Swan River
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Swan River clear sky chart from 2005-11-06 to 2026-06-16.

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	11% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	142874
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 34%	137995
Transparency	Day	44% 7% 16% 24% 9% 1	142580
	Night	48% 7% 14% 22% 9% 1	137875
Seeing	Day	15% 11% 31% 27% 13% 3	18561
	Night	14% 8% 31% 33% 13% 1	28022

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	3 13% 7% 15% 7% 6% 8% 5% 6% 8% 20%	4125
	11th to 20th	Day	5% 12% 4 13% 9% 8% 9% 9% 5% 5% 21%	4376
	21st to end	Day	5% 13% 5% 8% 6% 6% 7% 5% 6% 8% 32%	5188
February	1st to 10th	Day	7% 11% 5% 10% 6% 7% 8% 5% 6% 8% 26%	1680
	11th to 20th	Day	5% 9% 4 9% 7% 7% 7% 5% 6% 9% 33%	1767
	21st to end	Day	5% 9% 5% 12% 8% 7% 7% 6% 6% 7% 28%	1730
March	1st to 10th	Day	7% 11% 5% 12% 7% 6% 6% 5% 7% 7% 28%	4600
	11th to 20th	Day	8% 10% 5% 11% 6% 6% 5% 4 6% 7% 32%	4850
	21st to end	Day	8% 10% 3 7% 5% 4 5% 4 6% 8% 40%	5948
April	1st to 10th	Day	8% 10% 4 8% 6% 4 5% 5% 6% 7% 37%	2520
	11th to 20th	Day	13% 11% 4 11% 6% 5% 5% 4 5% 7% 29%	2686
	21st to end	Day	10% 9% 3 8% 5% 5% 5% 5% 6% 7% 38%	2777
May	1st to 10th	Day	12% 10% 3 7% 4 4 4 3 5% 7% 41%	8224
	11th to 20th	Day	20% 10% 3 7% 5% 4 4 3 5% 6% 33%	8722
	21st to end	Day	12% 8% 3 7% 4 4 4 3 5% 7% 43%	9683
June	1st to 10th	Day	18% 7% 3 6% 5% 4 5% 4 6% 7% 34%	3150
	11th to 20th	Day	20% 6% 2 7% 5% 5% 5% 4 6% 7% 32%	3250
	21st to end	Day	15% 6% 3 6% 5% 5% 6% 4 7% 8% 35%	3200
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 5% 7% 9% 45%	7157
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 7% 9% 46%	6800
	21st to end	Day	9% 4 2 5% 4 4 5% 5% 7% 9% 48%	7067
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 7% 9% 42%	4552
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 9% 44%	4402
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	4675
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 44%	2400
	11th to 20th	Day	13% 8% 3 6% 5% 4 4 4 6% 7% 39%	2271
	21st to end	Day	11% 7% 2 6% 4 4 4 3 5% 7% 45%	2200
October	1st to 10th	Day	14% 8% 2 7% 4 4 4 3 5% 7% 42%	3411
	11th to 20th	Day	10% 8% 2 6% 5% 4 6% 3 5% 9% 43%	2922
	21st to end	Day	10% 13% 4 9% 5% 4 6% 4 7% 6% 32%	3190
November	1st to 10th	Day	16% 19% 5% 9% 5% 4 3 3 3 5% 27%	1767
	11th to 20th	Day	9% 16% 5% 12% 7% 5% 6% 3 5% 6% 26%	1470
	21st to end	Day	8% 14% 7% 12% 7% 5% 5% 3 5% 6% 28%	1470
December	1st to 10th	Day	9% 13% 5% 14% 10% 9% 7% 5% 5% 6% 17%	2360
	11th to 20th	Day	8% 15% 5% 11% 7% 7% 6% 6% 6% 6% 23%	2709
	21st to end	Day	4 9% 6% 15% 6% 4 4 3 4 5% 40%	3575
January	1st to 10th	Night	5% 14% 7% 16% 9% 7% 6% 5% 6% 6% 18%	8799
	11th to 20th	Night	5% 12% 5% 14% 10% 8% 11% 6% 6% 5% 18%	8534
	21st to end	Night	4 12% 6% 13% 7% 8% 6% 5% 6% 6% 28%	9026
February	1st to 10th	Night	8% 12% 4 12% 6% 7% 8% 5% 6% 6% 26%	2940
	11th to 20th	Night	4 9% 4 11% 7% 6% 7% 5% 7% 7% 33%	2758
	21st to end	Night	4 11% 5% 13% 8% 6% 5% 3 6% 8% 32%	2076
March	1st to 10th	Night	8% 13% 5% 9% 5% 6% 5% 4 5% 9% 33%	5494
	11th to 20th	Night	6% 12% 5% 12% 7% 5% 5% 4 6% 7% 30%	5338
	21st to end	Night	8% 11% 4 7% 5% 5% 5% 4 6% 7% 39%	5240
April	1st to 10th	Night	7% 8% 3 9% 6% 5% 6% 4 6% 6% 40%	2100
	11th to 20th	Night	13% 9% 4 10% 6% 5% 4 4 6% 7% 33%	1885
	21st to end	Night	12% 8% 3 8% 5% 5% 4 5% 5% 7% 38%	1840
May	1st to 10th	Night	12% 11% 4 6% 4 3 4 4 5% 6% 43%	4724
	11th to 20th	Night	17% 9% 3 7% 5% 5% 6% 4 7% 7% 31%	4171
	21st to end	Night	13% 6% 2 5% 4 3 4 3 5% 6% 48%	4523
June	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 7% 39%	1470
	11th to 20th	Night	20% 5% 3 5% 5% 4 5% 5% 6% 8% 34%	1255
	21st to end	Night	14% 7% 3 6% 6% 6% 6% 4 5% 6% 38%	1200
July	1st to 10th	Night	7% 3 1 5% 4 4 5% 4 6% 10% 50%	2802
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 6% 9% 50%	3180
	21st to end	Night	8% 3 2 4 3 3 3 3 6% 8% 57%	3783
August	1st to 10th	Night	14% 5% 2 5% 3 4 3 4 5% 8% 47%	2608
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 7% 8% 47%	2657
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	3234
September	1st to 10th	Night	9% 6% 2 5% 4 3 3 4 5% 7% 50%	1998
	11th to 20th	Night	16% 8% 2 5% 4 3 4 3 5% 6% 43%	2128
	21st to end	Night	11% 6% 2 5% 4 4 4 4 5% 8% 47%	2200
October	1st to 10th	Night	11% 8% 2 5% 4 3 3 4 4 7% 48%	3736
	11th to 20th	Night	9% 10% 3 7% 5% 5% 6% 5% 7% 9% 36%	4227
	21st to end	Night	10% 12% 3 8% 5% 5% 4 3 5% 7% 38%	4610
November	1st to 10th	Night	13% 16% 6% 11% 7% 5% 5% 3 4 5% 25%	2746
	11th to 20th	Night	10% 16% 6% 11% 7% 5% 4 4 5% 6% 26%	3150
	21st to end	Night	9% 14% 6% 13% 9% 6% 5% 4 5% 6% 24%	3150
December	1st to 10th	Night	8% 16% 6% 15% 11% 7% 6% 4 4 6% 17%	5050
	11th to 20th	Night	7% 13% 5% 13% 7% 5% 8% 6% 6% 6% 25%	5718
	21st to end	Night	5% 10% 5% 15% 7% 5% 4 3 5% 6% 36%	7645

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	59% 7% 14% 16% 4	4125
	11th to 20th	Day	58% 5% 11% 19% 7%	4398
	21st to end	Day	47% 8% 16% 25% 4	5188
February	1st to 10th	Day	52% 10% 13% 17% 8%	1680
	11th to 20th	Day	44% 8% 14% 23% 11%	1775
	21st to end	Day	51% 8% 13% 19% 10%	1730
March	1st to 10th	Day	52% 7% 15% 18% 8%	4600
	11th to 20th	Day	47% 8% 15% 21% 9%	4852
	21st to end	Day	40% 6% 13% 27% 13%	5948
April	1st to 10th	Day	44% 7% 16% 19% 12% 1	2520
	11th to 20th	Day	54% 7% 12% 16% 9% 2	2686
	21st to end	Day	42% 8% 14% 25% 9% 1	2777
May	1st to 10th	Day	42% 6% 11% 27% 14%	8224
	11th to 20th	Day	51% 5% 13% 22% 8% 1	8726
	21st to end	Day	39% 6% 9% 23% 21% 2	9683
June	1st to 10th	Day	48% 8% 13% 21% 9% 1	3150
	11th to 20th	Day	50% 8% 15% 18% 9%	3250
	21st to end	Day	45% 10% 16% 22% 6%	3200
July	1st to 10th	Day	33% 7% 25% 29% 6%	7079
	11th to 20th	Day	33% 8% 19% 36% 3	6719
	21st to end	Day	32% 8% 23% 30% 7%	6870
August	1st to 10th	Day	36% 9% 21% 26% 8%	4536
	11th to 20th	Day	35% 9% 19% 32% 5%	4444
	21st to end	Day	36% 7% 23% 27% 7%	4674
September	1st to 10th	Day	37% 8% 18% 26% 9% 1	2400
	11th to 20th	Day	43% 8% 16% 26% 7%	2271
	21st to end	Day	38% 7% 14% 29% 11% 1	2200
October	1st to 10th	Day	42% 5% 17% 29% 6%	3411
	11th to 20th	Day	39% 6% 15% 33% 7%	2922
	21st to end	Day	50% 6% 19% 20% 5%	3190
November	1st to 10th	Day	60% 6% 11% 17% 6%	1767
	11th to 20th	Day	59% 8% 15% 13% 6%	1470
	21st to end	Day	57% 8% 13% 18% 4	1470
December	1st to 10th	Day	63% 9% 13% 11% 4	2360
	11th to 20th	Day	57% 8% 17% 14% 3	2710
	21st to end	Day	44% 7% 11% 22% 12% 3	3575
January	1st to 10th	Night	62% 7% 10% 16% 5%	8800
	11th to 20th	Night	63% 6% 10% 15% 6%	8574
	21st to end	Night	52% 8% 14% 21% 5%	9026
February	1st to 10th	Night	54% 9% 12% 15% 9% 1	2940
	11th to 20th	Night	47% 7% 15% 21% 10%	2760
	21st to end	Night	48% 6% 16% 21% 8%	2076
March	1st to 10th	Night	48% 6% 18% 22% 6%	5494
	11th to 20th	Night	49% 8% 10% 19% 13% 1	5338
	21st to end	Night	40% 7% 9% 29% 14%	5240
April	1st to 10th	Night	42% 6% 13% 21% 16% 2	2100
	11th to 20th	Night	49% 7% 13% 19% 10% 3	1885
	21st to end	Night	43% 7% 15% 24% 9% 2	1840
May	1st to 10th	Night	41% 6% 11% 25% 16% 2	4724
	11th to 20th	Night	50% 6% 14% 19% 10% 1	4171
	21st to end	Night	35% 7% 12% 20% 24% 2	4526
June	1st to 10th	Night	43% 7% 13% 24% 11% 1	1470
	11th to 20th	Night	45% 9% 18% 17% 10% 2	1255
	21st to end	Night	45% 9% 18% 20% 8%	1200
July	1st to 10th	Night	29% 7% 26% 32% 5%	2771
	11th to 20th	Night	29% 8% 20% 38% 4	3138
	21st to end	Night	26% 7% 25% 33% 8%	3676
August	1st to 10th	Night	34% 8% 20% 29% 10%	2600
	11th to 20th	Night	33% 6% 22% 32% 7%	2681
	21st to end	Night	38% 6% 19% 30% 7%	3234
September	1st to 10th	Night	32% 8% 16% 32% 11% 2	1998
	11th to 20th	Night	41% 8% 15% 29% 7%	2128
	21st to end	Night	35% 7% 13% 32% 11% 2	2200
October	1st to 10th	Night	36% 5% 18% 28% 13%	3736
	11th to 20th	Night	43% 6% 12% 30% 9%	4227
	21st to end	Night	46% 5% 17% 26% 6%	4610
November	1st to 10th	Night	60% 8% 11% 14% 6% 1	2746
	11th to 20th	Night	57% 8% 16% 14% 4	3150
	21st to end	Night	59% 8% 14% 14% 4	3150
December	1st to 10th	Night	67% 8% 10% 12% 3	5050
	11th to 20th	Night	54% 9% 15% 15% 8%	5716
	21st to end	Night	48% 6% 12% 20% 12% 3	7645

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	11% 13% 33% 30% 13%	330
	11th to 20th	Day	10% 12% 41% 29% 8% 1	330
	21st to end	Day	13% 10% 34% 33% 9% 2	363
February	1st to 10th	Day	13% 12% 34% 28% 13%	330
	11th to 20th	Day	7% 9% 35% 31% 15% 3	330
	21st to end	Day	6% 10% 39% 30% 14% 1	273
March	1st to 10th	Day	16% 10% 32% 29% 10% 3	330
	11th to 20th	Day	10% 13% 39% 27% 9% 2	330
	21st to end	Day	15% 15% 32% 27% 10% 2	519
April	1st to 10th	Day	12% 14% 36% 27% 11% 1	540
	11th to 20th	Day	16% 13% 31% 26% 13% 1	724
	21st to end	Day	13% 13% 33% 28% 11% 2	750
May	1st to 10th	Day	17% 13% 29% 29% 11% 1	753
	11th to 20th	Day	19% 14% 27% 23% 14% 2	775
	21st to end	Day	18% 10% 28% 26% 15% 3	844
June	1st to 10th	Day	21% 13% 30% 21% 10% 4	780
	11th to 20th	Day	20% 14% 29% 23% 11% 4	756
	21st to end	Day	18% 9% 27% 27% 13% 5%	730
July	1st to 10th	Day	10% 9% 27% 30% 19% 5%	721
	11th to 20th	Day	12% 10% 28% 29% 16% 5%	718
	21st to end	Day	10% 9% 25% 29% 20% 7%	794
August	1st to 10th	Day	12% 9% 27% 29% 19% 4	726
	11th to 20th	Day	14% 7% 25% 28% 19% 7%	715
	21st to end	Day	17% 8% 26% 30% 17% 1	803
September	1st to 10th	Day	15% 14% 32% 29% 8% 2	530
	11th to 20th	Day	19% 12% 30% 25% 8% 4	401
	21st to end	Day	15% 11% 32% 28% 12% 1	330
October	1st to 10th	Day	23% 11% 28% 26% 10% 2	330
	11th to 20th	Day	20% 14% 37% 20% 8% 1	330
	21st to end	Day	20% 12% 34% 22% 10% 2	363
November	1st to 10th	Day	25% 12% 29% 25% 6% 2	330
	11th to 20th	Day	19% 12% 36% 25% 8% 1	330
	21st to end	Day	15% 12% 40% 24% 8% 1	330
December	1st to 10th	Day	10% 12% 35% 30% 12% 1	330
	11th to 20th	Day	14% 14% 39% 27% 6%	330
	21st to end	Day	10% 7% 26% 40% 14% 2	363
January	1st to 10th	Night	11% 9% 38% 31% 10% 1	1050
	11th to 20th	Night	12% 8% 38% 30% 10% 1	1050
	21st to end	Night	13% 10% 37% 31% 8% 1	1155
February	1st to 10th	Night	12% 7% 33% 34% 13% 1	1050
	11th to 20th	Night	8% 8% 35% 33% 15% 2	992
	21st to end	Night	8% 7% 31% 36% 16% 1	692
March	1st to 10th	Night	15% 6% 28% 38% 12% 1	840
	11th to 20th	Night	11% 8% 35% 34% 12% 1	748
	21st to end	Night	13% 8% 30% 30% 17% 3	693
April	1st to 10th	Night	10% 4 32% 35% 18% 1	630
	11th to 20th	Night	16% 6% 31% 32% 13% 1	623
	21st to end	Night	13% 9% 35% 34% 8% 1	630
May	1st to 10th	Night	19% 7% 30% 33% 12% 1	630
	11th to 20th	Night	17% 9% 32% 30% 12%	621
	21st to end	Night	20% 8% 24% 35% 12% 1	683
June	1st to 10th	Night	19% 7% 29% 33% 12% 1	630
	11th to 20th	Night	20% 4 27% 33% 14% 1	435
	21st to end	Night	13% 7% 25% 39% 17% 1	400
July	1st to 10th	Night	9% 5% 25% 42% 19% 1	419
	11th to 20th	Night	10% 6% 28% 36% 18% 2	582
	21st to end	Night	9% 4 24% 39% 20% 4	657
August	1st to 10th	Night	10% 6% 22% 38% 22% 2	597
	11th to 20th	Night	14% 4 20% 35% 24% 3	594
	21st to end	Night	17% 4 26% 40% 13% 1	660
September	1st to 10th	Night	12% 6% 27% 38% 17% 1	600
	11th to 20th	Night	18% 7% 23% 38% 13% 1	600
	21st to end	Night	12% 6% 28% 42% 11% 1	600
October	1st to 10th	Night	19% 7% 29% 34% 10% 1	681
	11th to 20th	Night	20% 9% 28% 32% 10% 1	1000
	21st to end	Night	20% 8% 31% 29% 10% 1	1100
November	1st to 10th	Night	19% 11% 33% 28% 8% 1	1025
	11th to 20th	Night	18% 9% 37% 26% 9% 2	1050
	21st to end	Night	15% 12% 35% 28% 8% 1	1050
December	1st to 10th	Night	11% 9% 37% 28% 12% 3	1050
	11th to 20th	Night	15% 9% 35% 29% 11% 1	1050
	21st to end	Night	12% 7% 29% 37% 13% 3	1155

Page generated at 2026-06-13 01:31:41. Version ac07

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