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 2024-11-05.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	101060
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	97936
Transparency	Day	38% 6% 12% 23% 17% 4	104704
	Night	39% 5% 11% 23% 18% 4	102432
Seeing	Day	18% 11% 27% 25% 15% 5%	15131
	Night	17% 10% 29% 30% 13% 1	24776

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% 6% 2 4 3 3 3 3 4 5% 52%	2776
	11th to 20th	Day	14% 8% 3 6% 4 4 4 4 4 6% 44%	2840
	21st to end	Day	16% 10% 2 7% 4 4 4 3 5% 5% 39%	3124
February	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	1710
	11th to 20th	Day	21% 7% 2 3 3 3 4 3 5% 6% 43%	1721
	21st to end	Day	18% 7% 2 8% 5% 4 5% 3 5% 6% 37%	1579
March	1st to 10th	Day	20% 9% 3 6% 5% 3 4 3 5% 6% 36%	3435
	11th to 20th	Day	10% 4 2 5% 3 3 4 4 6% 7% 53%	3480
	21st to end	Day	21% 6% 2 6% 4 4 4 3 5% 6% 39%	3836
April	1st to 10th	Day	10% 5% 2 5% 4 3 4 4 5% 7% 51%	2280
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 6% 7% 48%	2254
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 7% 43%	2280
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 9% 32%	3976
	11th to 20th	Day	20% 7% 2 6% 4 4 4 4 7% 9% 32%	4099
	21st to end	Day	9% 4 2 5% 5% 4 5% 5% 7% 9% 46%	4823
June	1st to 10th	Day	11% 4 2 5% 5% 4 5% 5% 7% 10% 42%	2660
	11th to 20th	Day	11% 4 1 6% 5% 4 5% 5% 8% 10% 42%	2660
	21st to end	Day	8% 4 2 5% 5% 5% 5% 5% 7% 11% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 13% 40%	4378
	11th to 20th	Day	11% 5% 2 7% 6% 5% 5% 5% 8% 12% 35%	4120
	21st to end	Day	10% 6% 3 7% 6% 6% 6% 6% 8% 10% 34%	4466
August	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 5% 7% 11% 28%	4104
	11th to 20th	Day	8% 6% 2 8% 5% 5% 6% 5% 7% 10% 38%	3855
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 11% 50%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 5% 5% 4 6% 8% 45%	2280
	11th to 20th	Day	12% 7% 2 6% 4 3 4 3 5% 8% 47%	2280
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 6% 38%	1983
October	1st to 10th	Day	16% 5% 2 6% 4 4 4 2 4 6% 48%	1900
	11th to 20th	Day	16% 5% 1 3 4 3 3 3 4 5% 53%	1900
	21st to end	Day	13% 4 1 5% 3 3 3 2 4 7% 54%	2090
November	1st to 10th	Day	13% 6% 1 4 3 4 4 3 4 5% 54%	1884
	11th to 20th	Day	14% 6% 2 4 4 3 3 3 5% 6% 50%	1710
	21st to end	Day	14% 5% 2 4 3 3 3 3 4 6% 52%	1535
December	1st to 10th	Day	17% 7% 1 5% 4 3 4 4 6% 8% 41%	1520
	11th to 20th	Day	16% 6% 2 4 3 3 3 3 3 4 52%	1910
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 7% 40%	2772
January	1st to 10th	Night	15% 7% 2 6% 3 3 4 3 5% 6% 47%	4164
	11th to 20th	Night	17% 7% 3 5% 4 3 3 3 5% 5% 46%	4120
	21st to end	Night	20% 9% 3 5% 3 3 3 3 4 4 44%	4532
February	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	2470
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 4 6% 43%	2343
	21st to end	Night	17% 7% 2 6% 5% 3 3 2 4 6% 45%	1872
March	1st to 10th	Night	22% 7% 2 6% 4 3 4 3 5% 6% 38%	3418
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 5% 7% 51%	3480
	21st to end	Night	17% 7% 2 5% 3 4 3 3 4 5% 49%	3796
April	1st to 10th	Night	13% 7% 2 7% 5% 4 4 3 4 7% 46%	1900
	11th to 20th	Night	10% 7% 2 5% 4 3 4 3 6% 7% 50%	1875
	21st to end	Night	15% 6% 2 6% 4 3 4 3 6% 7% 44%	1900
May	1st to 10th	Night	16% 8% 2 7% 4 5% 4 3 6% 8% 35%	2947
	11th to 20th	Night	22% 8% 3 7% 5% 4 5% 3 5% 6% 33%	2790
	21st to end	Night	9% 4 2 6% 4 5% 4 4 6% 10% 44%	2732
June	1st to 10th	Night	10% 6% 2 5% 5% 5% 6% 4 8% 10% 40%	1520
	11th to 20th	Night	10% 6% 2 6% 4 5% 4 4 8% 10% 40%	1520
	21st to end	Night	8% 6% 2 5% 4 4 5% 6% 8% 9% 44%	1520
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 5% 7% 11% 40%	2576
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	2840
	21st to end	Night	11% 8% 3 9% 7% 7% 6% 5% 7% 7% 30%	3087
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 4 4 7% 10% 31%	2825
	11th to 20th	Night	10% 7% 2 8% 4 5% 5% 4 7% 10% 38%	2940
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 47%	3476
September	1st to 10th	Night	13% 5% 3 6% 4 4 5% 4 5% 7% 44%	1900
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	1900
	21st to end	Night	21% 9% 2 6% 3 3 4 3 5% 6% 36%	2190
October	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 4 6% 48%	2280
	11th to 20th	Night	15% 6% 2 5% 4 3 3 2 5% 6% 49%	2280
	21st to end	Night	14% 4 2 4 4 3 3 3 4 7% 53%	2508
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 5% 52%	2292
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 50%	2470
	21st to end	Night	13% 5% 1 4 3 3 4 3 5% 7% 50%	2644
December	1st to 10th	Night	17% 8% 2 5% 5% 4 4 3 5% 6% 42%	2660
	11th to 20th	Night	18% 6% 3 5% 3 3 3 3 4 5% 47%	3285
	21st to end	Night	15% 8% 3 6% 3 3 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% 11% 23% 15% 5%	2280
May	1st to 10th	Day	47% 9% 13% 19% 10% 2	4012
	11th to 20th	Day	47% 8% 12% 22% 9% 1	4103
	21st to end	Day	31% 8% 17% 24% 19% 2	4837
June	1st to 10th	Day	34% 9% 16% 25% 14% 1	2660
	11th to 20th	Day	35% 9% 16% 23% 16% 1	2660
	21st to end	Day	34% 8% 16% 24% 16% 2	2660
July	1st to 10th	Day	34% 7% 23% 18% 12% 5%	4309
	11th to 20th	Day	40% 5% 24% 22% 7% 1	4051
	21st to end	Day	44% 8% 13% 19% 15% 1	4324
August	1st to 10th	Day	48% 6% 17% 22% 6%	4091
	11th to 20th	Day	40% 6% 15% 25% 14%	3894
	21st to end	Day	31% 5% 16% 29% 18% 2	4180
September	1st to 10th	Day	37% 7% 14% 17% 21% 4	2280
	11th to 20th	Day	38% 5% 9% 26% 19% 4	2280
	21st to end	Day	47% 5% 9% 20% 15% 4	1983
October	1st to 10th	Day	38% 2 7% 19% 28% 5%	3160
	11th to 20th	Day	25% 3 5% 21% 33% 13%	3140
	21st to end	Day	21% 2 9% 29% 26% 13%	3428
November	1st to 10th	Day	33% 4 10% 25% 25% 4	1884
	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%	1900
May	1st to 10th	Night	45% 8% 15% 19% 11% 2	2977
	11th to 20th	Night	48% 9% 10% 22% 9% 1	2791
	21st to end	Night	32% 9% 17% 25% 16% 1	2739
June	1st to 10th	Night	37% 8% 14% 24% 15% 1	1520
	11th to 20th	Night	36% 9% 18% 20% 16% 1	1520
	21st to end	Night	32% 9% 15% 22% 18% 4	1520
July	1st to 10th	Night	38% 9% 20% 18% 12% 3	2535
	11th to 20th	Night	40% 8% 24% 20% 6% 1	2786
	21st to end	Night	51% 8% 13% 16% 12%	2988
August	1st to 10th	Night	48% 5% 17% 23% 6%	2814
	11th to 20th	Night	40% 6% 16% 25% 12%	2967
	21st to end	Night	34% 6% 11% 29% 19% 1	3476
September	1st to 10th	Night	39% 7% 15% 19% 15% 6%	1900
	11th to 20th	Night	36% 6% 10% 24% 21% 3	1900
	21st to end	Night	48% 5% 10% 18% 16% 4	2190
October	1st to 10th	Night	41% 3 10% 19% 23% 5%	3800
	11th to 20th	Night	26% 2 7% 22% 31% 12%	3779
	21st to end	Night	22% 2 6% 32% 26% 13%	4120
November	1st to 10th	Night	35% 4 9% 23% 24% 5%	2292
	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	22% 14% 33% 22% 7% 1	460
May	1st to 10th	Day	19% 15% 32% 23% 9% 2	463
	11th to 20th	Day	21% 9% 27% 28% 12% 3	487
	21st to end	Day	19% 4 22% 30% 20% 5%	529
June	1st to 10th	Day	11% 6% 27% 29% 23% 5%	490
	11th to 20th	Day	12% 4 24% 36% 17% 7%	490
	21st to end	Day	11% 5% 22% 35% 22% 6%	490
July	1st to 10th	Day	9% 2 17% 36% 27% 9%	485
	11th to 20th	Day	6% 2 15% 38% 30% 10%	484
	21st to end	Day	11% 2 20% 34% 23% 10%	532
August	1st to 10th	Day	14% 3 18% 32% 24% 8%	486
	11th to 20th	Day	11% 4 18% 31% 27% 10%	479
	21st to end	Day	12% 3 14% 31% 25% 15%	539
September	1st to 10th	Day	16% 3 17% 29% 19% 16%	490
	11th to 20th	Day	16% 4 18% 26% 25% 11%	490
	21st to end	Day	29% 7% 22% 27% 13% 3	490
October	1st to 10th	Day	20% 5% 22% 26% 20% 7%	490
	11th to 20th	Day	16% 11% 25% 28% 18% 2	490
	21st to end	Day	15% 10% 33% 27% 10% 4	539
November	1st to 10th	Day	18% 9% 31% 25% 14% 4	466
	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% 9% 39% 28% 7%	760
May	1st to 10th	Night	18% 9% 36% 28% 8%	655
	11th to 20th	Night	18% 5% 30% 34% 12% 1	552
	21st to end	Night	14% 2 22% 38% 22% 2	411
June	1st to 10th	Night	11% 2 21% 43% 22% 1	380
	11th to 20th	Night	12% 4 20% 40% 22% 2	380
	21st to end	Night	7% 4 20% 48% 20% 1	380
July	1st to 10th	Night	11% 3 17% 41% 26% 3	399
	11th to 20th	Night	6% 1 14% 41% 34% 5%	552
	21st to end	Night	8% 2 18% 44% 24% 4	624
August	1st to 10th	Night	12% 2 13% 44% 26% 3	567
	11th to 20th	Night	13% 3 14% 41% 26% 4	654
	21st to end	Night	12% 2 14% 37% 29% 6%	836
September	1st to 10th	Night	16% 2 14% 37% 27% 6%	760
	11th to 20th	Night	14% 4 14% 34% 30% 4	760
	21st to end	Night	26% 3 20% 33% 17% 1	760
October	1st to 10th	Night	17% 5% 21% 31% 21% 4	760
	11th to 20th	Night	18% 9% 25% 31% 16% 1	760
	21st to end	Night	16% 10% 30% 30% 14% 1	836
November	1st to 10th	Night	18% 8% 28% 32% 13% 2	760
	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 2024-11-03 02:11:29. Version ac07

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