Custer Institute
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Custer Institute clear sky chart from 2005-11-06 to 2024-11-06.

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	16% 7% 2 6% 4 4 4 3 5% 7% 42%	101191
	Night	18% 8% 3 6% 4 4 4 3 5% 7% 39%	97885
Transparency	Day	23% 46% 7% 13% 9% 1	104753
	Night	25% 46% 7% 12% 9% 1	102412
Seeing	Day	18% 6% 22% 31% 18% 5%	15115
	Night	20% 8% 28% 29% 13% 2	24119

By Month

Percentage Cloud Cover NE (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	16% 8% 2 6% 4 4 4 4 5% 6% 42%	2517
	11th to 20th	Day	18% 8% 3 7% 4 5% 4 4 6% 6% 37%	2520
	21st to end	Day	22% 11% 4 7% 4 3 4 3 4 6% 33%	2834
February	1st to 10th	Day	18% 6% 2 6% 4 5% 4 4 6% 7% 37%	1819
	11th to 20th	Day	15% 6% 2 6% 5% 4 3 3 5% 6% 44%	1848
	21st to end	Day	21% 8% 3 5% 3 4 4 3 5% 7% 38%	1570
March	1st to 10th	Day	26% 8% 2 5% 3 3 3 3 4 6% 38%	3116
	11th to 20th	Day	10% 4 1 5% 3 3 4 3 6% 8% 52%	3252
	21st to end	Day	20% 8% 3 5% 3 3 4 3 5% 6% 40%	4152
April	1st to 10th	Day	18% 8% 2 5% 4 3 4 3 5% 6% 42%	2280
	11th to 20th	Day	19% 6% 2 5% 4 3 3 3 5% 7% 44%	2254
	21st to end	Day	19% 6% 2 5% 3 4 4 3 6% 6% 43%	2399
May	1st to 10th	Day	25% 10% 3 6% 3 3 3 3 5% 7% 32%	4106
	11th to 20th	Day	22% 8% 2 7% 4 3 4 3 5% 6% 36%	4171
	21st to end	Day	13% 6% 2 5% 3 3 4 4 6% 8% 47%	4837
June	1st to 10th	Day	16% 6% 1 5% 4 4 4 4 6% 8% 43%	2660
	11th to 20th	Day	12% 6% 3 6% 4 4 5% 5% 6% 9% 43%	2660
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 9% 45%	2660
July	1st to 10th	Day	12% 8% 3 8% 5% 4 4 5% 6% 8% 38%	4440
	11th to 20th	Day	12% 6% 3 6% 4 3 4 4 6% 9% 42%	4440
	21st to end	Day	16% 8% 3 7% 4 4 5% 4 6% 7% 37%	4814
August	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 6% 7% 38%	4181
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 6% 8% 47%	3773
	21st to end	Day	6% 4 1 5% 3 4 5% 4 6% 9% 52%	4180
September	1st to 10th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 48%	2280
	11th to 20th	Day	11% 5% 2 4 3 3 2 3 5% 7% 53%	2126
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 6% 41%	2090
October	1st to 10th	Day	21% 5% 2 5% 3 3 3 3 5% 7% 44%	2090
	11th to 20th	Day	14% 6% 2 5% 3 3 4 4 6% 6% 48%	1877
	21st to end	Day	19% 6% 2 6% 4 3 5% 3 5% 7% 41%	1881
November	1st to 10th	Day	15% 7% 2 4 4 4 4 4 4 5% 49%	1717
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 5% 6% 45%	1710
	21st to end	Day	15% 9% 2 4 3 4 3 5% 5% 6% 43%	1710
December	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 5% 7% 35%	1549
	11th to 20th	Day	21% 8% 2 6% 3 2 2 2 5% 6% 42%	1906
	21st to end	Day	20% 11% 3 8% 4 5% 4 3 5% 5% 33%	2772
January	1st to 10th	Night	18% 6% 2 5% 3 4 4 3 5% 7% 44%	4429
	11th to 20th	Night	18% 9% 3 7% 3 3 3 3 4 4 42%	4440
	21st to end	Night	24% 11% 4 7% 4 4 3 4 4 6% 30%	4820
February	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 4 7% 37%	2359
	11th to 20th	Night	19% 8% 2 5% 4 4 4 3 6% 8% 37%	2218
	21st to end	Night	20% 7% 2 5% 4 4 4 3 5% 6% 39%	1884
March	1st to 10th	Night	25% 11% 3 6% 3 3 3 2 4 5% 35%	3724
	11th to 20th	Night	12% 6% 2 7% 4 4 4 4 6% 6% 44%	3705
	21st to end	Night	24% 8% 2 5% 3 3 4 3 4 5% 38%	3476
April	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 39%	1900
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 5% 6% 42%	1875
	21st to end	Night	20% 8% 3 7% 4 3 3 3 4 7% 39%	1781
May	1st to 10th	Night	22% 13% 5% 7% 5% 4 4 3 5% 6% 27%	2834
	11th to 20th	Night	25% 9% 3 7% 5% 5% 4 4 6% 7% 27%	2749
	21st to end	Night	16% 9% 2 5% 4 4 3 3 6% 9% 37%	2738
June	1st to 10th	Night	17% 8% 3 5% 4 4 4 3 5% 7% 39%	1520
	11th to 20th	Night	17% 5% 2 4 4 4 3 4 6% 9% 40%	1520
	21st to end	Night	12% 7% 3 5% 4 5% 5% 5% 6% 9% 38%	1520
July	1st to 10th	Night	14% 10% 4 9% 5% 4 5% 5% 5% 8% 31%	2520
	11th to 20th	Night	13% 7% 2 6% 6% 4 4 5% 7% 11% 34%	2520
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 8% 34%	2739
August	1st to 10th	Night	21% 7% 3 6% 4 5% 5% 5% 6% 8% 30%	2716
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 8% 39%	3051
	21st to end	Night	7% 4 2 5% 3 3 3 4 6% 9% 54%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 5% 5% 4 5% 5% 43%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 52%	2053
	21st to end	Night	18% 7% 3 6% 4 4 5% 4 5% 6% 39%	2090
October	1st to 10th	Night	20% 8% 3 6% 4 4 3 3 4 5% 40%	2090
	11th to 20th	Night	16% 6% 2 5% 3 3 3 2 4 6% 48%	2299
	21st to end	Night	20% 8% 3 7% 4 3 4 3 4 6% 39%	2717
November	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 5% 46%	2483
	11th to 20th	Night	19% 8% 2 5% 4 3 3 3 4 5% 43%	2470
	21st to end	Night	15% 5% 3 5% 5% 4 3 3 5% 8% 43%	2470
December	1st to 10th	Night	21% 9% 3 6% 4 4 4 3 5% 6% 35%	2627
	11th to 20th	Night	19% 9% 3 6% 3 3 4 4 5% 6% 38%	3288
	21st to end	Night	20% 10% 3 6% 4 4 4 3 5% 5% 35%	4884

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	28% 37% 5% 20% 9% 2	2518
	11th to 20th	Day	28% 39% 6% 20% 7% 1	2520
	21st to end	Day	38% 39% 7% 11% 5% 1	2834
February	1st to 10th	Day	17% 66% 7% 7% 3	1819
	11th to 20th	Day	16% 64% 2 9% 9% 1	1857
	21st to end	Day	17% 65% 5% 7% 5% 1	1570
March	1st to 10th	Day	34% 40% 7% 12% 6% 1	3119
	11th to 20th	Day	14% 41% 11% 24% 10%	3252
	21st to end	Day	28% 40% 4 12% 12% 4	4152
April	1st to 10th	Day	18% 64% 3 7% 6% 1	2280
	11th to 20th	Day	14% 65% 3 8% 9%	2254
	21st to end	Day	16% 65% 3 8% 7% 1	2399
May	1st to 10th	Day	35% 40% 13% 7% 5% 1	4120
	11th to 20th	Day	33% 40% 13% 11% 3	4175
	21st to end	Day	21% 40% 7% 17% 14% 1	4837
June	1st to 10th	Day	14% 65% 4 9% 7%	2660
	11th to 20th	Day	14% 65% 5% 8% 7% 1	2660
	21st to end	Day	12% 64% 5% 9% 9%	2660
July	1st to 10th	Day	32% 39% 7% 10% 11% 1	4370
	11th to 20th	Day	28% 36% 13% 14% 9%	4365
	21st to end	Day	33% 36% 8% 12% 9% 1	4661
August	1st to 10th	Day	33% 37% 11% 15% 4	4167
	11th to 20th	Day	23% 37% 10% 24% 5%	3810
	21st to end	Day	17% 36% 10% 23% 13% 1	4180
September	1st to 10th	Day	15% 60% 6% 11% 7%	2280
	11th to 20th	Day	15% 60% 4 9% 11% 1	2126
	21st to end	Day	21% 60% 4 8% 7%	2090
October	1st to 10th	Day	20% 36% 7% 14% 18% 5%	3480
	11th to 20th	Day	14% 36% 5% 15% 26% 3	3093
	21st to end	Day	17% 36% 11% 19% 16% 2	3081
November	1st to 10th	Day	18% 58% 7% 9% 8% 1	1717
	11th to 20th	Day	15% 59% 4 10% 11% 1	1710
	21st to end	Day	15% 60% 6% 12% 6% 2	1710
December	1st to 10th	Day	21% 59% 7% 6% 7%	1549
	11th to 20th	Day	22% 48% 6% 9% 13% 1	1906
	21st to end	Day	36% 38% 5% 16% 5%	2772
January	1st to 10th	Night	26% 37% 8% 21% 6% 3	4429
	11th to 20th	Night	28% 37% 8% 15% 10% 2	4440
	21st to end	Night	42% 39% 5% 9% 5% 1	4820
February	1st to 10th	Night	17% 65% 5% 9% 3 1	2359
	11th to 20th	Night	16% 63% 3 8% 8% 1	2220
	21st to end	Night	17% 64% 3 7% 6% 2	1884
March	1st to 10th	Night	39% 40% 4 10% 5% 1	3734
	11th to 20th	Night	22% 40% 10% 16% 10% 2	3706
	21st to end	Night	31% 40% 4 5% 16% 4	3476
April	1st to 10th	Night	17% 65% 3 8% 6% 1	1900
	11th to 20th	Night	14% 66% 5% 8% 7% 1	1875
	21st to end	Night	17% 65% 3 8% 6% 1	1781
May	1st to 10th	Night	37% 40% 10% 8% 3 1	2840
	11th to 20th	Night	39% 40% 11% 7% 2	2750
	21st to end	Night	27% 40% 9% 15% 8% 1	2739
June	1st to 10th	Night	16% 65% 2 8% 8% 1	1520
	11th to 20th	Night	13% 65% 5% 11% 6%	1520
	21st to end	Night	14% 63% 3 11% 7% 1	1520
July	1st to 10th	Night	34% 37% 8% 11% 9%	2480
	11th to 20th	Night	31% 36% 11% 14% 7%	2472
	21st to end	Night	36% 35% 9% 7% 12% 1	2651
August	1st to 10th	Night	39% 37% 10% 10% 4 1	2706
	11th to 20th	Night	28% 37% 10% 20% 4	3080
	21st to end	Night	14% 36% 9% 26% 15% 1	3476
September	1st to 10th	Night	16% 60% 4 12% 6% 1	1900
	11th to 20th	Night	15% 59% 4 9% 11% 1	2053
	21st to end	Night	22% 61% 4 7% 6%	2090
October	1st to 10th	Night	26% 36% 7% 16% 13% 2	3480
	11th to 20th	Night	12% 36% 6% 16% 27% 4	3822
	21st to end	Night	16% 37% 9% 19% 17% 2	4467
November	1st to 10th	Night	19% 57% 4 10% 8% 1	2483
	11th to 20th	Night	17% 60% 4 9% 8% 3	2470
	21st to end	Night	15% 60% 5% 13% 6% 1	2470
December	1st to 10th	Night	22% 60% 5% 8% 5%	2627
	11th to 20th	Night	24% 49% 6% 9% 11% 1	3288
	21st to end	Night	34% 37% 7% 15% 7%	4884

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	19% 13% 34% 22% 10% 1	180
	11th to 20th	Day	18% 22% 36% 21% 3 1	180
	21st to end	Day	22% 10% 33% 24% 9% 3	216
February	1st to 10th	Day	22% 11% 34% 27% 6%	379
	11th to 20th	Day	19% 12% 40% 26% 3	456
	21st to end	Day	23% 13% 31% 25% 8% 1	382
March	1st to 10th	Day	19% 9% 31% 32% 8%	460
	11th to 20th	Day	16% 9% 38% 29% 7% 1	460
	21st to end	Day	22% 8% 34% 26% 7% 2	506
April	1st to 10th	Day	23% 8% 30% 28% 9% 2	460
	11th to 20th	Day	22% 4 25% 37% 11% 1	452
	21st to end	Day	23% 7% 22% 34% 12% 2	460
May	1st to 10th	Day	25% 6% 23% 32% 12% 2	463
	11th to 20th	Day	20% 3 21% 36% 18% 2	487
	21st to end	Day	20% 2 12% 32% 26% 9%	529
June	1st to 10th	Day	17% 2 15% 35% 25% 6%	490
	11th to 20th	Day	13% 2 14% 37% 25% 8%	490
	21st to end	Day	13% 2 13% 36% 29% 7%	490
July	1st to 10th	Day	16% 1 9% 32% 32% 9%	485
	11th to 20th	Day	11% 1 8% 33% 36% 11%	484
	21st to end	Day	12% 2 9% 40% 32% 4	532
August	1st to 10th	Day	12% 2 9% 34% 32% 10%	486
	11th to 20th	Day	15% 1 9% 34% 31% 10%	479
	21st to end	Day	11% 2 11% 36% 28% 12%	539
September	1st to 10th	Day	18% 1 19% 25% 25% 12%	490
	11th to 20th	Day	13% 3 16% 31% 22% 14%	490
	21st to end	Day	22% 3 21% 32% 18% 4	490
October	1st to 10th	Day	26% 3 18% 25% 19% 9%	490
	11th to 20th	Day	15% 7% 26% 32% 16% 4	473
	21st to end	Day	23% 7% 27% 28% 14% 3	429
November	1st to 10th	Day	17% 7% 24% 32% 17% 3	381
	11th to 20th	Day	22% 13% 30% 26% 9% 1	370
	21st to end	Day	18% 15% 29% 28% 11% 1	370
December	1st to 10th	Day	21% 12% 35% 22% 10%	209
	11th to 20th	Day	24% 14% 33% 23% 5%	180
	21st to end	Day	22% 12% 33% 22% 8% 3	198
January	1st to 10th	Night	21% 16% 37% 21% 6%	760
	11th to 20th	Night	27% 14% 32% 22% 4	760
	21st to end	Night	21% 15% 37% 23% 3	836
February	1st to 10th	Night	21% 13% 37% 26% 4	760
	11th to 20th	Night	22% 13% 42% 21% 2	743
	21st to end	Night	25% 11% 37% 22% 5%	628
March	1st to 10th	Night	21% 10% 37% 26% 6%	760
	11th to 20th	Night	20% 8% 38% 27% 6% 1	760
	21st to end	Night	22% 8% 34% 26% 8% 1	836
April	1st to 10th	Night	24% 6% 30% 31% 8% 1	760
	11th to 20th	Night	21% 5% 30% 33% 11%	751
	21st to end	Night	22% 4 29% 34% 10% 1	641
May	1st to 10th	Night	24% 5% 22% 31% 15% 2	570
	11th to 20th	Night	23% 4 25% 36% 12%	524
	21st to end	Night	21% 2 14% 36% 25% 2	411
June	1st to 10th	Night	20% 4 19% 34% 21% 2	380
	11th to 20th	Night	18% 2 12% 41% 24% 3	380
	21st to end	Night	13% 1 20% 43% 21% 2	380
July	1st to 10th	Night	15% 2 12% 39% 28% 4	370
	11th to 20th	Night	13% 2 11% 41% 31% 3	368
	21st to end	Night	15% 1 14% 44% 24% 2	416
August	1st to 10th	Night	10% 1 13% 48% 26% 2	497
	11th to 20th	Night	14% 2 13% 35% 30% 6%	716
	21st to end	Night	14% 2 12% 38% 28% 7%	836
September	1st to 10th	Night	15% 3 21% 33% 24% 5%	760
	11th to 20th	Night	17% 2 15% 35% 24% 8%	760
	21st to end	Night	23% 3 19% 32% 20% 3	760
October	1st to 10th	Night	24% 8% 24% 25% 16% 4	760
	11th to 20th	Night	19% 8% 29% 30% 13% 1	760
	21st to end	Night	25% 9% 31% 24% 10% 1	836
November	1st to 10th	Night	17% 8% 33% 27% 13% 2	764
	11th to 20th	Night	21% 13% 32% 25% 9% 1	760
	21st to end	Night	18% 16% 32% 26% 7% 1	760
December	1st to 10th	Night	25% 14% 32% 23% 6% 1	760
	11th to 20th	Night	23% 16% 33% 23% 4 1	760
	21st to end	Night	25% 14% 35% 22% 4	836

Page generated at 2024-11-03 13:19:05. Version ac07

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