Spotted-Tail Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spotted-Tail Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	7% 5% 2 4 4 3 4 5% 6% 8% 53%	74802
	Night	7% 6% 2 5% 4 3 4 4 5% 8% 52%	74943
Transparency	Day	28% 6% 14% 27% 19% 6%	74506
	Night	30% 6% 14% 27% 18% 5%	74761
Seeing	Day	9% 8% 29% 31% 18% 5%	14609
	Night	9% 7% 35% 38% 10%	24726

By Month

Percentage Cloud Cover SW (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	4 6% 3 5% 5% 2 4 4 6% 7% 54%	1518
	11th to 20th	Day	4 7% 2 6% 6% 4 6% 4 4 7% 50%	1452
	21st to end	Day	4 8% 3 5% 4 3 4 4 6% 8% 51%	1660
February	1st to 10th	Day	5% 8% 5% 7% 6% 4 5% 4 4 8% 45%	1620
	11th to 20th	Day	5% 8% 4 6% 5% 3 4 5% 7% 7% 46%	1745
	21st to end	Day	5% 8% 3 6% 4 3 4 5% 5% 7% 51%	1480
March	1st to 10th	Day	6% 7% 3 5% 5% 3 4 5% 6% 7% 52%	1800
	11th to 20th	Day	7% 5% 3 5% 4 4 5% 5% 4 9% 50%	1856
	21st to end	Day	9% 6% 3 6% 5% 3 4 5% 5% 9% 45%	2371
April	1st to 10th	Day	10% 7% 2 5% 4 3 4 4 6% 7% 48%	2160
	11th to 20th	Day	14% 7% 3 5% 5% 3 4 5% 5% 8% 42%	2131
	21st to end	Day	11% 6% 3 5% 5% 3 4 5% 6% 8% 45%	2227
May	1st to 10th	Day	13% 6% 2 5% 5% 2 4 5% 6% 8% 44%	2340
	11th to 20th	Day	15% 6% 2 5% 5% 3 5% 5% 7% 8% 40%	2472
	21st to end	Day	10% 6% 2 4 5% 4 5% 7% 7% 9% 42%	2724
June	1st to 10th	Day	6% 2 1 4 4 3 5% 6% 6% 10% 53%	2520
	11th to 20th	Day	7% 4 2 4 4 3 4 5% 6% 9% 52%	2520
	21st to end	Day	4 2 1 3 4 3 4 5% 6% 9% 59%	2520
July	1st to 10th	Day	6% 2 1 3 4 2 5% 6% 8% 7% 54%	2520
	11th to 20th	Day	4 2 1 2 3 3 5% 4 5% 8% 62%	2520
	21st to end	Day	5% 3 1 4 4 3 5% 5% 7% 10% 53%	2742
August	1st to 10th	Day	4 3 1 3 4 3 5% 6% 7% 9% 54%	2472
	11th to 20th	Day	4 2 2 3 3 3 4 5% 6% 8% 61%	2123
	21st to end	Day	5% 3 1 4 3 3 4 4 5% 8% 61%	2376
September	1st to 10th	Day	6% 4 2 3 3 2 3 4 4 6% 62%	2160
	11th to 20th	Day	8% 2 1 3 3 2 3 4 4 7% 64%	2090
	21st to end	Day	7% 4 1 3 2 1 3 3 5% 7% 63%	1980
October	1st to 10th	Day	11% 5% 1 3 3 1 3 3 5% 7% 57%	1887
	11th to 20th	Day	7% 4 2 3 3 2 4 4 5% 7% 60%	1800
	21st to end	Day	6% 4 2 3 3 2 3 5% 5% 7% 59%	1799
November	1st to 10th	Day	4 5% 2 4 4 3 3 4 5% 9% 58%	1673
	11th to 20th	Day	4 4 2 4 3 3 4 5% 5% 8% 58%	1710
	21st to end	Day	5% 6% 2 4 3 1 4 4 5% 6% 60%	1594
December	1st to 10th	Day	4 5% 3 5% 5% 3 5% 5% 6% 9% 51%	1520
	11th to 20th	Day	4 5% 2 4 4 2 4 4 6% 9% 57%	1926
	21st to end	Day	6% 11% 3 6% 5% 3 4 4 4 7% 48%	2794
January	1st to 10th	Night	4 6% 2 6% 5% 2 4 4 5% 9% 53%	2647
	11th to 20th	Night	3 6% 2 4 6% 3 5% 6% 6% 7% 52%	2548
	21st to end	Night	4 7% 3 6% 5% 4 4 5% 6% 8% 48%	2709
February	1st to 10th	Night	4 8% 3 6% 6% 3 4 5% 6% 9% 46%	2340
	11th to 20th	Night	5% 8% 3 6% 6% 3 5% 5% 6% 8% 46%	2101
	21st to end	Night	4 9% 3 6% 5% 3 4 4 6% 5% 50%	1776
March	1st to 10th	Night	6% 8% 4 5% 4 3 4 3 5% 7% 52%	2160
	11th to 20th	Night	7% 6% 3 4 4 3 4 5% 6% 8% 51%	2102
	21st to end	Night	9% 8% 3 6% 5% 3 5% 4 5% 8% 45%	1984
April	1st to 10th	Night	12% 7% 2 4 4 3 4 4 5% 7% 48%	1800
	11th to 20th	Night	11% 6% 2 6% 5% 2 4 5% 5% 6% 46%	1777
	21st to end	Night	14% 7% 2 5% 6% 3 5% 4 6% 6% 43%	1731
May	1st to 10th	Night	14% 6% 2 6% 5% 3 4 4 4 7% 44%	1620
	11th to 20th	Night	14% 6% 3 5% 5% 3 3 5% 5% 5% 47%	1420
	21st to end	Night	13% 4 2 4 5% 3 4 5% 8% 8% 44%	1552
June	1st to 10th	Night	8% 4 2 4 4 3 4 5% 7% 10% 51%	1440
	11th to 20th	Night	7% 3 1 4 4 3 4 6% 8% 11% 49%	1440
	21st to end	Night	4 2 1 4 4 3 5% 5% 7% 11% 55%	1440
July	1st to 10th	Night	7% 3 2 3 4 3 5% 5% 7% 9% 51%	1440
	11th to 20th	Night	3 2 1 2 3 3 4 4 6% 11% 61%	1440
	21st to end	Night	3 2 1 4 4 3 5% 6% 7% 11% 53%	1576
August	1st to 10th	Night	6% 3 2 4 5% 3 6% 6% 7% 9% 50%	1462
	11th to 20th	Night	3 3 1 3 3 2 4 5% 5% 11% 60%	1778
	21st to end	Night	4 2 2 4 4 3 4 5% 6% 7% 59%	1980
September	1st to 10th	Night	7% 5% 2 4 4 2 3 3 5% 6% 60%	1800
	11th to 20th	Night	8% 3 1 3 3 2 3 4 5% 6% 62%	1869
	21st to end	Night	8% 3 1 4 2 2 4 5% 4 5% 61%	1980
October	1st to 10th	Night	11% 5% 2 3 3 2 3 4 4 6% 56%	2070
	11th to 20th	Night	8% 4 2 3 3 3 4 3 4 7% 59%	2160
	21st to end	Night	8% 5% 2 4 3 3 3 4 6% 7% 55%	2538
November	1st to 10th	Night	6% 5% 2 4 3 2 4 4 5% 8% 57%	2405
	11th to 20th	Night	5% 4 2 4 4 3 4 5% 6% 8% 56%	2470
	21st to end	Night	4 8% 2 4 4 3 3 4 5% 8% 56%	2584
December	1st to 10th	Night	5% 6% 3 5% 5% 3 4 5% 5% 8% 53%	2660
	11th to 20th	Night	4 5% 3 4 3 2 4 5% 5% 9% 56%	3271
	21st to end	Night	7% 9% 3 6% 5% 3 4 4 5% 8% 47%	4873

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	29% 6% 15% 32% 17% 1	1519
	11th to 20th	Day	33% 6% 15% 29% 15% 2	1452
	21st to end	Day	30% 7% 18% 29% 14% 3	1660
February	1st to 10th	Day	39% 6% 15% 23% 14% 2	1620
	11th to 20th	Day	35% 8% 12% 25% 16% 4	1755
	21st to end	Day	33% 5% 15% 27% 16% 5%	1480
March	1st to 10th	Day	31% 5% 13% 27% 20% 4	1800
	11th to 20th	Day	31% 5% 14% 24% 21% 4	1856
	21st to end	Day	35% 8% 16% 28% 12% 2	2371
April	1st to 10th	Day	35% 6% 17% 25% 12% 5%	2160
	11th to 20th	Day	40% 6% 14% 21% 16% 4	2134
	21st to end	Day	35% 7% 12% 26% 17% 1	2227
May	1st to 10th	Day	36% 8% 15% 29% 11% 1	2340
	11th to 20th	Day	41% 8% 17% 24% 9% 1	2476
	21st to end	Day	35% 10% 18% 23% 12% 2	2724
June	1st to 10th	Day	25% 8% 18% 29% 16% 4	2520
	11th to 20th	Day	27% 7% 13% 25% 20% 7%	2520
	21st to end	Day	20% 6% 11% 26% 27% 9%	2520
July	1st to 10th	Day	25% 8% 11% 25% 24% 8%	2451
	11th to 20th	Day	19% 5% 10% 24% 26% 15%	2445
	21st to end	Day	23% 8% 13% 25% 22% 9%	2590
August	1st to 10th	Day	23% 8% 16% 28% 16% 9%	2456
	11th to 20th	Day	20% 6% 13% 27% 24% 11%	2123
	21st to end	Day	22% 5% 9% 22% 29% 14%	2376
September	1st to 10th	Day	22% 5% 8% 19% 30% 16%	2160
	11th to 20th	Day	20% 5% 9% 25% 27% 14%	2090
	21st to end	Day	21% 4 9% 27% 32% 7%	1980
October	1st to 10th	Day	29% 5% 12% 24% 25% 6%	1887
	11th to 20th	Day	24% 6% 10% 28% 24% 8%	1800
	21st to end	Day	23% 5% 15% 31% 20% 6%	1799
November	1st to 10th	Day	24% 4 17% 27% 20% 8%	1673
	11th to 20th	Day	23% 5% 14% 31% 24% 4	1710
	21st to end	Day	25% 4 14% 29% 25% 4	1594
December	1st to 10th	Day	29% 6% 18% 27% 17% 3	1520
	11th to 20th	Day	25% 5% 14% 38% 17% 1	1924
	21st to end	Day	36% 7% 11% 30% 14% 1	2794
January	1st to 10th	Night	29% 6% 16% 30% 17% 2	2647
	11th to 20th	Night	29% 6% 15% 29% 18% 3	2548
	21st to end	Night	33% 6% 17% 30% 13% 2	2709
February	1st to 10th	Night	36% 7% 16% 27% 12% 2	2340
	11th to 20th	Night	35% 8% 14% 25% 15% 3	2101
	21st to end	Night	33% 6% 14% 28% 15% 4	1776
March	1st to 10th	Night	32% 5% 12% 29% 18% 4	2160
	11th to 20th	Night	30% 6% 14% 28% 19% 3	2102
	21st to end	Night	38% 6% 17% 24% 14% 1	1984
April	1st to 10th	Night	36% 5% 15% 23% 16% 5%	1800
	11th to 20th	Night	36% 6% 15% 24% 15% 3	1777
	21st to end	Night	41% 7% 10% 26% 15% 1	1731
May	1st to 10th	Night	40% 7% 15% 24% 13% 1	1620
	11th to 20th	Night	38% 8% 18% 26% 9% 1	1421
	21st to end	Night	35% 10% 20% 22% 10% 3	1552
June	1st to 10th	Night	27% 9% 24% 24% 12% 3	1440
	11th to 20th	Night	26% 9% 18% 25% 15% 7%	1440
	21st to end	Night	21% 7% 15% 28% 23% 5%	1440
July	1st to 10th	Night	26% 8% 14% 28% 18% 5%	1400
	11th to 20th	Night	18% 6% 15% 26% 23% 12%	1392
	21st to end	Night	22% 8% 19% 27% 18% 7%	1488
August	1st to 10th	Night	27% 10% 18% 26% 15% 5%	1454
	11th to 20th	Night	18% 6% 16% 25% 23% 10%	1778
	21st to end	Night	23% 6% 12% 22% 26% 12%	1980
September	1st to 10th	Night	25% 4 9% 24% 25% 13%	1800
	11th to 20th	Night	22% 6% 11% 26% 24% 11%	1869
	21st to end	Night	25% 6% 7% 28% 24% 9%	1980
October	1st to 10th	Night	29% 5% 14% 22% 25% 6%	2070
	11th to 20th	Night	25% 5% 11% 28% 21% 10%	2160
	21st to end	Night	28% 6% 13% 25% 19% 8%	2538
November	1st to 10th	Night	26% 5% 14% 26% 24% 5%	2405
	11th to 20th	Night	25% 5% 13% 30% 21% 6%	2470
	21st to end	Night	27% 5% 11% 30% 21% 5%	2584
December	1st to 10th	Night	30% 6% 15% 29% 18% 4	2660
	11th to 20th	Night	25% 5% 16% 36% 16% 1	3272
	21st to end	Night	35% 7% 14% 30% 12% 1	4873

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	7% 7% 37% 30% 15% 4	270
	11th to 20th	Day	10% 9% 36% 35% 9%	245
	21st to end	Day	9% 9% 52% 25% 3 1	264
February	1st to 10th	Day	10% 14% 53% 19% 4	240
	11th to 20th	Day	7% 18% 45% 23% 5% 3	240
	21st to end	Day	11% 16% 42% 26% 5%	198
March	1st to 10th	Day	8% 13% 35% 31% 13%	240
	11th to 20th	Day	10% 11% 40% 26% 11% 2	296
	21st to end	Day	15% 13% 35% 27% 9% 1	462
April	1st to 10th	Day	15% 16% 35% 23% 10% 1	420
	11th to 20th	Day	19% 18% 33% 20% 8% 2	412
	21st to end	Day	14% 14% 30% 24% 15% 2	487
May	1st to 10th	Day	15% 10% 30% 28% 14% 3	603
	11th to 20th	Day	18% 8% 22% 32% 15% 5%	625
	21st to end	Day	15% 9% 26% 29% 17% 5%	679
June	1st to 10th	Day	5% 4 22% 37% 23% 8%	630
	11th to 20th	Day	8% 7% 30% 34% 18% 3	630
	21st to end	Day	5% 5% 25% 34% 27% 4	630
July	1st to 10th	Day	5% 2 18% 39% 30% 6%	621
	11th to 20th	Day	5% 3 20% 37% 30% 6%	618
	21st to end	Day	7% 2 16% 36% 35% 5%	684
August	1st to 10th	Day	5% 2 17% 41% 29% 6%	599
	11th to 20th	Day	4 3 19% 38% 25% 11%	437
	21st to end	Day	5% 4 22% 32% 25% 13%	495
September	1st to 10th	Day	7% 5% 22% 31% 26% 9%	450
	11th to 20th	Day	6% 3 20% 30% 29% 11%	380
	21st to end	Day	7% 9% 24% 29% 21% 10%	270
October	1st to 10th	Day	14% 7% 24% 22% 21% 12%	270
	11th to 20th	Day	9% 12% 31% 26% 16% 7%	270
	21st to end	Day	9% 13% 33% 28% 13% 4	297
November	1st to 10th	Day	6% 8% 34% 34% 14% 5%	270
	11th to 20th	Day	6% 12% 34% 31% 15% 1	270
	21st to end	Day	11% 7% 36% 33% 9% 4	270
December	1st to 10th	Day	8% 10% 41% 30% 9% 1	270
	11th to 20th	Day	10% 10% 45% 25% 8% 1	270
	21st to end	Day	12% 13% 40% 25% 9% 1	297
January	1st to 10th	Night	6% 10% 39% 32% 11% 1	950
	11th to 20th	Night	6% 13% 41% 33% 6% 1	910
	21st to end	Night	8% 9% 49% 30% 5%	927
February	1st to 10th	Night	9% 8% 51% 27% 6%	720
	11th to 20th	Night	10% 9% 43% 33% 5%	704
	21st to end	Night	8% 10% 49% 28% 5%	592
March	1st to 10th	Night	9% 10% 40% 35% 6%	720
	11th to 20th	Night	10% 7% 37% 39% 7%	720
	21st to end	Night	13% 11% 38% 33% 4 1	598
April	1st to 10th	Night	15% 12% 36% 32% 5%	540
	11th to 20th	Night	13% 11% 40% 33% 4	533
	21st to end	Night	18% 11% 28% 35% 9%	540
May	1st to 10th	Night	17% 8% 32% 31% 12%	540
	11th to 20th	Night	18% 5% 26% 39% 12% 1	531
	21st to end	Night	13% 5% 33% 37% 12%	584
June	1st to 10th	Night	9% 5% 27% 40% 17% 1	540
	11th to 20th	Night	6% 5% 37% 44% 7%	540
	21st to end	Night	4 3 30% 53% 10%	540
July	1st to 10th	Night	7% 2 20% 50% 20% 1	525
	11th to 20th	Night	3 1 23% 57% 15%	522
	21st to end	Night	4 1 20% 54% 22%	591
August	1st to 10th	Night	6% 2 20% 53% 18%	537
	11th to 20th	Night	4 1 24% 54% 17%	534
	21st to end	Night	4 3 22% 54% 17%	594
September	1st to 10th	Night	7% 4 24% 46% 18% 1	540
	11th to 20th	Night	8% 4 21% 46% 19% 2	540
	21st to end	Night	11% 2 31% 44% 12%	540
October	1st to 10th	Night	14% 6% 28% 37% 14% 1	630
	11th to 20th	Night	9% 6% 33% 41% 11%	720
	21st to end	Night	11% 7% 36% 35% 10% 1	954
November	1st to 10th	Night	8% 6% 36% 41% 9% 1	925
	11th to 20th	Night	6% 11% 42% 35% 7%	950
	21st to end	Night	8% 9% 35% 41% 8%	950
December	1st to 10th	Night	8% 10% 47% 31% 5%	950
	11th to 20th	Night	8% 8% 40% 39% 5%	950
	21st to end	Night	11% 12% 41% 31% 6%	1045

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