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 2024-11-02.

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
Cloud Cover	Day	20% 18% 5% 10% 6% 5% 5% 3 5% 5% 18%	103209
	Night	12% 18% 6% 11% 6% 5% 4 3 4 5% 25%	95900
Transparency	Day	63% 7% 13% 11% 5% 1	102899
	Night	57% 7% 11% 13% 9% 3	95859
Seeing	Day	25% 8% 18% 25% 18% 5%	17988
	Night	22% 9% 21% 27% 18% 3	22301

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 17% 4 10% 6% 6% 5% 5% 6% 7% 31%	1269
	11th to 20th	Day	6% 17% 4 8% 4 4 4 4 5% 4 39%	1549
	21st to end	Day	4 18% 5% 13% 6% 6% 4 3 4 4 34%	2090
February	1st to 10th	Day	4 17% 7% 10% 5% 3 4 4 5% 5% 36%	1270
	11th to 20th	Day	6% 25% 10% 11% 5% 6% 4 4 5% 6% 17%	1478
	21st to end	Day	5% 15% 7% 12% 8% 6% 4 4 5% 5% 31%	1341
March	1st to 10th	Day	3 15% 8% 16% 7% 6% 4 3 4 5% 28%	3074
	11th to 20th	Day	3 16% 4 11% 8% 7% 7% 5% 6% 7% 27%	3246
	21st to end	Day	6% 26% 8% 16% 6% 4 4 3 4 5% 17%	4160
April	1st to 10th	Day	6% 23% 8% 14% 8% 5% 6% 4 4 5% 16%	2337
	11th to 20th	Day	5% 19% 5% 11% 6% 5% 5% 4 6% 6% 27%	2626
	21st to end	Day	9% 20% 5% 10% 6% 6% 5% 4 5% 6% 25%	2714
May	1st to 10th	Day	9% 29% 8% 15% 7% 5% 4 3 4 4 13%	5028
	11th to 20th	Day	8% 19% 6% 12% 7% 5% 5% 4 5% 6% 22%	5050
	21st to end	Day	20% 21% 6% 10% 6% 5% 4 3 5% 4 16%	6065
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	25% 19% 5% 9% 5% 4 5% 3 5% 5% 16%	1870
	11th to 20th	Day	15% 19% 5% 10% 6% 5% 4 3 4 6% 24%	1697
	21st to end	Day	11% 21% 5% 9% 5% 4 4 4 5% 5% 27%	1610
November	1st to 10th	Day	8% 21% 5% 11% 5% 4 6% 5% 6% 5% 24%	1299
	11th to 20th	Day	9% 17% 5% 10% 5% 3 3 2 4 4 37%	1121
	21st to end	Day	10% 19% 6% 13% 4 5% 4 4 4 5% 25%	950
December	1st to 10th	Day	11% 23% 5% 14% 5% 3 4 4 4 5% 21%	798
	11th to 20th	Day	3 20% 11% 18% 7% 5% 3 4 5% 4 20%	967
	21st to end	Day	2 16% 7% 11% 8% 6% 5% 3 6% 5% 31%	1397
January	1st to 10th	Night	3 17% 6% 11% 7% 6% 5% 4 5% 7% 30%	5686
	11th to 20th	Night	6% 16% 5% 10% 5% 4 4 3 4 4 39%	5363
	21st to end	Night	5% 16% 4 10% 6% 5% 5% 3 4 5% 37%	5577
February	1st to 10th	Night	4 18% 7% 10% 5% 3 3 3 4 4 38%	2907
	11th to 20th	Night	7% 24% 10% 13% 6% 5% 5% 3 4 5% 18%	2589
	21st to end	Night	6% 14% 7% 16% 7% 6% 4 3 5% 5% 27%	2113
March	1st to 10th	Night	3 13% 11% 15% 7% 5% 4 3 4 5% 29%	3780
	11th to 20th	Night	2 16% 6% 12% 6% 6% 6% 4 7% 6% 29%	3712
	21st to end	Night	5% 23% 9% 14% 5% 4 3 4 4 5% 24%	3471
April	1st to 10th	Night	6% 22% 8% 15% 8% 6% 5% 3 4 4 20%	1840
	11th to 20th	Night	6% 18% 6% 7% 7% 6% 5% 4 5% 5% 30%	1503
	21st to end	Night	9% 20% 6% 9% 5% 5% 3 3 5% 6% 29%	1466
May	1st to 10th	Night	11% 30% 8% 13% 6% 5% 4 3 3 4 13%	1942
	11th to 20th	Night	11% 20% 6% 15% 6% 6% 5% 4 4 6% 18%	1853
	21st to end	Night	20% 20% 6% 10% 6% 4 4 3 4 4 18%	1516
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	24% 21% 5% 8% 5% 4 4 3 3 5% 17%	2309
	11th to 20th	Night	15% 18% 5% 11% 6% 4 4 3 5% 5% 25%	2480
	21st to end	Night	11% 21% 5% 11% 5% 5% 5% 4 5% 5% 24%	2988
November	1st to 10th	Night	9% 21% 6% 10% 5% 5% 5% 3 5% 4 27%	2801
	11th to 20th	Night	7% 17% 4 10% 5% 5% 4 3 4 5% 34%	3059
	21st to end	Night	8% 21% 6% 13% 6% 5% 4 3 4 5% 25%	3230
December	1st to 10th	Night	11% 26% 6% 10% 6% 4 4 3 4 5% 21%	3382
	11th to 20th	Night	4 20% 9% 18% 7% 5% 5% 3 4 4 20%	4240
	21st to end	Night	3 16% 8% 13% 8% 5% 5% 4 5% 4 30%	6270

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	42% 10% 13% 17% 13% 4	1269
	11th to 20th	Day	45% 4 9% 21% 16% 6%	1564
	21st to end	Day	50% 6% 7% 13% 18% 6%	2090
February	1st to 10th	Day	46% 5% 12% 14% 16% 7%	1270
	11th to 20th	Day	62% 5% 11% 12% 7% 2	1485
	21st to end	Day	51% 8% 10% 16% 13% 2	1341
March	1st to 10th	Day	54% 4 7% 16% 13% 7%	3074
	11th to 20th	Day	43% 11% 10% 18% 15% 2	3246
	21st to end	Day	65% 6% 12% 9% 7% 2	4160
April	1st to 10th	Day	64% 6% 11% 13% 5% 1	2337
	11th to 20th	Day	51% 6% 14% 21% 9%	2626
	21st to end	Day	56% 6% 10% 19% 8% 1	2714
May	1st to 10th	Day	73% 7% 9% 10% 2	5028
	11th to 20th	Day	57% 8% 14% 18% 3	5053
	21st to end	Day	66% 8% 12% 13% 2	6066
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	66% 5% 15% 10% 3	1870
	11th to 20th	Day	57% 5% 15% 16% 5% 2	1697
	21st to end	Day	53% 6% 13% 19% 7% 2	1610
November	1st to 10th	Day	55% 6% 14% 15% 7% 3	1299
	11th to 20th	Day	50% 3 9% 22% 14% 1	1121
	21st to end	Day	58% 4 10% 12% 13% 3	950
December	1st to 10th	Day	63% 5% 10% 13% 9% 1	798
	11th to 20th	Day	63% 5% 10% 14% 6% 1	970
	21st to end	Day	46% 9% 10% 13% 15% 8%	1397
January	1st to 10th	Night	48% 8% 9% 19% 12% 4	5686
	11th to 20th	Night	45% 6% 10% 18% 18% 3	5411
	21st to end	Night	44% 9% 6% 13% 21% 7%	5577
February	1st to 10th	Night	48% 4 6% 17% 18% 7%	2907
	11th to 20th	Night	64% 4 8% 12% 9% 2	2593
	21st to end	Night	53% 6% 9% 16% 12% 2	2113
March	1st to 10th	Night	52% 6% 6% 16% 12% 8%	3780
	11th to 20th	Night	45% 8% 11% 16% 18% 2	3714
	21st to end	Night	58% 7% 13% 12% 8% 2	3471
April	1st to 10th	Night	63% 6% 11% 15% 4 1	1840
	11th to 20th	Night	50% 6% 16% 19% 9%	1503
	21st to end	Night	53% 5% 15% 20% 7%	1466
May	1st to 10th	Night	71% 5% 10% 13% 1	1942
	11th to 20th	Night	63% 5% 16% 13% 3	1854
	21st to end	Night	64% 7% 14% 14% 2	1516
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	69% 4 11% 13% 3	2309
	11th to 20th	Night	58% 6% 16% 13% 6% 1	2480
	21st to end	Night	57% 5% 13% 16% 8% 2	2988
November	1st to 10th	Night	56% 4 14% 15% 9% 2	2801
	11th to 20th	Night	49% 4 11% 23% 14%	3059
	21st to end	Night	58% 5% 9% 13% 12% 4	3230
December	1st to 10th	Night	63% 5% 9% 13% 10% 1	3382
	11th to 20th	Night	62% 7% 9% 13% 8% 1	4236
	21st to end	Night	50% 9% 8% 12% 14% 7%	6270

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	6% 14% 27% 27% 16% 11%	90
	11th to 20th	Day	15% 18% 21% 28% 12% 7%	227
	21st to end	Day	20% 10% 16% 28% 18% 6%	308
February	1st to 10th	Day	11% 11% 22% 29% 16% 10%	280
	11th to 20th	Day	21% 14% 22% 22% 16% 3	365
	21st to end	Day	14% 13% 25% 26% 15% 6%	307
March	1st to 10th	Day	14% 11% 25% 29% 16% 5%	370
	11th to 20th	Day	12% 8% 29% 29% 15% 7%	417
	21st to end	Day	13% 13% 24% 23% 23% 5%	616
April	1st to 10th	Day	15% 10% 28% 26% 17% 2	581
	11th to 20th	Day	16% 8% 19% 31% 22% 4	640
	21st to end	Day	17% 8% 20% 31% 21% 4	650
May	1st to 10th	Day	20% 12% 23% 25% 18% 2	653
	11th to 20th	Day	20% 10% 19% 27% 21% 3	675
	21st to end	Day	25% 7% 19% 29% 17% 3	852
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% 24% 19% 9%	429
November	1st to 10th	Day	20% 10% 18% 23% 19% 10%	364
	11th to 20th	Day	16% 7% 15% 28% 21% 12%	261
	21st to end	Day	18% 12% 14% 28% 16% 12%	90
December	1st to 10th	Day	27% 11% 10% 26% 26% 1	90
	11th to 20th	Day	20% 10% 29% 24% 10% 7%	90
	21st to end	Day	16% 13% 29% 21% 15% 5%	99
January	1st to 10th	Night	11% 11% 27% 31% 17% 4	940
	11th to 20th	Night	16% 15% 28% 24% 13% 4	920
	21st to end	Night	17% 10% 24% 31% 15% 3	935
February	1st to 10th	Night	11% 13% 30% 26% 17% 3	850
	11th to 20th	Night	19% 14% 29% 27% 11% 1	834
	21st to end	Night	13% 9% 32% 29% 13% 4	618
March	1st to 10th	Night	11% 11% 27% 34% 15% 2	660
	11th to 20th	Night	9% 10% 25% 32% 18% 5%	660
	21st to end	Night	13% 7% 25% 26% 25% 3	629
April	1st to 10th	Night	15% 9% 21% 31% 20% 3	570
	11th to 20th	Night	14% 6% 20% 28% 28% 4	563
	21st to end	Night	17% 6% 14% 34% 26% 2	570
May	1st to 10th	Night	21% 8% 16% 31% 22% 2	409
	11th to 20th	Night	17% 8% 15% 36% 20% 4	358
	21st to end	Night	23% 6% 15% 32% 24%	206
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% 7% 26% 29% 16% 2	835
	11th to 20th	Night	16% 9% 22% 31% 19% 4	850
	21st to end	Night	20% 13% 22% 26% 15% 5%	850
December	1st to 10th	Night	25% 10% 21% 27% 14% 3	922
	11th to 20th	Night	13% 9% 26% 31% 16% 5%	940
	21st to end	Night	15% 15% 26% 26% 13% 4	1034

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