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 2024-11-11.

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	4 8% 3 8% 6% 6% 5% 4 6% 6% 44%	105457
	Night	4 8% 3 9% 7% 7% 7% 6% 7% 6% 38%	102393
Transparency	Day	39% 9% 8% 18% 17% 9%	105200
	Night	44% 11% 7% 15% 15% 7%	102239
Seeing	Day	16% 23% 27% 21% 11% 3	14706
	Night	12% 21% 20% 28% 17% 2	23716

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	4 12% 7% 18% 10% 9% 8% 6% 6% 5% 16%	2539
	11th to 20th	Day	7% 12% 5% 13% 9% 9% 7% 6% 7% 6% 19%	2540
	21st to end	Day	5% 13% 4 13% 9% 9% 8% 7% 5% 6% 20%	3050
February	1st to 10th	Day	4 14% 6% 11% 7% 8% 7% 7% 7% 6% 22%	1710
	11th to 20th	Day	4 14% 5% 11% 7% 7% 6% 8% 9% 6% 25%	1665
	21st to end	Day	3 10% 6% 14% 8% 9% 9% 7% 7% 6% 20%	1550
March	1st to 10th	Day	2 9% 6% 12% 9% 8% 8% 7% 8% 7% 26%	3221
	11th to 20th	Day	2 10% 3 10% 7% 6% 5% 4 4 6% 42%	3490
	21st to end	Day	3 15% 6% 14% 10% 9% 8% 6% 7% 6% 16%	3965
April	1st to 10th	Day	7% 15% 6% 13% 9% 8% 6% 6% 7% 5% 19%	2280
	11th to 20th	Day	6% 15% 5% 10% 8% 7% 6% 5% 5% 5% 27%	2249
	21st to end	Day	3 13% 5% 11% 8% 6% 7% 5% 7% 6% 29%	2372
May	1st to 10th	Day	8% 13% 4 9% 6% 5% 5% 4 5% 5% 36%	4379
	11th to 20th	Day	5% 6% 3 6% 4 4 4 4 5% 6% 54%	4377
	21st to end	Day	7% 9% 4 10% 6% 6% 5% 5% 7% 8% 32%	4813
June	1st to 10th	Day	10% 11% 3 9% 6% 5% 5% 4 6% 7% 35%	2660
	11th to 20th	Day	7% 7% 3 9% 6% 5% 6% 5% 7% 6% 39%	2660
	21st to end	Day	4 4 1 5% 4 3 3 3 5% 5% 61%	2660
July	1st to 10th	Day	1 1 1 2 2 1 2 2 2 3 83%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	4420
	21st to end	Day	1 1 1 2 2 1 2 2 3 5% 80%	4790
August	1st to 10th	Day	2 2 1 4 2 2 3 2 4 5% 71%	4138
	11th to 20th	Day	2 2 1 4 3 3 4 3 5% 7% 68%	4026
	21st to end	Day	5% 3 1 3 2 3 3 3 4 6% 68%	4192
September	1st to 10th	Day	2 2 1 3 3 3 3 2 4 6% 70%	2280
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 3 5% 7% 51%	2280
	21st to end	Day	5% 6% 2 6% 4 5% 5% 4 5% 6% 52%	2021
October	1st to 10th	Day	4 5% 1 5% 3 4 5% 3 4 6% 58%	3180
	11th to 20th	Day	3 4 2 7% 6% 7% 10% 6% 8% 7% 40%	3161
	21st to end	Day	3 5% 3 10% 8% 9% 12% 7% 7% 7% 28%	3334
November	1st to 10th	Day	6% 10% 3 11% 8% 7% 8% 6% 9% 7% 24%	1753
	11th to 20th	Day	4 10% 3 11% 11% 9% 8% 7% 9% 7% 22%	1520
	21st to end	Day	4 12% 3 9% 9% 8% 7% 6% 8% 5% 28%	1520
December	1st to 10th	Day	4 8% 4 12% 7% 8% 6% 7% 8% 6% 31%	1520
	11th to 20th	Day	5% 10% 5% 10% 9% 9% 9% 7% 10% 7% 20%	1928
	21st to end	Day	4 16% 7% 13% 9% 7% 6% 5% 6% 5% 21%	2794
January	1st to 10th	Night	4 13% 5% 15% 10% 9% 9% 7% 7% 7% 13%	4406
	11th to 20th	Night	5% 11% 5% 14% 11% 8% 8% 7% 7% 6% 17%	4420
	21st to end	Night	6% 13% 5% 13% 10% 9% 8% 6% 6% 5% 20%	4610
February	1st to 10th	Night	5% 12% 6% 10% 6% 7% 7% 7% 10% 8% 22%	2470
	11th to 20th	Night	4 12% 4 10% 8% 7% 8% 7% 10% 6% 24%	2402
	21st to end	Night	4 10% 5% 13% 9% 11% 8% 7% 8% 5% 21%	1903
March	1st to 10th	Night	2 8% 4 9% 8% 10% 10% 9% 11% 8% 20%	3649
	11th to 20th	Night	1 6% 4 8% 7% 7% 7% 5% 6% 5% 43%	3490
	21st to end	Night	4 13% 5% 16% 11% 10% 8% 6% 8% 6% 14%	3688
April	1st to 10th	Night	5% 12% 5% 13% 9% 9% 8% 5% 5% 5% 24%	1900
	11th to 20th	Night	4 11% 5% 8% 7% 8% 7% 7% 8% 6% 27%	1879
	21st to end	Night	6% 12% 4 10% 7% 8% 7% 5% 8% 5% 27%	1805
May	1st to 10th	Night	8% 12% 6% 10% 6% 4 6% 4 6% 5% 33%	2582
	11th to 20th	Night	4 7% 2 6% 5% 5% 5% 4 7% 6% 47%	2516
	21st to end	Night	5% 10% 3 11% 6% 6% 5% 5% 7% 7% 35%	2762
June	1st to 10th	Night	10% 10% 4 9% 6% 4 5% 4 5% 6% 35%	1520
	11th to 20th	Night	6% 8% 3 8% 6% 5% 5% 5% 7% 7% 41%	1520
	21st to end	Night	4 5% 2 4 3 3 4 3 6% 6% 60%	1520
July	1st to 10th	Night	1 1 2 2 2 2 2 3 4 82%	2540
	11th to 20th	Night	1 2 1 2 2 2 2 2 2 5% 80%	2540
	21st to end	Night	2 1 2 1 1 2 3 3 3 81%	2761
August	1st to 10th	Night	2 2 1 3 2 2 3 3 4 7% 72%	2783
	11th to 20th	Night	1 2 1 3 3 3 5% 3 5% 7% 67%	2812
	21st to end	Night	5% 2 1 3 3 3 3 3 4 5% 69%	3484
September	1st to 10th	Night	1 2 1 2 2 3 3 2 4 6% 73%	1900
	11th to 20th	Night	4 4 2 5% 4 5% 5% 4 6% 6% 55%	1900
	21st to end	Night	5% 6% 2 6% 4 4 4 4 6% 7% 53%	2156
October	1st to 10th	Night	3 4 2 6% 4 5% 4 3 4 5% 61%	3800
	11th to 20th	Night	4 5% 2 6% 5% 7% 9% 7% 6% 8% 42%	3777
	21st to end	Night	3 5% 2 9% 7% 9% 14% 9% 7% 6% 28%	4229
November	1st to 10th	Night	6% 10% 3 12% 8% 8% 9% 6% 8% 7% 24%	2545
	11th to 20th	Night	6% 9% 3 11% 9% 9% 7% 7% 9% 7% 23%	2666
	21st to end	Night	2 10% 3 10% 9% 7% 7% 9% 8% 7% 29%	2660
December	1st to 10th	Night	5% 8% 3 9% 8% 8% 9% 8% 10% 7% 27%	2660
	11th to 20th	Night	4 12% 5% 12% 9% 8% 9% 7% 8% 6% 20%	3276
	21st to end	Night	4 10% 5% 11% 9% 9% 7% 7% 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	58% 21% 7% 7% 6% 1	2539
	11th to 20th	Day	57% 16% 8% 12% 4 4	2540
	21st to end	Day	60% 14% 6% 12% 6% 2	3050
February	1st to 10th	Day	63% 9% 9% 12% 7% 1	1710
	11th to 20th	Day	57% 11% 8% 12% 8% 3	1674
	21st to end	Day	63% 11% 8% 12% 6% 2	1550
March	1st to 10th	Day	45% 22% 7% 15% 11% 1	3221
	11th to 20th	Day	42% 9% 9% 23% 16% 1	3490
	21st to end	Day	57% 22% 6% 10% 5%	3965
April	1st to 10th	Day	64% 11% 7% 10% 7% 1	2280
	11th to 20th	Day	57% 9% 10% 13% 9% 3	2252
	21st to end	Day	53% 11% 11% 15% 9% 1	2372
May	1st to 10th	Day	42% 15% 7% 21% 13%	4379
	11th to 20th	Day	30% 8% 11% 30% 20% 1	4380
	21st to end	Day	40% 17% 20% 19% 4	4814
June	1st to 10th	Day	48% 9% 12% 23% 8% 1	2660
	11th to 20th	Day	44% 8% 12% 24% 9% 3	2660
	21st to end	Day	26% 4 11% 33% 20% 6%	2660
July	1st to 10th	Day	10% 2 6% 14% 24% 45%	4351
	11th to 20th	Day	13% 3 3 11% 22% 48%	4345
	21st to end	Day	10% 2 5% 18% 40% 25%	4639
August	1st to 10th	Day	17% 4 6% 19% 35% 19%	4122
	11th to 20th	Day	20% 2 5% 34% 32% 8%	4065
	21st to end	Day	23% 2 7% 25% 26% 17%	4191
September	1st to 10th	Day	18% 4 5% 24% 32% 18%	2280
	11th to 20th	Day	34% 5% 8% 22% 22% 9%	2280
	21st to end	Day	33% 7% 9% 22% 20% 9%	2021
October	1st to 10th	Day	30% 4 5% 15% 36% 10%	3180
	11th to 20th	Day	45% 7% 10% 21% 14% 3	3161
	21st to end	Day	55% 7% 6% 15% 15% 2	3334
November	1st to 10th	Day	58% 10% 7% 11% 12% 2	1753
	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	58% 22% 6% 8% 4 1	4406
	11th to 20th	Night	56% 16% 6% 11% 5% 4	4420
	21st to end	Night	64% 11% 7% 9% 6% 3	4610
February	1st to 10th	Night	60% 12% 9% 10% 8% 1	2470
	11th to 20th	Night	58% 10% 9% 11% 8% 4	2404
	21st to end	Night	60% 11% 7% 11% 8% 3	1903
March	1st to 10th	Night	47% 22% 7% 14% 9% 1	3649
	11th to 20th	Night	38% 14% 6% 23% 18% 1	3490
	21st to end	Night	57% 23% 7% 9% 3	3688
April	1st to 10th	Night	60% 12% 10% 10% 7% 1	1900
	11th to 20th	Night	56% 11% 8% 14% 8% 2	1879
	21st to end	Night	57% 11% 9% 13% 8% 2	1805
May	1st to 10th	Night	42% 19% 6% 21% 11%	2582
	11th to 20th	Night	29% 16% 8% 21% 25% 1	2516
	21st to end	Night	38% 22% 16% 19% 5%	2762
June	1st to 10th	Night	50% 9% 10% 22% 8% 1	1520
	11th to 20th	Night	42% 10% 9% 26% 10% 3	1520
	21st to end	Night	26% 6% 11% 30% 20% 8%	1520
July	1st to 10th	Night	10% 3 6% 12% 24% 46%	2500
	11th to 20th	Night	11% 3 3 12% 21% 49%	2492
	21st to end	Night	11% 2 4 15% 42% 25%	2674
August	1st to 10th	Night	16% 3 4 20% 40% 17%	2775
	11th to 20th	Night	21% 3 4 31% 32% 9%	2839
	21st to end	Night	22% 3 6% 24% 27% 17%	3484
September	1st to 10th	Night	16% 4 5% 25% 35% 16%	1900
	11th to 20th	Night	31% 7% 7% 21% 24% 11%	1900
	21st to end	Night	31% 8% 8% 21% 20% 12%	2156
October	1st to 10th	Night	30% 3 6% 16% 36% 10%	3800
	11th to 20th	Night	43% 6% 11% 22% 13% 5%	3777
	21st to end	Night	58% 6% 6% 10% 16% 4	4229
November	1st to 10th	Night	59% 10% 7% 9% 12% 3	2545
	11th to 20th	Night	61% 9% 7% 9% 9% 4	2666
	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	26% 26% 21% 18% 6% 3	180
	21st to end	Day	19% 29% 23% 17% 10% 2	350
February	1st to 10th	Day	22% 28% 25% 16% 7% 2	370
	11th to 20th	Day	21% 36% 22% 15% 4 1	365
	21st to end	Day	28% 28% 21% 18% 5%	373
March	1st to 10th	Day	20% 32% 25% 19% 4	460
	11th to 20th	Day	19% 25% 27% 20% 7% 1	460
	21st to end	Day	26% 33% 24% 13% 4	506
April	1st to 10th	Day	26% 25% 24% 16% 7% 2	460
	11th to 20th	Day	21% 33% 26% 12% 6% 2	452
	21st to end	Day	18% 32% 24% 18% 7% 2	460
May	1st to 10th	Day	18% 25% 27% 20% 7% 3	463
	11th to 20th	Day	22% 22% 22% 19% 12% 3	488
	21st to end	Day	19% 20% 26% 19% 12% 4	529
June	1st to 10th	Day	19% 20% 23% 21% 13% 4	490
	11th to 20th	Day	22% 24% 27% 17% 9% 2	490
	21st to end	Day	7% 18% 30% 28% 15% 3	490
July	1st to 10th	Day	5% 13% 39% 26% 14% 3	486
	11th to 20th	Day	2 15% 39% 24% 15% 4	484
	21st to end	Day	2 12% 39% 27% 16% 5%	531
August	1st to 10th	Day	3 13% 34% 30% 14% 7%	487
	11th to 20th	Day	4 11% 28% 33% 16% 8%	477
	21st to end	Day	8% 14% 32% 28% 12% 6%	539
September	1st to 10th	Day	6% 11% 32% 28% 17% 5%	490
	11th to 20th	Day	13% 16% 29% 29% 11% 3	490
	21st to end	Day	14% 20% 24% 28% 11% 2	490
October	1st to 10th	Day	15% 22% 26% 21% 16% 1	490
	11th to 20th	Day	15% 23% 22% 23% 14% 2	490
	21st to end	Day	21% 27% 21% 16% 12% 3	474
November	1st to 10th	Day	24% 22% 22% 19% 10% 2	294
	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% 12% 4	760
	21st to end	Night	14% 22% 23% 24% 12% 4	836
February	1st to 10th	Night	13% 29% 21% 25% 11% 1	760
	11th to 20th	Night	14% 28% 23% 24% 10% 1	744
	21st to end	Night	16% 29% 23% 24% 6%	628
March	1st to 10th	Night	14% 31% 21% 23% 10% 1	760
	11th to 20th	Night	9% 29% 23% 27% 11% 1	760
	21st to end	Night	12% 35% 23% 20% 9%	836
April	1st to 10th	Night	15% 28% 22% 22% 11% 2	760
	11th to 20th	Night	12% 28% 25% 19% 13% 3	751
	21st to end	Night	14% 24% 23% 25% 13% 2	665
May	1st to 10th	Night	13% 19% 22% 24% 19% 2	409
	11th to 20th	Night	15% 14% 14% 33% 21% 2	374
	21st to end	Night	14% 15% 16% 34% 21% 2	411
June	1st to 10th	Night	19% 13% 14% 32% 19% 2	380
	11th to 20th	Night	10% 17% 19% 34% 19% 1	380
	21st to end	Night	8% 6% 17% 43% 26% 1	380
July	1st to 10th	Night	3 5% 11% 48% 32% 2	370
	11th to 20th	Night	1 4 14% 55% 24% 1	368
	21st to end	Night	2 3 9% 48% 38% 1	416
August	1st to 10th	Night	2 2 11% 36% 43% 6%	528
	11th to 20th	Night	3 3 11% 33% 44% 7%	564
	21st to end	Night	3 6% 19% 42% 28% 2	822
September	1st to 10th	Night	3 8% 17% 38% 30% 4	760
	11th to 20th	Night	9% 9% 15% 32% 32% 4	760
	21st to end	Night	9% 12% 18% 32% 25% 4	760
October	1st to 10th	Night	11% 13% 21% 31% 21% 4	760
	11th to 20th	Night	12% 18% 16% 29% 20% 5%	760
	21st to end	Night	15% 24% 21% 22% 13% 5%	836
November	1st to 10th	Night	16% 29% 19% 25% 10% 1	780
	11th to 20th	Night	15% 31% 22% 19% 12% 1	762
	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 2024-11-08 03:44:47. Version ac07

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