Northstar Ski Resort
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northstar Ski Resort clear sky chart from 2005-11-06 to 2024-11-12.

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	3 7% 2 5% 3 3 3 3 4 6% 63%	105197
	Night	2 7% 3 5% 3 3 3 3 5% 6% 59%	102713
Transparency	Day	11% 47% 4 10% 14% 13%	104943
	Night	13% 48% 5% 11% 14% 10%	102556
Seeing	Day	14% 18% 29% 22% 13% 3	15091
	Night	11% 12% 21% 30% 22% 4	24415

By Month

Percentage Cloud Cover SW (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 12% 6% 8% 5% 4 4 4 7% 6% 42%	2539
	11th to 20th	Day	2 13% 3 7% 6% 5% 6% 5% 6% 7% 42%	2636
	21st to end	Day	6% 17% 5% 9% 4 4 5% 5% 5% 6% 35%	3146
February	1st to 10th	Day	4 10% 4 7% 5% 3 4 4 5% 6% 47%	1710
	11th to 20th	Day	2 11% 4 8% 4 4 3 3 6% 6% 48%	1835
	21st to end	Day	2 12% 5% 8% 4 3 4 3 6% 7% 46%	1570
March	1st to 10th	Day	2 18% 10% 14% 5% 4 4 4 4 6% 28%	3138
	11th to 20th	Day	2 10% 3 5% 4 3 3 3 4 5% 58%	3459
	21st to end	Day	2 15% 6% 9% 5% 4 4 4 5% 7% 39%	4058
April	1st to 10th	Day	3 12% 3 6% 4 3 4 4 5% 8% 47%	2280
	11th to 20th	Day	3 11% 2 6% 4 4 4 4 5% 6% 51%	2249
	21st to end	Day	4 9% 2 4 3 3 3 3 5% 7% 57%	2280
May	1st to 10th	Day	4 7% 1 4 2 2 2 2 4 6% 67%	3970
	11th to 20th	Day	2 4 1 3 2 2 2 3 4 5% 71%	4339
	21st to end	Day	4 5% 1 5% 3 3 3 3 5% 5% 61%	4813
June	1st to 10th	Day	4 4 2 5% 4 4 3 4 5% 7% 59%	2660
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 75%	2660
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 78%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 90%	4420
	11th to 20th	Day	2 2 1 3 3 2 3 2 4 5% 75%	4420
	21st to end	Day	1 1 1 1 1 2 2 2 3 4 82%	4652
August	1st to 10th	Day	2 2 1 3 2 2 2 2 3 5% 76%	4070
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 86%	3724
	21st to end	Day	1 1 1 1 1 2 1 2 2 87%	4180
September	1st to 10th	Day	2 1 1 2 2 1 2 4 4 82%	2280
	11th to 20th	Day	2 3 1 3 3 2 2 2 4 5% 72%	2260
	21st to end	Day	3 3 1 2 2 2 2 2 3 4 76%	2090
October	1st to 10th	Day	3 3 1 2 2 2 2 3 4 6% 71%	3194
	11th to 20th	Day	3 4 1 4 2 1 2 2 3 5% 73%	3161
	21st to end	Day	2 4 1 3 3 2 3 3 5% 7% 65%	3387
November	1st to 10th	Day	4 8% 2 6% 3 4 4 3 4 7% 56%	1764
	11th to 20th	Day	2 7% 1 4 4 3 4 4 6% 10% 54%	1719
	21st to end	Day	2 8% 2 7% 4 4 3 4 6% 9% 52%	1632
December	1st to 10th	Day	4 10% 4 6% 4 4 3 3 6% 8% 46%	1520
	11th to 20th	Day	7% 17% 5% 8% 5% 4 5% 4 6% 6% 33%	1928
	21st to end	Day	3 10% 3 7% 5% 4 4 4 8% 11% 40%	2794
January	1st to 10th	Night	2 7% 3 8% 5% 4 5% 5% 8% 9% 43%	4406
	11th to 20th	Night	2 13% 5% 9% 6% 5% 6% 4 6% 7% 38%	4327
	21st to end	Night	4 18% 6% 7% 5% 5% 5% 4 5% 6% 35%	4521
February	1st to 10th	Night	2 9% 4 9% 6% 4 4 3 6% 6% 46%	2470
	11th to 20th	Night	1 9% 3 7% 6% 4 5% 4 6% 8% 46%	2231
	21st to end	Night	2 9% 4 9% 5% 4 3 4 5% 8% 47%	1884
March	1st to 10th	Night	1 14% 8% 12% 5% 5% 4 4 6% 8% 33%	3736
	11th to 20th	Night	1 12% 3 6% 3 3 3 3 5% 5% 56%	3505
	21st to end	Night	2 16% 7% 11% 7% 4 5% 5% 6% 6% 32%	3595
April	1st to 10th	Night	3 9% 3 6% 5% 4 4 4 7% 7% 48%	1900
	11th to 20th	Night	2 10% 2 5% 4 3 4 4 5% 8% 53%	1879
	21st to end	Night	3 8% 2 4 2 3 3 3 4 7% 59%	1900
May	1st to 10th	Night	2 4 3 4 3 2 3 2 4 5% 67%	3002
	11th to 20th	Night	2 4 1 3 2 2 2 2 4 5% 73%	2552
	21st to end	Night	3 5% 2 3 2 3 3 3 4 6% 65%	2762
June	1st to 10th	Night	4 5% 2 4 3 4 4 3 5% 6% 61%	1520
	11th to 20th	Night	1 2 2 2 2 2 2 4 5% 78%	1520
	21st to end	Night	2 3 1 2 2 2 3 2 3 4 77%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	2540
	11th to 20th	Night	2 3 1 4 2 2 2 2 3 3 76%	2540
	21st to end	Night	2 1 1 1 2 2 2 2 3 4 80%	2901
August	1st to 10th	Night	2 2 1 2 2 2 2 2 3 6% 74%	2870
	11th to 20th	Night	1 1 1 2 1 1 1 2 2 3 86%	3126
	21st to end	Night	1 1 2 1 1 1 1 3 2 86%	3498
September	1st to 10th	Night	2 1 1 1 1 1 1 1 3 4 84%	1900
	11th to 20th	Night	2 3 1 2 2 2 2 1 4 5% 76%	1919
	21st to end	Night	2 2 1 3 2 2 2 2 4 4 76%	2090
October	1st to 10th	Night	2 3 1 2 2 2 2 2 4 4 77%	3782
	11th to 20th	Night	2 2 1 3 2 1 2 2 3 5% 78%	3777
	21st to end	Night	2 6% 1 4 3 3 3 3 5% 7% 64%	4178
November	1st to 10th	Night	3 6% 2 6% 4 3 3 4 5% 8% 56%	2535
	11th to 20th	Night	2 6% 2 5% 3 3 3 3 6% 9% 57%	2483
	21st to end	Night	2 6% 3 7% 4 3 4 3 5% 8% 54%	2546
December	1st to 10th	Night	4 9% 3 7% 4 3 4 4 6% 7% 48%	2660
	11th to 20th	Night	5% 18% 5% 9% 5% 4 4 4 6% 8% 34%	3276
	21st to end	Night	2 11% 4 10% 6% 6% 6% 5% 6% 8% 36%	4862

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	20% 46% 8% 15% 10% 1	2539
	11th to 20th	Day	28% 45% 10% 10% 6% 2	2636
	21st to end	Day	29% 45% 8% 13% 6% 1	3146
February	1st to 10th	Day	18% 66% 4 7% 3 1	1710
	11th to 20th	Day	14% 66% 3 9% 6% 1	1845
	21st to end	Day	11% 67% 5% 9% 6% 2	1570
March	1st to 10th	Day	29% 53% 7% 4 5% 3	3138
	11th to 20th	Day	8% 44% 6% 27% 14% 2	3459
	21st to end	Day	22% 48% 11% 13% 6% 1	4058
April	1st to 10th	Day	13% 66% 5% 10% 5% 1	2280
	11th to 20th	Day	9% 67% 4 11% 8% 1	2252
	21st to end	Day	10% 66% 6% 10% 7%	2280
May	1st to 10th	Day	9% 42% 6% 22% 17% 4	3970
	11th to 20th	Day	8% 39% 3 11% 32% 6%	4342
	21st to end	Day	9% 41% 5% 20% 23% 2	4814
June	1st to 10th	Day	5% 64% 3 9% 13% 5%	2660
	11th to 20th	Day	4 64% 2 10% 12% 7%	2660
	21st to end	Day	3 63% 1 4 11% 18%	2660
July	1st to 10th	Day	3 34% 1 3 7% 52%	4351
	11th to 20th	Day	10% 36% 2 6% 8% 39%	4345
	21st to end	Day	4 35% 1 5% 19% 36%	4505
August	1st to 10th	Day	9% 36% 1 5% 14% 35%	4055
	11th to 20th	Day	3 36% 1 3 19% 37%	3760
	21st to end	Day	4 35% 1 8% 28% 23%	4179
September	1st to 10th	Day	4 59% 1 3 16% 15%	2280
	11th to 20th	Day	7% 60% 3 10% 13% 7%	2260
	21st to end	Day	7% 59% 3 9% 14% 8%	2090
October	1st to 10th	Day	6% 36% 2 8% 26% 22%	3194
	11th to 20th	Day	10% 36% 2 8% 36% 7%	3161
	21st to end	Day	12% 37% 4 18% 21% 8%	3387
November	1st to 10th	Day	8% 57% 5% 13% 14% 4	1764
	11th to 20th	Day	11% 61% 5% 9% 11% 3	1719
	21st to end	Day	15% 62% 5% 8% 7% 3	1632
December	1st to 10th	Day	17% 62% 5% 9% 5% 1	1520
	11th to 20th	Day	28% 52% 3 7% 7% 3	1928
	21st to end	Day	17% 41% 13% 15% 13% 2	2794
January	1st to 10th	Night	19% 43% 14% 15% 8% 1	4406
	11th to 20th	Night	29% 45% 9% 8% 7% 2	4327
	21st to end	Night	28% 47% 7% 12% 5% 1	4521
February	1st to 10th	Night	16% 68% 4 6% 4 2	2470
	11th to 20th	Night	11% 67% 4 8% 8% 2	2232
	21st to end	Night	10% 67% 6% 9% 6% 2	1884
March	1st to 10th	Night	24% 51% 7% 10% 6% 2	3736
	11th to 20th	Night	12% 43% 5% 28% 11% 1	3505
	21st to end	Night	27% 50% 9% 10% 4 1	3595
April	1st to 10th	Night	12% 68% 5% 10% 6%	1900
	11th to 20th	Night	10% 66% 4 11% 7% 2	1879
	21st to end	Night	9% 66% 6% 12% 7%	1900
May	1st to 10th	Night	7% 44% 8% 21% 17% 3	3002
	11th to 20th	Night	6% 39% 4 13% 30% 8%	2552
	21st to end	Night	6% 42% 8% 25% 18% 2	2762
June	1st to 10th	Night	4 65% 3 11% 12% 5%	1520
	11th to 20th	Night	2 65% 3 11% 12% 8%	1520
	21st to end	Night	4 62% 2 4 11% 17%	1520
July	1st to 10th	Night	3 34% 1 2 7% 52%	2500
	11th to 20th	Night	11% 35% 1 6% 7% 41%	2492
	21st to end	Night	3 34% 2 5% 21% 35%	2810
August	1st to 10th	Night	9% 36% 1 5% 19% 30%	2861
	11th to 20th	Night	3 36% 1 4 23% 33%	3156
	21st to end	Night	4 35% 2 11% 27% 21%	3498
September	1st to 10th	Night	3 59% 1 4 19% 14%	1900
	11th to 20th	Night	5% 59% 2 10% 18% 5%	1919
	21st to end	Night	7% 59% 2 9% 16% 8%	2090
October	1st to 10th	Night	4 36% 2 7% 26% 25%	3782
	11th to 20th	Night	5% 38% 3 13% 34% 8%	3777
	21st to end	Night	13% 36% 3 19% 20% 8%	4178
November	1st to 10th	Night	9% 56% 4 13% 13% 4	2535
	11th to 20th	Night	10% 60% 4 11% 11% 3	2483
	21st to end	Night	14% 61% 4 8% 7% 5%	2546
December	1st to 10th	Night	16% 62% 5% 8% 6% 2	2660
	11th to 20th	Night	24% 53% 4 6% 10% 2	3276
	21st to end	Night	21% 44% 11% 11% 11% 2	4862

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	33% 21% 22% 13% 9% 1	180
	11th to 20th	Day	27% 19% 22% 18% 12% 2	237
	21st to end	Day	17% 23% 27% 21% 9% 3	407
February	1st to 10th	Day	23% 21% 26% 20% 8% 1	370
	11th to 20th	Day	24% 22% 25% 21% 7%	448
	21st to end	Day	23% 21% 32% 19% 5%	382
March	1st to 10th	Day	30% 28% 23% 12% 5% 1	460
	11th to 20th	Day	25% 23% 23% 17% 10% 2	460
	21st to end	Day	24% 28% 26% 16% 5%	506
April	1st to 10th	Day	20% 26% 28% 17% 8% 1	460
	11th to 20th	Day	23% 27% 25% 15% 8% 2	452
	21st to end	Day	16% 23% 28% 19% 12% 2	460
May	1st to 10th	Day	19% 24% 28% 20% 7% 2	463
	11th to 20th	Day	18% 22% 25% 21% 9% 5%	488
	21st to end	Day	15% 16% 26% 24% 15% 3	529
June	1st to 10th	Day	12% 19% 29% 24% 13% 4	490
	11th to 20th	Day	12% 20% 35% 19% 11% 3	490
	21st to end	Day	4 16% 32% 30% 14% 4	490
July	1st to 10th	Day	2 12% 41% 27% 15% 3	486
	11th to 20th	Day	2 10% 41% 27% 17% 4	484
	21st to end	Day	2 7% 38% 30% 17% 6%	531
August	1st to 10th	Day	6% 12% 32% 29% 16% 5%	487
	11th to 20th	Day	2 13% 34% 25% 19% 6%	477
	21st to end	Day	4 9% 39% 27% 18% 3	539
September	1st to 10th	Day	4 11% 36% 28% 19% 3	490
	11th to 20th	Day	9% 17% 29% 26% 16% 3	490
	21st to end	Day	6% 14% 35% 28% 16% 2	490
October	1st to 10th	Day	8% 18% 26% 26% 17% 4	490
	11th to 20th	Day	12% 14% 23% 27% 19% 4	490
	21st to end	Day	13% 19% 27% 20% 18% 3	502
November	1st to 10th	Day	17% 19% 19% 21% 22% 3	300
	11th to 20th	Day	13% 18% 24% 21% 17% 7%	273
	21st to end	Day	13% 22% 22% 23% 16% 2	232
December	1st to 10th	Day	25% 16% 21% 18% 15% 6%	180
	11th to 20th	Day	21% 25% 24% 12% 12% 6%	180
	21st to end	Day	27% 18% 25% 18% 11% 2	198
January	1st to 10th	Night	15% 16% 23% 24% 16% 7%	760
	11th to 20th	Night	16% 13% 21% 32% 13% 4	760
	21st to end	Night	13% 14% 26% 25% 18% 4	836
February	1st to 10th	Night	15% 14% 24% 29% 15% 3	760
	11th to 20th	Night	15% 16% 21% 28% 16% 5%	744
	21st to end	Night	19% 15% 21% 31% 11% 3	628
March	1st to 10th	Night	21% 19% 23% 22% 13% 3	760
	11th to 20th	Night	16% 19% 20% 21% 21% 3	760
	21st to end	Night	16% 24% 23% 24% 12% 1	836
April	1st to 10th	Night	11% 21% 26% 26% 15% 1	760
	11th to 20th	Night	16% 16% 22% 27% 14% 4	751
	21st to end	Night	11% 16% 22% 28% 19% 4	760
May	1st to 10th	Night	11% 18% 24% 25% 18% 3	660
	11th to 20th	Night	8% 10% 21% 34% 23% 3	397
	21st to end	Night	9% 9% 18% 34% 26% 5%	411
June	1st to 10th	Night	7% 9% 19% 36% 26% 4	380
	11th to 20th	Night	3 10% 21% 37% 28% 1	380
	21st to end	Night	4 4 18% 40% 31% 3	380
July	1st to 10th	Night	1 2 15% 48% 31% 2	370
	11th to 20th	Night	1 3 19% 39% 36% 1	368
	21st to end	Night	2 3 9% 42% 38% 6%	494
August	1st to 10th	Night	3 3 13% 39% 38% 4	576
	11th to 20th	Night	2 4 20% 38% 31% 5%	752
	21st to end	Night	2 4 21% 42% 28% 3	836
September	1st to 10th	Night	2 5% 19% 38% 32% 4	760
	11th to 20th	Night	4 9% 18% 30% 33% 7%	760
	21st to end	Night	5% 9% 19% 33% 25% 9%	760
October	1st to 10th	Night	7% 10% 18% 32% 25% 7%	760
	11th to 20th	Night	10% 6% 15% 37% 26% 6%	760
	21st to end	Night	11% 11% 18% 26% 25% 10%	836
November	1st to 10th	Night	14% 12% 18% 24% 26% 7%	780
	11th to 20th	Night	12% 15% 21% 29% 18% 5%	764
	21st to end	Night	12% 15% 20% 27% 21% 5%	760
December	1st to 10th	Night	19% 13% 20% 28% 16% 4	760
	11th to 20th	Night	19% 15% 22% 25% 14% 4	760
	21st to end	Night	18% 18% 26% 26% 11% 2	836

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