3 Herons Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3 Herons Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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% 11% 5% 8% 7% 3 5% 4 5% 6% 28%	74886
	Night	18% 13% 5% 9% 7% 4 5% 4 4 5% 26%	75156
Transparency	Day	57% 5% 10% 18% 8% 1	74591
	Night	60% 4 10% 17% 8% 1	74985
Seeing	Day	27% 15% 29% 19% 8% 2	13619
	Night	26% 13% 24% 26% 10% 1	22711

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	14% 16% 9% 14% 10% 4 7% 5% 3 4 13%	1517
	11th to 20th	Day	15% 17% 7% 14% 9% 4 5% 4 4 6% 17%	1520
	21st to end	Day	11% 14% 8% 14% 10% 4 8% 5% 5% 5% 18%	1632
February	1st to 10th	Day	13% 15% 6% 15% 11% 5% 5% 3 5% 4 17%	1562
	11th to 20th	Day	11% 16% 8% 14% 11% 4 5% 5% 3 4 18%	1726
	21st to end	Day	12% 16% 8% 11% 10% 6% 6% 4 4 4 20%	1480
March	1st to 10th	Day	13% 17% 8% 10% 9% 4 4 4 4 4 23%	1797
	11th to 20th	Day	16% 16% 7% 8% 9% 2 4 3 4 5% 26%	1845
	21st to end	Day	18% 15% 6% 9% 7% 3 3 3 3 5% 28%	2376
April	1st to 10th	Day	19% 15% 6% 9% 6% 3 3 3 4 5% 26%	2160
	11th to 20th	Day	18% 13% 3 7% 5% 3 4 4 4 5% 33%	2134
	21st to end	Day	24% 12% 3 7% 6% 3 4 4 4 5% 27%	2326
May	1st to 10th	Day	22% 11% 4 6% 5% 3 4 4 6% 5% 30%	2396
	11th to 20th	Day	18% 9% 4 6% 5% 2 4 5% 4 7% 34%	2478
	21st to end	Day	18% 7% 4 6% 5% 3 6% 6% 6% 8% 31%	2725
June	1st to 10th	Day	27% 7% 3 5% 5% 3 5% 5% 6% 7% 27%	2520
	11th to 20th	Day	19% 6% 2 6% 6% 3 5% 5% 6% 8% 33%	2520
	21st to end	Day	22% 7% 3 7% 5% 4 5% 5% 6% 8% 28%	2520
July	1st to 10th	Day	17% 5% 2 5% 6% 3 6% 5% 6% 8% 36%	2520
	11th to 20th	Day	14% 7% 3 5% 6% 4 6% 7% 6% 9% 32%	2520
	21st to end	Day	17% 6% 3 6% 7% 4 5% 6% 6% 8% 32%	2744
August	1st to 10th	Day	16% 8% 3 6% 5% 4 6% 6% 6% 7% 34%	2506
	11th to 20th	Day	16% 6% 4 5% 6% 4 6% 5% 6% 9% 34%	2248
	21st to end	Day	15% 7% 3 6% 6% 3 5% 6% 5% 9% 36%	2376
September	1st to 10th	Day	19% 6% 2 5% 4 2 4 4 6% 6% 41%	2160
	11th to 20th	Day	18% 7% 1 4 4 3 4 3 4 7% 45%	2047
	21st to end	Day	17% 8% 3 6% 6% 3 5% 4 5% 7% 36%	1980
October	1st to 10th	Day	25% 10% 3 7% 5% 3 4 5% 4 7% 27%	1920
	11th to 20th	Day	19% 11% 4 7% 6% 3 5% 5% 5% 5% 29%	1735
	21st to end	Day	27% 19% 4 5% 6% 4 3 3 3 4 22%	1782
November	1st to 10th	Day	17% 17% 4 7% 5% 3 5% 4 3 5% 28%	1671
	11th to 20th	Day	17% 17% 7% 8% 8% 3 5% 3 4 4 23%	1710
	21st to end	Day	17% 18% 6% 12% 6% 4 4 5% 4 5% 20%	1535
December	1st to 10th	Day	18% 20% 9% 13% 7% 3 4 3 4 4 14%	1520
	11th to 20th	Day	19% 16% 8% 11% 7% 3 5% 3 4 5% 21%	1906
	21st to end	Day	20% 15% 7% 14% 10% 4 6% 4 3 3 14%	2772
January	1st to 10th	Night	14% 20% 8% 15% 10% 5% 6% 5% 4 3 11%	2649
	11th to 20th	Night	14% 17% 7% 14% 9% 5% 6% 5% 4 4 16%	2660
	21st to end	Night	12% 14% 8% 14% 10% 5% 8% 5% 4 5% 18%	2868
February	1st to 10th	Night	14% 15% 7% 16% 10% 5% 6% 4 4 4 15%	2396
	11th to 20th	Night	13% 15% 7% 14% 11% 5% 6% 4 4 4 19%	2118
	21st to end	Night	13% 15% 6% 12% 9% 5% 5% 4 4 4 23%	1776
March	1st to 10th	Night	14% 16% 8% 10% 8% 3 5% 3 3 4 24%	2150
	11th to 20th	Night	17% 15% 6% 10% 7% 4 3 3 4 4 28%	2109
	21st to end	Night	17% 16% 6% 9% 5% 3 3 3 3 3 31%	1980
April	1st to 10th	Night	21% 13% 6% 9% 5% 3 4 3 4 5% 27%	1800
	11th to 20th	Night	22% 10% 4 8% 6% 3 5% 4 4 4 31%	1775
	21st to end	Night	23% 12% 4 5% 6% 3 4 4 4 5% 30%	1632
May	1st to 10th	Night	23% 11% 4 6% 4 3 4 4 5% 6% 31%	1580
	11th to 20th	Night	19% 11% 4 5% 4 3 4 4 3 8% 35%	1415
	21st to end	Night	18% 6% 3 6% 5% 4 5% 5% 5% 8% 35%	1550
June	1st to 10th	Night	28% 8% 3 5% 5% 3 4 5% 4 7% 29%	1440
	11th to 20th	Night	18% 6% 3 6% 5% 3 5% 5% 6% 9% 34%	1440
	21st to end	Night	21% 8% 3 5% 5% 3 5% 6% 6% 7% 31%	1440
July	1st to 10th	Night	16% 6% 2 5% 4 4 6% 5% 6% 8% 38%	1440
	11th to 20th	Night	12% 5% 3 6% 7% 4 4 6% 6% 10% 37%	1440
	21st to end	Night	15% 5% 2 5% 6% 4 6% 6% 6% 9% 36%	1574
August	1st to 10th	Night	14% 6% 2 5% 5% 3 5% 4 6% 10% 39%	1432
	11th to 20th	Night	15% 7% 4 7% 6% 3 4 4 5% 6% 40%	1632
	21st to end	Night	15% 8% 3 5% 5% 3 4 5% 6% 7% 38%	1980
September	1st to 10th	Night	17% 6% 2 5% 5% 3 5% 5% 4 7% 41%	1800
	11th to 20th	Night	17% 7% 2 4 4 2 3 4 5% 8% 45%	1912
	21st to end	Night	20% 7% 2 4 4 3 4 4 5% 6% 41%	1980
October	1st to 10th	Night	23% 8% 2 7% 5% 4 4 4 4 6% 32%	2038
	11th to 20th	Night	22% 11% 3 6% 6% 4 5% 4 3 6% 29%	2223
	21st to end	Night	26% 16% 4 7% 6% 3 4 4 4 5% 22%	2574
November	1st to 10th	Night	19% 15% 4 7% 6% 3 4 4 4 5% 29%	2407
	11th to 20th	Night	18% 15% 7% 10% 6% 3 3 3 4 5% 26%	2470
	21st to end	Night	17% 17% 7% 12% 7% 3 4 4 4 5% 20%	2644
December	1st to 10th	Night	19% 21% 9% 12% 8% 4 5% 3 3 4 12%	2660
	11th to 20th	Night	19% 20% 8% 13% 8% 4 4 3 3 3 15%	3288
	21st to end	Night	20% 18% 7% 16% 8% 3 4 3 3 3 14%	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	75% 4 7% 11% 3	1518
	11th to 20th	Day	70% 5% 7% 11% 6% 1	1520
	21st to end	Day	68% 3 11% 11% 6% 1	1632
February	1st to 10th	Day	72% 4 6% 12% 5% 1	1562
	11th to 20th	Day	70% 4 7% 13% 5% 1	1735
	21st to end	Day	68% 4 7% 11% 8% 2	1480
March	1st to 10th	Day	66% 4 9% 14% 6% 2	1800
	11th to 20th	Day	62% 4 11% 11% 11% 1	1845
	21st to end	Day	62% 4 6% 16% 10% 2	2376
April	1st to 10th	Day	62% 4 7% 18% 9% 2	2160
	11th to 20th	Day	54% 5% 7% 19% 13% 1	2134
	21st to end	Day	60% 4 9% 16% 8% 3	2326
May	1st to 10th	Day	55% 4 7% 21% 12% 1	2396
	11th to 20th	Day	49% 5% 8% 22% 13% 2	2482
	21st to end	Day	49% 7% 13% 20% 10% 1	2725
June	1st to 10th	Day	56% 5% 12% 18% 9%	2520
	11th to 20th	Day	48% 6% 11% 23% 12% 2	2520
	21st to end	Day	54% 6% 11% 20% 8% 1	2520
July	1st to 10th	Day	43% 6% 15% 24% 12%	2450
	11th to 20th	Day	45% 7% 14% 27% 7%	2445
	21st to end	Day	48% 6% 15% 23% 8% 1	2591
August	1st to 10th	Day	47% 6% 16% 23% 7%	2492
	11th to 20th	Day	46% 5% 17% 22% 9%	2248
	21st to end	Day	45% 6% 12% 24% 12%	2376
September	1st to 10th	Day	43% 4 12% 28% 12% 1	2160
	11th to 20th	Day	42% 4 10% 30% 14% 1	2047
	21st to end	Day	46% 5% 14% 23% 9% 2	1980
October	1st to 10th	Day	58% 4 10% 19% 9%	1920
	11th to 20th	Day	55% 5% 14% 18% 7%	1735
	21st to end	Day	69% 3 7% 14% 6% 2	1782
November	1st to 10th	Day	59% 4 11% 18% 8%	1671
	11th to 20th	Day	66% 4 10% 12% 7% 2	1710
	21st to end	Day	67% 5% 13% 12% 3	1535
December	1st to 10th	Day	75% 4 9% 9% 3	1520
	11th to 20th	Day	69% 3 8% 15% 5%	1906
	21st to end	Day	76% 4 6% 11% 3	2772
January	1st to 10th	Night	77% 4 8% 8% 3	2649
	11th to 20th	Night	72% 4 8% 10% 5% 1	2660
	21st to end	Night	69% 4 7% 11% 7% 1	2868
February	1st to 10th	Night	73% 3 7% 10% 5%	2396
	11th to 20th	Night	70% 3 7% 13% 7%	2120
	21st to end	Night	65% 4 7% 14% 8% 1	1776
March	1st to 10th	Night	64% 3 8% 15% 7% 2	2160
	11th to 20th	Night	61% 3 8% 16% 9% 2	2110
	21st to end	Night	58% 4 7% 17% 11% 2	1980
April	1st to 10th	Night	61% 3 8% 17% 10% 1	1800
	11th to 20th	Night	56% 3 8% 20% 12% 1	1775
	21st to end	Night	57% 4 9% 15% 13% 3	1632
May	1st to 10th	Night	55% 3 6% 19% 14% 2	1580
	11th to 20th	Night	49% 5% 8% 20% 15% 2	1416
	21st to end	Night	47% 5% 14% 20% 12% 2	1551
June	1st to 10th	Night	55% 4 10% 21% 8% 1	1440
	11th to 20th	Night	46% 7% 12% 23% 10% 2	1440
	21st to end	Night	50% 6% 13% 20% 9%	1440
July	1st to 10th	Night	42% 8% 15% 22% 13% 1	1400
	11th to 20th	Night	40% 7% 15% 28% 9%	1392
	21st to end	Night	43% 6% 16% 24% 9% 1	1486
August	1st to 10th	Night	39% 5% 20% 26% 9%	1422
	11th to 20th	Night	44% 5% 17% 23% 11%	1632
	21st to end	Night	44% 5% 14% 24% 12%	1980
September	1st to 10th	Night	42% 5% 15% 27% 10% 1	1800
	11th to 20th	Night	39% 4 14% 31% 11% 1	1912
	21st to end	Night	44% 4 12% 25% 12% 2	1980
October	1st to 10th	Night	53% 4 12% 21% 9% 1	2038
	11th to 20th	Night	57% 5% 12% 19% 7%	2223
	21st to end	Night	65% 4 9% 13% 7% 1	2574
November	1st to 10th	Night	58% 4 11% 18% 8% 1	2407
	11th to 20th	Night	62% 4 9% 15% 7% 2	2470
	21st to end	Night	69% 3 10% 14% 4	2644
December	1st to 10th	Night	78% 4 7% 9% 2	2660
	11th to 20th	Night	76% 4 6% 9% 5%	3288
	21st to end	Night	77% 3 6% 10% 3	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	26% 22% 30% 19% 4	180
	11th to 20th	Day	33% 22% 31% 10% 4 1	180
	21st to end	Day	24% 21% 37% 14% 4	188
February	1st to 10th	Day	36% 31% 26% 7% 1	214
	11th to 20th	Day	29% 27% 31% 10% 3	394
	21st to end	Day	30% 20% 32% 15% 3	346
March	1st to 10th	Day	32% 17% 30% 17% 4	420
	11th to 20th	Day	28% 25% 29% 14% 5%	420
	21st to end	Day	33% 19% 26% 12% 8% 1	462
April	1st to 10th	Day	37% 13% 26% 16% 5% 1	420
	11th to 20th	Day	33% 17% 25% 17% 6% 2	412
	21st to end	Day	33% 16% 28% 13% 8% 1	420
May	1st to 10th	Day	34% 16% 28% 14% 7% 2	423
	11th to 20th	Day	24% 16% 33% 17% 7% 2	447
	21st to end	Day	23% 17% 29% 23% 7% 1	485
June	1st to 10th	Day	32% 14% 31% 16% 7% 1	450
	11th to 20th	Day	23% 13% 30% 22% 10% 2	450
	21st to end	Day	25% 15% 32% 19% 8% 1	450
July	1st to 10th	Day	20% 9% 31% 27% 12% 2	445
	11th to 20th	Day	17% 8% 38% 25% 10% 3	444
	21st to end	Day	18% 10% 35% 26% 9% 2	488
August	1st to 10th	Day	19% 10% 28% 29% 12% 3	446
	11th to 20th	Day	19% 8% 29% 23% 15% 5%	439
	21st to end	Day	21% 14% 25% 23% 13% 5%	495
September	1st to 10th	Day	20% 10% 26% 22% 14% 6%	450
	11th to 20th	Day	21% 12% 23% 20% 16% 8%	450
	21st to end	Day	18% 10% 30% 23% 12% 6%	450
October	1st to 10th	Day	29% 10% 25% 18% 14% 4	450
	11th to 20th	Day	29% 16% 24% 20% 10% 1	417
	21st to end	Day	39% 15% 22% 19% 4	396
November	1st to 10th	Day	33% 15% 21% 23% 8% 1	365
	11th to 20th	Day	31% 22% 24% 15% 8% 1	370
	21st to end	Day	35% 14% 31% 18% 1	195
December	1st to 10th	Day	30% 20% 31% 16% 4	180
	11th to 20th	Day	34% 20% 32% 11% 2	180
	21st to end	Day	31% 23% 30% 13% 2 1	198
January	1st to 10th	Night	26% 21% 29% 21% 3	760
	11th to 20th	Night	28% 19% 28% 20% 4	760
	21st to end	Night	21% 18% 32% 25% 4 1	820
February	1st to 10th	Night	23% 20% 33% 21% 3	720
	11th to 20th	Night	23% 23% 29% 22% 3	703
	21st to end	Night	23% 20% 29% 24% 4	592
March	1st to 10th	Night	28% 15% 29% 24% 4 1	720
	11th to 20th	Night	29% 18% 25% 21% 7%	720
	21st to end	Night	29% 15% 25% 22% 8% 1	792
April	1st to 10th	Night	31% 15% 25% 23% 6%	720
	11th to 20th	Night	29% 14% 24% 23% 9%	711
	21st to end	Night	30% 14% 26% 22% 8%	552
May	1st to 10th	Night	30% 12% 25% 22% 11% 1	500
	11th to 20th	Night	23% 9% 27% 31% 9% 2	354
	21st to end	Night	19% 8% 23% 36% 13% 1	389
June	1st to 10th	Night	31% 6% 23% 29% 11% 1	360
	11th to 20th	Night	23% 5% 21% 34% 17% 1	360
	21st to end	Night	22% 6% 19% 40% 13% 1	360
July	1st to 10th	Night	16% 3 22% 38% 19% 1	350
	11th to 20th	Night	10% 3 20% 45% 20% 1	348
	21st to end	Night	16% 4 18% 45% 17%	394
August	1st to 10th	Night	17% 4 14% 43% 22% 1	358
	11th to 20th	Night	18% 4 20% 32% 23% 2	568
	21st to end	Night	20% 5% 17% 35% 22% 2	792
September	1st to 10th	Night	18% 6% 19% 36% 19% 2	720
	11th to 20th	Night	21% 5% 18% 31% 22% 3	720
	21st to end	Night	23% 7% 22% 27% 17% 3	720
October	1st to 10th	Night	28% 11% 17% 27% 15% 2	720
	11th to 20th	Night	29% 11% 21% 27% 11% 1	720
	21st to end	Night	34% 13% 25% 23% 5%	792
November	1st to 10th	Night	30% 15% 23% 24% 8% 1	740
	11th to 20th	Night	29% 14% 26% 21% 8% 1	760
	21st to end	Night	31% 17% 25% 19% 7% 1	760
December	1st to 10th	Night	32% 19% 25% 19% 4	760
	11th to 20th	Night	28% 21% 26% 20% 5%	760
	21st to end	Night	31% 21% 24% 20% 3	836

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