Herron Observing Field
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Herron Observing Field clear sky chart from 2005-11-06 to 2024-01-17.

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	13% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	74901
	Night	13% 6% 3 5% 5% 3 4 5% 5% 7% 44%	74932
Transparency	Day	41% 4 12% 21% 18% 4	74624
	Night	39% 4 10% 21% 20% 6%	74728
Seeing	Day	18% 9% 22% 23% 18% 10%	13740
	Night	17% 8% 22% 29% 21% 4	23836

By Month

Percentage Cloud Cover SE (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	20% 7% 3 5% 3 2 4 4 4 7% 41%	1706
	11th to 20th	Day	19% 7% 3 5% 4 2 4 4 4 5% 42%	1674
	21st to end	Day	18% 8% 3 4 4 3 4 4 4 4 44%	1858
February	1st to 10th	Day	17% 9% 3 5% 4 3 5% 4 5% 7% 38%	1800
	11th to 20th	Day	23% 6% 2 5% 4 2 4 4 5% 6% 39%	1748
	21st to end	Day	23% 7% 3 5% 4 2 4 3 4 5% 39%	1480
March	1st to 10th	Day	15% 6% 2 5% 5% 3 4 4 5% 7% 44%	1842
	11th to 20th	Day	20% 6% 3 5% 4 3 5% 4 5% 7% 38%	1980
	21st to end	Day	15% 6% 3 5% 6% 4 5% 5% 5% 7% 39%	2178
April	1st to 10th	Day	12% 6% 3 5% 5% 3 4 5% 6% 7% 45%	2047
	11th to 20th	Day	12% 7% 2 6% 5% 3 4 4 5% 6% 46%	2134
	21st to end	Day	11% 3 2 4 5% 3 5% 5% 4 9% 49%	2297
May	1st to 10th	Day	10% 6% 2 5% 4 3 4 4 6% 8% 48%	2340
	11th to 20th	Day	12% 5% 3 5% 7% 4 5% 6% 7% 9% 39%	2301
	21st to end	Day	10% 5% 2 6% 5% 3 5% 6% 7% 9% 42%	2531
June	1st to 10th	Day	10% 6% 3 7% 7% 5% 6% 5% 8% 10% 35%	2340
	11th to 20th	Day	6% 4 1 5% 5% 3 6% 7% 9% 13% 41%	2340
	21st to end	Day	7% 4 2 6% 6% 5% 7% 7% 8% 12% 37%	2340
July	1st to 10th	Day	9% 5% 3 6% 7% 5% 8% 8% 8% 11% 29%	2340
	11th to 20th	Day	9% 6% 3 7% 7% 5% 8% 8% 9% 11% 28%	2340
	21st to end	Day	7% 5% 3 6% 8% 5% 8% 9% 9% 11% 29%	2547
August	1st to 10th	Day	7% 6% 3 6% 7% 6% 7% 9% 8% 10% 30%	2327
	11th to 20th	Day	12% 6% 3 6% 7% 5% 6% 6% 8% 10% 31%	2153
	21st to end	Day	12% 6% 3 7% 8% 5% 7% 8% 7% 9% 29%	2376
September	1st to 10th	Day	10% 7% 3 6% 6% 3 5% 6% 6% 8% 40%	1996
	11th to 20th	Day	14% 7% 2 5% 4 3 4 5% 6% 8% 41%	1980
	21st to end	Day	13% 6% 2 4 5% 3 4 5% 5% 6% 48%	1980
October	1st to 10th	Day	12% 5% 2 3 4 2 4 3 4 6% 54%	1980
	11th to 20th	Day	12% 6% 2 3 3 2 3 3 5% 7% 53%	1887
	21st to end	Day	15% 5% 2 5% 4 3 3 2 4 4 54%	1980
November	1st to 10th	Day	13% 4 2 4 3 2 3 3 3 6% 56%	1672
	11th to 20th	Day	14% 5% 2 4 4 2 4 4 6% 4 53%	1710
	21st to end	Day	14% 6% 2 5% 4 2 4 5% 4 7% 46%	1710
December	1st to 10th	Day	22% 7% 3 5% 5% 3 4 4 5% 6% 37%	1710
	11th to 20th	Day	18% 6% 3 6% 6% 3 4 4 4 5% 42%	2153
	21st to end	Day	22% 7% 4 5% 4 3 5% 5% 5% 6% 35%	3124
January	1st to 10th	Night	20% 7% 3 5% 3 2 3 3 5% 6% 43%	2459
	11th to 20th	Night	19% 7% 4 6% 5% 3 4 4 4 6% 38%	2430
	21st to end	Night	17% 7% 3 5% 4 2 3 3 4 5% 46%	2494
February	1st to 10th	Night	17% 7% 3 5% 4 3 4 4 6% 6% 42%	2160
	11th to 20th	Night	21% 7% 3 5% 4 2 4 4 3 5% 42%	2098
	21st to end	Night	22% 7% 2 5% 5% 2 4 4 5% 5% 38%	1776
March	1st to 10th	Night	15% 7% 2 4 4 2 3 3 5% 6% 48%	2117
	11th to 20th	Night	16% 6% 2 6% 6% 3 4 5% 5% 8% 41%	1980
	21st to end	Night	14% 6% 3 5% 5% 3 5% 4 5% 6% 44%	2178
April	1st to 10th	Night	12% 4 2 4 4 3 5% 5% 6% 9% 47%	1926
	11th to 20th	Night	13% 5% 2 4 4 2 4 4 5% 6% 50%	1775
	21st to end	Night	8% 5% 3 4 4 3 4 4 5% 9% 51%	1661
May	1st to 10th	Night	9% 4 2 4 4 3 5% 5% 5% 7% 51%	1620
	11th to 20th	Night	10% 5% 3 6% 5% 4 6% 5% 5% 9% 42%	1592
	21st to end	Night	8% 6% 3 5% 5% 3 5% 5% 7% 10% 44%	1744
June	1st to 10th	Night	6% 5% 2 6% 6% 5% 7% 7% 9% 11% 36%	1620
	11th to 20th	Night	4 3 2 5% 6% 4 4 7% 7% 12% 45%	1620
	21st to end	Night	6% 4 2 5% 5% 4 7% 5% 9% 12% 42%	1620
July	1st to 10th	Night	8% 5% 3 6% 6% 4 5% 8% 7% 11% 36%	1620
	11th to 20th	Night	6% 5% 2 6% 7% 4 6% 6% 9% 12% 36%	1620
	21st to end	Night	6% 6% 3 8% 6% 4 7% 7% 7% 11% 35%	1771
August	1st to 10th	Night	7% 6% 3 5% 7% 3 6% 7% 7% 12% 37%	1611
	11th to 20th	Night	8% 6% 3 7% 7% 3 5% 6% 6% 9% 40%	1744
	21st to end	Night	12% 6% 3 6% 7% 4 6% 6% 6% 9% 35%	1980
September	1st to 10th	Night	8% 5% 2 6% 6% 3 6% 6% 7% 9% 42%	1944
	11th to 20th	Night	13% 6% 2 5% 5% 3 5% 4 6% 7% 45%	1980
	21st to end	Night	11% 4 2 4 5% 3 4 5% 5% 6% 51%	1980
October	1st to 10th	Night	11% 5% 2 3 4 2 3 3 4 6% 57%	1980
	11th to 20th	Night	10% 5% 3 4 4 2 3 3 3 6% 56%	2071
	21st to end	Night	13% 5% 2 4 3 2 3 3 4 6% 55%	2376
November	1st to 10th	Night	12% 5% 2 3 3 2 3 3 3 6% 57%	2401
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 4 6% 53%	2470
	21st to end	Night	16% 6% 2 4 4 3 4 5% 5% 6% 45%	2470
December	1st to 10th	Night	19% 7% 3 6% 5% 2 4 4 4 7% 39%	2470
	11th to 20th	Night	17% 8% 2 4 3 2 3 3 4 5% 46%	3042
	21st to end	Night	21% 9% 4 6% 5% 3 4 4 5% 7% 33%	4532

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	45% 2 7% 19% 22% 4	1707
	11th to 20th	Day	45% 2 8% 20% 21% 4	1674
	21st to end	Day	45% 2 5% 21% 22% 6%	1858
February	1st to 10th	Day	46% 3 7% 20% 20% 4	1800
	11th to 20th	Day	46% 2 6% 21% 19% 6%	1758
	21st to end	Day	49% 2 8% 15% 19% 7%	1480
March	1st to 10th	Day	40% 3 8% 19% 21% 9%	1842
	11th to 20th	Day	47% 3 11% 17% 17% 5%	1980
	21st to end	Day	45% 3 10% 23% 16% 3	2178
April	1st to 10th	Day	38% 3 11% 24% 19% 6%	2047
	11th to 20th	Day	39% 3 7% 20% 24% 7%	2134
	21st to end	Day	33% 3 10% 29% 21% 3	2297
May	1st to 10th	Day	34% 3 13% 25% 22% 4	2340
	11th to 20th	Day	41% 5% 16% 23% 14% 2	2305
	21st to end	Day	37% 5% 12% 27% 17% 2	2531
June	1st to 10th	Day	42% 5% 17% 23% 11% 2	2340
	11th to 20th	Day	31% 6% 21% 29% 12% 2	2340
	21st to end	Day	37% 6% 18% 24% 12% 2	2340
July	1st to 10th	Day	43% 7% 20% 20% 9% 1	2275
	11th to 20th	Day	42% 9% 22% 15% 9% 2	2271
	21st to end	Day	42% 8% 21% 21% 9%	2405
August	1st to 10th	Day	42% 8% 20% 24% 6% 1	2314
	11th to 20th	Day	45% 7% 17% 20% 9% 2	2153
	21st to end	Day	48% 7% 15% 18% 10% 2	2376
September	1st to 10th	Day	40% 4 15% 18% 18% 4	1996
	11th to 20th	Day	41% 4 11% 20% 22% 2	1980
	21st to end	Day	36% 4 10% 19% 24% 7%	1980
October	1st to 10th	Day	33% 3 9% 19% 28% 8%	1980
	11th to 20th	Day	31% 2 8% 25% 25% 9%	1887
	21st to end	Day	35% 2 5% 19% 29% 10%	1980
November	1st to 10th	Day	33% 2 6% 17% 33% 10%	1672
	11th to 20th	Day	35% 2 6% 19% 28% 10%	1710
	21st to end	Day	39% 3 6% 20% 25% 6%	1710
December	1st to 10th	Day	48% 2 6% 21% 17% 6%	1710
	11th to 20th	Day	46% 1 5% 17% 23% 7%	2150
	21st to end	Day	51% 1 7% 19% 19% 3	3124
January	1st to 10th	Night	44% 2 7% 19% 24% 5%	2459
	11th to 20th	Night	47% 3 7% 19% 20% 5%	2430
	21st to end	Night	42% 2 4 21% 26% 5%	2494
February	1st to 10th	Night	43% 3 7% 20% 21% 7%	2160
	11th to 20th	Night	47% 2 5% 19% 21% 7%	2099
	21st to end	Night	48% 2 6% 16% 19% 8%	1776
March	1st to 10th	Night	39% 2 7% 19% 21% 11%	2117
	11th to 20th	Night	42% 4 10% 18% 20% 6%	1980
	21st to end	Night	40% 3 10% 24% 19% 4	2178
April	1st to 10th	Night	36% 3 10% 21% 25% 6%	1926
	11th to 20th	Night	35% 2 7% 22% 24% 10%	1775
	21st to end	Night	31% 3 10% 30% 21% 5%	1661
May	1st to 10th	Night	32% 4 14% 24% 23% 4	1620
	11th to 20th	Night	39% 5% 17% 24% 15% 1	1593
	21st to end	Night	36% 5% 13% 26% 17% 3	1745
June	1st to 10th	Night	38% 9% 17% 22% 13% 2	1620
	11th to 20th	Night	30% 5% 23% 27% 14% 1	1620
	21st to end	Night	33% 7% 21% 22% 13% 4	1620
July	1st to 10th	Night	40% 7% 19% 21% 11% 1	1575
	11th to 20th	Night	37% 8% 23% 18% 11% 3	1566
	21st to end	Night	40% 8% 21% 22% 8% 2	1672
August	1st to 10th	Night	38% 9% 19% 27% 7% 1	1600
	11th to 20th	Night	41% 7% 18% 18% 15% 1	1744
	21st to end	Night	45% 7% 17% 18% 12% 2	1980
September	1st to 10th	Night	38% 5% 14% 20% 18% 5%	1944
	11th to 20th	Night	39% 5% 12% 22% 20% 3	1980
	21st to end	Night	35% 4 11% 20% 22% 9%	1980
October	1st to 10th	Night	30% 3 10% 20% 29% 8%	1980
	11th to 20th	Night	33% 2 7% 22% 27% 9%	2071
	21st to end	Night	32% 2 4 20% 30% 12%	2376
November	1st to 10th	Night	32% 2 7% 19% 28% 13%	2401
	11th to 20th	Night	34% 2 4 19% 29% 12%	2470
	21st to end	Night	39% 2 5% 20% 26% 7%	2470
December	1st to 10th	Night	46% 2 8% 20% 18% 5%	2470
	11th to 20th	Night	41% 1 7% 17% 27% 6%	3044
	21st to end	Night	52% 2 5% 22% 17% 2	4532

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	29% 20% 39% 10% 1	270
	11th to 20th	Day	31% 19% 32% 13% 5%	258
	21st to end	Day	25% 27% 35% 10% 3 1	264
February	1st to 10th	Day	27% 21% 34% 15% 3	240
	11th to 20th	Day	38% 24% 28% 10%	240
	21st to end	Day	35% 17% 36% 9% 2	198
March	1st to 10th	Day	26% 14% 39% 16% 5%	282
	11th to 20th	Day	25% 19% 29% 20% 7%	420
	21st to end	Day	20% 17% 35% 20% 6% 1	462
April	1st to 10th	Day	21% 13% 30% 21% 13% 1	420
	11th to 20th	Day	15% 13% 34% 23% 13% 1	412
	21st to end	Day	17% 10% 27% 28% 15% 3	420
May	1st to 10th	Day	15% 11% 31% 24% 14% 5%	423
	11th to 20th	Day	14% 3 23% 32% 16% 12%	447
	21st to end	Day	14% 5% 13% 31% 26% 11%	485
June	1st to 10th	Day	13% 2 14% 30% 26% 14%	450
	11th to 20th	Day	8% 4 11% 27% 26% 24%	450
	21st to end	Day	10% 4 16% 27% 26% 17%	450
July	1st to 10th	Day	11% 1 7% 26% 31% 24%	445
	11th to 20th	Day	10% 2 8% 20% 32% 29%	444
	21st to end	Day	8% 2 7% 19% 38% 27%	488
August	1st to 10th	Day	14% 3 10% 23% 26% 24%	446
	11th to 20th	Day	14% 3 5% 29% 32% 19%	439
	21st to end	Day	16% 2 8% 24% 30% 20%	495
September	1st to 10th	Day	12% 6% 9% 27% 28% 18%	450
	11th to 20th	Day	16% 5% 12% 25% 30% 12%	450
	21st to end	Day	15% 4 15% 29% 27% 10%	450
October	1st to 10th	Day	16% 4 22% 28% 21% 9%	450
	11th to 20th	Day	14% 10% 24% 26% 23% 4	450
	21st to end	Day	20% 8% 23% 28% 17% 3	495
November	1st to 10th	Day	20% 8% 32% 23% 12% 5%	270
	11th to 20th	Day	19% 19% 35% 19% 8% 1	270
	21st to end	Day	12% 16% 41% 21% 9% 1	270
December	1st to 10th	Day	29% 20% 27% 20% 2 1	270
	11th to 20th	Day	23% 19% 35% 17% 5%	270
	21st to end	Day	31% 16% 33% 16% 4	297
January	1st to 10th	Night	26% 16% 31% 21% 6%	760
	11th to 20th	Night	24% 16% 31% 22% 7%	748
	21st to end	Night	21% 17% 33% 26% 3	792
February	1st to 10th	Night	21% 14% 31% 28% 6%	720
	11th to 20th	Night	27% 16% 33% 23% 1	703
	21st to end	Night	26% 15% 34% 22% 3	592
March	1st to 10th	Night	18% 15% 34% 28% 4	720
	11th to 20th	Night	19% 13% 32% 25% 10% 1	720
	21st to end	Night	20% 11% 31% 30% 8% 1	792
April	1st to 10th	Night	15% 13% 28% 30% 13% 1	666
	11th to 20th	Night	17% 10% 27% 32% 14% 1	533
	21st to end	Night	12% 6% 25% 36% 19% 1	540
May	1st to 10th	Night	10% 9% 24% 32% 24% 1	540
	11th to 20th	Night	14% 4 21% 32% 25% 4	531
	21st to end	Night	12% 2 12% 34% 33% 7%	584
June	1st to 10th	Night	8% 2 10% 39% 36% 6%	540
	11th to 20th	Night	5% 2 12% 31% 36% 14%	540
	21st to end	Night	8% 1 8% 36% 38% 9%	540
July	1st to 10th	Night	9% 4 4 22% 46% 14%	525
	11th to 20th	Night	7% 2 5% 20% 52% 15%	522
	21st to end	Night	7% 3 4 24% 49% 13%	591
August	1st to 10th	Night	10% 2 4 26% 45% 12%	537
	11th to 20th	Night	11% 2 7% 28% 43% 8%	534
	21st to end	Night	15% 3 7% 25% 39% 10%	594
September	1st to 10th	Night	11% 2 8% 27% 41% 11%	684
	11th to 20th	Night	16% 3 11% 29% 36% 5%	720
	21st to end	Night	14% 2 15% 33% 33% 3	720
October	1st to 10th	Night	14% 3 21% 31% 24% 8%	720
	11th to 20th	Night	14% 6% 25% 32% 20% 3	720
	21st to end	Night	16% 9% 21% 35% 17% 3	792
November	1st to 10th	Night	16% 7% 22% 34% 20% 2	740
	11th to 20th	Night	18% 11% 28% 30% 13% 1	760
	21st to end	Night	21% 10% 26% 29% 13%	760
December	1st to 10th	Night	22% 15% 29% 27% 8% 1	760
	11th to 20th	Night	24% 9% 30% 28% 8% 1	760
	21st to end	Night	29% 12% 27% 26% 6%	836

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