Spenco Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spenco Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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% 7% 3 6% 5% 3 5% 5% 5% 8% 41%	74938
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	75124
Transparency	Day	27% 38% 8% 15% 10% 2	74642
	Night	27% 37% 7% 14% 11% 2	74941
Seeing	Day	15% 8% 22% 25% 20% 9%	14380
	Night	17% 7% 25% 31% 18% 2	24328

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% 9% 5% 9% 5% 3 3 4 4 5% 36%	1517
	11th to 20th	Day	19% 10% 4 8% 6% 3 4 4 5% 7% 30%	1520
	21st to end	Day	14% 11% 4 7% 6% 3 4 5% 4 5% 36%	1817
February	1st to 10th	Day	18% 11% 5% 6% 5% 2 4 4 5% 6% 35%	1800
	11th to 20th	Day	17% 9% 5% 7% 5% 3 4 4 6% 5% 34%	1748
	21st to end	Day	16% 10% 3 8% 6% 3 6% 4 5% 6% 33%	1480
March	1st to 10th	Day	17% 8% 3 6% 5% 3 4 5% 4 5% 41%	1800
	11th to 20th	Day	20% 8% 4 7% 5% 3 4 5% 5% 6% 33%	1853
	21st to end	Day	19% 9% 4 6% 6% 4 5% 5% 5% 6% 32%	2376
April	1st to 10th	Day	12% 6% 3 5% 5% 3 4 5% 5% 9% 44%	2160
	11th to 20th	Day	15% 7% 3 6% 6% 3 5% 5% 4 7% 41%	2134
	21st to end	Day	17% 7% 2 6% 5% 3 4 4 5% 7% 39%	2168
May	1st to 10th	Day	17% 6% 3 6% 5% 3 5% 5% 6% 9% 35%	2340
	11th to 20th	Day	17% 6% 3 6% 5% 3 5% 4 5% 8% 39%	2301
	21st to end	Day	8% 5% 2 6% 7% 5% 7% 7% 7% 11% 35%	2628
June	1st to 10th	Day	10% 5% 2 4 4 3 5% 6% 6% 8% 46%	2520
	11th to 20th	Day	5% 3 2 4 5% 4 6% 5% 7% 9% 49%	2520
	21st to end	Day	7% 4 2 4 5% 4 5% 7% 8% 10% 45%	2520
July	1st to 10th	Day	9% 5% 2 5% 5% 4 6% 6% 7% 10% 39%	2520
	11th to 20th	Day	6% 4 2 4 5% 3 5% 5% 8% 10% 47%	2520
	21st to end	Day	7% 4 2 5% 6% 4 6% 7% 7% 10% 44%	2721
August	1st to 10th	Day	7% 4 2 5% 5% 4 6% 5% 7% 10% 46%	2195
	11th to 20th	Day	10% 5% 2 5% 5% 2 5% 6% 7% 9% 44%	2125
	21st to end	Day	6% 4 2 5% 4 3 6% 5% 5% 10% 49%	2376
September	1st to 10th	Day	10% 5% 2 5% 5% 3 4 5% 5% 7% 48%	2160
	11th to 20th	Day	9% 4 2 5% 6% 4 5% 5% 5% 8% 48%	2021
	21st to end	Day	10% 4 2 5% 4 3 3 3 4 6% 55%	1980
October	1st to 10th	Day	10% 4 1 4 4 3 3 4 4 7% 55%	1980
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 49%	1818
	21st to end	Day	18% 7% 2 5% 3 3 4 4 5% 5% 45%	1831
November	1st to 10th	Day	9% 5% 2 4 3 2 4 5% 6% 7% 53%	1672
	11th to 20th	Day	15% 5% 4 5% 5% 2 3 4 4 6% 46%	1710
	21st to end	Day	20% 7% 3 5% 4 2 4 4 4 6% 42%	1710
December	1st to 10th	Day	18% 11% 4 5% 5% 2 4 3 4 5% 39%	1696
	11th to 20th	Day	15% 10% 5% 8% 7% 3 5% 5% 5% 6% 32%	1918
	21st to end	Day	23% 14% 5% 7% 6% 3 3 4 4 5% 26%	2783
January	1st to 10th	Night	16% 10% 4 6% 5% 3 3 4 4 6% 38%	2648
	11th to 20th	Night	19% 11% 4 8% 5% 2 4 4 5% 5% 33%	2660
	21st to end	Night	17% 9% 4 6% 6% 3 4 5% 4 6% 35%	2677
February	1st to 10th	Night	20% 10% 4 7% 5% 3 4 4 4 5% 32%	2160
	11th to 20th	Night	17% 7% 3 7% 5% 3 4 4 4 5% 39%	2098
	21st to end	Night	20% 9% 3 6% 5% 3 4 5% 4 5% 36%	1776
March	1st to 10th	Night	17% 7% 2 5% 4 3 4 5% 4 5% 44%	2160
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 4 6% 38%	2122
	21st to end	Night	20% 9% 3 6% 5% 4 5% 5% 4 7% 33%	1980
April	1st to 10th	Night	12% 6% 3 5% 5% 3 4 4 5% 7% 46%	1800
	11th to 20th	Night	16% 6% 3 5% 4 3 5% 4 4 7% 44%	1775
	21st to end	Night	19% 6% 3 6% 5% 2 4 5% 5% 8% 39%	1789
May	1st to 10th	Night	16% 6% 3 5% 5% 3 4 5% 4 7% 42%	1620
	11th to 20th	Night	11% 6% 3 6% 7% 4 6% 4 5% 7% 41%	1592
	21st to end	Night	7% 4 2 6% 6% 4 5% 6% 8% 11% 41%	1643
June	1st to 10th	Night	8% 6% 2 5% 4 3 5% 4 6% 10% 48%	1440
	11th to 20th	Night	5% 3 1 5% 5% 3 4 5% 6% 10% 54%	1440
	21st to end	Night	4 3 2 4 3 2 5% 5% 8% 10% 53%	1440
July	1st to 10th	Night	7% 5% 2 5% 4 2 4 6% 6% 10% 48%	1440
	11th to 20th	Night	6% 3 2 4 4 3 4 4 5% 9% 56%	1440
	21st to end	Night	7% 4 2 5% 4 4 5% 5% 6% 9% 49%	1597
August	1st to 10th	Night	6% 3 2 4 5% 3 4 5% 6% 10% 53%	1739
	11th to 20th	Night	5% 4 2 5% 5% 3 5% 5% 6% 10% 50%	1776
	21st to end	Night	6% 3 2 4 5% 3 4 4 5% 9% 53%	1980
September	1st to 10th	Night	10% 5% 3 5% 3 3 3 4 5% 6% 54%	1800
	11th to 20th	Night	9% 4 2 5% 4 2 4 5% 6% 8% 52%	1921
	21st to end	Night	10% 5% 2 3 4 2 3 5% 5% 6% 54%	1980
October	1st to 10th	Night	7% 5% 2 5% 4 2 3 4 4 6% 58%	1980
	11th to 20th	Night	15% 4 1 4 4 2 4 3 4 6% 52%	2139
	21st to end	Night	18% 8% 3 5% 4 2 3 3 4 5% 45%	2524
November	1st to 10th	Night	12% 5% 2 4 3 2 4 3 4 5% 56%	2406
	11th to 20th	Night	14% 5% 2 5% 6% 2 3 3 4 7% 50%	2470
	21st to end	Night	19% 5% 2 5% 4 3 5% 3 3 6% 46%	2470
December	1st to 10th	Night	18% 10% 3 6% 5% 3 4 4 5% 6% 37%	2479
	11th to 20th	Night	17% 9% 3 7% 5% 3 4 4 4 5% 38%	3279
	21st to end	Night	21% 14% 4 7% 6% 3 5% 5% 4 5% 26%	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	31% 41% 8% 14% 6%	1518
	11th to 20th	Day	34% 40% 8% 13% 6%	1520
	21st to end	Day	35% 37% 7% 17% 4 1	1817
February	1st to 10th	Day	34% 37% 7% 14% 7%	1800
	11th to 20th	Day	32% 37% 7% 14% 9% 2	1758
	21st to end	Day	35% 38% 7% 11% 7% 2	1480
March	1st to 10th	Day	29% 37% 6% 14% 13%	1800
	11th to 20th	Day	35% 37% 8% 12% 6% 1	1853
	21st to end	Day	37% 37% 8% 13% 4 1	2376
April	1st to 10th	Day	21% 38% 10% 18% 13% 1	2160
	11th to 20th	Day	28% 36% 7% 14% 12% 2	2134
	21st to end	Day	28% 39% 8% 17% 7% 1	2168
May	1st to 10th	Day	32% 39% 9% 14% 6% 1	2340
	11th to 20th	Day	29% 41% 7% 11% 11% 1	2305
	21st to end	Day	24% 40% 11% 13% 9% 2	2628
June	1st to 10th	Day	22% 40% 9% 16% 12% 2	2520
	11th to 20th	Day	17% 39% 10% 17% 15% 2	2520
	21st to end	Day	20% 38% 9% 15% 13% 5%	2520
July	1st to 10th	Day	21% 40% 10% 16% 10% 3	2451
	11th to 20th	Day	22% 41% 10% 13% 12% 2	2445
	21st to end	Day	21% 41% 11% 18% 7% 1	2570
August	1st to 10th	Day	20% 39% 10% 19% 12% 1	2182
	11th to 20th	Day	20% 39% 10% 18% 11% 2	2125
	21st to end	Day	19% 38% 8% 16% 14% 6%	2376
September	1st to 10th	Day	26% 39% 7% 13% 12% 3	2160
	11th to 20th	Day	23% 38% 5% 17% 14% 2	2021
	21st to end	Day	21% 37% 5% 14% 20% 4	1980
October	1st to 10th	Day	17% 36% 6% 16% 21% 4	1980
	11th to 20th	Day	24% 37% 6% 17% 13% 3	1818
	21st to end	Day	26% 38% 5% 16% 12% 2	1831
November	1st to 10th	Day	15% 41% 8% 19% 13% 4	1672
	11th to 20th	Day	25% 40% 5% 16% 12% 2	1710
	21st to end	Day	27% 39% 6% 15% 11% 2	1710
December	1st to 10th	Day	30% 40% 8% 16% 5%	1696
	11th to 20th	Day	35% 36% 6% 17% 6%	1915
	21st to end	Day	47% 29% 10% 9% 6%	2783
January	1st to 10th	Night	29% 41% 8% 14% 8%	2648
	11th to 20th	Night	34% 41% 7% 11% 6% 1	2660
	21st to end	Night	36% 36% 6% 14% 7% 1	2677
February	1st to 10th	Night	36% 36% 7% 14% 6% 1	2160
	11th to 20th	Night	31% 37% 5% 16% 9% 2	2099
	21st to end	Night	34% 37% 7% 13% 7% 2	1776
March	1st to 10th	Night	27% 37% 5% 14% 14% 2	2160
	11th to 20th	Night	32% 39% 10% 11% 7% 2	2122
	21st to end	Night	35% 37% 10% 12% 5% 1	1980
April	1st to 10th	Night	23% 37% 7% 18% 13% 2	1800
	11th to 20th	Night	25% 36% 7% 14% 14% 4	1775
	21st to end	Night	30% 37% 7% 15% 10% 1	1789
May	1st to 10th	Night	30% 38% 8% 15% 7% 2	1620
	11th to 20th	Night	29% 40% 6% 10% 13% 2	1593
	21st to end	Night	19% 40% 11% 14% 11% 4	1644
June	1st to 10th	Night	20% 38% 9% 15% 16% 2	1440
	11th to 20th	Night	15% 38% 10% 16% 17% 4	1440
	21st to end	Night	15% 37% 11% 17% 13% 7%	1440
July	1st to 10th	Night	18% 40% 10% 16% 11% 4	1400
	11th to 20th	Night	18% 40% 9% 17% 12% 5%	1392
	21st to end	Night	20% 42% 9% 21% 8% 1	1508
August	1st to 10th	Night	17% 38% 12% 16% 16% 1	1728
	11th to 20th	Night	16% 39% 9% 19% 15% 2	1776
	21st to end	Night	18% 37% 8% 15% 16% 5%	1980
September	1st to 10th	Night	23% 38% 5% 14% 14% 5%	1800
	11th to 20th	Night	22% 37% 6% 18% 13% 4	1921
	21st to end	Night	21% 37% 7% 13% 16% 6%	1980
October	1st to 10th	Night	18% 37% 5% 12% 23% 5%	1980
	11th to 20th	Night	24% 36% 5% 17% 14% 4	2139
	21st to end	Night	27% 38% 5% 15% 11% 4	2524
November	1st to 10th	Night	16% 40% 7% 16% 18% 3	2406
	11th to 20th	Night	23% 39% 6% 14% 14% 4	2470
	21st to end	Night	25% 40% 5% 14% 13% 2	2470
December	1st to 10th	Night	29% 41% 8% 15% 7%	2479
	11th to 20th	Night	33% 35% 6% 15% 10% 1	3281
	21st to end	Night	47% 28% 8% 10% 6% 1	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	25% 15% 31% 21% 8%	270
	11th to 20th	Day	24% 16% 36% 20% 4	270
	21st to end	Day	21% 21% 37% 16% 5%	282
February	1st to 10th	Day	25% 14% 40% 16% 4 1	240
	11th to 20th	Day	29% 22% 34% 12% 3	240
	21st to end	Day	27% 16% 32% 21% 4 1	198
March	1st to 10th	Day	20% 18% 39% 18% 5%	240
	11th to 20th	Day	27% 15% 34% 17% 6% 1	271
	21st to end	Day	28% 14% 29% 20% 9% 1	462
April	1st to 10th	Day	14% 15% 33% 26% 11% 1	420
	11th to 20th	Day	20% 14% 25% 25% 15% 1	412
	21st to end	Day	22% 9% 33% 23% 9% 4	420
May	1st to 10th	Day	20% 9% 28% 29% 11% 3	423
	11th to 20th	Day	18% 6% 24% 27% 19% 5%	447
	21st to end	Day	11% 4 21% 29% 23% 11%	582
June	1st to 10th	Day	13% 3 16% 27% 28% 13%	630
	11th to 20th	Day	6% 3 17% 33% 28% 12%	630
	21st to end	Day	7% 2 20% 34% 25% 11%	630
July	1st to 10th	Day	10% 3 11% 23% 28% 25%	621
	11th to 20th	Day	7% 3 12% 26% 35% 17%	618
	21st to end	Day	7% 3 10% 28% 35% 18%	662
August	1st to 10th	Day	9% 3 8% 28% 34% 18%	446
	11th to 20th	Day	16% 2 10% 26% 32% 15%	439
	21st to end	Day	9% 2 10% 21% 36% 22%	495
September	1st to 10th	Day	11% 2 11% 24% 35% 16%	450
	11th to 20th	Day	7% 2 10% 27% 34% 20%	450
	21st to end	Day	13% 6% 20% 22% 27% 13%	450
October	1st to 10th	Day	12% 3 20% 32% 24% 9%	450
	11th to 20th	Day	14% 10% 35% 25% 11% 5%	288
	21st to end	Day	25% 10% 29% 24% 11% 1	297
November	1st to 10th	Day	12% 6% 33% 29% 16% 4	270
	11th to 20th	Day	17% 15% 29% 25% 13% 2	270
	21st to end	Day	22% 10% 31% 25% 10% 1	270
December	1st to 10th	Day	21% 15% 31% 21% 8% 3	270
	11th to 20th	Day	22% 13% 28% 28% 9%	270
	21st to end	Day	28% 17% 32% 17% 6%	297
January	1st to 10th	Night	20% 14% 35% 27% 5%	950
	11th to 20th	Night	25% 14% 35% 22% 5%	950
	21st to end	Night	23% 10% 36% 28% 3	1018
February	1st to 10th	Night	25% 12% 34% 26% 3	720
	11th to 20th	Night	21% 14% 38% 26% 2	703
	21st to end	Night	24% 13% 37% 23% 2	592
March	1st to 10th	Night	22% 11% 35% 29% 4	720
	11th to 20th	Night	23% 8% 33% 28% 7%	713
	21st to end	Night	25% 8% 28% 31% 7% 1	594
April	1st to 10th	Night	15% 8% 34% 34% 9%	540
	11th to 20th	Night	21% 6% 29% 31% 13% 1	533
	21st to end	Night	20% 6% 31% 33% 10% 1	540
May	1st to 10th	Night	18% 5% 27% 36% 14% 1	540
	11th to 20th	Night	15% 4 22% 36% 20% 3	531
	21st to end	Night	8% 1 19% 38% 31% 3	584
June	1st to 10th	Night	11% 3 15% 39% 29% 3	540
	11th to 20th	Night	6% 1 11% 46% 33% 2	540
	21st to end	Night	4 1 16% 41% 34% 4	540
July	1st to 10th	Night	9% 1 8% 31% 42% 9%	525
	11th to 20th	Night	5% 3 10% 30% 46% 6%	522
	21st to end	Night	8% 1 9% 35% 39% 7%	591
August	1st to 10th	Night	8% 1 7% 35% 44% 6%	537
	11th to 20th	Night	7% 1 7% 38% 40% 6%	534
	21st to end	Night	8% 1 7% 32% 44% 8%	594
September	1st to 10th	Night	11% 2 10% 31% 39% 7%	540
	11th to 20th	Night	10% 2 10% 30% 41% 7%	661
	21st to end	Night	11% 3 20% 30% 29% 6%	720
October	1st to 10th	Night	9% 3 19% 37% 27% 5%	720
	11th to 20th	Night	19% 6% 25% 34% 16% 2	720
	21st to end	Night	21% 7% 31% 29% 11% 1	792
November	1st to 10th	Night	14% 5% 29% 35% 15% 1	740
	11th to 20th	Night	16% 8% 33% 30% 13%	760
	21st to end	Night	22% 8% 28% 32% 11%	760
December	1st to 10th	Night	24% 8% 29% 33% 6%	769
	11th to 20th	Night	24% 7% 31% 31% 7%	950
	21st to end	Night	27% 11% 32% 25% 4	1045

Page generated at 2024-01-24 08:37:33. Version ac07

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