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 2025-05-26.

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% 17% 6% 11% 6% 5% 5% 4 5% 5% 19%	116209
	Night	10% 17% 7% 13% 7% 5% 5% 3 5% 5% 24%	121812
Transparency	Day	62% 7% 12% 12% 5% 1	115899
	Night	58% 7% 10% 14% 9% 3	121771
Seeing	Day	25% 9% 18% 25% 18% 5%	18573
	Night	21% 9% 21% 27% 18% 3	23185

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	2 18% 7% 13% 6% 5% 5% 4 5% 6% 31%	1819
	11th to 20th	Day	4 16% 7% 16% 7% 5% 5% 4 5% 4 27%	2250
	21st to end	Day	8% 17% 5% 14% 7% 6% 5% 3 4 4 29%	2992
February	1st to 10th	Day	4 16% 7% 10% 4 3 4 4 5% 5% 37%	1337
	11th to 20th	Day	6% 24% 10% 12% 5% 6% 4 4 5% 6% 17%	1558
	21st to end	Day	5% 15% 7% 12% 9% 6% 4 4 4 5% 30%	1410
March	1st to 10th	Day	2 13% 10% 19% 9% 7% 5% 3 5% 5% 24%	4420
	11th to 20th	Day	2 14% 7% 20% 8% 7% 6% 5% 6% 6% 20%	4648
	21st to end	Day	4 20% 6% 15% 6% 5% 5% 4 6% 6% 22%	5964
April	1st to 10th	Day	5% 23% 9% 14% 8% 6% 6% 4 5% 5% 16%	2460
	11th to 20th	Day	5% 19% 6% 11% 7% 5% 5% 5% 6% 6% 26%	2766
	21st to end	Day	8% 21% 6% 11% 6% 6% 5% 4 5% 5% 24%	2857
May	1st to 10th	Day	9% 28% 8% 14% 8% 5% 4 3 4 4 13%	5187
	11th to 20th	Day	8% 20% 6% 13% 7% 5% 5% 4 5% 6% 22%	5211
	21st to end	Day	20% 21% 6% 10% 5% 5% 4 3 4 4 16%	6157
June	1st to 10th	Day	22% 16% 4 8% 6% 6% 6% 4 6% 6% 17%	3420
	11th to 20th	Day	28% 15% 3 8% 6% 5% 6% 4 5% 6% 15%	3420
	21st to end	Day	30% 15% 4 8% 5% 5% 5% 4 6% 5% 13%	3420
July	1st to 10th	Day	42% 15% 3 7% 4 4 4 3 4 4 8%	5700
	11th to 20th	Day	31% 21% 4 8% 5% 5% 5% 3 4 5% 10%	5646
	21st to end	Day	32% 13% 3 8% 5% 5% 5% 4 5% 7% 13%	5543
August	1st to 10th	Day	37% 16% 5% 9% 5% 4 4 2 4 4 11%	5046
	11th to 20th	Day	38% 13% 3 8% 4 5% 4 3 4 5% 13%	4456
	21st to end	Day	28% 14% 5% 11% 7% 6% 5% 4 5% 5% 10%	4884
September	1st to 10th	Day	38% 15% 3 10% 5% 4 3 3 3 3 11%	2365
	11th to 20th	Day	34% 19% 5% 9% 4 3 3 2 3 4 13%	2275
	21st to end	Day	26% 15% 5% 11% 5% 5% 4 3 4 5% 18%	2028
October	1st to 10th	Day	15% 14% 5% 12% 7% 7% 8% 5% 7% 6% 16%	3106
	11th to 20th	Day	9% 13% 4 8% 5% 5% 4 4 5% 7% 35%	2809
	21st to end	Day	11% 21% 6% 8% 4 3 3 3 4 5% 33%	2636
November	1st to 10th	Day	8% 20% 6% 13% 5% 4 6% 4 6% 5% 23%	1360
	11th to 20th	Day	8% 17% 5% 10% 5% 4 3 2 4 4 37%	1179
	21st to end	Day	9% 18% 6% 14% 5% 5% 4 4 4 5% 26%	1000
December	1st to 10th	Day	7% 18% 7% 19% 7% 6% 4 4 4 4 19%	1361
	11th to 20th	Day	2 17% 11% 19% 7% 6% 5% 4 6% 4 18%	1517
	21st to end	Day	2 14% 8% 13% 8% 6% 5% 3 5% 5% 31%	2002
January	1st to 10th	Night	2 17% 8% 13% 6% 5% 4 3 4 6% 30%	8146
	11th to 20th	Night	5% 18% 7% 15% 8% 5% 4 3 4 3 28%	7677
	21st to end	Night	9% 16% 6% 10% 7% 6% 5% 3 3 4 31%	7986
February	1st to 10th	Night	4 17% 7% 10% 5% 3 3 3 4 4 40%	3060
	11th to 20th	Night	7% 23% 10% 13% 6% 5% 5% 3 4 5% 19%	2729
	21st to end	Night	6% 14% 7% 16% 7% 6% 4 4 5% 5% 27%	2220
March	1st to 10th	Night	2 12% 11% 17% 7% 5% 5% 4 4 5% 27%	5434
	11th to 20th	Night	2 13% 7% 20% 7% 6% 6% 4 6% 6% 24%	5310
	21st to end	Night	3 18% 7% 14% 6% 5% 5% 4 5% 7% 26%	4967
April	1st to 10th	Night	6% 21% 8% 15% 8% 6% 5% 3 4 4 19%	1937
	11th to 20th	Night	6% 18% 6% 9% 8% 7% 5% 4 5% 5% 29%	1583
	21st to end	Night	9% 20% 6% 10% 6% 4 3 3 5% 6% 28%	1543
May	1st to 10th	Night	11% 29% 8% 13% 6% 5% 4 3 3 4 13%	2003
	11th to 20th	Night	10% 21% 6% 15% 6% 6% 5% 4 4 6% 18%	1912
	21st to end	Night	20% 20% 6% 10% 6% 4 4 3 4 4 18%	1541
June	1st to 10th	Night	22% 15% 5% 10% 8% 4 4 3 4 6% 19%	760
	11th to 20th	Night	26% 14% 4 9% 4 4 3 5% 7% 5% 19%	760
	21st to end	Night	29% 14% 5% 8% 5% 4 5% 4 4 5% 17%	760
July	1st to 10th	Night	42% 10% 2 6% 4 5% 4 5% 4 5% 13%	1270
	11th to 20th	Night	28% 21% 5% 7% 4 3 3 2 4 6% 16%	1324
	21st to end	Night	34% 13% 3 7% 4 3 3 3 5% 6% 18%	2022
August	1st to 10th	Night	36% 12% 3 8% 6% 3 3 3 4 5% 16%	1894
	11th to 20th	Night	39% 11% 3 6% 4 5% 4 3 5% 5% 14%	2380
	21st to end	Night	30% 18% 5% 10% 5% 4 4 3 4 4 14%	2783
September	1st to 10th	Night	37% 16% 4 9% 5% 4 2 3 3 3 13%	1814
	11th to 20th	Night	36% 16% 4 7% 4 3 4 3 5% 3 15%	1905
	21st to end	Night	27% 16% 5% 8% 5% 5% 4 4 4 4 18%	2151
October	1st to 10th	Night	15% 18% 6% 12% 6% 6% 7% 5% 6% 5% 15%	3855
	11th to 20th	Night	9% 11% 4 11% 7% 5% 5% 4 6% 6% 32%	4116
	21st to end	Night	10% 20% 5% 10% 5% 4 5% 3 5% 5% 27%	4923
November	1st to 10th	Night	9% 21% 6% 11% 6% 5% 5% 3 5% 4 26%	2937
	11th to 20th	Night	7% 17% 5% 10% 5% 5% 4 3 5% 5% 35%	3220
	21st to end	Night	7% 20% 6% 12% 6% 5% 4 3 4 5% 26%	3400
December	1st to 10th	Night	7% 20% 7% 16% 8% 6% 5% 3 5% 5% 18%	5814
	11th to 20th	Night	3 16% 10% 19% 7% 6% 6% 4 6% 5% 18%	6700
	21st to end	Night	2 12% 8% 16% 8% 5% 5% 4 5% 4 30%	8976

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% 8% 11% 13% 11% 8%	1819
	11th to 20th	Day	55% 6% 8% 16% 11% 4	2265
	21st to end	Day	57% 5% 5% 10% 18% 5%	2992
February	1st to 10th	Day	44% 5% 12% 16% 17% 7%	1337
	11th to 20th	Day	62% 5% 11% 12% 8% 3	1565
	21st to end	Day	52% 8% 9% 17% 12% 2	1410
March	1st to 10th	Day	58% 5% 6% 13% 12% 6%	4420
	11th to 20th	Day	52% 10% 9% 15% 13% 2	4648
	21st to end	Day	55% 8% 12% 13% 11% 2	5964
April	1st to 10th	Day	63% 7% 12% 13% 5%	2460
	11th to 20th	Day	51% 6% 14% 20% 8%	2766
	21st to end	Day	58% 6% 10% 18% 8% 1	2857
May	1st to 10th	Day	72% 7% 9% 10% 2	5187
	11th to 20th	Day	58% 7% 14% 18% 3	5214
	21st to end	Day	66% 8% 12% 13% 2	6158
June	1st to 10th	Day	62% 8% 17% 12% 1	3420
	11th to 20th	Day	63% 9% 16% 9% 3	3420
	21st to end	Day	67% 9% 19% 5%	3420
July	1st to 10th	Day	74% 9% 12% 5%	5611
	11th to 20th	Day	72% 8% 17% 3	5547
	21st to end	Day	63% 11% 20% 6%	5368
August	1st to 10th	Day	75% 6% 14% 5% 1	5027
	11th to 20th	Day	69% 9% 12% 9% 1	4500
	21st to end	Day	68% 9% 15% 6% 1	4883
September	1st to 10th	Day	73% 7% 10% 8% 2	2365
	11th to 20th	Day	73% 5% 12% 8% 1	2275
	21st to end	Day	65% 5% 15% 12% 3	2028
October	1st to 10th	Day	61% 7% 14% 14% 3	3106
	11th to 20th	Day	44% 6% 13% 22% 14% 1	2809
	21st to end	Day	52% 4 10% 24% 8% 1	2636
November	1st to 10th	Day	56% 6% 13% 15% 7% 3	1360
	11th to 20th	Day	50% 3 9% 22% 14% 1	1179
	21st to end	Day	57% 4 10% 12% 14% 3	1000
December	1st to 10th	Day	66% 5% 8% 12% 8% 1	1361
	11th to 20th	Day	64% 6% 10% 15% 4 1	1520
	21st to end	Day	48% 8% 9% 15% 14% 6%	2002
January	1st to 10th	Night	51% 7% 7% 15% 11% 10%	8146
	11th to 20th	Night	57% 6% 8% 14% 13% 2	7725
	21st to end	Night	52% 8% 5% 10% 20% 5%	7986
February	1st to 10th	Night	45% 4 6% 19% 18% 7%	3060
	11th to 20th	Night	64% 4 8% 11% 9% 3	2733
	21st to end	Night	54% 7% 9% 16% 12% 2	2220
March	1st to 10th	Night	55% 5% 6% 15% 13% 6%	5434
	11th to 20th	Night	52% 8% 9% 14% 16% 2	5312
	21st to end	Night	52% 8% 11% 14% 14% 2	4967
April	1st to 10th	Night	63% 6% 11% 14% 4 1	1937
	11th to 20th	Night	51% 7% 15% 19% 9%	1583
	21st to end	Night	55% 5% 15% 20% 6%	1543
May	1st to 10th	Night	71% 5% 10% 13% 1	2003
	11th to 20th	Night	63% 5% 16% 13% 3	1913
	21st to end	Night	64% 7% 14% 13% 2	1541
June	1st to 10th	Night	63% 6% 21% 10% 1	760
	11th to 20th	Night	62% 9% 18% 9% 2	760
	21st to end	Night	67% 6% 22% 5%	760
July	1st to 10th	Night	71% 9% 16% 4	1250
	11th to 20th	Night	68% 8% 18% 6%	1300
	21st to end	Night	64% 10% 20% 6%	1958
August	1st to 10th	Night	70% 7% 17% 6% 1	1888
	11th to 20th	Night	68% 10% 14% 6% 2	2402
	21st to end	Night	72% 8% 12% 6% 2	2783
September	1st to 10th	Night	75% 7% 10% 6% 2	1814
	11th to 20th	Night	72% 5% 13% 9% 1	1905
	21st to end	Night	67% 5% 14% 11% 4	2151
October	1st to 10th	Night	66% 6% 12% 13% 2	3855
	11th to 20th	Night	45% 8% 14% 20% 12% 1	4116
	21st to end	Night	56% 5% 9% 22% 7% 1	4923
November	1st to 10th	Night	57% 4 13% 15% 8% 2	2937
	11th to 20th	Night	48% 4 10% 23% 14%	3220
	21st to end	Night	56% 5% 9% 14% 12% 4	3400
December	1st to 10th	Night	65% 7% 7% 13% 8% 1	5814
	11th to 20th	Night	62% 7% 10% 14% 5%	6696
	21st to end	Night	51% 8% 8% 15% 13% 5%	8976

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% 10%	100
	11th to 20th	Day	17% 17% 22% 28% 11% 6%	245
	21st to end	Day	23% 10% 15% 28% 17% 6%	330
February	1st to 10th	Day	11% 11% 21% 30% 18% 10%	300
	11th to 20th	Day	20% 13% 22% 24% 18% 3	395
	21st to end	Day	14% 14% 25% 25% 16% 7%	331
March	1st to 10th	Day	13% 12% 26% 28% 16% 5%	400
	11th to 20th	Day	11% 8% 28% 30% 15% 7%	450
	21st to end	Day	12% 12% 24% 23% 23% 5%	660
April	1st to 10th	Day	15% 11% 29% 26% 16% 2	623
	11th to 20th	Day	16% 9% 20% 30% 21% 3	690
	21st to end	Day	17% 8% 20% 30% 21% 4	700
May	1st to 10th	Day	20% 12% 23% 25% 18% 2	703
	11th to 20th	Day	20% 11% 19% 26% 20% 4	725
	21st to end	Day	25% 8% 20% 29% 16% 3	880
June	1st to 10th	Day	24% 8% 15% 21% 24% 7%	870
	11th to 20th	Day	27% 6% 15% 24% 22% 6%	870
	21st to end	Day	29% 6% 16% 22% 22% 6%	870
July	1st to 10th	Day	31% 5% 12% 27% 20% 6%	856
	11th to 20th	Day	31% 6% 12% 27% 19% 4	852
	21st to end	Day	32% 7% 12% 25% 19% 5%	770
August	1st to 10th	Day	40% 5% 17% 21% 14% 4	676
	11th to 20th	Day	38% 8% 17% 20% 15% 3	665
	21st to end	Day	37% 6% 14% 24% 16% 3	748
September	1st to 10th	Day	35% 8% 15% 25% 13% 4	612
	11th to 20th	Day	43% 6% 12% 18% 16% 5%	575
	21st to end	Day	29% 9% 15% 24% 18% 6%	390
October	1st to 10th	Day	34% 7% 19% 19% 15% 6%	390
	11th to 20th	Day	24% 7% 16% 26% 21% 6%	390
	21st to end	Day	24% 9% 15% 25% 19% 9%	429
November	1st to 10th	Day	20% 12% 18% 22% 19% 10%	389
	11th to 20th	Day	15% 6% 16% 28% 23% 12%	279
	21st to end	Day	16% 12% 17% 29% 15% 11%	100
December	1st to 10th	Day	28% 12% 10% 25% 24% 1	100
	11th to 20th	Day	19% 11% 29% 25% 10% 6%	100
	21st to end	Day	15% 14% 28% 23% 15% 5%	110
January	1st to 10th	Night	12% 11% 27% 30% 17% 3	1000
	11th to 20th	Night	17% 15% 28% 24% 12% 4	977
	21st to end	Night	18% 11% 24% 30% 15% 3	990
February	1st to 10th	Night	11% 13% 30% 27% 16% 3	900
	11th to 20th	Night	18% 14% 29% 28% 10% 1	884
	21st to end	Night	12% 9% 32% 30% 13% 4	653
March	1st to 10th	Night	11% 11% 28% 34% 14% 2	700
	11th to 20th	Night	9% 10% 26% 33% 18% 4	700
	21st to end	Night	13% 8% 24% 27% 25% 3	662
April	1st to 10th	Night	15% 10% 23% 31% 19% 3	600
	11th to 20th	Night	15% 7% 20% 29% 26% 4	593
	21st to end	Night	17% 7% 15% 34% 25% 2	600
May	1st to 10th	Night	21% 9% 17% 31% 21% 2	430
	11th to 20th	Night	16% 8% 16% 36% 19% 4	377
	21st to end	Night	23% 6% 14% 32% 25%	211
June	1st to 10th	Night	24% 6% 11% 28% 25% 5%	190
	11th to 20th	Night	28% 4 7% 24% 32% 4	190
	21st to end	Night	32% 6% 7% 17% 34% 4	190
July	1st to 10th	Night	34% 2 6% 18% 35% 5%	185
	11th to 20th	Night	40% 5% 4 23% 24% 4	212
	21st to end	Night	38% 7% 6% 20% 24% 6%	416
August	1st to 10th	Night	39% 4 9% 25% 18% 4	378
	11th to 20th	Night	45% 6% 12% 18% 17% 2	512
	21st to end	Night	40% 4 9% 22% 22% 3	627
September	1st to 10th	Night	42% 6% 11% 23% 15% 4	570
	11th to 20th	Night	43% 5% 8% 20% 20% 3	570
	21st to end	Night	31% 7% 15% 21% 21% 5%	603
October	1st to 10th	Night	35% 7% 17% 20% 18% 3	699
	11th to 20th	Night	21% 7% 17% 29% 21% 4	860
	21st to end	Night	23% 8% 19% 27% 20% 2	946
November	1st to 10th	Night	20% 8% 26% 28% 15% 2	880
	11th to 20th	Night	15% 10% 22% 31% 18% 4	900
	21st to end	Night	19% 13% 23% 26% 14% 4	900
December	1st to 10th	Night	26% 10% 22% 26% 13% 3	980
	11th to 20th	Night	13% 9% 28% 30% 16% 5%	1000
	21st to end	Night	15% 15% 27% 26% 13% 4	1100

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