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 2024-11-11.

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 9% 5% 5% 4 4 5% 7% 29%	101529
	Night	17% 13% 5% 10% 5% 4 4 3 5% 6% 29%	97658
Transparency	Day	54% 4 12% 20% 9% 1	105038
	Night	56% 4 11% 19% 9% 1	102238
Seeing	Day	28% 16% 28% 17% 8% 3	14793
	Night	26% 16% 25% 23% 9% 1	23935

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% 18% 13% 13% 5% 4 4 3 3 4 21%	2517
	11th to 20th	Day	14% 18% 9% 15% 9% 5% 4 3 4 4 17%	2520
	21st to end	Day	18% 20% 10% 10% 9% 5% 4 3 3 4 14%	3004
February	1st to 10th	Day	16% 18% 5% 14% 9% 5% 4 3 5% 3 18%	1710
	11th to 20th	Day	11% 15% 6% 17% 7% 6% 4 3 4 5% 21%	1761
	21st to end	Day	18% 16% 4 13% 7% 4 4 3 3 4 23%	1570
March	1st to 10th	Day	26% 15% 4 10% 5% 3 3 2 3 4 25%	3116
	11th to 20th	Day	12% 21% 7% 11% 5% 4 3 3 4 5% 27%	3438
	21st to end	Day	20% 13% 5% 10% 4 3 3 3 3 4 31%	4065
April	1st to 10th	Day	17% 16% 4 9% 5% 4 3 3 4 5% 30%	2280
	11th to 20th	Day	17% 12% 3 7% 5% 4 4 3 5% 5% 33%	2254
	21st to end	Day	20% 14% 3 7% 4 4 4 3 5% 5% 31%	2289
May	1st to 10th	Day	26% 10% 3 7% 4 3 3 3 4 6% 30%	4318
	11th to 20th	Day	18% 9% 3 8% 6% 5% 5% 5% 6% 8% 27%	4398
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 7% 9% 38%	4837
June	1st to 10th	Day	19% 7% 2 7% 5% 5% 5% 4 7% 7% 30%	2660
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 7% 8% 30%	2660
	21st to end	Day	18% 7% 2 8% 6% 5% 5% 5% 6% 9% 31%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 6% 5% 5% 8% 10% 33%	4440
	11th to 20th	Day	10% 6% 3 7% 6% 7% 6% 6% 8% 11% 31%	4440
	21st to end	Day	13% 7% 3 8% 6% 6% 6% 5% 7% 8% 32%	4814
August	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 8% 28%	4238
	11th to 20th	Day	14% 9% 4 8% 5% 6% 6% 5% 8% 10% 26%	3821
	21st to end	Day	16% 9% 2 6% 4 5% 4 4 7% 9% 34%	4180
September	1st to 10th	Day	16% 8% 2 6% 4 5% 5% 4 5% 8% 38%	2280
	11th to 20th	Day	16% 8% 2 6% 4 3 3 3 5% 7% 44%	2247
	21st to end	Day	18% 7% 3 6% 5% 4 5% 4 5% 7% 35%	2090
October	1st to 10th	Day	20% 9% 3 9% 5% 4 4 3 5% 7% 31%	1900
	11th to 20th	Day	18% 11% 3 7% 4 5% 4 3 5% 6% 34%	1900
	21st to end	Day	23% 15% 3 6% 4 4 4 3 5% 6% 25%	1959
November	1st to 10th	Day	14% 14% 3 6% 4 3 4 3 6% 7% 35%	1761
	11th to 20th	Day	17% 17% 5% 10% 5% 4 4 4 4 4 26%	1684
	21st to end	Day	16% 17% 4 10% 7% 4 5% 4 5% 5% 23%	1520
December	1st to 10th	Day	18% 21% 8% 12% 6% 4 4 3 4 5% 16%	1520
	11th to 20th	Day	20% 18% 9% 12% 6% 3 3 2 3 4 21%	1906
	21st to end	Day	22% 21% 9% 10% 5% 4 3 3 4 4 15%	2772
January	1st to 10th	Night	15% 16% 9% 15% 6% 4 5% 3 4 5% 18%	4429
	11th to 20th	Night	12% 19% 8% 17% 9% 5% 3 3 3 4 18%	4440
	21st to end	Night	21% 18% 6% 15% 8% 5% 4 3 3 3 13%	4650
February	1st to 10th	Night	14% 17% 5% 15% 8% 5% 4 3 4 4 20%	2470
	11th to 20th	Night	12% 14% 6% 16% 10% 6% 5% 4 4 5% 18%	2304
	21st to end	Night	18% 13% 5% 12% 8% 4 4 3 4 6% 23%	1884
March	1st to 10th	Night	21% 14% 5% 11% 5% 4 4 3 4 5% 24%	3724
	11th to 20th	Night	13% 21% 6% 11% 6% 4 3 3 3 4 27%	3519
	21st to end	Night	16% 15% 5% 12% 5% 4 4 3 4 4 30%	3567
April	1st to 10th	Night	19% 16% 5% 7% 5% 4 4 3 4 6% 28%	1900
	11th to 20th	Night	19% 13% 3 6% 4 3 3 2 5% 7% 34%	1875
	21st to end	Night	21% 12% 4 7% 4 3 3 3 5% 5% 35%	1889
May	1st to 10th	Night	27% 11% 3 5% 3 3 3 3 4 7% 30%	2652
	11th to 20th	Night	17% 11% 3 8% 5% 4 5% 4 4 7% 32%	2495
	21st to end	Night	14% 9% 3 7% 4 5% 4 4 6% 7% 37%	2738
June	1st to 10th	Night	17% 7% 3 7% 5% 3 3 4 5% 8% 39%	1520
	11th to 20th	Night	17% 6% 3 5% 4 4 4 4 5% 9% 38%	1520
	21st to end	Night	12% 6% 2 6% 4 6% 5% 3 8% 8% 40%	1520
July	1st to 10th	Night	12% 6% 3 8% 6% 5% 4 4 7% 8% 37%	2520
	11th to 20th	Night	12% 7% 2 6% 4 5% 5% 4 5% 9% 40%	2520
	21st to end	Night	14% 7% 3 6% 4 4 5% 4 6% 7% 40%	2739
August	1st to 10th	Night	22% 9% 3 6% 3 4 4 3 4 7% 34%	2687
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 3 4 6% 38%	2974
	21st to end	Night	14% 8% 3 6% 4 3 3 3 5% 8% 42%	3476
September	1st to 10th	Night	16% 6% 2 6% 4 4 4 4 5% 7% 43%	1900
	11th to 20th	Night	16% 6% 2 4 4 3 3 3 5% 7% 48%	1933
	21st to end	Night	18% 9% 2 7% 4 4 4 3 5% 6% 39%	2090
October	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 5% 6% 34%	2277
	11th to 20th	Night	19% 11% 3 7% 5% 4 5% 3 5% 5% 34%	2280
	21st to end	Night	24% 15% 3 7% 4 4 3 3 4 7% 25%	2637
November	1st to 10th	Night	13% 14% 4 8% 4 4 4 3 5% 6% 35%	2537
	11th to 20th	Night	18% 15% 5% 9% 4 4 4 4 5% 6% 28%	2500
	21st to end	Night	14% 15% 6% 10% 6% 5% 5% 4 5% 6% 26%	2660
December	1st to 10th	Night	18% 20% 7% 12% 6% 4 4 3 4 4 18%	2660
	11th to 20th	Night	20% 16% 6% 13% 6% 4 4 3 4 5% 20%	3288
	21st to end	Night	24% 18% 9% 12% 5% 3 4 3 4 4 16%	4884

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	69% 3 9% 14% 4 1	2518
	11th to 20th	Day	73% 3 6% 13% 4 1	2520
	21st to end	Day	74% 3 8% 10% 4 1	3004
February	1st to 10th	Day	71% 4 8% 12% 5%	1710
	11th to 20th	Day	65% 5% 6% 16% 8% 1	1770
	21st to end	Day	65% 4 7% 16% 8%	1570
March	1st to 10th	Day	65% 3 10% 14% 8% 1	3119
	11th to 20th	Day	62% 2 13% 14% 8% 1	3438
	21st to end	Day	58% 3 7% 15% 16% 2	4065
April	1st to 10th	Day	58% 3 7% 21% 9% 2	2280
	11th to 20th	Day	53% 4 11% 19% 12% 2	2254
	21st to end	Day	55% 3 9% 19% 11% 2	2289
May	1st to 10th	Day	56% 3 10% 19% 11% 1	4332
	11th to 20th	Day	55% 4 13% 21% 7% 1	4402
	21st to end	Day	42% 6% 13% 26% 12% 1	4837
June	1st to 10th	Day	50% 5% 14% 19% 11% 1	2660
	11th to 20th	Day	50% 4 14% 22% 10% 1	2660
	21st to end	Day	50% 5% 14% 22% 9%	2660
July	1st to 10th	Day	44% 6% 16% 24% 10% 1	4370
	11th to 20th	Day	43% 6% 17% 24% 10%	4365
	21st to end	Day	48% 5% 16% 22% 9%	4661
August	1st to 10th	Day	54% 4 14% 19% 9%	4224
	11th to 20th	Day	51% 6% 17% 22% 5%	3860
	21st to end	Day	46% 5% 12% 29% 7% 1	4180
September	1st to 10th	Day	45% 5% 11% 27% 11% 1	2280
	11th to 20th	Day	42% 3 11% 24% 20% 1	2247
	21st to end	Day	48% 5% 11% 22% 13% 1	2090
October	1st to 10th	Day	51% 4 7% 24% 13% 1	3160
	11th to 20th	Day	46% 3 11% 22% 16% 3	3140
	21st to end	Day	45% 5% 14% 25% 10% 1	3210
November	1st to 10th	Day	48% 6% 13% 20% 12% 1	1761
	11th to 20th	Day	61% 3 10% 15% 9% 2	1684
	21st to end	Day	63% 3 13% 16% 5% 1	1520
December	1st to 10th	Day	72% 3 10% 10% 4	1520
	11th to 20th	Day	69% 2 11% 13% 4	1906
	21st to end	Day	73% 3 8% 10% 6%	2772
January	1st to 10th	Night	67% 5% 12% 10% 6%	4429
	11th to 20th	Night	72% 3 8% 12% 5% 1	4440
	21st to end	Night	76% 3 6% 10% 4 1	4650
February	1st to 10th	Night	67% 4 9% 13% 7%	2470
	11th to 20th	Night	68% 5% 8% 13% 6%	2306
	21st to end	Night	62% 3 10% 15% 8% 1	1884
March	1st to 10th	Night	64% 3 9% 16% 8% 1	3734
	11th to 20th	Night	63% 2 11% 17% 6% 1	3520
	21st to end	Night	58% 4 6% 16% 15% 2	3567
April	1st to 10th	Night	58% 4 11% 18% 8% 2	1900
	11th to 20th	Night	52% 3 11% 19% 14%	1875
	21st to end	Night	53% 3 10% 20% 11% 3	1889
May	1st to 10th	Night	55% 3 11% 18% 10% 2	2658
	11th to 20th	Night	52% 6% 14% 20% 7% 1	2496
	21st to end	Night	46% 4 15% 20% 14% 1	2739
June	1st to 10th	Night	45% 3 12% 26% 13%	1520
	11th to 20th	Night	44% 5% 16% 25% 10% 1	1520
	21st to end	Night	39% 6% 18% 26% 11% 1	1520
July	1st to 10th	Night	45% 5% 15% 21% 14% 1	2480
	11th to 20th	Night	42% 5% 16% 26% 12%	2472
	21st to end	Night	44% 4 13% 26% 12% 1	2651
August	1st to 10th	Night	50% 5% 16% 22% 7%	2677
	11th to 20th	Night	48% 4 16% 23% 8%	3001
	21st to end	Night	41% 4 13% 33% 9%	3476
September	1st to 10th	Night	41% 6% 13% 27% 13% 1	1900
	11th to 20th	Night	37% 4 11% 28% 20% 1	1933
	21st to end	Night	48% 3 11% 23% 14% 2	2090
October	1st to 10th	Night	55% 4 11% 21% 9% 1	3797
	11th to 20th	Night	49% 4 12% 17% 14% 4	3779
	21st to end	Night	48% 4 14% 26% 8%	4336
November	1st to 10th	Night	50% 5% 10% 20% 14% 2	2537
	11th to 20th	Night	58% 4 10% 17% 9% 2	2500
	21st to end	Night	58% 4 15% 17% 6%	2660
December	1st to 10th	Night	70% 4 11% 12% 3	2660
	11th to 20th	Night	67% 5% 10% 13% 5%	3288
	21st to end	Night	73% 3 9% 12% 4	4884

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	31% 24% 29% 12% 3 1	180
	11th to 20th	Day	46% 22% 23% 6% 2 1	180
	21st to end	Day	35% 24% 28% 10% 3	265
February	1st to 10th	Day	40% 29% 26% 5% 1	270
	11th to 20th	Day	39% 28% 25% 6% 1	368
	21st to end	Day	39% 21% 28% 10% 2	382
March	1st to 10th	Day	34% 22% 30% 10% 3	460
	11th to 20th	Day	38% 22% 25% 12% 3	460
	21st to end	Day	39% 22% 21% 11% 4 2	506
April	1st to 10th	Day	38% 20% 24% 13% 5%	460
	11th to 20th	Day	33% 21% 26% 11% 6% 3	452
	21st to end	Day	35% 17% 28% 12% 6% 2	460
May	1st to 10th	Day	39% 14% 24% 13% 6% 4	463
	11th to 20th	Day	23% 17% 33% 18% 6% 3	487
	21st to end	Day	19% 12% 33% 21% 11% 3	529
June	1st to 10th	Day	26% 16% 26% 21% 9% 2	490
	11th to 20th	Day	21% 13% 29% 26% 9% 2	490
	21st to end	Day	24% 12% 33% 22% 7% 2	490
July	1st to 10th	Day	17% 9% 28% 26% 15% 5%	485
	11th to 20th	Day	12% 8% 33% 29% 14% 4	484
	21st to end	Day	18% 8% 36% 25% 10% 3	532
August	1st to 10th	Day	17% 9% 26% 26% 15% 6%	486
	11th to 20th	Day	19% 7% 29% 25% 13% 7%	479
	21st to end	Day	18% 10% 29% 23% 14% 7%	539
September	1st to 10th	Day	23% 10% 23% 21% 16% 7%	490
	11th to 20th	Day	17% 7% 25% 21% 17% 12%	490
	21st to end	Day	21% 13% 30% 20% 12% 5%	490
October	1st to 10th	Day	28% 12% 25% 17% 12% 6%	490
	11th to 20th	Day	28% 16% 28% 18% 8% 2	490
	21st to end	Day	34% 16% 26% 18% 5%	469
November	1st to 10th	Day	31% 19% 22% 19% 8% 1	395
	11th to 20th	Day	35% 23% 21% 16% 4 1	344
	21st to end	Day	35% 22% 33% 8% 1	180
December	1st to 10th	Day	38% 22% 26% 9% 4 1	180
	11th to 20th	Day	39% 20% 29% 9% 2	180
	21st to end	Day	38% 24% 29% 7% 2	198
January	1st to 10th	Night	28% 26% 31% 12% 3	760
	11th to 20th	Night	31% 21% 28% 16% 3 1	760
	21st to end	Night	26% 26% 28% 17% 3	836
February	1st to 10th	Night	28% 23% 33% 15% 2	760
	11th to 20th	Night	29% 26% 31% 12% 2	743
	21st to end	Night	36% 21% 26% 14% 2	628
March	1st to 10th	Night	29% 21% 30% 17% 3	760
	11th to 20th	Night	32% 21% 27% 17% 3	760
	21st to end	Night	28% 21% 24% 19% 7% 1	836
April	1st to 10th	Night	28% 19% 29% 21% 3	760
	11th to 20th	Night	27% 19% 26% 20% 7% 1	751
	21st to end	Night	30% 15% 25% 21% 7% 1	749
May	1st to 10th	Night	31% 12% 20% 27% 9%	466
	11th to 20th	Night	20% 11% 32% 30% 6% 1	374
	21st to end	Night	16% 6% 27% 34% 16% 1	411
June	1st to 10th	Night	20% 9% 20% 38% 13% 1	380
	11th to 20th	Night	19% 7% 20% 37% 17%	380
	21st to end	Night	14% 6% 26% 39% 15% 1	380
July	1st to 10th	Night	14% 3 20% 43% 18% 2	370
	11th to 20th	Night	13% 3 20% 40% 22% 1	368
	21st to end	Night	16% 4 17% 45% 17% 2	416
August	1st to 10th	Night	15% 4 22% 36% 22% 1	481
	11th to 20th	Night	17% 5% 15% 37% 23% 4	675
	21st to end	Night	20% 6% 15% 35% 23% 2	836
September	1st to 10th	Night	18% 7% 23% 31% 19% 3	760
	11th to 20th	Night	19% 6% 19% 27% 24% 5%	760
	21st to end	Night	23% 9% 22% 28% 16% 3	760
October	1st to 10th	Night	27% 11% 20% 23% 16% 3	760
	11th to 20th	Night	26% 15% 24% 26% 8% 1	760
	21st to end	Night	34% 15% 23% 23% 4	836
November	1st to 10th	Night	25% 16% 28% 23% 8%	780
	11th to 20th	Night	30% 21% 24% 17% 6% 2	763
	21st to end	Night	27% 21% 28% 18% 5% 1	760
December	1st to 10th	Night	33% 25% 24% 14% 3	760
	11th to 20th	Night	32% 22% 27% 16% 2	760
	21st to end	Night	33% 26% 26% 13% 2	836

Page generated at 2024-11-08 06:44:45. Version ac07

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