Fishtrap
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fishtrap clear sky chart from 2005-11-06 to 2025-12-03.

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	8% 7% 3 7% 4 4 4 3 5% 7% 48%	130540
	Night	10% 8% 3 7% 4 4 4 3 5% 7% 43%	128837
Transparency	Day	36% 6% 11% 23% 17% 7%	130266
	Night	40% 6% 11% 21% 16% 6%	128688
Seeing	Day	12% 8% 23% 27% 22% 9%	15904
	Night	15% 6% 21% 35% 20% 3	25134

By Month

Percentage Cloud Cover NW (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	18% 18% 8% 15% 6% 5% 4 4 4 5% 13%	3639
	11th to 20th	Day	14% 9% 3 8% 5% 5% 5% 4 5% 6% 34%	3616
	21st to end	Day	18% 13% 5% 9% 4 3 3 3 4 5% 33%	4004
February	1st to 10th	Day	15% 12% 5% 10% 6% 7% 5% 4 5% 6% 26%	1635
	11th to 20th	Day	14% 13% 3 9% 6% 5% 6% 4 5% 6% 29%	1862
	21st to end	Day	12% 14% 5% 9% 4 5% 4 4 5% 6% 31%	1650
March	1st to 10th	Day	6% 9% 3 6% 5% 7% 8% 5% 7% 9% 35%	4518
	11th to 20th	Day	8% 8% 2 7% 5% 5% 4 4 5% 6% 44%	4656
	21st to end	Day	11% 11% 4 10% 7% 6% 6% 5% 6% 8% 27%	5969
April	1st to 10th	Day	9% 11% 3 8% 6% 6% 6% 4 7% 9% 31%	2400
	11th to 20th	Day	8% 8% 3 7% 5% 5% 5% 3 7% 7% 41%	2412
	21st to end	Day	9% 7% 3 7% 4 5% 4 5% 5% 8% 43%	2600
May	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 6% 8% 51%	6231
	11th to 20th	Day	8% 6% 2 8% 6% 5% 5% 4 6% 8% 41%	6277
	21st to end	Day	7% 6% 2 6% 4 5% 5% 4 6% 8% 48%	6903
June	1st to 10th	Day	11% 8% 3 6% 4 4 4 4 5% 7% 44%	2865
	11th to 20th	Day	9% 6% 3 5% 4 3 4 4 6% 8% 47%	3000
	21st to end	Day	5% 3 2 5% 3 4 4 3 5% 7% 60%	2868
July	1st to 10th	Day	3 2 1 2 2 2 2 2 4 4 75%	6320
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 4 81%	6320
	21st to end	Day	3 2 1 3 2 2 2 2 4 6% 75%	6874
August	1st to 10th	Day	3 3 1 3 2 2 3 2 4 5% 72%	4541
	11th to 20th	Day	2 2 1 3 2 2 3 3 5% 7% 69%	4296
	21st to end	Day	3 3 1 4 3 3 4 3 5% 6% 64%	4327
September	1st to 10th	Day	4 3 1 3 2 2 2 2 4 7% 70%	2400
	11th to 20th	Day	5% 4 2 5% 3 4 4 4 5% 7% 56%	2305
	21st to end	Day	5% 5% 2 6% 4 3 4 4 6% 7% 54%	2200
October	1st to 10th	Day	5% 4 1 5% 4 3 3 2 6% 7% 59%	3423
	11th to 20th	Day	8% 6% 2 6% 4 5% 6% 4 6% 7% 47%	3105
	21st to end	Day	12% 8% 3 6% 5% 4 5% 4 5% 7% 42%	3200
November	1st to 10th	Day	16% 10% 3 9% 5% 4 5% 4 5% 7% 33%	1854
	11th to 20th	Day	18% 11% 4 9% 5% 5% 5% 5% 5% 5% 29%	1774
	21st to end	Day	13% 15% 5% 11% 6% 4 5% 5% 6% 7% 24%	1638
December	1st to 10th	Day	12% 10% 5% 10% 8% 7% 6% 4 6% 7% 25%	2304
	11th to 20th	Day	19% 18% 6% 14% 6% 5% 5% 4 4 5% 15%	2642
	21st to end	Day	18% 16% 7% 13% 6% 5% 5% 4 5% 5% 16%	3912
January	1st to 10th	Night	22% 17% 6% 11% 5% 5% 4 4 5% 5% 15%	6306
	11th to 20th	Night	13% 10% 3 9% 5% 4 4 3 5% 7% 38%	6282
	21st to end	Night	15% 13% 4 7% 4 4 4 3 5% 6% 35%	6952
February	1st to 10th	Night	16% 15% 5% 8% 5% 4 4 4 6% 6% 29%	2761
	11th to 20th	Night	13% 12% 5% 8% 5% 4 4 4 6% 6% 34%	2425
	21st to end	Night	11% 13% 3 8% 5% 5% 4 4 6% 8% 33%	1980
March	1st to 10th	Night	7% 7% 3 8% 5% 5% 5% 4 6% 9% 43%	5376
	11th to 20th	Night	10% 10% 4 7% 5% 4 4 3 4 6% 44%	5339
	21st to end	Night	9% 8% 3 10% 7% 7% 7% 5% 9% 9% 26%	5006
April	1st to 10th	Night	10% 10% 3 7% 6% 5% 5% 5% 7% 8% 36%	2000
	11th to 20th	Night	8% 11% 3 7% 5% 3 4 3 5% 6% 46%	1938
	21st to end	Night	7% 7% 4 7% 5% 5% 5% 5% 6% 7% 42%	1800
May	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 6% 8% 52%	3718
	11th to 20th	Night	8% 7% 4 8% 6% 4 5% 3 6% 8% 42%	3616
	21st to end	Night	6% 5% 1 5% 4 4 4 4 6% 9% 52%	3972
June	1st to 10th	Night	9% 7% 2 6% 4 5% 4 4 5% 7% 48%	1530
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 4 5% 7% 49%	1400
	21st to end	Night	6% 3 2 5% 4 3 5% 4 7% 6% 56%	1523
July	1st to 10th	Night	3 2 2 1 2 3 2 4 5% 78%	3640
	11th to 20th	Night	1 1 1 2 1 2 2 1 3 4 82%	3640
	21st to end	Night	2 2 1 2 1 1 2 2 3 5% 78%	3969
August	1st to 10th	Night	4 3 1 3 1 2 3 2 4 6% 72%	2609
	11th to 20th	Night	2 3 2 4 3 4 4 4 5% 6% 64%	2756
	21st to end	Night	2 3 1 4 3 3 4 3 5% 6% 66%	3590
September	1st to 10th	Night	3 2 1 3 2 3 3 3 5% 7% 68%	2000
	11th to 20th	Night	4 4 2 6% 4 4 4 3 5% 6% 58%	2095
	21st to end	Night	8% 5% 2 5% 3 4 4 2 6% 7% 55%	2200
October	1st to 10th	Night	6% 3 2 4 3 3 3 3 5% 7% 62%	3760
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 6% 8% 50%	4047
	21st to end	Night	12% 5% 2 6% 5% 4 5% 4 6% 6% 47%	4600
November	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 6% 7% 33%	2665
	11th to 20th	Night	17% 9% 4 9% 6% 5% 5% 4 5% 7% 30%	2844
	21st to end	Night	16% 12% 5% 9% 6% 5% 4 4 5% 5% 29%	2956
December	1st to 10th	Night	9% 8% 4 8% 5% 5% 5% 4 6% 9% 36%	4921
	11th to 20th	Night	20% 15% 6% 14% 7% 5% 5% 3 5% 6% 13%	5550
	21st to end	Night	20% 16% 5% 13% 6% 5% 5% 3 4 5% 19%	7071

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	72% 9% 9% 8% 1	3639
	11th to 20th	Day	49% 6% 18% 20% 6% 1	3640
	21st to end	Day	55% 6% 13% 19% 7%	4004
February	1st to 10th	Day	58% 10% 14% 11% 6% 1	1635
	11th to 20th	Day	55% 8% 14% 16% 7%	1871
	21st to end	Day	52% 9% 12% 18% 8% 1	1650
March	1st to 10th	Day	42% 7% 12% 23% 16% 1	4518
	11th to 20th	Day	39% 5% 12% 32% 11%	4656
	21st to end	Day	52% 8% 15% 18% 5% 1	5969
April	1st to 10th	Day	49% 8% 14% 21% 8% 1	2400
	11th to 20th	Day	40% 5% 15% 24% 13% 3	2412
	21st to end	Day	38% 8% 14% 25% 14% 2	2600
May	1st to 10th	Day	28% 7% 9% 28% 27% 2	6231
	11th to 20th	Day	38% 6% 12% 27% 16% 2	6280
	21st to end	Day	31% 8% 15% 30% 14% 1	6904
June	1st to 10th	Day	39% 6% 10% 23% 18% 4	2865
	11th to 20th	Day	33% 7% 13% 26% 17% 4	3000
	21st to end	Day	23% 5% 11% 30% 24% 6%	2868
July	1st to 10th	Day	15% 2 4 21% 27% 32%	6251
	11th to 20th	Day	10% 2 4 18% 33% 34%	6245
	21st to end	Day	14% 1 4 27% 33% 20%	6681
August	1st to 10th	Day	16% 2 5% 18% 38% 21%	4524
	11th to 20th	Day	17% 3 6% 37% 30% 7%	4338
	21st to end	Day	21% 3 9% 30% 31% 7%	4326
September	1st to 10th	Day	16% 4 11% 28% 28% 13%	2400
	11th to 20th	Day	27% 5% 14% 25% 23% 6%	2305
	21st to end	Day	28% 7% 17% 30% 14% 4	2200
October	1st to 10th	Day	26% 3 11% 36% 22% 1	3423
	11th to 20th	Day	37% 4 18% 25% 15% 1	3105
	21st to end	Day	42% 5% 14% 25% 12% 2	3200
November	1st to 10th	Day	49% 8% 16% 20% 5% 1	1854
	11th to 20th	Day	57% 8% 16% 14% 4 1	1774
	21st to end	Day	57% 11% 16% 12% 4 1	1638
December	1st to 10th	Day	59% 6% 17% 15% 3	2304
	11th to 20th	Day	72% 6% 10% 9% 1	2644
	21st to end	Day	68% 8% 11% 9% 3	3912
January	1st to 10th	Night	69% 7% 11% 11% 1	6306
	11th to 20th	Night	47% 6% 19% 18% 10% 1	6320
	21st to end	Night	50% 8% 13% 20% 9%	6952
February	1st to 10th	Night	55% 7% 15% 15% 7% 1	2761
	11th to 20th	Night	49% 8% 18% 15% 9% 1	2427
	21st to end	Night	48% 7% 15% 21% 9% 1	1980
March	1st to 10th	Night	36% 6% 11% 26% 21% 1	5376
	11th to 20th	Night	41% 4 9% 29% 15% 1	5340
	21st to end	Night	49% 9% 13% 22% 6%	5006
April	1st to 10th	Night	43% 7% 13% 26% 11% 1	2000
	11th to 20th	Night	38% 6% 10% 25% 17% 3	1938
	21st to end	Night	38% 7% 11% 21% 19% 3	1800
May	1st to 10th	Night	28% 6% 8% 24% 30% 4	3715
	11th to 20th	Night	38% 5% 10% 25% 19% 2	3616
	21st to end	Night	27% 8% 15% 33% 17% 1	3972
June	1st to 10th	Night	35% 6% 9% 22% 23% 5%	1530
	11th to 20th	Night	34% 6% 7% 25% 22% 6%	1400
	21st to end	Night	26% 6% 8% 25% 25% 9%	1523
July	1st to 10th	Night	12% 2 3 16% 32% 35%	3600
	11th to 20th	Night	10% 1 3 13% 29% 44%	3592
	21st to end	Night	12% 1 3 20% 34% 30%	3857
August	1st to 10th	Night	16% 2 4 13% 37% 28%	2601
	11th to 20th	Night	22% 3 4 25% 35% 11%	2780
	21st to end	Night	19% 3 7% 27% 28% 15%	3590
September	1st to 10th	Night	17% 4 7% 25% 29% 18%	2000
	11th to 20th	Night	27% 4 10% 25% 24% 10%	2095
	21st to end	Night	28% 5% 12% 29% 18% 8%	2200
October	1st to 10th	Night	23% 3 8% 35% 28% 2	3760
	11th to 20th	Night	33% 4 15% 24% 22% 2	4047
	21st to end	Night	38% 4 10% 33% 13% 2	4600
November	1st to 10th	Night	48% 8% 16% 19% 7% 1	2665
	11th to 20th	Night	53% 8% 15% 17% 6% 1	2844
	21st to end	Night	56% 8% 15% 16% 6%	2956
December	1st to 10th	Night	45% 9% 18% 23% 4	4921
	11th to 20th	Night	72% 6% 12% 9% 2	5547
	21st to end	Night	67% 8% 11% 11% 3	7071

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	25% 7% 32% 21% 14% 3	200
	11th to 20th	Day	24% 6% 20% 27% 19% 5%	200
	21st to end	Day	25% 9% 23% 25% 16% 2	220
February	1st to 10th	Day	21% 7% 33% 27% 10% 2	235
	11th to 20th	Day	24% 12% 26% 23% 14% 2	450
	21st to end	Day	17% 14% 28% 24% 14% 3	414
March	1st to 10th	Day	16% 10% 34% 22% 15% 4	500
	11th to 20th	Day	15% 10% 24% 27% 17% 7%	500
	21st to end	Day	16% 15% 29% 25% 13% 2	550
April	1st to 10th	Day	15% 12% 25% 27% 15% 6%	500
	11th to 20th	Day	10% 15% 30% 24% 15% 6%	492
	21st to end	Day	9% 10% 26% 28% 19% 7%	500
May	1st to 10th	Day	9% 7% 22% 31% 23% 7%	503
	11th to 20th	Day	14% 6% 20% 28% 20% 11%	528
	21st to end	Day	9% 7% 18% 27% 24% 15%	573
June	1st to 10th	Day	12% 7% 21% 24% 25% 11%	595
	11th to 20th	Day	11% 7% 29% 31% 15% 6%	730
	21st to end	Day	5% 3 21% 28% 31% 12%	598
July	1st to 10th	Day	3 3 17% 35% 33% 7%	526
	11th to 20th	Day	2 4 18% 34% 31% 11%	524
	21st to end	Day	3 2 14% 29% 37% 16%	575
August	1st to 10th	Day	6% 4 13% 31% 31% 17%	527
	11th to 20th	Day	3 2 14% 29% 33% 20%	517
	21st to end	Day	7% 5% 19% 25% 29% 15%	583
September	1st to 10th	Day	5% 5% 17% 26% 29% 18%	530
	11th to 20th	Day	7% 4 22% 25% 28% 14%	530
	21st to end	Day	8% 8% 24% 27% 25% 8%	530
October	1st to 10th	Day	11% 6% 25% 26% 24% 8%	530
	11th to 20th	Day	14% 9% 21% 25% 24% 8%	438
	21st to end	Day	18% 10% 24% 31% 13% 4	363
November	1st to 10th	Day	21% 9% 23% 31% 14% 2	330
	11th to 20th	Day	25% 10% 21% 23% 16% 5%	269
	21st to end	Day	20% 10% 21% 31% 15% 4	220
December	1st to 10th	Day	18% 9% 20% 26% 18% 9%	204
	11th to 20th	Day	28% 10% 24% 24% 14% 1	200
	21st to end	Day	26% 7% 29% 25% 10% 3	220
January	1st to 10th	Night	29% 10% 21% 28% 11% 2	800
	11th to 20th	Night	23% 7% 22% 29% 18% 1	800
	21st to end	Night	20% 4 21% 36% 18% 2	880
February	1st to 10th	Night	22% 10% 24% 30% 13% 1	800
	11th to 20th	Night	20% 7% 23% 32% 17% 1	784
	21st to end	Night	17% 10% 22% 32% 15% 3	660
March	1st to 10th	Night	16% 7% 24% 34% 16% 2	800
	11th to 20th	Night	15% 8% 24% 32% 19% 2	800
	21st to end	Night	16% 7% 28% 34% 13% 2	880
April	1st to 10th	Night	16% 5% 25% 33% 19% 2	800
	11th to 20th	Night	11% 6% 33% 33% 14% 2	750
	21st to end	Night	9% 7% 25% 34% 22% 2	600
May	1st to 10th	Night	7% 4 28% 33% 24% 3	439
	11th to 20th	Night	11% 6% 16% 34% 26% 7%	394
	21st to end	Night	8% 5% 20% 37% 26% 3	433
June	1st to 10th	Night	12% 3 22% 36% 24% 3	400
	11th to 20th	Night	11% 5% 29% 35% 18% 3	400
	21st to end	Night	8% 4 16% 41% 29% 3	400
July	1st to 10th	Night	4 1 13% 43% 34% 4	390
	11th to 20th	Night	3 3 13% 50% 29% 2	388
	21st to end	Night	1 1 11% 45% 37% 5%	438
August	1st to 10th	Night	4 3 10% 38% 39% 6%	398
	11th to 20th	Night	3 2 10% 41% 34% 10%	497
	21st to end	Night	4 3 17% 39% 33% 5%	862
September	1st to 10th	Night	5% 2 13% 43% 32% 5%	800
	11th to 20th	Night	6% 4 16% 33% 33% 8%	800
	21st to end	Night	9% 5% 20% 32% 29% 5%	800
October	1st to 10th	Night	12% 4 18% 39% 25% 3	800
	11th to 20th	Night	13% 5% 22% 34% 23% 3	800
	21st to end	Night	15% 6% 25% 35% 14% 4	880
November	1st to 10th	Night	22% 7% 20% 38% 12% 1	820
	11th to 20th	Night	22% 8% 27% 29% 13% 2	840
	21st to end	Night	22% 5% 19% 32% 19% 2	856
December	1st to 10th	Night	19% 7% 20% 36% 16% 2	1011
	11th to 20th	Night	26% 7% 22% 32% 12% 1	1000
	21st to end	Night	31% 10% 19% 29% 10% 1	934

Page generated at 2025-11-29 22:00:50. Version ac07

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