Anchorage
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anchorage clear sky chart from 2005-11-06 to 2023-06-24.

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	18% 11% 4 7% 6% 3 5% 5% 5% 6% 26%	71503
	Night	18% 13% 4 7% 6% 3 4 4 5% 6% 24%	69705
Transparency	Day	56% 12% 15% 10% 4	71133
	Night	57% 10% 13% 11% 6% 1	69596
Seeing	Day	19% 7% 18% 26% 21% 6%	16152
	Night	20% 7% 20% 27% 19% 4	20702

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	8% 14% 6% 8% 7% 2 4 4 6% 12% 24%	733
	11th to 20th	Day	17% 14% 4 7% 5% 3 6% 4 6% 8% 20%	962
	21st to end	Day	15% 17% 4 10% 6% 3 6% 5% 5% 6% 18%	1188
February	1st to 10th	Day	11% 16% 5% 7% 4 2 4 4 4 7% 30%	1241
	11th to 20th	Day	16% 21% 5% 9% 7% 3 5% 4 4 4 18%	1399
	21st to end	Day	12% 14% 4 8% 7% 2 4 3 4 6% 30%	1292
March	1st to 10th	Day	12% 15% 3 6% 5% 2 3 4 4 4 36%	1766
	11th to 20th	Day	6% 9% 4 7% 5% 2 5% 5% 5% 7% 40%	1862
	21st to end	Day	8% 14% 5% 8% 6% 3 4 4 4 7% 32%	2358
April	1st to 10th	Day	10% 15% 6% 8% 7% 3 4 4 5% 6% 27%	2200
	11th to 20th	Day	8% 9% 3 7% 6% 3 5% 6% 5% 6% 36%	2486
	21st to end	Day	11% 10% 4 7% 6% 3 5% 5% 5% 7% 29%	2538
May	1st to 10th	Day	15% 12% 4 9% 7% 4 6% 4 5% 6% 23%	2847
	11th to 20th	Day	15% 10% 4 6% 6% 3 4 5% 5% 6% 31%	2833
	21st to end	Day	17% 8% 3 6% 5% 3 4 4 5% 7% 33%	3346
June	1st to 10th	Day	14% 10% 3 7% 6% 3 4 6% 7% 9% 26%	3240
	11th to 20th	Day	15% 6% 3 7% 6% 5% 5% 6% 6% 7% 28%	3240
	21st to end	Day	17% 7% 4 7% 5% 3 5% 6% 6% 7% 27%	3121
July	1st to 10th	Day	19% 8% 3 7% 6% 3 5% 5% 5% 7% 26%	3060
	11th to 20th	Day	18% 7% 3 7% 6% 3 5% 5% 6% 8% 26%	2971
	21st to end	Day	26% 8% 3 6% 5% 3 4 4 4 5% 25%	2970
August	1st to 10th	Day	27% 9% 3 6% 5% 3 5% 4 5% 6% 23%	2687
	11th to 20th	Day	30% 9% 3 6% 4 3 4 3 4 6% 21%	2382
	21st to end	Day	27% 8% 2 6% 6% 3 4 5% 5% 6% 23%	2618
September	1st to 10th	Day	30% 10% 3 7% 6% 3 3 4 5% 5% 18%	2095
	11th to 20th	Day	32% 9% 3 6% 6% 3 3 4 4 5% 19%	2040
	21st to end	Day	26% 10% 4 8% 5% 3 3 4 3 4 25%	1808
October	1st to 10th	Day	27% 12% 4 7% 5% 3 4 4 4 5% 21%	1692
	11th to 20th	Day	21% 11% 3 6% 7% 3 4 4 5% 5% 24%	1514
	21st to end	Day	17% 14% 3 7% 5% 2 3 6% 6% 6% 25%	1471
November	1st to 10th	Day	15% 15% 5% 9% 6% 1 4 6% 4 6% 24%	1228
	11th to 20th	Day	13% 11% 2 6% 6% 4 6% 6% 5% 6% 30%	1080
	21st to end	Day	15% 15% 4 7% 7% 3 5% 5% 6% 7% 20%	940
December	1st to 10th	Day	21% 16% 5% 10% 6% 3 3 3 4 6% 18%	783
	11th to 20th	Day	9% 17% 8% 8% 6% 5% 6% 5% 5% 7% 18%	720
	21st to end	Day	12% 17% 6% 11% 7% 6% 6% 5% 6% 5% 14%	792
January	1st to 10th	Night	9% 11% 5% 8% 6% 3 4 5% 7% 9% 27%	3212
	11th to 20th	Night	15% 16% 5% 7% 6% 2 4 4 5% 8% 22%	2997
	21st to end	Night	17% 15% 5% 8% 6% 3 4 4 6% 6% 20%	3168
February	1st to 10th	Night	12% 15% 4 8% 5% 2 4 5% 4 5% 29%	2718
	11th to 20th	Night	17% 21% 6% 9% 7% 3 3 3 4 5% 17%	2448
	21st to end	Night	12% 14% 5% 8% 7% 3 4 4 3 5% 30%	1964
March	1st to 10th	Night	10% 14% 4 7% 6% 3 3 3 5% 5% 34%	2194
	11th to 20th	Night	8% 10% 4 8% 5% 3 4 4 4 7% 37%	2095
	21st to end	Night	9% 14% 5% 8% 5% 2 3 3 4 5% 35%	1998
April	1st to 10th	Night	10% 19% 5% 8% 7% 4 5% 3 4 4 26%	1760
	11th to 20th	Night	11% 12% 3 6% 6% 4 5% 4 5% 6% 32%	1423
	21st to end	Night	10% 11% 5% 8% 7% 4 5% 4 4 6% 29%	1422
May	1st to 10th	Night	16% 12% 5% 7% 8% 4 6% 5% 6% 5% 21%	1112
	11th to 20th	Night	17% 9% 3 7% 7% 3 5% 4 5% 7% 27%	1062
	21st to end	Night	18% 8% 3 5% 6% 4 4 5% 6% 7% 29%	925
June	1st to 10th	Night	18% 11% 5% 8% 7% 4 4 5% 6% 4 21%	720
	11th to 20th	Night	19% 9% 3 9% 5% 4 5% 7% 5% 7% 21%	720
	21st to end	Night	20% 7% 4 8% 6% 5% 4 5% 7% 8% 20%	696
July	1st to 10th	Night	24% 10% 2 6% 6% 3 5% 4 5% 6% 22%	680
	11th to 20th	Night	26% 8% 4 7% 6% 3 5% 4 5% 6% 21%	768
	21st to end	Night	30% 9% 4 7% 5% 3 3 4 3 5% 21%	1107
August	1st to 10th	Night	28% 9% 4 5% 6% 3 4 4 5% 6% 20%	1030
	11th to 20th	Night	34% 9% 3 7% 5% 3 4 5% 4 4 18%	1298
	21st to end	Night	31% 9% 4 5% 5% 3 4 3 4 6% 22%	1496
September	1st to 10th	Night	35% 10% 4 5% 5% 3 4 4 3 5% 16%	1644
	11th to 20th	Night	35% 10% 2 6% 5% 3 3 4 4 4 19%	1700
	21st to end	Night	29% 10% 3 5% 4 3 5% 4 4 3 23%	1929
October	1st to 10th	Night	31% 12% 3 6% 5% 2 3 3 3 3 22%	2048
	11th to 20th	Night	24% 10% 3 7% 6% 3 4 4 5% 5% 24%	2223
	21st to end	Night	18% 15% 4 8% 5% 3 4 4 4 5% 24%	2643
November	1st to 10th	Night	18% 14% 4 7% 5% 3 5% 4 5% 6% 24%	2627
	11th to 20th	Night	12% 11% 3 6% 5% 3 5% 6% 6% 8% 30%	2880
	21st to end	Night	15% 16% 6% 9% 6% 2 4 4 4 7% 22%	3020
December	1st to 10th	Night	22% 16% 5% 7% 6% 3 4 4 4 6% 16%	3174
	11th to 20th	Night	12% 17% 6% 9% 7% 3 5% 4 5% 6% 20%	3240
	21st to end	Night	14% 16% 6% 9% 7% 3 6% 5% 5% 7% 16%	3564

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	46% 9% 15% 10% 14% 3	733
	11th to 20th	Day	55% 11% 14% 10% 6% 2	962
	21st to end	Day	62% 9% 13% 7% 5% 1	1188
February	1st to 10th	Day	50% 7% 12% 15% 9% 4	1241
	11th to 20th	Day	66% 9% 9% 9% 4	1406
	21st to end	Day	51% 9% 10% 17% 9% 1	1292
March	1st to 10th	Day	48% 8% 12% 12% 11% 5%	1766
	11th to 20th	Day	42% 9% 15% 16% 14% 1	1862
	21st to end	Day	51% 10% 14% 11% 9% 2	2358
April	1st to 10th	Day	57% 9% 14% 12% 5%	2200
	11th to 20th	Day	45% 11% 16% 18% 6%	2486
	21st to end	Day	52% 8% 13% 16% 7%	2538
May	1st to 10th	Day	59% 10% 14% 11% 3	2847
	11th to 20th	Day	52% 9% 15% 17% 5%	2836
	21st to end	Day	49% 10% 17% 18% 3	3347
June	1st to 10th	Day	52% 17% 18% 11% 1	3240
	11th to 20th	Day	52% 16% 18% 9% 2	3240
	21st to end	Day	54% 17% 20% 7%	3121
July	1st to 10th	Day	56% 15% 18% 9%	2971
	11th to 20th	Day	54% 18% 21% 4	2873
	21st to end	Day	60% 16% 16% 6%	2795
August	1st to 10th	Day	63% 14% 16% 5%	2668
	11th to 20th	Day	65% 11% 14% 6% 1	2382
	21st to end	Day	61% 14% 14% 6% 2	2618
September	1st to 10th	Day	68% 12% 9% 7% 1	2095
	11th to 20th	Day	66% 10% 13% 8%	2040
	21st to end	Day	64% 8% 13% 12% 1	1808
October	1st to 10th	Day	64% 10% 14% 9% 1	1692
	11th to 20th	Day	58% 12% 13% 12% 2	1514
	21st to end	Day	54% 13% 14% 9% 7%	1471
November	1st to 10th	Day	57% 10% 12% 10% 7% 1	1228
	11th to 20th	Day	48% 10% 19% 14% 6%	1080
	21st to end	Day	59% 9% 12% 10% 6% 1	940
December	1st to 10th	Day	64% 9% 14% 7% 3	783
	11th to 20th	Day	60% 11% 15% 8% 3	720
	21st to end	Day	65% 9% 13% 7% 3	792
January	1st to 10th	Night	46% 10% 14% 11% 13% 2	3212
	11th to 20th	Night	54% 9% 12% 12% 7% 3	2997
	21st to end	Night	58% 9% 12% 10% 7% 1	3168
February	1st to 10th	Night	50% 8% 12% 15% 8% 5%	2718
	11th to 20th	Night	65% 8% 10% 9% 4 1	2452
	21st to end	Night	53% 9% 10% 15% 9% 1	1964
March	1st to 10th	Night	48% 7% 12% 13% 12% 5%	2194
	11th to 20th	Night	42% 8% 11% 17% 18% 1	2097
	21st to end	Night	48% 10% 14% 13% 11% 2	1998
April	1st to 10th	Night	58% 8% 12% 15% 4	1760
	11th to 20th	Night	47% 9% 17% 17% 7%	1423
	21st to end	Night	51% 9% 13% 16% 9%	1422
May	1st to 10th	Night	58% 11% 13% 14% 2	1112
	11th to 20th	Night	53% 11% 11% 17% 5%	1063
	21st to end	Night	48% 12% 17% 18% 3	925
June	1st to 10th	Night	59% 12% 16% 10%	720
	11th to 20th	Night	56% 16% 15% 9% 3	720
	21st to end	Night	59% 16% 18% 6%	696
July	1st to 10th	Night	57% 16% 15% 10%	660
	11th to 20th	Night	59% 14% 20% 5%	743
	21st to end	Night	64% 10% 17% 7%	1042
August	1st to 10th	Night	62% 13% 18% 5%	1024
	11th to 20th	Night	67% 10% 12% 8% 1	1298
	21st to end	Night	62% 10% 15% 9% 2	1496
September	1st to 10th	Night	69% 12% 7% 8% 2	1644
	11th to 20th	Night	66% 11% 11% 9% 1	1700
	21st to end	Night	63% 9% 13% 12% 2	1929
October	1st to 10th	Night	65% 8% 11% 12% 2	2048
	11th to 20th	Night	59% 10% 15% 9% 4	2223
	21st to end	Night	59% 8% 11% 11% 8%	2643
November	1st to 10th	Night	57% 10% 12% 11% 7% 1	2627
	11th to 20th	Night	47% 9% 17% 16% 8%	2880
	21st to end	Night	57% 9% 14% 10% 6% 1	3020
December	1st to 10th	Night	64% 9% 12% 7% 5%	3174
	11th to 20th	Night	60% 10% 13% 10% 4	3240
	21st to end	Night	61% 12% 12% 8% 4	3564

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	5% 7% 21% 29% 27% 8%	94
	11th to 20th	Day	16% 8% 23% 28% 15% 7%	260
	21st to end	Day	18% 12% 20% 27% 12% 8%	286
February	1st to 10th	Day	13% 7% 17% 30% 17% 13%	260
	11th to 20th	Day	21% 11% 21% 29% 12% 3	335
	21st to end	Day	18% 8% 26% 27% 10% 8%	280
March	1st to 10th	Day	17% 7% 20% 27% 18% 6%	340
	11th to 20th	Day	8% 5% 20% 32% 24% 8%	402
	21st to end	Day	11% 8% 22% 27% 23% 7%	572
April	1st to 10th	Day	11% 8% 24% 30% 19% 4	528
	11th to 20th	Day	9% 9% 13% 28% 31% 7%	590
	21st to end	Day	14% 6% 19% 30% 23% 5%	600
May	1st to 10th	Day	17% 11% 23% 24% 20% 2	603
	11th to 20th	Day	17% 8% 19% 30% 19% 5%	625
	21st to end	Day	19% 8% 18% 30% 19% 3	765
June	1st to 10th	Day	12% 10% 16% 24% 27% 8%	810
	11th to 20th	Day	13% 5% 17% 31% 26% 6%	810
	21st to end	Day	15% 5% 14% 26% 28% 8%	771
July	1st to 10th	Day	17% 4 13% 27% 31% 5%	736
	11th to 20th	Day	21% 3 16% 28% 24% 6%	732
	21st to end	Day	24% 5% 15% 23% 22% 7%	638
August	1st to 10th	Day	27% 5% 15% 27% 20% 3	576
	11th to 20th	Day	27% 9% 19% 23% 16% 3	565
	21st to end	Day	28% 7% 13% 24% 19% 6%	638
September	1st to 10th	Day	29% 9% 16% 20% 20% 3	513
	11th to 20th	Day	30% 7% 16% 20% 19% 5%	500
	21st to end	Day	30% 6% 19% 15% 21% 5%	339
October	1st to 10th	Day	30% 7% 20% 21% 14% 4	330
	11th to 20th	Day	23% 6% 17% 27% 16% 7%	330
	21st to end	Day	22% 9% 19% 24% 19% 5%	363
November	1st to 10th	Day	21% 8% 25% 20% 18% 6%	333
	11th to 20th	Day	16% 5% 25% 23% 23% 5%	260
	21st to end	Day	15% 8% 25% 25% 17% 7%	120
December	1st to 10th	Day	22% 8% 16% 25% 21% 6%	80
	11th to 20th	Day	17% 6% 22% 37% 10% 6%	80
	21st to end	Day	15% 10% 34% 25% 11% 3	88
January	1st to 10th	Night	10% 6% 25% 32% 22% 3	880
	11th to 20th	Night	17% 9% 24% 28% 15% 3	852
	21st to end	Night	19% 9% 22% 28% 16% 4	880
February	1st to 10th	Night	15% 8% 20% 33% 18% 4	800
	11th to 20th	Night	22% 11% 25% 25% 14% 1	784
	21st to end	Night	15% 7% 27% 31% 12% 5%	550
March	1st to 10th	Night	14% 8% 23% 31% 19% 3	620
	11th to 20th	Night	9% 5% 21% 34% 23% 5%	620
	21st to end	Night	13% 7% 21% 25% 23% 9%	602
April	1st to 10th	Night	16% 9% 17% 31% 20% 4	540
	11th to 20th	Night	12% 7% 13% 28% 30% 8%	533
	21st to end	Night	13% 4 19% 30% 28% 3	540
May	1st to 10th	Night	17% 5% 19% 29% 24% 3	392
	11th to 20th	Night	16% 7% 14% 28% 30% 2	354
	21st to end	Night	19% 4 14% 33% 25% 2	239
June	1st to 10th	Night	23% 6% 14% 23% 28% 2	180
	11th to 20th	Night	18% 6% 15% 30% 26% 2	180
	21st to end	Night	20% 2 14% 28% 31% 2	174
July	1st to 10th	Night	23% 6% 10% 18% 37% 3	165
	11th to 20th	Night	31% 3 8% 23% 27% 5%	251
	21st to end	Night	30% 4 15% 18% 21% 9%	372
August	1st to 10th	Night	33% 5% 13% 25% 18% 4	338
	11th to 20th	Night	36% 5% 18% 21% 14% 3	459
	21st to end	Night	32% 5% 12% 21% 21% 6%	561
September	1st to 10th	Night	34% 9% 16% 20% 14% 3	510
	11th to 20th	Night	35% 8% 11% 16% 21% 5%	510
	21st to end	Night	31% 8% 14% 20% 19% 6%	542
October	1st to 10th	Night	32% 8% 17% 22% 16% 3	598
	11th to 20th	Night	22% 6% 18% 25% 20% 6%	760
	21st to end	Night	20% 8% 20% 27% 18% 3	836
November	1st to 10th	Night	22% 8% 22% 26% 16% 3	780
	11th to 20th	Night	14% 6% 23% 32% 19% 3	800
	21st to end	Night	19% 8% 23% 24% 18% 5%	800
December	1st to 10th	Night	23% 10% 21% 25% 15% 4	852
	11th to 20th	Night	14% 7% 25% 31% 16% 4	880
	21st to end	Night	18% 9% 28% 23% 14% 5%	968

Page generated at 2023-06-21 15:40:08. Version ac06

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