67% 4 9% 13% 6%	2600
	11th to 20th	Night	68% 5% 8% 13% 6%	2430
	21st to end	Night	63% 3 10% 15% 8% 1	1980
March	1st to 10th	Night	65% 3 11% 13% 8%	5374
	11th to 20th	Night	57% 2 17% 14% 6% 3	5034
	21st to end	Night	60% 4 5% 15% 14% 1	5105
April	1st to 10th	Night	59% 4 10% 18% 8% 1	2000
	11th to 20th	Night	53% 3 11% 19% 14%	1975
	21st to end	Night	52% 4 10% 20% 12% 3	1988
May	1st to 10th	Night	64% 3 9% 13% 9% 2	3794
	11th to 20th	Night	54% 6% 13% 19% 8% 1	3576
	21st to end	Night	57% 3 11% 17% 10% 1	3927
June	1st to 10th	Night	46% 3 12% 25% 14%	1600
	11th to 20th	Night	45% 5% 15% 24% 10% 1	1600
	21st to end	Night	39% 6% 18% 25% 11% 1	1600
July	1st to 10th	Night	44% 4 16% 21% 14% 1	3560
	11th to 20th	Night	45% 5% 14% 23% 12% 1	3552
	21st to end	Night	39% 4 14% 29% 13% 1	3809
August	1st to 10th	Night	50% 4 15% 22% 8% 1	2763
	11th to 20th	Night	48% 4 16% 24% 8%	3097
	21st to end	Night	41% 4 13% 33% 10%	3586
September	1st to 10th	Night	41% 5% 13% 27% 13% 1	2000
	11th to 20th	Night	36% 4 11% 29% 19% 1	2035
	21st to end	Night	48% 3 11% 23% 13% 2	2200
October	1st to 10th	Night	54% 4 11% 21% 10% 1	3917
	11th to 20th	Night	49% 3 12% 17% 14% 4	3899
	21st to end	Night	48% 4 14% 25% 8%	4475
November	1st to 10th	Night	51% 4 10% 20% 14% 2	2667
	11th to 20th	Night	58% 4 10% 17% 9% 2	2756
	21st to end	Night	60% 4 14% 17% 5%	2940
December	1st to 10th	Night	71% 4 9% 14% 2	4720
	11th to 20th	Night	66% 3 8% 13% 9%	5307
	21st to end	Night	71% 3 7% 9% 8% 2	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	33% 25% 28% 11% 3 1	200
	11th to 20th	Day	47% 22% 23% 7% 2 1	200
	21st to end	Day	38% 24% 26% 9% 3	295
February	1st to 10th	Day	42% 28% 24% 5% 1	300
	11th to 20th	Day	42% 27% 24% 6% 1	404
	21st to end	Day	39% 21% 28% 9% 2	414
March	1st to 10th	Day	34% 23% 30% 9% 3	500
	11th to 20th	Day	38% 22% 25% 12% 3	500
	21st to end	Day	39% 22% 22% 11% 4 2	550
April	1st to 10th	Day	40% 20% 23% 12% 5%	500
	11th to 20th	Day	35% 21% 25% 11% 5% 3	492
	21st to end	Day	35% 18% 28% 13% 5% 2	500
May	1st to 10th	Day	40% 14% 23% 13% 6% 3	503
	11th to 20th	Day	24% 17% 32% 18% 6% 3	527
	21st to end	Day	22% 12% 33% 20% 10% 3	573
June	1st to 10th	Day	26% 15% 26% 22% 9% 2	530
	11th to 20th	Day	22% 13% 29% 26% 8% 2	530
	21st to end	Day	24% 12% 33% 22% 7% 2	530
July	1st to 10th	Day	17% 10% 29% 26% 14% 4	525
	11th to 20th	Day	12% 8% 33% 28% 14% 4	524
	21st to end	Day	18% 8% 36% 25% 10% 3	576
August	1st to 10th	Day	16% 9% 26% 27% 17% 6%	526
	11th to 20th	Day	19% 6% 29% 26% 13% 7%	519
	21st to end	Day	17% 10% 30% 22% 14% 6%	583
September	1st to 10th	Day	22% 10% 24% 22% 16% 7%	530
	11th to 20th	Day	16% 7% 25% 21% 18% 12%	530
	21st to end	Day	21% 12% 29% 21% 12% 5%	530
October	1st to 10th	Day	27% 12% 25% 17% 14% 6%	530
	11th to 20th	Day	30% 16% 27% 18% 8% 2	530
	21st to end	Day	34% 17% 26% 18% 5%	506
November	1st to 10th	Day	33% 19% 22% 18% 8% 1	425
	11th to 20th	Day	36% 22% 21% 16% 5% 1	400
	21st to end	Day	37% 25% 30% 7% 1	220
December	1st to 10th	Day	37% 25% 25% 9% 3	220
	11th to 20th	Day	40% 21% 29% 9% 2	214
	21st to end	Day	35% 23% 31% 8% 2	220
January	1st to 10th	Night	28% 28% 30% 12% 2	800
	11th to 20th	Night	31% 22% 29% 16% 3 1	800
	21st to end	Night	26% 26% 28% 16% 3	880
February	1st to 10th	Night	29% 23% 31% 15% 2	800
	11th to 20th	Night	31% 25% 31% 12% 2	783
	21st to end	Night	36% 22% 26% 14% 2	660
March	1st to 10th	Night	29% 22% 30% 16% 3	800
	11th to 20th	Night	31% 21% 27% 17% 3	800
	21st to end	Night	29% 21% 25% 19% 6% 1	880
April	1st to 10th	Night	30% 18% 29% 20% 3	800
	11th to 20th	Night	28% 20% 25% 19% 7% 1	791
	21st to end	Night	30% 15% 26% 21% 7% 1	788
May	1st to 10th	Night	32% 13% 20% 26% 9%	490
	11th to 20th	Night	21% 10% 31% 30% 6% 1	394
	21st to end	Night	18% 6% 27% 33% 15% 1	433
June	1st to 10th	Night	21% 9% 20% 38% 12% 1	400
	11th to 20th	Night	20% 7% 21% 36% 16%	400
	21st to end	Night	14% 6% 26% 39% 16% 1	400
July	1st to 10th	Night	14% 3 20% 44% 18% 2	390
	11th to 20th	Night	14% 3 19% 40% 23% 1	388
	21st to end	Night	15% 4 17% 45% 16% 2	438
August	1st to 10th	Night	14% 4 22% 36% 23% 1	507
	11th to 20th	Night	17% 5% 14% 36% 24% 4	711
	21st to end	Night	20% 6% 16% 34% 22% 2	880
September	1st to 10th	Night	18% 7% 22% 31% 20% 3	800
	11th to 20th	Night	18% 6% 18% 28% 25% 5%	800
	21st to end	Night	22% 9% 22% 28% 16% 3	800
October	1st to 10th	Night	26% 11% 20% 23% 17% 3	800
	11th to 20th	Night	26% 15% 25% 26% 7% 1	800
	21st to end	Night	35% 14% 24% 23% 4	880
November	1st to 10th	Night	26% 16% 28% 23% 8%	820
	11th to 20th	Night	30% 21% 25% 17% 6% 1	840
	21st to end	Night	29% 21% 27% 18% 4	840
December	1st to 10th	Night	33% 25% 24% 15% 3	840
	11th to 20th	Night	33% 22% 27% 16% 2	829
	21st to end	Night	33% 25% 26% 13% 3	880

Page generated at 2025-12-15 01:13:49. Version ac07

Copyright © 2025 Attilla Danko.