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 2025-01-07.

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% 2 6% 5% 4 5% 4 6% 8% 40%	105040
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	103960
Transparency	Day	40% 4 11% 21% 19% 5%	108870
	Night	39% 3 9% 21% 21% 6%	108268
Seeing	Day	18% 9% 22% 23% 18% 9%	15046
	Night	17% 9% 22% 29% 20% 4	25110

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	19% 11% 4 6% 3 3 4 3 5% 5% 37%	2890
	11th to 20th	Day	15% 7% 2 6% 4 3 4 3 3 4 47%	2840
	21st to end	Day	26% 9% 3 5% 3 3 3 2 4 4 38%	3256
February	1st to 10th	Day	17% 9% 3 6% 4 4 4 3 5% 7% 38%	1900
	11th to 20th	Day	22% 6% 2 6% 4 3 3 3 5% 6% 39%	1848
	21st to end	Day	22% 7% 3 6% 3 4 4 3 4 5% 39%	1570
March	1st to 10th	Day	24% 10% 3 7% 4 3 3 3 5% 6% 32%	3204
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 6% 46%	3480
	21st to end	Day	18% 6% 3 6% 5% 5% 4 4 5% 6% 39%	3815
April	1st to 10th	Day	12% 6% 2 5% 5% 3 4 4 6% 7% 44%	2161
	11th to 20th	Day	12% 7% 2 7% 4 4 4 3 6% 6% 46%	2254
	21st to end	Day	11% 4 2 5% 4 4 5% 4 5% 8% 49%	2425
May	1st to 10th	Day	11% 7% 2 6% 4 4 4 4 6% 9% 41%	4082
	11th to 20th	Day	17% 7% 3 7% 5% 4 5% 4 7% 8% 34%	4081
	21st to end	Day	8% 5% 2 7% 5% 4 5% 5% 8% 10% 42%	4476
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 5% 4 8% 10% 34%	2470
	11th to 20th	Day	5% 4 1 6% 4 5% 5% 6% 9% 12% 42%	2470
	21st to end	Day	7% 4 2 7% 5% 6% 6% 5% 8% 12% 37%	2470
July	1st to 10th	Day	7% 6% 3 9% 6% 7% 9% 6% 9% 11% 26%	4120
	11th to 20th	Day	8% 5% 2 8% 6% 7% 7% 6% 8% 11% 32%	4120
	21st to end	Day	12% 9% 4 10% 9% 8% 7% 6% 8% 9% 19%	4466
August	1st to 10th	Day	5% 5% 2 6% 5% 5% 6% 6% 9% 11% 39%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 8% 10% 42%	3770
	21st to end	Day	7% 4 2 5% 4 5% 6% 5% 7% 8% 47%	4180
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 5% 6% 8% 41%	2106
	11th to 20th	Day	15% 8% 2 6% 4 4 4 4 6% 8% 39%	2090
	21st to end	Day	13% 6% 1 5% 4 4 4 4 5% 6% 47%	2090
October	1st to 10th	Day	12% 5% 2 4 3 3 3 3 4 5% 55%	2090
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 5% 7% 55%	1991
	21st to end	Day	14% 5% 2 5% 3 3 3 2 4 4 55%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 3 2 3 3 5% 54%	1762
	11th to 20th	Day	14% 5% 1 5% 3 3 3 3 6% 5% 52%	1800
	21st to end	Day	14% 6% 3 5% 4 4 3 4 4 7% 47%	1800
December	1st to 10th	Day	20% 6% 2 4 3 3 3 3 5% 6% 45%	2930
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 3 4 4 39%	3373
	21st to end	Day	22% 6% 3 5% 3 3 4 3 5% 6% 39%	4466
January	1st to 10th	Night	19% 8% 3 6% 3 3 4 3 5% 7% 40%	4188
	11th to 20th	Night	15% 8% 3 6% 4 4 3 3 4 5% 44%	4120
	21st to end	Night	25% 12% 3 5% 3 3 2 2 3 4 38%	4396
February	1st to 10th	Night	16% 8% 2 6% 4 4 3 3 6% 6% 42%	2280
	11th to 20th	Night	21% 8% 2 6% 4 3 4 3 3 4 42%	2218
	21st to end	Night	21% 7% 2 6% 4 4 3 4 5% 5% 38%	1884
March	1st to 10th	Night	24% 10% 3 6% 5% 3 3 3 5% 5% 34%	3648
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 5% 8% 49%	3480
	21st to end	Night	16% 7% 2 7% 4 4 4 4 5% 6% 41%	3817
April	1st to 10th	Night	12% 5% 1 5% 4 4 5% 4 6% 7% 47%	2033
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 6% 6% 49%	1875
	21st to end	Night	8% 5% 2 5% 4 4 3 4 5% 8% 51%	1753
May	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 4 6% 8% 41%	2813
	11th to 20th	Night	16% 8% 2 6% 4 5% 4 4 6% 8% 37%	2812
	21st to end	Night	5% 5% 2 5% 4 5% 5% 5% 8% 11% 45%	3079
June	1st to 10th	Night	7% 5% 2 7% 5% 7% 6% 6% 9% 10% 36%	1710
	11th to 20th	Night	4 3 2 6% 6% 5% 4 5% 8% 12% 44%	1710
	21st to end	Night	6% 5% 2 5% 5% 6% 5% 5% 9% 11% 42%	1710
July	1st to 10th	Night	9% 7% 3 9% 7% 6% 7% 6% 7% 10% 30%	2840
	11th to 20th	Night	6% 5% 2 7% 5% 5% 6% 6% 9% 13% 34%	2840
	21st to end	Night	11% 9% 4 11% 6% 6% 7% 6% 7% 10% 23%	3087
August	1st to 10th	Night	6% 5% 2 7% 5% 4 6% 5% 8% 13% 39%	2825
	11th to 20th	Night	8% 7% 3 6% 5% 4 4 4 4 8% 47%	3055
	21st to end	Night	7% 5% 2 5% 4 4 4 3 5% 8% 51%	3476
September	1st to 10th	Night	8% 6% 3 7% 5% 5% 6% 5% 7% 8% 41%	2053
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 3 6% 7% 43%	2090
	21st to end	Night	12% 4 2 5% 5% 4 4 4 5% 6% 49%	2090
October	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 6% 56%	2090
	11th to 20th	Night	10% 6% 2 5% 3 3 2 3 3 5% 58%	2187
	21st to end	Night	12% 5% 2 4 2 3 3 2 4 5% 56%	2508
November	1st to 10th	Night	12% 6% 2 4 3 2 3 3 4 6% 55%	2531
	11th to 20th	Night	14% 6% 2 4 3 3 3 2 4 6% 53%	2600
	21st to end	Night	15% 7% 2 4 4 3 4 4 5% 6% 46%	2600
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 6% 48%	4250
	11th to 20th	Night	18% 8% 2 6% 3 3 3 3 4 6% 43%	4822
	21st to end	Night	21% 7% 3 6% 4 4 3 3 4 6% 39%	6490

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	50% 2 4 17% 22% 5%	2891
	11th to 20th	Day	41% 1 7% 23% 23% 6%	2840
	21st to end	Day	51% 1 4 18% 22% 5%	3256
February	1st to 10th	Day	46% 2 7% 20% 21% 4	1900
	11th to 20th	Day	47% 2 6% 20% 19% 6%	1858
	21st to end	Day	49% 2 7% 15% 19% 8%	1570
March	1st to 10th	Day	55% 2 8% 13% 13% 9%	3204
	11th to 20th	Day	39% 2 7% 18% 24% 9%	3480
	21st to end	Day	46% 2 8% 22% 19% 4	3815
April	1st to 10th	Day	38% 3 10% 23% 20% 6%	2161
	11th to 20th	Day	38% 3 7% 19% 24% 8%	2254
	21st to end	Day	33% 3 10% 29% 21% 3	2425
May	1st to 10th	Day	40% 2 13% 24% 18% 2	4120
	11th to 20th	Day	47% 3 11% 26% 12% 1	4085
	21st to end	Day	36% 3 13% 25% 20% 2	4489
June	1st to 10th	Day	42% 5% 18% 23% 10% 2	2470
	11th to 20th	Day	30% 6% 20% 30% 13% 1	2470
	21st to end	Day	37% 6% 18% 24% 12% 3	2470
July	1st to 10th	Day	43% 11% 21% 20% 5% 1	4055
	11th to 20th	Day	40% 10% 22% 20% 7% 1	4051
	21st to end	Day	57% 9% 15% 14% 5%	4324
August	1st to 10th	Day	34% 7% 15% 34% 10%	4091
	11th to 20th	Day	33% 7% 17% 26% 15% 2	3807
	21st to end	Day	31% 7% 14% 25% 21% 3	4179
September	1st to 10th	Day	40% 4 15% 18% 19% 4	2106
	11th to 20th	Day	42% 4 11% 20% 21% 1	2090
	21st to end	Day	37% 4 11% 19% 23% 7%	2090
October	1st to 10th	Day	25% 3 6% 14% 39% 14%	3480
	11th to 20th	Day	21% 2 5% 16% 29% 28%	3283
	21st to end	Day	29% 3 5% 23% 28% 12%	3428
November	1st to 10th	Day	35% 2 6% 16% 31% 10%	1762
	11th to 20th	Day	35% 2 6% 19% 29% 10%	1800
	21st to end	Day	38% 3 6% 20% 27% 6%	1800
December	1st to 10th	Day	41% 2 5% 19% 26% 7%	2930
	11th to 20th	Day	49% 3 6% 18% 19% 6%	3370
	21st to end	Day	47% 2 6% 26% 17% 2	4466
January	1st to 10th	Night	45% 1 4 19% 26% 4	4188
	11th to 20th	Night	43% 2 5% 24% 19% 8%	4120
	21st to end	Night	52% 1 3 13% 25% 6%	4396
February	1st to 10th	Night	43% 2 7% 19% 21% 7%	2280
	11th to 20th	Night	47% 2 5% 18% 21% 8%	2219
	21st to end	Night	48% 2 6% 16% 18% 9%	1884
March	1st to 10th	Night	53% 2 8% 16% 15% 7%	3648
	11th to 20th	Night	34% 2 8% 18% 27% 11%	3480
	21st to end	Night	44% 2 6% 21% 20% 8%	3817
April	1st to 10th	Night	36% 3 10% 20% 25% 6%	2033
	11th to 20th	Night	36% 2 7% 21% 24% 10%	1875
	21st to end	Night	32% 3 10% 30% 20% 5%	1753
May	1st to 10th	Night	41% 3 14% 24% 17% 2	2840
	11th to 20th	Night	45% 3 12% 27% 12% 1	2813
	21st to end	Night	31% 3 15% 28% 19% 3	3087
June	1st to 10th	Night	38% 8% 17% 21% 13% 2	1710
	11th to 20th	Night	30% 6% 22% 26% 15% 1	1710
	21st to end	Night	33% 6% 20% 21% 14% 4	1710
July	1st to 10th	Night	46% 9% 19% 19% 6% 1	2795
	11th to 20th	Night	36% 12% 22% 21% 7% 2	2786
	21st to end	Night	55% 10% 15% 14% 5% 1	2988
August	1st to 10th	Night	35% 7% 17% 33% 8%	2814
	11th to 20th	Night	35% 6% 15% 27% 16% 1	3084
	21st to end	Night	31% 5% 13% 21% 26% 3	3476
September	1st to 10th	Night	39% 5% 14% 20% 18% 5%	2053
	11th to 20th	Night	40% 4 11% 22% 20% 2	2090
	21st to end	Night	36% 4 11% 20% 21% 8%	2090
October	1st to 10th	Night	24% 4 8% 14% 38% 12%	3480
	11th to 20th	Night	19% 1 4 18% 34% 24%	3634
	21st to end	Night	24% 2 3 24% 30% 17%	4120
November	1st to 10th	Night	34% 3 7% 18% 27% 12%	2531
	11th to 20th	Night	34% 2 4 19% 29% 12%	2600
	21st to end	Night	38% 2 5% 21% 26% 7%	2600
December	1st to 10th	Night	39% 2 5% 20% 27% 7%	4250
	11th to 20th	Night	43% 3 8% 16% 23% 6%	4824
	21st to end	Night	48% 2 4 28% 16% 2	6490

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	30% 21% 38% 9% 1	288
	11th to 20th	Day	32% 18% 31% 13% 5%	270
	21st to end	Day	26% 25% 34% 11% 3 1	297
February	1st to 10th	Day	27% 19% 36% 15% 3	270
	11th to 20th	Day	39% 21% 27% 12% 1	270
	21st to end	Day	36% 18% 36% 9% 2	225
March	1st to 10th	Day	30% 13% 36% 15% 5% 1	315
	11th to 20th	Day	24% 17% 32% 20% 6%	460
	21st to end	Day	21% 17% 36% 20% 6% 1	506
April	1st to 10th	Day	23% 13% 31% 20% 12% 1	460
	11th to 20th	Day	15% 13% 35% 24% 12% 1	452
	21st to end	Day	17% 9% 28% 28% 15% 3	460
May	1st to 10th	Day	16% 10% 31% 23% 14% 5%	463
	11th to 20th	Day	16% 3 23% 31% 15% 11%	487
	21st to end	Day	13% 5% 16% 32% 24% 10%	529
June	1st to 10th	Day	13% 2 15% 31% 26% 13%	490
	11th to 20th	Day	7% 4 13% 27% 26% 22%	490
	21st to end	Day	10% 3 16% 29% 26% 16%	490
July	1st to 10th	Day	10% 1 8% 26% 31% 24%	485
	11th to 20th	Day	9% 3 8% 20% 32% 28%	484
	21st to end	Day	10% 3 7% 20% 36% 25%	532
August	1st to 10th	Day	13% 3 11% 24% 26% 23%	486
	11th to 20th	Day	13% 3 6% 29% 31% 18%	479
	21st to end	Day	15% 2 7% 25% 30% 20%	539
September	1st to 10th	Day	12% 6% 11% 28% 27% 17%	490
	11th to 20th	Day	18% 4 13% 24% 29% 11%	490
	21st to end	Day	16% 5% 15% 28% 26% 10%	490
October	1st to 10th	Day	15% 4 22% 29% 21% 8%	490
	11th to 20th	Day	13% 11% 24% 26% 23% 4	490
	21st to end	Day	19% 8% 23% 28% 19% 3	539
November	1st to 10th	Day	20% 10% 32% 21% 12% 5%	300
	11th to 20th	Day	20% 17% 35% 18% 8% 1	300
	21st to end	Day	14% 17% 40% 20% 8% 1	300
December	1st to 10th	Day	29% 22% 28% 19% 2 1	300
	11th to 20th	Day	24% 18% 35% 19% 5%	300
	21st to end	Day	32% 15% 33% 16% 3 1	330
January	1st to 10th	Night	26% 17% 31% 20% 5%	785
	11th to 20th	Night	23% 16% 31% 23% 7%	760
	21st to end	Night	22% 17% 33% 25% 3	836
February	1st to 10th	Night	21% 13% 31% 29% 6%	760
	11th to 20th	Night	27% 16% 32% 24% 2	743
	21st to end	Night	25% 16% 34% 21% 3	628
March	1st to 10th	Night	20% 15% 34% 27% 5%	760
	11th to 20th	Night	18% 13% 33% 25% 10% 1	760
	21st to end	Night	20% 11% 30% 30% 8%	836
April	1st to 10th	Night	16% 13% 29% 29% 12% 1	703
	11th to 20th	Night	17% 10% 28% 32% 13% 1	563
	21st to end	Night	13% 6% 26% 36% 19% 1	570
May	1st to 10th	Night	11% 9% 23% 32% 24% 1	570
	11th to 20th	Night	15% 4 21% 32% 24% 4	561
	21st to end	Night	11% 2 13% 35% 32% 7%	617
June	1st to 10th	Night	8% 2 10% 39% 35% 6%	570
	11th to 20th	Night	5% 2 13% 31% 36% 13%	570
	21st to end	Night	8% 2 8% 36% 38% 9%	570
July	1st to 10th	Night	10% 4 5% 23% 45% 14%	555
	11th to 20th	Night	7% 2 5% 20% 52% 15%	552
	21st to end	Night	7% 4 4 24% 48% 12%	624
August	1st to 10th	Night	10% 2 5% 28% 44% 11%	567
	11th to 20th	Night	12% 2 7% 29% 42% 8%	564
	21st to end	Night	15% 3 7% 26% 39% 10%	627
September	1st to 10th	Night	11% 2 10% 27% 39% 11%	723
	11th to 20th	Night	17% 3 11% 30% 35% 5%	760
	21st to end	Night	15% 2 14% 33% 33% 3	760
October	1st to 10th	Night	13% 3 20% 33% 24% 7%	760
	11th to 20th	Night	14% 6% 24% 33% 20% 3	760
	21st to end	Night	15% 8% 21% 36% 17% 3	836
November	1st to 10th	Night	16% 7% 23% 33% 19% 2	780
	11th to 20th	Night	18% 11% 28% 29% 13% 1	800
	21st to end	Night	21% 12% 27% 28% 13%	800
December	1st to 10th	Night	22% 15% 29% 27% 7% 1	800
	11th to 20th	Night	24% 9% 30% 27% 8% 1	800
	21st to end	Night	29% 12% 27% 26% 6%	880

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