Fenwick Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fenwick Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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	14% 6% 2 5% 5% 3 4 5% 5% 8% 43%	83452
	Night	15% 6% 2 5% 5% 3 4 4 5% 7% 45%	85654
Transparency	Day	39% 6% 11% 23% 17% 4	83169
	Night	39% 6% 10% 22% 18% 4	85456
Seeing	Day	18% 11% 27% 25% 14% 5%	14355
	Night	17% 10% 29% 30% 13% 2	24153

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% 5% 2 3 4 2 3 3 4 5% 52%	2776
	11th to 20th	Day	14% 7% 3 5% 4 3 4 4 4 6% 44%	2840
	21st to end	Day	16% 9% 3 6% 5% 3 4 4 5% 6% 39%	3124
February	1st to 10th	Day	12% 7% 3 6% 5% 3 5% 5% 6% 7% 40%	1710
	11th to 20th	Day	21% 6% 2 3 3 2 4 4 5% 6% 43%	1721
	21st to end	Day	18% 7% 2 7% 5% 3 5% 4 5% 7% 37%	1579
March	1st to 10th	Day	20% 9% 3 5% 6% 3 4 4 4 7% 36%	3435
	11th to 20th	Day	10% 4 2 4 4 2 4 5% 6% 7% 53%	3480
	21st to end	Day	21% 6% 2 5% 5% 3 4 4 4 6% 40%	3836
April	1st to 10th	Day	10% 5% 2 5% 4 3 4 5% 5% 7% 51%	2280
	11th to 20th	Day	11% 6% 2 4 5% 2 4 4 5% 7% 48%	2254
	21st to end	Day	16% 6% 2 5% 5% 3 5% 5% 5% 7% 43%	2210
May	1st to 10th	Day	14% 5% 3 5% 5% 3 5% 5% 7% 8% 40%	2274
	11th to 20th	Day	17% 6% 3 5% 6% 4 4 5% 6% 7% 38%	2305
	21st to end	Day	13% 5% 3 5% 6% 4 4 5% 7% 8% 40%	2725
June	1st to 10th	Day	12% 4 2 5% 5% 3 5% 6% 6% 10% 43%	2520
	11th to 20th	Day	11% 3 2 5% 6% 3 5% 6% 8% 9% 41%	2520
	21st to end	Day	9% 4 2 5% 5% 4 5% 6% 7% 11% 42%	2520
July	1st to 10th	Day	8% 5% 2 6% 6% 4 6% 6% 7% 10% 39%	2486
	11th to 20th	Day	6% 3 2 6% 7% 4 6% 5% 8% 13% 40%	2340
	21st to end	Day	8% 4 2 6% 5% 5% 6% 7% 8% 12% 38%	2547
August	1st to 10th	Day	10% 6% 2 6% 7% 4 6% 6% 7% 10% 35%	2327
	11th to 20th	Day	10% 6% 2 8% 7% 4 6% 6% 7% 9% 35%	2198
	21st to end	Day	10% 6% 2 5% 5% 4 5% 5% 7% 11% 40%	2376
September	1st to 10th	Day	13% 5% 2 4 3 3 5% 5% 6% 9% 44%	2160
	11th to 20th	Day	11% 6% 3 5% 5% 3 4 3 5% 8% 48%	2160
	21st to end	Day	20% 7% 2 6% 4 3 4 4 4 6% 39%	1879
October	1st to 10th	Day	16% 5% 2 5% 5% 3 4 3 4 6% 48%	1800
	11th to 20th	Day	17% 5% 2 3 4 2 3 4 5% 5% 51%	1800
	21st to end	Day	14% 4 2 4 4 3 3 3 4 7% 52%	1980
November	1st to 10th	Day	13% 6% 2 3 3 2 4 4 4 5% 54%	1843
	11th to 20th	Day	14% 6% 2 4 4 3 3 4 5% 6% 50%	1710
	21st to end	Day	14% 5% 2 4 4 2 3 3 4 6% 52%	1535
December	1st to 10th	Day	17% 7% 2 5% 4 3 4 4 6% 9% 41%	1520
	11th to 20th	Day	16% 6% 2 4 3 2 3 4 3 4 53%	1910
	21st to end	Day	17% 6% 2 6% 5% 3 5% 4 5% 7% 40%	2772
January	1st to 10th	Night	15% 7% 2 5% 4 2 4 3 4 6% 47%	4164
	11th to 20th	Night	17% 7% 3 5% 4 2 4 4 5% 5% 46%	4120
	21st to end	Night	20% 9% 3 5% 4 2 3 4 3 4 44%	4532
February	1st to 10th	Night	16% 6% 2 5% 5% 3 4 4 5% 7% 43%	2470
	11th to 20th	Night	19% 7% 3 5% 4 2 3 4 4 6% 43%	2343
	21st to end	Night	17% 6% 2 5% 6% 2 3 3 4 6% 45%	1872
March	1st to 10th	Night	22% 7% 2 6% 4 2 4 4 5% 6% 38%	3418
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 5% 7% 51%	3480
	21st to end	Night	17% 6% 3 4 3 3 3 3 3 5% 49%	3796
April	1st to 10th	Night	14% 6% 3 5% 6% 2 4 3 4 7% 46%	1900
	11th to 20th	Night	10% 6% 2 4 4 3 3 4 5% 7% 50%	1875
	21st to end	Night	15% 5% 2 5% 5% 2 4 4 5% 7% 45%	1850
May	1st to 10th	Night	13% 6% 2 5% 5% 3 4 4 7% 9% 41%	1701
	11th to 20th	Night	16% 6% 3 5% 5% 3 5% 5% 5% 7% 39%	1584
	21st to end	Night	13% 5% 2 5% 6% 4 5% 5% 6% 9% 41%	1550
June	1st to 10th	Night	11% 4 3 5% 5% 3 6% 5% 7% 10% 40%	1440
	11th to 20th	Night	10% 5% 2 5% 5% 4 6% 5% 7% 10% 40%	1440
	21st to end	Night	8% 5% 2 4 5% 3 5% 6% 8% 9% 44%	1440
July	1st to 10th	Night	9% 5% 3 6% 6% 4 5% 4 6% 10% 42%	1468
	11th to 20th	Night	5% 3 3 5% 5% 4 7% 6% 8% 10% 44%	1620
	21st to end	Night	8% 4 3 5% 7% 5% 6% 6% 9% 9% 38%	1771
August	1st to 10th	Night	8% 5% 3 7% 6% 4 5% 5% 7% 11% 37%	1611
	11th to 20th	Night	12% 6% 2 6% 6% 4 6% 6% 8% 9% 35%	1683
	21st to end	Night	9% 5% 3 5% 5% 4 6% 6% 7% 10% 38%	1980
September	1st to 10th	Night	13% 5% 3 5% 5% 3 5% 5% 5% 8% 43%	1800
	11th to 20th	Night	11% 6% 3 5% 5% 3 4 5% 4 6% 48%	1800
	21st to end	Night	20% 8% 3 5% 4 2 5% 4 5% 7% 38%	2074
October	1st to 10th	Night	15% 6% 2 5% 4 3 3 4 4 6% 48%	2160
	11th to 20th	Night	16% 5% 2 4 5% 3 3 3 5% 6% 48%	2160
	21st to end	Night	15% 5% 2 4 4 2 3 4 4 6% 51%	2376
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 4 5% 52%	2233
	11th to 20th	Night	14% 6% 3 4 4 2 4 4 4 6% 51%	2470
	21st to end	Night	13% 4 2 4 4 2 4 5% 5% 7% 50%	2644
December	1st to 10th	Night	17% 7% 2 4 5% 3 4 4 5% 6% 42%	2660
	11th to 20th	Night	18% 6% 3 4 4 2 3 4 4 6% 47%	3285
	21st to end	Night	15% 7% 3 5% 4 2 4 4 5% 7% 42%	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	35% 3 6% 33% 18% 5%	2777
	11th to 20th	Day	40% 5% 7% 20% 22% 5%	2840
	21st to end	Day	47% 4 9% 20% 15% 3	3124
February	1st to 10th	Day	41% 5% 11% 26% 15% 2	1710
	11th to 20th	Day	41% 5% 6% 22% 20% 5%	1730
	21st to end	Day	48% 5% 11% 15% 17% 5%	1579
March	1st to 10th	Day	47% 5% 9% 19% 13% 6%	3435
	11th to 20th	Day	29% 6% 9% 25% 24% 9%	3480
	21st to end	Day	45% 6% 10% 19% 17% 3	3836
April	1st to 10th	Day	31% 6% 12% 23% 20% 8%	2280
	11th to 20th	Day	35% 5% 11% 21% 22% 7%	2254
	21st to end	Day	41% 6% 10% 22% 15% 6%	2210
May	1st to 10th	Day	40% 8% 15% 19% 15% 3	2274
	11th to 20th	Day	44% 7% 12% 23% 11% 2	2309
	21st to end	Day	40% 7% 15% 25% 12% 1	2725
June	1st to 10th	Day	35% 8% 16% 26% 14% 1	2520
	11th to 20th	Day	36% 9% 17% 21% 16% 1	2520
	21st to end	Day	34% 8% 16% 24% 15% 2	2520
July	1st to 10th	Day	37% 9% 14% 21% 16% 3	2417
	11th to 20th	Day	33% 8% 21% 25% 12% 2	2271
	21st to end	Day	36% 10% 14% 23% 15% 1	2405
August	1st to 10th	Day	41% 9% 16% 22% 10% 1	2314
	11th to 20th	Day	42% 10% 15% 21% 11% 1	2198
	21st to end	Day	37% 8% 17% 23% 13% 3	2376
September	1st to 10th	Day	37% 7% 15% 17% 20% 3	2160
	11th to 20th	Day	36% 5% 9% 26% 19% 4	2160
	21st to end	Day	46% 5% 9% 20% 16% 3	1879
October	1st to 10th	Day	39% 4 10% 24% 20% 3	1800
	11th to 20th	Day	35% 5% 8% 25% 24% 4	1800
	21st to end	Day	34% 4 6% 25% 25% 6%	1980
November	1st to 10th	Day	33% 4 9% 25% 25% 4	1843
	11th to 20th	Day	35% 4 10% 21% 23% 7%	1710
	21st to end	Day	34% 4 7% 25% 24% 5%	1535
December	1st to 10th	Day	40% 6% 11% 25% 14% 5%	1520
	11th to 20th	Day	36% 4 7% 30% 16% 7%	1906
	21st to end	Day	42% 6% 10% 27% 14% 1	2772
January	1st to 10th	Night	38% 4 8% 25% 20% 5%	4164
	11th to 20th	Night	41% 4 7% 22% 22% 4	4120
	21st to end	Night	45% 4 6% 23% 17% 5%	4532
February	1st to 10th	Night	41% 4 10% 24% 18% 3	2470
	11th to 20th	Night	44% 3 7% 20% 19% 6%	2345
	21st to end	Night	41% 4 9% 21% 17% 7%	1872
March	1st to 10th	Night	46% 5% 8% 22% 17% 2	3418
	11th to 20th	Night	33% 4 7% 21% 27% 8%	3480
	21st to end	Night	39% 4 8% 23% 21% 5%	3796
April	1st to 10th	Night	39% 5% 9% 18% 22% 6%	1900
	11th to 20th	Night	33% 5% 11% 23% 22% 6%	1875
	21st to end	Night	39% 5% 11% 23% 16% 5%	1850
May	1st to 10th	Night	39% 7% 14% 21% 15% 3	1701
	11th to 20th	Night	43% 9% 13% 20% 14% 2	1585
	21st to end	Night	38% 9% 14% 26% 12% 1	1551
June	1st to 10th	Night	37% 8% 14% 25% 15% 1	1440
	11th to 20th	Night	37% 10% 18% 18% 16% 1	1440
	21st to end	Night	32% 10% 15% 22% 18% 3	1440
July	1st to 10th	Night	39% 9% 15% 19% 15% 4	1427
	11th to 20th	Night	31% 11% 21% 24% 11% 2	1566
	21st to end	Night	36% 11% 16% 23% 14%	1672
August	1st to 10th	Night	38% 9% 17% 24% 11%	1600
	11th to 20th	Night	41% 11% 16% 19% 12% 1	1683
	21st to end	Night	38% 10% 14% 24% 12% 2	1980
September	1st to 10th	Night	39% 7% 15% 20% 14% 5%	1800
	11th to 20th	Night	35% 6% 10% 25% 21% 3	1800
	21st to end	Night	46% 5% 10% 18% 16% 4	2074
October	1st to 10th	Night	38% 4 9% 25% 19% 4	2160
	11th to 20th	Night	38% 4 7% 24% 22% 5%	2160
	21st to end	Night	35% 4 9% 24% 23% 6%	2376
November	1st to 10th	Night	36% 4 9% 22% 24% 5%	2233
	11th to 20th	Night	36% 4 8% 19% 23% 9%	2470
	21st to end	Night	33% 5% 9% 25% 22% 6%	2644
December	1st to 10th	Night	43% 5% 8% 23% 17% 4	2660
	11th to 20th	Night	39% 4 9% 21% 21% 6%	3288
	21st to end	Night	40% 6% 10% 25% 17% 2	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	26% 24% 32% 14% 4	254
	11th to 20th	Day	29% 28% 30% 10% 3	270
	21st to end	Day	23% 23% 40% 12% 2	297
February	1st to 10th	Day	21% 25% 39% 13% 3	270
	11th to 20th	Day	33% 23% 31% 11% 1	328
	21st to end	Day	30% 18% 31% 19% 2	382
March	1st to 10th	Day	22% 21% 34% 18% 5%	460
	11th to 20th	Day	22% 24% 35% 15% 4 1	460
	21st to end	Day	25% 14% 40% 16% 5% 1	506
April	1st to 10th	Day	22% 19% 33% 18% 7% 1	460
	11th to 20th	Day	18% 13% 35% 21% 9% 4	452
	21st to end	Day	23% 14% 33% 21% 8% 1	437
May	1st to 10th	Day	18% 15% 33% 23% 9% 2	423
	11th to 20th	Day	21% 9% 28% 28% 12% 2	447
	21st to end	Day	20% 4 21% 29% 21% 5%	485
June	1st to 10th	Day	12% 6% 24% 30% 22% 5%	450
	11th to 20th	Day	13% 5% 24% 35% 16% 6%	450
	21st to end	Day	11% 5% 22% 35% 21% 6%	450
July	1st to 10th	Day	10% 2 17% 34% 27% 10%	445
	11th to 20th	Day	5% 2 14% 38% 30% 10%	444
	21st to end	Day	10% 2 21% 34% 24% 10%	488
August	1st to 10th	Day	14% 3 17% 31% 26% 9%	446
	11th to 20th	Day	11% 4 16% 30% 29% 10%	439
	21st to end	Day	12% 3 15% 31% 24% 15%	495
September	1st to 10th	Day	18% 3 16% 28% 19% 17%	450
	11th to 20th	Day	15% 4 17% 27% 25% 11%	450
	21st to end	Day	28% 7% 21% 27% 14% 4	450
October	1st to 10th	Day	21% 5% 22% 24% 20% 7%	450
	11th to 20th	Day	16% 9% 26% 28% 19% 2	450
	21st to end	Day	16% 11% 33% 27% 9% 4	495
November	1st to 10th	Day	18% 9% 31% 24% 13% 4	449
	11th to 20th	Day	16% 14% 33% 26% 11% 1	370
	21st to end	Day	19% 14% 31% 24% 10% 2	195
December	1st to 10th	Day	23% 18% 36% 15% 7% 1	180
	11th to 20th	Day	24% 15% 33% 17% 9% 1	180
	21st to end	Day	23% 17% 30% 21% 9% 1	198
January	1st to 10th	Night	21% 17% 39% 19% 3	760
	11th to 20th	Night	21% 18% 33% 25% 4	760
	21st to end	Night	21% 16% 38% 23% 2	836
February	1st to 10th	Night	19% 14% 39% 24% 3	760
	11th to 20th	Night	24% 20% 36% 18% 2	743
	21st to end	Night	22% 17% 37% 21% 3	628
March	1st to 10th	Night	20% 16% 36% 25% 3	760
	11th to 20th	Night	22% 18% 36% 19% 5% 1	760
	21st to end	Night	19% 15% 39% 23% 4	836
April	1st to 10th	Night	18% 19% 38% 21% 4	760
	11th to 20th	Night	14% 13% 39% 26% 8%	751
	21st to end	Night	17% 10% 38% 29% 7%	740
May	1st to 10th	Night	17% 10% 37% 28% 9%	621
	11th to 20th	Night	17% 5% 31% 34% 12% 1	523
	21st to end	Night	15% 3 21% 38% 22% 2	389
June	1st to 10th	Night	12% 2 21% 42% 22% 1	360
	11th to 20th	Night	12% 4 19% 40% 22% 3	360
	21st to end	Night	8% 4 19% 48% 21% 1	360
July	1st to 10th	Night	11% 3 17% 40% 26% 3	377
	11th to 20th	Night	5% 13% 42% 35% 5%	522
	21st to end	Night	7% 2 17% 45% 25% 4	591
August	1st to 10th	Night	11% 2 12% 45% 28% 3	537
	11th to 20th	Night	13% 3 13% 41% 26% 4	619
	21st to end	Night	12% 2 14% 36% 29% 6%	792
September	1st to 10th	Night	16% 2 13% 36% 28% 6%	720
	11th to 20th	Night	13% 4 15% 34% 31% 4	720
	21st to end	Night	24% 3 21% 34% 17% 1	720
October	1st to 10th	Night	17% 5% 21% 32% 21% 4	720
	11th to 20th	Night	19% 9% 24% 31% 16% 2	720
	21st to end	Night	16% 10% 30% 29% 13% 1	792
November	1st to 10th	Night	18% 8% 27% 31% 13% 2	740
	11th to 20th	Night	18% 12% 31% 29% 9% 1	760
	21st to end	Night	14% 14% 32% 30% 9% 1	760
December	1st to 10th	Night	21% 14% 34% 24% 8%	760
	11th to 20th	Night	19% 12% 34% 30% 6%	760
	21st to end	Night	21% 14% 29% 32% 5%	836

Page generated at 2024-04-22 09:25:44. Version ac07

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