The Star Deck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Star Deck Observatory clear sky chart from 2005-11-06 to 2024-08-30.

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% 9% 4 7% 5% 4 4 4 5% 7% 35%	95460
	Night	15% 10% 4 7% 4 4 4 3 5% 6% 37%	93581
Transparency	Day	48% 3 11% 23% 13% 2	95219
	Night	49% 3 10% 23% 14% 2	93428
Seeing	Day	17% 8% 25% 25% 17% 7%	15310
	Night	19% 7% 27% 31% 15% 2	25098

By Month

Percentage Cloud Cover SE (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	15% 16% 9% 11% 5% 5% 4 4 5% 5% 22%	2527
	11th to 20th	Day	17% 13% 6% 14% 6% 7% 7% 5% 5% 4 16%	2530
	21st to end	Day	26% 20% 9% 10% 4 3 2 2 3 5% 15%	2951
February	1st to 10th	Day	18% 14% 6% 11% 6% 4 4 3 4 5% 26%	1881
	11th to 20th	Day	15% 13% 4 11% 5% 5% 4 4 4 6% 29%	1848
	21st to end	Day	18% 15% 5% 10% 4 3 3 3 3 5% 31%	1570
March	1st to 10th	Day	21% 19% 5% 8% 4 3 3 3 3 5% 27%	3120
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 5% 8% 41%	3383
	21st to end	Day	16% 10% 4 9% 6% 5% 5% 3 5% 7% 30%	3939
April	1st to 10th	Day	15% 11% 3 8% 5% 5% 4 4 5% 7% 34%	2280
	11th to 20th	Day	17% 8% 4 6% 4 5% 4 4 5% 7% 37%	2254
	21st to end	Day	19% 8% 3 7% 4 4 4 4 5% 7% 34%	2459
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4082
	11th to 20th	Day	19% 7% 3 6% 5% 4 4 4 5% 7% 37%	4081
	21st to end	Day	9% 7% 3 6% 5% 5% 6% 5% 7% 10% 37%	4525
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 5% 8% 41%	2660
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 7% 10% 44%	2660
	21st to end	Day	9% 6% 2 6% 5% 4 6% 4 7% 9% 42%	2660
July	1st to 10th	Day	13% 6% 2 7% 4 4 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	7% 4 1 5% 4 4 4 4 8% 11% 46%	4440
	21st to end	Day	9% 7% 2 7% 5% 6% 6% 5% 7% 9% 35%	4629
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 7% 10% 44%	2457
	11th to 20th	Day	10% 6% 2 6% 4 4 6% 5% 7% 11% 39%	2355
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 7% 9% 48%	2485
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 45%	2116
	11th to 20th	Day	11% 5% 2 5% 3 4 4 5% 5% 8% 47%	1980
	21st to end	Day	14% 6% 2 4 4 4 3 3 5% 7% 49%	1980
October	1st to 10th	Day	13% 5% 2 6% 4 5% 4 3 5% 7% 46%	1980
	11th to 20th	Day	15% 6% 2 5% 4 4 5% 4 6% 6% 43%	1846
	21st to end	Day	20% 9% 3 7% 4 5% 3 3 4 6% 34%	1782
November	1st to 10th	Day	9% 6% 2 5% 4 4 3 3 5% 6% 51%	1672
	11th to 20th	Day	15% 11% 4 7% 4 3 5% 3 5% 6% 35%	1710
	21st to end	Day	21% 11% 4 6% 5% 4 4 3 5% 5% 34%	1710
December	1st to 10th	Day	20% 15% 5% 9% 6% 4 4 3 5% 5% 25%	1710
	11th to 20th	Day	15% 17% 6% 10% 4 4 4 3 4 5% 27%	1975
	21st to end	Day	23% 21% 9% 10% 5% 4 3 2 4 4 15%	2783
January	1st to 10th	Night	18% 16% 7% 11% 5% 4 4 3 5% 5% 22%	4428
	11th to 20th	Night	15% 19% 7% 13% 6% 6% 5% 3 4 4 18%	4440
	21st to end	Night	26% 18% 7% 8% 4 3 3 2 3 4 21%	4695
February	1st to 10th	Night	21% 12% 5% 9% 6% 4 3 3 4 5% 28%	2295
	11th to 20th	Night	14% 12% 5% 11% 5% 3 4 3 5% 6% 32%	2218
	21st to end	Night	19% 14% 4 9% 4 5% 4 3 4 5% 30%	1884
March	1st to 10th	Night	20% 13% 3 7% 4 4 3 3 4 5% 34%	3733
	11th to 20th	Night	13% 7% 3 6% 4 3 4 3 5% 6% 46%	3576
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 37%	3707
April	1st to 10th	Night	14% 10% 2 7% 4 4 5% 3 5% 6% 38%	1900
	11th to 20th	Night	16% 8% 3 6% 5% 4 3 3 5% 8% 40%	1875
	21st to end	Night	18% 7% 2 7% 4 6% 4 3 6% 6% 36%	1720
May	1st to 10th	Night	18% 11% 4 8% 5% 5% 5% 4 6% 7% 27%	2813
	11th to 20th	Night	19% 7% 2 7% 4 3 4 3 5% 7% 41%	2812
	21st to end	Night	10% 5% 2 7% 4 4 5% 4 6% 9% 44%	3028
June	1st to 10th	Night	9% 7% 2 5% 4 4 4 3 5% 7% 48%	1520
	11th to 20th	Night	9% 6% 2 6% 3 4 4 4 6% 10% 46%	1520
	21st to end	Night	7% 3 2 5% 6% 5% 4 6% 8% 11% 44%	1520
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	2530
	11th to 20th	Night	6% 3 1 4 3 3 4 4 6% 8% 56%	2530
	21st to end	Night	11% 6% 2 6% 5% 5% 4 5% 6% 10% 39%	2929
August	1st to 10th	Night	7% 5% 2 6% 4 5% 4 4 6% 8% 50%	1701
	11th to 20th	Night	6% 5% 2 7% 4 4 5% 4 6% 8% 49%	1762
	21st to end	Night	6% 4 1 5% 4 3 5% 4 6% 8% 53%	2080
September	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 5% 8% 47%	1824
	11th to 20th	Night	8% 4 1 5% 4 4 4 4 6% 7% 54%	1980
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 6% 50%	1980
October	1st to 10th	Night	10% 4 2 5% 4 3 4 3 4 7% 53%	1980
	11th to 20th	Night	16% 7% 3 5% 4 4 4 2 5% 6% 44%	2110
	21st to end	Night	19% 8% 3 6% 4 4 4 4 4 6% 39%	2574
November	1st to 10th	Night	13% 8% 3 5% 3 2 3 3 5% 5% 49%	2406
	11th to 20th	Night	14% 9% 3 8% 4 4 3 2 5% 7% 39%	2470
	21st to end	Night	19% 12% 4 8% 5% 4 3 3 4 6% 32%	2470
December	1st to 10th	Night	17% 13% 5% 10% 6% 3 4 4 5% 5% 28%	2470
	11th to 20th	Night	18% 15% 4 9% 5% 4 3 3 4 5% 31%	3217
	21st to end	Night	23% 20% 6% 10% 5% 4 4 3 4 4 17%	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	65% 1 6% 20% 7%	2528
	11th to 20th	Day	72% 2 10% 11% 5%	2530
	21st to end	Day	75% 2 6% 14% 3 1	2951
February	1st to 10th	Day	62% 2 8% 16% 11% 1	1881
	11th to 20th	Day	57% 3 9% 20% 10% 1	1858
	21st to end	Day	58% 1 9% 22% 8% 2	1570
March	1st to 10th	Day	62% 2 6% 18% 10% 1	3120
	11th to 20th	Day	44% 2 10% 29% 14% 1	3383
	21st to end	Day	55% 2 13% 23% 6% 1	3939
April	1st to 10th	Day	51% 3 11% 21% 14% 1	2280
	11th to 20th	Day	48% 3 11% 25% 11% 3	2254
	21st to end	Day	51% 3 11% 22% 11% 2	2459
May	1st to 10th	Day	56% 2 12% 23% 7%	4120
	11th to 20th	Day	48% 3 13% 25% 11% 1	4085
	21st to end	Day	42% 4 11% 24% 18% 2	4538
June	1st to 10th	Day	40% 5% 13% 25% 16% 3	2660
	11th to 20th	Day	34% 4 12% 27% 20% 2	2660
	21st to end	Day	38% 5% 16% 20% 16% 4	2660
July	1st to 10th	Day	42% 4 11% 23% 17% 2	4371
	11th to 20th	Day	30% 5% 15% 30% 19% 1	4365
	21st to end	Day	42% 7% 15% 18% 14% 3	4482
August	1st to 10th	Day	34% 5% 14% 31% 15% 2	2444
	11th to 20th	Day	38% 6% 13% 27% 15% 1	2355
	21st to end	Day	32% 4 12% 28% 17% 7%	2485
September	1st to 10th	Day	38% 4 11% 26% 17% 5%	2116
	11th to 20th	Day	36% 4 11% 26% 21% 3	1980
	21st to end	Day	35% 3 9% 27% 25% 1	1980
October	1st to 10th	Day	38% 3 11% 25% 21% 3	1980
	11th to 20th	Day	40% 3 9% 26% 20% 2	1846
	21st to end	Day	53% 2 9% 22% 13% 1	1782
November	1st to 10th	Day	35% 2 12% 30% 18% 4	1672
	11th to 20th	Day	49% 2 9% 22% 14% 4	1710
	21st to end	Day	53% 2 9% 23% 11% 2	1710
December	1st to 10th	Day	62% 3 9% 18% 8%	1710
	11th to 20th	Day	61% 2 7% 22% 8% 1	1972
	21st to end	Day	75% 1 7% 10% 6%	2783
January	1st to 10th	Night	65% 2 9% 16% 8%	4428
	11th to 20th	Night	71% 1 8% 14% 5%	4440
	21st to end	Night	69% 1 7% 15% 7%	4695
February	1st to 10th	Night	60% 2 10% 17% 9% 1	2295
	11th to 20th	Night	54% 3 10% 20% 13% 1	2219
	21st to end	Night	58% 2 9% 18% 10% 2	1884
March	1st to 10th	Night	54% 2 7% 25% 11% 1	3733
	11th to 20th	Night	41% 2 10% 26% 19% 2	3576
	21st to end	Night	49% 2 10% 29% 8% 1	3707
April	1st to 10th	Night	47% 2 11% 24% 14% 2	1900
	11th to 20th	Night	45% 2 8% 22% 17% 5%	1875
	21st to end	Night	48% 3 9% 22% 13% 4	1720
May	1st to 10th	Night	56% 3 10% 22% 7% 1	2840
	11th to 20th	Night	45% 2 13% 28% 11% 1	2813
	21st to end	Night	37% 3 14% 26% 16% 3	3036
June	1st to 10th	Night	36% 3 11% 26% 21% 3	1520
	11th to 20th	Night	35% 3 11% 25% 22% 4	1520
	21st to end	Night	31% 8% 14% 26% 18% 4	1520
July	1st to 10th	Night	35% 5% 11% 27% 17% 5%	2490
	11th to 20th	Night	25% 4 15% 31% 23% 2	2482
	21st to end	Night	40% 5% 15% 22% 15% 4	2836
August	1st to 10th	Night	32% 4 14% 31% 18% 2	1690
	11th to 20th	Night	32% 4 13% 29% 19% 2	1762
	21st to end	Night	28% 5% 13% 27% 22% 5%	2080
September	1st to 10th	Night	35% 3 11% 24% 19% 7%	1824
	11th to 20th	Night	29% 4 9% 30% 25% 3	1980
	21st to end	Night	35% 3 9% 25% 24% 4	1980
October	1st to 10th	Night	32% 3 11% 26% 25% 4	1980
	11th to 20th	Night	42% 3 7% 27% 20% 2	2110
	21st to end	Night	47% 2 8% 25% 15% 2	2574
November	1st to 10th	Night	38% 2 9% 25% 22% 3	2406
	11th to 20th	Night	46% 2 8% 23% 17% 3	2470
	21st to end	Night	55% 2 9% 23% 11%	2470
December	1st to 10th	Night	57% 3 9% 20% 10% 1	2470
	11th to 20th	Night	57% 1 8% 21% 12% 1	3219
	21st to end	Night	73% 2 8% 11% 6%	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	24% 16% 31% 23% 6% 1	270
	11th to 20th	Day	24% 16% 38% 16% 5% 1	270
	21st to end	Day	31% 13% 28% 22% 5%	297
February	1st to 10th	Day	28% 10% 36% 21% 4	270
	11th to 20th	Day	22% 18% 33% 20% 7%	270
	21st to end	Day	28% 9% 38% 21% 3	225
March	1st to 10th	Day	23% 16% 36% 21% 4	270
	11th to 20th	Day	26% 13% 34% 19% 9%	402
	21st to end	Day	29% 12% 32% 18% 7% 1	506
April	1st to 10th	Day	18% 13% 32% 25% 9% 2	460
	11th to 20th	Day	22% 12% 29% 24% 10% 3	452
	21st to end	Day	24% 11% 36% 19% 7% 2	460
May	1st to 10th	Day	23% 10% 29% 26% 10% 3	463
	11th to 20th	Day	21% 6% 27% 24% 15% 7%	487
	21st to end	Day	14% 4 24% 25% 21% 12%	552
June	1st to 10th	Day	13% 5% 23% 31% 22% 7%	680
	11th to 20th	Day	12% 4 20% 34% 23% 8%	680
	21st to end	Day	11% 5% 24% 29% 22% 9%	680
July	1st to 10th	Day	13% 2 13% 26% 26% 20%	671
	11th to 20th	Day	9% 3 16% 33% 27% 13%	668
	21st to end	Day	11% 1 13% 30% 30% 14%	630
August	1st to 10th	Day	10% 2 10% 26% 32% 19%	486
	11th to 20th	Day	14% 4 14% 28% 28% 13%	479
	21st to end	Day	8% 2 10% 28% 34% 17%	532
September	1st to 10th	Day	12% 3 14% 27% 31% 13%	450
	11th to 20th	Day	10% 3 12% 26% 31% 18%	450
	21st to end	Day	16% 6% 20% 23% 23% 12%	450
October	1st to 10th	Day	13% 6% 21% 34% 17% 9%	450
	11th to 20th	Day	14% 12% 34% 27% 13% 1	406
	21st to end	Day	29% 11% 37% 20% 3	297
November	1st to 10th	Day	14% 14% 29% 25% 15% 3	270
	11th to 20th	Day	21% 14% 29% 21% 12% 3	270
	21st to end	Day	25% 8% 36% 24% 6% 1	270
December	1st to 10th	Day	21% 14% 40% 19% 6% 1	270
	11th to 20th	Day	21% 14% 34% 25% 4 2	270
	21st to end	Day	29% 17% 32% 18% 4 1	297
January	1st to 10th	Night	23% 12% 34% 25% 5%	950
	11th to 20th	Night	27% 11% 37% 21% 4	950
	21st to end	Night	25% 11% 33% 27% 4	934
February	1st to 10th	Night	28% 10% 33% 26% 3	760
	11th to 20th	Night	21% 13% 37% 27% 3	743
	21st to end	Night	26% 11% 37% 24% 2	628
March	1st to 10th	Night	23% 12% 33% 26% 6%	760
	11th to 20th	Night	20% 11% 37% 25% 6% 1	760
	21st to end	Night	26% 7% 32% 29% 6% 1	777
April	1st to 10th	Night	18% 10% 36% 27% 8% 1	570
	11th to 20th	Night	18% 9% 34% 30% 9% 1	563
	21st to end	Night	22% 6% 39% 25% 8% 1	570
May	1st to 10th	Night	20% 7% 31% 29% 12%	570
	11th to 20th	Night	21% 3 21% 34% 18% 2	561
	21st to end	Night	11% 2 20% 39% 26% 2	617
June	1st to 10th	Night	11% 4 19% 47% 18% 1	570
	11th to 20th	Night	11% 4 17% 44% 22% 2	570
	21st to end	Night	7% 2 22% 38% 29% 2	570
July	1st to 10th	Night	11% 3 8% 36% 36% 7%	555
	11th to 20th	Night	8% 3 12% 38% 34% 5%	552
	21st to end	Night	10% 3 10% 42% 33% 3	624
August	1st to 10th	Night	7% 3 9% 41% 35% 5%	567
	11th to 20th	Night	7% 2 11% 43% 31% 6%	564
	21st to end	Night	7% 2 11% 36% 39% 5%	624
September	1st to 10th	Night	13% 2 12% 36% 32% 5%	564
	11th to 20th	Night	9% 3 14% 34% 35% 6%	720
	21st to end	Night	13% 3 19% 34% 26% 3	720
October	1st to 10th	Night	12% 4 24% 34% 21% 5%	720
	11th to 20th	Night	19% 8% 29% 29% 14% 1	720
	21st to end	Night	22% 8% 35% 27% 7% 1	792
November	1st to 10th	Night	17% 7% 32% 30% 14% 1	740
	11th to 20th	Night	18% 13% 32% 29% 8% 1	760
	21st to end	Night	25% 9% 30% 29% 8%	760
December	1st to 10th	Night	23% 9% 37% 24% 6%	760
	11th to 20th	Night	26% 8% 33% 26% 7% 1	888
	21st to end	Night	31% 12% 30% 23% 4	1045

Page generated at 2024-08-27 09:49:38. Version ac07

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