Nelson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nelson clear sky chart from 2005-11-06 to 2025-05-26.

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	10% 14% 5% 9% 5% 4 4 3 5% 6% 33%	117486
	Night	11% 16% 5% 10% 5% 4 5% 4 5% 6% 31%	120486
Transparency	Day	49% 9% 13% 17% 9% 2	117248
	Night	51% 10% 14% 16% 8% 2	120371
Seeing	Day	18% 14% 28% 24% 12% 3	15636
	Night	19% 7% 25% 31% 16% 1	24813

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	9% 26% 13% 19% 5% 4 3 2 3 3 14%	3436
	11th to 20th	Day	8% 16% 4 7% 4 5% 5% 3 5% 8% 34%	3616
	21st to end	Day	12% 27% 5% 7% 4 2 3 3 4 5% 27%	4004
February	1st to 10th	Day	14% 22% 6% 11% 5% 5% 4 4 5% 5% 19%	1600
	11th to 20th	Day	9% 25% 7% 11% 6% 4 4 3 4 5% 22%	1785
	21st to end	Day	9% 22% 8% 12% 7% 5% 6% 4 5% 4 19%	1650
March	1st to 10th	Day	8% 19% 7% 13% 6% 5% 4 3 5% 7% 23%	4518
	11th to 20th	Day	8% 22% 6% 8% 4 4 4 3 4 6% 31%	4687
	21st to end	Day	11% 26% 7% 13% 6% 5% 4 3 4 5% 16%	5918
April	1st to 10th	Day	14% 21% 5% 12% 6% 5% 4 4 5% 5% 19%	2400
	11th to 20th	Day	12% 16% 5% 11% 6% 5% 5% 3 5% 6% 23%	2475
	21st to end	Day	13% 15% 5% 12% 7% 5% 5% 3 5% 7% 23%	2606
May	1st to 10th	Day	10% 14% 4 11% 7% 7% 5% 4 6% 6% 28%	4560
	11th to 20th	Day	11% 13% 4 10% 7% 6% 6% 5% 7% 8% 25%	4517
	21st to end	Day	16% 14% 5% 12% 8% 7% 7% 5% 5% 5% 18%	5175
June	1st to 10th	Day	22% 11% 4 9% 6% 6% 4 4 5% 5% 24%	2850
	11th to 20th	Day	18% 13% 5% 11% 7% 7% 5% 4 5% 5% 20%	2850
	21st to end	Day	11% 7% 3 9% 6% 4 5% 4 5% 8% 37%	2850
July	1st to 10th	Day	4 4 2 4 3 3 4 3 6% 6% 61%	4740
	11th to 20th	Day	4 3 1 3 3 3 3 2 4 5% 69%	4438
	21st to end	Day	3 2 1 4 4 4 4 4 6% 7% 62%	4790
August	1st to 10th	Day	3 4 1 4 3 3 4 3 7% 9% 58%	4401
	11th to 20th	Day	5% 3 1 4 4 4 5% 4 6% 8% 56%	4216
	21st to end	Day	7% 5% 2 7% 4 4 6% 4 7% 8% 46%	4341
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 5% 8% 49%	2280
	11th to 20th	Day	12% 8% 4 7% 4 5% 4 3 4 6% 44%	2180
	21st to end	Day	16% 8% 3 7% 5% 4 4 4 5% 6% 38%	2090
October	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 4 5% 6% 42%	3340
	11th to 20th	Day	17% 10% 4 7% 4 4 3 4 5% 7% 35%	2920
	21st to end	Day	17% 18% 6% 11% 5% 4 5% 3 4 4 24%	3101
November	1st to 10th	Day	22% 22% 5% 9% 5% 4 4 3 4 5% 17%	1764
	11th to 20th	Day	20% 22% 6% 9% 5% 3 4 3 4 4 20%	1558
	21st to end	Day	16% 24% 6% 12% 6% 4 3 3 4 6% 17%	1400
December	1st to 10th	Day	10% 13% 5% 9% 4 4 4 4 8% 8% 30%	2290
	11th to 20th	Day	10% 27% 7% 15% 8% 5% 5% 3 3 4 14%	2642
	21st to end	Day	9% 28% 11% 18% 6% 4 3 3 3 3 13%	3498
January	1st to 10th	Night	11% 27% 11% 17% 6% 4 3 2 3 2 14%	6540
	11th to 20th	Night	9% 14% 5% 11% 5% 5% 5% 4 5% 6% 32%	6282
	21st to end	Night	12% 24% 7% 9% 4 4 4 3 4 5% 24%	6952
February	1st to 10th	Night	14% 22% 7% 12% 7% 3 5% 4 4 6% 18%	2800
	11th to 20th	Night	12% 22% 7% 10% 4 4 4 3 4 7% 24%	2498
	21st to end	Night	9% 23% 7% 13% 6% 4 5% 3 5% 5% 20%	1980
March	1st to 10th	Night	8% 16% 7% 11% 5% 5% 5% 3 4 6% 29%	5376
	11th to 20th	Night	8% 19% 5% 11% 5% 5% 4 3 4 5% 32%	5312
	21st to end	Night	12% 22% 6% 11% 7% 6% 6% 4 5% 5% 16%	5055
April	1st to 10th	Night	10% 19% 4 8% 5% 4 5% 4 6% 8% 28%	2000
	11th to 20th	Night	13% 13% 4 8% 5% 5% 4 3 6% 7% 33%	1876
	21st to end	Night	12% 14% 4 8% 4 5% 4 4 5% 7% 33%	1790
May	1st to 10th	Night	9% 9% 4 8% 6% 6% 6% 5% 7% 7% 34%	2620
	11th to 20th	Night	8% 8% 3 7% 6% 4 5% 4 7% 10% 38%	2596
	21st to end	Night	18% 15% 4 9% 5% 6% 6% 3 6% 6% 22%	2516
June	1st to 10th	Night	17% 7% 4 8% 5% 5% 6% 4 6% 7% 32%	1330
	11th to 20th	Night	15% 10% 4 8% 6% 5% 5% 5% 7% 7% 29%	1330
	21st to end	Night	8% 6% 2 5% 6% 4 4 5% 6% 6% 46%	1330
July	1st to 10th	Night	2 2 1 3 3 2 3 3 4 7% 69%	2220
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 5% 71%	2505
	21st to end	Night	2 2 1 3 3 3 3 3 5% 8% 68%	2762
August	1st to 10th	Night	3 3 1 3 3 3 4 3 6% 9% 63%	2529
	11th to 20th	Night	3 3 1 3 3 4 4 4 6% 9% 59%	2615
	21st to end	Night	6% 5% 2 4 4 4 5% 4 7% 9% 50%	3328
September	1st to 10th	Night	6% 3 2 4 4 4 4 4 7% 7% 55%	1900
	11th to 20th	Night	9% 8% 3 6% 4 4 4 3 6% 7% 46%	2000
	21st to end	Night	13% 7% 2 7% 3 3 4 4 6% 8% 43%	2090
October	1st to 10th	Night	8% 7% 3 6% 4 4 5% 5% 7% 8% 45%	3638
	11th to 20th	Night	18% 9% 2 6% 3 4 3 3 5% 7% 39%	4010
	21st to end	Night	15% 14% 5% 11% 5% 4 5% 4 4 5% 27%	4457
November	1st to 10th	Night	23% 19% 6% 8% 5% 4 5% 3 4 6% 16%	2535
	11th to 20th	Night	18% 24% 5% 10% 5% 4 3 3 5% 5% 17%	2816
	21st to end	Night	17% 23% 6% 11% 5% 4 5% 3 4 6% 16%	3000
December	1st to 10th	Night	11% 14% 5% 9% 5% 4 6% 4 6% 7% 29%	4890
	11th to 20th	Night	11% 24% 9% 17% 6% 5% 5% 4 4 4 10%	5550
	21st to end	Night	10% 27% 9% 17% 6% 5% 5% 4 3 4 10%	7458

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% 11% 8% 8% 1	3436
	11th to 20th	Day	45% 10% 22% 15% 7% 1	3640
	21st to end	Day	61% 9% 8% 16% 6%	4004
February	1st to 10th	Day	66% 9% 11% 7% 5% 2	1600
	11th to 20th	Day	63% 10% 13% 9% 4 1	1794
	21st to end	Day	64% 10% 10% 11% 4	1650
March	1st to 10th	Day	58% 9% 9% 12% 12%	4518
	11th to 20th	Day	53% 7% 18% 17% 4	4687
	21st to end	Day	67% 11% 11% 9% 3	5918
April	1st to 10th	Day	65% 9% 13% 10% 3	2400
	11th to 20th	Day	60% 8% 14% 15% 4	2475
	21st to end	Day	60% 8% 14% 15% 3 1	2606
May	1st to 10th	Day	48% 14% 15% 19% 4	4560
	11th to 20th	Day	45% 14% 16% 21% 4	4520
	21st to end	Day	57% 17% 14% 10% 2	5176
June	1st to 10th	Day	59% 9% 11% 11% 8% 2	2850
	11th to 20th	Day	64% 9% 11% 11% 6%	2850
	21st to end	Day	44% 9% 13% 20% 12% 2	2850
July	1st to 10th	Day	23% 6% 11% 22% 27% 10%	4666
	11th to 20th	Day	19% 4 7% 17% 32% 21%	4362
	21st to end	Day	21% 5% 10% 26% 25% 13%	4639
August	1st to 10th	Day	22% 6% 12% 30% 24% 6%	4384
	11th to 20th	Day	25% 8% 18% 35% 14% 1	4258
	21st to end	Day	33% 9% 18% 27% 13% 1	4340
September	1st to 10th	Day	32% 8% 13% 27% 16% 4	2280
	11th to 20th	Day	42% 7% 12% 23% 13% 2	2180
	21st to end	Day	46% 9% 16% 18% 10% 1	2090
October	1st to 10th	Day	40% 8% 19% 29% 4	3340
	11th to 20th	Day	48% 8% 14% 24% 7%	2920
	21st to end	Day	58% 12% 12% 13% 4	3101
November	1st to 10th	Day	69% 12% 10% 8% 2	1764
	11th to 20th	Day	69% 9% 9% 8% 3 1	1558
	21st to end	Day	66% 13% 13% 6% 3	1400
December	1st to 10th	Day	48% 15% 23% 11% 2	2290
	11th to 20th	Day	71% 12% 10% 5% 1	2644
	21st to end	Day	75% 10% 6% 5% 3	3498
January	1st to 10th	Night	73% 10% 6% 9% 1	6540
	11th to 20th	Night	49% 10% 16% 17% 7% 1	6320
	21st to end	Night	62% 9% 12% 11% 5%	6952
February	1st to 10th	Night	66% 9% 12% 7% 5% 1	2800
	11th to 20th	Night	60% 9% 15% 10% 6% 1	2500
	21st to end	Night	61% 11% 12% 10% 5% 1	1980
March	1st to 10th	Night	51% 11% 13% 13% 11%	5376
	11th to 20th	Night	54% 6% 18% 18% 4	5313
	21st to end	Night	62% 13% 13% 9% 3	5055
April	1st to 10th	Night	52% 11% 20% 13% 5%	2000
	11th to 20th	Night	50% 9% 17% 18% 6%	1876
	21st to end	Night	47% 9% 16% 21% 7%	1790
May	1st to 10th	Night	42% 15% 15% 21% 7%	2620
	11th to 20th	Night	34% 12% 19% 27% 8%	2596
	21st to end	Night	51% 14% 15% 16% 4	2516
June	1st to 10th	Night	47% 11% 15% 17% 8% 1	1330
	11th to 20th	Night	51% 11% 14% 17% 7% 1	1330
	21st to end	Night	34% 8% 17% 24% 16% 1	1330
July	1st to 10th	Night	15% 5% 13% 26% 26% 15%	2185
	11th to 20th	Night	14% 5% 9% 17% 32% 22%	2459
	21st to end	Night	15% 4 10% 27% 30% 14%	2674
August	1st to 10th	Night	17% 5% 13% 32% 27% 6%	2521
	11th to 20th	Night	20% 6% 21% 36% 15% 2	2639
	21st to end	Night	26% 9% 20% 27% 16% 2	3328
September	1st to 10th	Night	24% 8% 15% 29% 20% 5%	1900
	11th to 20th	Night	36% 7% 13% 26% 16% 3	2000
	21st to end	Night	37% 10% 22% 19% 11% 1	2090
October	1st to 10th	Night	34% 8% 20% 32% 5%	3638
	11th to 20th	Night	44% 7% 14% 26% 7%	4010
	21st to end	Night	55% 10% 15% 17% 3	4457
November	1st to 10th	Night	66% 10% 12% 8% 3	2535
	11th to 20th	Night	69% 10% 11% 7% 4	2816
	21st to end	Night	67% 10% 11% 9% 2	3000
December	1st to 10th	Night	51% 14% 20% 13% 2	4890
	11th to 20th	Night	75% 11% 9% 4 1	5547
	21st to end	Night	76% 10% 6% 5% 2	7458

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% 11% 33% 21% 10% 3	200
	11th to 20th	Day	19% 11% 30% 26% 13% 2	200
	21st to end	Day	21% 10% 35% 23% 10% 2	220
February	1st to 10th	Day	21% 16% 37% 19% 8% 1	200
	11th to 20th	Day	18% 16% 35% 21% 9% 1	384
	21st to end	Day	17% 20% 31% 24% 7%	414
March	1st to 10th	Day	25% 21% 25% 19% 9% 2	500
	11th to 20th	Day	23% 18% 24% 19% 14% 2	500
	21st to end	Day	26% 18% 29% 16% 9% 2	550
April	1st to 10th	Day	25% 18% 28% 19% 7% 2	500
	11th to 20th	Day	21% 21% 26% 21% 8% 3	492
	21st to end	Day	18% 16% 30% 24% 11% 3	500
May	1st to 10th	Day	14% 15% 33% 25% 11% 2	503
	11th to 20th	Day	20% 15% 24% 25% 13% 3	528
	21st to end	Day	22% 14% 19% 25% 16% 4	722
June	1st to 10th	Day	23% 14% 25% 24% 11% 2	680
	11th to 20th	Day	20% 16% 28% 22% 11% 3	680
	21st to end	Day	13% 11% 25% 28% 20% 3	680
July	1st to 10th	Day	9% 9% 27% 31% 22% 3	671
	11th to 20th	Day	8% 13% 35% 29% 12% 3	501
	21st to end	Day	7% 12% 34% 29% 15% 4	531
August	1st to 10th	Day	7% 10% 31% 30% 16% 6%	487
	11th to 20th	Day	5% 10% 31% 28% 17% 8%	477
	21st to end	Day	11% 13% 28% 27% 15% 6%	539
September	1st to 10th	Day	15% 12% 28% 23% 17% 4	490
	11th to 20th	Day	16% 14% 28% 25% 12% 4	490
	21st to end	Day	24% 18% 22% 22% 11% 3	490
October	1st to 10th	Day	20% 13% 30% 22% 12% 3	490
	11th to 20th	Day	25% 14% 22% 23% 12% 4	352
	21st to end	Day	28% 13% 33% 17% 6% 2	330
November	1st to 10th	Day	28% 14% 22% 27% 8% 1	300
	11th to 20th	Day	28% 15% 24% 22% 9% 2	215
	21st to end	Day	22% 14% 26% 21% 12% 5%	200
December	1st to 10th	Day	19% 11% 25% 23% 19% 5%	200
	11th to 20th	Day	17% 16% 30% 28% 9% 2	200
	21st to end	Day	17% 12% 27% 31% 10% 2	220
January	1st to 10th	Night	30% 11% 27% 24% 7% 1	904
	11th to 20th	Night	22% 9% 32% 27% 8% 2	800
	21st to end	Night	18% 9% 33% 31% 9% 1	880
February	1st to 10th	Night	23% 10% 31% 27% 9%	800
	11th to 20th	Night	21% 5% 34% 30% 10%	784
	21st to end	Night	18% 9% 30% 30% 12% 2	660
March	1st to 10th	Night	22% 10% 25% 29% 14% 1	800
	11th to 20th	Night	25% 10% 23% 27% 15% 1	800
	21st to end	Night	22% 9% 28% 29% 11% 2	880
April	1st to 10th	Night	19% 8% 24% 31% 16% 1	800
	11th to 20th	Night	20% 6% 27% 32% 14% 1	688
	21st to end	Night	17% 9% 24% 35% 14% 1	590
May	1st to 10th	Night	12% 5% 23% 38% 22%	400
	11th to 20th	Night	15% 5% 23% 32% 23% 1	394
	21st to end	Night	15% 10% 21% 30% 23% 1	422
June	1st to 10th	Night	18% 6% 21% 35% 19% 1	380
	11th to 20th	Night	15% 5% 26% 34% 18% 1	380
	21st to end	Night	9% 6% 22% 40% 23% 2	380
July	1st to 10th	Night	5% 1 14% 45% 34% 1	370
	11th to 20th	Night	3 2 18% 45% 29% 2	368
	21st to end	Night	4 2 14% 45% 34% 1	416
August	1st to 10th	Night	5% 2 14% 38% 39% 2	378
	11th to 20th	Night	6% 1 16% 38% 34% 5%	442
	21st to end	Night	8% 4 17% 39% 30% 2	731
September	1st to 10th	Night	8% 2 22% 38% 28% 3	760
	11th to 20th	Night	12% 4 17% 32% 31% 4	760
	21st to end	Night	16% 6% 19% 39% 18% 2	760
October	1st to 10th	Night	17% 6% 22% 35% 19% 1	760
	11th to 20th	Night	23% 6% 21% 35% 14% 2	760
	21st to end	Night	23% 9% 29% 29% 7% 3	836
November	1st to 10th	Night	31% 9% 25% 31% 5%	780
	11th to 20th	Night	30% 11% 26% 20% 12%	850
	21st to end	Night	27% 9% 25% 26% 11% 2	1000
December	1st to 10th	Night	23% 6% 32% 26% 12% 1	1000
	11th to 20th	Night	24% 9% 31% 28% 8%	1000
	21st to end	Night	24% 9% 34% 25% 6% 1	1100

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