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 2024-01-16.

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% 6% 2 5% 4 3 4 4 5% 7% 43%	74821
	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 40%	74962
Transparency	Day	40% 5% 13% 24% 16% 2	74526
	Night	44% 4 12% 22% 16% 2	74791
Seeing	Day	18% 6% 23% 31% 18% 5%	13987
	Night	20% 8% 28% 29% 12% 2	23123

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	15% 7% 2 4 5% 3 4 5% 6% 8% 40%	1517
	11th to 20th	Day	17% 8% 3 6% 4 3 5% 4 6% 7% 38%	1480
	21st to end	Day	16% 7% 3 5% 5% 3 4 5% 5% 8% 39%	1618
February	1st to 10th	Day	19% 6% 3 6% 4 4 5% 5% 6% 8% 36%	1724
	11th to 20th	Day	16% 6% 2 6% 5% 3 4 3 5% 7% 43%	1748
	21st to end	Day	21% 8% 3 5% 4 3 4 4 4 7% 38%	1480
March	1st to 10th	Day	16% 5% 2 5% 5% 3 3 4 5% 8% 45%	1797
	11th to 20th	Day	15% 6% 2 5% 4 3 4 4 6% 7% 45%	1850
	21st to end	Day	19% 7% 3 6% 4 2 4 3 4 6% 43%	2362
April	1st to 10th	Day	17% 7% 2 4 4 3 4 4 4 6% 43%	2160
	11th to 20th	Day	18% 5% 2 4 4 3 3 4 5% 7% 45%	2134
	21st to end	Day	19% 5% 2 4 3 3 4 4 5% 7% 43%	2272
May	1st to 10th	Day	22% 8% 3 5% 4 2 3 4 4 7% 39%	2340
	11th to 20th	Day	17% 6% 2 5% 4 2 4 3 6% 7% 43%	2349
	21st to end	Day	14% 6% 2 4 4 2 4 5% 6% 9% 45%	2725
June	1st to 10th	Day	17% 5% 2 4 4 3 4 5% 6% 10% 41%	2520
	11th to 20th	Day	13% 6% 3 5% 4 3 5% 5% 6% 9% 41%	2520
	21st to end	Day	11% 6% 2 5% 4 2 5% 5% 5% 9% 46%	2520
July	1st to 10th	Day	12% 5% 2 4 4 3 4 5% 6% 9% 47%	2520
	11th to 20th	Day	10% 5% 3 5% 5% 3 4 5% 6% 8% 46%	2520
	21st to end	Day	13% 4 2 5% 4 3 5% 5% 6% 9% 44%	2744
August	1st to 10th	Day	10% 5% 2 4 3 3 4 6% 7% 8% 47%	2375
	11th to 20th	Day	12% 6% 2 4 4 3 4 5% 6% 8% 46%	2156
	21st to end	Day	11% 5% 2 5% 5% 4 4 5% 5% 9% 46%	2376
September	1st to 10th	Day	14% 6% 2 5% 5% 2 3 4 5% 6% 48%	2160
	11th to 20th	Day	12% 5% 2 4 4 2 3 3 5% 8% 54%	2015
	21st to end	Day	18% 6% 2 4 5% 2 4 4 5% 6% 43%	1980
October	1st to 10th	Day	22% 5% 2 4 4 2 3 3 4 6% 45%	1980
	11th to 20th	Day	15% 6% 2 4 4 2 4 5% 6% 7% 46%	1779
	21st to end	Day	19% 5% 3 6% 4 2 4 4 5% 7% 40%	1782
November	1st to 10th	Day	15% 7% 2 3 4 3 4 4 4 5% 49%	1671
	11th to 20th	Day	17% 6% 3 4 4 2 4 4 5% 7% 45%	1710
	21st to end	Day	15% 8% 3 4 3 3 4 5% 6% 7% 43%	1710
December	1st to 10th	Day	20% 7% 3 6% 5% 3 4 5% 5% 7% 35%	1549
	11th to 20th	Day	21% 7% 2 5% 4 2 3 3 4 6% 43%	1906
	21st to end	Day	20% 11% 3 8% 4 4 5% 3 5% 5% 33%	2772
January	1st to 10th	Night	17% 7% 3 5% 5% 3 4 4 6% 8% 40%	2649
	11th to 20th	Night	21% 8% 3 6% 4 3 4 5% 4 6% 37%	2591
	21st to end	Night	15% 8% 3 5% 4 3 4 4 5% 7% 41%	2736
February	1st to 10th	Night	18% 8% 3 6% 5% 3 4 5% 5% 6% 36%	2235
	11th to 20th	Night	19% 7% 2 4 4 3 4 5% 6% 9% 36%	2098
	21st to end	Night	21% 6% 2 5% 5% 3 4 4 4 6% 40%	1776
March	1st to 10th	Night	17% 7% 2 5% 4 2 4 4 4 7% 43%	2150
	11th to 20th	Night	17% 7% 2 6% 4 2 4 4 6% 7% 40%	2107
	21st to end	Night	18% 8% 3 5% 3 2 4 4 4 6% 43%	1990
April	1st to 10th	Night	19% 8% 3 5% 5% 3 4 4 4 6% 40%	1800
	11th to 20th	Night	18% 7% 2 5% 4 2 4 3 4 6% 43%	1775
	21st to end	Night	21% 7% 3 6% 4 2 3 3 4 7% 39%	1688
May	1st to 10th	Night	21% 9% 3 5% 5% 4 6% 4 5% 6% 33%	1620
	11th to 20th	Night	19% 7% 3 5% 6% 3 4 4 6% 8% 34%	1557
	21st to end	Night	18% 7% 3 5% 5% 4 4 4 5% 10% 35%	1550
June	1st to 10th	Night	18% 7% 3 4 5% 3 5% 4 5% 7% 38%	1440
	11th to 20th	Night	18% 5% 2 4 5% 3 4 4 6% 9% 39%	1440
	21st to end	Night	12% 6% 3 4 5% 4 6% 5% 5% 9% 40%	1440
July	1st to 10th	Night	11% 5% 3 6% 6% 4 6% 6% 6% 9% 38%	1440
	11th to 20th	Night	12% 6% 3 5% 4 2 5% 6% 7% 10% 39%	1440
	21st to end	Night	12% 5% 3 5% 5% 4 6% 5% 6% 10% 40%	1574
August	1st to 10th	Night	7% 4 2 4 6% 5% 6% 7% 7% 10% 43%	1544
	11th to 20th	Night	11% 5% 2 6% 7% 4 6% 6% 6% 8% 39%	1741
	21st to end	Night	11% 5% 3 5% 5% 3 4 5% 6% 9% 46%	1980
September	1st to 10th	Night	15% 6% 3 5% 6% 3 5% 5% 5% 6% 41%	1800
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 7% 51%	1944
	21st to end	Night	18% 6% 3 5% 5% 3 5% 4 5% 6% 40%	1980
October	1st to 10th	Night	21% 7% 3 5% 5% 3 4 3 4 5% 40%	1980
	11th to 20th	Night	17% 7% 2 4 4 2 3 3 4 7% 46%	2177
	21st to end	Night	21% 8% 3 6% 5% 2 4 4 4 6% 37%	2574
November	1st to 10th	Night	15% 6% 3 5% 4 3 5% 4 4 6% 45%	2407
	11th to 20th	Night	19% 7% 3 5% 4 2 3 4 4 5% 43%	2470
	21st to end	Night	16% 5% 3 5% 5% 3 4 4 5% 8% 44%	2470
December	1st to 10th	Night	21% 8% 3 6% 4 3 4 4 5% 6% 35%	2627
	11th to 20th	Night	20% 8% 3 6% 4 2 4 4 5% 6% 38%	3288
	21st to end	Night	20% 10% 3 6% 4 3 5% 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	39% 4 11% 28% 15% 3	1518
	11th to 20th	Day	46% 5% 14% 21% 13% 2	1480
	21st to end	Day	42% 4 10% 25% 16% 3	1618
February	1st to 10th	Day	46% 4 14% 22% 13% 1	1724
	11th to 20th	Day	41% 3 11% 26% 16% 3	1757
	21st to end	Day	46% 3 11% 21% 15% 3	1480
March	1st to 10th	Day	39% 4 15% 23% 15% 3	1800
	11th to 20th	Day	39% 3 13% 26% 17% 2	1850
	21st to end	Day	45% 3 10% 19% 18% 5%	2362
April	1st to 10th	Day	41% 4 11% 22% 18% 4	2160
	11th to 20th	Day	40% 3 11% 22% 21% 3	2134
	21st to end	Day	42% 4 10% 23% 16% 4	2272
May	1st to 10th	Day	46% 3 13% 21% 15% 2	2340
	11th to 20th	Day	41% 4 11% 22% 18% 3	2353
	21st to end	Day	37% 6% 18% 23% 14% 2	2725
June	1st to 10th	Day	40% 6% 14% 25% 13% 1	2520
	11th to 20th	Day	40% 5% 15% 25% 14% 1	2520
	21st to end	Day	36% 7% 15% 26% 16% 1	2520
July	1st to 10th	Day	34% 6% 17% 23% 18% 2	2450
	11th to 20th	Day	35% 5% 15% 28% 14% 2	2445
	21st to end	Day	36% 6% 14% 23% 17% 3	2591
August	1st to 10th	Day	34% 6% 18% 28% 15% 1	2361
	11th to 20th	Day	36% 5% 16% 27% 14% 1	2156
	21st to end	Day	35% 6% 13% 23% 20% 3	2376
September	1st to 10th	Day	37% 5% 14% 22% 19% 2	2160
	11th to 20th	Day	31% 3 11% 26% 25% 3	2015
	21st to end	Day	41% 5% 9% 21% 21% 2	1980
October	1st to 10th	Day	42% 5% 10% 23% 18% 3	1980
	11th to 20th	Day	37% 4 13% 28% 18% 1	1779
	21st to end	Day	44% 5% 10% 22% 17% 2	1782
November	1st to 10th	Day	38% 4 14% 22% 19% 3	1671
	11th to 20th	Day	39% 5% 11% 23% 20% 3	1710
	21st to end	Day	41% 4 13% 27% 12% 3	1710
December	1st to 10th	Day	48% 4 14% 18% 15% 1	1549
	11th to 20th	Day	43% 3 16% 18% 18% 2	1906
	21st to end	Day	53% 5% 10% 23% 9%	2772
January	1st to 10th	Night	42% 4 11% 24% 16% 3	2649
	11th to 20th	Night	48% 4 13% 23% 11% 2	2591
	21st to end	Night	41% 3 11% 24% 19% 3	2736
February	1st to 10th	Night	49% 4 12% 22% 12% 2	2235
	11th to 20th	Night	45% 4 10% 22% 15% 4	2100
	21st to end	Night	46% 3 8% 23% 16% 3	1776
March	1st to 10th	Night	41% 4 13% 22% 16% 4	2160
	11th to 20th	Night	44% 4 11% 21% 16% 4	2108
	21st to end	Night	43% 4 9% 18% 20% 6%	1990
April	1st to 10th	Night	46% 4 10% 20% 16% 4	1800
	11th to 20th	Night	43% 5% 10% 20% 20% 2	1775
	21st to end	Night	47% 3 9% 21% 15% 5%	1688
May	1st to 10th	Night	51% 5% 10% 16% 15% 2	1620
	11th to 20th	Night	47% 5% 10% 19% 15% 3	1558
	21st to end	Night	45% 7% 15% 19% 13% 1	1551
June	1st to 10th	Night	45% 4 12% 20% 16% 2	1440
	11th to 20th	Night	42% 5% 15% 25% 13% 1	1440
	21st to end	Night	39% 5% 14% 25% 15%	1440
July	1st to 10th	Night	42% 6% 15% 21% 15% 2	1400
	11th to 20th	Night	39% 6% 15% 28% 11% 1	1392
	21st to end	Night	39% 6% 15% 24% 14% 2	1486
August	1st to 10th	Night	34% 7% 18% 25% 14% 2	1534
	11th to 20th	Night	42% 7% 14% 25% 11% 1	1741
	21st to end	Night	35% 6% 13% 24% 20% 1	1980
September	1st to 10th	Night	42% 6% 10% 24% 17% 2	1800
	11th to 20th	Night	34% 3 11% 24% 25% 3	1944
	21st to end	Night	44% 4 8% 20% 21% 3	1980
October	1st to 10th	Night	49% 4 8% 21% 15% 3	1980
	11th to 20th	Night	40% 3 12% 27% 17% 1	2177
	21st to end	Night	48% 4 11% 20% 15% 1	2574
November	1st to 10th	Night	40% 4 12% 22% 19% 4	2407
	11th to 20th	Night	42% 4 9% 24% 18% 3	2470
	21st to end	Night	39% 5% 13% 29% 13% 2	2470
December	1st to 10th	Night	49% 4 12% 20% 14% 1	2627
	11th to 20th	Night	46% 4 13% 18% 17% 1	3288
	21st to end	Night	50% 4 11% 23% 11% 1	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% 35% 22% 3 1	170
	21st to end	Day	21% 10% 35% 23% 8% 3	192
February	1st to 10th	Day	24% 11% 36% 23% 5%	344
	11th to 20th	Day	20% 12% 41% 25% 3	416
	21st to end	Day	23% 13% 33% 23% 6%	346
March	1st to 10th	Day	17% 10% 33% 33% 7%	420
	11th to 20th	Day	17% 9% 38% 28% 7% 1	420
	21st to end	Day	22% 8% 35% 26% 8% 2	462
April	1st to 10th	Day	22% 8% 31% 28% 9% 2	420
	11th to 20th	Day	22% 4 25% 36% 12% 1	412
	21st to end	Day	23% 8% 24% 34% 10% 2	420
May	1st to 10th	Day	25% 6% 25% 32% 10% 3	423
	11th to 20th	Day	19% 3 22% 36% 17% 2	447
	21st to end	Day	20% 2 12% 30% 27% 9%	485
June	1st to 10th	Day	18% 2 15% 33% 25% 6%	450
	11th to 20th	Day	15% 2 15% 37% 24% 6%	450
	21st to end	Day	13% 2 13% 36% 29% 7%	450
July	1st to 10th	Day	15% 2 10% 32% 32% 9%	445
	11th to 20th	Day	11% 1 8% 32% 35% 12%	444
	21st to end	Day	11% 2 10% 41% 32% 4	488
August	1st to 10th	Day	11% 2 9% 34% 33% 10%	446
	11th to 20th	Day	15% 1 9% 34% 31% 10%	439
	21st to end	Day	12% 2 11% 37% 26% 11%	495
September	1st to 10th	Day	18% 1 19% 25% 24% 12%	450
	11th to 20th	Day	14% 3 17% 33% 22% 11%	450
	21st to end	Day	21% 3 22% 32% 18% 4	450
October	1st to 10th	Day	27% 4 18% 26% 18% 8%	450
	11th to 20th	Day	16% 8% 25% 31% 17% 3	435
	21st to end	Day	24% 7% 28% 27% 12% 2	396
November	1st to 10th	Day	17% 7% 24% 32% 16% 3	365
	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% 23% 4	741
	21st to end	Night	20% 16% 38% 23% 3 1	792
February	1st to 10th	Night	22% 13% 37% 25% 3	720
	11th to 20th	Night	23% 12% 41% 22% 2	703
	21st to end	Night	25% 11% 38% 22% 4	592
March	1st to 10th	Night	21% 10% 38% 26% 5%	720
	11th to 20th	Night	20% 8% 37% 26% 7% 2	720
	21st to end	Night	22% 7% 34% 27% 8% 1	792
April	1st to 10th	Night	24% 7% 31% 30% 9% 1	720
	11th to 20th	Night	21% 5% 30% 33% 12%	711
	21st to end	Night	23% 4 29% 33% 10% 1	608
May	1st to 10th	Night	24% 5% 23% 31% 15% 2	540
	11th to 20th	Night	22% 3 26% 36% 12%	496
	21st to end	Night	22% 2 14% 35% 25% 3	389
June	1st to 10th	Night	21% 4 19% 33% 21% 2	360
	11th to 20th	Night	19% 2 13% 41% 23% 3	360
	21st to end	Night	13% 1 18% 44% 21% 3	360
July	1st to 10th	Night	14% 2 13% 41% 26% 4	350
	11th to 20th	Night	13% 2 11% 40% 31% 3	348
	21st to end	Night	13% 1 14% 45% 24% 3	394
August	1st to 10th	Night	8% 1 13% 50% 26% 2	470
	11th to 20th	Night	13% 2 14% 35% 30% 5%	677
	21st to end	Night	14% 3 12% 37% 27% 7%	792
September	1st to 10th	Night	16% 3 21% 33% 23% 5%	720
	11th to 20th	Night	18% 2 16% 35% 22% 7%	720
	21st to end	Night	22% 3 20% 31% 21% 3	720
October	1st to 10th	Night	24% 8% 24% 24% 14% 5%	720
	11th to 20th	Night	20% 8% 28% 30% 13% 1	720
	21st to end	Night	26% 9% 32% 23% 9% 1	792
November	1st to 10th	Night	18% 8% 33% 26% 13% 2	740
	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 2024-01-13 01:38:09. Version ac07

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