Snow College Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Snow College Planetarium clear sky chart from 2005-11-06 to 2024-07-05.

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	4 10% 4 7% 5% 4 4 4 5% 7% 46%	89601
	Night	4 9% 3 7% 4 4 4 4 5% 7% 49%	89835
Transparency	Day	36% 7% 10% 19% 19% 9%	89327
	Night	35% 6% 10% 20% 19% 9%	89630
Seeing	Day	9% 14% 29% 30% 14% 4	14284
	Night	9% 7% 26% 35% 21% 4	24691

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	4 18% 7% 14% 5% 4 5% 3 5% 6% 30%	2858
	11th to 20th	Day	6% 20% 7% 13% 5% 4 3 2 3 4 32%	2860
	21st to end	Day	5% 14% 7% 10% 4 3 3 3 4 6% 41%	3146
February	1st to 10th	Day	6% 14% 5% 13% 6% 5% 3 3 5% 5% 35%	1710
	11th to 20th	Day	7% 14% 6% 9% 4 3 4 3 3 6% 40%	1745
	21st to end	Day	3 16% 4 12% 5% 4 3 3 4 7% 38%	1570
March	1st to 10th	Day	4 14% 7% 10% 5% 4 5% 4 4 6% 38%	3401
	11th to 20th	Day	2 12% 5% 8% 5% 4 4 4 6% 8% 40%	3490
	21st to end	Day	4 20% 7% 13% 7% 5% 5% 3 5% 5% 25%	3825
April	1st to 10th	Day	6% 12% 3 8% 5% 5% 4 3 5% 6% 42%	2104
	11th to 20th	Day	5% 11% 4 8% 6% 4 5% 3 6% 5% 43%	2352
	21st to end	Day	6% 13% 3 6% 4 4 4 4 5% 7% 44%	2470
May	1st to 10th	Day	6% 15% 3 8% 5% 4 4 3 5% 7% 39%	4110
	11th to 20th	Day	3 5% 2 6% 4 5% 5% 4 7% 8% 50%	4071
	21st to end	Day	5% 6% 2 6% 4 5% 5% 4 6% 8% 47%	4477
June	1st to 10th	Day	3 3 1 5% 3 5% 5% 3 6% 7% 59%	2470
	11th to 20th	Day	5% 3 1 4 3 2 4 3 5% 7% 62%	2470
	21st to end	Day	2 2 1 3 3 3 4 3 5% 7% 66%	2470
July	1st to 10th	Day	3 3 1 5% 4 4 5% 5% 6% 8% 56%	2394
	11th to 20th	Day	2 5% 2 6% 6% 6% 6% 5% 8% 8% 48%	2340
	21st to end	Day	4 5% 2 7% 5% 6% 7% 5% 8% 10% 42%	2545
August	1st to 10th	Day	3 4 1 5% 4 6% 6% 5% 7% 8% 50%	2328
	11th to 20th	Day	3 3 1 4 5% 5% 4 5% 6% 7% 58%	2299
	21st to end	Day	3 3 2 6% 4 5% 5% 4 6% 7% 54%	2528
September	1st to 10th	Day	3 3 1 4 4 4 4 4 6% 6% 61%	2030
	11th to 20th	Day	4 4 1 5% 3 3 3 4 5% 5% 62%	1980
	21st to end	Day	4 4 2 4 3 3 4 2 4 5% 66%	1980
October	1st to 10th	Day	5% 7% 2 5% 3 4 4 3 4 6% 55%	1890
	11th to 20th	Day	5% 6% 2 4 3 3 3 2 5% 6% 63%	1800
	21st to end	Day	4 7% 2 5% 3 2 3 3 5% 7% 58%	1980
November	1st to 10th	Day	6% 7% 4 6% 3 3 4 4 5% 6% 52%	1720
	11th to 20th	Day	7% 9% 2 5% 4 3 3 3 6% 6% 52%	1710
	21st to end	Day	5% 9% 5% 7% 4 3 3 4 4 6% 50%	1705
December	1st to 10th	Day	5% 12% 4 8% 5% 3 3 2 5% 6% 46%	1520
	11th to 20th	Day	5% 13% 4 12% 6% 5% 5% 4 5% 7% 36%	2107
	21st to end	Day	4 13% 5% 9% 5% 4 4 4 6% 9% 37%	3146
January	1st to 10th	Night	3 13% 11% 15% 5% 3 3 2 4 5% 35%	4097
	11th to 20th	Night	7% 21% 5% 12% 5% 3 4 3 4 5% 31%	4110
	21st to end	Night	4 16% 5% 8% 4 4 4 3 6% 6% 40%	4521
February	1st to 10th	Night	5% 12% 4 9% 6% 4 4 4 5% 6% 42%	2470
	11th to 20th	Night	5% 12% 4 10% 4 3 3 3 4 7% 43%	2318
	21st to end	Night	4 14% 4 10% 5% 4 4 4 5% 6% 42%	1884
March	1st to 10th	Night	3 15% 4 8% 4 4 4 3 5% 8% 41%	3473
	11th to 20th	Night	2 10% 3 8% 5% 4 4 4 5% 7% 48%	3490
	21st to end	Night	4 17% 5% 10% 5% 4 4 4 5% 6% 35%	3828
April	1st to 10th	Night	5% 10% 4 6% 4 4 4 4 5% 6% 48%	2075
	11th to 20th	Night	6% 9% 3 6% 5% 4 4 4 6% 8% 46%	1774
	21st to end	Night	6% 10% 3 5% 4 4 4 4 5% 8% 47%	1710
May	1st to 10th	Night	5% 12% 4 8% 4 4 5% 4 5% 7% 43%	2860
	11th to 20th	Night	3 4 1 4 3 5% 4 4 7% 7% 58%	2834
	21st to end	Night	6% 5% 2 5% 3 4 3 3 5% 8% 58%	3110
June	1st to 10th	Night	2 4 1 4 3 4 4 3 6% 8% 62%	1710
	11th to 20th	Night	3 3 1 3 3 3 4 3 5% 6% 67%	1710
	21st to end	Night	2 2 1 3 3 3 4 3 5% 5% 68%	1710
July	1st to 10th	Night	2 3 1 4 4 4 5% 4 7% 8% 59%	1660
	11th to 20th	Night	3 3 3 6% 6% 6% 6% 4 8% 8% 47%	1620
	21st to end	Night	2 4 2 7% 6% 5% 7% 5% 10% 12% 41%	1773
August	1st to 10th	Night	2 3 1 4 4 5% 6% 6% 7% 9% 52%	1611
	11th to 20th	Night	2 3 2 4 3 4 4 5% 6% 8% 59%	1602
	21st to end	Night	2 3 1 5% 3 3 4 5% 6% 7% 61%	1826
September	1st to 10th	Night	2 2 1 2 3 2 4 4 6% 5% 69%	1927
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 6% 68%	1980
	21st to end	Night	5% 3 1 3 2 2 3 3 3 5% 70%	1980
October	1st to 10th	Night	5% 5% 2 4 3 4 4 4 4 6% 59%	2070
	11th to 20th	Night	4 5% 2 4 3 2 3 3 5% 6% 63%	2160
	21st to end	Night	5% 6% 2 5% 3 2 3 2 5% 6% 61%	2376
November	1st to 10th	Night	5% 7% 3 4 4 2 4 3 5% 8% 56%	2355
	11th to 20th	Night	6% 7% 2 5% 3 3 4 3 5% 6% 55%	2470
	21st to end	Night	5% 9% 4 8% 4 3 3 3 5% 6% 49%	2470
December	1st to 10th	Night	7% 12% 3 7% 4 3 3 3 5% 6% 49%	2637
	11th to 20th	Night	7% 12% 4 9% 4 3 4 4 6% 7% 38%	3113
	21st to end	Night	4 13% 4 13% 5% 5% 4 5% 6% 7% 34%	4521

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	49% 7% 14% 17% 11% 2	2859
	11th to 20th	Day	55% 5% 13% 16% 8% 3	2860
	21st to end	Day	42% 5% 17% 21% 11% 4	3146
February	1st to 10th	Day	51% 5% 11% 16% 12% 4	1710
	11th to 20th	Day	46% 4 11% 18% 17% 4	1754
	21st to end	Day	45% 7% 9% 22% 14% 3	1570
March	1st to 10th	Day	43% 9% 8% 22% 12% 7%	3401
	11th to 20th	Day	40% 14% 13% 22% 8% 3	3490
	21st to end	Day	58% 9% 9% 16% 7% 2	3825
April	1st to 10th	Day	42% 7% 9% 20% 18% 4	2104
	11th to 20th	Day	42% 5% 9% 21% 19% 3	2355
	21st to end	Day	41% 6% 8% 26% 18% 2	2470
May	1st to 10th	Day	39% 8% 10% 21% 18% 4	4110
	11th to 20th	Day	28% 6% 9% 30% 25% 2	4074
	21st to end	Day	31% 9% 9% 22% 26% 3	4477
June	1st to 10th	Day	24% 7% 9% 21% 30% 10%	2470
	11th to 20th	Day	24% 4 7% 12% 29% 23%	2470
	21st to end	Day	19% 6% 6% 11% 24% 34%	2470
July	1st to 10th	Day	29% 7% 8% 11% 17% 28%	2330
	11th to 20th	Day	33% 10% 10% 13% 15% 19%	2271
	21st to end	Day	36% 10% 12% 14% 14% 15%	2404
August	1st to 10th	Day	33% 8% 12% 12% 16% 18%	2313
	11th to 20th	Day	27% 7% 9% 10% 24% 22%	2299
	21st to end	Day	30% 8% 9% 15% 24% 14%	2528
September	1st to 10th	Day	25% 7% 10% 11% 24% 23%	2030
	11th to 20th	Day	25% 7% 8% 15% 24% 21%	1980
	21st to end	Day	24% 4 6% 19% 32% 15%	1980
October	1st to 10th	Day	32% 7% 7% 18% 25% 11%	1890
	11th to 20th	Day	26% 5% 5% 22% 33% 10%	1800
	21st to end	Day	27% 5% 9% 29% 23% 6%	1980
November	1st to 10th	Day	32% 7% 10% 20% 24% 8%	1720
	11th to 20th	Day	30% 4 11% 26% 23% 6%	1710
	21st to end	Day	36% 4 11% 22% 19% 9%	1705
December	1st to 10th	Day	39% 6% 16% 23% 10% 6%	1520
	11th to 20th	Day	47% 5% 10% 19% 15% 4	2106
	21st to end	Day	43% 6% 10% 13% 16% 11%	3146
January	1st to 10th	Night	49% 5% 13% 14% 16% 2	4097
	11th to 20th	Night	54% 6% 11% 16% 9% 3	4110
	21st to end	Night	45% 5% 13% 21% 11% 5%	4521
February	1st to 10th	Night	45% 5% 11% 20% 15% 4	2470
	11th to 20th	Night	43% 6% 7% 19% 19% 5%	2319
	21st to end	Night	42% 5% 10% 20% 17% 5%	1884
March	1st to 10th	Night	38% 6% 11% 25% 16% 4	3473
	11th to 20th	Night	34% 12% 12% 27% 14% 2	3490
	21st to end	Night	47% 8% 10% 16% 16% 2	3828
April	1st to 10th	Night	36% 6% 11% 20% 20% 7%	2075
	11th to 20th	Night	36% 6% 11% 23% 22% 3	1774
	21st to end	Night	36% 6% 10% 28% 17% 3	1710
May	1st to 10th	Night	34% 8% 9% 23% 20% 5%	2860
	11th to 20th	Night	22% 7% 10% 30% 28% 4	2835
	21st to end	Night	23% 7% 10% 28% 24% 7%	3110
June	1st to 10th	Night	22% 5% 10% 24% 29% 11%	1710
	11th to 20th	Night	20% 4 8% 14% 33% 22%	1710
	21st to end	Night	18% 5% 5% 10% 24% 38%	1710
July	1st to 10th	Night	24% 6% 9% 12% 20% 30%	1615
	11th to 20th	Night	32% 7% 11% 15% 16% 19%	1566
	21st to end	Night	32% 11% 15% 16% 12% 14%	1674
August	1st to 10th	Night	28% 8% 12% 19% 15% 18%	1602
	11th to 20th	Night	24% 8% 10% 13% 20% 25%	1602
	21st to end	Night	22% 6% 10% 18% 25% 18%	1826
September	1st to 10th	Night	17% 5% 10% 15% 25% 28%	1927
	11th to 20th	Night	18% 6% 8% 21% 22% 25%	1980
	21st to end	Night	19% 3 5% 18% 35% 20%	1980
October	1st to 10th	Night	28% 5% 7% 21% 28% 11%	2070
	11th to 20th	Night	24% 5% 6% 20% 34% 10%	2160
	21st to end	Night	26% 6% 8% 29% 24% 7%	2376
November	1st to 10th	Night	28% 5% 9% 26% 23% 9%	2355
	11th to 20th	Night	30% 4 10% 26% 24% 5%	2470
	21st to end	Night	36% 4 8% 21% 21% 10%	2470
December	1st to 10th	Night	38% 5% 12% 23% 16% 6%	2637
	11th to 20th	Night	44% 6% 10% 21% 16% 3	3113
	21st to end	Night	46% 6% 10% 15% 12% 10%	4521

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	20% 11% 27% 26% 13% 3	270
	11th to 20th	Day	13% 13% 29% 27% 14% 3	270
	21st to end	Day	8% 9% 39% 24% 17% 2	297
February	1st to 10th	Day	12% 18% 35% 26% 9%	270
	11th to 20th	Day	15% 17% 34% 24% 9% 1	350
	21st to end	Day	10% 24% 39% 22% 5% 1	382
March	1st to 10th	Day	16% 23% 28% 23% 9% 1	460
	11th to 20th	Day	12% 17% 35% 24% 11% 2	460
	21st to end	Day	15% 25% 37% 20% 3	506
April	1st to 10th	Day	17% 23% 27% 21% 10% 2	460
	11th to 20th	Day	14% 25% 33% 20% 6% 1	452
	21st to end	Day	17% 22% 28% 21% 10% 2	460
May	1st to 10th	Day	11% 22% 34% 24% 8% 2	463
	11th to 20th	Day	10% 14% 28% 30% 13% 4	488
	21st to end	Day	10% 18% 26% 29% 14% 3	529
June	1st to 10th	Day	4 11% 32% 34% 16% 3	490
	11th to 20th	Day	8% 17% 32% 28% 12% 2	490
	21st to end	Day	4 10% 37% 36% 11% 2	490
July	1st to 10th	Day	4 6% 24% 42% 19% 4	463
	11th to 20th	Day	3 3 26% 42% 20% 5%	444
	21st to end	Day	5% 4 15% 44% 24% 8%	487
August	1st to 10th	Day	4 4 29% 40% 19% 4	447
	11th to 20th	Day	5% 5% 28% 35% 20% 6%	437
	21st to end	Day	5% 4 29% 36% 19% 6%	495
September	1st to 10th	Day	4 8% 27% 32% 19% 10%	450
	11th to 20th	Day	5% 10% 29% 32% 17% 6%	450
	21st to end	Day	8% 11% 24% 29% 20% 8%	450
October	1st to 10th	Day	9% 14% 25% 28% 16% 9%	360
	11th to 20th	Day	6% 10% 22% 29% 22% 10%	270
	21st to end	Day	9% 13% 28% 31% 16% 2	297
November	1st to 10th	Day	12% 16% 26% 21% 19% 6%	270
	11th to 20th	Day	7% 9% 31% 32% 16% 6%	270
	21st to end	Day	10% 10% 23% 31% 20% 6%	270
December	1st to 10th	Day	12% 11% 29% 31% 12% 5%	270
	11th to 20th	Day	13% 11% 28% 32% 14% 1	270
	21st to end	Day	12% 12% 32% 28% 13% 1	297
January	1st to 10th	Night	13% 7% 27% 33% 16% 5%	760
	11th to 20th	Night	11% 7% 29% 33% 16% 4	760
	21st to end	Night	10% 9% 30% 34% 15% 3	836
February	1st to 10th	Night	12% 7% 37% 31% 9% 4	760
	11th to 20th	Night	10% 8% 38% 32% 10% 2	744
	21st to end	Night	11% 9% 35% 35% 9%	628
March	1st to 10th	Night	11% 10% 32% 33% 13% 2	760
	11th to 20th	Night	8% 6% 31% 34% 19% 2	760
	21st to end	Night	10% 12% 36% 30% 10% 2	836
April	1st to 10th	Night	12% 12% 33% 30% 12% 1	745
	11th to 20th	Night	10% 11% 34% 33% 11% 1	563
	21st to end	Night	12% 12% 29% 32% 14% 2	570
May	1st to 10th	Night	11% 10% 32% 32% 13% 2	570
	11th to 20th	Night	8% 5% 25% 38% 20% 4	561
	21st to end	Night	8% 10% 23% 35% 22% 3	617
June	1st to 10th	Night	5% 5% 24% 41% 25% 1	570
	11th to 20th	Night	5% 9% 26% 40% 19% 1	570
	21st to end	Night	3 3 25% 43% 24% 3	570
July	1st to 10th	Night	4 1 16% 36% 38% 4	539
	11th to 20th	Night	4 2 11% 39% 39% 5%	522
	21st to end	Night	3 1 9% 36% 45% 6%	591
August	1st to 10th	Night	4 1 14% 46% 32% 3	537
	11th to 20th	Night	3 1 14% 45% 33% 4	534
	21st to end	Night	3 3 15% 37% 36% 6%	594
September	1st to 10th	Night	3 2 13% 37% 37% 8%	667
	11th to 20th	Night	2 3 18% 37% 32% 8%	720
	21st to end	Night	7% 4 17% 32% 32% 9%	720
October	1st to 10th	Night	9% 5% 25% 34% 25% 3	720
	11th to 20th	Night	6% 5% 22% 37% 24% 5%	720
	21st to end	Night	8% 5% 27% 33% 21% 5%	792
November	1st to 10th	Night	8% 8% 24% 35% 21% 5%	740
	11th to 20th	Night	10% 5% 30% 32% 18% 5%	760
	21st to end	Night	10% 7% 25% 33% 21% 5%	760
December	1st to 10th	Night	13% 7% 30% 34% 14% 3	927
	11th to 20th	Night	14% 7% 29% 34% 15% 1	832
	21st to end	Night	16% 10% 31% 31% 10% 2	836

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