Blue River
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue River clear sky chart from 2005-11-06 to 2026-06-01.

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% 15% 4 10% 6% 5% 5% 4 5% 6% 27%	135582
	Night	14% 19% 5% 10% 6% 5% 5% 4 5% 6% 22%	136247
Transparency	Day	57% 8% 14% 15% 5%	135284
	Night	63% 7% 12% 13% 5% 1	136124
Seeing	Day	22% 12% 25% 25% 13% 3	17990
	Night	26% 8% 24% 28% 13% 1	26360

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	8% 33% 9% 16% 9% 6% 5% 3 3 3 5%	3665
	11th to 20th	Day	12% 15% 4 10% 4 3 3 4 6% 7% 33%	4254
	21st to end	Day	13% 28% 4 10% 3 4 4 4 4 5% 19%	5186
February	1st to 10th	Day	13% 26% 7% 10% 5% 4 4 3 4 4 19%	1680
	11th to 20th	Day	11% 18% 7% 14% 7% 5% 4 3 4 5% 21%	1806
	21st to end	Day	8% 23% 7% 14% 7% 6% 4 3 4 4 21%	1678
March	1st to 10th	Day	8% 22% 6% 13% 6% 6% 5% 4 5% 6% 19%	4618
	11th to 20th	Day	7% 21% 6% 10% 6% 6% 5% 4 5% 5% 25%	4829
	21st to end	Day	9% 21% 5% 12% 6% 5% 5% 4 6% 7% 20%	6072
April	1st to 10th	Day	10% 21% 6% 12% 9% 6% 6% 4 5% 5% 16%	2520
	11th to 20th	Day	11% 19% 4 9% 7% 5% 5% 3 6% 7% 23%	2569
	21st to end	Day	13% 15% 4 9% 6% 5% 6% 4 5% 7% 25%	2929
May	1st to 10th	Day	10% 12% 3 9% 5% 5% 5% 3 6% 8% 33%	6460
	11th to 20th	Day	12% 13% 5% 11% 6% 6% 6% 5% 6% 7% 22%	6453
	21st to end	Day	19% 12% 3 8% 5% 6% 5% 5% 7% 8% 23%	7653
June	1st to 10th	Day	23% 11% 4 9% 6% 6% 5% 4 4 5% 24%	3201
	11th to 20th	Day	25% 13% 5% 10% 7% 6% 6% 4 6% 5% 14%	3200
	21st to end	Day	19% 10% 4 10% 7% 6% 5% 4 6% 5% 24%	3200
July	1st to 10th	Day	11% 7% 3 6% 5% 4 5% 4 6% 8% 41%	7240
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 6% 7% 46%	6978
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	6887
August	1st to 10th	Day	8% 4 2 6% 4 4 4 4 6% 8% 50%	4541
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 11% 35%	4470
	21st to end	Day	19% 7% 2 6% 4 5% 4 4 5% 7% 36%	4524
September	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 3 6% 6% 37%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 5% 6% 35%	2375
	21st to end	Day	23% 12% 3 8% 4 5% 4 4 5% 6% 26%	2181
October	1st to 10th	Day	21% 15% 5% 10% 6% 6% 5% 4 4 4 20%	3280
	11th to 20th	Day	27% 17% 4 8% 5% 3 4 3 4 5% 22%	3079
	21st to end	Day	15% 20% 6% 13% 6% 4 4 4 5% 6% 17%	3200
November	1st to 10th	Day	22% 23% 6% 13% 6% 5% 4 2 4 4 12%	1690
	11th to 20th	Day	20% 25% 7% 11% 6% 4 4 3 4 4 14%	1575
	21st to end	Day	14% 29% 5% 12% 6% 4 4 3 4 4 15%	1470
December	1st to 10th	Day	17% 19% 5% 16% 7% 7% 4 3 4 4 14%	2339
	11th to 20th	Day	15% 31% 8% 15% 6% 4 3 2 3 4 10%	2322
	21st to end	Day	11% 30% 8% 13% 7% 4 4 3 5% 4 11%	3058
January	1st to 10th	Night	9% 32% 11% 18% 7% 5% 5% 2 2 3 5%	9269
	11th to 20th	Night	12% 14% 5% 11% 6% 6% 5% 4 6% 6% 25%	8641
	21st to end	Night	15% 27% 5% 8% 5% 5% 4 3 4 5% 19%	8994
February	1st to 10th	Night	14% 25% 7% 12% 7% 4 4 3 4 6% 15%	2940
	11th to 20th	Night	11% 22% 7% 12% 6% 5% 4 3 5% 5% 20%	2699
	21st to end	Night	6% 25% 5% 11% 7% 6% 5% 4 5% 6% 19%	2127
March	1st to 10th	Night	8% 22% 6% 11% 5% 5% 5% 4 5% 7% 22%	5496
	11th to 20th	Night	7% 23% 6% 9% 5% 4 5% 5% 7% 7% 22%	5388
	21st to end	Night	10% 21% 5% 11% 5% 5% 5% 4 6% 6% 21%	5144
April	1st to 10th	Night	11% 21% 5% 9% 5% 5% 5% 4 6% 6% 23%	2100
	11th to 20th	Night	11% 18% 3 9% 6% 6% 5% 4 6% 6% 27%	2002
	21st to end	Night	11% 15% 3 9% 6% 6% 5% 4 5% 5% 32%	1688
May	1st to 10th	Night	11% 11% 3 7% 6% 6% 5% 4 7% 8% 33%	3720
	11th to 20th	Night	12% 13% 4 11% 6% 6% 6% 5% 6% 7% 25%	3659
	21st to end	Night	15% 14% 4 9% 6% 5% 4 4 8% 5% 24%	3459
June	1st to 10th	Night	21% 9% 3 7% 6% 6% 4 3 6% 8% 28%	1200
	11th to 20th	Night	22% 13% 4 9% 5% 7% 6% 5% 5% 7% 17%	1200
	21st to end	Night	15% 8% 3 7% 6% 6% 7% 4 7% 8% 31%	1200
July	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 5% 6% 49%	2720
	11th to 20th	Night	8% 7% 3 6% 4 4 4 3 5% 6% 51%	2966
	21st to end	Night	12% 8% 3 7% 4 4 4 3 5% 8% 41%	3941
August	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 7% 8% 49%	2609
	11th to 20th	Night	11% 5% 2 7% 4 5% 5% 4 7% 10% 40%	2577
	21st to end	Night	18% 7% 2 6% 4 5% 5% 4 7% 8% 34%	3389
September	1st to 10th	Night	16% 7% 3 6% 5% 3 4 3 5% 7% 42%	2000
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 5% 7% 39%	2025
	21st to end	Night	22% 11% 3 7% 4 4 4 4 5% 6% 30%	2215
October	1st to 10th	Night	20% 14% 4 11% 7% 7% 5% 3 5% 4 20%	3920
	11th to 20th	Night	21% 13% 3 8% 6% 5% 5% 4 6% 7% 22%	4072
	21st to end	Night	18% 20% 5% 11% 5% 5% 5% 3 5% 6% 18%	4600
November	1st to 10th	Night	25% 23% 5% 10% 5% 4 5% 3 3 4 11%	2825
	11th to 20th	Night	20% 25% 5% 11% 5% 5% 4 3 3 3 15%	3024
	21st to end	Night	15% 25% 7% 13% 6% 5% 4 3 4 4 11%	3150
December	1st to 10th	Night	13% 20% 6% 16% 9% 7% 5% 4 5% 4 12%	5058
	11th to 20th	Night	15% 33% 9% 14% 7% 5% 4 3 3 2 6%	6090
	21st to end	Night	14% 29% 7% 13% 6% 5% 4 4 5% 5% 8%	8140

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	86% 5% 5% 4 1	3665
	11th to 20th	Day	52% 8% 17% 15% 6% 3	4275
	21st to end	Day	68% 6% 13% 10% 2	5187
February	1st to 10th	Day	71% 5% 8% 8% 7% 1	1680
	11th to 20th	Day	67% 7% 11% 9% 5% 1	1815
	21st to end	Day	67% 5% 11% 10% 5% 1	1678
March	1st to 10th	Day	65% 6% 11% 10% 7%	4618
	11th to 20th	Day	61% 8% 10% 17% 3	4829
	21st to end	Day	62% 9% 10% 12% 6%	6072
April	1st to 10th	Day	71% 7% 12% 8% 2	2520
	11th to 20th	Day	60% 7% 16% 13% 3	2569
	21st to end	Day	59% 8% 14% 15% 4	2929
May	1st to 10th	Day	47% 7% 17% 24% 4	6460
	11th to 20th	Day	56% 9% 18% 14% 3	6456
	21st to end	Day	54% 9% 21% 13% 2	7654
June	1st to 10th	Day	62% 8% 10% 12% 7% 2	3201
	11th to 20th	Day	71% 8% 10% 11% 1	3200
	21st to end	Day	59% 9% 13% 14% 5% 1	3200
July	1st to 10th	Day	40% 7% 15% 23% 15% 1	7161
	11th to 20th	Day	36% 7% 16% 29% 11%	6892
	21st to end	Day	39% 7% 23% 22% 8% 1	6693
August	1st to 10th	Day	31% 7% 14% 38% 10%	4524
	11th to 20th	Day	38% 11% 24% 20% 6%	4512
	21st to end	Day	47% 9% 21% 17% 5% 1	4523
September	1st to 10th	Day	48% 7% 17% 20% 7% 1	2400
	11th to 20th	Day	49% 9% 13% 20% 7% 1	2375
	21st to end	Day	59% 10% 13% 11% 6% 1	2181
October	1st to 10th	Day	65% 10% 13% 10% 2	3280
	11th to 20th	Day	68% 5% 17% 9% 1	3079
	21st to end	Day	68% 7% 16% 6% 3	3200
November	1st to 10th	Day	79% 6% 6% 7% 3	1690
	11th to 20th	Day	76% 6% 8% 6% 3 1	1575
	21st to end	Day	74% 7% 6% 4 7% 1	1470
December	1st to 10th	Day	74% 8% 10% 6% 2	2339
	11th to 20th	Day	80% 4 7% 6% 3	2324
	21st to end	Day	74% 7% 9% 8% 2	3058
January	1st to 10th	Night	87% 4 5% 3 1	9270
	11th to 20th	Night	58% 7% 13% 14% 5% 3	8682
	21st to end	Night	70% 5% 10% 12% 4	8994
February	1st to 10th	Night	72% 4 8% 7% 7% 1	2940
	11th to 20th	Night	67% 6% 10% 10% 6% 1	2701
	21st to end	Night	64% 7% 11% 11% 5% 2	2127
March	1st to 10th	Night	59% 6% 12% 14% 8%	5496
	11th to 20th	Night	59% 9% 14% 14% 4	5389
	21st to end	Night	61% 8% 12% 13% 6% 1	5144
April	1st to 10th	Night	64% 6% 14% 12% 3	2100
	11th to 20th	Night	55% 8% 15% 14% 7%	2002
	21st to end	Night	54% 7% 11% 21% 7%	1688
May	1st to 10th	Night	46% 9% 21% 17% 7%	3717
	11th to 20th	Night	55% 10% 16% 15% 5%	3659
	21st to end	Night	55% 8% 20% 15% 2	3459
June	1st to 10th	Night	54% 9% 13% 16% 6% 2	1200
	11th to 20th	Night	65% 10% 11% 12% 3	1200
	21st to end	Night	50% 11% 16% 16% 6%	1200
July	1st to 10th	Night	36% 6% 13% 24% 21% 1	2690
	11th to 20th	Night	34% 7% 12% 34% 13% 1	2929
	21st to end	Night	40% 6% 19% 24% 9% 1	3830
August	1st to 10th	Night	31% 9% 14% 39% 7%	2601
	11th to 20th	Night	37% 8% 26% 23% 5% 1	2601
	21st to end	Night	47% 9% 26% 12% 6% 1	3389
September	1st to 10th	Night	42% 9% 16% 24% 8% 1	2000
	11th to 20th	Night	46% 7% 14% 23% 9% 1	2025
	21st to end	Night	55% 8% 15% 13% 8% 1	2215
October	1st to 10th	Night	65% 9% 12% 12% 2	3920
	11th to 20th	Night	63% 6% 16% 13% 2	4072
	21st to end	Night	69% 6% 14% 8% 3	4600
November	1st to 10th	Night	77% 6% 7% 8% 2	2825
	11th to 20th	Night	75% 7% 7% 6% 5% 1	3024
	21st to end	Night	77% 5% 7% 5% 5% 1	3150
December	1st to 10th	Night	73% 9% 9% 6% 3	5058
	11th to 20th	Night	85% 4 4 4 2	6087
	21st to end	Night	77% 9% 8% 5% 2	8140

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	23% 10% 29% 26% 10% 2	220
	11th to 20th	Day	28% 10% 25% 26% 10% 1	220
	21st to end	Day	24% 12% 27% 28% 7% 1	242
February	1st to 10th	Day	21% 15% 35% 19% 9% 2	220
	11th to 20th	Day	17% 17% 33% 23% 8% 2	386
	21st to end	Day	19% 22% 29% 20% 9% 1	419
March	1st to 10th	Day	24% 18% 27% 19% 10% 1	540
	11th to 20th	Day	25% 20% 25% 19% 11% 1	540
	21st to end	Day	27% 21% 24% 14% 12% 2	594
April	1st to 10th	Day	23% 19% 30% 18% 9% 1	540
	11th to 20th	Day	21% 17% 27% 22% 10% 3	532
	21st to end	Day	20% 17% 25% 20% 14% 4	540
May	1st to 10th	Day	18% 13% 29% 26% 13% 2	543
	11th to 20th	Day	18% 10% 21% 26% 20% 5%	578
	21st to end	Day	20% 8% 21% 27% 18% 6%	875
June	1st to 10th	Day	25% 10% 27% 26% 11% 1	931
	11th to 20th	Day	27% 11% 22% 26% 12% 2	930
	21st to end	Day	22% 6% 23% 28% 17% 3	930
July	1st to 10th	Day	19% 7% 21% 33% 17% 3	916
	11th to 20th	Day	15% 9% 26% 30% 16% 4	807
	21st to end	Day	12% 7% 22% 33% 21% 4	588
August	1st to 10th	Day	10% 5% 23% 37% 20% 4	527
	11th to 20th	Day	12% 10% 28% 31% 16% 3	517
	21st to end	Day	17% 9% 26% 29% 16% 3	583
September	1st to 10th	Day	19% 10% 28% 25% 15% 5%	530
	11th to 20th	Day	23% 9% 24% 26% 16% 2	530
	21st to end	Day	32% 15% 22% 20% 10% 2	530
October	1st to 10th	Day	33% 10% 22% 20% 11% 4	530
	11th to 20th	Day	32% 14% 22% 18% 12% 2	425
	21st to end	Day	33% 11% 28% 21% 6% 1	363
November	1st to 10th	Day	34% 17% 27% 15% 6% 1	242
	11th to 20th	Day	37% 12% 20% 22% 6% 2	220
	21st to end	Day	28% 17% 23% 19% 9% 4	220
December	1st to 10th	Day	25% 11% 19% 24% 19% 3	220
	11th to 20th	Day	27% 14% 25% 24% 9% 1	220
	21st to end	Day	21% 7% 37% 22% 11% 1	242
January	1st to 10th	Night	33% 13% 24% 22% 7% 1	1128
	11th to 20th	Night	30% 8% 30% 25% 7%	925
	21st to end	Night	27% 9% 30% 27% 7%	924
February	1st to 10th	Night	29% 9% 30% 22% 10% 1	840
	11th to 20th	Night	24% 7% 30% 31% 8%	824
	21st to end	Night	20% 9% 26% 29% 15% 1	692
March	1st to 10th	Night	23% 10% 28% 27% 10%	840
	11th to 20th	Night	24% 12% 25% 28% 11%	840
	21st to end	Night	23% 10% 25% 26% 14% 3	924
April	1st to 10th	Night	20% 9% 25% 29% 15% 2	840
	11th to 20th	Night	19% 9% 25% 30% 17%	754
	21st to end	Night	18% 7% 21% 31% 20% 2	428
May	1st to 10th	Night	16% 6% 22% 36% 19%	420
	11th to 20th	Night	18% 5% 21% 35% 20% 2	414
	21st to end	Night	22% 4 18% 32% 21% 3	455
June	1st to 10th	Night	23% 7% 24% 31% 14% 2	400
	11th to 20th	Night	27% 6% 24% 26% 15% 2	400
	21st to end	Night	19% 4 21% 31% 21% 5%	400
July	1st to 10th	Night	20% 5% 19% 31% 24% 2	390
	11th to 20th	Night	14% 6% 20% 34% 24% 2	388
	21st to end	Night	13% 5% 13% 38% 28% 3	438
August	1st to 10th	Night	11% 3 17% 33% 33% 3	398
	11th to 20th	Night	14% 2 18% 37% 26% 3	396
	21st to end	Night	19% 3 17% 39% 20% 1	739
September	1st to 10th	Night	20% 4 17% 34% 22% 3	800
	11th to 20th	Night	22% 4 17% 30% 23% 4	800
	21st to end	Night	30% 9% 18% 28% 13% 2	800
October	1st to 10th	Night	28% 5% 20% 31% 14% 2	800
	11th to 20th	Night	33% 6% 21% 29% 10% 1	800
	21st to end	Night	36% 9% 23% 23% 8% 1	880
November	1st to 10th	Night	38% 10% 25% 21% 5%	820
	11th to 20th	Night	36% 12% 24% 20% 7%	924
	21st to end	Night	31% 11% 27% 24% 6% 2	1050
December	1st to 10th	Night	29% 7% 30% 25% 8%	1053
	11th to 20th	Night	34% 10% 28% 21% 6%	1160
	21st to end	Night	29% 10% 31% 24% 6% 1	1276

Page generated at 2026-05-28 22:53:30. Version ac07

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