Star Watcher Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Watcher 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	15% 6% 2 5% 5% 2 4 4 5% 8% 43%	74783
	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 41%	74930
Transparency	Day	40% 7% 12% 23% 16% 2	74488
	Night	42% 6% 11% 23% 16% 2	74759
Seeing	Day	18% 8% 27% 29% 15% 3	13945
	Night	20% 8% 30% 29% 11% 1	23120

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	14% 7% 3 5% 5% 3 5% 5% 5% 7% 42%	1517
	11th to 20th	Day	17% 8% 4 5% 5% 3 4 5% 6% 6% 38%	1454
	21st to end	Day	16% 6% 3 5% 4 4 4 5% 5% 8% 41%	1653
February	1st to 10th	Day	18% 7% 3 5% 4 3 4 4 5% 7% 39%	1692
	11th to 20th	Day	15% 5% 3 6% 5% 2 3 4 4 8% 44%	1748
	21st to end	Day	19% 8% 2 5% 5% 2 4 4 4 6% 39%	1480
March	1st to 10th	Day	14% 7% 2 5% 4 3 4 4 4 8% 44%	1797
	11th to 20th	Day	14% 6% 2 5% 5% 3 5% 4 6% 8% 43%	1848
	21st to end	Day	18% 8% 3 5% 5% 2 3 4 5% 5% 42%	2376
April	1st to 10th	Day	16% 7% 2 5% 4 2 4 5% 5% 7% 43%	2160
	11th to 20th	Day	16% 6% 2 5% 4 2 4 4 5% 6% 45%	2134
	21st to end	Day	19% 5% 2 4 4 2 3 5% 5% 8% 42%	2240
May	1st to 10th	Day	22% 7% 2 4 5% 2 3 5% 5% 7% 39%	2340
	11th to 20th	Day	19% 5% 2 4 5% 2 4 4 5% 8% 42%	2384
	21st to end	Day	13% 6% 2 4 4 2 4 5% 6% 9% 44%	2725
June	1st to 10th	Day	16% 5% 2 5% 5% 2 4 4 6% 9% 41%	2520
	11th to 20th	Day	13% 6% 3 4 5% 3 5% 5% 6% 8% 42%	2520
	21st to end	Day	10% 6% 3 5% 5% 3 4 4 5% 9% 46%	2520
July	1st to 10th	Day	10% 4 2 5% 4 3 5% 5% 7% 10% 46%	2520
	11th to 20th	Day	10% 5% 3 5% 5% 2 4 6% 6% 9% 45%	2520
	21st to end	Day	12% 5% 2 5% 5% 3 4 5% 7% 9% 43%	2744
August	1st to 10th	Day	9% 5% 2 4 6% 3 4 6% 7% 10% 43%	2392
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 4 6% 8% 45%	2126
	21st to end	Day	11% 5% 2 5% 5% 3 4 4 6% 9% 45%	2376
September	1st to 10th	Day	13% 5% 2 4 5% 3 4 5% 6% 7% 47%	2160
	11th to 20th	Day	12% 6% 2 4 3 2 3 4 4 7% 53%	2037
	21st to end	Day	17% 6% 2 5% 5% 3 4 4 5% 6% 42%	1980
October	1st to 10th	Day	22% 6% 2 3 3 2 4 3 4 7% 44%	1954
	11th to 20th	Day	15% 5% 2 4 5% 2 3 5% 6% 8% 45%	1799
	21st to end	Day	19% 6% 2 4 5% 3 4 5% 5% 6% 40%	1782
November	1st to 10th	Day	14% 6% 2 5% 4 3 4 4 4 5% 49%	1671
	11th to 20th	Day	17% 6% 2 5% 4 2 3 3 4 8% 46%	1710
	21st to end	Day	15% 6% 3 5% 5% 2 4 5% 5% 7% 44%	1706
December	1st to 10th	Day	18% 6% 3 6% 5% 3 5% 5% 6% 8% 36%	1520
	11th to 20th	Day	21% 7% 3 4 3 1 3 3 5% 7% 44%	1906
	21st to end	Day	18% 11% 3 7% 6% 3 4 4 5% 6% 32%	2772
January	1st to 10th	Night	16% 6% 3 5% 4 3 4 4 5% 8% 42%	2649
	11th to 20th	Night	20% 8% 3 5% 4 3 4 4 4 7% 38%	2546
	21st to end	Night	14% 8% 3 5% 5% 2 3 4 5% 7% 42%	2700
February	1st to 10th	Night	16% 9% 3 5% 5% 3 4 4 5% 7% 37%	2267
	11th to 20th	Night	18% 8% 3 5% 5% 2 4 4 6% 6% 39%	2098
	21st to end	Night	21% 7% 2 5% 4 3 4 4 5% 6% 40%	1776
March	1st to 10th	Night	17% 6% 2 4 4 2 4 4 5% 7% 44%	2150
	11th to 20th	Night	17% 6% 3 5% 5% 4 4 5% 5% 6% 42%	2106
	21st to end	Night	17% 9% 3 5% 4 3 4 4 5% 6% 41%	1980
April	1st to 10th	Night	19% 6% 3 6% 6% 3 4 4 5% 6% 39%	1800
	11th to 20th	Night	17% 6% 3 6% 5% 3 3 4 4 7% 42%	1775
	21st to end	Night	19% 8% 3 6% 4 3 4 3 4 6% 41%	1719
May	1st to 10th	Night	21% 9% 2 6% 6% 3 5% 4 5% 6% 34%	1620
	11th to 20th	Night	20% 7% 2 5% 6% 3 4 4 6% 8% 35%	1524
	21st to end	Night	17% 6% 3 6% 5% 3 4 5% 6% 8% 38%	1550
June	1st to 10th	Night	19% 6% 3 4 6% 3 4 4 4 8% 40%	1440
	11th to 20th	Night	16% 5% 2 4 4 3 4 5% 6% 9% 40%	1440
	21st to end	Night	11% 5% 3 5% 5% 4 5% 5% 6% 10% 42%	1440
July	1st to 10th	Night	10% 6% 3 6% 5% 3 5% 6% 7% 10% 39%	1440
	11th to 20th	Night	12% 6% 2 5% 5% 4 6% 6% 6% 9% 40%	1440
	21st to end	Night	11% 5% 3 5% 5% 4 5% 6% 6% 9% 41%	1574
August	1st to 10th	Night	6% 4 2 5% 5% 4 7% 6% 8% 10% 43%	1523
	11th to 20th	Night	11% 6% 3 5% 6% 4 6% 6% 7% 9% 38%	1774
	21st to end	Night	12% 5% 3 4 4 3 4 5% 7% 8% 45%	1980
September	1st to 10th	Night	15% 4 3 6% 5% 4 4 5% 5% 5% 44%	1800
	11th to 20th	Night	14% 4 3 4 4 2 4 4 5% 6% 51%	1921
	21st to end	Night	17% 6% 3 5% 5% 3 4 4 5% 7% 42%	1980
October	1st to 10th	Night	22% 7% 3 5% 4 3 4 3 4 6% 39%	2003
	11th to 20th	Night	16% 5% 2 4 5% 2 4 4 4 6% 47%	2160
	21st to end	Night	22% 7% 2 5% 4 3 4 4 5% 6% 39%	2574
November	1st to 10th	Night	14% 6% 2 5% 5% 3 4 3 5% 6% 47%	2407
	11th to 20th	Night	19% 6% 3 4 4 2 3 4 4 6% 46%	2470
	21st to end	Night	15% 5% 3 4 5% 2 5% 4 5% 7% 45%	2472
December	1st to 10th	Night	20% 8% 2 5% 5% 3 4 4 5% 6% 38%	2660
	11th to 20th	Night	19% 7% 2 5% 5% 3 4 4 5% 6% 41%	3288
	21st to end	Night	20% 9% 3 5% 5% 3 4 4 4 6% 37%	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	41% 5% 12% 27% 13% 3	1518
	11th to 20th	Day	45% 6% 14% 20% 13% 2	1454
	21st to end	Day	42% 5% 10% 25% 15% 2	1653
February	1st to 10th	Day	45% 5% 12% 22% 13% 1	1692
	11th to 20th	Day	41% 5% 10% 25% 16% 3	1757
	21st to end	Day	47% 4 10% 21% 15% 3	1480
March	1st to 10th	Day	40% 5% 14% 22% 16% 3	1800
	11th to 20th	Day	40% 5% 12% 25% 17% 2	1848
	21st to end	Day	45% 5% 9% 19% 18% 5%	2376
April	1st to 10th	Day	43% 6% 9% 21% 17% 4	2160
	11th to 20th	Day	40% 6% 10% 21% 19% 3	2134
	21st to end	Day	41% 6% 10% 22% 17% 5%	2240
May	1st to 10th	Day	45% 7% 10% 21% 14% 3	2340
	11th to 20th	Day	40% 7% 11% 21% 17% 3	2388
	21st to end	Day	36% 9% 18% 22% 14% 2	2725
June	1st to 10th	Day	40% 8% 13% 23% 14% 2	2520
	11th to 20th	Day	40% 8% 12% 25% 14% 1	2520
	21st to end	Day	35% 9% 16% 24% 15% 1	2520
July	1st to 10th	Day	35% 10% 14% 23% 17% 1	2450
	11th to 20th	Day	34% 8% 15% 26% 14% 2	2445
	21st to end	Day	37% 8% 13% 22% 17% 3	2591
August	1st to 10th	Day	34% 8% 17% 25% 15% 1	2378
	11th to 20th	Day	36% 9% 14% 25% 15% 1	2126
	21st to end	Day	35% 9% 12% 22% 20% 3	2376
September	1st to 10th	Day	37% 7% 14% 21% 19% 2	2160
	11th to 20th	Day	32% 6% 11% 24% 25% 2	2037
	21st to end	Day	42% 7% 8% 20% 21% 3	1980
October	1st to 10th	Day	41% 7% 10% 21% 19% 2	1954
	11th to 20th	Day	36% 7% 12% 26% 18% 2	1799
	21st to end	Day	44% 5% 11% 22% 17% 2	1782
November	1st to 10th	Day	39% 5% 13% 22% 19% 2	1671
	11th to 20th	Day	39% 4 11% 22% 21% 3	1710
	21st to end	Day	39% 5% 14% 27% 12% 2	1706
December	1st to 10th	Day	44% 5% 16% 19% 14% 1	1520
	11th to 20th	Day	42% 4 15% 18% 19% 2	1906
	21st to end	Day	51% 6% 10% 23% 9% 1	2772
January	1st to 10th	Night	40% 4 12% 24% 17% 3	2649
	11th to 20th	Night	46% 6% 13% 23% 10% 2	2546
	21st to end	Night	41% 3 10% 24% 19% 2	2700
February	1st to 10th	Night	46% 6% 12% 22% 12% 2	2267
	11th to 20th	Night	45% 5% 10% 21% 15% 4	2100
	21st to end	Night	45% 4 7% 24% 17% 4	1776
March	1st to 10th	Night	40% 4 12% 24% 16% 4	2160
	11th to 20th	Night	42% 5% 10% 23% 16% 4	2107
	21st to end	Night	44% 5% 8% 19% 19% 6%	1980
April	1st to 10th	Night	47% 4 9% 20% 17% 3	1800
	11th to 20th	Night	43% 5% 11% 19% 21% 1	1775
	21st to end	Night	45% 5% 8% 21% 15% 5%	1719
May	1st to 10th	Night	51% 6% 9% 17% 15% 2	1620
	11th to 20th	Night	46% 7% 9% 19% 16% 3	1525
	21st to end	Night	44% 9% 15% 19% 12% 1	1551
June	1st to 10th	Night	44% 7% 10% 20% 17% 2	1440
	11th to 20th	Night	40% 8% 13% 26% 13% 1	1440
	21st to end	Night	37% 10% 13% 26% 13% 1	1440
July	1st to 10th	Night	39% 10% 15% 20% 14% 2	1400
	11th to 20th	Night	39% 9% 12% 27% 12% 1	1392
	21st to end	Night	37% 9% 14% 24% 14% 2	1486
August	1st to 10th	Night	33% 9% 18% 24% 14% 1	1513
	11th to 20th	Night	40% 9% 15% 24% 11% 1	1774
	21st to end	Night	35% 9% 13% 23% 18% 2	1980
September	1st to 10th	Night	41% 8% 10% 24% 16% 2	1800
	11th to 20th	Night	33% 5% 11% 23% 24% 3	1921
	21st to end	Night	42% 8% 7% 20% 20% 3	1980
October	1st to 10th	Night	46% 5% 11% 21% 14% 3	2003
	11th to 20th	Night	38% 6% 12% 28% 16% 1	2160
	21st to end	Night	46% 5% 11% 21% 16% 1	2574
November	1st to 10th	Night	39% 5% 12% 22% 19% 3	2407
	11th to 20th	Night	42% 4 9% 24% 18% 4	2470
	21st to end	Night	38% 4 13% 29% 14% 2	2472
December	1st to 10th	Night	46% 5% 13% 21% 14% 1	2660
	11th to 20th	Night	44% 4 12% 20% 18% 2	3288
	21st to end	Night	47% 5% 10% 24% 12% 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	21% 17% 35% 21% 7% 1	180
	11th to 20th	Day	15% 26% 34% 21% 4 1	164
	21st to end	Day	20% 11% 42% 18% 7% 1	207
February	1st to 10th	Day	27% 13% 39% 19% 3	312
	11th to 20th	Day	19% 15% 41% 22% 2	416
	21st to end	Day	25% 14% 35% 21% 5%	346
March	1st to 10th	Day	15% 16% 37% 26% 5%	420
	11th to 20th	Day	19% 9% 40% 26% 5% 1	420
	21st to end	Day	21% 12% 38% 20% 7% 1	462
April	1st to 10th	Day	20% 13% 34% 22% 10% 1	420
	11th to 20th	Day	19% 9% 32% 26% 12% 1	412
	21st to end	Day	25% 10% 30% 28% 6% 2	420
May	1st to 10th	Day	28% 9% 28% 25% 9% 1	423
	11th to 20th	Day	19% 7% 28% 33% 12% 1	447
	21st to end	Day	19% 4 17% 29% 26% 5%	485
June	1st to 10th	Day	18% 4 20% 34% 20% 3	450
	11th to 20th	Day	15% 5% 21% 38% 17% 4	450
	21st to end	Day	12% 4 18% 38% 23% 4	450
July	1st to 10th	Day	13% 2 15% 36% 27% 6%	445
	11th to 20th	Day	8% 3 14% 37% 29% 9%	444
	21st to end	Day	11% 4 15% 42% 24% 3	488
August	1st to 10th	Day	11% 3 15% 37% 27% 7%	446
	11th to 20th	Day	14% 2 11% 39% 27% 8%	439
	21st to end	Day	11% 4 18% 35% 24% 8%	495
September	1st to 10th	Day	17% 4 20% 26% 25% 8%	450
	11th to 20th	Day	14% 3 22% 30% 20% 11%	450
	21st to end	Day	21% 6% 24% 28% 17% 5%	450
October	1st to 10th	Day	28% 5% 21% 21% 19% 6%	450
	11th to 20th	Day	15% 9% 31% 27% 16% 3	449
	21st to end	Day	22% 10% 32% 23% 11% 2	396
November	1st to 10th	Day	16% 10% 25% 34% 12% 3	365
	11th to 20th	Day	21% 15% 31% 24% 9% 1	370
	21st to end	Day	16% 14% 33% 27% 10% 1	366
December	1st to 10th	Day	19% 15% 37% 22% 6% 1	180
	11th to 20th	Day	24% 22% 29% 20% 4 1	180
	21st to end	Day	24% 14% 39% 15% 7% 1	198
January	1st to 10th	Night	19% 13% 40% 22% 6%	760
	11th to 20th	Night	26% 12% 34% 24% 4	728
	21st to end	Night	18% 13% 42% 24% 3	792
February	1st to 10th	Night	21% 13% 37% 27% 3	720
	11th to 20th	Night	24% 13% 41% 20% 2	703
	21st to end	Night	24% 14% 36% 23% 3	592
March	1st to 10th	Night	21% 11% 39% 23% 5%	720
	11th to 20th	Night	21% 9% 41% 24% 5% 1	720
	21st to end	Night	20% 10% 40% 23% 8%	792
April	1st to 10th	Night	22% 8% 38% 26% 6%	720
	11th to 20th	Night	21% 8% 32% 31% 8%	711
	21st to end	Night	22% 6% 32% 32% 7% 1	639
May	1st to 10th	Night	25% 6% 30% 28% 11% 1	540
	11th to 20th	Night	22% 6% 35% 29% 8%	463
	21st to end	Night	21% 3 17% 40% 18% 1	389
June	1st to 10th	Night	21% 3 21% 34% 19% 1	360
	11th to 20th	Night	19% 2 16% 44% 19% 1	360
	21st to end	Night	11% 3 21% 47% 18% 1	360
July	1st to 10th	Night	12% 3 20% 41% 21% 2	350
	11th to 20th	Night	13% 3 14% 42% 25% 2	348
	21st to end	Night	14% 3 15% 47% 19% 1	394
August	1st to 10th	Night	7% 2 17% 47% 27% 1	449
	11th to 20th	Night	14% 3 14% 40% 27% 2	710
	21st to end	Night	13% 4 14% 39% 27% 4	792
September	1st to 10th	Night	16% 3 21% 35% 20% 5%	720
	11th to 20th	Night	16% 3 20% 33% 20% 7%	720
	21st to end	Night	21% 4 23% 31% 17% 4	720
October	1st to 10th	Night	24% 7% 26% 23% 16% 3	720
	11th to 20th	Night	19% 9% 27% 32% 11% 1	720
	21st to end	Night	25% 8% 31% 26% 9%	792
November	1st to 10th	Night	15% 9% 34% 28% 13% 2	740
	11th to 20th	Night	21% 12% 33% 24% 9% 1	760
	21st to end	Night	18% 13% 33% 26% 8% 1	760
December	1st to 10th	Night	24% 14% 33% 22% 7% 1	760
	11th to 20th	Night	21% 14% 35% 25% 4	760
	21st to end	Night	23% 11% 38% 24% 3	836

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