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 2025-12-03.

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	4 7% 3 8% 6% 6% 6% 5% 6% 6% 45%	136203
	Night	4 7% 3 8% 6% 7% 7% 6% 6% 6% 39%	131807
Transparency	Day	41% 7% 7% 19% 17% 9%	131008
	Night	45% 9% 7% 16% 16% 8%	127996
Seeing	Day	17% 22% 27% 21% 10% 3	16018
	Night	13% 21% 20% 27% 17% 2	25052

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% 6% 17% 11% 11% 9% 5% 5% 5% 15%	3639
	11th to 20th	Day	5% 9% 4 9% 7% 7% 5% 5% 6% 6% 38%	3640
	21st to end	Day	3 10% 3 10% 7% 7% 8% 6% 5% 6% 35%	4372
February	1st to 10th	Day	4 14% 6% 12% 8% 8% 7% 8% 7% 7% 21%	1800
	11th to 20th	Day	4 14% 5% 11% 7% 7% 6% 8% 8% 6% 25%	1755
	21st to end	Day	3 10% 5% 14% 8% 10% 10% 6% 7% 6% 21%	1629
March	1st to 10th	Day	3 8% 5% 11% 8% 7% 8% 8% 8% 8% 25%	4640
	11th to 20th	Day	2 11% 3 12% 10% 8% 9% 6% 4 4 30%	5000
	21st to end	Day	2 11% 5% 13% 11% 11% 12% 8% 7% 7% 14%	5678
April	1st to 10th	Day	7% 15% 6% 13% 9% 8% 7% 6% 7% 5% 19%	2400
	11th to 20th	Day	5% 14% 5% 10% 8% 7% 6% 5% 6% 5% 30%	2369
	21st to end	Day	3 12% 4 11% 7% 6% 7% 5% 7% 6% 31%	2497
May	1st to 10th	Day	7% 10% 3 7% 5% 6% 5% 4 5% 5% 44%	6266
	11th to 20th	Day	4 6% 3 8% 6% 7% 10% 5% 5% 6% 39%	6277
	21st to end	Day	5% 7% 3 7% 5% 6% 6% 5% 7% 9% 41%	6902
June	1st to 10th	Day	9% 10% 3 8% 6% 5% 5% 4 6% 7% 38%	2800
	11th to 20th	Day	7% 7% 3 9% 6% 5% 6% 5% 7% 6% 40%	2800
	21st to end	Day	4 4 1 5% 4 4 4 3 5% 5% 61%	2800
July	1st to 10th	Day	1 2 1 2 2 2 2 2 3 4 82%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 2 3 83%	6320
	21st to end	Day	2 1 1 2 2 1 2 2 4 6% 78%	6880
August	1st to 10th	Day	2 2 1 4 2 3 3 3 5% 6% 70%	5850
	11th to 20th	Day	1 1 1 3 3 3 4 3 5% 7% 69%	5796
	21st to end	Day	5% 3 1 4 3 3 4 3 5% 7% 62%	5996
September	1st to 10th	Day	2 3 1 4 3 3 3 2 5% 6% 68%	2400
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 3 5% 7% 52%	2400
	21st to end	Day	5% 6% 2 6% 4 5% 5% 3 5% 6% 53%	2127
October	1st to 10th	Day	4 5% 1 5% 4 4 5% 3 4 6% 58%	3280
	11th to 20th	Day	3 4 2 7% 6% 7% 10% 6% 8% 7% 40%	3261
	21st to end	Day	3 5% 3 10% 8% 9% 12% 7% 8% 7% 28%	3440
November	1st to 10th	Day	6% 10% 3 11% 8% 7% 9% 6% 9% 7% 24%	1842
	11th to 20th	Day	4 9% 3 11% 11% 10% 9% 7% 9% 7% 20%	1680
	21st to end	Day	4 11% 3 9% 9% 8% 8% 7% 8% 5% 27%	1680
December	1st to 10th	Day	3 5% 3 8% 6% 8% 7% 7% 6% 6% 43%	2636
	11th to 20th	Day	4 11% 5% 13% 10% 9% 10% 7% 8% 6% 16%	3028
	21st to end	Day	3 13% 6% 13% 10% 9% 12% 7% 6% 5% 16%	4004
January	1st to 10th	Night	4 12% 5% 14% 9% 10% 12% 7% 7% 7% 13%	6306
	11th to 20th	Night	3 8% 4 10% 8% 6% 6% 6% 7% 8% 33%	6320
	21st to end	Night	4 10% 4 10% 7% 7% 7% 4 5% 5% 37%	6583
February	1st to 10th	Night	5% 12% 6% 10% 6% 7% 7% 7% 10% 7% 22%	2600
	11th to 20th	Night	4 12% 4 11% 8% 7% 9% 8% 10% 6% 23%	2532
	21st to end	Night	4 9% 4 12% 9% 11% 8% 8% 8% 5% 22%	2000
March	1st to 10th	Night	4 9% 4 8% 7% 9% 11% 9% 10% 8% 21%	5250
	11th to 20th	Night	1 6% 4 10% 8% 8% 9% 12% 6% 5% 30%	5000
	21st to end	Night	3 9% 4 13% 10% 11% 13% 9% 8% 6% 14%	5284
April	1st to 10th	Night	5% 12% 5% 13% 9% 9% 8% 6% 6% 6% 23%	2000
	11th to 20th	Night	4 11% 5% 8% 7% 8% 7% 7% 8% 6% 29%	1979
	21st to end	Night	6% 12% 4 9% 7% 9% 7% 5% 8% 5% 28%	1900
May	1st to 10th	Night	7% 9% 4 8% 5% 4 5% 4 5% 5% 43%	3696
	11th to 20th	Night	4 7% 2 7% 7% 8% 13% 6% 6% 6% 34%	3616
	21st to end	Night	5% 8% 3 8% 5% 5% 5% 5% 7% 8% 41%	3972
June	1st to 10th	Night	9% 10% 4 9% 6% 4 5% 4 5% 6% 38%	1600
	11th to 20th	Night	5% 8% 2 8% 6% 5% 5% 5% 8% 7% 42%	1600
	21st to end	Night	4 5% 2 4 3 3 4 3 6% 6% 60%	1600
July	1st to 10th	Night	1 2 1 3 2 3 3 2 3 5% 74%	3640
	11th to 20th	Night	1 2 1 1 2 2 2 2 2 3 83%	3640
	21st to end	Night	2 1 1 2 2 2 2 3 3 4 78%	3970
August	1st to 10th	Night	1 2 1 3 3 3 5% 4 5% 8% 64%	3942
	11th to 20th	Night	2 2 1 3 3 3 5% 3 4 7% 67%	4052
	21st to end	Night	5% 3 1 4 3 3 4 4 4 6% 64%	5001
September	1st to 10th	Night	1 2 1 3 3 4 3 3 5% 6% 70%	2000
	11th to 20th	Night	4 4 2 5% 5% 5% 5% 4 6% 6% 54%	2000
	21st to end	Night	5% 5% 2 6% 4 4 4 4 6% 7% 54%	2270
October	1st to 10th	Night	3 4 2 6% 4 5% 4 3 4 5% 60%	3920
	11th to 20th	Night	4 5% 2 6% 5% 7% 9% 7% 6% 8% 41%	3897
	21st to end	Night	3 5% 2 9% 7% 8% 14% 9% 7% 6% 29%	4365
November	1st to 10th	Night	6% 10% 3 12% 8% 8% 9% 5% 8% 7% 24%	2676
	11th to 20th	Night	5% 9% 3 10% 9% 9% 9% 7% 9% 7% 23%	2940
	21st to end	Night	2 9% 3 9% 9% 7% 8% 9% 8% 7% 29%	2940
December	1st to 10th	Night	3 5% 2 6% 5% 6% 7% 7% 8% 7% 45%	4588
	11th to 20th	Night	4 11% 4 12% 9% 9% 11% 9% 8% 7% 16%	5176
	21st to end	Night	6% 9% 4 11% 10% 10% 10% 9% 6% 5% 20%	6952

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	63% 15% 6% 6% 7% 2	3639
	11th to 20th	Day	42% 11% 7% 16% 16% 7%	3640
	21st to end	Day	48% 10% 4 10% 19% 9%	4372
February	1st to 10th	Day	63% 9% 9% 11% 8% 1	1800
	11th to 20th	Day	58% 10% 8% 12% 8% 4	1764
	21st to end	Day	62% 10% 7% 12% 6% 3	1629
March	1st to 10th	Day	49% 16% 6% 15% 10% 3	4640
	11th to 20th	Day	59% 6% 7% 16% 12% 1	5000
	21st to end	Day	63% 16% 6% 11% 4	5678
April	1st to 10th	Day	64% 11% 7% 10% 6% 1	2400
	11th to 20th	Day	55% 9% 9% 15% 10% 3	2372
	21st to end	Day	52% 10% 11% 16% 9% 1	2497
May	1st to 10th	Day	39% 11% 6% 23% 18% 3	6266
	11th to 20th	Day	47% 6% 9% 23% 14% 1	6280
	21st to end	Day	36% 12% 20% 26% 5% 1	6904
June	1st to 10th	Day	46% 8% 12% 25% 8% 1	2800
	11th to 20th	Day	43% 8% 12% 24% 11% 3	2800
	21st to end	Day	27% 4 11% 32% 20% 6%	2800
July	1st to 10th	Day	11% 2 5% 23% 28% 32%	6251
	11th to 20th	Day	11% 2 3 11% 24% 49%	6245
	21st to end	Day	13% 1 4 32% 33% 17%	6681
August	1st to 10th	Day	17% 4 6% 21% 35% 18%	4254
	11th to 20th	Day	20% 2 5% 34% 32% 8%	4195
	21st to end	Day	23% 2 7% 25% 25% 17%	4323
September	1st to 10th	Day	19% 3 6% 24% 31% 17%	2400
	11th to 20th	Day	33% 5% 8% 23% 22% 9%	2400
	21st to end	Day	32% 6% 8% 22% 21% 10%	2127
October	1st to 10th	Day	31% 4 5% 16% 35% 9%	3280
	11th to 20th	Day	45% 7% 10% 21% 14% 3	3261
	21st to end	Day	56% 6% 6% 15% 14% 2	3440
November	1st to 10th	Day	59% 10% 7% 11% 12% 2	1842
	11th to 20th	Day	61% 8% 10% 11% 7% 4	1680
	21st to end	Day	56% 8% 7% 12% 13% 5%	1680
December	1st to 10th	Day	42% 6% 6% 14% 21% 11%	2636
	11th to 20th	Day	69% 7% 7% 11% 5%	3028
	21st to end	Day	69% 8% 5% 10% 6% 1	4004
January	1st to 10th	Night	63% 16% 5% 7% 7% 3	6306
	11th to 20th	Night	43% 13% 5% 15% 16% 8%	6320
	21st to end	Night	50% 8% 5% 8% 17% 13%	6583
February	1st to 10th	Night	60% 12% 8% 10% 8% 1	2600
	11th to 20th	Night	59% 10% 9% 11% 8% 4	2534
	21st to end	Night	59% 11% 7% 11% 9% 3	2000
March	1st to 10th	Night	51% 17% 6% 13% 8% 4	5250
	11th to 20th	Night	54% 10% 6% 17% 13% 1	5000
	21st to end	Night	62% 17% 6% 12% 2	5284
April	1st to 10th	Night	61% 11% 10% 10% 7% 1	2000
	11th to 20th	Night	54% 11% 8% 16% 9% 2	1979
	21st to end	Night	57% 10% 8% 14% 9% 2	1900
May	1st to 10th	Night	37% 15% 5% 21% 21% 2	3693
	11th to 20th	Night	46% 13% 7% 17% 17% 1	3616
	21st to end	Night	35% 15% 14% 29% 6% 2	3972
June	1st to 10th	Night	47% 8% 10% 25% 9% 1	1600
	11th to 20th	Night	42% 9% 9% 26% 11% 3	1600
	21st to end	Night	26% 6% 11% 30% 19% 7%	1600
July	1st to 10th	Night	16% 2 6% 21% 23% 32%	3600
	11th to 20th	Night	10% 2 3 11% 25% 49%	3592
	21st to end	Night	14% 1 4 30% 33% 18%	3857
August	1st to 10th	Night	16% 3 4 21% 39% 17%	2863
	11th to 20th	Night	21% 3 4 31% 32% 9%	2929
	21st to end	Night	22% 3 7% 24% 27% 17%	3594
September	1st to 10th	Night	18% 4 6% 25% 33% 15%	2000
	11th to 20th	Night	31% 6% 7% 21% 24% 11%	2000
	21st to end	Night	31% 8% 7% 21% 21% 13%	2270
October	1st to 10th	Night	30% 3 6% 16% 35% 10%	3920
	11th to 20th	Night	44% 6% 11% 22% 13% 5%	3897
	21st to end	Night	58% 6% 6% 10% 16% 4	4365
November	1st to 10th	Night	60% 9% 8% 9% 11% 3	2676
	11th to 20th	Night	63% 8% 7% 9% 9% 3	2940
	21st to end	Night	54% 7% 7% 11% 14% 7%	2940
December	1st to 10th	Night	38% 8% 5% 13% 23% 13%	4588
	11th to 20th	Night	69% 7% 9% 9% 6% 1	5176
	21st to end	Night	64% 10% 5% 13% 7% 1	6952

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% 31% 22% 8% 7% 1	200
	11th to 20th	Day	24% 25% 24% 18% 7% 4	200
	21st to end	Day	20% 28% 22% 18% 10% 2	380
February	1st to 10th	Day	26% 27% 25% 15% 6% 2	400
	11th to 20th	Day	24% 35% 22% 15% 4 1	395
	21st to end	Day	29% 26% 22% 18% 4	404
March	1st to 10th	Day	21% 32% 26% 18% 3	500
	11th to 20th	Day	23% 26% 26% 18% 7% 1	500
	21st to end	Day	27% 33% 23% 13% 4	550
April	1st to 10th	Day	26% 25% 26% 15% 6% 2	500
	11th to 20th	Day	21% 32% 27% 13% 6% 2	492
	21st to end	Day	17% 31% 26% 18% 7% 2	500
May	1st to 10th	Day	19% 25% 27% 19% 7% 3	503
	11th to 20th	Day	25% 22% 21% 18% 11% 3	528
	21st to end	Day	19% 20% 26% 19% 12% 4	573
June	1st to 10th	Day	18% 21% 23% 22% 13% 4	530
	11th to 20th	Day	22% 25% 27% 17% 8% 2	530
	21st to end	Day	8% 18% 30% 28% 14% 3	530
July	1st to 10th	Day	5% 13% 39% 25% 14% 3	526
	11th to 20th	Day	3 15% 40% 24% 15% 4	524
	21st to end	Day	3 12% 38% 26% 16% 5%	575
August	1st to 10th	Day	3 14% 34% 30% 13% 6%	527
	11th to 20th	Day	4 12% 28% 32% 15% 8%	517
	21st to end	Day	8% 13% 33% 28% 12% 7%	583
September	1st to 10th	Day	7% 12% 32% 29% 16% 5%	530
	11th to 20th	Day	13% 15% 29% 29% 12% 3	530
	21st to end	Day	15% 19% 24% 28% 12% 3	530
October	1st to 10th	Day	16% 22% 27% 20% 15% 1	530
	11th to 20th	Day	17% 24% 21% 23% 13% 2	530
	21st to end	Day	22% 27% 21% 16% 11% 3	514
November	1st to 10th	Day	26% 22% 22% 18% 10% 2	323
	11th to 20th	Day	24% 24% 27% 13% 10% 2	220
	21st to end	Day	23% 25% 20% 21% 9% 1	220
December	1st to 10th	Day	19% 22% 20% 21% 12% 6%	204
	11th to 20th	Day	23% 34% 23% 13% 6% 2	200
	21st to end	Day	28% 28% 20% 15% 8% 1	220
January	1st to 10th	Night	21% 30% 21% 16% 10% 2	800
	11th to 20th	Night	17% 26% 22% 19% 12% 4	800
	21st to end	Night	15% 22% 23% 24% 12% 4	880
February	1st to 10th	Night	15% 29% 21% 24% 10% 1	800
	11th to 20th	Night	16% 28% 23% 23% 9% 1	784
	21st to end	Night	18% 28% 24% 24% 6%	660
March	1st to 10th	Night	16% 30% 21% 23% 10% 1	800
	11th to 20th	Night	12% 29% 22% 26% 11% 1	800
	21st to end	Night	14% 34% 23% 19% 9%	880
April	1st to 10th	Night	16% 27% 23% 22% 11% 2	800
	11th to 20th	Night	12% 27% 25% 19% 13% 3	791
	21st to end	Night	13% 23% 24% 25% 13% 2	700
May	1st to 10th	Night	13% 19% 22% 25% 19% 2	430
	11th to 20th	Night	18% 14% 14% 32% 20% 2	394
	21st to end	Night	14% 15% 15% 35% 20% 2	433
June	1st to 10th	Night	18% 13% 15% 33% 19% 3	400
	11th to 20th	Night	10% 18% 20% 34% 18% 1	400
	21st to end	Night	8% 6% 18% 42% 26% 1	400
July	1st to 10th	Night	3 5% 13% 46% 31% 2	390
	11th to 20th	Night	2 4 15% 54% 24% 1	388
	21st to end	Night	2 3 10% 49% 36% 1	438
August	1st to 10th	Night	3 2 11% 37% 42% 6%	556
	11th to 20th	Night	3 4 12% 33% 42% 6%	594
	21st to end	Night	3 6% 19% 43% 27% 2	866
September	1st to 10th	Night	3 7% 17% 39% 30% 4	800
	11th to 20th	Night	9% 9% 16% 32% 32% 4	800
	21st to end	Night	10% 12% 18% 32% 25% 4	800
October	1st to 10th	Night	11% 13% 22% 30% 20% 3	800
	11th to 20th	Night	13% 18% 17% 29% 19% 5%	800
	21st to end	Night	16% 23% 21% 23% 12% 5%	880
November	1st to 10th	Night	18% 27% 19% 25% 10% 1	820
	11th to 20th	Night	19% 30% 22% 18% 11% 1	840
	21st to end	Night	12% 28% 22% 23% 12% 3	840
December	1st to 10th	Night	14% 24% 22% 24% 14% 3	808
	11th to 20th	Night	18% 27% 22% 21% 10% 1	800
	21st to end	Night	20% 33% 21% 18% 7% 1	880

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