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 2025-02-21.

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% 36%	112827
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 33%	114481
Transparency	Day	45% 8% 16% 24% 7%	112575
	Night	48% 7% 14% 21% 8% 1	114381
Seeing	Day	15% 11% 30% 28% 13% 3	16656
	Night	14% 7% 31% 33% 13% 1	26297

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	4 13% 8% 17% 7% 6% 8% 5% 6% 8% 18%	3179
	11th to 20th	Day	5% 10% 4 14% 9% 9% 7% 11% 6% 6% 21%	3360
	21st to end	Day	5% 15% 6% 10% 7% 6% 6% 5% 6% 7% 27%	4004
February	1st to 10th	Day	7% 11% 5% 10% 6% 7% 8% 5% 7% 8% 26%	1600
	11th to 20th	Day	4 9% 4 9% 7% 7% 7% 5% 6% 9% 34%	1680
	21st to end	Day	5% 9% 5% 11% 8% 7% 6% 5% 6% 7% 30%	1570
March	1st to 10th	Day	8% 10% 5% 10% 6% 5% 4 4 6% 7% 34%	3130
	11th to 20th	Day	8% 9% 5% 11% 6% 5% 5% 4 6% 7% 34%	3311
	21st to end	Day	6% 9% 3 7% 5% 4 5% 4 7% 8% 42%	4058
April	1st to 10th	Day	9% 9% 4 8% 6% 4 5% 5% 6% 7% 37%	2280
	11th to 20th	Day	13% 11% 4 11% 6% 5% 5% 4 5% 7% 29%	2428
	21st to end	Day	10% 9% 3 7% 5% 5% 5% 5% 5% 7% 38%	2513
May	1st to 10th	Day	16% 10% 3 7% 4 4 4 4 7% 7% 35%	4430
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 5% 6% 36%	4668
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 6% 7% 31%	5173
June	1st to 10th	Day	18% 7% 3 6% 5% 4 5% 4 6% 7% 34%	2850
	11th to 20th	Day	20% 6% 2 7% 5% 5% 5% 4 6% 7% 32%	3011
	21st to end	Day	15% 6% 3 6% 5% 5% 6% 4 7% 8% 35%	3040
July	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 7% 10% 41%	5003
	11th to 20th	Day	9% 5% 2 4 3 4 4 4 7% 9% 49%	4750
	21st to end	Day	7% 5% 2 5% 4 4 5% 5% 7% 8% 48%	4915
August	1st to 10th	Day	11% 4 2 6% 4 4 5% 5% 7% 9% 42%	4412
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 9% 45%	4264
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 44%	4532
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 44%	2280
	11th to 20th	Day	14% 8% 3 6% 4 4 4 4 6% 8% 41%	2158
	21st to end	Day	12% 8% 2 6% 4 4 4 3 5% 8% 44%	2090
October	1st to 10th	Day	14% 8% 2 7% 4 4 4 3 5% 7% 41%	3307
	11th to 20th	Day	9% 8% 2 6% 4 4 5% 3 5% 9% 43%	2832
	21st to end	Day	10% 13% 4 9% 5% 4 5% 4 7% 7% 32%	3091
November	1st to 10th	Day	16% 20% 5% 9% 5% 4 4 3 3 5% 26%	1681
	11th to 20th	Day	9% 17% 5% 13% 7% 6% 5% 3 5% 6% 25%	1400
	21st to end	Day	8% 15% 7% 12% 7% 5% 5% 3 5% 6% 27%	1400
December	1st to 10th	Day	9% 13% 4 14% 10% 9% 7% 5% 5% 6% 17%	2290
	11th to 20th	Day	7% 15% 5% 12% 7% 7% 6% 7% 6% 6% 23%	2639
	21st to end	Day	3 9% 6% 15% 6% 4 4 3 4 5% 40%	3498
January	1st to 10th	Night	4 13% 8% 17% 10% 7% 7% 5% 7% 6% 16%	6787
	11th to 20th	Night	4 11% 5% 15% 10% 9% 12% 6% 6% 5% 16%	6554
	21st to end	Night	5% 15% 8% 15% 7% 7% 6% 4 5% 5% 23%	6974
February	1st to 10th	Night	8% 12% 4 11% 6% 7% 8% 5% 6% 6% 27%	2800
	11th to 20th	Night	4 9% 4 11% 7% 6% 7% 5% 7% 7% 33%	2624
	21st to end	Night	4 11% 5% 13% 8% 5% 5% 3 6% 8% 33%	1884
March	1st to 10th	Night	7% 12% 5% 8% 5% 6% 5% 4 5% 8% 34%	3734
	11th to 20th	Night	7% 11% 5% 12% 7% 5% 5% 3 6% 7% 32%	3652
	21st to end	Night	6% 8% 4 6% 5% 5% 5% 4 6% 7% 45%	3580
April	1st to 10th	Night	7% 8% 3 9% 6% 5% 6% 4 6% 6% 41%	1900
	11th to 20th	Night	14% 9% 4 9% 6% 5% 4 4 5% 7% 33%	1703
	21st to end	Night	13% 8% 2 8% 5% 5% 5% 5% 5% 7% 39%	1665
May	1st to 10th	Night	19% 11% 3 6% 4 4 4 4 5% 6% 34%	2540
	11th to 20th	Night	12% 10% 3 7% 6% 5% 6% 3 7% 7% 34%	2238
	21st to end	Night	18% 8% 2 5% 4 4 4 3 6% 7% 38%	2414
June	1st to 10th	Night	16% 8% 2 6% 4 5% 4 4 5% 7% 40%	1330
	11th to 20th	Night	20% 5% 3 6% 4 4 5% 5% 6% 7% 35%	1163
	21st to end	Night	13% 7% 3 6% 6% 6% 6% 4 5% 6% 39%	1140
July	1st to 10th	Night	10% 4 2 6% 5% 5% 5% 5% 7% 11% 42%	1954
	11th to 20th	Night	8% 3 2 4 4 4 4 4 6% 9% 53%	2220
	21st to end	Night	6% 3 2 4 3 4 3 3 7% 9% 57%	2633
August	1st to 10th	Night	14% 5% 2 5% 3 4 4 4 5% 8% 47%	2528
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 7% 9% 49%	2575
	21st to end	Night	16% 6% 2 5% 4 3 3 3 5% 7% 46%	3135
September	1st to 10th	Night	9% 6% 1 5% 5% 3 3 4 5% 8% 50%	1898
	11th to 20th	Night	16% 7% 2 5% 4 3 4 3 5% 6% 45%	2021
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 8% 45%	2090
October	1st to 10th	Night	11% 8% 2 5% 4 3 3 4 4 7% 48%	3621
	11th to 20th	Night	9% 10% 3 7% 5% 5% 6% 5% 7% 9% 36%	4097
	21st to end	Night	10% 12% 3 8% 5% 5% 4 3 5% 7% 38%	4467
November	1st to 10th	Night	13% 17% 6% 11% 6% 5% 5% 3 4 5% 24%	2612
	11th to 20th	Night	11% 16% 6% 12% 7% 5% 4 4 6% 6% 25%	3000
	21st to end	Night	10% 14% 6% 13% 8% 6% 5% 4 5% 6% 24%	3000
December	1st to 10th	Night	8% 17% 6% 16% 11% 7% 6% 4 4 6% 16%	4900
	11th to 20th	Night	7% 12% 5% 13% 7% 5% 8% 6% 6% 6% 24%	5568
	21st to end	Night	5% 9% 5% 15% 7% 5% 4 3 5% 5% 37%	7480

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	61% 8% 12% 16% 4	3179
	11th to 20th	Day	57% 6% 11% 19% 6%	3382
	21st to end	Day	52% 9% 17% 19% 3	4004
February	1st to 10th	Day	51% 10% 13% 17% 9%	1600
	11th to 20th	Day	44% 8% 14% 23% 10%	1688
	21st to end	Day	49% 8% 13% 20% 10%	1570
March	1st to 10th	Day	48% 7% 14% 22% 9%	3130
	11th to 20th	Day	46% 10% 17% 20% 7% 1	3313
	21st to end	Day	36% 7% 14% 28% 14%	4058
April	1st to 10th	Day	44% 8% 15% 20% 13% 1	2280
	11th to 20th	Day	54% 7% 11% 16% 9% 2	2428
	21st to end	Day	42% 8% 15% 25% 9% 1	2513
May	1st to 10th	Day	45% 9% 12% 26% 8%	4430
	11th to 20th	Day	46% 7% 12% 23% 10% 1	4672
	21st to end	Day	50% 8% 11% 23% 6%	5173
June	1st to 10th	Day	48% 8% 13% 21% 8% 1	2850
	11th to 20th	Day	51% 8% 14% 18% 9%	3011
	21st to end	Day	45% 11% 16% 22% 6%	3040
July	1st to 10th	Day	37% 9% 24% 24% 5%	4925
	11th to 20th	Day	30% 8% 20% 37% 5%	4669
	21st to end	Day	31% 8% 24% 29% 7% 1	4760
August	1st to 10th	Day	36% 9% 22% 26% 7%	4396
	11th to 20th	Day	34% 9% 19% 33% 5%	4306
	21st to end	Day	37% 7% 23% 27% 7%	4531
September	1st to 10th	Day	37% 9% 17% 26% 10% 1	2280
	11th to 20th	Day	41% 8% 16% 27% 7%	2158
	21st to end	Day	39% 8% 14% 28% 11%	2090
October	1st to 10th	Day	43% 5% 17% 29% 6%	3307
	11th to 20th	Day	38% 6% 16% 33% 7%	2832
	21st to end	Day	50% 6% 19% 20% 5%	3091
November	1st to 10th	Day	61% 6% 11% 17% 5%	1681
	11th to 20th	Day	59% 8% 15% 12% 5%	1400
	21st to end	Day	58% 8% 13% 17% 4	1400
December	1st to 10th	Day	63% 9% 13% 11% 4	2290
	11th to 20th	Day	57% 8% 18% 14% 3	2640
	21st to end	Day	43% 7% 11% 22% 12% 3	3498
January	1st to 10th	Night	64% 8% 10% 14% 4	6787
	11th to 20th	Night	64% 7% 9% 13% 6%	6594
	21st to end	Night	59% 9% 14% 15% 3	6974
February	1st to 10th	Night	53% 9% 12% 16% 9% 1	2800
	11th to 20th	Night	47% 7% 15% 21% 10%	2626
	21st to end	Night	48% 7% 16% 22% 9%	1884
March	1st to 10th	Night	47% 7% 16% 23% 6% 1	3734
	11th to 20th	Night	48% 9% 11% 19% 12% 1	3652
	21st to end	Night	34% 8% 10% 31% 17%	3580
April	1st to 10th	Night	42% 7% 13% 21% 16% 2	1900
	11th to 20th	Night	49% 7% 13% 19% 10% 3	1703
	21st to end	Night	42% 8% 15% 23% 9% 2	1665
May	1st to 10th	Night	48% 8% 11% 20% 11% 1	2540
	11th to 20th	Night	46% 8% 13% 19% 12% 2	2238
	21st to end	Night	42% 10% 16% 22% 9%	2414
June	1st to 10th	Night	42% 8% 14% 24% 11% 1	1330
	11th to 20th	Night	45% 9% 17% 17% 10% 2	1163
	21st to end	Night	44% 9% 18% 20% 8%	1140
July	1st to 10th	Night	36% 9% 27% 23% 5%	1923
	11th to 20th	Night	26% 8% 21% 38% 6%	2178
	21st to end	Night	24% 8% 27% 32% 9%	2550
August	1st to 10th	Night	34% 8% 20% 29% 10%	2520
	11th to 20th	Night	31% 5% 23% 33% 7%	2599
	21st to end	Night	38% 6% 19% 30% 7%	3135
September	1st to 10th	Night	31% 8% 16% 32% 11% 2	1898
	11th to 20th	Night	40% 8% 14% 30% 8%	2021
	21st to end	Night	37% 7% 14% 31% 11% 2	2090
October	1st to 10th	Night	37% 5% 18% 27% 13%	3621
	11th to 20th	Night	43% 6% 12% 30% 9%	4097
	21st to end	Night	46% 5% 17% 26% 6%	4467
November	1st to 10th	Night	61% 8% 12% 14% 5% 1	2612
	11th to 20th	Night	58% 8% 16% 14% 3	3000
	21st to end	Night	60% 8% 14% 14% 4	3000
December	1st to 10th	Night	68% 8% 10% 11% 3	4900
	11th to 20th	Night	54% 9% 15% 14% 8%	5566
	21st to end	Night	47% 6% 12% 20% 12% 3	7480

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% 32% 30% 14%	300
	11th to 20th	Day	9% 9% 40% 32% 8% 1	300
	21st to end	Day	14% 9% 33% 34% 9% 2	330
February	1st to 10th	Day	12% 11% 35% 29% 13%	300
	11th to 20th	Day	6% 9% 34% 32% 15% 3	300
	21st to end	Day	6% 9% 39% 30% 15% 2	225
March	1st to 10th	Day	16% 9% 30% 30% 11% 3	270
	11th to 20th	Day	9% 13% 37% 29% 9% 2	270
	21st to end	Day	15% 13% 31% 29% 10% 2	438
April	1st to 10th	Day	12% 12% 37% 28% 11% 1	460
	11th to 20th	Day	17% 13% 30% 25% 14% 1	626
	21st to end	Day	13% 14% 32% 29% 11% 1	650
May	1st to 10th	Day	19% 13% 28% 29% 11% 1	653
	11th to 20th	Day	17% 14% 27% 25% 15% 2	675
	21st to end	Day	19% 11% 27% 25% 14% 3	734
June	1st to 10th	Day	20% 13% 29% 23% 11% 4	680
	11th to 20th	Day	20% 14% 28% 23% 11% 4	680
	21st to end	Day	18% 9% 28% 27% 14% 5%	680
July	1st to 10th	Day	11% 9% 26% 30% 19% 5%	671
	11th to 20th	Day	11% 10% 29% 29% 16% 5%	668
	21st to end	Day	10% 9% 24% 28% 21% 7%	739
August	1st to 10th	Day	11% 9% 28% 28% 18% 4	676
	11th to 20th	Day	14% 7% 24% 28% 19% 8%	665
	21st to end	Day	18% 8% 27% 29% 17% 2	748
September	1st to 10th	Day	15% 15% 33% 28% 7% 2	490
	11th to 20th	Day	20% 13% 31% 25% 8% 3	368
	21st to end	Day	17% 10% 33% 26% 12% 1	300
October	1st to 10th	Day	23% 11% 27% 26% 11% 2	300
	11th to 20th	Day	18% 15% 37% 20% 9% 1	300
	21st to end	Day	21% 12% 35% 22% 8% 2	330
November	1st to 10th	Day	26% 12% 29% 26% 6% 2	300
	11th to 20th	Day	18% 10% 37% 26% 7% 1	300
	21st to end	Day	16% 12% 39% 23% 8% 1	300
December	1st to 10th	Day	10% 12% 35% 30% 12% 1	300
	11th to 20th	Day	14% 12% 39% 28% 7%	300
	21st to end	Day	9% 7% 26% 40% 15% 3	330
January	1st to 10th	Night	12% 8% 37% 32% 10% 1	1000
	11th to 20th	Night	12% 7% 38% 31% 11% 2	1000
	21st to end	Night	14% 10% 35% 32% 9% 1	1100
February	1st to 10th	Night	11% 7% 34% 34% 13% 1	1000
	11th to 20th	Night	8% 7% 35% 33% 15% 2	944
	21st to end	Night	7% 7% 31% 37% 17% 1	628
March	1st to 10th	Night	15% 6% 26% 40% 13% 1	760
	11th to 20th	Night	11% 8% 34% 33% 12% 1	677
	21st to end	Night	12% 7% 29% 31% 18% 3	627
April	1st to 10th	Night	10% 5% 31% 35% 18% 2	570
	11th to 20th	Night	17% 6% 32% 31% 13% 1	563
	21st to end	Night	14% 8% 36% 33% 8% 1	570
May	1st to 10th	Night	19% 7% 29% 32% 12% 1	570
	11th to 20th	Night	15% 9% 32% 30% 13% 1	561
	21st to end	Night	22% 8% 24% 33% 12% 1	617
June	1st to 10th	Night	18% 7% 28% 33% 13% 1	570
	11th to 20th	Night	20% 4 27% 33% 14% 1	403
	21st to end	Night	12% 7% 26% 38% 17% 1	380
July	1st to 10th	Night	9% 6% 25% 41% 19% 1	397
	11th to 20th	Night	9% 6% 28% 36% 19% 2	552
	21st to end	Night	8% 5% 24% 38% 21% 4	624
August	1st to 10th	Night	9% 6% 23% 39% 21% 2	567
	11th to 20th	Night	13% 3 20% 35% 26% 3	564
	21st to end	Night	17% 4 26% 39% 14% 1	627
September	1st to 10th	Night	12% 6% 28% 37% 17% 1	570
	11th to 20th	Night	18% 7% 24% 38% 13% 1	570
	21st to end	Night	12% 6% 29% 40% 12% 1	570
October	1st to 10th	Night	20% 7% 28% 33% 10% 1	646
	11th to 20th	Night	20% 9% 28% 32% 10% 1	950
	21st to end	Night	20% 8% 31% 29% 10% 1	1045
November	1st to 10th	Night	20% 10% 33% 28% 8% 1	975
	11th to 20th	Night	18% 8% 37% 27% 9% 2	1000
	21st to end	Night	16% 13% 35% 27% 8% 2	1000
December	1st to 10th	Night	11% 9% 36% 28% 12% 4	1000
	11th to 20th	Night	16% 9% 34% 29% 11% 2	1000
	21st to end	Night	12% 6% 28% 37% 14% 3	1100

Page generated at 2025-02-18 02:26:30. Version ac07

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