Nike Site Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nike Site Park clear sky chart from 2005-11-06 to 2025-10-04.

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	17% 9% 3 8% 5% 5% 4 4 5% 7% 33%	131028
	Night	19% 10% 3 8% 5% 5% 4 3 5% 6% 32%	127039
Transparency	Day	30% 40% 10% 13% 6% 1	130743
	Night	34% 40% 9% 12% 5% 1	126901
Seeing	Day	20% 10% 20% 25% 18% 6%	16295
	Night	25% 8% 17% 28% 20% 3	24588

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	32% 13% 4 12% 6% 5% 5% 4 5% 4 10%	3251
	11th to 20th	Day	19% 9% 5% 14% 7% 5% 5% 4 5% 6% 21%	3616
	21st to end	Day	28% 12% 4 9% 5% 5% 4 3 4 5% 22%	4165
February	1st to 10th	Day	26% 14% 4 9% 7% 5% 5% 4 5% 6% 15%	1800
	11th to 20th	Day	27% 13% 5% 10% 6% 4 3 3 5% 5% 19%	1755
	21st to end	Day	25% 12% 3 8% 5% 6% 5% 4 5% 5% 21%	1547
March	1st to 10th	Day	18% 12% 4 11% 7% 6% 6% 5% 7% 7% 17%	4648
	11th to 20th	Day	19% 13% 4 9% 5% 5% 4 4 4 6% 27%	5000
	21st to end	Day	21% 12% 4 11% 8% 7% 6% 4 6% 6% 15%	5652
April	1st to 10th	Day	22% 11% 4 9% 7% 5% 5% 5% 7% 7% 19%	2400
	11th to 20th	Day	18% 11% 3 8% 6% 5% 5% 4 5% 7% 29%	2401
	21st to end	Day	16% 11% 4 9% 6% 5% 5% 4 6% 7% 27%	2630
May	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 6% 7% 42%	6320
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 4 6% 7% 30%	6277
	21st to end	Day	15% 9% 3 8% 5% 4 4 3 7% 9% 32%	6903
June	1st to 10th	Day	16% 9% 2 7% 5% 5% 6% 4 7% 9% 31%	2840
	11th to 20th	Day	15% 10% 4 9% 7% 6% 7% 5% 7% 8% 24%	3000
	21st to end	Day	12% 7% 3 7% 5% 4 6% 5% 6% 7% 40%	3000
July	1st to 10th	Day	6% 4 2 4 3 3 3 3 5% 6% 62%	6780
	11th to 20th	Day	4 3 1 4 3 2 2 2 4 5% 68%	6467
	21st to end	Day	8% 5% 2 5% 3 3 3 3 5% 7% 56%	6874
August	1st to 10th	Day	4 5% 1 5% 3 3 3 3 5% 7% 59%	4427
	11th to 20th	Day	10% 7% 2 7% 5% 4 4 3 6% 8% 43%	4156
	21st to end	Day	14% 9% 3 7% 3 4 4 3 4 6% 42%	4477
September	1st to 10th	Day	9% 6% 2 6% 4 4 4 4 5% 7% 50%	2400
	11th to 20th	Day	12% 8% 2 8% 5% 4 5% 4 6% 7% 39%	2400
	21st to end	Day	15% 7% 3 7% 6% 4 5% 3 6% 7% 37%	2111
October	1st to 10th	Day	13% 8% 3 6% 5% 5% 6% 4 8% 10% 34%	3211
	11th to 20th	Day	24% 10% 3 9% 5% 5% 4 3 5% 8% 25%	3161
	21st to end	Day	25% 11% 4 9% 5% 5% 4 3 6% 7% 21%	3270
November	1st to 10th	Day	29% 12% 4 8% 4 5% 4 5% 4 6% 18%	1608
	11th to 20th	Day	29% 12% 3 9% 5% 5% 4 3 3 5% 21%	1600
	21st to end	Day	28% 11% 3 9% 6% 4 5% 4 6% 6% 18%	1600
December	1st to 10th	Day	21% 7% 3 12% 9% 6% 5% 5% 5% 7% 18%	2620
	11th to 20th	Day	32% 12% 4 11% 7% 5% 4 4 5% 4 11%	3025
	21st to end	Day	30% 14% 5% 13% 6% 5% 5% 4 4 5% 11%	3636
January	1st to 10th	Night	31% 15% 5% 12% 7% 5% 4 3 4 3 10%	6660
	11th to 20th	Night	19% 9% 4 9% 6% 5% 5% 4 5% 6% 29%	6282
	21st to end	Night	26% 11% 4 9% 6% 6% 4 3 4 5% 23%	6798
February	1st to 10th	Night	27% 13% 4 8% 6% 4 4 3 4 6% 21%	2600
	11th to 20th	Night	24% 11% 3 9% 5% 4 4 3 5% 7% 26%	2532
	21st to end	Night	24% 12% 3 8% 4 5% 5% 4 5% 6% 23%	2082
March	1st to 10th	Night	17% 12% 4 10% 6% 6% 6% 5% 7% 8% 20%	5244
	11th to 20th	Night	21% 10% 3 7% 5% 4 4 4 5% 5% 32%	4999
	21st to end	Night	25% 14% 4 10% 5% 5% 5% 4 6% 5% 16%	5322
April	1st to 10th	Night	19% 10% 4 9% 5% 5% 5% 5% 6% 7% 24%	2000
	11th to 20th	Night	14% 9% 4 8% 5% 4 5% 5% 6% 7% 33%	1947
	21st to end	Night	17% 10% 3 7% 6% 5% 4 3 7% 7% 31%	1770
May	1st to 10th	Night	13% 8% 3 6% 4 5% 4 3 4 4 45%	3640
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 5% 7% 7% 35%	3616
	21st to end	Night	7% 9% 3 9% 6% 6% 7% 5% 7% 9% 32%	3972
June	1st to 10th	Night	13% 7% 3 8% 5% 4 5% 4 5% 7% 39%	1555
	11th to 20th	Night	12% 7% 3 8% 6% 6% 6% 4 6% 10% 33%	1400
	21st to end	Night	6% 7% 2 7% 5% 5% 4 4 6% 7% 47%	1400
July	1st to 10th	Night	5% 5% 2 4 4 3 3 2 5% 5% 62%	3180
	11th to 20th	Night	2 2 1 2 2 2 3 3 3 4 77%	3492
	21st to end	Night	5% 3 2 5% 3 3 3 4 5% 6% 61%	3969
August	1st to 10th	Night	2 2 1 4 3 3 2 2 4 6% 70%	2724
	11th to 20th	Night	8% 6% 3 7% 3 4 4 3 5% 7% 52%	2902
	21st to end	Night	13% 6% 2 6% 4 3 3 3 5% 7% 48%	3438
September	1st to 10th	Night	7% 4 2 3 3 4 4 3 4 7% 59%	2000
	11th to 20th	Night	13% 7% 3 6% 4 3 4 3 5% 7% 45%	2000
	21st to end	Night	14% 7% 3 6% 5% 4 4 3 4 7% 42%	2286
October	1st to 10th	Night	12% 7% 2 7% 5% 4 5% 4 5% 9% 41%	3836
	11th to 20th	Night	23% 9% 3 7% 4 4 4 3 5% 6% 31%	3777
	21st to end	Night	25% 11% 3 8% 5% 4 4 3 5% 6% 26%	4292
November	1st to 10th	Night	30% 9% 3 7% 6% 5% 5% 4 5% 6% 23%	2687
	11th to 20th	Night	29% 11% 3 8% 5% 4 4 3 4 5% 24%	2800
	21st to end	Night	27% 11% 3 8% 6% 5% 5% 4 5% 6% 21%	2800
December	1st to 10th	Night	20% 10% 3 13% 8% 6% 4 3 4 6% 23%	4560
	11th to 20th	Night	32% 14% 5% 11% 6% 6% 4 3 3 4 11%	5180
	21st to end	Night	31% 15% 5% 11% 6% 5% 4 3 4 4 12%	7297

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	59% 28% 6% 5% 1	3251
	11th to 20th	Day	42% 29% 10% 14% 4	3640
	21st to end	Day	45% 29% 7% 12% 7%	4165
February	1st to 10th	Day	28% 62% 4 3 3 1	1800
	11th to 20th	Day	31% 61% 2 3 3	1764
	21st to end	Day	25% 63% 3 6% 3	1547
March	1st to 10th	Day	43% 32% 11% 12% 3	4648
	11th to 20th	Day	40% 31% 10% 17% 2	5000
	21st to end	Day	50% 31% 10% 8% 1	5652
April	1st to 10th	Day	24% 65% 5% 6%	2400
	11th to 20th	Day	18% 65% 6% 8% 3	2401
	21st to end	Day	20% 63% 7% 7% 3	2630
May	1st to 10th	Day	27% 30% 11% 21% 10% 1	6320
	11th to 20th	Day	36% 33% 14% 15% 3	6280
	21st to end	Day	32% 31% 19% 14% 3	6904
June	1st to 10th	Day	14% 64% 9% 9% 3	2840
	11th to 20th	Day	19% 64% 6% 8% 3	3000
	21st to end	Day	16% 63% 8% 10% 3	3000
July	1st to 10th	Day	14% 28% 13% 17% 16% 12%	6706
	11th to 20th	Day	10% 27% 10% 24% 25% 5%	6388
	21st to end	Day	20% 28% 17% 27% 8% 1	6681
August	1st to 10th	Day	12% 36% 7% 25% 16% 3	4410
	11th to 20th	Day	26% 41% 13% 15% 4 1	4195
	21st to end	Day	30% 38% 6% 18% 8%	4476
September	1st to 10th	Day	18% 59% 9% 9% 4 1	2400
	11th to 20th	Day	20% 60% 5% 7% 7%	2400
	21st to end	Day	19% 59% 9% 10% 4	2111
October	1st to 10th	Day	24% 41% 12% 20% 3	3211
	11th to 20th	Day	38% 39% 9% 9% 6%	3161
	21st to end	Day	40% 39% 8% 7% 5% 1	3270
November	1st to 10th	Day	31% 58% 6% 5% 1	1608
	11th to 20th	Day	27% 58% 5% 6% 3 1	1600
	21st to end	Day	25% 59% 4 6% 5%	1600
December	1st to 10th	Day	38% 40% 5% 10% 6%	2620
	11th to 20th	Day	53% 34% 7% 6% 1	3028
	21st to end	Day	56% 28% 10% 4 2	3636
January	1st to 10th	Night	62% 28% 5% 4 1 1	6660
	11th to 20th	Night	36% 31% 8% 20% 5%	6320
	21st to end	Night	44% 31% 8% 14% 4	6798
February	1st to 10th	Night	27% 62% 4 3 3	2600
	11th to 20th	Night	28% 62% 4 4 2 1	2534
	21st to end	Night	22% 63% 6% 6% 2	2082
March	1st to 10th	Night	40% 33% 11% 12% 4	5244
	11th to 20th	Night	36% 30% 13% 19% 1	5000
	21st to end	Night	51% 32% 10% 8%	5322
April	1st to 10th	Night	22% 66% 6% 6%	2000
	11th to 20th	Night	18% 64% 7% 6% 3	1947
	21st to end	Night	19% 63% 7% 8% 2	1770
May	1st to 10th	Night	29% 29% 7% 27% 6%	3637
	11th to 20th	Night	33% 35% 14% 15% 3	3616
	21st to end	Night	36% 33% 17% 13% 1	3972
June	1st to 10th	Night	14% 63% 8% 11% 3	1555
	11th to 20th	Night	16% 64% 8% 11% 2	1400
	21st to end	Night	14% 62% 10% 11% 3	1400
July	1st to 10th	Night	17% 29% 13% 16% 14% 12%	3145
	11th to 20th	Night	7% 27% 12% 20% 31% 3	3448
	21st to end	Night	18% 28% 18% 26% 9% 1	3857
August	1st to 10th	Night	10% 36% 7% 28% 18% 1	2716
	11th to 20th	Night	23% 41% 18% 13% 4 1	2929
	21st to end	Night	26% 38% 8% 19% 7% 1	3438
September	1st to 10th	Night	15% 59% 10% 11% 6%	2000
	11th to 20th	Night	20% 59% 5% 9% 6% 1	2000
	21st to end	Night	19% 59% 7% 12% 3	2286
October	1st to 10th	Night	23% 40% 15% 21% 1	3836
	11th to 20th	Night	36% 39% 9% 13% 3	3777
	21st to end	Night	40% 39% 7% 10% 5%	4292
November	1st to 10th	Night	28% 58% 6% 6% 1	2687
	11th to 20th	Night	28% 58% 4 5% 3 2	2800
	21st to end	Night	26% 58% 4 5% 6% 1	2800
December	1st to 10th	Night	39% 40% 6% 12% 4	4560
	11th to 20th	Night	56% 33% 6% 5% 1	5176
	21st to end	Night	56% 29% 7% 6% 1	7297

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	35% 13% 18% 19% 12% 5%	200
	11th to 20th	Day	30% 14% 18% 19% 15% 5%	200
	21st to end	Day	32% 12% 22% 15% 15% 4	290
February	1st to 10th	Day	36% 14% 16% 19% 11% 4	400
	11th to 20th	Day	39% 12% 20% 18% 9% 2	395
	21st to end	Day	32% 17% 21% 21% 9% 1	363
March	1st to 10th	Day	26% 15% 24% 22% 10% 1	500
	11th to 20th	Day	27% 16% 17% 22% 13% 5%	500
	21st to end	Day	29% 15% 25% 20% 9% 2	550
April	1st to 10th	Day	29% 16% 20% 22% 10% 4	500
	11th to 20th	Day	21% 15% 29% 21% 9% 6%	492
	21st to end	Day	19% 13% 27% 24% 15% 2	500
May	1st to 10th	Day	15% 11% 24% 30% 15% 5%	503
	11th to 20th	Day	21% 9% 22% 22% 18% 7%	528
	21st to end	Day	18% 9% 21% 28% 17% 6%	573
June	1st to 10th	Day	19% 9% 22% 26% 18% 7%	570
	11th to 20th	Day	17% 13% 22% 28% 16% 3	730
	21st to end	Day	11% 7% 19% 34% 22% 7%	730
July	1st to 10th	Day	9% 4 17% 33% 28% 8%	721
	11th to 20th	Day	6% 4 15% 35% 30% 10%	589
	21st to end	Day	6% 3 12% 30% 38% 12%	575
August	1st to 10th	Day	8% 4 12% 30% 34% 12%	527
	11th to 20th	Day	8% 4 11% 31% 32% 14%	517
	21st to end	Day	12% 8% 18% 30% 24% 8%	583
September	1st to 10th	Day	10% 5% 21% 33% 24% 7%	530
	11th to 20th	Day	15% 8% 18% 28% 23% 8%	530
	21st to end	Day	18% 8% 22% 29% 19% 4	530
October	1st to 10th	Day	21% 12% 19% 22% 19% 7%	503
	11th to 20th	Day	26% 12% 16% 24% 17% 6%	490
	21st to end	Day	30% 14% 19% 19% 12% 5%	436
November	1st to 10th	Day	34% 13% 20% 20% 11% 3	220
	11th to 20th	Day	33% 16% 23% 12% 10% 8%	200
	21st to end	Day	32% 10% 22% 18% 9% 11%	200
December	1st to 10th	Day	35% 14% 16% 14% 15% 9%	200
	11th to 20th	Day	35% 19% 16% 21% 8% 2	200
	21st to end	Day	39% 10% 20% 16% 12% 2	220
January	1st to 10th	Night	41% 11% 17% 17% 13% 2	879
	11th to 20th	Night	34% 10% 13% 25% 17% 2	800
	21st to end	Night	31% 9% 16% 25% 16% 3	880
February	1st to 10th	Night	34% 12% 17% 22% 13% 2	800
	11th to 20th	Night	30% 11% 19% 24% 14% 2	784
	21st to end	Night	33% 11% 20% 22% 12% 2	660
March	1st to 10th	Night	28% 12% 19% 25% 15% 2	800
	11th to 20th	Night	29% 12% 19% 25% 14% 2	800
	21st to end	Night	30% 12% 20% 26% 10% 1	880
April	1st to 10th	Night	26% 9% 22% 28% 14% 2	800
	11th to 20th	Night	19% 10% 23% 30% 14% 3	759
	21st to end	Night	22% 9% 22% 32% 13% 2	570
May	1st to 10th	Night	17% 11% 18% 36% 18% 1	400
	11th to 20th	Night	15% 7% 19% 31% 25% 3	394
	21st to end	Night	13% 7% 18% 35% 24% 3	433
June	1st to 10th	Night	13% 7% 25% 32% 23% 1	400
	11th to 20th	Night	15% 8% 25% 34% 18% 1	400
	21st to end	Night	10% 7% 16% 40% 26% 2	400
July	1st to 10th	Night	7% 3 15% 38% 35% 2	390
	11th to 20th	Night	6% 3 11% 46% 31% 3	388
	21st to end	Night	5% 3 10% 37% 41% 3	438
August	1st to 10th	Night	5% 2 10% 34% 41% 7%	461
	11th to 20th	Night	8% 2 9% 29% 46% 6%	594
	21st to end	Night	12% 2 13% 34% 34% 4	775
September	1st to 10th	Night	11% 2 14% 37% 32% 5%	800
	11th to 20th	Night	15% 5% 14% 27% 33% 7%	800
	21st to end	Night	18% 6% 16% 35% 22% 3	800
October	1st to 10th	Night	19% 5% 15% 29% 27% 4	772
	11th to 20th	Night	26% 7% 15% 27% 21% 4	760
	21st to end	Night	28% 9% 20% 25% 16% 2	836
November	1st to 10th	Night	33% 9% 20% 26% 12%	780
	11th to 20th	Night	35% 11% 19% 21% 12% 1	800
	21st to end	Night	31% 9% 20% 22% 15% 3	800
December	1st to 10th	Night	30% 9% 16% 26% 18% 1	800
	11th to 20th	Night	35% 11% 19% 24% 11% 2	800
	21st to end	Night	42% 11% 18% 19% 9% 1	955

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