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 2024-04-25.

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	19% 9% 4 7% 6% 4 5% 5% 5% 7% 30%	83334
	Night	21% 9% 4 7% 6% 3 5% 4 5% 6% 30%	85757
Transparency	Day	39% 33% 11% 13% 4 1	83052
	Night	41% 32% 10% 12% 4 1	85619
Seeing	Day	21% 10% 19% 25% 18% 6%	14326
	Night	25% 8% 17% 27% 20% 3	22921

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	34% 13% 4 9% 7% 4 5% 5% 4 4 12%	2263
	11th to 20th	Day	26% 9% 4 9% 6% 3 4 4 4 7% 23%	2516
	21st to end	Day	37% 13% 5% 9% 6% 3 5% 4 4 4 10%	2899
February	1st to 10th	Day	28% 13% 5% 8% 7% 4 5% 4 4 6% 15%	1710
	11th to 20th	Day	27% 12% 5% 9% 7% 3 3 5% 5% 5% 20%	1665
	21st to end	Day	24% 12% 4 7% 6% 4 6% 5% 5% 6% 21%	1472
March	1st to 10th	Day	17% 10% 4 9% 10% 5% 7% 6% 6% 7% 18%	3229
	11th to 20th	Day	18% 9% 3 6% 6% 3 5% 4 4 6% 36%	3490
	21st to end	Day	20% 11% 5% 8% 9% 5% 6% 6% 6% 7% 19%	3939
April	1st to 10th	Day	23% 10% 5% 8% 8% 3 5% 6% 6% 7% 20%	2280
	11th to 20th	Day	19% 10% 4 7% 6% 4 5% 5% 5% 7% 27%	2279
	21st to end	Day	17% 10% 5% 8% 6% 4 5% 5% 6% 7% 27%	2421
May	1st to 10th	Day	14% 8% 4 8% 6% 4 6% 5% 6% 7% 34%	2520
	11th to 20th	Day	16% 9% 3 6% 6% 4 6% 6% 6% 8% 29%	2477
	21st to end	Day	18% 10% 4 8% 7% 4 5% 4 5% 7% 28%	2723
June	1st to 10th	Day	16% 8% 3 7% 6% 4 6% 5% 6% 9% 28%	2556
	11th to 20th	Day	15% 10% 4 8% 8% 4 7% 6% 6% 8% 23%	2700
	21st to end	Day	12% 6% 3 6% 5% 3 5% 6% 6% 7% 41%	2700
July	1st to 10th	Day	8% 5% 3 6% 5% 3 5% 5% 5% 7% 49%	2700
	11th to 20th	Day	9% 5% 2 4 4 3 5% 4 6% 8% 51%	2583
	21st to end	Day	6% 4 2 3 3 2 3 4 5% 5% 63%	2741
August	1st to 10th	Day	6% 6% 2 5% 4 3 4 4 5% 7% 54%	2449
	11th to 20th	Day	7% 5% 2 5% 4 2 3 4 6% 7% 57%	2298
	21st to end	Day	9% 8% 3 5% 5% 3 4 5% 5% 8% 46%	2471
September	1st to 10th	Day	9% 6% 2 5% 5% 3 4 4 5% 7% 50%	2160
	11th to 20th	Day	11% 7% 3 7% 6% 3 5% 5% 6% 7% 40%	2160
	21st to end	Day	16% 7% 3 6% 7% 3 4 4 6% 7% 37%	1901
October	1st to 10th	Day	17% 10% 4 6% 6% 3 4 5% 7% 7% 31%	1800
	11th to 20th	Day	21% 9% 3 9% 5% 3 5% 4 6% 7% 27%	1800
	21st to end	Day	28% 10% 3 7% 5% 4 4 4 6% 6% 23%	1883
November	1st to 10th	Day	30% 11% 5% 7% 5% 4 5% 5% 4 6% 19%	1526
	11th to 20th	Day	30% 11% 4 7% 6% 3 5% 4 3 5% 22%	1520
	21st to end	Day	28% 10% 3 8% 6% 3 5% 5% 7% 6% 19%	1520
December	1st to 10th	Day	28% 9% 4 8% 7% 3 4 4 5% 6% 22%	1520
	11th to 20th	Day	31% 12% 6% 9% 9% 4 5% 4 4 4 12%	1925
	21st to end	Day	26% 12% 5% 12% 9% 4 6% 5% 3 5% 12%	2538
January	1st to 10th	Night	33% 14% 4 9% 8% 4 5% 4 4 4 12%	4662
	11th to 20th	Night	27% 11% 4 6% 5% 2 4 4 5% 7% 26%	4382
	21st to end	Night	34% 12% 5% 9% 7% 4 4 3 4 6% 12%	4760
February	1st to 10th	Night	28% 12% 4 7% 7% 3 4 4 4 6% 21%	2470
	11th to 20th	Night	23% 11% 4 8% 6% 3 5% 4 4 7% 26%	2402
	21st to end	Night	24% 11% 4 7% 5% 4 5% 5% 5% 6% 23%	1981
March	1st to 10th	Night	19% 13% 4 10% 8% 5% 6% 6% 6% 6% 17%	3643
	11th to 20th	Night	17% 7% 3 6% 5% 3 4 4 5% 6% 42%	3489
	21st to end	Night	23% 14% 4 9% 7% 4 5% 5% 5% 5% 18%	3713
April	1st to 10th	Night	20% 10% 4 8% 6% 4 5% 6% 6% 7% 25%	1900
	11th to 20th	Night	15% 8% 4 8% 6% 3 5% 6% 5% 8% 32%	1849
	21st to end	Night	17% 9% 4 6% 7% 3 5% 4 7% 6% 31%	1639
May	1st to 10th	Night	13% 8% 3 6% 7% 4 6% 5% 6% 6% 36%	1440
	11th to 20th	Night	13% 6% 2 6% 6% 4 6% 6% 7% 8% 35%	1416
	21st to end	Night	10% 7% 4 8% 8% 4 7% 6% 5% 6% 34%	1552
June	1st to 10th	Night	13% 6% 3 8% 6% 4 4 6% 5% 7% 38%	1400
	11th to 20th	Night	13% 7% 3 7% 6% 5% 6% 6% 6% 10% 32%	1260
	21st to end	Night	6% 6% 3 6% 5% 3 4 5% 5% 7% 49%	1260
July	1st to 10th	Night	6% 5% 2 4 5% 2 4 3 5% 6% 57%	1260
	11th to 20th	Night	5% 4 2 4 4 2 3 4 3 6% 63%	1376
	21st to end	Night	4 2 1 3 3 1 3 3 4 5% 70%	1576
August	1st to 10th	Night	3 3 2 4 4 2 4 3 5% 8% 61%	1488
	11th to 20th	Night	5% 3 1 3 3 2 3 3 4 7% 68%	1602
	21st to end	Night	8% 4 2 4 4 3 4 5% 6% 8% 53%	1885
September	1st to 10th	Night	7% 4 2 3 4 3 4 3 4 7% 60%	1800
	11th to 20th	Night	12% 6% 3 5% 6% 2 4 4 5% 7% 46%	1800
	21st to end	Night	14% 7% 3 5% 6% 4 4 4 4 7% 42%	2056
October	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 4 5% 7% 39%	2160
	11th to 20th	Night	22% 8% 3 7% 6% 3 4 4 4 6% 34%	2160
	21st to end	Night	24% 9% 2 6% 5% 3 4 4 5% 6% 32%	2472
November	1st to 10th	Night	30% 8% 3 6% 6% 3 5% 5% 5% 6% 23%	2549
	11th to 20th	Night	28% 10% 4 7% 6% 3 5% 4 4 5% 25%	2660
	21st to end	Night	27% 10% 3 7% 6% 3 6% 5% 5% 6% 22%	2660
December	1st to 10th	Night	26% 10% 4 8% 5% 3 5% 4 4 6% 24%	2660
	11th to 20th	Night	27% 12% 5% 9% 8% 5% 5% 5% 4 4 15%	3280
	21st to end	Night	28% 13% 6% 10% 8% 4 5% 4 4 4 15%	5095

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	64% 24% 6% 6%	2263
	11th to 20th	Day	49% 25% 10% 13% 3 1	2540
	21st to end	Day	65% 21% 7% 5% 2	2899
February	1st to 10th	Day	45% 36% 10% 7% 2	1710
	11th to 20th	Day	43% 35% 8% 10% 4	1674
	21st to end	Day	47% 35% 8% 7% 3	1472
March	1st to 10th	Day	50% 23% 14% 12% 2	3229
	11th to 20th	Day	40% 22% 13% 23% 2	3490
	21st to end	Day	55% 23% 10% 11% 1	3939
April	1st to 10th	Day	46% 35% 11% 8% 1	2280
	11th to 20th	Day	39% 34% 9% 11% 6% 1	2279
	21st to end	Day	40% 36% 10% 10% 3 1	2421
May	1st to 10th	Day	37% 36% 12% 12% 3	2520
	11th to 20th	Day	32% 38% 15% 11% 4	2480
	21st to end	Day	42% 37% 10% 9% 2	2724
June	1st to 10th	Day	39% 36% 12% 9% 4	2556
	11th to 20th	Day	40% 38% 13% 8% 2	2700
	21st to end	Day	27% 36% 17% 14% 5% 2	2700
July	1st to 10th	Day	21% 38% 11% 19% 9% 1	2626
	11th to 20th	Day	21% 37% 9% 22% 11% 1	2504
	21st to end	Day	15% 37% 9% 24% 12% 3	2589
August	1st to 10th	Day	21% 36% 13% 22% 8% 1	2432
	11th to 20th	Day	19% 35% 14% 20% 10% 3	2298
	21st to end	Day	23% 37% 10% 22% 7% 1	2471
September	1st to 10th	Day	19% 35% 12% 21% 9% 4	2160
	11th to 20th	Day	27% 37% 11% 15% 8% 2	2160
	21st to end	Day	31% 36% 12% 13% 7% 2	1901
October	1st to 10th	Day	34% 37% 9% 14% 5%	1800
	11th to 20th	Day	39% 36% 11% 11% 3 1	1800
	21st to end	Day	40% 37% 8% 10% 4	1883
November	1st to 10th	Day	42% 39% 7% 7% 4	1526
	11th to 20th	Day	43% 40% 7% 7% 3	1520
	21st to end	Day	44% 40% 6% 8% 2	1520
December	1st to 10th	Day	40% 39% 8% 8% 4	1520
	11th to 20th	Day	54% 31% 7% 7% 1	1928
	21st to end	Day	57% 25% 12% 6% 1	2538
January	1st to 10th	Night	64% 25% 6% 5%	4662
	11th to 20th	Night	44% 25% 7% 19% 3 1	4420
	21st to end	Night	62% 21% 7% 6% 2 1	4760
February	1st to 10th	Night	46% 35% 9% 8% 3	2470
	11th to 20th	Night	39% 36% 10% 9% 6% 1	2404
	21st to end	Night	45% 35% 8% 8% 4	1981
March	1st to 10th	Night	51% 23% 11% 13% 2	3643
	11th to 20th	Night	35% 22% 18% 22% 2	3490
	21st to end	Night	59% 22% 11% 8%	3713
April	1st to 10th	Night	43% 37% 11% 8% 2	1900
	11th to 20th	Night	34% 37% 11% 11% 6% 2	1849
	21st to end	Night	38% 36% 12% 9% 4 1	1639
May	1st to 10th	Night	36% 37% 12% 12% 4	1440
	11th to 20th	Night	31% 37% 14% 12% 5% 1	1416
	21st to end	Night	37% 38% 12% 11% 2	1552
June	1st to 10th	Night	34% 38% 11% 14% 4	1400
	11th to 20th	Night	34% 39% 17% 9% 1	1260
	21st to end	Night	25% 35% 18% 13% 6% 3	1260
July	1st to 10th	Night	17% 38% 13% 21% 8% 4	1225
	11th to 20th	Night	16% 37% 9% 25% 11% 1	1332
	21st to end	Night	9% 38% 10% 24% 16% 4	1488
August	1st to 10th	Night	16% 37% 13% 22% 11% 1	1480
	11th to 20th	Night	14% 36% 13% 21% 12% 4	1602
	21st to end	Night	21% 38% 11% 21% 9% 1	1885
September	1st to 10th	Night	16% 37% 11% 22% 11% 5%	1800
	11th to 20th	Night	26% 36% 10% 16% 11% 3	1800
	21st to end	Night	28% 36% 11% 15% 8% 2	2056
October	1st to 10th	Night	31% 37% 10% 15% 7% 1	2160
	11th to 20th	Night	35% 37% 10% 14% 4	2160
	21st to end	Night	34% 37% 10% 12% 7%	2472
November	1st to 10th	Night	40% 41% 8% 7% 3 1	2549
	11th to 20th	Night	41% 39% 9% 8% 3	2660
	21st to end	Night	41% 40% 9% 7% 3	2660
December	1st to 10th	Night	40% 40% 8% 9% 4	2660
	11th to 20th	Night	51% 32% 10% 6% 1	3276
	21st to end	Night	58% 25% 7% 8% 2	5095

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% 12% 16% 19% 12% 6%	180
	11th to 20th	Day	33% 14% 18% 16% 14% 5%	180
	21st to end	Day	34% 13% 22% 15% 14% 3	264
February	1st to 10th	Day	38% 13% 15% 19% 11% 4	370
	11th to 20th	Day	40% 13% 19% 18% 9% 2	365
	21st to end	Day	31% 17% 21% 20% 9% 1	336
March	1st to 10th	Day	27% 15% 25% 22% 10% 1	460
	11th to 20th	Day	28% 15% 16% 22% 14% 5%	460
	21st to end	Day	28% 15% 25% 20% 10% 2	506
April	1st to 10th	Day	29% 16% 20% 21% 10% 3	460
	11th to 20th	Day	22% 15% 30% 19% 8% 6%	452
	21st to end	Day	19% 14% 28% 23% 14% 1	437
May	1st to 10th	Day	15% 12% 23% 30% 15% 5%	423
	11th to 20th	Day	21% 7% 21% 23% 19% 7%	448
	21st to end	Day	18% 8% 20% 28% 18% 7%	485
June	1st to 10th	Day	20% 9% 22% 24% 18% 7%	486
	11th to 20th	Day	17% 12% 23% 28% 17% 3	630
	21st to end	Day	10% 7% 19% 34% 23% 7%	630
July	1st to 10th	Day	11% 4 16% 32% 29% 9%	621
	11th to 20th	Day	7% 5% 15% 34% 31% 9%	503
	21st to end	Day	5% 3 12% 30% 39% 11%	487
August	1st to 10th	Day	8% 4 13% 27% 34% 14%	447
	11th to 20th	Day	7% 3 8% 31% 34% 16%	437
	21st to end	Day	11% 8% 19% 30% 23% 8%	495
September	1st to 10th	Day	11% 6% 24% 33% 21% 6%	450
	11th to 20th	Day	15% 8% 17% 27% 24% 9%	450
	21st to end	Day	18% 10% 22% 28% 19% 4	450
October	1st to 10th	Day	21% 12% 18% 22% 19% 8%	450
	11th to 20th	Day	26% 11% 15% 25% 17% 6%	450
	21st to end	Day	32% 15% 17% 19% 12% 5%	398
November	1st to 10th	Day	35% 13% 19% 20% 11% 3	198
	11th to 20th	Day	35% 14% 21% 11% 11% 8%	180
	21st to end	Day	33% 10% 23% 14% 9% 11%	180
December	1st to 10th	Day	37% 14% 17% 12% 12% 7%	180
	11th to 20th	Day	34% 19% 16% 20% 9% 2	180
	21st to end	Day	37% 10% 21% 17% 13% 2	198
January	1st to 10th	Night	42% 11% 16% 16% 13% 2	832
	11th to 20th	Night	36% 10% 13% 23% 16% 2	760
	21st to end	Night	32% 9% 16% 25% 16% 3	836
February	1st to 10th	Night	34% 11% 17% 22% 14% 2	760
	11th to 20th	Night	29% 11% 19% 24% 15% 2	744
	21st to end	Night	33% 11% 21% 21% 12% 2	628
March	1st to 10th	Night	28% 12% 19% 24% 15% 2	760
	11th to 20th	Night	28% 12% 19% 24% 15% 2	760
	21st to end	Night	29% 12% 20% 27% 10% 1	836
April	1st to 10th	Night	26% 9% 22% 27% 14% 2	760
	11th to 20th	Night	20% 10% 23% 29% 15% 3	721
	21st to end	Night	22% 8% 23% 34% 12% 1	529
May	1st to 10th	Night	18% 11% 18% 34% 18% 1	360
	11th to 20th	Night	16% 5% 18% 32% 26% 3	354
	21st to end	Night	13% 6% 17% 35% 25% 3	389
June	1st to 10th	Night	13% 6% 26% 31% 23% 1	360
	11th to 20th	Night	15% 9% 24% 32% 18% 1	360
	21st to end	Night	11% 7% 15% 39% 27% 2	360
July	1st to 10th	Night	7% 4 14% 37% 35% 2	350
	11th to 20th	Night	7% 3 11% 46% 30% 3	348
	21st to end	Night	5% 3 10% 37% 42% 3	394
August	1st to 10th	Night	5% 2 10% 34% 41% 8%	414
	11th to 20th	Night	6% 2 9% 29% 49% 6%	534
	21st to end	Night	12% 2 13% 35% 34% 4	697
September	1st to 10th	Night	11% 3 14% 37% 31% 4	720
	11th to 20th	Night	15% 4 14% 25% 35% 7%	720
	21st to end	Night	19% 6% 15% 35% 22% 3	720
October	1st to 10th	Night	20% 5% 14% 29% 28% 5%	720
	11th to 20th	Night	26% 7% 15% 27% 21% 4	720
	21st to end	Night	29% 8% 19% 25% 17% 2	792
November	1st to 10th	Night	33% 9% 20% 26% 12%	740
	11th to 20th	Night	34% 11% 19% 22% 13% 1	760
	21st to end	Night	32% 9% 20% 21% 16% 3	760
December	1st to 10th	Night	31% 9% 17% 26% 16% 1	760
	11th to 20th	Night	35% 11% 19% 23% 11% 2	760
	21st to end	Night	42% 11% 17% 20% 10% 1	903

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