Northwest River Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northwest River Park clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	119792
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	119331
Transparency	Day	40% 5% 12% 22% 17% 4	123426
	Night	40% 4 11% 22% 18% 5%	123834
Seeing	Day	16% 10% 25% 26% 16% 6%	16230
	Night	16% 8% 26% 31% 16% 2	25807

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	14% 5% 2 4 3 3 3 3 4 5% 55%	3827
	11th to 20th	Day	12% 6% 3 6% 3 3 3 3 4 5% 53%	4060
	21st to end	Day	11% 10% 3 7% 4 4 4 3 4 4 44%	4466
February	1st to 10th	Day	13% 8% 3 6% 4 4 4 4 5% 7% 40%	1800
	11th to 20th	Day	20% 7% 2 5% 4 4 3 3 4 5% 42%	1935
	21st to end	Day	16% 7% 2 6% 4 3 4 3 6% 8% 41%	1650
March	1st to 10th	Day	20% 8% 2 6% 3 4 4 3 5% 5% 39%	4745
	11th to 20th	Day	11% 6% 2 6% 3 3 3 3 4 6% 52%	4980
	21st to end	Day	13% 6% 2 5% 4 5% 4 3 4 6% 47%	5648
April	1st to 10th	Day	12% 7% 1 4 4 4 3 3 5% 7% 50%	2400
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 3 4 7% 51%	2374
	21st to end	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 44%	2400
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 4 6% 8% 35%	5405
	11th to 20th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 9% 30%	5978
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 9% 40%	6934
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 6% 6% 7% 10% 39%	2800
	11th to 20th	Day	10% 6% 2 6% 4 5% 6% 5% 7% 10% 39%	2800
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 11% 40%	2800
July	1st to 10th	Day	8% 5% 1 7% 4 6% 6% 6% 9% 12% 35%	4580
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 10% 31%	4327
	21st to end	Day	9% 7% 3 9% 7% 7% 6% 6% 8% 9% 30%	4466
August	1st to 10th	Day	17% 8% 3 9% 6% 6% 6% 5% 8% 9% 21%	4024
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 4 8% 11% 34%	3730
	21st to end	Day	9% 5% 2 6% 5% 6% 6% 4 8% 11% 39%	4180
September	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	2280
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	2280
	21st to end	Day	21% 7% 3 6% 5% 4 4 3 5% 7% 34%	2045
October	1st to 10th	Day	17% 6% 2 5% 3 4 5% 3 4 6% 45%	1900
	11th to 20th	Day	13% 6% 2 6% 3 3 4 4 4 5% 49%	1900
	21st to end	Day	11% 6% 2 5% 4 4 4 3 5% 6% 51%	2090
November	1st to 10th	Day	16% 5% 3 5% 4 3 3 3 5% 6% 47%	1860
	11th to 20th	Day	16% 7% 2 5% 3 4 3 3 4 7% 47%	1800
	21st to end	Day	14% 4 2 4 5% 4 3 3 5% 6% 49%	1778
December	1st to 10th	Day	11% 6% 3 5% 4 3 4 3 4 6% 53%	2600
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 5% 5% 44%	2990
	21st to end	Day	13% 5% 2 5% 3 4 4 3 5% 7% 49%	3960
January	1st to 10th	Night	12% 6% 3 5% 3 3 3 3 4 5% 53%	6101
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 5% 51%	5900
	21st to end	Night	14% 7% 3 6% 4 4 4 3 5% 6% 45%	6490
February	1st to 10th	Night	18% 7% 2 5% 4 3 4 3 6% 6% 42%	2600
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 4 5% 40%	2349
	21st to end	Night	15% 8% 2 6% 3 4 4 2 4 5% 46%	1980
March	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 4 5% 42%	5106
	11th to 20th	Night	14% 6% 3 6% 4 4 4 4 4 5% 48%	4980
	21st to end	Night	15% 8% 2 5% 4 3 3 3 4 5% 48%	5270
April	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 6% 6% 45%	2000
	11th to 20th	Night	12% 6% 3 5% 4 3 3 3 4 8% 49%	1975
	21st to end	Night	14% 6% 2 5% 4 3 4 3 4 6% 48%	2000
May	1st to 10th	Night	12% 8% 3 7% 6% 5% 5% 5% 6% 9% 34%	4490
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 4 5% 7% 32%	3955
	21st to end	Night	14% 9% 3 8% 4 4 5% 4 6% 8% 37%	3920
June	1st to 10th	Night	10% 6% 2 6% 5% 4 5% 5% 8% 10% 41%	1600
	11th to 20th	Night	7% 6% 3 9% 4 5% 5% 4 7% 11% 39%	1600
	21st to end	Night	7% 5% 2 5% 5% 4 6% 5% 7% 10% 45%	1600
July	1st to 10th	Night	7% 6% 3 6% 4 6% 6% 5% 8% 12% 38%	2600
	11th to 20th	Night	9% 7% 2 7% 6% 5% 6% 5% 7% 10% 35%	2847
	21st to end	Night	14% 8% 3 9% 6% 5% 6% 5% 6% 8% 30%	3087
August	1st to 10th	Night	17% 8% 4 8% 5% 5% 5% 4 7% 8% 29%	2874
	11th to 20th	Night	9% 7% 3 8% 5% 5% 6% 4 6% 9% 38%	3097
	21st to end	Night	9% 5% 2 6% 4 4 5% 5% 7% 9% 43%	3476
September	1st to 10th	Night	10% 6% 2 5% 4 5% 5% 3 6% 10% 44%	1900
	11th to 20th	Night	13% 7% 3 6% 4 3 4 3 5% 8% 43%	1900
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 8% 36%	2130
October	1st to 10th	Night	13% 7% 2 5% 4 4 5% 4 5% 5% 48%	2280
	11th to 20th	Night	14% 7% 2 5% 4 2 3 3 4 6% 51%	2280
	21st to end	Night	11% 5% 2 5% 4 3 3 3 4 6% 53%	2508
November	1st to 10th	Night	16% 6% 2 6% 3 3 3 3 4 7% 46%	2436
	11th to 20th	Night	16% 7% 2 5% 4 3 4 4 4 5% 46%	2600
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 7% 50%	2619
December	1st to 10th	Night	9% 5% 2 6% 4 4 3 3 5% 5% 53%	4580
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 4 5% 5% 38%	5205
	21st to end	Night	12% 6% 2 5% 3 4 4 4 5% 6% 50%	6996

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	34% 1 5% 26% 20% 14%	3828
	11th to 20th	Day	35% 2 5% 18% 34% 6%	4060
	21st to end	Day	44% 2 7% 24% 19% 4	4466
February	1st to 10th	Day	43% 3 11% 25% 15% 3	1800
	11th to 20th	Day	46% 3 8% 18% 20% 5%	1944
	21st to end	Day	42% 3 12% 20% 18% 6%	1650
March	1st to 10th	Day	47% 2 7% 20% 15% 8%	4745
	11th to 20th	Day	35% 3 7% 19% 26% 11%	4980
	21st to end	Day	39% 3 13% 21% 19% 5%	5648
April	1st to 10th	Day	37% 2 13% 22% 22% 5%	2400
	11th to 20th	Day	35% 3 10% 27% 20% 5%	2374
	21st to end	Day	37% 4 12% 23% 18% 5%	2400
May	1st to 10th	Day	45% 5% 18% 20% 11% 1	5437
	11th to 20th	Day	51% 5% 15% 20% 10% 1	5982
	21st to end	Day	40% 4 14% 27% 13% 2	6949
June	1st to 10th	Day	38% 6% 15% 25% 14% 2	2800
	11th to 20th	Day	38% 6% 17% 24% 14% 1	2800
	21st to end	Day	35% 7% 16% 27% 14% 1	2800
July	1st to 10th	Day	35% 11% 20% 22% 10% 2	4510
	11th to 20th	Day	46% 9% 22% 18% 5% 1	4255
	21st to end	Day	47% 9% 13% 19% 11% 1	4324
August	1st to 10th	Day	54% 8% 17% 16% 5%	4011
	11th to 20th	Day	40% 8% 16% 24% 11%	3766
	21st to end	Day	35% 6% 15% 28% 14% 2	4180
September	1st to 10th	Day	41% 7% 16% 17% 17% 3	2280
	11th to 20th	Day	45% 5% 10% 20% 18% 2	2280
	21st to end	Day	50% 5% 8% 20% 15% 2	2045
October	1st to 10th	Day	32% 6% 8% 25% 26% 3	3160
	11th to 20th	Day	27% 4 6% 21% 32% 10%	3140
	21st to end	Day	23% 3 9% 28% 29% 8%	3428
November	1st to 10th	Day	40% 3 8% 26% 20% 3	1860
	11th to 20th	Day	40% 2 11% 21% 21% 6%	1800
	21st to end	Day	38% 2 8% 26% 21% 4	1778
December	1st to 10th	Day	34% 4 9% 28% 23% 4	2600
	11th to 20th	Day	43% 4 11% 21% 15% 6%	2986
	21st to end	Day	36% 4 12% 30% 17% 1	3960
January	1st to 10th	Night	35% 2 4 22% 27% 9%	6101
	11th to 20th	Night	37% 2 5% 20% 30% 6%	5900
	21st to end	Night	41% 3 7% 24% 20% 5%	6490
February	1st to 10th	Night	43% 3 9% 24% 18% 3	2600
	11th to 20th	Night	46% 3 7% 21% 18% 5%	2351
	21st to end	Night	41% 2 7% 22% 19% 7%	1980
March	1st to 10th	Night	46% 2 8% 19% 16% 8%	5106
	11th to 20th	Night	40% 3 8% 16% 23% 11%	4980
	21st to end	Night	40% 3 9% 25% 16% 6%	5270
April	1st to 10th	Night	40% 3 12% 17% 23% 4	2000
	11th to 20th	Night	37% 2 10% 27% 19% 6%	1975
	21st to end	Night	38% 4 10% 25% 19% 3	2000
May	1st to 10th	Night	46% 6% 17% 20% 9% 1	4520
	11th to 20th	Night	52% 5% 11% 23% 8% 1	3956
	21st to end	Night	46% 5% 14% 22% 11% 1	3927
June	1st to 10th	Night	38% 6% 15% 27% 14% 1	1600
	11th to 20th	Night	40% 6% 17% 23% 13% 1	1600
	21st to end	Night	33% 7% 16% 28% 14% 2	1600
July	1st to 10th	Night	37% 12% 18% 21% 10% 2	2560
	11th to 20th	Night	41% 10% 25% 18% 6% 1	2796
	21st to end	Night	51% 8% 12% 18% 11%	2988
August	1st to 10th	Night	50% 7% 20% 18% 4	2863
	11th to 20th	Night	42% 7% 15% 24% 12%	3127
	21st to end	Night	35% 6% 13% 28% 17% 2	3476
September	1st to 10th	Night	37% 6% 19% 20% 15% 3	1900
	11th to 20th	Night	42% 5% 10% 22% 19% 2	1900
	21st to end	Night	49% 4 10% 20% 15% 2	2130
October	1st to 10th	Night	38% 5% 7% 24% 24% 3	3800
	11th to 20th	Night	28% 3 6% 19% 33% 12%	3779
	21st to end	Night	21% 2 7% 31% 28% 11%	4120
November	1st to 10th	Night	39% 3 7% 24% 24% 4	2436
	11th to 20th	Night	42% 2 9% 20% 19% 7%	2600
	21st to end	Night	35% 3 9% 27% 22% 4	2619
December	1st to 10th	Night	34% 3 7% 24% 26% 6%	4580
	11th to 20th	Night	48% 4 10% 16% 15% 6%	5208
	21st to end	Night	36% 3 14% 28% 17% 1	6996

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	18% 22% 41% 15% 4	250
	11th to 20th	Day	16% 24% 43% 16% 2	300
	21st to end	Day	18% 19% 42% 18% 3 1	330
February	1st to 10th	Day	18% 19% 40% 19% 4	300
	11th to 20th	Day	29% 19% 36% 13% 3	483
	21st to end	Day	20% 19% 35% 22% 4 1	414
March	1st to 10th	Day	20% 19% 35% 20% 6%	500
	11th to 20th	Day	20% 20% 35% 20% 4 2	500
	21st to end	Day	16% 18% 39% 20% 7% 1	550
April	1st to 10th	Day	17% 18% 40% 18% 6% 2	500
	11th to 20th	Day	12% 15% 33% 26% 10% 3	492
	21st to end	Day	15% 11% 33% 26% 11% 3	500
May	1st to 10th	Day	16% 13% 33% 25% 9% 3	503
	11th to 20th	Day	18% 9% 26% 29% 13% 5%	527
	21st to end	Day	19% 5% 23% 28% 18% 7%	573
June	1st to 10th	Day	12% 4 20% 33% 25% 6%	530
	11th to 20th	Day	12% 5% 20% 29% 25% 8%	530
	21st to end	Day	8% 4 19% 35% 27% 8%	530
July	1st to 10th	Day	11% 3 9% 34% 32% 10%	525
	11th to 20th	Day	9% 2 7% 30% 37% 15%	524
	21st to end	Day	10% 2 14% 35% 29% 8%	532
August	1st to 10th	Day	14% 3 12% 35% 27% 9%	486
	11th to 20th	Day	11% 3 10% 30% 32% 14%	479
	21st to end	Day	14% 3 11% 32% 26% 15%	539
September	1st to 10th	Day	14% 3 18% 29% 21% 16%	490
	11th to 20th	Day	19% 5% 14% 30% 23% 10%	490
	21st to end	Day	25% 8% 19% 23% 20% 6%	490
October	1st to 10th	Day	21% 5% 19% 28% 19% 9%	490
	11th to 20th	Day	16% 10% 23% 29% 17% 5%	490
	21st to end	Day	14% 11% 27% 26% 17% 6%	539
November	1st to 10th	Day	19% 9% 27% 27% 14% 4	446
	11th to 20th	Day	19% 15% 27% 28% 10% 2	400
	21st to end	Day	15% 17% 28% 22% 16% 2	378
December	1st to 10th	Day	19% 19% 36% 18% 7% 2	200
	11th to 20th	Day	17% 18% 34% 22% 9% 1	200
	21st to end	Day	21% 13% 35% 21% 9% 2	220
January	1st to 10th	Night	19% 17% 35% 24% 5%	800
	11th to 20th	Night	20% 16% 32% 26% 6%	800
	21st to end	Night	18% 15% 40% 25% 3	880
February	1st to 10th	Night	23% 11% 35% 27% 4	800
	11th to 20th	Night	24% 17% 38% 19% 2	783
	21st to end	Night	20% 15% 35% 23% 5%	660
March	1st to 10th	Night	19% 13% 36% 26% 6%	800
	11th to 20th	Night	21% 11% 38% 24% 6% 1	800
	21st to end	Night	20% 11% 38% 25% 6%	880
April	1st to 10th	Night	17% 13% 38% 26% 6%	800
	11th to 20th	Night	14% 10% 35% 30% 10%	791
	21st to end	Night	16% 7% 32% 36% 9% 1	800
May	1st to 10th	Night	16% 10% 31% 32% 10% 1	800
	11th to 20th	Night	18% 5% 25% 36% 15% 1	566
	21st to end	Night	14% 4 15% 40% 23% 4	433
June	1st to 10th	Night	12% 4 17% 44% 22% 2	400
	11th to 20th	Night	8% 3 16% 42% 27% 4	400
	21st to end	Night	8% 2 13% 46% 30% 1	400
July	1st to 10th	Night	8% 3 11% 43% 32% 4	390
	11th to 20th	Night	7% 3 4 41% 38% 7%	528
	21st to end	Night	10% 3 11% 42% 30% 5%	624
August	1st to 10th	Night	12% 4 8% 42% 30% 5%	590
	11th to 20th	Night	11% 3 9% 39% 33% 6%	750
	21st to end	Night	12% 3 8% 32% 39% 7%	836
September	1st to 10th	Night	12% 3 11% 35% 30% 9%	760
	11th to 20th	Night	16% 4 12% 33% 30% 4	760
	21st to end	Night	24% 4 16% 31% 24% 2	760
October	1st to 10th	Night	16% 4 17% 33% 24% 6%	760
	11th to 20th	Night	18% 6% 23% 33% 19% 2	760
	21st to end	Night	14% 8% 23% 32% 18% 5%	836
November	1st to 10th	Night	18% 7% 25% 31% 16% 3	780
	11th to 20th	Night	20% 9% 32% 28% 11% 1	800
	21st to end	Night	14% 10% 30% 33% 13% 1	800
December	1st to 10th	Night	18% 12% 34% 28% 9% 1	800
	11th to 20th	Night	17% 10% 36% 29% 8%	800
	21st to end	Night	18% 10% 30% 33% 8%	880

Page generated at 2025-07-18 01:10:03. Version ac07

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