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 2024-01-17.

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	14% 10% 4 8% 6% 4 5% 5% 5% 7% 32%	74913
	Night	13% 11% 5% 9% 7% 4 5% 5% 5% 6% 30%	74892
Transparency	Day	52% 4 12% 22% 9% 1	74608
	Night	54% 4 12% 20% 9% 1	74731
Seeing	Day	19% 15% 33% 21% 9% 2	14983
	Night	19% 13% 33% 25% 9% 1	25029

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	10% 11% 9% 11% 9% 5% 5% 7% 6% 6% 22%	1328
	11th to 20th	Day	10% 14% 7% 12% 8% 5% 8% 8% 5% 6% 16%	1463
	21st to end	Day	9% 12% 6% 15% 11% 6% 8% 8% 7% 6% 13%	1641
February	1st to 10th	Day	7% 14% 7% 9% 7% 4 9% 8% 7% 6% 24%	1620
	11th to 20th	Day	10% 10% 5% 8% 9% 6% 6% 5% 5% 7% 29%	1574
	21st to end	Day	8% 14% 7% 8% 9% 3 5% 4 4 6% 32%	1385
March	1st to 10th	Day	9% 11% 6% 9% 6% 4 5% 6% 5% 6% 33%	1856
	11th to 20th	Day	11% 12% 6% 8% 7% 3 4 4 4 7% 34%	1980
	21st to end	Day	16% 11% 5% 8% 7% 3 4 4 4 6% 33%	2246
April	1st to 10th	Day	15% 12% 4 7% 8% 4 5% 5% 5% 6% 30%	2160
	11th to 20th	Day	16% 12% 3 7% 5% 4 4 7% 5% 8% 29%	2162
	21st to end	Day	18% 11% 4 7% 6% 3 4 4 6% 7% 31%	2381
May	1st to 10th	Day	16% 10% 4 7% 6% 4 5% 4 4 6% 34%	2520
	11th to 20th	Day	15% 7% 3 6% 6% 4 5% 6% 6% 7% 34%	2479
	21st to end	Day	17% 7% 3 5% 6% 3 5% 4 5% 8% 36%	2725
June	1st to 10th	Day	14% 6% 3 6% 5% 3 4 6% 6% 9% 39%	2591
	11th to 20th	Day	15% 7% 4 5% 5% 3 6% 5% 6% 8% 37%	2700
	21st to end	Day	17% 5% 2 5% 5% 3 5% 5% 7% 10% 35%	2700
July	1st to 10th	Day	9% 5% 2 4 4 3 5% 5% 6% 10% 47%	2700
	11th to 20th	Day	12% 6% 3 6% 5% 3 5% 6% 5% 8% 41%	2615
	21st to end	Day	9% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	2743
August	1st to 10th	Day	8% 6% 3 6% 5% 4 6% 6% 8% 9% 41%	2473
	11th to 20th	Day	13% 6% 2 6% 5% 3 4 6% 6% 8% 40%	2301
	21st to end	Day	13% 6% 2 5% 5% 3 4 5% 6% 8% 44%	2476
September	1st to 10th	Day	14% 7% 3 5% 6% 3 5% 5% 5% 7% 40%	2160
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 5% 6% 8% 36%	2160
	21st to end	Day	20% 9% 3 6% 5% 3 4 4 4 6% 36%	1889
October	1st to 10th	Day	18% 12% 3 6% 4 3 4 4 5% 7% 32%	1800
	11th to 20th	Day	19% 17% 4 7% 6% 3 5% 4 5% 5% 25%	1800
	21st to end	Day	21% 18% 6% 8% 5% 3 4 3 3 5% 26%	1853
November	1st to 10th	Day	14% 15% 8% 8% 8% 4 4 4 5% 5% 25%	1519
	11th to 20th	Day	16% 15% 7% 15% 8% 4 4 4 3 4 20%	1520
	21st to end	Day	16% 14% 7% 15% 10% 4 5% 4 4 4 17%	1520
December	1st to 10th	Day	9% 15% 8% 13% 12% 6% 6% 6% 5% 5% 15%	1520
	11th to 20th	Day	10% 15% 10% 13% 9% 5% 6% 6% 5% 5% 17%	1911
	21st to end	Day	21% 21% 8% 9% 9% 4 3 3 4 4 14%	2442
January	1st to 10th	Night	8% 12% 6% 12% 10% 5% 6% 7% 6% 6% 20%	2837
	11th to 20th	Night	11% 13% 7% 10% 11% 6% 7% 7% 6% 6% 18%	2631
	21st to end	Night	11% 15% 8% 14% 8% 5% 5% 6% 8% 5% 16%	2714
February	1st to 10th	Night	8% 11% 6% 9% 8% 6% 8% 6% 6% 7% 24%	2340
	11th to 20th	Night	8% 9% 4 9% 8% 5% 6% 6% 6% 6% 32%	2273
	21st to end	Night	9% 13% 6% 11% 9% 4 5% 5% 4 6% 28%	1870
March	1st to 10th	Night	11% 10% 6% 10% 7% 4 5% 5% 5% 6% 30%	2105
	11th to 20th	Night	10% 11% 4 8% 6% 3 5% 5% 5% 6% 37%	1979
	21st to end	Night	14% 10% 4 8% 6% 3 5% 5% 5% 6% 34%	2109
April	1st to 10th	Night	16% 12% 5% 8% 5% 3 4 4 4 7% 31%	1800
	11th to 20th	Night	16% 12% 4 7% 6% 3 5% 4 4 7% 32%	1745
	21st to end	Night	21% 10% 4 6% 5% 3 6% 4 5% 5% 32%	1577
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 6% 7% 5% 35%	1440
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 5% 5% 8% 38%	1416
	21st to end	Night	15% 7% 3 4 4 3 4 5% 6% 7% 41%	1551
June	1st to 10th	Night	15% 6% 3 5% 5% 3 4 6% 6% 6% 40%	1365
	11th to 20th	Night	12% 5% 2 5% 5% 3 4 5% 4 8% 45%	1260
	21st to end	Night	12% 5% 3 5% 4 3 5% 5% 8% 8% 42%	1260
July	1st to 10th	Night	6% 3 2 4 4 3 3 5% 6% 11% 52%	1260
	11th to 20th	Night	8% 4 2 5% 4 3 5% 5% 5% 8% 51%	1343
	21st to end	Night	7% 4 2 5% 3 3 4 4 5% 9% 54%	1575
August	1st to 10th	Night	8% 4 2 4 4 3 5% 5% 7% 8% 51%	1462
	11th to 20th	Night	13% 5% 2 4 3 3 4 5% 5% 7% 50%	1600
	21st to end	Night	13% 6% 2 4 4 2 3 4 5% 8% 49%	1880
September	1st to 10th	Night	12% 6% 3 5% 5% 2 4 4 5% 7% 46%	1800
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 6% 8% 47%	1800
	21st to end	Night	19% 9% 4 5% 4 3 3 5% 5% 6% 36%	2051
October	1st to 10th	Night	17% 13% 3 5% 5% 3 5% 4 4 6% 35%	2160
	11th to 20th	Night	19% 15% 4 5% 5% 3 3 5% 5% 6% 30%	2160
	21st to end	Night	20% 16% 5% 8% 6% 3 5% 4 4 6% 24%	2502
November	1st to 10th	Night	13% 16% 6% 10% 6% 4 3 4 5% 6% 26%	2556
	11th to 20th	Night	16% 15% 7% 13% 10% 4 4 3 4 5% 18%	2660
	21st to end	Night	12% 13% 6% 14% 12% 5% 5% 5% 4 6% 19%	2660
December	1st to 10th	Night	10% 14% 7% 14% 13% 5% 7% 6% 5% 4 15%	2660
	11th to 20th	Night	11% 14% 7% 13% 11% 5% 6% 6% 5% 5% 16%	3283
	21st to end	Night	22% 22% 7% 11% 8% 4 4 4 4 3 13%	5208

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% 2 8% 18% 8%	1329
	11th to 20th	Day	67% 5% 6% 16% 6%	1463
	21st to end	Day	69% 4 7% 12% 8%	1641
February	1st to 10th	Day	57% 5% 11% 14% 12% 2	1620
	11th to 20th	Day	55% 3 11% 20% 10% 1	1583
	21st to end	Day	55% 4 10% 18% 12% 2	1385
March	1st to 10th	Day	51% 3 13% 21% 11% 1	1858
	11th to 20th	Day	52% 3 13% 19% 12% 1	1980
	21st to end	Day	55% 3 11% 19% 10% 2	2246
April	1st to 10th	Day	56% 4 8% 18% 13% 2	2160
	11th to 20th	Day	55% 3 12% 20% 10% 1	2162
	21st to end	Day	54% 3 10% 21% 12% 1	2381
May	1st to 10th	Day	52% 4 10% 23% 10% 2	2520
	11th to 20th	Day	49% 4 12% 24% 10% 2	2483
	21st to end	Day	48% 4 11% 23% 12% 1	2725
June	1st to 10th	Day	43% 5% 16% 25% 11% 1	2591
	11th to 20th	Day	45% 4 12% 24% 14%	2700
	21st to end	Day	44% 5% 17% 27% 7%	2700
July	1st to 10th	Day	33% 4 17% 32% 13% 1	2626
	11th to 20th	Day	40% 5% 15% 29% 11%	2534
	21st to end	Day	37% 5% 15% 31% 11% 1	2591
August	1st to 10th	Day	37% 5% 17% 29% 10% 1	2458
	11th to 20th	Day	42% 5% 16% 28% 9%	2301
	21st to end	Day	38% 4 16% 29% 12%	2476
September	1st to 10th	Day	43% 4 15% 28% 10%	2160
	11th to 20th	Day	46% 4 19% 25% 6%	2160
	21st to end	Day	51% 4 14% 26% 5%	1889
October	1st to 10th	Day	52% 4 17% 21% 6%	1800
	11th to 20th	Day	62% 4 10% 18% 6% 1	1800
	21st to end	Day	64% 3 9% 18% 5%	1853
November	1st to 10th	Day	61% 4 12% 16% 6% 1	1519
	11th to 20th	Day	68% 3 9% 13% 6%	1520
	21st to end	Day	71% 4 10% 10% 5% 1	1520
December	1st to 10th	Day	69% 4 10% 14% 3	1520
	11th to 20th	Day	70% 2 9% 14% 4 1	1912
	21st to end	Day	76% 1 4 11% 6%	2442
January	1st to 10th	Night	63% 3 9% 18% 7%	2837
	11th to 20th	Night	67% 3 9% 15% 6% 1	2631
	21st to end	Night	67% 3 7% 16% 7%	2714
February	1st to 10th	Night	58% 3 11% 17% 10% 1	2340
	11th to 20th	Night	52% 4 12% 20% 12% 1	2274
	21st to end	Night	59% 2 9% 19% 9% 2	1870
March	1st to 10th	Night	54% 4 10% 18% 11% 2	2116
	11th to 20th	Night	48% 4 12% 22% 14% 1	1980
	21st to end	Night	51% 4 12% 19% 11% 3	2109
April	1st to 10th	Night	54% 3 9% 19% 14% 1	1800
	11th to 20th	Night	54% 3 9% 20% 12% 2	1745
	21st to end	Night	55% 3 8% 21% 12% 1	1577
May	1st to 10th	Night	48% 5% 12% 22% 13% 1	1440
	11th to 20th	Night	45% 5% 11% 25% 12% 2	1417
	21st to end	Night	41% 5% 12% 27% 13% 2	1552
June	1st to 10th	Night	42% 5% 16% 26% 11% 1	1365
	11th to 20th	Night	37% 4 14% 26% 17% 2	1260
	21st to end	Night	38% 5% 18% 28% 9% 1	1260
July	1st to 10th	Night	26% 4 18% 37% 14%	1225
	11th to 20th	Night	31% 5% 16% 30% 16% 1	1301
	21st to end	Night	30% 4 18% 33% 15% 1	1487
August	1st to 10th	Night	30% 6% 18% 33% 11% 2	1453
	11th to 20th	Night	34% 5% 17% 33% 11%	1600
	21st to end	Night	35% 4 18% 28% 15% 1	1880
September	1st to 10th	Night	39% 4 15% 30% 12%	1800
	11th to 20th	Night	35% 5% 22% 29% 8%	1800
	21st to end	Night	47% 4 14% 28% 7%	2051
October	1st to 10th	Night	52% 3 12% 26% 8%	2160
	11th to 20th	Night	54% 4 13% 23% 6%	2160
	21st to end	Night	60% 5% 11% 18% 5%	2502
November	1st to 10th	Night	59% 3 12% 20% 7%	2556
	11th to 20th	Night	69% 3 11% 12% 5%	2660
	21st to end	Night	67% 4 13% 11% 4	2660
December	1st to 10th	Night	70% 4 11% 12% 3	2660
	11th to 20th	Night	69% 3 10% 13% 5%	3281
	21st to end	Night	78% 2 5% 10% 5%	5208

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	19% 11% 45% 20% 5%	258
	21st to end	Day	14% 16% 41% 24% 6%	264
February	1st to 10th	Day	14% 20% 49% 13% 5%	240
	11th to 20th	Day	18% 28% 35% 11% 7% 2	240
	21st to end	Day	20% 22% 42% 13% 2 1	198
March	1st to 10th	Day	13% 22% 37% 23% 5% 1	240
	11th to 20th	Day	20% 19% 35% 15% 10% 1	240
	21st to end	Day	22% 18% 33% 16% 8% 2	332
April	1st to 10th	Day	23% 19% 32% 20% 6% 1	420
	11th to 20th	Day	25% 19% 28% 20% 8% 1	440
	21st to end	Day	26% 16% 30% 20% 8% 1	600
May	1st to 10th	Day	22% 20% 32% 17% 7% 2	603
	11th to 20th	Day	20% 17% 31% 20% 9% 3	625
	21st to end	Day	22% 13% 30% 24% 9% 2	679
June	1st to 10th	Day	16% 17% 33% 23% 8% 4	630
	11th to 20th	Day	17% 15% 39% 20% 7% 2	630
	21st to end	Day	19% 13% 30% 23% 11% 4	630
July	1st to 10th	Day	12% 10% 33% 26% 14% 5%	621
	11th to 20th	Day	16% 10% 32% 23% 14% 5%	618
	21st to end	Day	11% 10% 31% 26% 17% 5%	684
August	1st to 10th	Day	12% 9% 32% 27% 16% 4	626
	11th to 20th	Day	16% 8% 28% 29% 15% 4	615
	21st to end	Day	19% 12% 30% 22% 16% 2	595
September	1st to 10th	Day	17% 12% 34% 27% 9% 2	450
	11th to 20th	Day	21% 12% 32% 21% 11% 3	450
	21st to end	Day	28% 17% 29% 18% 8%	301
October	1st to 10th	Day	24% 18% 31% 16% 7% 3	270
	11th to 20th	Day	26% 20% 36% 15% 2	270
	21st to end	Day	33% 25% 31% 10% 2	297
November	1st to 10th	Day	22% 21% 37% 16% 3 1	270
	11th to 20th	Day	27% 23% 33% 14% 3	270
	21st to end	Day	27% 19% 37% 14% 1	270
December	1st to 10th	Day	20% 18% 42% 17% 3	270
	11th to 20th	Day	23% 19% 40% 14% 3	270
	21st to end	Day	25% 16% 40% 14% 3 1	297
January	1st to 10th	Night	13% 17% 38% 23% 8% 1	950
	11th to 20th	Night	18% 15% 40% 21% 6% 1	933
	21st to end	Night	18% 15% 40% 22% 5%	990
February	1st to 10th	Night	15% 16% 39% 23% 7% 1	900
	11th to 20th	Night	15% 19% 35% 21% 8% 2	879
	21st to end	Night	15% 16% 38% 21% 9% 1	686
March	1st to 10th	Night	16% 15% 34% 25% 8% 1	676
	11th to 20th	Night	14% 18% 37% 22% 7% 1	540
	21st to end	Night	20% 12% 30% 25% 11% 3	594
April	1st to 10th	Night	21% 13% 37% 24% 6%	540
	11th to 20th	Night	21% 14% 32% 24% 8%	533
	21st to end	Night	27% 10% 28% 25% 9% 1	540
May	1st to 10th	Night	21% 12% 33% 26% 6% 1	540
	11th to 20th	Night	19% 13% 30% 28% 9% 1	531
	21st to end	Night	19% 11% 30% 32% 8% 1	584
June	1st to 10th	Night	16% 8% 34% 31% 9% 1	540
	11th to 20th	Night	15% 8% 35% 34% 9%	540
	21st to end	Night	15% 8% 28% 34% 15% 1	540
July	1st to 10th	Night	7% 5% 29% 39% 18% 1	525
	11th to 20th	Night	11% 8% 30% 34% 16% 1	522
	21st to end	Night	9% 3 30% 37% 19% 2	591
August	1st to 10th	Night	10% 4 30% 39% 17% 1	537
	11th to 20th	Night	15% 8% 21% 33% 20% 3	534
	21st to end	Night	15% 10% 28% 29% 17% 1	594
September	1st to 10th	Night	16% 7% 28% 34% 14% 1	540
	11th to 20th	Night	16% 8% 32% 28% 15% 1	540
	21st to end	Night	23% 13% 32% 21% 9% 2	643
October	1st to 10th	Night	25% 14% 30% 24% 7%	720
	11th to 20th	Night	28% 16% 31% 19% 6%	720
	21st to end	Night	28% 15% 33% 20% 4	792
November	1st to 10th	Night	21% 17% 34% 22% 5% 1	890
	11th to 20th	Night	25% 18% 32% 19% 6% 1	950
	21st to end	Night	21% 21% 39% 16% 4	950
December	1st to 10th	Night	19% 17% 37% 21% 6%	950
	11th to 20th	Night	21% 17% 35% 20% 6% 1	950
	21st to end	Night	28% 14% 32% 20% 5% 1	1045

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