DeJoVu Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the DeJoVu Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	131966
	Night	12% 11% 5% 11% 6% 5% 5% 4 5% 6% 29%	130451
Transparency	Day	51% 4 13% 22% 10% 1	135432
	Night	55% 3 11% 20% 9% 1	135071
Seeing	Day	20% 16% 34% 20% 8% 2	18080
	Night	19% 14% 34% 25% 9% 1	28141

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	5% 12% 6% 20% 10% 6% 6% 4 4 5% 21%	4124
	11th to 20th	Day	6% 15% 7% 15% 9% 7% 10% 8% 9% 3 11%	4604
	21st to end	Day	4 12% 9% 16% 7% 5% 6% 6% 7% 6% 20%	5353
February	1st to 10th	Day	7% 16% 6% 12% 5% 6% 8% 6% 7% 5% 22%	1890
	11th to 20th	Day	9% 10% 4 10% 8% 7% 5% 4 6% 7% 29%	1844
	21st to end	Day	7% 15% 5% 11% 7% 5% 5% 3 4 6% 31%	1619
March	1st to 10th	Day	9% 11% 4 9% 4 5% 4 4 4 7% 39%	4737
	11th to 20th	Day	7% 10% 7% 11% 6% 6% 5% 4 6% 7% 29%	5090
	21st to end	Day	18% 13% 5% 11% 4 3 4 2 4 5% 31%	5768
April	1st to 10th	Day	15% 13% 4 8% 6% 5% 5% 4 5% 6% 28%	2520
	11th to 20th	Day	16% 13% 3 8% 5% 5% 5% 5% 5% 7% 28%	2478
	21st to end	Day	19% 12% 3 8% 5% 4 4 3 5% 7% 30%	2647
May	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 4 6% 46%	6348
	11th to 20th	Day	13% 9% 3 7% 5% 4 5% 4 6% 7% 38%	6319
	21st to end	Day	13% 8% 3 6% 4 4 4 3 5% 7% 42%	6949
June	1st to 10th	Day	13% 6% 3 7% 4 4 5% 4 6% 9% 39%	2879
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 4 6% 8% 35%	3000
	21st to end	Day	17% 6% 2 6% 4 5% 5% 4 7% 10% 34%	3000
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 8% 11% 39%	6799
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 9% 41%	6585
	21st to end	Day	10% 6% 2 6% 5% 4 5% 4 6% 9% 41%	6926
August	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 5% 8% 8% 37%	4499
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 9% 43%	4171
	21st to end	Day	15% 7% 2 7% 4 3 4 3 6% 9% 40%	4487
September	1st to 10th	Day	13% 8% 3 6% 5% 4 6% 4 5% 7% 40%	2400
	11th to 20th	Day	15% 9% 3 6% 5% 5% 4 4 6% 8% 37%	2400
	21st to end	Day	18% 9% 3 6% 4 4 4 3 5% 6% 38%	2097
October	1st to 10th	Day	17% 11% 3 6% 4 4 4 3 6% 8% 34%	2000
	11th to 20th	Day	18% 17% 4 9% 4 4 5% 3 5% 5% 26%	2000
	21st to end	Day	20% 18% 5% 9% 4 4 3 2 3 5% 26%	2058
November	1st to 10th	Day	13% 17% 7% 10% 6% 5% 4 3 5% 5% 24%	1682
	11th to 20th	Day	15% 17% 6% 15% 7% 4 3 4 3 4 22%	1680
	21st to end	Day	15% 16% 7% 18% 9% 5% 4 3 4 4 16%	1680
December	1st to 10th	Day	10% 18% 6% 21% 14% 5% 4 3 4 4 12%	2690
	11th to 20th	Day	8% 18% 9% 14% 5% 8% 5% 4 5% 4 20%	3068
	21st to end	Day	17% 22% 6% 16% 9% 5% 4 3 4 4 11%	3575
January	1st to 10th	Night	6% 12% 6% 20% 10% 7% 6% 4 5% 5% 20%	8800
	11th to 20th	Night	6% 13% 7% 15% 11% 10% 9% 8% 6% 3 13%	8294
	21st to end	Night	6% 13% 9% 14% 6% 6% 8% 5% 6% 8% 19%	8860
February	1st to 10th	Night	8% 12% 6% 11% 7% 8% 7% 5% 6% 7% 24%	2730
	11th to 20th	Night	7% 10% 4 11% 7% 6% 7% 5% 5% 6% 31%	2663
	21st to end	Night	7% 14% 5% 12% 9% 6% 5% 4 5% 6% 27%	2186
March	1st to 10th	Night	10% 9% 3 9% 5% 5% 5% 4 5% 7% 37%	5352
	11th to 20th	Night	8% 13% 5% 15% 8% 6% 5% 4 6% 6% 23%	5089
	21st to end	Night	20% 14% 4 10% 5% 4 4 3 3 4 30%	5404
April	1st to 10th	Night	16% 14% 4 9% 4 4 4 3 4 7% 31%	2100
	11th to 20th	Night	15% 13% 4 8% 5% 5% 4 3 5% 6% 32%	2001
	21st to end	Night	20% 11% 4 7% 5% 4 4 3 4 5% 32%	1751
May	1st to 10th	Night	16% 6% 2 5% 4 4 3 3 4 5% 48%	3613
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 44%	3596
	21st to end	Night	15% 7% 2 5% 3 4 4 4 5% 7% 43%	3949
June	1st to 10th	Night	15% 6% 3 6% 5% 4 4 5% 6% 6% 40%	1516
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 5% 8% 44%	1400
	21st to end	Night	12% 6% 3 6% 4 4 5% 4 8% 7% 41%	1400
July	1st to 10th	Night	9% 4 2 5% 4 4 4 4 7% 9% 49%	3180
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 8% 50%	3393
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 6% 8% 43%	3948
August	1st to 10th	Night	13% 5% 2 6% 3 4 5% 4 7% 8% 44%	2657
	11th to 20th	Night	13% 5% 2 4 3 4 5% 4 6% 9% 46%	2888
	21st to end	Night	16% 7% 2 6% 3 3 4 3 5% 7% 44%	3416
September	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 5% 7% 45%	2000
	11th to 20th	Night	12% 5% 2 6% 3 4 4 3 6% 8% 47%	2000
	21st to end	Night	17% 9% 3 6% 4 4 3 4 5% 6% 40%	2280
October	1st to 10th	Night	16% 12% 3 6% 4 4 4 3 4 6% 37%	2400
	11th to 20th	Night	18% 16% 3 6% 4 4 4 3 5% 6% 30%	2400
	21st to end	Night	20% 16% 4 9% 5% 4 4 3 4 6% 25%	2781
November	1st to 10th	Night	13% 17% 5% 11% 6% 4 4 3 5% 5% 26%	2833
	11th to 20th	Night	16% 17% 6% 14% 7% 5% 4 3 4 5% 20%	2940
	21st to end	Night	11% 14% 5% 18% 10% 6% 5% 3 4 5% 18%	2940
December	1st to 10th	Night	11% 17% 7% 22% 11% 7% 5% 3 4 3 10%	4720
	11th to 20th	Night	10% 15% 7% 15% 8% 6% 6% 5% 5% 5% 17%	5343
	21st to end	Night	19% 21% 7% 17% 6% 4 4 3 4 4 13%	7628

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% 2 6% 15% 9% 1	4125
	11th to 20th	Day	72% 3 4 17% 5%	4635
	21st to end	Day	63% 2 10% 17% 8%	5353
February	1st to 10th	Day	59% 4 10% 14% 11% 1	1890
	11th to 20th	Day	53% 3 11% 21% 11% 1	1853
	21st to end	Day	55% 3 11% 18% 12% 1	1619
March	1st to 10th	Day	47% 2 12% 22% 12% 4	4739
	11th to 20th	Day	53% 3 12% 20% 11% 1	5090
	21st to end	Day	58% 2 6% 20% 13% 1	5768
April	1st to 10th	Day	57% 3 7% 18% 13% 1	2520
	11th to 20th	Day	56% 3 11% 20% 9% 1	2478
	21st to end	Day	55% 3 9% 21% 12% 1	2647
May	1st to 10th	Day	41% 3 12% 25% 18% 1	6357
	11th to 20th	Day	46% 3 11% 23% 14% 2	6323
	21st to end	Day	42% 3 8% 28% 17% 1	6949
June	1st to 10th	Day	43% 4 16% 24% 12% 1	2879
	11th to 20th	Day	46% 4 13% 24% 13%	3000
	21st to end	Day	45% 5% 17% 25% 8% 1	3000
July	1st to 10th	Day	37% 7% 20% 27% 8%	6726
	11th to 20th	Day	38% 5% 19% 31% 6%	6504
	21st to end	Day	39% 6% 16% 29% 10%	6728
August	1st to 10th	Day	42% 5% 16% 26% 10% 1	4484
	11th to 20th	Day	37% 5% 22% 29% 7%	4210
	21st to end	Day	41% 5% 20% 26% 8%	4486
September	1st to 10th	Day	43% 5% 15% 27% 10%	2400
	11th to 20th	Day	45% 4 21% 24% 5%	2400
	21st to end	Day	48% 4 13% 27% 7%	2097
October	1st to 10th	Day	40% 3 18% 28% 11% 1	3260
	11th to 20th	Day	53% 4 12% 21% 11%	3240
	21st to end	Day	61% 3 11% 20% 4	3296
November	1st to 10th	Day	62% 4 12% 15% 7% 1	1682
	11th to 20th	Day	66% 3 9% 15% 6%	1680
	21st to end	Day	72% 4 9% 10% 4 1	1680
December	1st to 10th	Day	77% 3 7% 12% 2	2690
	11th to 20th	Day	68% 2 7% 13% 10%	3069
	21st to end	Day	78% 2 4 11% 5%	3575
January	1st to 10th	Night	67% 2 8% 13% 9%	8800
	11th to 20th	Night	75% 2 5% 11% 7%	8338
	21st to end	Night	64% 2 8% 17% 9%	8860
February	1st to 10th	Night	59% 3 10% 17% 10% 1	2730
	11th to 20th	Night	53% 3 11% 20% 12% 1	2664
	21st to end	Night	60% 2 9% 19% 8% 2	2186
March	1st to 10th	Night	47% 4 13% 15% 16% 5%	5363
	11th to 20th	Night	61% 3 11% 16% 8% 1	5090
	21st to end	Night	60% 2 7% 17% 12% 1	5404
April	1st to 10th	Night	54% 3 8% 19% 14% 1	2100
	11th to 20th	Night	53% 3 9% 21% 12% 1	2001
	21st to end	Night	54% 3 7% 23% 12% 1	1751
May	1st to 10th	Night	39% 3 11% 29% 17% 1	3620
	11th to 20th	Night	41% 3 12% 23% 18% 3	3597
	21st to end	Night	41% 4 10% 29% 15% 1	3950
June	1st to 10th	Night	42% 4 15% 26% 11% 1	1516
	11th to 20th	Night	38% 4 14% 26% 16% 2	1400
	21st to end	Night	39% 5% 18% 28% 9% 1	1400
July	1st to 10th	Night	31% 6% 21% 31% 10%	3145
	11th to 20th	Night	32% 4 21% 33% 9%	3351
	21st to end	Night	39% 5% 16% 29% 9%	3832
August	1st to 10th	Night	37% 6% 17% 27% 12% 1	2648
	11th to 20th	Night	34% 6% 20% 33% 7%	2915
	21st to end	Night	40% 5% 20% 26% 9%	3416
September	1st to 10th	Night	39% 4 16% 30% 11%	2000
	11th to 20th	Night	35% 5% 23% 29% 7%	2000
	21st to end	Night	45% 4 13% 30% 8%	2280
October	1st to 10th	Night	36% 2 10% 36% 14% 1	3920
	11th to 20th	Night	51% 3 12% 24% 10%	3899
	21st to end	Night	59% 4 11% 20% 6%	4493
November	1st to 10th	Night	60% 3 11% 19% 7%	2833
	11th to 20th	Night	67% 3 11% 13% 5% 1	2940
	21st to end	Night	69% 4 12% 11% 4	2940
December	1st to 10th	Night	79% 3 7% 9% 2	4720
	11th to 20th	Night	68% 3 8% 13% 8%	5341
	21st to end	Night	77% 2 5% 12% 5%	7628

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	13% 21% 37% 23% 6% 1	330
	11th to 20th	Day	19% 11% 45% 20% 4	330
	21st to end	Day	13% 16% 42% 23% 6%	363
February	1st to 10th	Day	15% 22% 45% 12% 5% 1	330
	11th to 20th	Day	16% 24% 37% 15% 7% 2	330
	21st to end	Day	20% 23% 42% 12% 2 1	273
March	1st to 10th	Day	18% 22% 36% 19% 4 1	330
	11th to 20th	Day	21% 19% 39% 14% 7% 1	330
	21st to end	Day	23% 21% 34% 14% 6% 1	443
April	1st to 10th	Day	24% 21% 32% 18% 5% 1	540
	11th to 20th	Day	25% 20% 29% 19% 6% 1	548
	21st to end	Day	26% 16% 30% 19% 7% 1	700
May	1st to 10th	Day	21% 19% 33% 17% 7% 2	703
	11th to 20th	Day	20% 18% 30% 21% 9% 2	725
	21st to end	Day	21% 13% 31% 24% 9% 2	789
June	1st to 10th	Day	16% 17% 33% 23% 8% 3	730
	11th to 20th	Day	18% 16% 38% 20% 6% 2	730
	21st to end	Day	20% 13% 32% 22% 10% 4	730
July	1st to 10th	Day	12% 9% 33% 25% 15% 5%	721
	11th to 20th	Day	16% 11% 31% 23% 14% 5%	718
	21st to end	Day	11% 10% 32% 25% 16% 5%	794
August	1st to 10th	Day	13% 10% 32% 25% 17% 3	726
	11th to 20th	Day	16% 8% 28% 30% 14% 5%	715
	21st to end	Day	19% 12% 30% 22% 15% 2	693
September	1st to 10th	Day	17% 12% 32% 26% 9% 2	530
	11th to 20th	Day	21% 12% 32% 23% 10% 2	530
	21st to end	Day	25% 16% 29% 20% 9% 1	363
October	1st to 10th	Day	25% 18% 33% 15% 7% 2	330
	11th to 20th	Day	27% 21% 35% 15% 2	330
	21st to end	Day	30% 23% 32% 12% 2 1	363
November	1st to 10th	Day	25% 21% 36% 14% 3 1	330
	11th to 20th	Day	28% 24% 32% 12% 3 1	330
	21st to end	Day	26% 19% 37% 16% 1	330
December	1st to 10th	Day	19% 17% 45% 15% 3	330
	11th to 20th	Day	22% 19% 41% 14% 3	330
	21st to end	Day	26% 15% 40% 15% 3 1	363
January	1st to 10th	Night	13% 18% 38% 23% 7% 1	1050
	11th to 20th	Night	18% 14% 41% 21% 6% 1	1050
	21st to end	Night	17% 16% 40% 22% 5%	1155
February	1st to 10th	Night	16% 17% 38% 22% 7% 1	1050
	11th to 20th	Night	14% 19% 36% 22% 7% 2	1029
	21st to end	Night	15% 15% 39% 21% 8% 1	802
March	1st to 10th	Night	17% 15% 36% 24% 7% 1	788
	11th to 20th	Night	15% 20% 36% 23% 6% 1	630
	21st to end	Night	22% 13% 30% 23% 9% 2	693
April	1st to 10th	Night	23% 13% 36% 23% 5%	630
	11th to 20th	Night	21% 14% 33% 24% 7%	611
	21st to end	Night	26% 11% 28% 25% 9% 1	600
May	1st to 10th	Night	21% 12% 34% 26% 7% 1	600
	11th to 20th	Night	19% 13% 31% 27% 9% 1	591
	21st to end	Night	19% 10% 30% 32% 8%	650
June	1st to 10th	Night	17% 8% 33% 31% 9% 1	600
	11th to 20th	Night	16% 8% 35% 34% 8%	600
	21st to end	Night	15% 8% 29% 34% 13% 1	600
July	1st to 10th	Night	8% 5% 28% 39% 19% 1	585
	11th to 20th	Night	11% 8% 29% 35% 17% 1	582
	21st to end	Night	9% 4 30% 37% 19% 2	657
August	1st to 10th	Night	11% 4 28% 39% 17% 1	597
	11th to 20th	Night	15% 8% 21% 33% 20% 3	594
	21st to end	Night	15% 10% 28% 30% 15% 1	660
September	1st to 10th	Night	16% 7% 28% 33% 14% 2	600
	11th to 20th	Night	15% 8% 33% 30% 14% 1	600
	21st to end	Night	21% 12% 32% 23% 10% 2	715
October	1st to 10th	Night	24% 16% 30% 24% 7%	800
	11th to 20th	Night	27% 16% 32% 19% 6%	800
	21st to end	Night	28% 15% 33% 20% 4	880
November	1st to 10th	Night	22% 17% 33% 22% 5% 1	987
	11th to 20th	Night	25% 18% 31% 19% 6% 1	1050
	21st to end	Night	21% 21% 39% 16% 3	1050
December	1st to 10th	Night	19% 17% 37% 21% 6%	1050
	11th to 20th	Night	21% 18% 35% 20% 5% 1	1050
	21st to end	Night	27% 15% 32% 20% 6% 1	1155

Page generated at 2026-04-14 01:44:29. Version ac07

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