808.5 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 808.5 Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	79252
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 45%	80890
Transparency	Day	38% 7% 11% 20% 19% 5%	78975
	Night	38% 7% 11% 18% 20% 6%	80686
Seeing	Day	16% 9% 24% 24% 19% 8%	14476
	Night	16% 8% 24% 31% 19% 2	24661

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	17% 4 2 4 3 2 3 3 3 6% 51%	2836
	11th to 20th	Day	13% 6% 3 4 4 2 4 3 4 5% 52%	2840
	21st to end	Day	20% 9% 2 5% 4 2 3 4 4 4 42%	3124
February	1st to 10th	Day	15% 6% 3 6% 5% 2 4 5% 5% 6% 43%	1710
	11th to 20th	Day	20% 6% 2 4 4 2 4 4 4 5% 46%	1803
	21st to end	Day	18% 7% 3 5% 5% 3 4 4 4 5% 40%	1639
March	1st to 10th	Day	16% 5% 2 4 4 2 3 3 5% 6% 51%	2090
	11th to 20th	Day	16% 6% 2 5% 4 3 5% 4 6% 7% 42%	2090
	21st to end	Day	16% 7% 2 6% 4 2 5% 6% 5% 7% 41%	2299
April	1st to 10th	Day	11% 4 2 4 4 3 4 5% 6% 8% 51%	2010
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 7% 47%	2134
	21st to end	Day	13% 4 2 4 5% 2 5% 5% 5% 6% 50%	2263
May	1st to 10th	Day	9% 5% 2 5% 5% 4 6% 5% 7% 7% 46%	2340
	11th to 20th	Day	13% 5% 2 5% 5% 3 5% 7% 6% 8% 41%	2301
	21st to end	Day	10% 6% 3 6% 6% 4 6% 5% 6% 9% 40%	2531
June	1st to 10th	Day	10% 4 2 6% 6% 3 6% 7% 7% 12% 38%	2340
	11th to 20th	Day	9% 4 2 5% 6% 4 5% 6% 7% 10% 41%	2340
	21st to end	Day	6% 4 2 5% 7% 4 7% 7% 8% 10% 39%	2340
July	1st to 10th	Day	8% 5% 3 6% 7% 4 6% 7% 8% 12% 34%	2340
	11th to 20th	Day	9% 5% 2 7% 6% 4 7% 8% 8% 12% 33%	2340
	21st to end	Day	6% 5% 3 6% 8% 5% 6% 8% 8% 10% 35%	2547
August	1st to 10th	Day	11% 6% 2 6% 6% 5% 7% 7% 8% 11% 31%	2327
	11th to 20th	Day	12% 6% 2 6% 8% 5% 7% 6% 8% 10% 29%	2303
	21st to end	Day	10% 7% 3 5% 5% 5% 7% 6% 6% 10% 36%	2394
September	1st to 10th	Day	8% 4 2 6% 5% 4 6% 6% 6% 9% 43%	2033
	11th to 20th	Day	12% 6% 2 5% 5% 4 4 5% 6% 8% 43%	1980
	21st to end	Day	19% 6% 2 4 5% 3 4 5% 5% 6% 41%	1980
October	1st to 10th	Day	14% 6% 2 5% 4 2 2 3 4 6% 52%	1854
	11th to 20th	Day	13% 4 2 4 3 2 3 4 4 5% 56%	1800
	21st to end	Day	13% 4 2 4 4 2 3 3 4 5% 56%	1980
November	1st to 10th	Day	14% 4 2 4 3 2 3 3 4 6% 57%	1857
	11th to 20th	Day	13% 5% 2 4 3 2 3 3 5% 6% 53%	1790
	21st to end	Day	15% 5% 2 4 4 2 4 4 5% 6% 50%	1710
December	1st to 10th	Day	18% 7% 3 5% 4 3 4 4 6% 7% 39%	1710
	11th to 20th	Day	16% 4 2 4 3 1 3 3 4 7% 52%	2153
	21st to end	Day	18% 6% 3 5% 4 3 4 4 4 6% 44%	3124
January	1st to 10th	Night	17% 5% 2 5% 4 3 3 3 4 5% 49%	4109
	11th to 20th	Night	11% 6% 3 5% 5% 3 4 5% 5% 6% 47%	4120
	21st to end	Night	21% 8% 3 4 4 2 3 3 3 4 45%	4532
February	1st to 10th	Night	16% 6% 3 6% 5% 3 3 4 5% 5% 46%	2470
	11th to 20th	Night	20% 7% 3 5% 4 2 3 4 4 5% 43%	2260
	21st to end	Night	16% 7% 2 4 4 3 3 4 6% 6% 45%	1813
March	1st to 10th	Night	15% 6% 3 4 4 2 3 3 5% 6% 49%	2090
	11th to 20th	Night	15% 5% 3 5% 4 2 4 4 6% 8% 43%	2090
	21st to end	Night	15% 6% 2 5% 5% 3 5% 5% 5% 6% 42%	2299
April	1st to 10th	Night	11% 6% 2 4 4 3 4 4 5% 7% 50%	1994
	11th to 20th	Night	11% 5% 2 4 5% 3 4 4 4 7% 52%	1775
	21st to end	Night	12% 4 2 5% 5% 2 4 4 6% 7% 50%	1710
May	1st to 10th	Night	10% 6% 2 5% 4 2 4 5% 5% 8% 48%	1620
	11th to 20th	Night	13% 6% 3 7% 6% 4 5% 6% 6% 8% 36%	1592
	21st to end	Night	11% 5% 2 5% 5% 4 4 6% 7% 9% 41%	1744
June	1st to 10th	Night	10% 6% 2 8% 6% 4 6% 6% 7% 9% 36%	1620
	11th to 20th	Night	11% 5% 2 6% 7% 3 6% 7% 8% 11% 35%	1620
	21st to end	Night	7% 5% 2 6% 6% 4 7% 6% 9% 13% 35%	1620
July	1st to 10th	Night	7% 6% 4 7% 7% 5% 6% 7% 8% 13% 32%	1620
	11th to 20th	Night	7% 5% 3 5% 6% 4 6% 7% 9% 12% 36%	1620
	21st to end	Night	5% 5% 3 8% 7% 5% 6% 8% 8% 12% 33%	1771
August	1st to 10th	Night	11% 5% 3 5% 6% 3 6% 7% 9% 12% 31%	1611
	11th to 20th	Night	10% 7% 3 7% 7% 4 6% 6% 7% 9% 33%	1598
	21st to end	Night	9% 6% 2 7% 7% 5% 6% 7% 8% 10% 33%	1944
September	1st to 10th	Night	9% 4 2 5% 6% 3 5% 5% 7% 9% 45%	1923
	11th to 20th	Night	13% 6% 3 5% 5% 3 4 5% 6% 7% 43%	1980
	21st to end	Night	18% 5% 2 5% 4 4 5% 5% 6% 5% 39%	1980
October	1st to 10th	Night	14% 5% 2 4 4 2 4 4 4 6% 52%	2106
	11th to 20th	Night	14% 5% 2 4 3 2 2 3 5% 5% 54%	2160
	21st to end	Night	12% 6% 2 3 4 2 3 4 4 5% 57%	2376
November	1st to 10th	Night	13% 6% 2 4 4 2 3 2 3 6% 54%	2221
	11th to 20th	Night	13% 6% 2 4 4 2 3 4 4 7% 51%	2388
	21st to end	Night	14% 4 2 4 4 3 4 4 5% 7% 49%	2470
December	1st to 10th	Night	17% 8% 3 5% 5% 3 4 4 4 6% 42%	2470
	11th to 20th	Night	17% 6% 2 4 4 2 2 2 3 5% 51%	3042
	21st to end	Night	19% 5% 2 5% 5% 2 4 4 5% 6% 44%	4532

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	36% 3 6% 25% 23% 6%	2837
	11th to 20th	Day	35% 5% 6% 16% 31% 7%	2840
	21st to end	Day	44% 5% 6% 16% 24% 6%	3124
February	1st to 10th	Day	40% 5% 10% 21% 19% 5%	1710
	11th to 20th	Day	41% 4 8% 18% 23% 6%	1813
	21st to end	Day	46% 5% 7% 16% 19% 7%	1639
March	1st to 10th	Day	36% 3 9% 20% 24% 8%	2090
	11th to 20th	Day	40% 5% 11% 19% 18% 7%	2090
	21st to end	Day	42% 7% 10% 19% 19% 3	2299
April	1st to 10th	Day	31% 6% 11% 21% 26% 6%	2010
	11th to 20th	Day	36% 5% 9% 19% 21% 9%	2134
	21st to end	Day	34% 6% 11% 23% 22% 5%	2263
May	1st to 10th	Day	35% 8% 13% 22% 19% 3	2340
	11th to 20th	Day	39% 11% 14% 18% 15% 2	2305
	21st to end	Day	38% 11% 13% 22% 15% 1	2531
June	1st to 10th	Day	38% 11% 13% 25% 12% 1	2340
	11th to 20th	Day	36% 10% 18% 21% 14% 1	2340
	21st to end	Day	35% 12% 15% 21% 14% 2	2340
July	1st to 10th	Day	39% 13% 18% 17% 10% 3	2275
	11th to 20th	Day	40% 13% 22% 17% 7% 1	2271
	21st to end	Day	38% 13% 18% 21% 8% 1	2405
August	1st to 10th	Day	43% 12% 18% 19% 8% 1	2314
	11th to 20th	Day	45% 13% 16% 16% 9% 1	2303
	21st to end	Day	41% 12% 16% 18% 9% 3	2394
September	1st to 10th	Day	35% 10% 15% 19% 18% 3	2033
	11th to 20th	Day	38% 8% 12% 18% 20% 4	1980
	21st to end	Day	44% 6% 9% 19% 17% 5%	1980
October	1st to 10th	Day	34% 5% 8% 21% 26% 5%	1854
	11th to 20th	Day	31% 5% 8% 25% 23% 8%	1800
	21st to end	Day	32% 4 7% 21% 27% 10%	1980
November	1st to 10th	Day	31% 4 7% 20% 29% 10%	1857
	11th to 20th	Day	31% 4 10% 20% 27% 8%	1790
	21st to end	Day	36% 4 9% 21% 22% 8%	1710
December	1st to 10th	Day	44% 6% 9% 19% 18% 4	1710
	11th to 20th	Day	33% 4 6% 21% 28% 8%	2150
	21st to end	Day	40% 6% 9% 22% 20% 3	3124
January	1st to 10th	Night	37% 5% 8% 18% 26% 7%	4109
	11th to 20th	Night	36% 6% 7% 16% 27% 7%	4120
	21st to end	Night	45% 3 5% 16% 25% 7%	4532
February	1st to 10th	Night	40% 5% 9% 19% 21% 7%	2470
	11th to 20th	Night	44% 4 7% 16% 22% 7%	2261
	21st to end	Night	39% 5% 10% 19% 20% 7%	1813
March	1st to 10th	Night	37% 4 8% 18% 22% 10%	2090
	11th to 20th	Night	39% 5% 12% 16% 21% 8%	2090
	21st to end	Night	42% 6% 10% 18% 19% 5%	2299
April	1st to 10th	Night	34% 5% 10% 20% 24% 7%	1994
	11th to 20th	Night	32% 6% 11% 20% 22% 9%	1775
	21st to end	Night	32% 8% 10% 24% 22% 5%	1710
May	1st to 10th	Night	33% 8% 15% 23% 18% 4	1620
	11th to 20th	Night	43% 9% 13% 19% 15% 1	1593
	21st to end	Night	35% 12% 15% 24% 12% 2	1745
June	1st to 10th	Night	41% 10% 14% 21% 11% 2	1620
	11th to 20th	Night	40% 11% 17% 20% 11% 1	1620
	21st to end	Night	36% 14% 18% 18% 11% 3	1620
July	1st to 10th	Night	41% 13% 20% 16% 8% 3	1575
	11th to 20th	Night	36% 13% 23% 19% 7% 2	1566
	21st to end	Night	38% 14% 21% 18% 9% 1	1672
August	1st to 10th	Night	40% 13% 22% 18% 6% 1	1600
	11th to 20th	Night	45% 10% 19% 15% 9% 2	1598
	21st to end	Night	40% 13% 16% 22% 7% 2	1944
September	1st to 10th	Night	34% 10% 14% 20% 15% 6%	1923
	11th to 20th	Night	38% 9% 14% 17% 19% 3	1980
	21st to end	Night	43% 8% 10% 16% 17% 5%	1980
October	1st to 10th	Night	34% 6% 10% 19% 25% 6%	2106
	11th to 20th	Night	33% 5% 8% 23% 23% 9%	2160
	21st to end	Night	30% 4 8% 20% 27% 11%	2376
November	1st to 10th	Night	33% 4 7% 17% 26% 13%	2221
	11th to 20th	Night	34% 4 7% 17% 25% 13%	2388
	21st to end	Night	35% 5% 6% 20% 27% 8%	2470
December	1st to 10th	Night	45% 5% 9% 17% 19% 6%	2470
	11th to 20th	Night	37% 4 8% 17% 24% 9%	3044
	21st to end	Night	40% 5% 9% 20% 23% 3	4532

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	22% 19% 39% 17% 3	270
	11th to 20th	Day	24% 22% 32% 17% 5%	270
	21st to end	Day	20% 23% 38% 15% 3	297
February	1st to 10th	Day	21% 18% 40% 14% 7%	270
	11th to 20th	Day	34% 15% 39% 10% 2	270
	21st to end	Day	22% 26% 31% 17% 3	294
March	1st to 10th	Day	20% 17% 33% 21% 8% 1	460
	11th to 20th	Day	19% 17% 40% 18% 5% 1	460
	21st to end	Day	21% 15% 36% 21% 7% 1	506
April	1st to 10th	Day	18% 18% 31% 22% 10% 2	428
	11th to 20th	Day	14% 17% 30% 21% 15% 3	412
	21st to end	Day	16% 10% 30% 25% 15% 4	420
May	1st to 10th	Day	11% 12% 35% 22% 16% 4	423
	11th to 20th	Day	14% 5% 21% 28% 23% 8%	447
	21st to end	Day	14% 4 18% 26% 27% 12%	485
June	1st to 10th	Day	11% 4 18% 31% 28% 9%	450
	11th to 20th	Day	12% 5% 18% 23% 30% 12%	450
	21st to end	Day	8% 2 15% 33% 30% 11%	450
July	1st to 10th	Day	8% 2 11% 22% 38% 19%	445
	11th to 20th	Day	10% 1 9% 27% 34% 19%	444
	21st to end	Day	7% 1 10% 29% 38% 15%	488
August	1st to 10th	Day	15% 3 13% 23% 26% 18%	446
	11th to 20th	Day	13% 3 8% 28% 30% 19%	439
	21st to end	Day	13% 2 11% 27% 29% 20%	495
September	1st to 10th	Day	10% 2 10% 34% 27% 18%	450
	11th to 20th	Day	14% 4 13% 26% 29% 13%	450
	21st to end	Day	21% 4 19% 26% 24% 6%	450
October	1st to 10th	Day	17% 5% 20% 25% 24% 8%	450
	11th to 20th	Day	14% 8% 24% 26% 22% 7%	450
	21st to end	Day	16% 6% 28% 27% 18% 5%	495
November	1st to 10th	Day	19% 6% 26% 30% 16% 4	455
	11th to 20th	Day	17% 14% 32% 25% 11% 1	350
	21st to end	Day	13% 12% 37% 24% 13% 2	270
December	1st to 10th	Day	23% 14% 37% 19% 7%	270
	11th to 20th	Day	21% 11% 34% 24% 9% 1	270
	21st to end	Day	24% 17% 35% 16% 6% 3	297
January	1st to 10th	Night	21% 14% 33% 27% 6%	760
	11th to 20th	Night	19% 14% 33% 28% 6%	760
	21st to end	Night	20% 11% 37% 29% 3	836
February	1st to 10th	Night	18% 11% 38% 29% 4	760
	11th to 20th	Night	25% 12% 39% 22% 2	743
	21st to end	Night	20% 17% 35% 24% 4	628
March	1st to 10th	Night	18% 10% 36% 31% 5%	760
	11th to 20th	Night	18% 14% 36% 24% 8% 1	760
	21st to end	Night	18% 12% 34% 28% 7%	836
April	1st to 10th	Night	15% 13% 34% 29% 9% 1	729
	11th to 20th	Night	14% 13% 29% 29% 14%	711
	21st to end	Night	14% 7% 28% 38% 13%	630
May	1st to 10th	Night	11% 10% 29% 36% 13%	540
	11th to 20th	Night	15% 5% 22% 36% 21% 1	531
	21st to end	Night	13% 3 12% 39% 31% 2	584
June	1st to 10th	Night	11% 2 17% 40% 25% 4	540
	11th to 20th	Night	13% 3 14% 34% 29% 6%	540
	21st to end	Night	9% 1 15% 37% 33% 5%	540
July	1st to 10th	Night	8% 2 8% 35% 38% 8%	525
	11th to 20th	Night	8% 2 7% 34% 40% 8%	522
	21st to end	Night	7% 2 10% 33% 42% 5%	591
August	1st to 10th	Night	11% 2 9% 29% 42% 6%	537
	11th to 20th	Night	13% 3 10% 30% 38% 7%	534
	21st to end	Night	11% 2 10% 28% 41% 8%	756
September	1st to 10th	Night	11% 1 10% 31% 41% 6%	720
	11th to 20th	Night	15% 2 13% 28% 36% 5%	720
	21st to end	Night	20% 4 15% 33% 25% 3	720
October	1st to 10th	Night	16% 4 19% 27% 28% 6%	720
	11th to 20th	Night	16% 6% 23% 30% 22% 2	720
	21st to end	Night	14% 7% 24% 31% 21% 2	792
November	1st to 10th	Night	16% 6% 25% 32% 18% 3	740
	11th to 20th	Night	15% 11% 30% 32% 11% 1	760
	21st to end	Night	16% 9% 27% 34% 14% 1	760
December	1st to 10th	Night	19% 12% 29% 33% 7%	760
	11th to 20th	Night	20% 8% 27% 35% 10%	760
	21st to end	Night	24% 13% 24% 30% 9%	836

Page generated at 2024-03-31 06:08:53. Version ac07

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