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 2024-01-13.

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% 11% 4 8% 6% 3 5% 5% 5% 7% 29%	74743
	Night	17% 12% 5% 8% 6% 3 4 4 5% 6% 30%	74990
Transparency	Day	55% 4 11% 19% 10% 1	74448
	Night	56% 4 11% 18% 9% 1	74819
Seeing	Day	28% 16% 27% 17% 8% 3	13644
	Night	26% 16% 25% 23% 9% 1	22923

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% 15% 8% 12% 7% 3 6% 5% 4 6% 21%	1517
	11th to 20th	Day	16% 17% 6% 14% 10% 3 5% 4 5% 5% 17%	1456
	21st to end	Day	11% 15% 8% 12% 11% 5% 5% 4 4 5% 21%	1718
February	1st to 10th	Day	17% 16% 7% 13% 11% 4 5% 3 5% 3 16%	1620
	11th to 20th	Day	12% 14% 7% 15% 8% 4 5% 4 4 5% 20%	1666
	21st to end	Day	20% 15% 5% 13% 9% 3 4 4 3 4 21%	1480
March	1st to 10th	Day	14% 12% 7% 13% 7% 3 4 4 5% 6% 25%	1797
	11th to 20th	Day	15% 16% 7% 10% 6% 3 4 4 4 5% 27%	1952
	21st to end	Day	20% 13% 5% 7% 6% 3 3 4 4 5% 31%	2317
April	1st to 10th	Day	17% 13% 6% 8% 6% 3 4 3 4 5% 31%	2160
	11th to 20th	Day	17% 11% 4 6% 6% 3 4 4 5% 5% 34%	2134
	21st to end	Day	21% 14% 3 6% 5% 3 4 4 4 5% 31%	2168
May	1st to 10th	Day	21% 12% 3 7% 5% 3 4 5% 4 6% 30%	2454
	11th to 20th	Day	19% 8% 3 5% 6% 4 4 5% 5% 7% 34%	2478
	21st to end	Day	14% 7% 3 6% 5% 4 5% 5% 6% 8% 36%	2725
June	1st to 10th	Day	20% 6% 3 6% 6% 3 6% 5% 6% 8% 31%	2520
	11th to 20th	Day	18% 8% 3 6% 6% 4 6% 5% 6% 9% 30%	2520
	21st to end	Day	18% 6% 2 6% 7% 3 6% 6% 6% 9% 32%	2520
July	1st to 10th	Day	13% 7% 3 7% 6% 4 5% 5% 7% 9% 33%	2520
	11th to 20th	Day	12% 6% 3 5% 6% 4 6% 5% 6% 9% 37%	2520
	21st to end	Day	14% 6% 3 6% 6% 4 6% 6% 7% 9% 33%	2744
August	1st to 10th	Day	13% 6% 3 7% 6% 4 7% 6% 7% 9% 33%	2405
	11th to 20th	Day	14% 8% 3 6% 6% 5% 6% 6% 6% 9% 29%	2178
	21st to end	Day	16% 8% 2 5% 6% 4 6% 4 7% 8% 33%	2376
September	1st to 10th	Day	16% 7% 3 5% 5% 4 5% 6% 4 8% 37%	2160
	11th to 20th	Day	16% 8% 2 5% 4 3 3 4 4 7% 43%	2129
	21st to end	Day	18% 6% 3 5% 5% 3 5% 6% 5% 7% 37%	1980
October	1st to 10th	Day	21% 9% 3 8% 6% 3 4 3 5% 7% 31%	1800
	11th to 20th	Day	18% 11% 3 6% 5% 4 4 4 5% 7% 33%	1800
	21st to end	Day	25% 15% 4 5% 5% 4 5% 4 5% 6% 24%	1856
November	1st to 10th	Day	15% 13% 3 6% 5% 3 4 4 6% 7% 35%	1671
	11th to 20th	Day	17% 17% 5% 9% 6% 3 4 5% 4 4 26%	1684
	21st to end	Day	16% 17% 5% 9% 8% 3 6% 5% 4 5% 23%	1520
December	1st to 10th	Day	19% 18% 10% 12% 7% 3 4 4 4 5% 16%	1520
	11th to 20th	Day	20% 17% 9% 11% 6% 2 3 2 3 4 22%	1906
	21st to end	Day	23% 20% 10% 9% 6% 3 4 3 4 4 15%	2772
January	1st to 10th	Night	13% 15% 8% 12% 7% 3 6% 4 5% 6% 19%	2649
	11th to 20th	Night	16% 16% 7% 12% 10% 4 5% 4 4 5% 18%	2560
	21st to end	Night	11% 14% 8% 12% 11% 6% 6% 5% 4 5% 18%	2636
February	1st to 10th	Night	15% 15% 7% 13% 10% 4 5% 4 4 4 19%	2340
	11th to 20th	Night	12% 13% 6% 14% 11% 5% 6% 5% 4 5% 18%	2179
	21st to end	Night	19% 13% 5% 12% 8% 3 5% 4 4 6% 22%	1776
March	1st to 10th	Night	15% 13% 7% 12% 7% 3 3 4 4 6% 25%	2150
	11th to 20th	Night	15% 14% 6% 10% 8% 3 4 4 4 4 28%	2005
	21st to end	Night	18% 12% 5% 7% 5% 3 4 5% 5% 5% 31%	2039
April	1st to 10th	Night	19% 14% 6% 6% 5% 3 4 4 4 6% 28%	1800
	11th to 20th	Night	19% 11% 4 6% 5% 2 4 3 5% 7% 35%	1775
	21st to end	Night	22% 11% 4 6% 5% 3 3 4 4 5% 34%	1790
May	1st to 10th	Night	24% 13% 4 5% 5% 2 3 4 4 6% 30%	1522
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 6% 5% 7% 37%	1415
	21st to end	Night	14% 6% 3 6% 5% 3 4 5% 6% 8% 40%	1550
June	1st to 10th	Night	17% 6% 3 7% 5% 3 3 4 5% 8% 39%	1440
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 5% 5% 8% 39%	1440
	21st to end	Night	12% 5% 3 5% 5% 4 6% 4 8% 8% 41%	1440
July	1st to 10th	Night	11% 4 2 5% 5% 3 5% 5% 7% 10% 42%	1440
	11th to 20th	Night	10% 6% 2 5% 5% 4 6% 6% 6% 8% 44%	1440
	21st to end	Night	13% 5% 3 5% 5% 3 5% 5% 6% 8% 42%	1574
August	1st to 10th	Night	12% 6% 2 5% 5% 3 4 6% 6% 10% 42%	1529
	11th to 20th	Night	14% 8% 2 6% 6% 4 6% 5% 6% 7% 37%	1703
	21st to end	Night	17% 6% 2 4 5% 3 3 4 4 9% 42%	1980
September	1st to 10th	Night	16% 6% 2 5% 5% 3 5% 5% 5% 7% 42%	1800
	11th to 20th	Night	16% 6% 2 4 4 2 3 4 4 7% 48%	1831
	21st to end	Night	17% 8% 2 5% 5% 2 5% 4 5% 6% 41%	1980
October	1st to 10th	Night	21% 8% 3 7% 5% 3 4 4 5% 6% 35%	2157
	11th to 20th	Night	19% 10% 3 6% 6% 3 5% 4 5% 6% 33%	2160
	21st to end	Night	26% 14% 3 7% 5% 3 4 4 4 7% 24%	2498
November	1st to 10th	Night	13% 13% 4 7% 5% 3 5% 4 5% 6% 35%	2407
	11th to 20th	Night	18% 14% 5% 8% 5% 3 4 5% 4 6% 28%	2493
	21st to end	Night	14% 14% 7% 9% 6% 4 5% 5% 4 6% 26%	2660
December	1st to 10th	Night	18% 18% 9% 11% 6% 3 4 4 4 5% 18%	2660
	11th to 20th	Night	20% 15% 7% 12% 8% 3 3 4 4 5% 20%	3288
	21st to end	Night	24% 17% 9% 10% 6% 2 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	64% 4 10% 15% 6% 1	1518
	11th to 20th	Day	71% 4 7% 10% 5% 2	1456
	21st to end	Day	66% 3 10% 13% 7% 1	1718
February	1st to 10th	Day	73% 4 8% 10% 5%	1620
	11th to 20th	Day	65% 5% 6% 15% 7% 1	1675
	21st to end	Day	66% 4 7% 14% 8%	1480
March	1st to 10th	Day	60% 4 12% 15% 8% 1	1800
	11th to 20th	Day	61% 3 8% 15% 10% 2	1952
	21st to end	Day	58% 3 8% 15% 14% 3	2317
April	1st to 10th	Day	57% 3 7% 21% 9% 2	2160
	11th to 20th	Day	52% 4 10% 19% 13% 2	2134
	21st to end	Day	56% 3 9% 19% 10% 3	2168
May	1st to 10th	Day	56% 4 8% 20% 10% 2	2454
	11th to 20th	Day	49% 4 10% 24% 12% 2	2482
	21st to end	Day	46% 6% 15% 22% 9% 2	2725
June	1st to 10th	Day	50% 5% 14% 19% 11% 1	2520
	11th to 20th	Day	51% 4 13% 22% 10% 1	2520
	21st to end	Day	49% 5% 14% 22% 9%	2520
July	1st to 10th	Day	46% 6% 13% 23% 11% 1	2450
	11th to 20th	Day	42% 5% 18% 26% 9%	2445
	21st to end	Day	46% 6% 15% 22% 10% 1	2591
August	1st to 10th	Day	46% 5% 14% 23% 11% 1	2391
	11th to 20th	Day	50% 5% 15% 24% 6%	2178
	21st to end	Day	46% 6% 12% 24% 11% 1	2376
September	1st to 10th	Day	46% 5% 12% 26% 11% 1	2160
	11th to 20th	Day	43% 3 11% 23% 19% 1	2129
	21st to end	Day	46% 5% 12% 23% 14% 1	1980
October	1st to 10th	Day	54% 4 9% 20% 11% 2	1800
	11th to 20th	Day	51% 4 12% 23% 9% 1	1800
	21st to end	Day	62% 4 11% 14% 8% 2	1856
November	1st to 10th	Day	48% 6% 13% 19% 12% 1	1671
	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	65% 5% 9% 12% 8% 1	2649
	11th to 20th	Night	70% 4 10% 10% 5% 1	2560
	21st to end	Night	67% 4 8% 12% 7% 1	2636
February	1st to 10th	Night	69% 4 9% 11% 7%	2340
	11th to 20th	Night	68% 5% 8% 13% 6%	2181
	21st to end	Night	63% 3 9% 15% 8% 1	1776
March	1st to 10th	Night	62% 3 10% 17% 7% 1	2160
	11th to 20th	Night	61% 3 10% 17% 9% 1	2006
	21st to end	Night	55% 4 8% 17% 14% 2	2039
April	1st to 10th	Night	57% 4 11% 18% 8% 1	1800
	11th to 20th	Night	51% 3 12% 20% 14%	1775
	21st to end	Night	54% 4 10% 19% 11% 3	1790
May	1st to 10th	Night	56% 4 9% 16% 14% 2	1522
	11th to 20th	Night	46% 5% 11% 24% 12% 2	1416
	21st to end	Night	43% 5% 16% 23% 10% 2	1551
June	1st to 10th	Night	45% 3 13% 26% 13%	1440
	11th to 20th	Night	45% 5% 16% 24% 10% 1	1440
	21st to end	Night	38% 6% 18% 27% 11% 1	1440
July	1st to 10th	Night	38% 6% 15% 26% 14% 2	1400
	11th to 20th	Night	38% 6% 17% 28% 10%	1392
	21st to end	Night	40% 5% 16% 24% 13% 1	1486
August	1st to 10th	Night	38% 6% 18% 28% 10% 1	1519
	11th to 20th	Night	46% 5% 17% 22% 9%	1703
	21st to end	Night	41% 4 14% 28% 13% 1	1980
September	1st to 10th	Night	42% 6% 13% 27% 11% 1	1800
	11th to 20th	Night	38% 4 11% 26% 20% 1	1831
	21st to end	Night	46% 3 11% 24% 14% 2	1980
October	1st to 10th	Night	51% 4 12% 21% 11% 1	2157
	11th to 20th	Night	51% 4 14% 20% 9% 1	2160
	21st to end	Night	62% 4 10% 15% 8% 1	2498
November	1st to 10th	Night	50% 5% 10% 19% 14% 2	2407
	11th to 20th	Night	58% 4 10% 17% 9% 2	2493
	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	32% 23% 29% 12% 3 1	180
	11th to 20th	Day	45% 22% 24% 7% 2 1	164
	21st to end	Day	34% 26% 27% 10% 4	236
February	1st to 10th	Day	42% 30% 24% 3	240
	11th to 20th	Day	38% 29% 26% 5% 1	333
	21st to end	Day	40% 22% 26% 9% 2	346
March	1st to 10th	Day	33% 22% 31% 10% 3	420
	11th to 20th	Day	37% 22% 25% 12% 4	420
	21st to end	Day	39% 21% 21% 12% 5% 3	462
April	1st to 10th	Day	36% 21% 24% 13% 6%	420
	11th to 20th	Day	32% 20% 26% 12% 6% 3	412
	21st to end	Day	37% 16% 27% 12% 5% 2	420
May	1st to 10th	Day	38% 14% 25% 13% 6% 4	423
	11th to 20th	Day	23% 18% 33% 16% 6% 3	447
	21st to end	Day	19% 12% 33% 20% 12% 4	485
June	1st to 10th	Day	26% 16% 25% 21% 9% 2	450
	11th to 20th	Day	22% 14% 28% 25% 8% 2	450
	21st to end	Day	23% 12% 33% 23% 7% 2	450
July	1st to 10th	Day	18% 9% 27% 27% 15% 5%	445
	11th to 20th	Day	12% 8% 32% 29% 15% 5%	444
	21st to end	Day	17% 9% 36% 26% 9% 3	488
August	1st to 10th	Day	15% 9% 26% 26% 16% 7%	446
	11th to 20th	Day	19% 7% 29% 26% 13% 7%	439
	21st to end	Day	18% 10% 28% 23% 14% 7%	495
September	1st to 10th	Day	23% 10% 23% 21% 16% 7%	450
	11th to 20th	Day	18% 8% 27% 20% 16% 10%	450
	21st to end	Day	20% 13% 30% 19% 12% 5%	450
October	1st to 10th	Day	29% 11% 24% 16% 12% 6%	450
	11th to 20th	Day	28% 16% 27% 19% 8% 2	450
	21st to end	Day	36% 16% 26% 17% 4	432
November	1st to 10th	Day	31% 18% 21% 21% 9% 1	365
	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	30% 22% 29% 17% 3 1	732
	21st to end	Night	25% 26% 29% 17% 3	792
February	1st to 10th	Night	29% 23% 32% 14% 2	720
	11th to 20th	Night	29% 26% 31% 12% 2	703
	21st to end	Night	36% 22% 26% 14% 2	592
March	1st to 10th	Night	28% 22% 29% 17% 3	720
	11th to 20th	Night	32% 21% 27% 17% 3	720
	21st to end	Night	28% 20% 25% 20% 7% 1	792
April	1st to 10th	Night	28% 19% 28% 21% 3	720
	11th to 20th	Night	27% 19% 26% 20% 8% 1	711
	21st to end	Night	31% 15% 26% 21% 7% 1	710
May	1st to 10th	Night	31% 12% 20% 27% 10%	442
	11th to 20th	Night	20% 11% 32% 30% 6% 1	354
	21st to end	Night	16% 6% 28% 33% 16% 1	389
June	1st to 10th	Night	20% 9% 19% 39% 13% 1	360
	11th to 20th	Night	20% 7% 19% 37% 17%	360
	21st to end	Night	12% 6% 25% 41% 15% 1	360
July	1st to 10th	Night	14% 3 20% 43% 19% 2	350
	11th to 20th	Night	13% 3 19% 41% 23% 1	348
	21st to end	Night	15% 4 17% 45% 16% 2	394
August	1st to 10th	Night	13% 4 23% 36% 23% 2	455
	11th to 20th	Night	17% 5% 14% 37% 23% 4	639
	21st to end	Night	19% 6% 15% 35% 22% 2	792
September	1st to 10th	Night	18% 6% 22% 32% 19% 3	720
	11th to 20th	Night	20% 7% 20% 27% 22% 5%	720
	21st to end	Night	22% 9% 22% 28% 16% 3	720
October	1st to 10th	Night	27% 12% 20% 23% 16% 3	720
	11th to 20th	Night	26% 15% 25% 27% 7% 1	720
	21st to end	Night	36% 15% 23% 22% 4	792
November	1st to 10th	Night	24% 16% 28% 23% 8%	740
	11th to 20th	Night	30% 21% 24% 17% 6% 2	760
	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 2024-01-10 10:16:10. Version ac07

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