Hatcher Pass Lodge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hatcher Pass Lodge clear sky chart from 2005-11-06 to 2026-09-29.

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% 19% 6% 12% 6% 5% 5% 3 5% 5% 16%	156416
	Night	10% 17% 7% 13% 6% 5% 5% 3 5% 5% 24%	143888
Transparency	Day	64% 8% 12% 10% 5% 1	156059
	Night	58% 7% 10% 13% 9% 3	143830
Seeing	Day	25% 9% 18% 24% 17% 5%	20564
	Night	21% 10% 22% 27% 17% 3	24719

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	1 17% 8% 13% 5% 5% 5% 3 4 5% 34%	2361
	11th to 20th	Day	4 19% 7% 18% 6% 4 4 3 5% 4 26%	2924
	21st to end	Day	7% 16% 5% 13% 6% 5% 5% 3 4 4 31%	3871
February	1st to 10th	Day	4 17% 7% 10% 5% 3 4 4 5% 6% 36%	1404
	11th to 20th	Day	6% 24% 9% 12% 6% 6% 4 4 5% 6% 18%	1638
	21st to end	Day	4 14% 6% 12% 9% 6% 4 5% 5% 5% 30%	1478
March	1st to 10th	Day	2 13% 9% 19% 9% 7% 5% 3 5% 5% 24%	4519
	11th to 20th	Day	2 13% 7% 20% 8% 7% 6% 5% 6% 6% 20%	4750
	21st to end	Day	4 19% 6% 15% 6% 5% 5% 4 6% 6% 23%	6096
April	1st to 10th	Day	5% 22% 9% 15% 8% 6% 6% 4 5% 5% 16%	2583
	11th to 20th	Day	4 20% 6% 12% 6% 5% 5% 5% 5% 6% 25%	2906
	21st to end	Day	8% 21% 6% 11% 6% 6% 5% 4 5% 5% 24%	3000
May	1st to 10th	Day	7% 30% 10% 16% 7% 5% 4 3 3 4 11%	9337
	11th to 20th	Day	9% 28% 8% 14% 7% 5% 4 3 4 4 15%	9434
	21st to end	Day	15% 27% 8% 14% 6% 4 4 3 4 3 11%	11337
June	1st to 10th	Day	20% 17% 4 8% 6% 6% 6% 4 6% 6% 17%	3780
	11th to 20th	Day	26% 14% 3 8% 5% 5% 6% 4 5% 6% 16%	3780
	21st to end	Day	31% 15% 4 9% 5% 5% 5% 4 5% 5% 12%	3780
July	1st to 10th	Day	35% 19% 4 10% 5% 5% 4 3 4 4 6%	10620
	11th to 20th	Day	30% 24% 5% 9% 5% 4 4 3 4 4 7%	10514
	21st to end	Day	29% 14% 4 10% 5% 5% 5% 4 5% 6% 13%	10336
August	1st to 10th	Day	32% 16% 4 8% 4 4 4 3 5% 5% 14%	7396
	11th to 20th	Day	34% 14% 3 8% 4 5% 5% 3 5% 6% 13%	6572
	21st to end	Day	28% 15% 5% 12% 6% 5% 5% 4 6% 5% 9%	7103
September	1st to 10th	Day	39% 15% 3 10% 5% 4 3 3 3 3 11%	2613
	11th to 20th	Day	32% 19% 5% 9% 4 3 4 2 4 4 13%	2515
	21st to end	Day	24% 15% 5% 11% 5% 4 4 3 4 5% 17%	2223
October	1st to 10th	Day	15% 14% 5% 12% 7% 7% 8% 5% 6% 6% 16%	3204
	11th to 20th	Day	10% 13% 4 8% 5% 5% 4 4 5% 7% 35%	2898
	21st to end	Day	11% 20% 6% 8% 4 3 3 3 4 5% 32%	2721
November	1st to 10th	Day	8% 20% 6% 13% 5% 4 6% 4 6% 5% 23%	1429
	11th to 20th	Day	8% 16% 5% 10% 6% 4 4 2 4 4 37%	1238
	21st to end	Day	9% 18% 7% 14% 5% 6% 4 4 4 4 26%	1050
December	1st to 10th	Day	7% 18% 6% 19% 7% 6% 4 4 4 4 19%	1403
	11th to 20th	Day	2 16% 11% 18% 7% 6% 5% 4 6% 4 20%	1557
	21st to end	Day	2 14% 8% 13% 8% 6% 5% 3 5% 5% 32%	2046
January	1st to 10th	Night	2 14% 8% 13% 6% 5% 4 3 5% 5% 33%	10564
	11th to 20th	Night	4 19% 8% 16% 7% 5% 5% 3 4 3 26%	10005
	21st to end	Night	7% 14% 6% 11% 7% 6% 5% 3 4 5% 32%	10342
February	1st to 10th	Night	3 17% 7% 10% 5% 4 3 3 4 4 39%	3213
	11th to 20th	Night	6% 22% 10% 14% 6% 5% 5% 3 4 5% 19%	2869
	21st to end	Night	6% 13% 7% 15% 7% 6% 4 4 5% 5% 28%	2327
March	1st to 10th	Night	2 12% 11% 17% 7% 5% 5% 4 4 5% 27%	5555
	11th to 20th	Night	2 13% 7% 20% 7% 6% 6% 4 6% 6% 24%	5427
	21st to end	Night	3 17% 7% 14% 6% 5% 5% 4 5% 7% 26%	5077
April	1st to 10th	Night	5% 20% 8% 16% 8% 6% 5% 3 4 4 20%	2034
	11th to 20th	Night	6% 18% 7% 9% 7% 7% 5% 3 5% 5% 28%	1663
	21st to end	Night	8% 21% 6% 12% 6% 4 3 3 5% 6% 26%	1620
May	1st to 10th	Night	7% 28% 11% 16% 7% 5% 4 3 4 4 12%	3598
	11th to 20th	Night	8% 26% 7% 17% 7% 6% 5% 3 4 5% 12%	3453
	21st to end	Night	14% 24% 8% 13% 7% 6% 4 3 4 3 13%	2852
June	1st to 10th	Night	20% 16% 5% 10% 8% 5% 4 3 4 6% 20%	840
	11th to 20th	Night	24% 14% 4 10% 4 4 4 5% 7% 5% 19%	840
	21st to end	Night	28% 15% 5% 9% 5% 4 5% 4 5% 5% 16%	840
July	1st to 10th	Night	37% 17% 3 8% 5% 5% 5% 3 3 5% 8%	2370
	11th to 20th	Night	27% 24% 5% 9% 4 4 4 3 4 6% 11%	2466
	21st to end	Night	31% 13% 4 9% 5% 4 4 3 4 5% 18%	3770
August	1st to 10th	Night	31% 12% 3 8% 5% 3 3 4 6% 6% 18%	2774
	11th to 20th	Night	36% 13% 4 8% 5% 5% 4 3 5% 4 13%	3491
	21st to end	Night	29% 19% 5% 10% 5% 4 4 3 4 3 12%	4045
September	1st to 10th	Night	37% 16% 4 8% 6% 5% 3 3 3 3 13%	2006
	11th to 20th	Night	33% 18% 5% 8% 4 4 4 3 5% 3 14%	2105
	21st to end	Night	26% 16% 5% 9% 5% 5% 5% 3 4 4 17%	2353
October	1st to 10th	Night	15% 18% 6% 12% 6% 6% 7% 5% 6% 5% 15%	3977
	11th to 20th	Night	9% 12% 4 11% 7% 5% 5% 4 6% 6% 31%	4247
	21st to end	Night	10% 20% 6% 11% 5% 4 5% 3 5% 5% 26%	5080
November	1st to 10th	Night	8% 20% 6% 11% 6% 5% 5% 3 5% 4 25%	3088
	11th to 20th	Night	7% 17% 5% 10% 5% 5% 4 3 5% 5% 34%	3381
	21st to end	Night	7% 20% 6% 12% 6% 5% 5% 3 5% 5% 26%	3570
December	1st to 10th	Night	6% 20% 7% 16% 8% 6% 5% 3 5% 5% 18%	5992
	11th to 20th	Night	3 16% 10% 19% 7% 6% 5% 4 6% 5% 20%	6880
	21st to end	Night	2 12% 8% 16% 8% 5% 5% 4 5% 4 30%	9174

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	48% 7% 8% 11% 11% 14%	2361
	11th to 20th	Day	57% 6% 7% 15% 11% 4	2939
	21st to end	Day	53% 5% 5% 14% 18% 4	3872
February	1st to 10th	Day	45% 5% 12% 16% 16% 6%	1404
	11th to 20th	Day	62% 5% 10% 12% 8% 3	1645
	21st to end	Day	50% 8% 10% 16% 14% 2	1478
March	1st to 10th	Day	58% 5% 6% 13% 12% 6%	4519
	11th to 20th	Day	52% 10% 9% 14% 14% 2	4750
	21st to end	Day	54% 8% 12% 13% 11% 2	6096
April	1st to 10th	Day	63% 7% 12% 14% 4	2583
	11th to 20th	Day	53% 6% 14% 19% 8%	2906
	21st to end	Day	58% 6% 9% 18% 7% 1	3000
May	1st to 10th	Day	73% 6% 8% 9% 4	9337
	11th to 20th	Day	68% 6% 10% 13% 2	9437
	21st to end	Day	71% 8% 9% 11% 1	11338
June	1st to 10th	Day	62% 8% 16% 13% 1	3780
	11th to 20th	Day	60% 10% 18% 9% 3	3780
	21st to end	Day	67% 10% 18% 5%	3780
July	1st to 10th	Day	72% 12% 11% 5%	10531
	11th to 20th	Day	75% 8% 13% 3	10415
	21st to end	Day	64% 12% 17% 7% 1	10113
August	1st to 10th	Day	67% 8% 19% 5%	7377
	11th to 20th	Day	67% 10% 16% 6% 1	6616
	21st to end	Day	69% 10% 14% 6% 1	7102
September	1st to 10th	Day	73% 8% 10% 7% 2	2613
	11th to 20th	Day	72% 6% 13% 9% 1	2515
	21st to end	Day	65% 5% 15% 12% 3	2223
October	1st to 10th	Day	61% 7% 14% 15% 3	3204
	11th to 20th	Day	45% 6% 13% 22% 13% 1	2898
	21st to end	Day	52% 5% 10% 24% 8% 1	2721
November	1st to 10th	Day	57% 6% 12% 15% 7% 3	1429
	11th to 20th	Day	50% 3 9% 22% 15% 1	1238
	21st to end	Day	57% 4 10% 13% 13% 3	1050
December	1st to 10th	Day	65% 5% 8% 12% 8% 2	1403
	11th to 20th	Day	62% 6% 10% 15% 5% 2	1560
	21st to end	Day	47% 8% 9% 15% 15% 6%	2046
January	1st to 10th	Night	49% 6% 6% 13% 10% 16%	10564
	11th to 20th	Night	60% 6% 7% 12% 12% 2	10053
	21st to end	Night	50% 7% 5% 16% 18% 4	10342
February	1st to 10th	Night	45% 4 6% 19% 18% 7%	3213
	11th to 20th	Night	63% 5% 8% 11% 9% 3	2873
	21st to end	Night	52% 7% 9% 16% 13% 3	2327
March	1st to 10th	Night	55% 5% 5% 14% 13% 6%	5555
	11th to 20th	Night	51% 8% 9% 14% 17% 2	5429
	21st to end	Night	51% 8% 11% 14% 14% 2	5077
April	1st to 10th	Night	62% 6% 11% 16% 4 1	2034
	11th to 20th	Night	52% 6% 14% 18% 9%	1663
	21st to end	Night	56% 5% 14% 19% 6%	1620
May	1st to 10th	Night	73% 6% 7% 13% 2	3595
	11th to 20th	Night	70% 6% 13% 8% 2	3454
	21st to end	Night	71% 7% 11% 10% 1	2856
June	1st to 10th	Night	63% 5% 20% 11% 1	840
	11th to 20th	Night	60% 11% 17% 9% 2	840
	21st to end	Night	67% 7% 21% 4	840
July	1st to 10th	Night	74% 10% 11% 5%	2350
	11th to 20th	Night	72% 10% 14% 4	2442
	21st to end	Night	64% 10% 16% 9% 1	3688
August	1st to 10th	Night	64% 9% 21% 5%	2768
	11th to 20th	Night	71% 10% 14% 4 1	3513
	21st to end	Night	71% 10% 12% 6% 1	4045
September	1st to 10th	Night	75% 7% 10% 6% 2	2006
	11th to 20th	Night	72% 5% 12% 10% 1	2105
	21st to end	Night	67% 5% 14% 11% 3	2353
October	1st to 10th	Night	67% 6% 12% 13% 2	3977
	11th to 20th	Night	46% 8% 14% 20% 12% 1	4247
	21st to end	Night	56% 5% 9% 22% 7% 1	5080
November	1st to 10th	Night	57% 4 13% 15% 9% 2	3088
	11th to 20th	Night	48% 4 10% 23% 14%	3381
	21st to end	Night	56% 5% 9% 14% 12% 4	3570
December	1st to 10th	Night	64% 7% 7% 12% 8% 2	5992
	11th to 20th	Night	61% 7% 10% 14% 6% 2	6876
	21st to end	Night	50% 8% 8% 15% 13% 6%	9174

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	9% 14% 25% 26% 16% 9%	110
	11th to 20th	Day	18% 17% 22% 27% 10% 6%	262
	21st to end	Day	22% 11% 15% 27% 18% 7%	352
February	1st to 10th	Day	11% 12% 21% 28% 19% 9%	320
	11th to 20th	Day	20% 13% 24% 23% 17% 3	425
	21st to end	Day	13% 15% 27% 24% 15% 6%	355
March	1st to 10th	Day	13% 12% 25% 30% 16% 4	430
	11th to 20th	Day	10% 8% 28% 32% 15% 7%	482
	21st to end	Day	11% 12% 25% 24% 23% 5%	704
April	1st to 10th	Day	14% 13% 28% 25% 17% 3	665
	11th to 20th	Day	17% 11% 20% 29% 20% 3	740
	21st to end	Day	19% 10% 20% 29% 19% 3	750
May	1st to 10th	Day	22% 13% 22% 24% 17% 2	753
	11th to 20th	Day	21% 12% 20% 24% 19% 3	775
	21st to end	Day	25% 9% 20% 28% 15% 3	974
June	1st to 10th	Day	25% 9% 16% 21% 22% 7%	990
	11th to 20th	Day	25% 7% 17% 24% 21% 6%	990
	21st to end	Day	29% 7% 15% 22% 20% 6%	990
July	1st to 10th	Day	31% 6% 14% 25% 18% 5%	976
	11th to 20th	Day	33% 8% 13% 24% 17% 4	972
	21st to end	Day	32% 8% 14% 23% 18% 5%	883
August	1st to 10th	Day	39% 5% 17% 21% 14% 4	776
	11th to 20th	Day	37% 8% 18% 20% 14% 3	765
	21st to end	Day	37% 8% 15% 22% 15% 2	858
September	1st to 10th	Day	36% 11% 15% 23% 12% 3	698
	11th to 20th	Day	41% 6% 14% 18% 15% 5%	655
	21st to end	Day	29% 10% 15% 23% 18% 6%	445
October	1st to 10th	Day	35% 8% 19% 19% 14% 5%	420
	11th to 20th	Day	24% 8% 16% 26% 20% 6%	420
	21st to end	Day	23% 9% 15% 26% 18% 8%	462
November	1st to 10th	Day	19% 11% 18% 25% 18% 10%	418
	11th to 20th	Day	14% 6% 16% 29% 23% 11%	298
	21st to end	Day	15% 13% 16% 28% 17% 11%	110
December	1st to 10th	Day	29% 11% 9% 26% 23% 2	110
	11th to 20th	Day	19% 12% 30% 25% 9% 5%	110
	21st to end	Day	16% 12% 32% 21% 14% 5%	121
January	1st to 10th	Night	11% 11% 29% 30% 16% 3	1060
	11th to 20th	Night	17% 16% 29% 23% 12% 3	1035
	21st to end	Night	18% 11% 24% 30% 15% 3	1045
February	1st to 10th	Night	11% 13% 30% 27% 16% 3	950
	11th to 20th	Night	17% 14% 31% 27% 10% 1	934
	21st to end	Night	12% 10% 33% 29% 12% 3	688
March	1st to 10th	Night	10% 12% 28% 34% 14% 2	740
	11th to 20th	Night	8% 9% 26% 33% 19% 4	740
	21st to end	Night	12% 8% 25% 27% 25% 3	695
April	1st to 10th	Night	14% 9% 23% 31% 19% 3	630
	11th to 20th	Night	15% 7% 21% 28% 26% 4	623
	21st to end	Night	18% 8% 16% 33% 24% 2	630
May	1st to 10th	Night	22% 10% 16% 29% 21% 2	451
	11th to 20th	Night	16% 10% 16% 35% 19% 4	396
	21st to end	Night	22% 6% 15% 32% 25%	228
June	1st to 10th	Night	25% 6% 12% 29% 25% 4	210
	11th to 20th	Night	27% 4 8% 26% 31% 4	210
	21st to end	Night	31% 6% 7% 19% 33% 4	210
July	1st to 10th	Night	32% 3 8% 20% 32% 4	205
	11th to 20th	Night	40% 6% 6% 23% 23% 3	235
	21st to end	Night	37% 7% 7% 21% 23% 5%	460
August	1st to 10th	Night	39% 4 9% 26% 19% 4	418
	11th to 20th	Night	43% 6% 12% 19% 17% 2	566
	21st to end	Night	40% 5% 11% 22% 21% 2	693
September	1st to 10th	Night	42% 7% 12% 22% 13% 3	630
	11th to 20th	Night	42% 5% 11% 20% 19% 3	630
	21st to end	Night	31% 8% 15% 22% 21% 4	661
October	1st to 10th	Night	35% 8% 18% 20% 17% 3	741
	11th to 20th	Night	21% 7% 19% 28% 20% 4	910
	21st to end	Night	23% 9% 19% 27% 20% 2	1001
November	1st to 10th	Night	19% 8% 27% 29% 15% 2	930
	11th to 20th	Night	15% 10% 22% 31% 19% 3	950
	21st to end	Night	18% 14% 23% 26% 15% 4	950
December	1st to 10th	Night	26% 11% 22% 26% 12% 3	1038
	11th to 20th	Night	13% 10% 28% 29% 15% 5%	1060
	21st to end	Night	14% 16% 28% 25% 13% 4	1166

Page generated at 2026-09-26 00:18:38. Version ac07

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