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 2024-12-24.

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	13% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	102327
	Night	13% 12% 4 10% 6% 5% 5% 4 5% 6% 31%	97912
Transparency	Day	51% 4 13% 23% 9% 1	105810
	Night	54% 3 12% 21% 9% 1	102516
Seeing	Day	20% 16% 34% 20% 9% 2	16316
	Night	19% 13% 33% 25% 9% 1	26289

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	7% 15% 7% 17% 9% 6% 5% 4 4 5% 20%	2218
	11th to 20th	Day	6% 11% 5% 13% 12% 10% 11% 7% 7% 5% 14%	2481
	21st to end	Day	7% 15% 12% 20% 8% 6% 6% 4 5% 5% 13%	2882
February	1st to 10th	Day	7% 15% 5% 12% 5% 5% 9% 6% 7% 6% 24%	1710
	11th to 20th	Day	9% 11% 5% 10% 9% 7% 5% 4 5% 7% 29%	1664
	21st to end	Day	7% 15% 5% 10% 8% 4 5% 4 4 6% 32%	1469
March	1st to 10th	Day	9% 11% 4 8% 4 5% 5% 3 4 6% 40%	3232
	11th to 20th	Day	6% 11% 9% 12% 6% 6% 4 3 5% 7% 32%	3480
	21st to end	Day	17% 14% 6% 10% 5% 4 3 2 4 5% 30%	3937
April	1st to 10th	Day	15% 12% 3 8% 6% 5% 5% 4 6% 6% 29%	2280
	11th to 20th	Day	16% 12% 3 8% 5% 5% 5% 5% 5% 7% 29%	2284
	21st to end	Day	19% 12% 3 8% 5% 4 3 3 5% 7% 31%	2514
May	1st to 10th	Day	18% 9% 2 6% 4 4 4 3 5% 7% 36%	4428
	11th to 20th	Day	15% 7% 3 7% 5% 5% 6% 4 6% 7% 35%	4399
	21st to end	Day	19% 9% 3 7% 5% 4 4 3 5% 7% 35%	4837
June	1st to 10th	Day	14% 6% 3 7% 4 4 5% 4 6% 8% 39%	2735
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 4 6% 8% 36%	2850
	21st to end	Day	17% 6% 2 6% 4 5% 5% 4 7% 10% 34%	2850
July	1st to 10th	Day	9% 5% 2 5% 4 5% 6% 5% 8% 11% 39%	4750
	11th to 20th	Day	9% 6% 2 5% 4 4 5% 4 6% 10% 45%	4600
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	4814
August	1st to 10th	Day	10% 7% 2 7% 5% 6% 6% 5% 8% 9% 37%	4361
	11th to 20th	Day	13% 5% 2 5% 3 4 5% 4 6% 9% 43%	4041
	21st to end	Day	15% 7% 2 7% 4 3 4 3 6% 8% 40%	4350
September	1st to 10th	Day	13% 7% 2 6% 5% 4 6% 4 5% 7% 41%	2280
	11th to 20th	Day	15% 8% 2 6% 5% 5% 4 4 6% 8% 37%	2280
	21st to end	Day	19% 9% 3 7% 4 4 4 3 5% 6% 37%	1993
October	1st to 10th	Day	18% 11% 3 7% 4 4 4 4 6% 7% 33%	1900
	11th to 20th	Day	18% 17% 3 9% 4 4 5% 3 5% 5% 26%	1900
	21st to end	Day	20% 19% 5% 9% 4 4 3 2 3 5% 26%	1955
November	1st to 10th	Day	14% 17% 7% 10% 6% 5% 4 3 5% 4 25%	1600
	11th to 20th	Day	16% 17% 6% 16% 7% 4 3 4 3 4 20%	1600
	21st to end	Day	15% 16% 7% 18% 7% 5% 5% 3 4 4 16%	1600
December	1st to 10th	Day	9% 18% 6% 16% 11% 6% 5% 5% 5% 5% 15%	1600
	11th to 20th	Day	9% 17% 9% 15% 7% 6% 6% 4 5% 5% 17%	1990
	21st to end	Day	21% 22% 7% 11% 8% 4 3 3 4 4 13%	2463
January	1st to 10th	Night	8% 15% 6% 16% 9% 6% 5% 4 5% 5% 21%	4737
	11th to 20th	Night	6% 11% 6% 14% 11% 11% 11% 5% 4 4 16%	4472
	21st to end	Night	9% 18% 14% 16% 6% 4 4 4 5% 4 16%	4784
February	1st to 10th	Night	8% 12% 6% 11% 6% 8% 7% 5% 6% 7% 25%	2470
	11th to 20th	Night	8% 9% 4 11% 7% 6% 7% 5% 5% 6% 32%	2403
	21st to end	Night	8% 14% 5% 13% 8% 6% 5% 4 4 5% 28%	1984
March	1st to 10th	Night	10% 11% 4 10% 5% 5% 4 4 5% 7% 36%	3636
	11th to 20th	Night	8% 10% 4 15% 8% 6% 5% 4 5% 6% 29%	3479
	21st to end	Night	20% 13% 5% 11% 5% 4 4 3 4 4 29%	3693
April	1st to 10th	Night	15% 13% 4 9% 5% 4 4 3 5% 7% 32%	1900
	11th to 20th	Night	15% 12% 3 8% 5% 5% 4 3 5% 6% 33%	1843
	21st to end	Night	20% 11% 3 6% 5% 5% 4 3 4 5% 32%	1664
May	1st to 10th	Night	19% 7% 2 6% 4 4 4 4 6% 6% 37%	2523
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 4 5% 7% 40%	2506
	21st to end	Night	19% 8% 2 6% 3 4 4 4 5% 6% 39%	2750
June	1st to 10th	Night	15% 6% 3 6% 5% 4 4 5% 6% 7% 40%	1440
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 5% 8% 44%	1330
	21st to end	Night	12% 5% 3 5% 4 4 5% 4 8% 7% 41%	1330
July	1st to 10th	Night	8% 4 2 5% 4 3 4 5% 7% 11% 47%	2220
	11th to 20th	Night	7% 4 1 4 3 4 4 3 6% 9% 55%	2368
	21st to end	Night	9% 5% 2 6% 4 3 4 3 6% 8% 50%	2750
August	1st to 10th	Night	13% 5% 2 6% 3 4 5% 4 7% 8% 44%	2575
	11th to 20th	Night	13% 5% 2 4 3 4 5% 4 6% 9% 46%	2798
	21st to end	Night	16% 7% 2 6% 3 3 4 3 5% 7% 44%	3311
September	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 6% 8% 47%	1900
	21st to end	Night	18% 9% 3 6% 4 4 3 4 5% 6% 39%	2166
October	1st to 10th	Night	17% 12% 3 6% 4 5% 4 3 4 6% 36%	2280
	11th to 20th	Night	18% 16% 3 6% 4 4 4 3 5% 6% 30%	2280
	21st to end	Night	20% 16% 4 9% 5% 4 4 3 4 6% 25%	2642
November	1st to 10th	Night	13% 18% 5% 11% 6% 4 4 3 5% 5% 26%	2695
	11th to 20th	Night	16% 17% 6% 15% 8% 5% 4 3 4 5% 18%	2800
	21st to end	Night	12% 14% 6% 17% 10% 6% 5% 4 4 5% 18%	2800
December	1st to 10th	Night	10% 16% 7% 17% 11% 7% 6% 4 5% 4 14%	2800
	11th to 20th	Night	11% 15% 6% 15% 9% 7% 7% 4 5% 5% 16%	3423
	21st to end	Night	22% 23% 6% 13% 6% 5% 4 3 4 3 13%	5260

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	68% 2 7% 15% 8%	2219
	11th to 20th	Day	71% 3 5% 17% 3	2481
	21st to end	Day	74% 3 7% 12% 4	2882
February	1st to 10th	Day	57% 5% 10% 15% 11% 2	1710
	11th to 20th	Day	55% 3 11% 20% 10% 1	1673
	21st to end	Day	54% 3 12% 17% 12% 2	1469
March	1st to 10th	Day	47% 2 10% 26% 15% 1	3234
	11th to 20th	Day	52% 2 12% 21% 11%	3480
	21st to end	Day	59% 2 8% 21% 9% 1	3937
April	1st to 10th	Day	56% 4 8% 18% 13% 2	2280
	11th to 20th	Day	55% 3 11% 20% 10% 1	2284
	21st to end	Day	54% 3 9% 21% 12% 1	2514
May	1st to 10th	Day	49% 3 11% 22% 14% 1	4440
	11th to 20th	Day	48% 3 12% 25% 10% 2	4403
	21st to end	Day	51% 3 10% 24% 11% 1	4837
June	1st to 10th	Day	43% 5% 16% 24% 12% 1	2735
	11th to 20th	Day	46% 4 12% 24% 13%	2850
	21st to end	Day	44% 5% 17% 26% 8%	2850
July	1st to 10th	Day	37% 6% 20% 26% 10%	4676
	11th to 20th	Day	32% 5% 19% 34% 9%	4519
	21st to end	Day	39% 5% 14% 32% 9%	4662
August	1st to 10th	Day	42% 5% 16% 26% 10% 1	4346
	11th to 20th	Day	36% 5% 22% 29% 7%	4080
	21st to end	Day	41% 5% 20% 26% 8%	4349
September	1st to 10th	Day	42% 4 16% 28% 10%	2280
	11th to 20th	Day	45% 4 21% 25% 6%	2280
	21st to end	Day	49% 4 13% 26% 7%	1993
October	1st to 10th	Day	41% 3 18% 28% 10%	3160
	11th to 20th	Day	53% 4 12% 21% 10%	3140
	21st to end	Day	61% 3 12% 20% 4	3193
November	1st to 10th	Day	62% 4 12% 16% 6% 1	1600
	11th to 20th	Day	68% 4 9% 14% 6%	1600
	21st to end	Day	72% 4 9% 10% 4 1	1600
December	1st to 10th	Day	70% 4 10% 14% 3	1600
	11th to 20th	Day	70% 2 9% 14% 5% 1	1991
	21st to end	Day	76% 1 4 11% 6%	2463
January	1st to 10th	Night	65% 2 9% 18% 5%	4737
	11th to 20th	Night	72% 2 9% 13% 4	4472
	21st to end	Night	71% 2 9% 14% 4	4784
February	1st to 10th	Night	57% 3 11% 18% 10% 1	2470
	11th to 20th	Night	52% 3 12% 20% 11% 1	2404
	21st to end	Night	59% 2 9% 19% 9% 2	1984
March	1st to 10th	Night	48% 3 13% 17% 16% 2	3647
	11th to 20th	Night	56% 3 13% 19% 8% 1	3480
	21st to end	Night	61% 3 8% 16% 10% 2	3693
April	1st to 10th	Night	53% 3 9% 19% 15% 1	1900
	11th to 20th	Night	53% 3 9% 21% 12% 2	1843
	21st to end	Night	54% 3 7% 23% 12% 1	1664
May	1st to 10th	Night	47% 4 10% 26% 13%	2530
	11th to 20th	Night	44% 3 13% 24% 12% 3	2507
	21st to end	Night	47% 4 12% 25% 10% 2	2751
June	1st to 10th	Night	42% 5% 15% 26% 11% 1	1440
	11th to 20th	Night	38% 4 14% 25% 17% 2	1330
	21st to end	Night	38% 5% 18% 28% 9% 1	1330
July	1st to 10th	Night	30% 7% 23% 31% 10%	2185
	11th to 20th	Night	25% 5% 20% 39% 11% 1	2326
	21st to end	Night	33% 5% 16% 34% 11% 1	2662
August	1st to 10th	Night	37% 6% 17% 27% 12% 2	2566
	11th to 20th	Night	34% 6% 20% 33% 7%	2825
	21st to end	Night	40% 5% 20% 26% 9%	3311
September	1st to 10th	Night	39% 4 15% 30% 11%	1900
	11th to 20th	Night	35% 5% 23% 29% 8%	1900
	21st to end	Night	46% 4 14% 29% 8%	2166
October	1st to 10th	Night	37% 2 10% 37% 14% 1	3800
	11th to 20th	Night	51% 3 12% 24% 10%	3779
	21st to end	Night	59% 4 12% 20% 6%	4354
November	1st to 10th	Night	59% 3 12% 19% 7%	2695
	11th to 20th	Night	69% 3 11% 12% 5%	2800
	21st to end	Night	69% 4 12% 11% 4	2800
December	1st to 10th	Night	71% 4 10% 12% 3	2800
	11th to 20th	Night	70% 3 10% 13% 5%	3421
	21st to end	Night	78% 2 5% 10% 5%	5260

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% 19% 37% 23% 6%	270
	11th to 20th	Day	18% 11% 45% 20% 5%	270
	21st to end	Day	13% 14% 42% 25% 6%	297
February	1st to 10th	Day	13% 20% 46% 14% 5% 1	270
	11th to 20th	Day	18% 27% 34% 12% 7% 2	270
	21st to end	Day	20% 22% 42% 13% 2 1	225
March	1st to 10th	Day	14% 22% 37% 22% 4 1	270
	11th to 20th	Day	19% 19% 37% 16% 9% 1	270
	21st to end	Day	22% 18% 35% 15% 8% 2	369
April	1st to 10th	Day	22% 18% 33% 19% 6% 1	460
	11th to 20th	Day	25% 19% 28% 20% 7% 1	482
	21st to end	Day	26% 16% 30% 20% 8% 1	650
May	1st to 10th	Day	22% 20% 33% 17% 6% 2	653
	11th to 20th	Day	20% 18% 30% 21% 9% 3	675
	21st to end	Day	22% 13% 30% 24% 8% 2	734
June	1st to 10th	Day	17% 17% 32% 23% 8% 4	680
	11th to 20th	Day	19% 16% 38% 19% 6% 2	680
	21st to end	Day	20% 13% 31% 22% 10% 4	680
July	1st to 10th	Day	12% 10% 32% 25% 15% 6%	671
	11th to 20th	Day	16% 10% 31% 23% 15% 5%	668
	21st to end	Day	11% 10% 32% 25% 17% 5%	739
August	1st to 10th	Day	13% 9% 33% 26% 16% 3	676
	11th to 20th	Day	16% 7% 27% 30% 15% 4	665
	21st to end	Day	20% 11% 30% 22% 16% 2	644
September	1st to 10th	Day	16% 12% 34% 27% 10% 2	490
	11th to 20th	Day	21% 12% 33% 21% 11% 3	490
	21st to end	Day	26% 17% 30% 19% 8%	332
October	1st to 10th	Day	27% 17% 31% 15% 7% 3	300
	11th to 20th	Day	25% 21% 38% 14% 2	300
	21st to end	Day	32% 24% 31% 11% 2	330
November	1st to 10th	Day	24% 20% 37% 15% 3 1	300
	11th to 20th	Day	28% 23% 33% 13% 3	300
	21st to end	Day	27% 20% 36% 15% 1	300
December	1st to 10th	Day	20% 18% 43% 16% 3	300
	11th to 20th	Day	21% 20% 42% 14% 3	300
	21st to end	Day	24% 16% 41% 14% 3 1	306
January	1st to 10th	Night	13% 17% 38% 23% 8% 1	950
	11th to 20th	Night	18% 15% 40% 21% 6% 1	950
	21st to end	Night	18% 15% 40% 22% 5%	1045
February	1st to 10th	Night	15% 16% 38% 23% 8% 1	950
	11th to 20th	Night	14% 19% 35% 22% 8% 2	929
	21st to end	Night	15% 16% 38% 22% 9% 1	728
March	1st to 10th	Night	16% 15% 35% 25% 8% 1	713
	11th to 20th	Night	14% 18% 37% 23% 7% 1	570
	21st to end	Night	21% 12% 30% 24% 10% 3	627
April	1st to 10th	Night	20% 13% 37% 24% 6%	570
	11th to 20th	Night	21% 14% 33% 24% 8%	563
	21st to end	Night	26% 10% 28% 25% 9% 1	570
May	1st to 10th	Night	21% 12% 33% 26% 6% 1	570
	11th to 20th	Night	19% 13% 30% 27% 9% 1	561
	21st to end	Night	19% 11% 31% 31% 8%	617
June	1st to 10th	Night	16% 9% 34% 31% 9% 1	570
	11th to 20th	Night	15% 8% 34% 34% 8%	570
	21st to end	Night	15% 7% 28% 35% 14% 1	570
July	1st to 10th	Night	8% 5% 29% 38% 20% 1	555
	11th to 20th	Night	11% 8% 29% 34% 17% 1	552
	21st to end	Night	9% 4 30% 36% 20% 2	624
August	1st to 10th	Night	11% 4 29% 40% 16% 1	567
	11th to 20th	Night	15% 8% 20% 33% 21% 3	564
	21st to end	Night	15% 10% 29% 29% 16% 1	627
September	1st to 10th	Night	16% 7% 27% 34% 14% 1	570
	11th to 20th	Night	15% 8% 34% 28% 15% 1	570
	21st to end	Night	22% 13% 32% 22% 9% 2	679
October	1st to 10th	Night	24% 15% 30% 24% 7%	760
	11th to 20th	Night	27% 16% 32% 19% 6%	760
	21st to end	Night	28% 15% 33% 20% 4	836
November	1st to 10th	Night	22% 17% 33% 22% 5% 1	939
	11th to 20th	Night	25% 17% 32% 19% 6% 1	1000
	21st to end	Night	21% 21% 40% 16% 4	1000
December	1st to 10th	Night	19% 17% 36% 22% 6%	1000
	11th to 20th	Night	21% 17% 35% 20% 6% 1	1000
	21st to end	Night	27% 14% 32% 20% 5% 1	1063

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