Dee Wright Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dee 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	5% 8% 4 7% 7% 4 6% 6% 6% 6% 41%	74754
	Night	5% 8% 4 7% 8% 5% 7% 7% 7% 6% 36%	75002
Transparency	Day	44% 7% 9% 18% 16% 6%	74457
	Night	48% 9% 8% 15% 14% 6%	74820
Seeing	Day	16% 22% 27% 22% 11% 3	13560
	Night	12% 21% 19% 28% 18% 3	22716

By Month

Percentage Cloud Cover SW (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	6% 11% 5% 11% 11% 6% 9% 8% 8% 7% 18%	1519
	11th to 20th	Day	3 8% 5% 9% 10% 8% 8% 8% 8% 7% 26%	1456
	21st to end	Day	5% 11% 4 8% 9% 5% 7% 8% 5% 7% 31%	1728
February	1st to 10th	Day	4 13% 6% 10% 8% 5% 8% 9% 7% 6% 23%	1620
	11th to 20th	Day	4 12% 7% 9% 8% 5% 6% 9% 8% 6% 26%	1575
	21st to end	Day	4 9% 7% 12% 9% 8% 10% 9% 7% 6% 19%	1461
March	1st to 10th	Day	4 11% 6% 10% 12% 6% 8% 8% 7% 7% 20%	1844
	11th to 20th	Day	3 12% 4 9% 12% 7% 9% 7% 6% 8% 23%	1980
	21st to end	Day	6% 15% 7% 12% 11% 6% 8% 7% 6% 6% 16%	2254
April	1st to 10th	Day	8% 14% 6% 11% 11% 6% 7% 7% 7% 5% 19%	2160
	11th to 20th	Day	7% 13% 6% 9% 10% 5% 6% 6% 6% 5% 27%	2129
	21st to end	Day	4 11% 5% 9% 9% 4 7% 7% 7% 6% 30%	2247
May	1st to 10th	Day	7% 11% 4 7% 6% 5% 6% 5% 5% 7% 37%	2492
	11th to 20th	Day	9% 9% 4 8% 8% 5% 6% 6% 6% 7% 30%	2477
	21st to end	Day	11% 10% 5% 9% 7% 4 5% 5% 7% 7% 29%	2723
June	1st to 10th	Day	11% 9% 4 8% 7% 3 5% 5% 6% 8% 35%	2520
	11th to 20th	Day	8% 7% 3 8% 7% 4 6% 6% 7% 7% 39%	2520
	21st to end	Day	5% 3 1 5% 4 3 3 4 5% 5% 61%	2520
July	1st to 10th	Day	2 2 1 2 3 2 3 3 4 5% 73%	2520
	11th to 20th	Day	3 2 1 3 3 2 3 3 3 5% 73%	2520
	21st to end	Day	2 1 1 1 2 1 2 2 3 5% 79%	2741
August	1st to 10th	Day	1 1 1 2 2 2 4 4 5% 7% 70%	2362
	11th to 20th	Day	2 2 1 2 3 2 3 3 4 6% 72%	2298
	21st to end	Day	3 3 1 2 3 1 3 3 4 6% 72%	2388
September	1st to 10th	Day	2 2 1 2 3 2 3 3 4 6% 71%	2160
	11th to 20th	Day	5% 5% 2 6% 6% 4 5% 4 5% 7% 52%	2160
	21st to end	Day	5% 6% 2 6% 5% 4 5% 5% 6% 6% 51%	1915
October	1st to 10th	Day	8% 9% 2 6% 6% 4 5% 5% 4 5% 46%	1800
	11th to 20th	Day	6% 6% 2 6% 8% 5% 6% 7% 6% 6% 42%	1800
	21st to end	Day	5% 7% 4 8% 8% 6% 7% 8% 8% 6% 36%	1919
November	1st to 10th	Day	7% 10% 3 9% 10% 5% 8% 8% 9% 7% 24%	1664
	11th to 20th	Day	4 9% 4 9% 12% 7% 8% 8% 9% 8% 22%	1520
	21st to end	Day	5% 10% 5% 7% 10% 7% 7% 8% 8% 5% 28%	1520
December	1st to 10th	Day	4 7% 4 10% 9% 5% 7% 9% 8% 6% 31%	1520
	11th to 20th	Day	5% 10% 5% 8% 11% 6% 9% 10% 9% 7% 20%	1928
	21st to end	Day	5% 15% 7% 12% 10% 5% 7% 6% 6% 5% 21%	2794
January	1st to 10th	Night	4 11% 5% 11% 12% 6% 9% 9% 8% 7% 18%	2646
	11th to 20th	Night	4 7% 4 7% 11% 6% 8% 10% 10% 7% 25%	2560
	21st to end	Night	4 9% 4 9% 10% 6% 8% 8% 7% 6% 29%	2624
February	1st to 10th	Night	6% 10% 6% 9% 7% 5% 8% 9% 10% 8% 23%	2340
	11th to 20th	Night	4 10% 5% 9% 9% 6% 8% 9% 10% 6% 24%	2272
	21st to end	Night	4 9% 5% 11% 10% 8% 9% 9% 8% 5% 21%	1794
March	1st to 10th	Night	5% 11% 5% 8% 9% 6% 9% 11% 9% 7% 20%	2112
	11th to 20th	Night	3 8% 5% 7% 11% 7% 11% 9% 9% 8% 23%	1980
	21st to end	Night	5% 10% 6% 13% 12% 8% 9% 8% 9% 6% 15%	2102
April	1st to 10th	Night	6% 11% 5% 11% 11% 6% 9% 7% 6% 5% 23%	1800
	11th to 20th	Night	5% 10% 6% 8% 8% 5% 9% 8% 8% 6% 27%	1779
	21st to end	Night	7% 12% 4 8% 9% 6% 9% 6% 8% 5% 27%	1710
May	1st to 10th	Night	6% 10% 4 7% 6% 3 7% 6% 7% 6% 36%	1468
	11th to 20th	Night	8% 9% 3 6% 8% 5% 7% 7% 8% 8% 31%	1416
	21st to end	Night	9% 9% 4 9% 7% 5% 6% 6% 8% 8% 30%	1552
June	1st to 10th	Night	10% 9% 5% 8% 7% 3 5% 5% 5% 7% 36%	1440
	11th to 20th	Night	6% 7% 3 6% 7% 3 5% 6% 7% 7% 41%	1440
	21st to end	Night	4 5% 2 4 3 2 4 3 5% 6% 61%	1440
July	1st to 10th	Night	2 2 1 3 3 3 4 3 4 6% 71%	1440
	11th to 20th	Night	2 1 1 2 3 2 3 3 3 6% 74%	1440
	21st to end	Night	1 1 1 2 2 1 2 3 3 5% 80%	1576
August	1st to 10th	Night	2 2 1 2 2 1 2 3 4 8% 72%	1574
	11th to 20th	Night	2 2 1 2 2 2 3 3 3 6% 74%	1602
	21st to end	Night	2 2 1 3 3 2 3 4 4 5% 72%	1966
September	1st to 10th	Night	1 2 1 2 2 2 3 3 4 5% 74%	1800
	11th to 20th	Night	4 3 2 4 5% 4 5% 5% 6% 6% 56%	1800
	21st to end	Night	5% 6% 2 5% 5% 3 4 5% 6% 7% 51%	2042
October	1st to 10th	Night	6% 6% 3 7% 6% 4 5% 6% 5% 5% 48%	2160
	11th to 20th	Night	6% 6% 3 6% 6% 4 6% 7% 5% 7% 44%	2160
	21st to end	Night	5% 8% 3 9% 7% 5% 8% 7% 7% 5% 37%	2435
November	1st to 10th	Night	7% 9% 4 10% 9% 6% 8% 7% 8% 7% 24%	2414
	11th to 20th	Night	6% 8% 3 9% 11% 6% 9% 8% 9% 7% 23%	2660
	21st to end	Night	2 8% 5% 8% 10% 6% 6% 11% 8% 7% 29%	2660
December	1st to 10th	Night	5% 7% 3 8% 8% 6% 9% 9% 9% 7% 27%	2660
	11th to 20th	Night	4 11% 5% 11% 10% 6% 8% 10% 8% 6% 20%	3276
	21st to end	Night	5% 10% 5% 9% 10% 7% 8% 9% 7% 6% 24%	4862

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	66% 10% 8% 8% 7% 1	1519
	11th to 20th	Day	56% 8% 8% 13% 7% 7%	1456
	21st to end	Day	55% 7% 7% 16% 11% 4	1728
February	1st to 10th	Day	62% 9% 9% 12% 7% 1	1620
	11th to 20th	Day	58% 10% 8% 13% 8% 3	1584
	21st to end	Day	64% 10% 7% 11% 6% 2	1461
March	1st to 10th	Day	62% 10% 9% 13% 6% 1	1844
	11th to 20th	Day	59% 10% 8% 16% 6% 1	1980
	21st to end	Day	69% 9% 7% 11% 4	2254
April	1st to 10th	Day	65% 11% 6% 10% 7% 1	2160
	11th to 20th	Day	58% 8% 10% 13% 8% 3	2132
	21st to end	Day	53% 9% 11% 15% 9% 2	2247
May	1st to 10th	Day	46% 8% 12% 23% 10%	2492
	11th to 20th	Day	53% 8% 11% 20% 8% 1	2480
	21st to end	Day	54% 9% 13% 18% 6%	2724
June	1st to 10th	Day	49% 8% 12% 24% 7% 1	2520
	11th to 20th	Day	45% 8% 13% 24% 8% 2	2520
	21st to end	Day	26% 4 11% 33% 20% 6%	2520
July	1st to 10th	Day	17% 4 10% 23% 27% 19%	2451
	11th to 20th	Day	17% 4 6% 17% 32% 24%	2445
	21st to end	Day	11% 3 3 18% 42% 23%	2589
August	1st to 10th	Day	14% 6% 10% 29% 28% 13%	2345
	11th to 20th	Day	16% 3 8% 26% 34% 12%	2298
	21st to end	Day	16% 4 7% 25% 35% 12%	2388
September	1st to 10th	Day	17% 4 5% 24% 31% 18%	2160
	11th to 20th	Day	33% 5% 8% 21% 23% 9%	2160
	21st to end	Day	34% 7% 8% 22% 19% 9%	1915
October	1st to 10th	Day	41% 5% 7% 17% 22% 7%	1800
	11th to 20th	Day	43% 7% 9% 16% 20% 5%	1800
	21st to end	Day	47% 9% 9% 12% 19% 4	1919
November	1st to 10th	Day	58% 11% 7% 11% 12% 2	1664
	11th to 20th	Day	59% 9% 10% 12% 7% 4	1520
	21st to end	Day	56% 8% 8% 11% 12% 5%	1520
December	1st to 10th	Day	50% 10% 10% 16% 11% 3	1520
	11th to 20th	Day	61% 10% 9% 12% 7% 1	1928
	21st to end	Day	61% 12% 5% 14% 8% 1	2794
January	1st to 10th	Night	64% 12% 8% 8% 6% 2	2646
	11th to 20th	Night	56% 8% 8% 12% 8% 7%	2560
	21st to end	Night	56% 7% 7% 14% 11% 5%	2624
February	1st to 10th	Night	60% 11% 9% 10% 8% 1	2340
	11th to 20th	Night	58% 10% 9% 12% 8% 4	2274
	21st to end	Night	62% 10% 7% 11% 8% 3	1794
March	1st to 10th	Night	62% 10% 7% 14% 6% 2	2112
	11th to 20th	Night	58% 9% 8% 16% 7% 1	1980
	21st to end	Night	66% 12% 8% 10% 4	2102
April	1st to 10th	Night	62% 10% 10% 10% 7% 1	1800
	11th to 20th	Night	57% 11% 9% 13% 8% 2	1779
	21st to end	Night	58% 9% 9% 14% 8% 2	1710
May	1st to 10th	Night	46% 10% 10% 22% 11% 1	1468
	11th to 20th	Night	50% 10% 11% 20% 8% 1	1416
	21st to end	Night	53% 11% 12% 17% 8%	1552
June	1st to 10th	Night	50% 8% 11% 23% 7% 1	1440
	11th to 20th	Night	42% 10% 10% 27% 9% 2	1440
	21st to end	Night	26% 6% 11% 29% 20% 8%	1440
July	1st to 10th	Night	18% 5% 10% 21% 26% 19%	1400
	11th to 20th	Night	14% 5% 6% 19% 30% 27%	1392
	21st to end	Night	11% 3 4 17% 41% 24%	1488
August	1st to 10th	Night	13% 5% 7% 30% 33% 12%	1566
	11th to 20th	Night	15% 4 6% 26% 35% 13%	1602
	21st to end	Night	15% 6% 7% 24% 36% 12%	1966
September	1st to 10th	Night	15% 4 6% 25% 34% 16%	1800
	11th to 20th	Night	29% 7% 7% 21% 25% 11%	1800
	21st to end	Night	33% 9% 7% 20% 20% 12%	2042
October	1st to 10th	Night	40% 5% 8% 14% 25% 9%	2160
	11th to 20th	Night	41% 8% 10% 16% 18% 8%	2160
	21st to end	Night	47% 10% 7% 10% 19% 6%	2435
November	1st to 10th	Night	59% 10% 7% 9% 11% 4	2414
	11th to 20th	Night	62% 9% 7% 9% 9% 4	2660
	21st to end	Night	54% 8% 7% 11% 13% 8%	2660
December	1st to 10th	Night	52% 11% 9% 14% 12% 3	2660
	11th to 20th	Night	62% 9% 9% 10% 9% 1	3276
	21st to end	Night	55% 13% 6% 15% 8% 1	4862

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	27% 34% 22% 8% 7% 1	180
	11th to 20th	Day	24% 27% 20% 19% 7% 3	164
	21st to end	Day	18% 28% 23% 19% 11% 2	320
February	1st to 10th	Day	22% 29% 26% 16% 7% 1	340
	11th to 20th	Day	21% 36% 22% 16% 4 1	335
	21st to end	Day	27% 30% 21% 17% 5%	338
March	1st to 10th	Day	20% 32% 25% 19% 4	420
	11th to 20th	Day	20% 26% 28% 21% 6%	420
	21st to end	Day	27% 32% 23% 14% 4	462
April	1st to 10th	Day	27% 25% 23% 16% 7% 2	420
	11th to 20th	Day	22% 33% 25% 12% 6% 2	412
	21st to end	Day	18% 31% 25% 17% 7% 2	420
May	1st to 10th	Day	17% 25% 27% 21% 7% 3	423
	11th to 20th	Day	24% 23% 21% 18% 11% 3	448
	21st to end	Day	19% 19% 26% 20% 12% 4	485
June	1st to 10th	Day	19% 20% 24% 21% 13% 4	450
	11th to 20th	Day	21% 24% 28% 17% 9% 2	450
	21st to end	Day	6% 16% 30% 29% 16% 3	450
July	1st to 10th	Day	5% 13% 39% 26% 14% 3	446
	11th to 20th	Day	2 14% 39% 25% 16% 4	444
	21st to end	Day	1 10% 39% 28% 16% 5%	487
August	1st to 10th	Day	2 12% 35% 31% 14% 7%	447
	11th to 20th	Day	3 9% 28% 35% 16% 9%	437
	21st to end	Day	6% 14% 33% 29% 12% 6%	495
September	1st to 10th	Day	6% 12% 31% 29% 17% 5%	450
	11th to 20th	Day	10% 15% 29% 30% 12% 3	450
	21st to end	Day	15% 19% 23% 29% 12% 2	450
October	1st to 10th	Day	14% 23% 24% 22% 16% 1	450
	11th to 20th	Day	14% 22% 21% 25% 15% 3	450
	21st to end	Day	19% 28% 20% 16% 13% 3	434
November	1st to 10th	Day	22% 23% 24% 20% 10% 2	265
	11th to 20th	Day	18% 25% 28% 14% 12% 2	180
	21st to end	Day	21% 26% 20% 22% 9% 1	180
December	1st to 10th	Day	19% 24% 21% 21% 11% 5%	180
	11th to 20th	Day	17% 37% 24% 14% 7% 2	180
	21st to end	Day	22% 30% 21% 16% 9% 1	198
January	1st to 10th	Night	19% 31% 21% 16% 11% 3	760
	11th to 20th	Night	16% 27% 21% 19% 13% 4	732
	21st to end	Night	14% 22% 22% 25% 12% 4	792
February	1st to 10th	Night	13% 29% 21% 25% 11% 1	720
	11th to 20th	Night	14% 27% 23% 25% 10% 1	704
	21st to end	Night	16% 30% 23% 24% 6% 1	592
March	1st to 10th	Night	14% 31% 21% 23% 10% 1	720
	11th to 20th	Night	9% 30% 22% 27% 11% 1	720
	21st to end	Night	12% 35% 23% 20% 10%	792
April	1st to 10th	Night	16% 28% 22% 22% 11% 2	720
	11th to 20th	Night	13% 28% 24% 19% 13% 4	711
	21st to end	Night	14% 23% 22% 25% 14% 2	630
May	1st to 10th	Night	12% 19% 21% 26% 20% 3	388
	11th to 20th	Night	16% 15% 14% 32% 21% 2	354
	21st to end	Night	14% 15% 16% 33% 21% 2	389
June	1st to 10th	Night	19% 13% 15% 31% 20% 3	360
	11th to 20th	Night	10% 17% 19% 34% 20% 1	360
	21st to end	Night	8% 6% 15% 43% 28% 1	360
July	1st to 10th	Night	3 5% 10% 48% 32% 2	350
	11th to 20th	Night	1 4 13% 56% 26% 1	348
	21st to end	Night	1 2 8% 48% 39% 1	394
August	1st to 10th	Night	2 2 10% 34% 45% 6%	500
	11th to 20th	Night	3 3 8% 33% 46% 7%	534
	21st to end	Night	3 6% 18% 43% 28% 2	778
September	1st to 10th	Night	3 8% 16% 38% 31% 4	720
	11th to 20th	Night	7% 8% 15% 32% 33% 4	720
	21st to end	Night	9% 12% 18% 32% 26% 4	720
October	1st to 10th	Night	11% 14% 20% 30% 21% 4	720
	11th to 20th	Night	11% 19% 15% 29% 20% 5%	720
	21st to end	Night	14% 23% 21% 22% 14% 5%	792
November	1st to 10th	Night	15% 29% 19% 26% 10% 1	740
	11th to 20th	Night	15% 32% 22% 19% 12% 1	760
	21st to end	Night	11% 29% 21% 24% 13% 3	760
December	1st to 10th	Night	14% 25% 22% 24% 13% 3	760
	11th to 20th	Night	16% 28% 22% 22% 11% 1	760
	21st to end	Night	17% 34% 22% 19% 8% 1	836

Page generated at 2024-01-10 07:16:07. Version ac07

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