Prudhoe Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prudhoe Bay clear sky chart from 2005-11-06 to 2026-01-02.

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
Transparency	Day	67% 6% 12% 10% 4	137696
	Night	65% 6% 9% 11% 8% 2	131351
Seeing	Day	17% 4 12% 28% 28% 12%	21708
	Night	12% 3 15% 30% 28% 11%	24083

By Month

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	21st to end	Day	66% 5% 7% 15% 6% 1	951
	February	1st to 10th	Day	57% 5% 12% 13% 13% 1	839
11th to 20th		Day	58% 8% 12% 14% 7% 1	1181
21st to end		Day	58% 9% 7% 19% 7%	1282
March	1st to 10th	Day	59% 5% 9% 12% 12% 2	4001
	11th to 20th	Day	52% 6% 11% 15% 16%	4610
	21st to end	Day	57% 6% 10% 12% 15% 1	5969
April	1st to 10th	Day	72% 8% 7% 7% 5% 1	2733
	11th to 20th	Day	65% 7% 10% 11% 6% 1	2968
	21st to end	Day	68% 7% 9% 11% 4	3326
May	1st to 10th	Day	77% 4 5% 5% 8%	8457
	11th to 20th	Day	76% 5% 8% 9% 2	9806
	21st to end	Day	81% 4 8% 6% 1	11880
June	1st to 10th	Day	64% 8% 10% 14% 4	4800
	11th to 20th	Day	44% 9% 20% 24% 3	4800
	21st to end	Day	50% 11% 21% 16% 2	4800
July	1st to 10th	Day	51% 7% 15% 15% 11% 1	10761
	11th to 20th	Day	58% 8% 22% 11% 1	10745
	21st to end	Day	67% 7% 16% 10%	10690
August	1st to 10th	Day	70% 8% 13% 9% 1	6095
	11th to 20th	Day	77% 6% 12% 6%	5451
	21st to end	Day	75% 6% 13% 5%	5457
September	1st to 10th	Day	78% 6% 11% 4 1	2745
	11th to 20th	Day	78% 7% 10% 6%	2423
	21st to end	Day	78% 5% 9% 7% 1	2122
October	1st to 10th	Day	71% 5% 10% 12% 2	3019
	11th to 20th	Day	80% 5% 10% 4	2460
	21st to end	Day	78% 5% 8% 8% 2	2123
November	1st to 10th	Day	75% 8% 12% 3 2	835
	11th to 20th	Day	81% 5% 10% 4 1	367
January	1st to 10th	Night	43% 4 5% 16% 23% 10%	10890
	11th to 20th	Night	53% 7% 12% 13% 9% 7%	10838
	21st to end	Night	66% 6% 9% 14% 5% 1	10067
February	1st to 10th	Night	55% 7% 13% 14% 10% 1	3560
	11th to 20th	Night	60% 8% 10% 15% 6% 1	3118
	21st to end	Night	63% 7% 8% 16% 5% 1	2348
March	1st to 10th	Night	62% 7% 7% 11% 13%	5862
	11th to 20th	Night	48% 6% 12% 15% 18% 1	5348
	21st to end	Night	61% 6% 11% 9% 12% 1	4962
April	1st to 10th	Night	72% 7% 8% 6% 6%	1665
	11th to 20th	Night	68% 6% 10% 10% 5% 1	1378
	21st to end	Night	73% 5% 6% 13% 4	1072
May	1st to 10th	Night	80% 3 4 4 8%	1519
	11th to 20th	Night	81% 4 6% 8% 1	377
July	21st to end	Night	83% 8% 6% 3	300
	August	1st to 10th	Night	66% 6% 14% 11% 3	1038
11th to 20th		Night	79% 6% 10% 5%	1670
21st to end		Night	75% 6% 14% 4	2451
September	1st to 10th	Night	77% 7% 11% 4 1	1655
	11th to 20th	Night	78% 6% 10% 6%	1975
	21st to end	Night	79% 6% 9% 6% 1	2275
October	1st to 10th	Night	69% 6% 11% 12% 2	4169
	11th to 20th	Night	78% 6% 9% 7%	4687
	21st to end	Night	79% 5% 8% 7% 1	5659
November	1st to 10th	Night	74% 7% 9% 6% 2 1	3660
	11th to 20th	Night	82% 5% 8% 4 1	4261
	21st to end	Night	77% 4 7% 7% 4 1	5040
December	1st to 10th	Night	76% 6% 8% 7% 3	8080
	11th to 20th	Night	56% 7% 10% 16% 11% 1	9195
	21st to end	Night	66% 5% 6% 14% 9% 1	12232

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	21st to end	Day	11% 2 19% 39% 24% 5%	83
	February	1st to 10th	Day	4 1 16% 39% 33% 7%	139
11th to 20th		Day	7% 5% 15% 34% 28% 11%	295
21st to end		Day	8% 4 13% 33% 31% 11%	313
March	1st to 10th	Day	5% 2 15% 33% 29% 17%	400
	11th to 20th	Day	4 4 15% 26% 25% 26%	418
	21st to end	Day	6% 4 17% 24% 32% 17%	660
April	1st to 10th	Day	5% 3 15% 32% 30% 15%	693
	11th to 20th	Day	7% 4 14% 31% 27% 16%	690
	21st to end	Day	6% 2 10% 27% 32% 24%	726
May	1st to 10th	Day	10% 3 11% 25% 33% 18%	995
	11th to 20th	Day	13% 4 13% 26% 30% 15%	1178
	21st to end	Day	16% 4 11% 25% 31% 13%	1439
June	1st to 10th	Day	15% 4 14% 29% 26% 12%	1330
	11th to 20th	Day	10% 2 11% 33% 32% 12%	1330
	21st to end	Day	13% 3 9% 33% 31% 11%	1330
July	1st to 10th	Day	13% 3 9% 34% 32% 9%	1306
	11th to 20th	Day	20% 3 9% 31% 28% 9%	1300
	21st to end	Day	28% 3 8% 26% 27% 8%	1331
August	1st to 10th	Day	28% 4 15% 24% 21% 7%	1007
	11th to 20th	Day	39% 5% 12% 23% 15% 6%	798
	21st to end	Day	35% 4 14% 18% 21% 8%	803
September	1st to 10th	Day	34% 3 9% 18% 25% 10%	730
	11th to 20th	Day	31% 5% 12% 20% 24% 9%	632
	21st to end	Day	26% 3 14% 20% 24% 13%	420
October	1st to 10th	Day	20% 4 16% 24% 22% 13%	420
	11th to 20th	Day	16% 6% 14% 28% 26% 10%	420
	21st to end	Day	13% 6% 15% 27% 27% 12%	313
November	1st to 10th	Day	12% 2 17% 32% 31% 6%	110
	11th to 20th	Day	4 2 13% 32% 35% 13%	99
January	1st to 10th	Night	5% 4 17% 32% 29% 13%	1309
	11th to 20th	Night	7% 4 20% 32% 26% 11%	1295
	21st to end	Night	5% 3 17% 32% 30% 13%	1290
February	1st to 10th	Night	3 2 15% 38% 31% 11%	1000
	11th to 20th	Night	8% 4 17% 33% 29% 10%	884
	21st to end	Night	6% 4 18% 34% 28% 10%	743
March	1st to 10th	Night	5% 3 12% 34% 33% 14%	795
	11th to 20th	Night	3 2 13% 28% 35% 18%	690
	21st to end	Night	5% 4 17% 25% 33% 16%	660
April	1st to 10th	Night	9% 3 16% 30% 31% 12%	600
	11th to 20th	Night	8% 3 15% 33% 30% 12%	521
	21st to end	Night	10% 1 11% 30% 33% 15%	300
May	1st to 10th	Night	12% 4 11% 30% 30% 15%	200
	11th to 20th	Night	17% 7% 9% 31% 29% 7%	103
July	21st to end	Night	34% 8% 32% 25% 2	65
	August	1st to 10th	Night	32% 5% 11% 29% 21% 4	199
11th to 20th		Night	40% 2 11% 25% 17% 6%	301
21st to end		Night	33% 3 9% 21% 26% 8%	544
September	1st to 10th	Night	32% 3 11% 22% 25% 8%	600
	11th to 20th	Night	36% 3 13% 20% 23% 6%	600
	21st to end	Night	25% 3 12% 28% 23% 9%	641
October	1st to 10th	Night	22% 4 16% 25% 23% 10%	855
	11th to 20th	Night	22% 3 13% 28% 26% 8%	910
	21st to end	Night	19% 3 14% 27% 26% 11%	1001
November	1st to 10th	Night	15% 3 15% 31% 25% 10%	1046
	11th to 20th	Night	10% 3 12% 29% 32% 14%	1273
	21st to end	Night	12% 3 13% 28% 30% 15%	1380
December	1st to 10th	Night	11% 3 18% 33% 26% 8%	1380
	11th to 20th	Night	7% 3 18% 28% 32% 12%	1380
	21st to end	Night	5% 4 15% 32% 29% 15%	1518

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