Whatt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whatt Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	12% 9% 3 9% 5% 5% 5% 4 5% 7% 36%	138620
	Night	12% 11% 4 10% 6% 5% 5% 4 5% 6% 32%	134414
Transparency	Day	47% 6% 13% 23% 10% 1	142071
	Night	52% 6% 12% 20% 9% 1	139049
Seeing	Day	17% 14% 34% 24% 10% 2	18458
	Night	18% 12% 32% 27% 10% 1	28332

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	6% 14% 7% 18% 10% 7% 5% 3 5% 6% 19%	4691
	11th to 20th	Day	6% 15% 5% 13% 9% 10% 9% 7% 6% 5% 13%	4679
	21st to end	Day	5% 11% 6% 15% 7% 8% 7% 4 8% 6% 22%	5272
February	1st to 10th	Day	7% 14% 6% 12% 6% 5% 6% 5% 5% 5% 29%	1890
	11th to 20th	Day	8% 13% 5% 11% 7% 5% 5% 4 5% 4 34%	1844
	21st to end	Day	8% 13% 5% 13% 7% 7% 5% 4 5% 6% 28%	1708
March	1st to 10th	Day	13% 9% 2 7% 5% 4 4 4 5% 8% 39%	4578
	11th to 20th	Day	9% 11% 5% 11% 6% 5% 5% 4 5% 7% 32%	5085
	21st to end	Day	21% 12% 4 10% 4 3 3 2 4 5% 31%	5902
April	1st to 10th	Day	16% 13% 4 10% 6% 4 4 4 5% 5% 30%	2520
	11th to 20th	Day	18% 12% 4 7% 5% 5% 4 3 5% 5% 32%	2494
	21st to end	Day	20% 11% 3 7% 5% 4 3 3 4 5% 34%	2724
May	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 49%	8241
	11th to 20th	Day	14% 9% 2 6% 4 4 4 3 5% 6% 41%	8222
	21st to end	Day	12% 7% 3 7% 4 4 4 3 5% 7% 45%	9061
June	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 7% 9% 42%	2940
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	2940
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 9% 38%	2940
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 6% 9% 47%	6499
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 6% 9% 48%	6431
	21st to end	Day	9% 5% 2 5% 3 3 4 4 6% 9% 51%	6926
August	1st to 10th	Day	10% 6% 2 8% 5% 5% 6% 4 7% 8% 40%	4563
	11th to 20th	Day	14% 4 2 5% 3 3 3 3 5% 7% 50%	4171
	21st to end	Day	13% 6% 3 6% 4 4 4 4 6% 8% 43%	4346
September	1st to 10th	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 44%	2400
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 5% 9% 42%	2400
	21st to end	Day	16% 9% 3 7% 4 4 4 3 4 6% 41%	2126
October	1st to 10th	Day	17% 8% 2 7% 4 4 4 4 5% 6% 39%	2000
	11th to 20th	Day	17% 14% 4 8% 4 5% 5% 3 4 7% 29%	2000
	21st to end	Day	17% 14% 4 9% 5% 5% 4 3 5% 5% 28%	2018
November	1st to 10th	Day	12% 14% 5% 9% 6% 5% 5% 3 5% 7% 29%	1805
	11th to 20th	Day	12% 17% 6% 13% 7% 5% 6% 3 4 5% 23%	1680
	21st to end	Day	14% 13% 5% 15% 8% 5% 4 4 6% 4 22%	1680
December	1st to 10th	Day	8% 15% 7% 26% 10% 6% 5% 4 4 5% 12%	2690
	11th to 20th	Day	9% 16% 7% 15% 9% 6% 7% 3 4 5% 19%	3084
	21st to end	Day	20% 19% 6% 14% 6% 5% 4 3 4 3 16%	4070
January	1st to 10th	Night	8% 14% 6% 17% 9% 6% 5% 3 4 6% 21%	8233
	11th to 20th	Night	7% 15% 7% 16% 9% 8% 9% 6% 5% 4 13%	8236
	21st to end	Night	6% 12% 5% 12% 9% 9% 6% 5% 8% 6% 22%	8938
February	1st to 10th	Night	7% 15% 5% 12% 7% 5% 5% 4 5% 6% 28%	2730
	11th to 20th	Night	9% 10% 3 10% 8% 7% 6% 5% 5% 7% 31%	2663
	21st to end	Night	10% 13% 4 10% 6% 5% 4 4 5% 7% 30%	2097
March	1st to 10th	Night	10% 8% 3 7% 4 4 4 4 5% 7% 45%	5482
	11th to 20th	Night	11% 11% 5% 11% 7% 5% 4 4 6% 7% 29%	5137
	21st to end	Night	22% 13% 4 9% 5% 4 4 3 4 4 29%	5271
April	1st to 10th	Night	19% 13% 4 7% 5% 5% 4 3 4 6% 30%	2100
	11th to 20th	Night	19% 10% 4 8% 4 3 4 3 6% 5% 33%	2075
	21st to end	Night	20% 10% 2 6% 4 3 4 3 5% 5% 36%	1892
May	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 5% 7% 44%	4688
	11th to 20th	Night	18% 7% 3 7% 4 4 4 3 4 5% 43%	4670
	21st to end	Night	10% 7% 2 7% 4 4 5% 3 5% 8% 44%	5148
June	1st to 10th	Night	12% 6% 3 6% 5% 5% 5% 4 6% 7% 41%	1680
	11th to 20th	Night	12% 6% 2 7% 4 5% 5% 5% 7% 7% 39%	1680
	21st to end	Night	13% 7% 2 6% 4 4 4 3 7% 7% 42%	1680
July	1st to 10th	Night	8% 6% 3 5% 4 4 5% 4 6% 8% 46%	3700
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 6% 8% 52%	3660
	21st to end	Night	10% 6% 2 6% 4 3 5% 3 5% 7% 48%	3948
August	1st to 10th	Night	11% 6% 2 6% 4 4 5% 3 6% 7% 45%	2597
	11th to 20th	Night	14% 4 2 5% 2 3 3 3 5% 8% 51%	2884
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 43%	3552
September	1st to 10th	Night	11% 4 2 6% 4 4 5% 3 5% 7% 49%	2000
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 5% 7% 46%	2000
	21st to end	Night	16% 7% 3 6% 3 4 3 2 5% 8% 42%	2249
October	1st to 10th	Night	16% 8% 2 6% 4 3 4 3 6% 7% 43%	2400
	11th to 20th	Night	16% 11% 4 7% 5% 5% 4 4 5% 6% 33%	2400
	21st to end	Night	19% 12% 3 8% 5% 4 5% 4 5% 6% 28%	2821
November	1st to 10th	Night	14% 13% 4 8% 6% 5% 4 4 5% 7% 30%	2710
	11th to 20th	Night	12% 14% 6% 11% 6% 5% 6% 4 5% 5% 26%	2940
	21st to end	Night	13% 12% 5% 13% 8% 5% 5% 3 5% 6% 24%	2940
December	1st to 10th	Night	10% 14% 7% 22% 9% 6% 5% 3 4 5% 15%	4720
	11th to 20th	Night	10% 16% 6% 15% 7% 7% 7% 3 5% 6% 18%	5343
	21st to end	Night	23% 18% 6% 11% 5% 4 4 3 4 5% 18%	7150

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	67% 6% 10% 13% 4	4692
	11th to 20th	Day	71% 7% 9% 10% 3	4710
	21st to end	Day	60% 6% 11% 18% 5%	5272
February	1st to 10th	Day	57% 7% 11% 15% 10%	1890
	11th to 20th	Day	55% 7% 9% 21% 8%	1853
	21st to end	Day	57% 6% 10% 15% 10% 1	1708
March	1st to 10th	Day	43% 6% 16% 21% 10% 2	4580
	11th to 20th	Day	51% 5% 15% 17% 11% 1	5085
	21st to end	Day	56% 5% 11% 17% 11%	5902
April	1st to 10th	Day	57% 5% 9% 17% 11% 1	2520
	11th to 20th	Day	54% 5% 10% 19% 10% 1	2494
	21st to end	Day	54% 5% 10% 21% 10% 1	2724
May	1st to 10th	Day	35% 6% 11% 29% 18%	8250
	11th to 20th	Day	43% 5% 11% 23% 12% 5%	8226
	21st to end	Day	38% 5% 10% 27% 18% 1	9061
June	1st to 10th	Day	38% 7% 14% 24% 15% 1	2940
	11th to 20th	Day	43% 7% 14% 22% 14% 1	2940
	21st to end	Day	41% 8% 16% 25% 9% 1	2940
July	1st to 10th	Day	32% 7% 19% 31% 10% 1	6431
	11th to 20th	Day	31% 7% 18% 32% 11% 1	6356
	21st to end	Day	29% 6% 17% 32% 16% 1	6728
August	1st to 10th	Day	40% 8% 16% 24% 11% 1	4548
	11th to 20th	Day	33% 5% 20% 32% 10%	4210
	21st to end	Day	37% 7% 17% 27% 10% 1	4345
September	1st to 10th	Day	38% 6% 16% 30% 10%	2400
	11th to 20th	Day	39% 8% 18% 27% 7%	2400
	21st to end	Day	45% 7% 12% 26% 10%	2126
October	1st to 10th	Day	36% 4 16% 30% 12% 1	3260
	11th to 20th	Day	49% 5% 10% 24% 11%	3240
	21st to end	Day	52% 6% 14% 22% 6%	3230
November	1st to 10th	Day	56% 5% 13% 17% 8% 1	1805
	11th to 20th	Day	66% 4 11% 14% 5%	1680
	21st to end	Day	65% 6% 12% 13% 3 1	1680
December	1st to 10th	Day	77% 5% 8% 9% 1	2690
	11th to 20th	Day	68% 6% 9% 11% 5%	3085
	21st to end	Day	74% 5% 7% 13% 1	4070
January	1st to 10th	Night	65% 6% 10% 14% 4	8233
	11th to 20th	Night	74% 7% 9% 8% 2	8280
	21st to end	Night	59% 8% 10% 16% 7%	8938
February	1st to 10th	Night	58% 6% 10% 18% 8% 1	2730
	11th to 20th	Night	53% 8% 12% 19% 8% 1	2664
	21st to end	Night	54% 6% 11% 20% 7% 1	2097
March	1st to 10th	Night	39% 5% 16% 21% 17% 2	5493
	11th to 20th	Night	54% 7% 12% 20% 7%	5138
	21st to end	Night	60% 5% 8% 15% 12% 1	5271
April	1st to 10th	Night	56% 5% 8% 18% 13% 1	2100
	11th to 20th	Night	51% 5% 10% 21% 11% 2	2075
	21st to end	Night	50% 4 8% 24% 13% 1	1892
May	1st to 10th	Night	38% 7% 12% 27% 16% 1	4695
	11th to 20th	Night	44% 5% 8% 24% 13% 6%	4671
	21st to end	Night	37% 6% 11% 28% 16% 1	5149
June	1st to 10th	Night	40% 7% 14% 25% 12% 2	1680
	11th to 20th	Night	41% 8% 13% 21% 17% 2	1680
	21st to end	Night	41% 7% 13% 27% 12% 1	1680
July	1st to 10th	Night	35% 8% 19% 27% 9% 2	3660
	11th to 20th	Night	29% 7% 19% 32% 11% 1	3612
	21st to end	Night	35% 6% 14% 26% 17% 1	3832
August	1st to 10th	Night	37% 6% 14% 27% 14% 2	2588
	11th to 20th	Night	33% 6% 18% 31% 11% 1	2911
	21st to end	Night	41% 7% 16% 25% 10% 1	3552
September	1st to 10th	Night	34% 6% 15% 32% 12% 1	2000
	11th to 20th	Night	36% 7% 17% 33% 8%	2000
	21st to end	Night	42% 7% 15% 27% 10%	2249
October	1st to 10th	Night	31% 4 11% 32% 20% 1	3920
	11th to 20th	Night	48% 5% 12% 22% 13% 1	3899
	21st to end	Night	54% 5% 10% 23% 7%	4559
November	1st to 10th	Night	54% 5% 13% 20% 7% 1	2710
	11th to 20th	Night	62% 5% 13% 14% 6% 1	2940
	21st to end	Night	62% 6% 15% 12% 4 1	2940
December	1st to 10th	Night	73% 6% 11% 9% 1	4720
	11th to 20th	Night	69% 6% 10% 13% 2	5341
	21st to end	Night	70% 6% 9% 13% 3	7150

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	14% 18% 40% 22% 6% 2	330
	11th to 20th	Day	17% 19% 35% 24% 5%	330
	21st to end	Day	16% 15% 41% 23% 4 1	363
February	1st to 10th	Day	15% 20% 42% 17% 4 1	330
	11th to 20th	Day	14% 24% 39% 15% 6% 1	330
	21st to end	Day	18% 21% 47% 12% 1	273
March	1st to 10th	Day	19% 18% 39% 19% 5%	330
	11th to 20th	Day	17% 17% 46% 12% 7%	330
	21st to end	Day	24% 18% 33% 17% 6% 2	499
April	1st to 10th	Day	21% 22% 32% 19% 4 1	540
	11th to 20th	Day	24% 19% 30% 19% 8%	532
	21st to end	Day	26% 13% 31% 22% 8% 1	658
May	1st to 10th	Day	22% 14% 33% 23% 7% 2	753
	11th to 20th	Day	17% 18% 33% 22% 8% 2	775
	21st to end	Day	18% 11% 32% 27% 10% 3	844
June	1st to 10th	Day	12% 13% 35% 28% 10% 2	780
	11th to 20th	Day	16% 13% 36% 27% 6% 2	780
	21st to end	Day	16% 13% 31% 25% 13% 2	780
July	1st to 10th	Day	8% 5% 31% 33% 18% 5%	771
	11th to 20th	Day	9% 8% 33% 30% 16% 4	744
	21st to end	Day	9% 6% 33% 31% 16% 6%	794
August	1st to 10th	Day	12% 8% 27% 33% 16% 3	726
	11th to 20th	Day	13% 7% 25% 33% 18% 4	715
	21st to end	Day	16% 9% 27% 28% 16% 3	604
September	1st to 10th	Day	13% 13% 30% 25% 16% 3	530
	11th to 20th	Day	13% 10% 31% 31% 13% 3	530
	21st to end	Day	20% 14% 26% 24% 13% 3	451
October	1st to 10th	Day	21% 15% 28% 24% 8% 3	330
	11th to 20th	Day	23% 21% 35% 16% 3 1	330
	21st to end	Day	24% 21% 37% 13% 4 1	363
November	1st to 10th	Day	22% 21% 38% 14% 5%	330
	11th to 20th	Day	22% 26% 36% 12% 4 1	330
	21st to end	Day	22% 21% 38% 15% 2 1	330
December	1st to 10th	Day	15% 20% 42% 18% 5% 1	330
	11th to 20th	Day	21% 22% 38% 15% 3	330
	21st to end	Day	25% 18% 38% 15% 3 1	363
January	1st to 10th	Night	15% 15% 40% 24% 6%	1050
	11th to 20th	Night	19% 12% 41% 21% 6% 1	1050
	21st to end	Night	16% 14% 40% 24% 5%	1155
February	1st to 10th	Night	15% 16% 39% 23% 6% 1	1050
	11th to 20th	Night	13% 17% 37% 24% 8% 1	1029
	21st to end	Night	19% 12% 36% 27% 6%	713
March	1st to 10th	Night	16% 13% 34% 30% 5% 1	840
	11th to 20th	Night	18% 16% 34% 24% 7% 1	652
	21st to end	Night	20% 10% 31% 26% 10% 3	693
April	1st to 10th	Night	27% 10% 30% 26% 6% 1	630
	11th to 20th	Night	26% 10% 24% 29% 9%	623
	21st to end	Night	26% 9% 23% 30% 12% 1	630
May	1st to 10th	Night	19% 9% 31% 30% 9% 2	630
	11th to 20th	Night	17% 10% 28% 33% 10% 2	621
	21st to end	Night	18% 6% 24% 36% 16% 1	683
June	1st to 10th	Night	14% 6% 28% 37% 15% 1	630
	11th to 20th	Night	16% 7% 26% 38% 12% 1	630
	21st to end	Night	15% 7% 24% 36% 15% 2	630
July	1st to 10th	Night	6% 5% 23% 40% 24% 2	615
	11th to 20th	Night	11% 4 27% 39% 18% 1	597
	21st to end	Night	10% 4 25% 40% 19% 3	657
August	1st to 10th	Night	11% 4 22% 41% 20% 2	597
	11th to 20th	Night	13% 5% 21% 35% 21% 3	594
	21st to end	Night	13% 7% 23% 32% 22% 2	660
September	1st to 10th	Night	12% 8% 25% 33% 20% 2	600
	11th to 20th	Night	12% 7% 27% 35% 17% 2	600
	21st to end	Night	19% 9% 29% 28% 13% 1	774
October	1st to 10th	Night	21% 11% 28% 26% 13% 1	800
	11th to 20th	Night	23% 17% 32% 21% 6%	800
	21st to end	Night	27% 12% 29% 26% 6%	880
November	1st to 10th	Night	23% 13% 35% 24% 5% 1	864
	11th to 20th	Night	20% 19% 34% 18% 8%	1050
	21st to end	Night	21% 18% 37% 20% 4	1050
December	1st to 10th	Night	18% 16% 40% 20% 5% 1	1050
	11th to 20th	Night	20% 20% 32% 22% 5% 1	1050
	21st to end	Night	28% 14% 35% 18% 4	1155

Page generated at 2026-07-13 02:14:53. Version ac07

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