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 2026-09-29.

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	9% 12% 4 9% 5% 5% 5% 4 5% 6% 37%	153731
	Night	10% 14% 5% 9% 5% 4 5% 4 5% 6% 34%	146399
Transparency	Day	45% 8% 14% 20% 10% 3	153452
	Night	48% 9% 14% 18% 9% 2	146262
Seeing	Day	18% 15% 28% 24% 12% 3	17477
	Night	19% 7% 26% 32% 15% 1	26409

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% 24% 12% 19% 6% 5% 4 2 3 3 12%	4466
	11th to 20th	Day	7% 15% 4 7% 3 4 5% 3 5% 7% 41%	4716
	21st to end	Day	10% 23% 5% 8% 4 3 4 3 4 5% 29%	5186
February	1st to 10th	Day	13% 22% 6% 11% 5% 5% 5% 4 5% 6% 18%	1680
	11th to 20th	Day	9% 25% 7% 12% 6% 4 4 3 4 5% 22%	1876
	21st to end	Day	9% 21% 8% 12% 7% 5% 6% 4 5% 4 19%	1730
March	1st to 10th	Day	8% 19% 7% 13% 6% 5% 4 3 5% 7% 23%	4618
	11th to 20th	Day	8% 22% 6% 9% 4 4 4 3 4 6% 30%	4790
	21st to end	Day	10% 26% 6% 13% 6% 5% 4 3 5% 5% 16%	6049
April	1st to 10th	Day	13% 21% 5% 11% 6% 5% 4 4 5% 5% 20%	2520
	11th to 20th	Day	12% 17% 5% 12% 7% 5% 5% 3 6% 6% 23%	2600
	21st to end	Day	13% 15% 5% 12% 7% 5% 5% 3 5% 7% 24%	2736
May	1st to 10th	Day	6% 9% 3 7% 5% 6% 5% 4 6% 8% 41%	8195
	11th to 20th	Day	10% 13% 5% 12% 7% 6% 6% 4 6% 7% 22%	8160
	21st to end	Day	11% 9% 4 9% 7% 6% 6% 5% 6% 7% 29%	9531
June	1st to 10th	Day	21% 10% 4 9% 6% 6% 4 4 5% 5% 27%	3150
	11th to 20th	Day	17% 11% 4 10% 7% 7% 5% 4 5% 6% 24%	3150
	21st to end	Day	11% 7% 3 9% 6% 5% 5% 4 5% 8% 37%	3150
July	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 7% 64%	8820
	11th to 20th	Day	3 3 1 4 3 4 4 3 5% 6% 65%	8266
	21st to end	Day	4 3 1 4 3 3 4 3 6% 7% 63%	8916
August	1st to 10th	Day	3 3 1 4 3 3 4 3 6% 7% 64%	6441
	11th to 20th	Day	6% 3 1 5% 4 4 5% 4 6% 9% 53%	6197
	21st to end	Day	6% 4 2 6% 4 5% 6% 4 6% 8% 48%	6306
September	1st to 10th	Day	8% 5% 2 6% 4 4 4 4 5% 9% 49%	2520
	11th to 20th	Day	11% 7% 3 7% 4 5% 3 3 5% 6% 45%	2409
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 5% 6% 39%	2289
October	1st to 10th	Day	9% 7% 3 7% 5% 5% 6% 4 5% 6% 42%	3445
	11th to 20th	Day	16% 11% 4 7% 4 4 4 4 5% 7% 35%	3013
	21st to end	Day	16% 18% 6% 11% 5% 4 5% 4 4 4 24%	3200
November	1st to 10th	Day	22% 21% 5% 9% 5% 4 4 3 4 5% 17%	1854
	11th to 20th	Day	21% 23% 6% 9% 5% 3 4 3 4 4 19%	1635
	21st to end	Day	15% 24% 6% 12% 6% 4 3 3 4 6% 17%	1470
December	1st to 10th	Day	10% 14% 6% 9% 4 4 4 4 8% 8% 29%	2360
	11th to 20th	Day	10% 27% 7% 15% 8% 5% 5% 3 3 4 13%	2712
	21st to end	Day	9% 27% 11% 18% 6% 4 3 3 3 3 13%	3575
January	1st to 10th	Night	9% 26% 11% 19% 7% 4 4 3 3 3 13%	8472
	11th to 20th	Night	8% 13% 4 9% 4 4 4 4 5% 6% 37%	8182
	21st to end	Night	10% 21% 6% 9% 5% 5% 4 3 4 5% 29%	8994
February	1st to 10th	Night	13% 21% 7% 12% 8% 4 5% 4 4 6% 17%	2940
	11th to 20th	Night	11% 22% 7% 10% 5% 4 4 3 4 7% 24%	2626
	21st to end	Night	9% 22% 7% 12% 6% 5% 5% 3 5% 5% 20%	2076
March	1st to 10th	Night	8% 16% 7% 11% 5% 5% 5% 3 4 5% 30%	5496
	11th to 20th	Night	9% 19% 5% 11% 5% 5% 3 3 4 5% 32%	5429
	21st to end	Night	12% 22% 6% 11% 7% 6% 6% 4 5% 5% 16%	5166
April	1st to 10th	Night	10% 19% 4 8% 5% 4 5% 4 6% 8% 28%	2100
	11th to 20th	Night	13% 13% 4 8% 5% 5% 4 4 6% 7% 32%	1971
	21st to end	Night	11% 14% 4 8% 4 5% 4 4 6% 7% 34%	1880
May	1st to 10th	Night	7% 6% 2 6% 4 5% 5% 5% 7% 7% 45%	4724
	11th to 20th	Night	9% 11% 4 10% 7% 5% 6% 5% 6% 8% 28%	4700
	21st to end	Night	13% 11% 3 8% 5% 5% 6% 4 6% 6% 34%	4640
June	1st to 10th	Night	15% 7% 3 9% 5% 5% 6% 5% 5% 8% 32%	1470
	11th to 20th	Night	13% 10% 3 8% 6% 5% 4 5% 7% 6% 32%	1470
	21st to end	Night	8% 6% 2 6% 6% 4 4 5% 6% 6% 45%	1470
July	1st to 10th	Night	4 4 1 3 2 2 4 3 5% 6% 66%	4140
	11th to 20th	Night	3 2 1 3 3 3 4 3 5% 6% 66%	4677
	21st to end	Night	3 2 1 3 2 3 3 3 5% 8% 68%	5152
August	1st to 10th	Night	2 2 1 3 3 2 3 2 5% 7% 69%	3709
	11th to 20th	Night	3 3 1 4 3 4 5% 4 7% 11% 57%	3841
	21st to end	Night	5% 4 2 4 3 3 5% 4 7% 10% 52%	4850
September	1st to 10th	Night	6% 3 2 4 4 4 4 4 6% 8% 55%	2100
	11th to 20th	Night	9% 8% 2 5% 4 4 4 3 6% 7% 47%	2210
	21st to end	Night	12% 6% 2 7% 3 3 4 4 6% 8% 45%	2288
October	1st to 10th	Night	8% 7% 3 6% 4 4 5% 5% 7% 8% 45%	3753
	11th to 20th	Night	18% 9% 3 6% 3 4 3 3 5% 7% 39%	4137
	21st to end	Night	14% 14% 5% 11% 5% 4 6% 4 4 5% 28%	4600
November	1st to 10th	Night	22% 19% 6% 8% 5% 4 5% 3 5% 7% 17%	2665
	11th to 20th	Night	19% 24% 5% 10% 5% 4 3 3 5% 5% 16%	2958
	21st to end	Night	16% 24% 6% 11% 5% 3 5% 3 4 6% 16%	3150
December	1st to 10th	Night	11% 14% 5% 9% 5% 4 6% 4 6% 7% 28%	5040
	11th to 20th	Night	11% 25% 9% 17% 7% 5% 5% 4 4 4 10%	5700
	21st to end	Night	10% 27% 9% 17% 6% 5% 5% 4 4 4 10%	7623

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	74% 10% 8% 7% 1	4466
	11th to 20th	Day	40% 9% 22% 20% 8% 1	4740
	21st to end	Day	57% 9% 10% 17% 7%	5187
February	1st to 10th	Day	65% 10% 11% 7% 5% 2	1680
	11th to 20th	Day	63% 9% 14% 9% 4 1	1885
	21st to end	Day	64% 10% 11% 11% 4	1730
March	1st to 10th	Day	58% 9% 10% 12% 12%	4618
	11th to 20th	Day	54% 7% 18% 17% 4	4790
	21st to end	Day	66% 11% 11% 9% 3	6049
April	1st to 10th	Day	63% 9% 14% 11% 3	2520
	11th to 20th	Day	60% 8% 14% 15% 3	2600
	21st to end	Day	59% 8% 14% 16% 3 1	2736
May	1st to 10th	Day	36% 9% 19% 25% 9%	8195
	11th to 20th	Day	53% 11% 14% 18% 3	8163
	21st to end	Day	46% 13% 19% 20% 2	9532
June	1st to 10th	Day	58% 8% 10% 13% 10% 2	3150
	11th to 20th	Day	59% 8% 11% 13% 8%	3150
	21st to end	Day	44% 9% 13% 20% 12% 1	3150
July	1st to 10th	Day	20% 5% 12% 27% 27% 9%	8746
	11th to 20th	Day	21% 4 11% 23% 24% 18%	8190
	21st to end	Day	20% 4 10% 30% 26% 9%	8723
August	1st to 10th	Day	18% 5% 11% 27% 29% 10%	6424
	11th to 20th	Day	27% 8% 21% 33% 10% 1	6239
	21st to end	Day	31% 8% 17% 32% 11% 1	6305
September	1st to 10th	Day	32% 8% 14% 27% 15% 4	2520
	11th to 20th	Day	40% 7% 13% 25% 13% 2	2409
	21st to end	Day	45% 9% 16% 19% 10% 1	2289
October	1st to 10th	Day	40% 8% 20% 29% 4	3445
	11th to 20th	Day	48% 8% 13% 24% 7%	3013
	21st to end	Day	58% 12% 12% 13% 4	3200
November	1st to 10th	Day	68% 11% 10% 9% 2	1854
	11th to 20th	Day	69% 10% 9% 8% 3 1	1635
	21st to end	Day	66% 13% 13% 6% 3	1470
December	1st to 10th	Day	49% 15% 23% 11% 2	2360
	11th to 20th	Day	72% 12% 10% 5% 1	2714
	21st to end	Day	75% 10% 6% 5% 3	3575
January	1st to 10th	Night	73% 10% 8% 7% 1	8473
	11th to 20th	Night	45% 9% 18% 20% 8% 1	8220
	21st to end	Night	57% 9% 13% 16% 6%	8994
February	1st to 10th	Night	66% 9% 11% 7% 5% 1	2940
	11th to 20th	Night	59% 9% 15% 11% 6% 1	2628
	21st to end	Night	60% 12% 12% 10% 5% 1	2076
March	1st to 10th	Night	51% 11% 14% 13% 11%	5496
	11th to 20th	Night	54% 6% 18% 18% 4	5430
	21st to end	Night	61% 13% 13% 10% 3	5166
April	1st to 10th	Night	50% 11% 20% 13% 5%	2100
	11th to 20th	Night	50% 9% 17% 18% 6%	1971
	21st to end	Night	46% 9% 16% 21% 8% 1	1880
May	1st to 10th	Night	32% 10% 18% 29% 10%	4721
	11th to 20th	Night	45% 10% 19% 21% 4	4700
	21st to end	Night	43% 10% 18% 25% 3	4644
June	1st to 10th	Night	46% 11% 15% 17% 9% 2	1470
	11th to 20th	Night	48% 10% 13% 19% 10% 1	1470
	21st to end	Night	36% 8% 16% 23% 15% 1	1470
July	1st to 10th	Night	19% 4 13% 27% 25% 12%	4105
	11th to 20th	Night	18% 4 12% 21% 26% 18%	4631
	21st to end	Night	16% 4 10% 32% 29% 10%	5040
August	1st to 10th	Night	15% 4 10% 30% 31% 9%	3701
	11th to 20th	Night	20% 6% 29% 32% 11% 1	3865
	21st to end	Night	24% 8% 19% 31% 16% 2	4850
September	1st to 10th	Night	25% 7% 16% 30% 19% 4	2100
	11th to 20th	Night	35% 7% 12% 27% 16% 2	2210
	21st to end	Night	36% 9% 22% 21% 11% 1	2288
October	1st to 10th	Night	34% 8% 21% 33% 5%	3753
	11th to 20th	Night	44% 7% 14% 26% 8%	4137
	21st to end	Night	55% 10% 15% 17% 3	4600
November	1st to 10th	Night	66% 10% 12% 8% 3	2665
	11th to 20th	Night	69% 10% 10% 7% 4	2958
	21st to end	Night	67% 11% 11% 9% 2	3150
December	1st to 10th	Night	52% 14% 19% 13% 2	5040
	11th to 20th	Night	75% 11% 9% 4 1	5697
	21st to end	Night	76% 11% 6% 5% 2	7623

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	22% 13% 32% 20% 9% 3	220
	11th to 20th	Day	18% 11% 31% 27% 11% 2	220
	21st to end	Day	19% 10% 34% 24% 10% 2	242
February	1st to 10th	Day	19% 16% 36% 20% 8% 1	220
	11th to 20th	Day	18% 16% 34% 22% 9% 1	415
	21st to end	Day	19% 19% 31% 23% 7%	446
March	1st to 10th	Day	25% 21% 25% 19% 8% 1	540
	11th to 20th	Day	27% 18% 22% 18% 13% 2	540
	21st to end	Day	26% 19% 28% 16% 8% 2	594
April	1st to 10th	Day	24% 18% 28% 19% 8% 2	540
	11th to 20th	Day	21% 21% 26% 21% 8% 4	532
	21st to end	Day	18% 15% 29% 24% 11% 3	540
May	1st to 10th	Day	14% 15% 34% 25% 10% 2	543
	11th to 20th	Day	20% 16% 24% 25% 12% 3	568
	21st to end	Day	21% 14% 21% 25% 15% 4	804
June	1st to 10th	Day	23% 14% 26% 23% 11% 2	780
	11th to 20th	Day	20% 16% 29% 22% 10% 2	780
	21st to end	Day	14% 12% 25% 28% 18% 3	780
July	1st to 10th	Day	9% 11% 26% 31% 20% 3	771
	11th to 20th	Day	9% 15% 35% 27% 11% 3	583
	21st to end	Day	7% 12% 33% 28% 15% 4	619
August	1st to 10th	Day	7% 10% 32% 30% 16% 5%	567
	11th to 20th	Day	7% 11% 31% 28% 16% 7%	557
	21st to end	Day	11% 13% 28% 29% 15% 5%	627
September	1st to 10th	Day	15% 12% 27% 25% 16% 5%	570
	11th to 20th	Day	15% 13% 27% 26% 14% 4	570
	21st to end	Day	23% 18% 25% 22% 11% 2	563
October	1st to 10th	Day	18% 14% 31% 22% 12% 3	530
	11th to 20th	Day	25% 15% 23% 21% 11% 4	385
	21st to end	Day	27% 13% 33% 18% 6% 2	363
November	1st to 10th	Day	30% 15% 21% 26% 8% 1	330
	11th to 20th	Day	29% 14% 24% 22% 9% 2	236
	21st to end	Day	21% 16% 26% 21% 11% 5%	220
December	1st to 10th	Day	20% 12% 25% 22% 17% 5%	220
	11th to 20th	Day	20% 17% 28% 25% 8% 1	220
	21st to end	Day	18% 12% 27% 30% 11% 2	242
January	1st to 10th	Night	29% 11% 27% 25% 7% 1	949
	11th to 20th	Night	22% 9% 33% 27% 8% 1	840
	21st to end	Night	18% 9% 33% 31% 9% 1	924
February	1st to 10th	Night	22% 10% 30% 28% 9%	840
	11th to 20th	Night	20% 5% 34% 30% 10%	824
	21st to end	Night	18% 9% 30% 30% 12% 2	692
March	1st to 10th	Night	22% 10% 26% 29% 13% 1	840
	11th to 20th	Night	27% 10% 23% 25% 15% 1	840
	21st to end	Night	21% 9% 28% 28% 11% 2	924
April	1st to 10th	Night	18% 8% 25% 32% 16% 1	840
	11th to 20th	Night	20% 7% 27% 33% 14% 1	723
	21st to end	Night	16% 9% 24% 36% 14% 1	620
May	1st to 10th	Night	11% 5% 23% 38% 22%	420
	11th to 20th	Night	15% 5% 24% 33% 22% 1	414
	21st to end	Night	15% 11% 21% 31% 22% 1	455
June	1st to 10th	Night	17% 6% 21% 36% 18% 1	420
	11th to 20th	Night	14% 5% 29% 35% 17% 1	420
	21st to end	Night	9% 5% 22% 40% 22% 1	420
July	1st to 10th	Night	5% 2 15% 46% 31%	410
	11th to 20th	Night	4 3 19% 45% 27% 2	408
	21st to end	Night	4 2 15% 47% 31% 1	460
August	1st to 10th	Night	6% 2 14% 39% 37% 2	418
	11th to 20th	Night	6% 1 17% 40% 32% 4	489
	21st to end	Night	8% 4 16% 40% 29% 2	809
September	1st to 10th	Night	8% 2 21% 38% 28% 3	840
	11th to 20th	Night	12% 4 17% 32% 31% 3	840
	21st to end	Night	15% 6% 21% 40% 17% 2	832
October	1st to 10th	Night	17% 6% 23% 36% 18% 1	800
	11th to 20th	Night	22% 6% 22% 34% 14% 2	800
	21st to end	Night	24% 9% 29% 29% 7% 2	880
November	1st to 10th	Night	30% 9% 25% 30% 5%	820
	11th to 20th	Night	31% 11% 26% 20% 11%	893
	21st to end	Night	27% 9% 26% 26% 11% 2	1050
December	1st to 10th	Night	23% 6% 32% 26% 11% 1	1050
	11th to 20th	Night	26% 9% 31% 26% 7%	1050
	21st to end	Night	24% 10% 34% 26% 6% 1	1155

Page generated at 2026-09-26 01:18:41. Version ac07

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