Holcomb Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Holcomb Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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	18% 10% 4 7% 6% 3 5% 5% 5% 7% 31%	74912
	Night	20% 12% 5% 8% 6% 3 4 4 4 5% 29%	75211
Transparency	Day	53% 8% 11% 18% 9% 1	74617
	Night	57% 7% 10% 16% 8% 1	75040
Seeing	Day	25% 17% 28% 19% 8% 2	13617
	Night	26% 14% 26% 24% 9% 1	22743

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% 19% 7% 9% 9% 3 7% 5% 4 4 17%	1517
	11th to 20th	Day	18% 15% 6% 12% 7% 4 5% 5% 4 6% 20%	1520
	21st to end	Day	12% 17% 8% 10% 7% 5% 5% 4 5% 5% 21%	1664
February	1st to 10th	Day	16% 14% 6% 12% 9% 4 5% 5% 4 5% 20%	1584
	11th to 20th	Day	12% 16% 6% 12% 8% 4 5% 4 4 5% 23%	1712
	21st to end	Day	17% 14% 7% 9% 8% 4 5% 4 3 5% 24%	1480
March	1st to 10th	Day	15% 14% 6% 10% 8% 4 5% 4 4 4 27%	1797
	11th to 20th	Day	18% 13% 6% 9% 7% 3 4 4 4 5% 28%	1852
	21st to end	Day	19% 12% 5% 8% 6% 3 3 4 3 5% 32%	2376
April	1st to 10th	Day	22% 12% 4 7% 5% 3 3 3 4 6% 31%	2160
	11th to 20th	Day	19% 9% 4 5% 6% 4 4 4 3 5% 36%	2134
	21st to end	Day	25% 10% 3 6% 4 3 4 3 3 6% 32%	2308
May	1st to 10th	Day	22% 8% 3 6% 5% 4 4 4 5% 6% 32%	2412
	11th to 20th	Day	18% 7% 3 6% 5% 3 5% 4 4 8% 38%	2478
	21st to end	Day	18% 6% 2 5% 6% 3 5% 6% 5% 9% 35%	2725
June	1st to 10th	Day	24% 6% 3 5% 5% 3 6% 6% 6% 9% 27%	2520
	11th to 20th	Day	18% 6% 2 5% 5% 3 5% 5% 6% 10% 36%	2520
	21st to end	Day	21% 6% 3 5% 6% 4 5% 5% 6% 8% 31%	2520
July	1st to 10th	Day	14% 5% 2 6% 5% 3 5% 5% 6% 9% 40%	2520
	11th to 20th	Day	12% 6% 3 6% 6% 3 6% 6% 7% 8% 37%	2520
	21st to end	Day	15% 5% 2 4 5% 3 6% 6% 6% 9% 36%	2744
August	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 6% 7% 8% 39%	2506
	11th to 20th	Day	14% 5% 2 5% 6% 4 4 5% 6% 10% 38%	2234
	21st to end	Day	13% 5% 2 5% 6% 3 5% 5% 6% 9% 41%	2376
September	1st to 10th	Day	16% 6% 2 5% 3 3 3 5% 5% 7% 44%	2160
	11th to 20th	Day	17% 6% 2 3 4 2 3 3 4 6% 49%	2060
	21st to end	Day	15% 6% 2 5% 6% 3 4 5% 5% 7% 40%	1980
October	1st to 10th	Day	25% 8% 3 6% 7% 3 3 4 5% 5% 31%	1900
	11th to 20th	Day	20% 9% 3 6% 5% 3 4 5% 4 6% 32%	1752
	21st to end	Day	28% 14% 3 6% 5% 3 4 4 4 4 25%	1782
November	1st to 10th	Day	19% 13% 4 6% 5% 3 5% 4 5% 6% 31%	1671
	11th to 20th	Day	19% 15% 5% 9% 5% 4 5% 4 5% 5% 25%	1710
	21st to end	Day	20% 15% 7% 8% 6% 2 4 5% 5% 5% 23%	1520
December	1st to 10th	Day	23% 19% 6% 10% 7% 3 3 3 4 5% 17%	1520
	11th to 20th	Day	22% 15% 5% 8% 6% 4 4 5% 5% 5% 22%	1906
	21st to end	Day	25% 15% 5% 10% 9% 4 5% 4 3 4 16%	2772
January	1st to 10th	Night	19% 17% 8% 13% 8% 4 4 4 4 4 15%	2649
	11th to 20th	Night	18% 17% 7% 12% 6% 4 5% 3 4 4 18%	2660
	21st to end	Night	13% 15% 7% 11% 8% 4 5% 4 4 5% 23%	2919
February	1st to 10th	Night	16% 15% 7% 14% 9% 4 6% 4 4 4 18%	2373
	11th to 20th	Night	14% 14% 7% 12% 9% 4 5% 5% 4 4 21%	2132
	21st to end	Night	15% 16% 6% 11% 7% 3 4 3 4 4 26%	1776
March	1st to 10th	Night	17% 17% 6% 8% 7% 2 4 3 3 4 27%	2150
	11th to 20th	Night	19% 15% 6% 8% 6% 4 3 4 3 4 29%	2104
	21st to end	Night	22% 13% 4 7% 5% 2 3 3 4 4 33%	1980
April	1st to 10th	Night	24% 12% 5% 7% 6% 3 4 4 4 5% 28%	1800
	11th to 20th	Night	25% 10% 4 6% 6% 3 3 3 4 5% 31%	1775
	21st to end	Night	24% 10% 4 6% 5% 3 4 3 4 4 33%	1652
May	1st to 10th	Night	25% 8% 3 6% 4 3 4 3 4 7% 31%	1563
	11th to 20th	Night	20% 8% 3 5% 5% 2 4 4 4 6% 38%	1415
	21st to end	Night	17% 5% 2 6% 4 4 5% 5% 6% 9% 38%	1550
June	1st to 10th	Night	25% 6% 4 5% 5% 2 4 5% 5% 7% 31%	1440
	11th to 20th	Night	19% 5% 2 6% 5% 3 5% 5% 5% 9% 36%	1440
	21st to end	Night	20% 7% 4 6% 4 3 4 5% 6% 6% 34%	1440
July	1st to 10th	Night	15% 4 3 5% 4 2 6% 6% 6% 8% 41%	1440
	11th to 20th	Night	13% 4 2 4 6% 4 5% 5% 7% 9% 40%	1440
	21st to end	Night	14% 5% 2 5% 4 4 6% 5% 8% 9% 38%	1574
August	1st to 10th	Night	13% 5% 2 4 5% 4 5% 5% 7% 8% 43%	1432
	11th to 20th	Night	14% 6% 3 6% 5% 3 5% 5% 5% 8% 40%	1646
	21st to end	Night	14% 7% 2 6% 4 4 4 5% 6% 7% 41%	1980
September	1st to 10th	Night	15% 5% 3 4 6% 3 3 5% 5% 7% 43%	1800
	11th to 20th	Night	16% 6% 3 5% 4 2 3 3 4 6% 47%	1899
	21st to end	Night	19% 5% 2 4 4 3 5% 5% 5% 6% 42%	1980
October	1st to 10th	Night	25% 7% 3 5% 5% 3 5% 4 4 6% 33%	2056
	11th to 20th	Night	24% 9% 3 6% 5% 3 4 3 5% 6% 31%	2206
	21st to end	Night	29% 12% 3 6% 5% 3 4 4 4 5% 25%	2574
November	1st to 10th	Night	21% 12% 4 6% 6% 2 4 4 5% 5% 31%	2407
	11th to 20th	Night	20% 13% 5% 9% 5% 4 4 4 4 5% 28%	2470
	21st to end	Night	21% 16% 6% 10% 5% 3 3 4 5% 4 23%	2657
December	1st to 10th	Night	22% 22% 7% 10% 7% 3 5% 4 3 3 14%	2660
	11th to 20th	Night	24% 15% 7% 8% 6% 4 5% 4 3 5% 18%	3288
	21st to end	Night	24% 17% 6% 11% 8% 3 4 3 3 4 17%	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	70% 7% 8% 12% 3	1518
	11th to 20th	Day	65% 7% 10% 11% 6% 1	1520
	21st to end	Day	64% 7% 11% 10% 6% 1	1664
February	1st to 10th	Day	66% 7% 7% 13% 5% 2	1584
	11th to 20th	Day	64% 7% 9% 14% 6% 1	1721
	21st to end	Day	65% 6% 8% 11% 8% 1	1480
March	1st to 10th	Day	62% 6% 10% 13% 7% 2	1800
	11th to 20th	Day	59% 6% 11% 11% 11% 2	1852
	21st to end	Day	57% 5% 8% 16% 11% 3	2376
April	1st to 10th	Day	57% 6% 7% 18% 10% 2	2160
	11th to 20th	Day	52% 6% 8% 19% 14% 2	2134
	21st to end	Day	57% 5% 9% 16% 11% 2	2308
May	1st to 10th	Day	53% 6% 8% 19% 13% 2	2412
	11th to 20th	Day	47% 7% 8% 20% 16% 2	2482
	21st to end	Day	46% 11% 12% 19% 11% 1	2725
June	1st to 10th	Day	53% 10% 11% 17% 10% 1	2520
	11th to 20th	Day	44% 9% 11% 22% 12% 2	2520
	21st to end	Day	51% 9% 11% 19% 9% 1	2520
July	1st to 10th	Day	41% 9% 12% 25% 13%	2450
	11th to 20th	Day	41% 11% 12% 28% 8%	2445
	21st to end	Day	43% 11% 14% 23% 9% 1	2591
August	1st to 10th	Day	42% 11% 15% 24% 9%	2492
	11th to 20th	Day	42% 12% 14% 23% 10%	2234
	21st to end	Day	40% 11% 12% 24% 13% 1	2376
September	1st to 10th	Day	38% 10% 11% 27% 13% 1	2160
	11th to 20th	Day	38% 6% 10% 31% 14% 1	2060
	21st to end	Day	42% 10% 14% 22% 10% 2	1980
October	1st to 10th	Day	56% 7% 10% 18% 9%	1900
	11th to 20th	Day	53% 8% 14% 18% 7%	1752
	21st to end	Day	63% 7% 9% 13% 6% 1	1782
November	1st to 10th	Day	56% 8% 12% 17% 7%	1671
	11th to 20th	Day	61% 7% 11% 12% 8% 1	1710
	21st to end	Day	64% 8% 12% 12% 4	1520
December	1st to 10th	Day	71% 7% 9% 10% 3	1520
	11th to 20th	Day	66% 7% 10% 12% 5%	1906
	21st to end	Day	72% 8% 9% 9% 3	2772
January	1st to 10th	Night	73% 7% 9% 7% 3	2649
	11th to 20th	Night	70% 6% 9% 9% 5%	2660
	21st to end	Night	64% 7% 9% 12% 7% 1	2919
February	1st to 10th	Night	70% 6% 7% 10% 5% 1	2373
	11th to 20th	Night	66% 6% 8% 13% 7%	2134
	21st to end	Night	63% 5% 8% 14% 9% 1	1776
March	1st to 10th	Night	62% 5% 9% 15% 8% 1	2160
	11th to 20th	Night	59% 6% 8% 16% 9% 2	2105
	21st to end	Night	56% 5% 8% 18% 11% 3	1980
April	1st to 10th	Night	60% 5% 7% 16% 11% 1	1800
	11th to 20th	Night	57% 5% 9% 17% 12% 1	1775
	21st to end	Night	56% 5% 8% 15% 13% 3	1652
May	1st to 10th	Night	54% 6% 6% 17% 15% 3	1563
	11th to 20th	Night	48% 6% 9% 19% 17% 2	1416
	21st to end	Night	43% 9% 14% 20% 13% 2	1551
June	1st to 10th	Night	53% 8% 10% 19% 9% 1	1440
	11th to 20th	Night	46% 7% 12% 22% 11% 3	1440
	21st to end	Night	48% 10% 12% 21% 9%	1440
July	1st to 10th	Night	39% 10% 15% 22% 14% 1	1400
	11th to 20th	Night	38% 11% 14% 27% 11% 1	1392
	21st to end	Night	41% 10% 14% 24% 10% 1	1486
August	1st to 10th	Night	37% 10% 17% 26% 10%	1422
	11th to 20th	Night	42% 10% 13% 23% 11% 1	1646
	21st to end	Night	41% 9% 13% 24% 13%	1980
September	1st to 10th	Night	40% 9% 12% 27% 11% 1	1800
	11th to 20th	Night	38% 7% 13% 30% 12% 1	1899
	21st to end	Night	41% 8% 12% 24% 12% 2	1980
October	1st to 10th	Night	53% 7% 10% 20% 10% 1	2056
	11th to 20th	Night	55% 7% 12% 19% 6%	2206
	21st to end	Night	61% 7% 10% 13% 7% 1	2574
November	1st to 10th	Night	55% 6% 12% 17% 8% 1	2407
	11th to 20th	Night	58% 6% 11% 15% 7% 2	2470
	21st to end	Night	65% 7% 11% 13% 3	2657
December	1st to 10th	Night	76% 6% 8% 8% 2	2660
	11th to 20th	Night	69% 8% 9% 9% 5%	3288
	21st to end	Night	73% 7% 7% 11% 2	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	25% 26% 26% 20% 4	180
	11th to 20th	Day	33% 25% 30% 7% 4 1	180
	21st to end	Day	23% 24% 34% 14% 4	196
February	1st to 10th	Day	34% 34% 23% 7% 1	224
	11th to 20th	Day	28% 26% 34% 9% 2 1	380
	21st to end	Day	31% 21% 33% 12% 3	346
March	1st to 10th	Day	31% 19% 29% 18% 3	420
	11th to 20th	Day	27% 25% 27% 15% 5% 1	420
	21st to end	Day	32% 20% 25% 13% 7% 2	462
April	1st to 10th	Day	34% 17% 26% 15% 6% 1	420
	11th to 20th	Day	31% 24% 20% 16% 8% 2	412
	21st to end	Day	32% 19% 28% 14% 6% 2	420
May	1st to 10th	Day	30% 19% 26% 16% 8% 2	423
	11th to 20th	Day	22% 15% 33% 22% 6% 2	447
	21st to end	Day	20% 17% 32% 21% 9% 1	485
June	1st to 10th	Day	29% 12% 32% 19% 7% 1	450
	11th to 20th	Day	20% 15% 32% 21% 10% 2	450
	21st to end	Day	22% 14% 32% 22% 9% 1	450
July	1st to 10th	Day	17% 10% 28% 30% 13% 3	445
	11th to 20th	Day	15% 9% 35% 28% 12% 2	444
	21st to end	Day	19% 7% 34% 30% 10% 1	488
August	1st to 10th	Day	15% 9% 30% 26% 17% 2	446
	11th to 20th	Day	17% 10% 24% 30% 14% 5%	439
	21st to end	Day	15% 15% 28% 26% 11% 5%	495
September	1st to 10th	Day	17% 14% 30% 21% 13% 6%	450
	11th to 20th	Day	18% 15% 20% 22% 18% 7%	450
	21st to end	Day	14% 15% 31% 21% 14% 4	450
October	1st to 10th	Day	28% 14% 25% 15% 15% 3	450
	11th to 20th	Day	29% 17% 26% 20% 7% 1	426
	21st to end	Day	38% 17% 24% 17% 3	396
November	1st to 10th	Day	31% 18% 26% 18% 6% 1	365
	11th to 20th	Day	31% 28% 22% 13% 6% 1	370
	21st to end	Day	36% 21% 27% 14% 1	180
December	1st to 10th	Day	29% 24% 29% 14% 3	180
	11th to 20th	Day	29% 28% 31% 10% 2	180
	21st to end	Day	28% 30% 28% 11% 3 1	198
January	1st to 10th	Night	29% 21% 30% 17% 3	760
	11th to 20th	Night	30% 19% 30% 18% 3	760
	21st to end	Night	23% 18% 30% 25% 4	835
February	1st to 10th	Night	25% 17% 35% 22% 2	720
	11th to 20th	Night	26% 19% 33% 19% 3	703
	21st to end	Night	27% 19% 29% 20% 5%	592
March	1st to 10th	Night	29% 15% 27% 26% 2 1	720
	11th to 20th	Night	29% 20% 25% 21% 6% 1	720
	21st to end	Night	31% 16% 24% 20% 8% 1	792
April	1st to 10th	Night	30% 16% 25% 25% 5%	720
	11th to 20th	Night	31% 17% 22% 22% 8%	711
	21st to end	Night	28% 17% 26% 21% 7% 1	572
May	1st to 10th	Night	31% 11% 25% 24% 8% 1	483
	11th to 20th	Night	24% 8% 32% 27% 8% 1	354
	21st to end	Night	18% 9% 27% 34% 11% 1	389
June	1st to 10th	Night	28% 6% 26% 31% 9% 1	360
	11th to 20th	Night	19% 7% 26% 34% 14% 1	360
	21st to end	Night	20% 5% 27% 37% 11% 1	360
July	1st to 10th	Night	16% 3 23% 41% 17% 1	350
	11th to 20th	Night	10% 4 24% 41% 19% 2	348
	21st to end	Night	15% 6% 21% 42% 15%	394
August	1st to 10th	Night	14% 4 18% 41% 23% 1	358
	11th to 20th	Night	17% 5% 20% 34% 22% 3	582
	21st to end	Night	16% 6% 24% 31% 21% 2	792
September	1st to 10th	Night	17% 8% 24% 33% 16% 2	720
	11th to 20th	Night	19% 8% 19% 30% 21% 4	720
	21st to end	Night	20% 9% 27% 24% 18% 2	720
October	1st to 10th	Night	29% 12% 20% 23% 15% 1	720
	11th to 20th	Night	30% 12% 23% 25% 8% 1	720
	21st to end	Night	33% 15% 27% 20% 5%	792
November	1st to 10th	Night	29% 18% 25% 20% 7% 1	740
	11th to 20th	Night	30% 18% 26% 18% 8% 1	760
	21st to end	Night	33% 20% 24% 17% 5% 1	760
December	1st to 10th	Night	35% 20% 28% 14% 4	760
	11th to 20th	Night	28% 22% 28% 17% 4	760
	21st to end	Night	33% 17% 30% 17% 3	836

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