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 2026-05-02.

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% 7% 2 6% 4 4 5% 4 6% 8% 40%	131825
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	130676
Transparency	Day	40% 4 11% 21% 18% 5%	135611
	Night	40% 4 9% 21% 20% 6%	134959
Seeing	Day	18% 10% 23% 23% 18% 9%	16778
	Night	17% 9% 22% 29% 20% 4	26857

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% 11% 3 6% 3 3 3 2 3 4 42%	5258
	11th to 20th	Day	11% 7% 3 5% 3 2 3 2 3 4 57%	5280
	21st to end	Day	23% 11% 3 6% 3 3 3 3 4 5% 35%	6038
February	1st to 10th	Day	16% 9% 3 6% 4 4 4 3 5% 6% 40%	2100
	11th to 20th	Day	23% 7% 2 6% 4 3 3 3 5% 6% 39%	2048
	21st to end	Day	21% 7% 3 6% 3 4 4 3 4 5% 40%	1730
March	1st to 10th	Day	21% 9% 3 7% 4 4 4 3 5% 6% 33%	4708
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 50%	5090
	21st to end	Day	15% 6% 3 6% 4 5% 4 3 4 5% 44%	5586
April	1st to 10th	Day	12% 6% 2 6% 5% 4 4 5% 6% 7% 43%	2389
	11th to 20th	Day	10% 6% 2 7% 4 4 4 3 6% 6% 48%	2494
	21st to end	Day	11% 4 2 5% 4 4 5% 4 5% 9% 46%	2681
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 9% 35%	5875
	11th to 20th	Day	17% 8% 3 7% 6% 5% 5% 5% 7% 8% 28%	5861
	21st to end	Day	13% 7% 3 7% 5% 4 4 4 6% 8% 39%	6433
June	1st to 10th	Day	9% 6% 3 7% 6% 6% 5% 4 8% 10% 34%	2600
	11th to 20th	Day	6% 4 1 6% 5% 5% 5% 7% 9% 12% 41%	2600
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 8% 12% 37%	2600
July	1st to 10th	Day	6% 5% 2 8% 6% 7% 8% 5% 9% 11% 33%	5900
	11th to 20th	Day	6% 4 2 7% 6% 6% 7% 6% 9% 13% 34%	5900
	21st to end	Day	10% 7% 3 9% 7% 7% 7% 6% 9% 11% 24%	6424
August	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 6% 9% 11% 38%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 8% 10% 41%	3891
	21st to end	Day	7% 4 2 5% 5% 5% 6% 5% 7% 8% 47%	4312
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 5% 6% 8% 41%	2217
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 9% 40%	2200
	21st to end	Day	12% 6% 1 5% 5% 4 4 4 5% 7% 46%	2200
October	1st to 10th	Day	12% 5% 2 4 3 3 4 3 5% 6% 54%	2200
	11th to 20th	Day	11% 5% 2 4 3 2 3 2 5% 7% 56%	2096
	21st to end	Day	15% 5% 2 5% 3 3 3 2 4 4 54%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 2 3 3 5% 55%	1852
	11th to 20th	Day	13% 5% 1 5% 3 3 3 3 5% 5% 53%	1890
	21st to end	Day	14% 6% 2 5% 4 4 3 4 4 7% 47%	1890
December	1st to 10th	Day	20% 6% 2 5% 3 3 3 3 5% 6% 44%	3020
	11th to 20th	Day	15% 9% 3 8% 5% 4 4 3 4 4 40%	3463
	21st to end	Day	21% 6% 3 5% 3 3 4 4 5% 6% 39%	4565
January	1st to 10th	Night	18% 7% 3 6% 4 3 3 2 5% 6% 42%	7635
	11th to 20th	Night	14% 8% 3 6% 3 3 3 3 4 5% 49%	7680
	21st to end	Night	22% 11% 3 6% 4 3 2 2 3 4 41%	8126
February	1st to 10th	Night	15% 8% 3 6% 4 4 3 3 6% 6% 43%	2520
	11th to 20th	Night	22% 8% 2 6% 4 3 4 3 4 4 41%	2458
	21st to end	Night	20% 7% 2 6% 4 4 3 4 5% 6% 39%	2076
March	1st to 10th	Night	21% 10% 3 7% 5% 4 4 3 5% 6% 34%	5364
	11th to 20th	Night	10% 5% 2 5% 4 3 4 4 5% 7% 51%	5090
	21st to end	Night	13% 7% 2 6% 4 4 4 4 5% 6% 45%	5588
April	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 46%	2247
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 51%	2075
	21st to end	Night	9% 6% 3 6% 4 4 3 4 5% 8% 49%	1937
May	1st to 10th	Night	11% 8% 3 8% 5% 5% 6% 4 6% 9% 35%	4042
	11th to 20th	Night	20% 8% 2 7% 5% 5% 5% 4 6% 8% 29%	4032
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 7% 10% 41%	4421
June	1st to 10th	Night	7% 5% 2 7% 5% 6% 6% 6% 9% 10% 36%	1800
	11th to 20th	Night	5% 3 2 6% 6% 5% 5% 5% 8% 12% 43%	1800
	21st to end	Night	6% 4 2 5% 5% 5% 5% 5% 9% 11% 43%	1800
July	1st to 10th	Night	8% 6% 3 8% 6% 6% 6% 6% 8% 10% 34%	4058
	11th to 20th	Night	5% 5% 2 7% 6% 6% 6% 5% 10% 13% 34%	4057
	21st to end	Night	10% 8% 4 10% 6% 6% 6% 5% 7% 9% 30%	4427
August	1st to 10th	Night	7% 5% 2 7% 5% 4 6% 5% 8% 13% 38%	2915
	11th to 20th	Night	8% 7% 3 7% 5% 4 4 4 5% 8% 46%	3153
	21st to end	Night	8% 5% 2 5% 4 5% 4 3 5% 9% 51%	3586
September	1st to 10th	Night	8% 6% 2 7% 5% 5% 6% 5% 7% 8% 40%	2161
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 6% 7% 44%	2200
	21st to end	Night	12% 4 2 5% 5% 4 4 4 5% 6% 48%	2200
October	1st to 10th	Night	11% 5% 2 4 4 3 3 3 4 6% 55%	2200
	11th to 20th	Night	9% 6% 2 5% 3 3 2 3 3 5% 59%	2302
	21st to end	Night	13% 6% 2 4 2 3 3 2 4 5% 56%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 2 3 3 3 6% 55%	2661
	11th to 20th	Night	13% 6% 2 4 3 3 3 2 4 6% 54%	2730
	21st to end	Night	15% 7% 2 5% 4 3 4 4 5% 6% 46%	2730
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 6% 48%	4380
	11th to 20th	Night	18% 8% 2 6% 3 3 3 3 4 6% 43%	4952
	21st to end	Night	21% 7% 3 6% 4 4 3 3 4 6% 40%	6633

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% 2 5% 22% 19% 4	5259
	11th to 20th	Day	33% 1 4 19% 31% 11%	5280
	21st to end	Day	53% 2 4 15% 19% 7%	6038
February	1st to 10th	Day	44% 3 7% 21% 21% 4	2100
	11th to 20th	Day	47% 2 6% 21% 19% 6%	2058
	21st to end	Day	47% 2 7% 16% 20% 8%	1730
March	1st to 10th	Day	53% 2 7% 12% 18% 8%	4708
	11th to 20th	Day	36% 3 6% 16% 24% 16%	5090
	21st to end	Day	43% 2 7% 24% 19% 4	5586
April	1st to 10th	Day	38% 3 11% 23% 19% 5%	2389
	11th to 20th	Day	36% 3 7% 21% 26% 7%	2494
	21st to end	Day	35% 4 11% 28% 20% 3	2681
May	1st to 10th	Day	46% 4 13% 22% 14% 2	5910
	11th to 20th	Day	52% 5% 14% 20% 9% 1	5865
	21st to end	Day	43% 3 10% 24% 18% 2	6447
June	1st to 10th	Day	42% 5% 18% 23% 10% 2	2600
	11th to 20th	Day	31% 6% 20% 29% 13% 1	2600
	21st to end	Day	36% 6% 18% 25% 12% 3	2600
July	1st to 10th	Day	38% 10% 19% 25% 7% 1	5835
	11th to 20th	Day	35% 12% 28% 19% 5% 1	5831
	21st to end	Day	49% 10% 18% 18% 4	6240
August	1st to 10th	Day	35% 7% 15% 33% 10%	4221
	11th to 20th	Day	33% 7% 17% 26% 14% 2	3928
	21st to end	Day	31% 7% 14% 25% 21% 3	4311
September	1st to 10th	Day	39% 4 15% 18% 19% 4	2217
	11th to 20th	Day	41% 5% 11% 22% 20% 1	2200
	21st to end	Day	37% 4 12% 18% 23% 7%	2200
October	1st to 10th	Day	25% 3 6% 14% 38% 14%	3590
	11th to 20th	Day	20% 2 4 17% 30% 27%	3388
	21st to end	Day	29% 3 4 23% 28% 12%	3538
November	1st to 10th	Day	34% 2 6% 17% 31% 10%	1852
	11th to 20th	Day	34% 2 6% 20% 28% 10%	1890
	21st to end	Day	38% 3 6% 20% 26% 6%	1890
December	1st to 10th	Day	42% 2 5% 19% 25% 7%	3020
	11th to 20th	Day	48% 3 6% 17% 20% 6%	3460
	21st to end	Day	47% 2 6% 26% 17% 2	4565
January	1st to 10th	Night	43% 2 7% 21% 22% 4	7636
	11th to 20th	Night	39% 2 3 20% 28% 8%	7680
	21st to end	Night	49% 2 3 16% 23% 8%	8126
February	1st to 10th	Night	42% 2 7% 20% 22% 7%	2520
	11th to 20th	Night	48% 2 5% 19% 20% 7%	2459
	21st to end	Night	47% 2 5% 17% 19% 10%	2076
March	1st to 10th	Night	52% 2 7% 15% 18% 6%	5364
	11th to 20th	Night	33% 2 6% 16% 27% 16%	5090
	21st to end	Night	41% 2 6% 23% 20% 8%	5588
April	1st to 10th	Night	36% 3 10% 21% 24% 6%	2247
	11th to 20th	Night	33% 2 7% 23% 26% 9%	2075
	21st to end	Night	34% 3 10% 28% 20% 5%	1937
May	1st to 10th	Night	45% 5% 16% 21% 12% 2	4069
	11th to 20th	Night	52% 4 14% 21% 9%	4033
	21st to end	Night	38% 4 13% 26% 17% 2	4429
June	1st to 10th	Night	38% 8% 17% 22% 13% 2	1800
	11th to 20th	Night	30% 6% 22% 26% 15% 1	1800
	21st to end	Night	33% 7% 20% 22% 15% 4	1800
July	1st to 10th	Night	42% 9% 17% 24% 7% 1	4015
	11th to 20th	Night	36% 13% 26% 19% 5% 1	4006
	21st to end	Night	49% 9% 19% 18% 5% 1	4297
August	1st to 10th	Night	36% 7% 17% 32% 8%	2904
	11th to 20th	Night	36% 6% 15% 27% 15% 1	3182
	21st to end	Night	32% 5% 13% 21% 26% 3	3586
September	1st to 10th	Night	39% 6% 14% 19% 17% 5%	2161
	11th to 20th	Night	39% 5% 12% 23% 20% 2	2200
	21st to end	Night	37% 4 11% 19% 21% 8%	2200
October	1st to 10th	Night	25% 4 8% 14% 38% 12%	3590
	11th to 20th	Night	19% 1 4 18% 34% 24%	3749
	21st to end	Night	24% 2 3 24% 30% 17%	4252
November	1st to 10th	Night	33% 3 7% 18% 28% 12%	2661
	11th to 20th	Night	33% 2 4 19% 30% 11%	2730
	21st to end	Night	38% 2 5% 21% 26% 7%	2730
December	1st to 10th	Night	39% 2 5% 19% 27% 7%	4380
	11th to 20th	Night	43% 3 8% 16% 24% 6%	4954
	21st to end	Night	47% 2 4 29% 16% 2	6633

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	31% 20% 38% 10% 1	330
	11th to 20th	Day	32% 21% 29% 13% 4	330
	21st to end	Day	29% 24% 33% 10% 3 1	363
February	1st to 10th	Day	25% 19% 38% 16% 2	330
	11th to 20th	Day	39% 21% 28% 11% 1	330
	21st to end	Day	33% 18% 36% 10% 3	273
March	1st to 10th	Day	29% 15% 34% 16% 5% 1	380
	11th to 20th	Day	25% 17% 32% 19% 6% 1	540
	21st to end	Day	20% 17% 36% 21% 5% 1	594
April	1st to 10th	Day	22% 12% 31% 22% 11% 1	540
	11th to 20th	Day	15% 12% 36% 24% 12% 2	532
	21st to end	Day	17% 10% 26% 29% 15% 3	540
May	1st to 10th	Day	17% 12% 31% 22% 13% 5%	507
	11th to 20th	Day	17% 4 25% 30% 14% 11%	527
	21st to end	Day	16% 6% 17% 30% 23% 10%	573
June	1st to 10th	Day	13% 3 16% 31% 25% 12%	530
	11th to 20th	Day	8% 4 13% 29% 25% 21%	530
	21st to end	Day	10% 3 15% 29% 27% 16%	530
July	1st to 10th	Day	9% 1 8% 27% 31% 23%	525
	11th to 20th	Day	9% 3 7% 20% 34% 27%	524
	21st to end	Day	9% 3 6% 21% 38% 23%	576
August	1st to 10th	Day	16% 3 11% 23% 25% 21%	526
	11th to 20th	Day	13% 3 6% 28% 32% 18%	519
	21st to end	Day	15% 3 8% 26% 30% 19%	583
September	1st to 10th	Day	11% 6% 12% 29% 26% 16%	530
	11th to 20th	Day	17% 5% 12% 25% 30% 12%	530
	21st to end	Day	15% 5% 16% 28% 26% 10%	530
October	1st to 10th	Day	15% 4 23% 30% 20% 8%	530
	11th to 20th	Day	13% 10% 23% 26% 24% 4	530
	21st to end	Day	20% 8% 25% 27% 17% 3	583
November	1st to 10th	Day	20% 10% 32% 22% 12% 5%	330
	11th to 20th	Day	18% 17% 36% 19% 8% 1	330
	21st to end	Day	15% 18% 38% 20% 8% 1	330
December	1st to 10th	Day	30% 22% 27% 18% 2 1	330
	11th to 20th	Day	25% 18% 33% 19% 5%	330
	21st to end	Day	30% 15% 35% 17% 3 1	363
January	1st to 10th	Night	26% 17% 31% 21% 5%	840
	11th to 20th	Night	23% 17% 31% 22% 7%	840
	21st to end	Night	22% 18% 32% 25% 3	924
February	1st to 10th	Night	21% 14% 31% 29% 5%	840
	11th to 20th	Night	27% 17% 32% 22% 1	823
	21st to end	Night	25% 16% 34% 22% 3	692
March	1st to 10th	Night	20% 14% 33% 27% 5%	840
	11th to 20th	Night	18% 13% 33% 26% 9% 1	840
	21st to end	Night	19% 12% 31% 30% 8%	924
April	1st to 10th	Night	15% 13% 30% 29% 12% 1	777
	11th to 20th	Night	16% 9% 28% 32% 14% 1	623
	21st to end	Night	13% 6% 26% 36% 18% 1	630
May	1st to 10th	Night	11% 9% 23% 33% 23% 1	603
	11th to 20th	Night	16% 5% 21% 32% 23% 4	591
	21st to end	Night	12% 3 14% 35% 30% 7%	650
June	1st to 10th	Night	8% 2 11% 39% 34% 6%	600
	11th to 20th	Night	6% 2 13% 31% 36% 12%	600
	21st to end	Night	8% 2 8% 35% 39% 9%	600
July	1st to 10th	Night	9% 4 5% 23% 46% 14%	585
	11th to 20th	Night	7% 2 4 20% 52% 15%	582
	21st to end	Night	7% 4 4 24% 49% 12%	657
August	1st to 10th	Night	11% 3 6% 28% 42% 11%	597
	11th to 20th	Night	12% 3 8% 28% 41% 9%	594
	21st to end	Night	15% 3 7% 26% 39% 10%	660
September	1st to 10th	Night	11% 2 10% 29% 38% 10%	761
	11th to 20th	Night	16% 3 11% 30% 36% 5%	800
	21st to end	Night	15% 2 14% 34% 32% 3	800
October	1st to 10th	Night	13% 3 20% 33% 23% 7%	800
	11th to 20th	Night	13% 6% 24% 33% 20% 3	800
	21st to end	Night	16% 9% 21% 35% 17% 3	880
November	1st to 10th	Night	16% 7% 22% 34% 19% 2	820
	11th to 20th	Night	18% 10% 28% 30% 13% 1	840
	21st to end	Night	20% 12% 27% 28% 13%	840
December	1st to 10th	Night	23% 15% 29% 27% 7%	840
	11th to 20th	Night	25% 9% 30% 28% 8% 1	840
	21st to end	Night	28% 11% 28% 27% 6%	924

Page generated at 2026-04-29 02:21:46. Version ac07

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