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 2024-03-06.

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% 5% 8% 6% 3 5% 4 5% 6% 29%	78106
	Night	14% 17% 6% 8% 6% 3 4 5% 5% 6% 27%	81351
Transparency	Day	55% 9% 12% 15% 8% 1	77826
	Night	56% 9% 12% 14% 7% 1	81212
Seeing	Day	18% 14% 28% 24% 13% 3	14019
	Night	20% 7% 25% 31% 16% 1	23289

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	13% 30% 13% 15% 7% 3 4 3 3 3 7%	2392
	11th to 20th	Day	12% 22% 6% 9% 6% 4 4 4 5% 7% 21%	2516
	21st to end	Day	17% 35% 8% 7% 5% 2 3 3 3 4 11%	2794
February	1st to 10th	Day	15% 22% 7% 10% 6% 2 5% 4 5% 5% 18%	1520
	11th to 20th	Day	10% 25% 7% 10% 7% 3 4 3 4 5% 22%	1692
	21st to end	Day	10% 20% 9% 11% 7% 3 6% 5% 5% 4 19%	1570
March	1st to 10th	Day	15% 22% 7% 13% 7% 3 4 5% 4 5% 16%	1851
	11th to 20th	Day	18% 20% 5% 8% 6% 3 5% 4 4 7% 19%	1849
	21st to end	Day	16% 26% 7% 10% 7% 3 4 4 4 4 15%	2351
April	1st to 10th	Day	15% 20% 6% 11% 7% 4 4 5% 5% 5% 18%	2160
	11th to 20th	Day	14% 16% 7% 10% 8% 4 5% 4 5% 6% 21%	2224
	21st to end	Day	15% 15% 5% 10% 8% 4 5% 4 5% 7% 22%	2344
May	1st to 10th	Day	12% 13% 4 8% 6% 4 5% 4 6% 7% 30%	2520
	11th to 20th	Day	14% 12% 4 8% 8% 4 5% 5% 5% 7% 28%	2477
	21st to end	Day	19% 11% 5% 9% 8% 5% 5% 5% 5% 5% 22%	2883
June	1st to 10th	Day	23% 10% 5% 8% 6% 4 5% 5% 5% 5% 24%	2700
	11th to 20th	Day	19% 12% 5% 10% 8% 5% 6% 5% 5% 5% 20%	2700
	21st to end	Day	11% 6% 4 7% 7% 3 5% 5% 5% 8% 38%	2700
July	1st to 10th	Day	7% 6% 2 5% 5% 3 5% 6% 7% 8% 45%	2700
	11th to 20th	Day	6% 4 2 4 6% 3 5% 5% 6% 8% 51%	2538
	21st to end	Day	5% 2 1 4 4 3 4 4 5% 7% 61%	2741
August	1st to 10th	Day	5% 5% 2 5% 5% 3 5% 5% 6% 8% 52%	2507
	11th to 20th	Day	6% 3 1 4 4 3 4 4 5% 8% 57%	2413
	21st to end	Day	8% 5% 2 6% 5% 3 5% 5% 6% 8% 46%	2472
September	1st to 10th	Day	9% 5% 2 6% 5% 3 4 5% 5% 8% 49%	2160
	11th to 20th	Day	12% 8% 4 7% 5% 3 3 4 4 6% 44%	2066
	21st to end	Day	17% 9% 3 6% 6% 3 5% 4 5% 6% 37%	1980
October	1st to 10th	Day	15% 10% 4 7% 6% 3 5% 5% 4 6% 35%	1895
	11th to 20th	Day	19% 13% 4 7% 5% 3 3 4 4 6% 32%	1670
	21st to end	Day	21% 17% 4 8% 5% 3 4 4 4 4 26%	1782
November	1st to 10th	Day	24% 20% 6% 8% 5% 3 4 4 4 5% 16%	1674
	11th to 20th	Day	21% 20% 6% 8% 5% 3 4 4 4 4 21%	1481
	21st to end	Day	17% 21% 7% 9% 7% 3 3 3 5% 6% 18%	1330
December	1st to 10th	Day	14% 18% 8% 10% 6% 2 4 4 6% 6% 21%	1330
	11th to 20th	Day	13% 30% 8% 11% 7% 4 5% 3 2 5% 13%	1682
	21st to end	Day	12% 26% 8% 11% 6% 3 4 5% 3 5% 18%	2442
January	1st to 10th	Night	15% 31% 12% 14% 6% 3 3 3 3 3 9%	4570
	11th to 20th	Night	13% 18% 7% 12% 7% 4 5% 5% 5% 6% 19%	4382
	21st to end	Night	18% 31% 10% 10% 6% 3 4 3 3 5% 8%	4862
February	1st to 10th	Night	15% 22% 7% 10% 8% 2 4 5% 4 6% 17%	2660
	11th to 20th	Night	13% 22% 7% 8% 6% 3 4 4 4 7% 24%	2371
	21st to end	Night	10% 21% 7% 11% 7% 3 5% 5% 5% 6% 21%	1884
March	1st to 10th	Night	15% 18% 7% 11% 6% 3 4 5% 4 5% 20%	2225
	11th to 20th	Night	17% 21% 6% 7% 5% 3 3 4 4 6% 24%	2110
	21st to end	Night	14% 18% 5% 8% 7% 4 5% 5% 5% 6% 23%	2003
April	1st to 10th	Night	11% 18% 5% 7% 5% 3 5% 5% 6% 8% 28%	1800
	11th to 20th	Night	15% 12% 4 7% 6% 3 5% 4 6% 6% 31%	1687
	21st to end	Night	12% 13% 5% 7% 5% 3 5% 5% 6% 7% 32%	1612
May	1st to 10th	Night	9% 9% 4 5% 6% 4 6% 6% 7% 7% 36%	1440
	11th to 20th	Night	11% 8% 3 7% 8% 3 5% 7% 5% 8% 36%	1416
	21st to end	Night	16% 11% 4 7% 6% 4 7% 5% 6% 6% 29%	1392
June	1st to 10th	Night	17% 7% 4 7% 5% 3 6% 6% 6% 8% 32%	1260
	11th to 20th	Night	15% 9% 4 7% 7% 4 5% 6% 6% 8% 29%	1260
	21st to end	Night	8% 6% 2 5% 7% 3 5% 5% 6% 7% 47%	1260
July	1st to 10th	Night	4 3 2 4 6% 3 4 5% 5% 8% 55%	1260
	11th to 20th	Night	4 3 1 3 4 3 5% 4 7% 8% 58%	1419
	21st to end	Night	2 1 1 2 3 2 3 4 4 8% 69%	1576
August	1st to 10th	Night	4 3 1 3 4 2 4 4 5% 8% 62%	1432
	11th to 20th	Night	5% 3 1 3 3 2 4 4 5% 8% 62%	1486
	21st to end	Night	6% 4 1 3 3 2 4 5% 7% 9% 56%	1880
September	1st to 10th	Night	7% 3 2 3 5% 3 4 5% 7% 8% 54%	1800
	11th to 20th	Night	9% 8% 3 5% 5% 3 4 4 5% 7% 46%	1894
	21st to end	Night	13% 7% 3 6% 4 2 4 5% 5% 8% 43%	1980
October	1st to 10th	Night	13% 10% 3 6% 6% 3 4 5% 5% 6% 39%	2063
	11th to 20th	Night	19% 10% 2 6% 4 4 4 5% 5% 7% 34%	2290
	21st to end	Night	18% 15% 5% 8% 6% 2 3 4 4 6% 28%	2574
November	1st to 10th	Night	24% 18% 6% 7% 5% 3 5% 5% 4 7% 16%	2405
	11th to 20th	Night	20% 22% 6% 9% 6% 3 3 4 5% 5% 18%	2674
	21st to end	Night	18% 21% 6% 9% 6% 2 5% 4 4 6% 16%	2850
December	1st to 10th	Night	17% 19% 8% 10% 7% 3 4 4 4 6% 19%	2850
	11th to 20th	Night	16% 26% 9% 13% 7% 3 4 5% 5% 4 9%	3510
	21st to end	Night	14% 25% 7% 11% 7% 3 5% 6% 4 6% 12%	5214

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	79% 12% 5% 3 1	2392
	11th to 20th	Day	59% 9% 16% 10% 4 1	2540
	21st to end	Day	78% 9% 6% 5% 2	2794
February	1st to 10th	Day	67% 10% 11% 7% 4 2	1520
	11th to 20th	Day	64% 10% 14% 8% 4 1	1701
	21st to end	Day	65% 10% 10% 11% 4	1570
March	1st to 10th	Day	71% 8% 11% 6% 4	1851
	11th to 20th	Day	65% 8% 10% 13% 3	1849
	21st to end	Day	73% 8% 10% 6% 2	2351
April	1st to 10th	Day	67% 9% 12% 10% 3	2160
	11th to 20th	Day	63% 8% 13% 13% 3	2224
	21st to end	Day	62% 7% 14% 14% 2 1	2344
May	1st to 10th	Day	51% 9% 18% 17% 5% 1	2520
	11th to 20th	Day	54% 9% 14% 18% 5%	2480
	21st to end	Day	61% 10% 14% 12% 3	2884
June	1st to 10th	Day	60% 8% 11% 12% 7% 2	2700
	11th to 20th	Day	65% 9% 11% 10% 5%	2700
	21st to end	Day	44% 9% 14% 20% 12% 1	2700
July	1st to 10th	Day	34% 9% 14% 22% 16% 5%	2626
	11th to 20th	Day	30% 8% 12% 22% 22% 6%	2462
	21st to end	Day	23% 5% 11% 25% 28% 8%	2589
August	1st to 10th	Day	30% 8% 13% 24% 19% 7%	2490
	11th to 20th	Day	24% 8% 13% 30% 22% 2	2413
	21st to end	Day	33% 9% 14% 29% 14% 2	2472
September	1st to 10th	Day	33% 8% 13% 27% 16% 4	2160
	11th to 20th	Day	43% 7% 12% 23% 14% 2	2066
	21st to end	Day	48% 9% 15% 17% 10% 1	1980
October	1st to 10th	Day	49% 8% 15% 23% 5%	1895
	11th to 20th	Day	53% 8% 15% 19% 6%	1670
	21st to end	Day	59% 10% 14% 13% 4 1	1782
November	1st to 10th	Day	69% 12% 9% 8% 2	1674
	11th to 20th	Day	68% 10% 9% 9% 3 1	1481
	21st to end	Day	65% 13% 13% 6% 3	1330
December	1st to 10th	Day	62% 11% 13% 10% 3	1330
	11th to 20th	Day	75% 11% 8% 4 2	1684
	21st to end	Day	69% 11% 8% 7% 5%	2442
January	1st to 10th	Night	80% 9% 6% 3 2	4570
	11th to 20th	Night	62% 8% 10% 13% 5% 2	4420
	21st to end	Night	80% 9% 5% 3 2	4862
February	1st to 10th	Night	67% 9% 12% 7% 3 1	2660
	11th to 20th	Night	60% 10% 15% 10% 5%	2373
	21st to end	Night	60% 12% 12% 10% 6% 1	1884
March	1st to 10th	Night	61% 12% 14% 8% 5%	2225
	11th to 20th	Night	60% 8% 12% 13% 6% 1	2111
	21st to end	Night	59% 10% 15% 10% 5%	2003
April	1st to 10th	Night	52% 10% 20% 13% 5%	1800
	11th to 20th	Night	52% 8% 16% 17% 6%	1687
	21st to end	Night	49% 9% 17% 19% 6%	1612
May	1st to 10th	Night	43% 11% 17% 20% 9%	1440
	11th to 20th	Night	43% 10% 20% 19% 7%	1416
	21st to end	Night	52% 11% 14% 19% 4	1392
June	1st to 10th	Night	48% 11% 15% 16% 8% 2	1260
	11th to 20th	Night	51% 11% 14% 17% 7%	1260
	21st to end	Night	34% 8% 18% 23% 15% 1	1260
July	1st to 10th	Night	25% 8% 18% 24% 19% 6%	1225
	11th to 20th	Night	20% 8% 14% 26% 23% 8%	1373
	21st to end	Night	14% 6% 12% 29% 32% 8%	1488
August	1st to 10th	Night	20% 7% 14% 29% 22% 8%	1424
	11th to 20th	Night	19% 6% 14% 36% 22% 4	1486
	21st to end	Night	22% 7% 16% 33% 19% 3	1880
September	1st to 10th	Night	25% 8% 15% 28% 20% 4	1800
	11th to 20th	Night	36% 7% 13% 25% 16% 3	1894
	21st to end	Night	38% 10% 21% 19% 11% 2	1980
October	1st to 10th	Night	45% 7% 16% 25% 7%	2063
	11th to 20th	Night	48% 8% 17% 19% 8%	2290
	21st to end	Night	57% 8% 13% 17% 4 1	2574
November	1st to 10th	Night	67% 11% 11% 8% 3	2405
	11th to 20th	Night	68% 10% 11% 7% 4	2674
	21st to end	Night	67% 10% 11% 10% 3	2850
December	1st to 10th	Night	66% 9% 11% 11% 3	2850
	11th to 20th	Night	78% 12% 5% 4 1	3507
	21st to end	Night	71% 13% 7% 6% 3	5214

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	26% 10% 31% 21% 9% 3	180
	11th to 20th	Day	19% 12% 29% 25% 12% 2	180
	21st to end	Day	23% 10% 34% 21% 10% 3	198
February	1st to 10th	Day	22% 15% 38% 19% 5% 1	180
	11th to 20th	Day	19% 17% 33% 20% 9% 1	351
	21st to end	Day	17% 19% 31% 25% 8% 1	382
March	1st to 10th	Day	26% 22% 26% 16% 8% 2	441
	11th to 20th	Day	25% 18% 23% 19% 14% 2	420
	21st to end	Day	26% 18% 27% 17% 10% 3	462
April	1st to 10th	Day	25% 18% 28% 20% 7% 2	420
	11th to 20th	Day	23% 21% 25% 19% 9% 4	412
	21st to end	Day	19% 16% 30% 22% 10% 3	420
May	1st to 10th	Day	15% 13% 33% 26% 11% 2	423
	11th to 20th	Day	20% 14% 24% 25% 14% 3	448
	21st to end	Day	23% 13% 18% 24% 17% 4	645
June	1st to 10th	Day	24% 14% 24% 24% 12% 3	630
	11th to 20th	Day	20% 15% 28% 22% 11% 3	630
	21st to end	Day	13% 10% 25% 28% 21% 3	630
July	1st to 10th	Day	9% 9% 26% 31% 22% 3	621
	11th to 20th	Day	9% 15% 34% 28% 12% 3	461
	21st to end	Day	7% 12% 32% 30% 15% 4	487
August	1st to 10th	Day	7% 10% 32% 30% 16% 6%	447
	11th to 20th	Day	5% 11% 31% 29% 17% 8%	437
	21st to end	Day	10% 13% 28% 28% 15% 6%	495
September	1st to 10th	Day	16% 13% 29% 23% 16% 4	450
	11th to 20th	Day	17% 14% 28% 25% 13% 4	450
	21st to end	Day	24% 17% 22% 22% 12% 3	450
October	1st to 10th	Day	20% 13% 30% 22% 13% 3	450
	11th to 20th	Day	24% 15% 23% 21% 13% 4	320
	21st to end	Day	28% 12% 33% 18% 6% 2	297
November	1st to 10th	Day	30% 14% 21% 26% 9% 1	270
	11th to 20th	Day	27% 13% 23% 24% 10% 2	194
	21st to end	Day	23% 12% 26% 21% 13% 6%	180
December	1st to 10th	Day	19% 12% 26% 22% 17% 5%	180
	11th to 20th	Day	17% 16% 29% 29% 8% 1	180
	21st to end	Day	18% 11% 27% 32% 11% 2	198
January	1st to 10th	Night	31% 11% 26% 24% 7% 1	859
	11th to 20th	Night	23% 9% 31% 27% 8% 2	760
	21st to end	Night	19% 9% 32% 31% 9% 1	836
February	1st to 10th	Night	24% 9% 31% 27% 8%	760
	11th to 20th	Night	22% 5% 34% 29% 10%	744
	21st to end	Night	17% 9% 30% 30% 12% 2	628
March	1st to 10th	Night	23% 9% 26% 27% 13% 2	742
	11th to 20th	Night	27% 9% 23% 26% 15% 1	720
	21st to end	Night	22% 8% 27% 29% 12% 2	792
April	1st to 10th	Night	19% 7% 24% 31% 17% 1	720
	11th to 20th	Night	21% 6% 26% 31% 15% 1	619
	21st to end	Night	17% 9% 25% 34% 14% 1	532
May	1st to 10th	Night	12% 5% 22% 38% 23%	360
	11th to 20th	Night	15% 4 23% 32% 25% 1	354
	21st to end	Night	16% 9% 22% 30% 23% 1	389
June	1st to 10th	Night	19% 6% 20% 35% 19% 1	360
	11th to 20th	Night	15% 5% 26% 34% 18% 1	360
	21st to end	Night	9% 5% 21% 40% 24% 2	360
July	1st to 10th	Night	6% 1 14% 45% 33% 1	350
	11th to 20th	Night	3 3 20% 45% 28% 2	348
	21st to end	Night	3 2 14% 45% 35% 1	394
August	1st to 10th	Night	5% 2 15% 36% 40% 3	358
	11th to 20th	Night	6% 1 15% 38% 35% 5%	418
	21st to end	Night	7% 4 17% 39% 31% 2	692
September	1st to 10th	Night	8% 2 22% 38% 26% 3	720
	11th to 20th	Night	13% 4 17% 31% 31% 4	720
	21st to end	Night	16% 6% 20% 38% 18% 3	720
October	1st to 10th	Night	18% 6% 21% 35% 19% 1	720
	11th to 20th	Night	23% 6% 20% 34% 15% 2	720
	21st to end	Night	24% 8% 29% 29% 7% 3	792
November	1st to 10th	Night	32% 9% 24% 30% 5%	740
	11th to 20th	Night	31% 10% 27% 20% 12%	807
	21st to end	Night	28% 8% 25% 26% 11% 2	950
December	1st to 10th	Night	24% 6% 33% 25% 11% 1	950
	11th to 20th	Night	25% 8% 31% 28% 8%	950
	21st to end	Night	25% 8% 34% 26% 6%	1045

Page generated at 2024-03-03 03:28:59. Version ac07

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