Sitka
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sitka clear sky chart from 2005-11-06 to 2024-07-16.

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	34% 22% 4 8% 4 3 3 2 3 4 13%	89502
	Night	30% 23% 5% 8% 4 3 3 2 3 4 15%	90022
Transparency	Day	76% 7% 7% 7% 3 1	89179
	Night	75% 6% 6% 7% 5% 1	89926
Seeing	Day	43% 13% 18% 17% 8% 2	16378
	Night	44% 11% 16% 16% 10% 2	23312

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	20% 25% 5% 8% 4 5% 4 4 4 4 17%	1899
	11th to 20th	Day	30% 25% 3 4 2 3 2 2 3 3 24%	1882
	21st to end	Day	42% 25% 5% 7% 3 3 2 2 2 2 7%	2388
February	1st to 10th	Day	25% 27% 4 9% 4 2 3 1 3 2 19%	1462
	11th to 20th	Day	28% 27% 5% 6% 3 4 2 2 3 3 18%	1545
	21st to end	Day	16% 26% 6% 6% 3 4 3 3 3 5% 25%	1418
March	1st to 10th	Day	13% 31% 6% 9% 3 3 3 3 3 4 21%	3126
	11th to 20th	Day	22% 33% 6% 9% 4 3 2 2 2 3 14%	3321
	21st to end	Day	15% 24% 5% 9% 6% 4 4 3 4 4 20%	4101
April	1st to 10th	Day	22% 34% 6% 8% 5% 3 2 2 3 2 13%	2280
	11th to 20th	Day	21% 22% 5% 9% 6% 4 4 3 3 4 19%	2519
	21st to end	Day	25% 25% 5% 10% 5% 4 3 3 3 4 14%	2660
May	1st to 10th	Day	32% 23% 4 7% 4 3 3 2 3 5% 14%	4487
	11th to 20th	Day	29% 30% 3 8% 4 3 3 2 3 3 12%	4995
	21st to end	Day	28% 25% 5% 10% 6% 4 4 3 4 4 10%	5509
June	1st to 10th	Day	33% 17% 5% 9% 6% 4 4 3 4 4 10%	3040
	11th to 20th	Day	38% 17% 4 9% 5% 4 4 3 4 5% 10%	3040
	21st to end	Day	43% 17% 4 8% 5% 4 4 2 3 3 9%	3040
July	1st to 10th	Day	38% 15% 3 7% 4 3 3 3 4 5% 13%	3040
	11th to 20th	Day	47% 16% 4 8% 4 4 3 2 3 3 6%	2965
	21st to end	Day	47% 14% 4 7% 4 3 3 2 3 3 10%	3092
August	1st to 10th	Day	43% 13% 4 9% 4 4 3 3 4 4 10%	2603
	11th to 20th	Day	51% 13% 3 7% 3 4 3 2 3 3 8%	2476
	21st to end	Day	53% 13% 3 6% 4 3 3 2 2 2 9%	2686
September	1st to 10th	Day	52% 11% 4 7% 3 3 3 3 3 4 8%	2160
	11th to 20th	Day	42% 13% 3 6% 3 3 2 2 3 5% 17%	2160
	21st to end	Day	53% 13% 3 8% 3 3 3 2 3 2 7%	1918
October	1st to 10th	Day	45% 16% 4 6% 3 3 3 2 4 3 11%	1800
	11th to 20th	Day	50% 16% 3 6% 4 3 2 2 3 2 7%	1674
	21st to end	Day	40% 18% 2 5% 3 3 3 2 3 5% 15%	1652
November	1st to 10th	Day	36% 20% 3 7% 3 3 2 2 3 3 16%	1424
	11th to 20th	Day	36% 23% 3 6% 3 3 2 1 3 4 16%	1325
	21st to end	Day	34% 28% 4 7% 5% 3 2 2 2 2 11%	1140
December	1st to 10th	Day	27% 25% 4 8% 4 3 2 2 4 4 15%	1140
	11th to 20th	Day	22% 27% 5% 10% 5% 3 4 2 3 4 16%	1445
	21st to end	Day	30% 32% 6% 9% 3 2 2 2 2 2 8%	2090
January	1st to 10th	Night	21% 25% 5% 9% 4 4 5% 3 3 4 16%	5046
	11th to 20th	Night	29% 21% 5% 7% 3 3 3 2 2 3 22%	5016
	21st to end	Night	41% 27% 4 7% 3 2 2 2 2 2 8%	5273
February	1st to 10th	Night	27% 22% 5% 8% 4 4 3 2 3 3 18%	2718
	11th to 20th	Night	26% 29% 5% 7% 3 2 3 2 3 3 17%	2521
	21st to end	Night	17% 28% 6% 7% 4 3 3 2 3 4 24%	2036
March	1st to 10th	Night	13% 31% 5% 7% 4 4 3 2 4 4 24%	3738
	11th to 20th	Night	23% 31% 6% 9% 5% 4 4 3 2 3 11%	3638
	21st to end	Night	18% 21% 4 7% 4 3 4 3 5% 6% 25%	3539
April	1st to 10th	Night	22% 30% 6% 7% 4 4 4 2 3 3 16%	1900
	11th to 20th	Night	18% 22% 6% 10% 6% 4 3 4 3 4 20%	1608
	21st to end	Night	24% 23% 5% 9% 5% 4 3 3 3 4 16%	1520
May	1st to 10th	Night	29% 21% 3 6% 4 4 3 3 4 4 19%	2481
	11th to 20th	Night	20% 26% 5% 9% 5% 3 3 3 5% 5% 16%	1897
	21st to end	Night	22% 22% 5% 11% 6% 6% 4 3 4 5% 13%	2066
June	1st to 10th	Night	28% 17% 4 10% 7% 5% 5% 3 4 5% 13%	1140
	11th to 20th	Night	33% 15% 5% 10% 6% 5% 4 3 4 4 10%	1140
	21st to end	Night	36% 17% 4 9% 5% 4 4 2 5% 5% 10%	1140
July	1st to 10th	Night	31% 16% 5% 10% 4 5% 4 3 6% 4 13%	1140
	11th to 20th	Night	39% 17% 5% 8% 5% 3 4 2 4 5% 9%	1116
	21st to end	Night	39% 15% 5% 8% 4 3 3 3 2 4 13%	1222
August	1st to 10th	Night	39% 15% 3 8% 4 3 4 3 4 4 13%	1335
	11th to 20th	Night	48% 13% 3 7% 5% 3 2 2 3 3 13%	1424
	21st to end	Night	46% 12% 4 8% 5% 2 4 3 2 3 11%	1668
September	1st to 10th	Night	50% 13% 3 7% 4 4 3 2 2 3 9%	1800
	11th to 20th	Night	38% 12% 3 8% 4 4 4 3 3 4 19%	1800
	21st to end	Night	49% 13% 4 8% 4 4 2 2 3 3 9%	2038
October	1st to 10th	Night	41% 12% 3 8% 5% 4 3 3 4 3 14%	2160
	11th to 20th	Night	46% 17% 3 7% 4 3 3 3 3 2 9%	2286
	21st to end	Night	37% 17% 4 7% 3 3 3 2 3 4 18%	2703
November	1st to 10th	Night	36% 19% 4 6% 4 3 3 3 3 3 16%	2653
	11th to 20th	Night	30% 23% 4 7% 4 4 3 3 3 4 15%	2854
	21st to end	Night	34% 25% 5% 7% 4 3 3 3 3 3 11%	3040
December	1st to 10th	Night	30% 26% 5% 7% 4 3 3 3 3 3 14%	3040
	11th to 20th	Night	25% 27% 4 7% 5% 3 4 2 3 3 16%	3760
	21st to end	Night	30% 30% 6% 8% 4 3 3 2 3 3 10%	5566

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	73% 7% 6% 7% 5% 2	1899
	11th to 20th	Day	69% 4 4 8% 14% 1	1900
	21st to end	Day	87% 4 4 4 2	2388
February	1st to 10th	Day	74% 4 6% 7% 7% 3	1462
	11th to 20th	Day	74% 6% 4 6% 8% 2	1553
	21st to end	Day	65% 6% 6% 13% 7% 3	1418
March	1st to 10th	Day	67% 5% 7% 9% 9% 2	3126
	11th to 20th	Day	78% 4 4 8% 5%	3321
	21st to end	Day	65% 8% 9% 15% 3	4101
April	1st to 10th	Day	77% 7% 4 7% 4	2280
	11th to 20th	Day	70% 6% 6% 10% 7% 1	2519
	21st to end	Day	75% 7% 7% 8% 4	2660
May	1st to 10th	Day	72% 6% 8% 10% 3	4487
	11th to 20th	Day	76% 8% 7% 7% 2	4998
	21st to end	Day	77% 7% 8% 6% 2	5510
June	1st to 10th	Day	77% 9% 8% 4 2	3040
	11th to 20th	Day	76% 10% 9% 5% 1	3040
	21st to end	Day	81% 7% 8% 3 1	3040
July	1st to 10th	Day	71% 10% 13% 5% 1	2961
	11th to 20th	Day	86% 7% 6% 1	2878
	21st to end	Day	79% 7% 9% 4	2920
August	1st to 10th	Day	79% 8% 8% 4 1	2585
	11th to 20th	Day	83% 6% 7% 4 1	2476
	21st to end	Day	83% 6% 5% 5% 1	2686
September	1st to 10th	Day	81% 7% 6% 6%	2160
	11th to 20th	Day	72% 6% 7% 11% 5%	2160
	21st to end	Day	85% 5% 4 5% 1	1918
October	1st to 10th	Day	78% 7% 5% 5% 4 1	1800
	11th to 20th	Day	85% 6% 4 4 2	1674
	21st to end	Day	73% 6% 7% 8% 5%	1652
November	1st to 10th	Day	75% 6% 6% 9% 3	1424
	11th to 20th	Day	76% 5% 6% 8% 5% 1	1325
	21st to end	Day	82% 6% 3 4 3 1	1140
December	1st to 10th	Day	75% 8% 8% 5% 4 1	1140
	11th to 20th	Day	75% 7% 5% 6% 6%	1448
	21st to end	Day	84% 6% 3 3 2 2	2090
January	1st to 10th	Night	74% 7% 5% 7% 6% 1	5046
	11th to 20th	Night	70% 4 5% 7% 13% 2	5060
	21st to end	Night	87% 4 3 4 2	5273
February	1st to 10th	Night	74% 5% 5% 7% 7% 2	2718
	11th to 20th	Night	76% 3 4 6% 9% 2	2524
	21st to end	Night	68% 4 6% 11% 9% 2	2036
March	1st to 10th	Night	66% 5% 7% 11% 10% 2	3738
	11th to 20th	Night	81% 4 5% 6% 4	3640
	21st to end	Night	61% 7% 9% 19% 4	3539
April	1st to 10th	Night	75% 6% 8% 5% 4 1	1900
	11th to 20th	Night	69% 7% 7% 7% 10%	1608
	21st to end	Night	73% 6% 8% 9% 4	1520
May	1st to 10th	Night	69% 7% 8% 13% 4	2481
	11th to 20th	Night	66% 11% 9% 9% 3 1	1897
	21st to end	Night	72% 8% 11% 7% 2	2066
June	1st to 10th	Night	75% 9% 9% 5% 2 1	1140
	11th to 20th	Night	77% 8% 9% 5% 1	1140
	21st to end	Night	78% 6% 11% 3 1	1140
July	1st to 10th	Night	72% 10% 11% 6% 1	1110
	11th to 20th	Night	78% 10% 9% 2	1080
	21st to end	Night	78% 7% 9% 4 1	1154
August	1st to 10th	Night	77% 8% 10% 4 1	1328
	11th to 20th	Night	80% 6% 8% 5% 1	1424
	21st to end	Night	80% 7% 6% 6% 1	1668
September	1st to 10th	Night	82% 7% 5% 5%	1800
	11th to 20th	Night	69% 7% 8% 10% 6% 1	1800
	21st to end	Night	83% 6% 3 5% 2	2038
October	1st to 10th	Night	76% 6% 7% 7% 3 1	2160
	11th to 20th	Night	84% 6% 4 3 2	2286
	21st to end	Night	73% 6% 6% 9% 6%	2703
November	1st to 10th	Night	75% 7% 6% 9% 3	2653
	11th to 20th	Night	76% 7% 6% 6% 4 1	2854
	21st to end	Night	80% 6% 5% 4 3 1	3040
December	1st to 10th	Night	77% 8% 7% 5% 4	3040
	11th to 20th	Night	76% 6% 5% 6% 8% 1	3756
	21st to end	Night	82% 5% 3 4 4 2	5566

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	34% 14% 23% 16% 11% 1	180
	11th to 20th	Day	47% 17% 16% 14% 6%	180
	21st to end	Day	51% 14% 17% 13% 5%	379
February	1st to 10th	Day	42% 16% 21% 14% 4 1	370
	11th to 20th	Day	42% 16% 19% 15% 5% 1	365
	21st to end	Day	33% 21% 26% 14% 5% 2	307
March	1st to 10th	Day	34% 16% 23% 17% 9% 2	370
	11th to 20th	Day	36% 21% 23% 15% 4 1	370
	21st to end	Day	35% 19% 19% 15% 11% 2	488
April	1st to 10th	Day	36% 20% 22% 15% 8%	460
	11th to 20th	Day	31% 12% 21% 20% 14% 3	596
	21st to end	Day	41% 15% 19% 15% 9% 2	650
May	1st to 10th	Day	44% 12% 22% 16% 5% 1	653
	11th to 20th	Day	33% 11% 20% 22% 12% 3	675
	21st to end	Day	36% 11% 21% 21% 9% 2	735
June	1st to 10th	Day	38% 16% 17% 18% 10% 2	680
	11th to 20th	Day	40% 12% 16% 22% 8% 2	680
	21st to end	Day	46% 10% 14% 19% 9% 3	680
July	1st to 10th	Day	35% 9% 18% 24% 11% 3	671
	11th to 20th	Day	49% 11% 15% 18% 6% 1	645
	21st to end	Day	48% 8% 12% 19% 10% 2	684
August	1st to 10th	Day	43% 9% 19% 20% 8% 1	626
	11th to 20th	Day	54% 10% 15% 14% 6% 1	615
	21st to end	Day	53% 10% 14% 15% 7% 1	671
September	1st to 10th	Day	49% 10% 16% 14% 8% 2	450
	11th to 20th	Day	40% 11% 19% 18% 10% 2	450
	21st to end	Day	58% 17% 13% 9% 2 1	412
October	1st to 10th	Day	49% 11% 17% 15% 7% 2	360
	11th to 20th	Day	58% 15% 18% 6% 3	360
	21st to end	Day	44% 15% 20% 15% 6%	396
November	1st to 10th	Day	51% 15% 18% 11% 5%	302
	11th to 20th	Day	53% 8% 17% 14% 6% 2	180
	21st to end	Day	58% 16% 13% 11% 2	180
December	1st to 10th	Day	49% 19% 12% 12% 7% 2	180
	11th to 20th	Day	42% 17% 22% 11% 7% 1	180
	21st to end	Day	46% 14% 23% 8% 6% 4	198
January	1st to 10th	Night	41% 15% 21% 15% 8% 1	850
	11th to 20th	Night	51% 16% 18% 10% 5% 1	850
	21st to end	Night	52% 15% 14% 12% 6% 1	935
February	1st to 10th	Night	43% 14% 18% 16% 7% 1	850
	11th to 20th	Night	43% 14% 18% 16% 8%	834
	21st to end	Night	37% 15% 23% 17% 7% 1	700
March	1st to 10th	Night	36% 13% 22% 17% 11% 2	760
	11th to 20th	Night	38% 17% 22% 16% 7% 1	665
	21st to end	Night	34% 10% 19% 19% 15% 5%	627
April	1st to 10th	Night	39% 14% 16% 16% 13% 4	570
	11th to 20th	Night	29% 12% 22% 20% 15% 3	563
	21st to end	Night	38% 12% 16% 16% 14% 4	570
May	1st to 10th	Night	47% 7% 15% 15% 13% 3	537
	11th to 20th	Night	28% 7% 13% 25% 22% 4	374
	21st to end	Night	29% 9% 13% 24% 23% 2	411
June	1st to 10th	Night	37% 7% 13% 17% 23% 3	380
	11th to 20th	Night	37% 5% 15% 24% 18% 1	380
	21st to end	Night	40% 7% 15% 17% 19% 2	380
July	1st to 10th	Night	36% 7% 11% 23% 22% 1	370
	11th to 20th	Night	47% 4 10% 19% 16% 4	360
	21st to end	Night	43% 4 11% 22% 18% 2	434
August	1st to 10th	Night	46% 5% 12% 17% 15% 4	537
	11th to 20th	Night	53% 6% 9% 14% 15% 4	534
	21st to end	Night	52% 5% 11% 17% 12% 2	594
September	1st to 10th	Night	54% 8% 9% 16% 11% 2	540
	11th to 20th	Night	43% 8% 11% 18% 16% 4	540
	21st to end	Night	57% 10% 13% 12% 6% 2	705
October	1st to 10th	Night	48% 7% 12% 20% 12% 1	720
	11th to 20th	Night	58% 10% 16% 11% 3 2	720
	21st to end	Night	46% 10% 17% 18% 9% 1	857
November	1st to 10th	Night	48% 15% 19% 12% 5%	830
	11th to 20th	Night	45% 14% 18% 15% 8% 1	850
	21st to end	Night	52% 14% 17% 12% 4	850
December	1st to 10th	Night	47% 14% 17% 13% 7% 1	850
	11th to 20th	Night	44% 16% 19% 14% 7% 1	850
	21st to end	Night	47% 15% 17% 14% 5% 2	935

Page generated at 2024-07-13 06:19:53. Version ac07

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