West Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Sky Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	18% 7% 3 5% 5% 3 5% 5% 5% 7% 37%	74516
	Night	18% 8% 3 5% 5% 3 5% 4 5% 7% 38%	75210
Transparency	Day	46% 6% 11% 21% 13% 2	74222
	Night	46% 6% 11% 22% 13% 2	75039
Seeing	Day	25% 16% 29% 20% 9% 2	13791
	Night	23% 12% 28% 27% 9% 1	22847

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% 9% 3 6% 5% 4 5% 5% 6% 7% 36%	1517
	11th to 20th	Day	17% 11% 3 5% 5% 3 6% 4 5% 6% 35%	1464
	21st to end	Day	12% 9% 4 7% 6% 4 5% 6% 5% 5% 38%	1584
February	1st to 10th	Day	16% 11% 3 6% 5% 4 6% 6% 4 6% 33%	1566
	11th to 20th	Day	12% 10% 4 6% 7% 4 5% 5% 6% 6% 35%	1748
	21st to end	Day	15% 9% 5% 7% 8% 3 6% 5% 5% 5% 32%	1480
March	1st to 10th	Day	14% 9% 4 6% 6% 3 7% 5% 5% 6% 35%	1797
	11th to 20th	Day	15% 8% 4 6% 5% 3 5% 3 4 7% 39%	1923
	21st to end	Day	16% 8% 3 7% 7% 3 4 4 4 4 41%	2254
April	1st to 10th	Day	19% 10% 4 6% 5% 2 4 3 4 7% 34%	2160
	11th to 20th	Day	18% 5% 2 6% 6% 4 5% 4 5% 5% 38%	2212
	21st to end	Day	25% 8% 3 6% 6% 2 4 5% 4 5% 31%	2340
May	1st to 10th	Day	25% 7% 3 6% 5% 3 5% 4 5% 7% 31%	2340
	11th to 20th	Day	20% 7% 2 7% 6% 3 4 5% 4 7% 36%	2374
	21st to end	Day	17% 7% 3 4 5% 4 5% 5% 6% 7% 37%	2725
June	1st to 10th	Day	25% 6% 3 5% 5% 4 5% 6% 6% 7% 29%	2520
	11th to 20th	Day	18% 6% 3 5% 5% 2 5% 6% 5% 7% 38%	2520
	21st to end	Day	22% 6% 3 6% 6% 3 6% 5% 5% 7% 29%	2520
July	1st to 10th	Day	17% 6% 2 5% 4 3 5% 5% 6% 9% 36%	2520
	11th to 20th	Day	14% 6% 2 6% 6% 5% 6% 6% 7% 9% 33%	2520
	21st to end	Day	15% 5% 3 5% 6% 3 6% 6% 8% 9% 35%	2744
August	1st to 10th	Day	14% 5% 3 6% 6% 4 5% 6% 7% 8% 37%	2377
	11th to 20th	Day	14% 6% 3 6% 7% 4 6% 5% 6% 8% 36%	2304
	21st to end	Day	13% 5% 2 4 5% 2 5% 5% 6% 9% 45%	2453
September	1st to 10th	Day	13% 5% 2 4 5% 3 3 5% 6% 8% 46%	2136
	11th to 20th	Day	13% 5% 2 4 4 2 3 3 4 7% 52%	1980
	21st to end	Day	16% 6% 2 3 4 3 4 4 5% 9% 44%	1980
October	1st to 10th	Day	21% 6% 3 6% 5% 2 4 4 5% 6% 39%	1980
	11th to 20th	Day	18% 7% 2 5% 4 3 5% 4 5% 7% 39%	1720
	21st to end	Day	25% 9% 3 5% 5% 3 3 4 3 6% 32%	1782
November	1st to 10th	Day	17% 7% 2 4 4 3 4 4 5% 6% 44%	1671
	11th to 20th	Day	21% 4 2 4 4 3 5% 5% 7% 7% 39%	1710
	21st to end	Day	21% 7% 2 4 4 2 4 4 5% 7% 39%	1692
December	1st to 10th	Day	22% 10% 3 6% 6% 2 4 4 6% 5% 32%	1357
	11th to 20th	Day	22% 9% 3 6% 4 2 4 4 4 5% 36%	1774
	21st to end	Day	23% 12% 3 6% 5% 3 4 5% 5% 5% 29%	2772
January	1st to 10th	Night	15% 10% 3 5% 5% 3 4 5% 5% 7% 37%	2649
	11th to 20th	Night	17% 12% 3 7% 6% 4 5% 4 5% 6% 31%	2574
	21st to end	Night	12% 8% 4 7% 6% 4 5% 5% 5% 7% 38%	2772
February	1st to 10th	Night	14% 10% 4 7% 6% 4 6% 5% 6% 7% 32%	2388
	11th to 20th	Night	10% 9% 4 7% 7% 3 5% 4 5% 6% 40%	2098
	21st to end	Night	13% 10% 3 6% 6% 3 5% 5% 5% 6% 38%	1776
March	1st to 10th	Night	13% 11% 3 5% 5% 3 5% 6% 6% 6% 35%	2150
	11th to 20th	Night	16% 8% 4 7% 6% 3 5% 5% 5% 6% 36%	2035
	21st to end	Night	18% 7% 3 6% 4 3 5% 5% 4 6% 39%	2098
April	1st to 10th	Night	20% 11% 3 5% 5% 3 5% 4 4 6% 35%	1800
	11th to 20th	Night	18% 6% 3 6% 5% 3 5% 5% 4 7% 37%	1697
	21st to end	Night	25% 7% 3 5% 6% 2 5% 4 4 6% 33%	1620
May	1st to 10th	Night	21% 10% 3 5% 5% 3 5% 5% 5% 5% 34%	1620
	11th to 20th	Night	22% 7% 2 5% 4 3 4 5% 4 8% 35%	1534
	21st to end	Night	18% 9% 4 5% 5% 4 4 4 6% 6% 35%	1550
June	1st to 10th	Night	22% 8% 4 4 5% 2 5% 5% 5% 7% 34%	1440
	11th to 20th	Night	19% 7% 1 5% 4 2 4 4 5% 10% 38%	1440
	21st to end	Night	21% 6% 3 5% 4 3 4 5% 6% 8% 35%	1440
July	1st to 10th	Night	15% 5% 3 4 5% 3 7% 3 6% 9% 39%	1440
	11th to 20th	Night	14% 6% 3 5% 5% 3 4 5% 6% 10% 41%	1440
	21st to end	Night	14% 5% 2 4 5% 3 4 5% 6% 8% 42%	1574
August	1st to 10th	Night	14% 5% 2 5% 5% 3 4 5% 6% 7% 44%	1542
	11th to 20th	Night	14% 5% 2 5% 6% 4 5% 5% 6% 9% 39%	1596
	21st to end	Night	14% 6% 2 4 4 2 3 4 6% 8% 46%	1899
September	1st to 10th	Night	13% 5% 2 4 4 3 4 3 5% 7% 50%	1823
	11th to 20th	Night	17% 5% 2 3 3 2 3 4 5% 6% 51%	1980
	21st to end	Night	16% 6% 2 4 4 2 4 4 4 7% 46%	1980
October	1st to 10th	Night	19% 7% 2 5% 5% 3 4 5% 5% 6% 38%	1980
	11th to 20th	Night	17% 7% 2 5% 5% 3 5% 5% 5% 7% 38%	2237
	21st to end	Night	25% 8% 3 4 5% 3 4 4 5% 6% 34%	2574
November	1st to 10th	Night	17% 7% 2 5% 5% 3 5% 4 5% 6% 41%	2407
	11th to 20th	Night	17% 7% 2 4 5% 3 5% 4 5% 7% 41%	2470
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 6% 7% 40%	2484
December	1st to 10th	Night	20% 10% 4 7% 6% 3 5% 4 5% 5% 31%	2815
	11th to 20th	Night	21% 8% 3 5% 5% 3 4 4 5% 5% 36%	3404
	21st to end	Night	23% 10% 4 7% 4 3 4 4 4 5% 33%	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	45% 6% 10% 22% 15% 1	1518
	11th to 20th	Day	48% 6% 13% 18% 12% 2	1464
	21st to end	Day	46% 5% 10% 20% 18% 2	1584
February	1st to 10th	Day	50% 6% 10% 19% 12% 2	1566
	11th to 20th	Day	49% 5% 12% 19% 12% 3	1757
	21st to end	Day	52% 5% 10% 16% 12% 5%	1480
March	1st to 10th	Day	50% 6% 11% 16% 15% 4	1800
	11th to 20th	Day	45% 5% 10% 18% 17% 5%	1923
	21st to end	Day	47% 4 8% 17% 19% 5%	2254
April	1st to 10th	Day	50% 4 9% 18% 15% 5%	2160
	11th to 20th	Day	45% 7% 8% 22% 14% 3	2212
	21st to end	Day	55% 5% 8% 17% 12% 4	2340
May	1st to 10th	Day	53% 5% 8% 19% 13% 2	2340
	11th to 20th	Day	48% 5% 8% 19% 16% 3	2378
	21st to end	Day	45% 7% 12% 22% 13% 2	2725
June	1st to 10th	Day	53% 7% 12% 19% 9%	2520
	11th to 20th	Day	44% 7% 11% 20% 16% 2	2520
	21st to end	Day	53% 8% 11% 17% 10% 1	2520
July	1st to 10th	Day	42% 8% 13% 23% 13% 1	2450
	11th to 20th	Day	45% 9% 12% 25% 8% 1	2445
	21st to end	Day	41% 9% 14% 24% 11% 1	2591
August	1st to 10th	Day	42% 9% 14% 25% 10%	2363
	11th to 20th	Day	44% 8% 13% 25% 10% 1	2304
	21st to end	Day	34% 7% 15% 24% 18% 1	2453
September	1st to 10th	Day	35% 7% 14% 26% 17% 1	2136
	11th to 20th	Day	33% 5% 9% 30% 20% 3	1980
	21st to end	Day	38% 5% 12% 27% 17% 1	1980
October	1st to 10th	Day	47% 5% 10% 23% 14% 1	1980
	11th to 20th	Day	44% 5% 14% 26% 10% 1	1720
	21st to end	Day	53% 4 11% 17% 12% 2	1782
November	1st to 10th	Day	40% 4 14% 24% 15% 3	1671
	11th to 20th	Day	42% 6% 13% 19% 17% 3	1710
	21st to end	Day	45% 4 18% 21% 11% 1	1692
December	1st to 10th	Day	53% 5% 11% 20% 10% 1	1357
	11th to 20th	Day	50% 4 9% 18% 17% 3	1775
	21st to end	Day	56% 6% 10% 17% 9% 2	2772
January	1st to 10th	Night	46% 6% 12% 22% 13% 2	2649
	11th to 20th	Night	54% 6% 12% 17% 10% 2	2574
	21st to end	Night	45% 5% 9% 21% 17% 3	2772
February	1st to 10th	Night	50% 7% 12% 17% 13% 3	2388
	11th to 20th	Night	46% 5% 10% 20% 15% 3	2100
	21st to end	Night	45% 6% 10% 21% 14% 4	1776
March	1st to 10th	Night	47% 6% 10% 21% 14% 3	2160
	11th to 20th	Night	48% 5% 12% 17% 14% 4	2036
	21st to end	Night	45% 5% 9% 19% 18% 4	2098
April	1st to 10th	Night	51% 4 8% 18% 16% 3	1800
	11th to 20th	Night	45% 5% 10% 22% 17% 1	1697
	21st to end	Night	52% 5% 12% 16% 12% 4	1620
May	1st to 10th	Night	52% 6% 7% 18% 14% 2	1620
	11th to 20th	Night	48% 7% 9% 20% 13% 4	1535
	21st to end	Night	49% 6% 13% 20% 11% 1	1551
June	1st to 10th	Night	50% 6% 11% 21% 10%	1440
	11th to 20th	Night	42% 7% 12% 23% 14% 2	1440
	21st to end	Night	46% 8% 15% 20% 10% 1	1440
July	1st to 10th	Night	40% 8% 16% 23% 12% 1	1400
	11th to 20th	Night	39% 7% 16% 28% 10%	1392
	21st to end	Night	38% 9% 15% 25% 12% 1	1486
August	1st to 10th	Night	39% 7% 15% 28% 10%	1532
	11th to 20th	Night	41% 8% 15% 22% 13%	1596
	21st to end	Night	36% 7% 13% 28% 16% 1	1899
September	1st to 10th	Night	34% 6% 13% 31% 15%	1823
	11th to 20th	Night	34% 5% 11% 31% 18% 2	1980
	21st to end	Night	37% 6% 11% 29% 15% 2	1980
October	1st to 10th	Night	45% 6% 12% 22% 13% 1	1980
	11th to 20th	Night	43% 7% 12% 26% 10% 1	2237
	21st to end	Night	51% 6% 10% 20% 12% 2	2574
November	1st to 10th	Night	43% 5% 11% 26% 14% 1	2407
	11th to 20th	Night	43% 5% 12% 22% 16% 2	2470
	21st to end	Night	43% 6% 14% 23% 12% 1	2484
December	1st to 10th	Night	53% 5% 10% 20% 10% 1	2815
	11th to 20th	Night	49% 5% 10% 18% 17% 1	3404
	21st to end	Night	53% 6% 10% 21% 10% 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	32% 16% 30% 17% 4	180
	11th to 20th	Day	30% 16% 34% 17% 2 2	166
	21st to end	Day	20% 24% 33% 16% 6%	176
February	1st to 10th	Day	31% 18% 31% 17% 3	278
	11th to 20th	Day	28% 25% 30% 14% 2	416
	21st to end	Day	23% 23% 30% 19% 4	346
March	1st to 10th	Day	25% 21% 35% 15% 4	420
	11th to 20th	Day	27% 23% 31% 14% 4 1	420
	21st to end	Day	26% 24% 26% 14% 7% 2	462
April	1st to 10th	Day	34% 20% 24% 15% 5% 2	420
	11th to 20th	Day	29% 22% 26% 14% 7% 2	412
	21st to end	Day	36% 20% 25% 11% 6% 3	420
May	1st to 10th	Day	37% 16% 25% 17% 3 2	423
	11th to 20th	Day	23% 20% 33% 15% 6% 2	447
	21st to end	Day	23% 14% 34% 21% 6% 2	485
June	1st to 10th	Day	25% 16% 32% 18% 7% 2	450
	11th to 20th	Day	22% 14% 32% 21% 9% 3	450
	21st to end	Day	22% 12% 34% 21% 10% 2	450
July	1st to 10th	Day	20% 11% 31% 26% 10% 3	445
	11th to 20th	Day	17% 7% 32% 28% 12% 4	444
	21st to end	Day	16% 8% 33% 28% 12% 3	488
August	1st to 10th	Day	14% 9% 29% 29% 14% 5%	446
	11th to 20th	Day	16% 9% 28% 27% 15% 5%	439
	21st to end	Day	15% 11% 28% 28% 14% 5%	495
September	1st to 10th	Day	18% 10% 24% 26% 17% 6%	450
	11th to 20th	Day	19% 11% 28% 18% 14% 10%	450
	21st to end	Day	17% 14% 25% 25% 12% 6%	450
October	1st to 10th	Day	23% 15% 21% 22% 16% 3	450
	11th to 20th	Day	26% 13% 27% 21% 11% 2	372
	21st to end	Day	37% 12% 24% 19% 7% 1	396
November	1st to 10th	Day	27% 14% 26% 21% 11% 1	365
	11th to 20th	Day	29% 14% 28% 21% 7% 1	370
	21st to end	Day	33% 18% 24% 18% 7%	352
December	1st to 10th	Day	27% 22% 29% 14% 8%	180
	11th to 20th	Day	33% 20% 33% 10% 4	180
	21st to end	Day	34% 19% 27% 16% 4 1	198
January	1st to 10th	Night	22% 17% 33% 24% 4	760
	11th to 20th	Night	26% 17% 27% 26% 4	736
	21st to end	Night	19% 19% 35% 24% 4 1	792
February	1st to 10th	Night	21% 18% 33% 24% 3	720
	11th to 20th	Night	20% 19% 37% 21% 3	703
	21st to end	Night	23% 16% 31% 26% 4	592
March	1st to 10th	Night	24% 17% 34% 21% 3	720
	11th to 20th	Night	24% 18% 30% 22% 7%	720
	21st to end	Night	25% 16% 30% 22% 6% 1	792
April	1st to 10th	Night	29% 15% 28% 24% 4	720
	11th to 20th	Night	22% 15% 33% 26% 5%	633
	21st to end	Night	30% 14% 30% 20% 6%	540
May	1st to 10th	Night	27% 11% 27% 28% 6% 1	540
	11th to 20th	Night	25% 12% 29% 28% 6%	473
	21st to end	Night	21% 7% 26% 36% 9%	389
June	1st to 10th	Night	25% 6% 29% 31% 9%	360
	11th to 20th	Night	21% 6% 26% 33% 14%	360
	21st to end	Night	21% 3 25% 40% 11%	360
July	1st to 10th	Night	18% 3 23% 39% 17%	350
	11th to 20th	Night	17% 1 23% 41% 18% 1	348
	21st to end	Night	15% 3 22% 40% 20% 1	394
August	1st to 10th	Night	15% 3 22% 38% 20% 1	468
	11th to 20th	Night	17% 4 27% 34% 18% 2	532
	21st to end	Night	15% 6% 21% 36% 19% 2	711
September	1st to 10th	Night	15% 4 28% 32% 19% 2	720
	11th to 20th	Night	20% 7% 24% 26% 20% 4	720
	21st to end	Night	18% 8% 26% 29% 18% 1	720
October	1st to 10th	Night	24% 8% 20% 33% 15% 1	720
	11th to 20th	Night	23% 10% 28% 26% 12% 2	720
	21st to end	Night	31% 11% 28% 26% 5%	792
November	1st to 10th	Night	23% 11% 29% 29% 8% 1	740
	11th to 20th	Night	22% 15% 29% 25% 8% 1	760
	21st to end	Night	24% 15% 26% 28% 6% 1	760
December	1st to 10th	Night	28% 13% 34% 19% 6%	834
	11th to 20th	Night	24% 20% 28% 23% 4	812
	21st to end	Night	29% 17% 27% 24% 3	836

Page generated at 2024-01-11 13:31:57. Version ac07

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