Dee-Top Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dee-Top Observatory clear sky chart from 2005-11-06 to 2024-11-06.

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	16% 11% 4 8% 5% 5% 4 4 5% 7% 30%	101475
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	97606
Transparency	Day	51% 6% 11% 20% 10% 1	105010
	Night	52% 6% 12% 19% 10% 1	102160
Seeing	Day	25% 17% 28% 18% 9% 3	14792
	Night	23% 17% 28% 23% 9% 1	23987

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% 17% 13% 12% 5% 4 4 2 3 4 22%	2517
	11th to 20th	Day	14% 15% 9% 16% 7% 6% 4 4 4 4 18%	2520
	21st to end	Day	19% 17% 9% 11% 8% 5% 4 3 4 5% 16%	3098
February	1st to 10th	Day	15% 17% 6% 11% 8% 5% 4 4 5% 5% 19%	1710
	11th to 20th	Day	13% 14% 6% 12% 7% 5% 5% 4 5% 5% 24%	1740
	21st to end	Day	18% 13% 5% 13% 7% 3 4 3 4 5% 25%	1570
March	1st to 10th	Day	22% 18% 5% 10% 5% 3 3 2 3 4 26%	3143
	11th to 20th	Day	11% 17% 6% 10% 5% 4 4 3 4 6% 29%	3480
	21st to end	Day	19% 12% 4 9% 5% 4 4 3 4 4 31%	4024
April	1st to 10th	Day	17% 15% 3 8% 4 4 3 4 5% 6% 31%	2280
	11th to 20th	Day	16% 10% 4 9% 5% 4 4 4 5% 5% 33%	2254
	21st to end	Day	18% 13% 4 7% 4 4 4 3 5% 6% 33%	2280
May	1st to 10th	Day	26% 9% 2 6% 4 4 4 3 5% 6% 30%	4291
	11th to 20th	Day	20% 9% 3 9% 6% 5% 5% 5% 5% 7% 26%	4398
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 9% 40%	4837
June	1st to 10th	Day	17% 6% 2 7% 6% 5% 4 5% 6% 9% 32%	2660
	11th to 20th	Day	16% 7% 3 6% 6% 5% 5% 5% 6% 10% 33%	2660
	21st to end	Day	14% 8% 3 6% 5% 5% 5% 5% 6% 9% 35%	2660
July	1st to 10th	Day	9% 6% 3 7% 6% 5% 6% 5% 8% 11% 34%	4440
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 11% 35%	4440
	21st to end	Day	12% 8% 3 8% 5% 6% 6% 5% 7% 9% 32%	4814
August	1st to 10th	Day	20% 7% 3 8% 5% 5% 5% 4 6% 7% 30%	4118
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 9% 31%	3855
	21st to end	Day	16% 8% 2 6% 4 4 4 4 6% 8% 38%	4180
September	1st to 10th	Day	17% 8% 3 5% 4 4 4 4 5% 7% 40%	2280
	11th to 20th	Day	14% 7% 2 5% 4 4 4 4 5% 7% 44%	2271
	21st to end	Day	18% 9% 3 7% 4 4 5% 4 6% 7% 34%	2049
October	1st to 10th	Day	17% 9% 3 8% 4 4 5% 3 5% 7% 33%	1900
	11th to 20th	Day	16% 12% 4 7% 5% 4 4 3 6% 6% 34%	1900
	21st to end	Day	22% 13% 3 7% 5% 5% 4 3 5% 6% 27%	2003
November	1st to 10th	Day	14% 11% 4 7% 5% 4 4 3 5% 6% 38%	1717
	11th to 20th	Day	15% 16% 4 10% 5% 3 3 3 4 4 31%	1668
	21st to end	Day	17% 16% 4 9% 5% 5% 4 4 5% 6% 25%	1520
December	1st to 10th	Day	17% 17% 7% 11% 5% 4 5% 3 4 4 20%	1520
	11th to 20th	Day	21% 16% 8% 11% 4 2 3 2 3 5% 25%	1906
	21st to end	Day	23% 19% 7% 11% 6% 5% 4 3 3 4 15%	2772
January	1st to 10th	Night	15% 14% 8% 14% 6% 4 4 3 5% 6% 21%	4429
	11th to 20th	Night	12% 16% 7% 16% 7% 5% 4 2 4 5% 20%	4440
	21st to end	Night	22% 16% 6% 14% 9% 4 3 2 3 3 17%	4555
February	1st to 10th	Night	15% 14% 5% 11% 7% 5% 5% 4 4 6% 24%	2470
	11th to 20th	Night	13% 14% 6% 13% 8% 5% 6% 5% 5% 5% 22%	2324
	21st to end	Night	19% 13% 3 11% 6% 4 4 3 4 6% 27%	1884
March	1st to 10th	Night	21% 15% 5% 10% 5% 4 3 3 4 5% 28%	3693
	11th to 20th	Night	13% 17% 6% 9% 6% 4 4 3 4 5% 28%	3479
	21st to end	Night	15% 15% 3 9% 5% 4 4 4 4 4 33%	3608
April	1st to 10th	Night	19% 13% 4 8% 5% 3 4 4 4 6% 30%	1900
	11th to 20th	Night	20% 10% 3 7% 5% 3 3 3 5% 6% 35%	1875
	21st to end	Night	17% 10% 3 7% 5% 4 4 2 5% 7% 36%	1900
May	1st to 10th	Night	28% 8% 3 6% 4 3 4 3 4 5% 32%	2678
	11th to 20th	Night	19% 9% 4 7% 5% 4 4 4 5% 7% 32%	2495
	21st to end	Night	12% 8% 3 7% 5% 4 5% 4 6% 8% 38%	2738
June	1st to 10th	Night	16% 7% 3 7% 4 3 3 3 6% 10% 38%	1520
	11th to 20th	Night	14% 7% 3 5% 4 4 5% 4 5% 8% 41%	1520
	21st to end	Night	13% 4 3 5% 4 5% 5% 4 6% 9% 41%	1520
July	1st to 10th	Night	9% 7% 3 7% 6% 5% 5% 4 6% 8% 39%	2520
	11th to 20th	Night	11% 6% 3 5% 5% 5% 4 4 7% 10% 41%	2520
	21st to end	Night	15% 7% 2 7% 5% 4 5% 4 6% 7% 39%	2739
August	1st to 10th	Night	20% 8% 3 7% 4 5% 5% 3 5% 8% 32%	2807
	11th to 20th	Night	16% 8% 3 7% 4 5% 4 3 5% 5% 40%	2940
	21st to end	Night	13% 7% 3 5% 4 4 4 4 5% 7% 44%	3476
September	1st to 10th	Night	14% 7% 3 6% 5% 3 4 3 5% 7% 44%	1900
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 4 6% 51%	1908
	21st to end	Night	17% 8% 3 7% 4 4 4 3 5% 6% 39%	2128
October	1st to 10th	Night	18% 9% 2 7% 4 5% 4 3 5% 6% 36%	2280
	11th to 20th	Night	19% 9% 2 6% 4 3 4 3 5% 6% 39%	2280
	21st to end	Night	23% 12% 4 7% 5% 4 4 3 4 7% 29%	2593
November	1st to 10th	Night	12% 12% 3 7% 4 4 4 4 5% 6% 39%	2483
	11th to 20th	Night	18% 12% 3 7% 4 4 4 3 6% 6% 31%	2512
	21st to end	Night	14% 13% 5% 7% 6% 4 5% 4 6% 7% 28%	2660
December	1st to 10th	Night	18% 17% 6% 10% 6% 4 4 4 5% 5% 21%	2660
	11th to 20th	Night	19% 14% 5% 11% 6% 4 5% 4 4 5% 23%	3288
	21st to end	Night	24% 15% 8% 10% 4 4 4 3 4 5% 19%	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% 6% 9% 16% 4 1	2518
	11th to 20th	Day	69% 5% 8% 12% 5% 1	2520
	21st to end	Day	70% 5% 9% 12% 4 1	3098
February	1st to 10th	Day	65% 6% 8% 14% 6% 1	1710
	11th to 20th	Day	60% 6% 7% 20% 7%	1749
	21st to end	Day	61% 5% 9% 17% 8% 1	1570
March	1st to 10th	Day	63% 4 10% 16% 7% 1	3146
	11th to 20th	Day	54% 7% 12% 18% 8% 1	3480
	21st to end	Day	54% 5% 8% 15% 16% 2	4024
April	1st to 10th	Day	55% 4 7% 22% 9% 3	2280
	11th to 20th	Day	51% 6% 10% 18% 13% 2	2254
	21st to end	Day	52% 5% 9% 19% 12% 4	2280
May	1st to 10th	Day	54% 6% 10% 18% 10% 2	4305
	11th to 20th	Day	55% 8% 11% 19% 7% 1	4402
	21st to end	Day	40% 7% 12% 25% 15% 1	4837
June	1st to 10th	Day	48% 7% 12% 20% 13%	2660
	11th to 20th	Day	46% 8% 13% 21% 10% 1	2660
	21st to end	Day	45% 7% 13% 25% 10% 1	2660
July	1st to 10th	Day	41% 9% 16% 23% 10% 1	4370
	11th to 20th	Day	39% 10% 16% 25% 10%	4365
	21st to end	Day	46% 8% 14% 19% 11% 1	4661
August	1st to 10th	Day	53% 7% 13% 18% 9%	4104
	11th to 20th	Day	46% 8% 15% 24% 6%	3894
	21st to end	Day	42% 7% 11% 32% 8% 1	4180
September	1st to 10th	Day	44% 5% 12% 25% 12% 1	2280
	11th to 20th	Day	39% 6% 11% 22% 21% 1	2271
	21st to end	Day	49% 5% 11% 21% 13% 1	2049
October	1st to 10th	Day	47% 6% 11% 22% 14% 1	3160
	11th to 20th	Day	44% 5% 8% 24% 14% 4	3140
	21st to end	Day	41% 5% 16% 26% 12% 1	3280
November	1st to 10th	Day	47% 5% 13% 21% 13% 2	1717
	11th to 20th	Day	57% 4 10% 18% 9% 2	1668
	21st to end	Day	58% 6% 14% 16% 5% 1	1520
December	1st to 10th	Day	65% 6% 13% 10% 5% 1	1520
	11th to 20th	Day	64% 4 13% 15% 4	1906
	21st to end	Day	72% 5% 7% 11% 4	2772
January	1st to 10th	Night	61% 8% 12% 14% 5% 1	4429
	11th to 20th	Night	65% 6% 12% 13% 4 1	4440
	21st to end	Night	73% 4 6% 12% 5% 1	4555
February	1st to 10th	Night	59% 5% 12% 16% 8%	2470
	11th to 20th	Night	61% 7% 10% 15% 6% 1	2326
	21st to end	Night	58% 4 11% 16% 9% 1	1884
March	1st to 10th	Night	60% 4 10% 17% 9% 1	3703
	11th to 20th	Night	57% 6% 12% 19% 6% 1	3480
	21st to end	Night	52% 5% 9% 18% 15% 2	3608
April	1st to 10th	Night	57% 5% 8% 18% 11% 2	1900
	11th to 20th	Night	50% 5% 10% 19% 15%	1875
	21st to end	Night	48% 6% 10% 20% 13% 3	1900
May	1st to 10th	Night	54% 5% 8% 21% 9% 2	2684
	11th to 20th	Night	51% 8% 14% 18% 7% 1	2496
	21st to end	Night	42% 8% 14% 20% 14% 1	2739
June	1st to 10th	Night	42% 5% 13% 24% 15% 1	1520
	11th to 20th	Night	40% 7% 16% 25% 10% 1	1520
	21st to end	Night	38% 9% 15% 25% 13% 1	1520
July	1st to 10th	Night	41% 9% 14% 19% 15% 1	2480
	11th to 20th	Night	37% 8% 15% 26% 13%	2472
	21st to end	Night	43% 7% 13% 20% 16% 1	2651
August	1st to 10th	Night	51% 8% 15% 19% 7%	2797
	11th to 20th	Night	45% 7% 14% 26% 9%	2967
	21st to end	Night	39% 6% 11% 32% 11% 1	3476
September	1st to 10th	Night	41% 6% 13% 25% 14% 1	1900
	11th to 20th	Night	35% 5% 11% 27% 20% 1	1908
	21st to end	Night	46% 5% 11% 22% 15% 1	2128
October	1st to 10th	Night	50% 6% 10% 22% 11% 1	3800
	11th to 20th	Night	42% 5% 13% 19% 16% 5%	3779
	21st to end	Night	43% 5% 16% 25% 11% 1	4266
November	1st to 10th	Night	45% 6% 12% 20% 15% 2	2483
	11th to 20th	Night	51% 6% 12% 18% 11% 2	2512
	21st to end	Night	53% 6% 15% 19% 6%	2660
December	1st to 10th	Night	64% 7% 11% 14% 4	2660
	11th to 20th	Night	61% 6% 12% 15% 6%	3288
	21st to end	Night	67% 6% 9% 14% 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% 25% 31% 11% 1 1	180
	11th to 20th	Day	42% 24% 26% 6% 1 1	180
	21st to end	Day	26% 30% 32% 8% 3	292
February	1st to 10th	Day	33% 34% 26% 7%	270
	11th to 20th	Day	38% 29% 26% 5% 1	347
	21st to end	Day	36% 24% 28% 10% 1	382
March	1st to 10th	Day	36% 19% 29% 13% 3	460
	11th to 20th	Day	32% 27% 25% 13% 3	460
	21st to end	Day	35% 25% 26% 9% 4 1	506
April	1st to 10th	Day	36% 21% 29% 10% 4 1	460
	11th to 20th	Day	29% 26% 27% 9% 7% 2	452
	21st to end	Day	30% 20% 31% 12% 5% 2	460
May	1st to 10th	Day	35% 17% 27% 10% 8% 3	463
	11th to 20th	Day	22% 16% 34% 18% 7% 3	487
	21st to end	Day	17% 14% 31% 23% 11% 4	529
June	1st to 10th	Day	21% 13% 32% 21% 10% 2	490
	11th to 20th	Day	19% 15% 26% 25% 13% 2	490
	21st to end	Day	18% 15% 33% 23% 8% 2	490
July	1st to 10th	Day	14% 7% 28% 30% 15% 5%	485
	11th to 20th	Day	11% 10% 27% 30% 17% 5%	484
	21st to end	Day	16% 9% 35% 27% 10% 4	532
August	1st to 10th	Day	15% 11% 25% 25% 18% 7%	486
	11th to 20th	Day	18% 8% 26% 25% 17% 6%	479
	21st to end	Day	16% 10% 27% 23% 14% 9%	539
September	1st to 10th	Day	19% 11% 25% 20% 17% 7%	490
	11th to 20th	Day	16% 9% 26% 22% 17% 11%	490
	21st to end	Day	25% 12% 28% 20% 11% 5%	490
October	1st to 10th	Day	24% 13% 24% 21% 13% 5%	490
	11th to 20th	Day	26% 17% 30% 19% 6% 2	490
	21st to end	Day	33% 18% 25% 17% 6%	492
November	1st to 10th	Day	28% 19% 23% 20% 9% 2	381
	11th to 20th	Day	34% 23% 22% 13% 6%	328
	21st to end	Day	36% 23% 33% 8% 1	180
December	1st to 10th	Day	33% 31% 26% 5% 4 2	180
	11th to 20th	Day	37% 25% 25% 12% 1	180
	21st to end	Day	34% 29% 26% 9% 2 1	198
January	1st to 10th	Night	25% 27% 32% 13% 3	760
	11th to 20th	Night	28% 23% 28% 18% 3	760
	21st to end	Night	20% 27% 35% 16% 2	836
February	1st to 10th	Night	27% 21% 33% 18% 1	760
	11th to 20th	Night	29% 27% 31% 12% 1	743
	21st to end	Night	32% 22% 30% 12% 3	628
March	1st to 10th	Night	27% 25% 29% 17% 3	760
	11th to 20th	Night	28% 24% 28% 17% 3	760
	21st to end	Night	26% 23% 30% 15% 6%	836
April	1st to 10th	Night	28% 22% 29% 18% 3	760
	11th to 20th	Night	25% 20% 31% 18% 6%	751
	21st to end	Night	25% 18% 30% 22% 5%	760
May	1st to 10th	Night	30% 13% 25% 24% 9%	479
	11th to 20th	Night	17% 11% 33% 29% 7% 1	374
	21st to end	Night	15% 8% 29% 35% 13% 1	411
June	1st to 10th	Night	17% 7% 27% 36% 12% 1	380
	11th to 20th	Night	17% 8% 25% 36% 13% 1	380
	21st to end	Night	14% 6% 26% 44% 10%	380
July	1st to 10th	Night	11% 4 22% 41% 21% 1	370
	11th to 20th	Night	13% 1 23% 43% 18% 1	368
	21st to end	Night	13% 6% 22% 41% 17% 1	416
August	1st to 10th	Night	13% 4 22% 36% 23% 2	549
	11th to 20th	Night	16% 5% 13% 39% 23% 4	654
	21st to end	Night	15% 6% 19% 34% 24% 2	836
September	1st to 10th	Night	18% 8% 23% 33% 16% 3	760
	11th to 20th	Night	15% 7% 20% 29% 24% 5%	760
	21st to end	Night	21% 9% 24% 28% 15% 2	760
October	1st to 10th	Night	23% 12% 25% 23% 16% 1	760
	11th to 20th	Night	25% 13% 29% 25% 8% 1	760
	21st to end	Night	31% 15% 29% 19% 6%	836
November	1st to 10th	Night	21% 16% 32% 23% 8%	764
	11th to 20th	Night	28% 20% 28% 18% 5% 1	760
	21st to end	Night	23% 22% 31% 20% 4	760
December	1st to 10th	Night	29% 23% 30% 15% 4	760
	11th to 20th	Night	27% 24% 28% 19% 2	760
	21st to end	Night	28% 25% 32% 13% 3	836

Page generated at 2024-11-03 13:19:04. Version ac07

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