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 2024-11-02.

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	11% 14% 4 9% 5% 4 4 3 5% 6% 33%	101635
	Night	12% 17% 5% 9% 5% 4 4 3 5% 6% 30%	97382
Transparency	Day	49% 9% 12% 16% 10% 3	101397
	Night	52% 10% 13% 15% 8% 2	97267
Seeing	Day	18% 14% 28% 24% 13% 3	15011
	Night	20% 7% 25% 31% 16% 1	24009

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	12% 31% 12% 17% 5% 4 4 2 3 2 7%	2392
	11th to 20th	Day	11% 23% 5% 10% 5% 5% 4 3 5% 7% 21%	2516
	21st to end	Day	17% 37% 7% 8% 4 2 3 2 4 4 11%	2794
February	1st to 10th	Day	15% 23% 6% 11% 5% 4 4 4 5% 5% 18%	1520
	11th to 20th	Day	10% 26% 7% 11% 6% 4 3 3 4 5% 22%	1692
	21st to end	Day	10% 22% 8% 12% 7% 5% 6% 4 4 4 19%	1570
March	1st to 10th	Day	10% 22% 9% 16% 6% 4 4 3 4 4 20%	3138
	11th to 20th	Day	10% 19% 4 6% 3 3 3 2 4 6% 38%	3268
	21st to end	Day	12% 28% 7% 12% 6% 5% 4 3 4 4 15%	4127
April	1st to 10th	Day	14% 22% 5% 12% 6% 5% 4 4 5% 5% 18%	2280
	11th to 20th	Day	13% 17% 5% 12% 6% 6% 5% 3 5% 6% 22%	2350
	21st to end	Day	14% 16% 5% 12% 7% 5% 5% 3 5% 7% 21%	2475
May	1st to 10th	Day	10% 14% 4 11% 7% 7% 5% 3 6% 6% 27%	4420
	11th to 20th	Day	11% 13% 4 9% 7% 6% 6% 5% 7% 8% 26%	4377
	21st to end	Day	17% 14% 5% 12% 8% 7% 7% 5% 5% 5% 17%	5099
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	14% 10% 3 8% 6% 5% 5% 4 5% 6% 35%	2000
	11th to 20th	Day	18% 14% 4 8% 4 4 3 3 4 6% 32%	1762
	21st to end	Day	20% 18% 4 8% 5% 4 4 3 4 4 26%	1881
November	1st to 10th	Day	23% 22% 5% 9% 5% 4 4 3 4 5% 16%	1683
	11th to 20th	Day	21% 22% 5% 8% 5% 3 4 3 4 4 21%	1481
	21st to end	Day	16% 24% 5% 10% 6% 4 3 3 5% 6% 18%	1330
December	1st to 10th	Day	14% 19% 7% 11% 6% 3 4 3 6% 6% 21%	1330
	11th to 20th	Day	12% 31% 7% 12% 6% 5% 4 3 2 5% 13%	1682
	21st to end	Day	11% 27% 8% 12% 5% 4 4 3 3 4 18%	2442
January	1st to 10th	Night	15% 32% 12% 15% 5% 3 3 2 3 2 9%	4570
	11th to 20th	Night	12% 19% 6% 14% 5% 5% 4 4 5% 6% 19%	4382
	21st to end	Night	18% 32% 9% 11% 5% 4 3 2 3 5% 8%	4862
February	1st to 10th	Night	14% 23% 7% 11% 7% 3 4 4 4 5% 17%	2660
	11th to 20th	Night	13% 23% 7% 9% 4 4 4 3 4 7% 24%	2371
	21st to end	Night	9% 23% 6% 12% 6% 4 5% 3 5% 5% 21%	1884
March	1st to 10th	Night	10% 18% 10% 14% 5% 4 4 3 4 5% 23%	3736
	11th to 20th	Night	11% 21% 4 5% 3 3 2 2 3 5% 40%	3711
	21st to end	Night	12% 24% 6% 11% 7% 6% 5% 4 5% 5% 16%	3524
April	1st to 10th	Night	11% 20% 4 7% 5% 4 4 4 5% 8% 27%	1900
	11th to 20th	Night	13% 13% 4 8% 6% 5% 4 4 6% 7% 31%	1781
	21st to end	Night	12% 15% 4 8% 4 5% 4 4 6% 7% 31%	1701
May	1st to 10th	Night	9% 10% 4 8% 6% 6% 5% 5% 7% 7% 33%	2540
	11th to 20th	Night	8% 7% 3 7% 6% 4 5% 4 7% 10% 39%	2516
	21st to end	Night	18% 15% 4 9% 5% 6% 6% 3 6% 6% 22%	2476
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	13% 10% 3 7% 5% 4 4 4 5% 7% 39%	2178
	11th to 20th	Night	19% 10% 3 7% 4 4 3 4 5% 7% 34%	2418
	21st to end	Night	18% 16% 5% 9% 4 4 4 3 4 6% 27%	2717
November	1st to 10th	Night	24% 19% 6% 7% 4 4 5% 3 4 7% 16%	2418
	11th to 20th	Night	19% 24% 5% 10% 5% 4 3 3 5% 5% 18%	2674
	21st to end	Night	18% 23% 6% 10% 5% 3 5% 3 4 6% 16%	2850
December	1st to 10th	Night	17% 21% 7% 11% 6% 4 3 3 5% 6% 19%	2850
	11th to 20th	Night	15% 27% 8% 14% 6% 4 4 4 5% 4 8%	3510
	21st to end	Night	13% 26% 6% 13% 6% 4 5% 4 4 5% 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	64% 10% 8% 8% 9%	3138
	11th to 20th	Day	47% 7% 22% 21% 3	3268
	21st to end	Day	68% 12% 9% 8% 3	4127
April	1st to 10th	Day	66% 9% 13% 10% 3	2280
	11th to 20th	Day	61% 9% 13% 14% 3	2350
	21st to end	Day	62% 8% 13% 13% 3 1	2475
May	1st to 10th	Day	49% 14% 15% 18% 3	4420
	11th to 20th	Day	44% 14% 16% 22% 5%	4380
	21st to end	Day	58% 17% 13% 10% 2	5100
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	48% 8% 16% 23% 5%	2000
	11th to 20th	Day	53% 8% 15% 19% 6%	1762
	21st to end	Day	59% 10% 13% 13% 4 1	1881
November	1st to 10th	Day	69% 12% 8% 8% 2	1683
	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	55% 14% 14% 11% 6%	3736
	11th to 20th	Night	46% 7% 23% 21% 4	3712
	21st to end	Night	62% 15% 12% 8% 4	3524
April	1st to 10th	Night	52% 11% 20% 12% 5%	1900
	11th to 20th	Night	51% 9% 16% 18% 6%	1781
	21st to end	Night	48% 9% 17% 19% 7%	1701
May	1st to 10th	Night	43% 15% 15% 21% 6%	2540
	11th to 20th	Night	32% 12% 20% 27% 8%	2516
	21st to end	Night	52% 15% 14% 15% 4	2476
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	44% 7% 17% 26% 6%	2178
	11th to 20th	Night	47% 8% 17% 19% 8%	2418
	21st to end	Night	57% 9% 14% 16% 4	2717
November	1st to 10th	Night	67% 11% 11% 7% 3	2418
	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% 25% 17% 8% 2	460
	11th to 20th	Day	23% 17% 23% 19% 15% 3	460
	21st to end	Day	24% 18% 29% 17% 9% 2	506
April	1st to 10th	Day	25% 18% 28% 20% 7% 2	460
	11th to 20th	Day	21% 21% 25% 21% 9% 4	452
	21st to end	Day	18% 16% 30% 23% 10% 3	460
May	1st to 10th	Day	15% 13% 33% 26% 11% 2	463
	11th to 20th	Day	19% 15% 23% 26% 14% 3	488
	21st to end	Day	22% 14% 20% 24% 16% 4	698
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% 12% 34% 18% 6% 2	330
November	1st to 10th	Day	30% 14% 21% 26% 8% 1	273
	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	22% 10% 25% 28% 13% 1	760
	11th to 20th	Night	26% 9% 22% 26% 16% 1	760
	21st to end	Night	22% 8% 27% 30% 12% 2	836
April	1st to 10th	Night	20% 8% 24% 31% 16% 1	760
	11th to 20th	Night	20% 7% 26% 32% 14% 1	653
	21st to end	Night	17% 9% 25% 34% 13% 1	561
May	1st to 10th	Night	12% 5% 22% 38% 23%	380
	11th to 20th	Night	14% 4 24% 33% 24% 1	374
	21st to end	Night	16% 10% 21% 29% 23% 1	411
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% 8% 2	836
November	1st to 10th	Night	32% 9% 24% 30% 5%	744
	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

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