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 2025-02-06.

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	17% 12% 4 10% 6% 5% 5% 4 5% 6% 27%	108384
	Night	16% 14% 5% 12% 6% 5% 4 3 4 5% 24%	109858
Transparency	Day	57% 5% 11% 19% 8% 1	111900
	Night	62% 4 10% 16% 8% 1	114506
Seeing	Day	27% 16% 29% 18% 8% 2	14911
	Night	26% 13% 24% 26% 10% 1	23997

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	12% 17% 9% 22% 11% 6% 4 3 3 3 11%	3597
	11th to 20th	Day	9% 14% 8% 22% 13% 6% 5% 3 4 4 13%	3570
	21st to end	Day	10% 18% 9% 13% 7% 7% 6% 4 5% 5% 16%	3960
February	1st to 10th	Day	13% 16% 5% 16% 10% 6% 4 3 5% 4 19%	1690
	11th to 20th	Day	11% 17% 7% 17% 10% 6% 4 4 3 4 17%	1825
	21st to end	Day	12% 17% 6% 13% 8% 6% 5% 3 4 4 21%	1570
March	1st to 10th	Day	21% 17% 5% 10% 7% 4 4 4 3 4 20%	3116
	11th to 20th	Day	13% 22% 7% 13% 6% 4 4 2 4 4 21%	3247
	21st to end	Day	17% 14% 5% 12% 5% 4 3 3 3 4 30%	4166
April	1st to 10th	Day	19% 17% 5% 10% 5% 4 3 2 4 5% 26%	2280
	11th to 20th	Day	18% 14% 3 7% 5% 4 4 3 4 5% 32%	2254
	21st to end	Day	23% 12% 3 9% 5% 4 4 3 5% 5% 28%	2456
May	1st to 10th	Day	25% 12% 3 8% 5% 5% 4 3 5% 5% 26%	4218
	11th to 20th	Day	18% 10% 4 8% 6% 5% 6% 5% 5% 7% 27%	4398
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 8% 33%	4837
June	1st to 10th	Day	26% 8% 3 6% 5% 4 5% 4 6% 6% 27%	2660
	11th to 20th	Day	18% 7% 2 7% 5% 5% 4 4 7% 8% 33%	2660
	21st to end	Day	23% 8% 3 7% 5% 5% 4 5% 6% 7% 27%	2660
July	1st to 10th	Day	13% 6% 2 7% 6% 5% 6% 5% 8% 10% 32%	4440
	11th to 20th	Day	12% 7% 3 7% 6% 6% 6% 6% 7% 9% 31%	4440
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	18% 9% 3 8% 4 5% 5% 5% 6% 7% 29%	4423
	11th to 20th	Day	17% 8% 4 7% 5% 5% 5% 4 6% 8% 30%	3933
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 5% 8% 38%	4180
September	1st to 10th	Day	18% 6% 2 6% 4 3 4 3 6% 6% 42%	2280
	11th to 20th	Day	17% 7% 1 5% 4 4 4 3 4 7% 45%	2160
	21st to end	Day	17% 9% 3 6% 5% 3 5% 3 6% 7% 36%	2090
October	1st to 10th	Day	23% 10% 3 8% 4 4 4 3 4 7% 28%	2026
	11th to 20th	Day	19% 12% 4 7% 5% 4 4 4 5% 5% 30%	1831
	21st to end	Day	25% 20% 4 6% 5% 5% 3 2 3 4 24%	1881
November	1st to 10th	Day	16% 19% 4 8% 4 5% 4 3 4 5% 28%	1761
	11th to 20th	Day	17% 18% 6% 9% 7% 4 4 2 5% 4 24%	1800
	21st to end	Day	17% 19% 6% 13% 4 6% 4 4 4 5% 19%	1615
December	1st to 10th	Day	12% 20% 8% 24% 8% 6% 4 2 3 3 9%	2600
	11th to 20th	Day	16% 14% 9% 16% 6% 5% 4 2 4 4 19%	2986
	21st to end	Day	15% 15% 7% 15% 7% 5% 6% 3 3 4 18%	3960
January	1st to 10th	Night	10% 17% 8% 25% 11% 7% 4 2 2 3 10%	6349
	11th to 20th	Night	8% 18% 7% 23% 12% 6% 5% 3 4 3 11%	6315
	21st to end	Night	11% 15% 6% 14% 10% 9% 7% 5% 4 4 15%	6996
February	1st to 10th	Night	14% 17% 6% 17% 9% 7% 4 3 4 4 16%	2598
	11th to 20th	Night	12% 16% 6% 16% 10% 6% 5% 3 4 3 19%	2239
	21st to end	Night	13% 15% 6% 13% 7% 6% 5% 3 4 4 23%	1884
March	1st to 10th	Night	21% 17% 5% 9% 6% 4 3 2 4 4 24%	3724
	11th to 20th	Night	14% 20% 7% 12% 5% 5% 3 2 4 4 25%	3707
	21st to end	Night	16% 19% 5% 12% 5% 4 3 2 3 2 29%	3466
April	1st to 10th	Night	20% 15% 5% 10% 4 4 4 2 4 5% 27%	1900
	11th to 20th	Night	22% 11% 4 9% 5% 4 4 2 4 5% 29%	1875
	21st to end	Night	22% 13% 4 6% 5% 4 4 3 4 5% 31%	1722
May	1st to 10th	Night	25% 13% 4 6% 4 4 4 3 4 6% 26%	2752
	11th to 20th	Night	20% 11% 4 7% 5% 4 4 3 4 6% 32%	2495
	21st to end	Night	17% 6% 2 7% 5% 5% 5% 4 6% 8% 36%	2738
June	1st to 10th	Night	27% 8% 3 6% 4 4 4 4 4 7% 29%	1520
	11th to 20th	Night	18% 7% 3 7% 4 4 5% 4 6% 8% 34%	1520
	21st to end	Night	22% 9% 3 6% 4 4 5% 5% 6% 7% 30%	1520
July	1st to 10th	Night	15% 7% 3 7% 5% 5% 6% 5% 7% 7% 33%	2520
	11th to 20th	Night	16% 9% 3 7% 5% 5% 4 3 5% 8% 35%	2520
	21st to end	Night	16% 7% 3 7% 6% 6% 5% 4 5% 7% 35%	2739
August	1st to 10th	Night	22% 10% 3 7% 4 3 4 3 5% 7% 32%	2506
	11th to 20th	Night	16% 8% 3 9% 6% 4 4 3 5% 5% 36%	2847
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 7% 40%	3476
September	1st to 10th	Night	16% 6% 2 6% 4 4 5% 4 5% 6% 42%	1900
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 5% 7% 46%	2019
	21st to end	Night	19% 8% 2 5% 3 4 4 3 5% 7% 41%	2090
October	1st to 10th	Night	23% 9% 2 7% 5% 5% 4 3 4 6% 32%	2152
	11th to 20th	Night	22% 12% 3 7% 5% 4 5% 3 3 6% 30%	2347
	21st to end	Night	25% 16% 3 8% 5% 4 3 3 4 5% 23%	2717
November	1st to 10th	Night	19% 16% 4 8% 4 4 4 3 4 6% 28%	2537
	11th to 20th	Night	17% 17% 6% 11% 5% 3 4 3 4 5% 26%	2600
	21st to end	Night	16% 19% 6% 14% 5% 5% 3 3 4 5% 19%	2784
December	1st to 10th	Night	13% 19% 10% 23% 7% 5% 4 3 3 3 8%	4580
	11th to 20th	Night	18% 21% 6% 16% 6% 5% 4 3 3 3 16%	5208
	21st to end	Night	15% 17% 6% 16% 7% 6% 6% 2 3 3 18%	6996

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	80% 4 7% 8% 1	3598
	11th to 20th	Day	75% 5% 6% 8% 4 1	3600
	21st to end	Day	69% 4 9% 11% 6%	3960
February	1st to 10th	Day	70% 4 6% 14% 5% 1	1690
	11th to 20th	Day	71% 4 7% 12% 5% 1	1834
	21st to end	Day	66% 4 8% 12% 8% 2	1570
March	1st to 10th	Day	69% 3 8% 10% 7% 1	3119
	11th to 20th	Day	67% 4 9% 10% 9% 1	3247
	21st to end	Day	58% 4 5% 16% 16% 1	4166
April	1st to 10th	Day	62% 3 7% 18% 9% 1	2280
	11th to 20th	Day	55% 5% 7% 19% 13% 1	2254
	21st to end	Day	59% 4 9% 16% 9% 3	2456
May	1st to 10th	Day	61% 5% 8% 19% 7% 1	4232
	11th to 20th	Day	54% 7% 13% 17% 8% 1	4402
	21st to end	Day	45% 7% 12% 26% 10% 1	4837
June	1st to 10th	Day	55% 5% 11% 19% 9%	2660
	11th to 20th	Day	47% 6% 11% 23% 12% 1	2660
	21st to end	Day	55% 5% 11% 20% 8% 1	2660
July	1st to 10th	Day	44% 6% 16% 25% 10%	4370
	11th to 20th	Day	44% 7% 16% 29% 5%	4365
	21st to end	Day	46% 5% 11% 29% 7%	4661
August	1st to 10th	Day	52% 5% 16% 21% 7%	4409
	11th to 20th	Day	50% 5% 17% 22% 5%	3972
	21st to end	Day	43% 5% 16% 21% 14% 1	4180
September	1st to 10th	Day	42% 4 12% 29% 12% 1	2280
	11th to 20th	Day	41% 4 10% 30% 14% 1	2160
	21st to end	Day	46% 5% 14% 23% 10% 1	2090
October	1st to 10th	Day	48% 5% 12% 25% 9%	3364
	11th to 20th	Day	53% 4 13% 21% 9%	3021
	21st to end	Day	52% 3 14% 23% 7% 1	3081
November	1st to 10th	Day	59% 4 11% 18% 8%	1761
	11th to 20th	Day	64% 4 9% 13% 8% 2	1800
	21st to end	Day	68% 4 13% 11% 4	1615
December	1st to 10th	Day	80% 3 8% 6% 2	2600
	11th to 20th	Day	67% 5% 7% 17% 4	2986
	21st to end	Day	70% 4 6% 11% 7% 1	3960
January	1st to 10th	Night	80% 4 8% 6% 1	6349
	11th to 20th	Night	78% 4 8% 7% 3	6360
	21st to end	Night	71% 4 8% 8% 8% 1	6996
February	1st to 10th	Night	72% 4 8% 12% 5%	2598
	11th to 20th	Night	70% 3 7% 13% 7%	2241
	21st to end	Night	64% 4 7% 15% 8% 1	1884
March	1st to 10th	Night	64% 4 8% 14% 9% 1	3734
	11th to 20th	Night	64% 4 7% 15% 10% 1	3708
	21st to end	Night	61% 5% 5% 15% 13% 1	3466
April	1st to 10th	Night	61% 3 8% 17% 10% 1	1900
	11th to 20th	Night	57% 3 8% 20% 11% 1	1875
	21st to end	Night	56% 4 9% 15% 13% 2	1722
May	1st to 10th	Night	60% 4 9% 18% 8% 1	2758
	11th to 20th	Night	53% 6% 12% 19% 9% 1	2496
	21st to end	Night	45% 6% 14% 19% 15% 1	2739
June	1st to 10th	Night	55% 4 10% 21% 9% 1	1520
	11th to 20th	Night	46% 7% 13% 22% 10% 2	1520
	21st to end	Night	52% 6% 13% 20% 9%	1520
July	1st to 10th	Night	47% 6% 15% 21% 11%	2480
	11th to 20th	Night	46% 5% 14% 28% 7%	2472
	21st to end	Night	48% 6% 12% 23% 10%	2651
August	1st to 10th	Night	52% 4 15% 23% 6%	2496
	11th to 20th	Night	50% 5% 18% 20% 8%	2874
	21st to end	Night	43% 4 15% 23% 14% 1	3476
September	1st to 10th	Night	42% 5% 14% 28% 11% 1	1900
	11th to 20th	Night	38% 4 15% 31% 11% 1	2019
	21st to end	Night	43% 4 12% 25% 13% 2	2090
October	1st to 10th	Night	55% 4 10% 22% 8%	3594
	11th to 20th	Night	53% 3 10% 21% 12%	3896
	21st to end	Night	55% 4 13% 19% 8% 1	4467
November	1st to 10th	Night	58% 4 12% 18% 8% 1	2537
	11th to 20th	Night	62% 4 9% 16% 8% 2	2600
	21st to end	Night	68% 3 11% 14% 3	2784
December	1st to 10th	Night	81% 4 8% 6% 1	4580
	11th to 20th	Night	74% 4 5% 10% 6%	5208
	21st to end	Night	72% 3 6% 11% 7% 1	6996

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% 25% 31% 17% 4	200
	11th to 20th	Day	32% 23% 32% 10% 4 1	200
	21st to end	Day	27% 21% 35% 13% 4	220
February	1st to 10th	Day	35% 29% 27% 8% 2	252
	11th to 20th	Day	29% 27% 32% 10% 3	433
	21st to end	Day	31% 19% 32% 15% 3	382
March	1st to 10th	Day	34% 17% 30% 17% 3	460
	11th to 20th	Day	29% 24% 29% 13% 4	460
	21st to end	Day	34% 20% 26% 11% 8% 1	506
April	1st to 10th	Day	37% 14% 25% 16% 5% 1	460
	11th to 20th	Day	36% 18% 23% 15% 5% 2	452
	21st to end	Day	33% 17% 28% 13% 8% 1	460
May	1st to 10th	Day	34% 16% 27% 15% 6% 2	463
	11th to 20th	Day	24% 15% 33% 18% 8% 2	487
	21st to end	Day	23% 18% 30% 22% 6% 1	529
June	1st to 10th	Day	30% 13% 32% 17% 7% 1	490
	11th to 20th	Day	23% 13% 31% 23% 9% 2	490
	21st to end	Day	26% 17% 31% 18% 7% 1	490
July	1st to 10th	Day	19% 9% 33% 25% 12% 2	485
	11th to 20th	Day	17% 9% 38% 25% 9% 3	484
	21st to end	Day	17% 10% 35% 26% 9% 2	532
August	1st to 10th	Day	20% 10% 28% 28% 12% 3	486
	11th to 20th	Day	20% 8% 30% 23% 15% 5%	479
	21st to end	Day	21% 13% 26% 23% 12% 5%	539
September	1st to 10th	Day	20% 11% 27% 21% 14% 6%	490
	11th to 20th	Day	20% 11% 22% 21% 18% 9%	490
	21st to end	Day	18% 10% 30% 23% 12% 6%	490
October	1st to 10th	Day	28% 10% 27% 19% 13% 3	490
	11th to 20th	Day	30% 15% 23% 20% 10% 1	453
	21st to end	Day	39% 15% 24% 18% 4	429
November	1st to 10th	Day	34% 15% 21% 22% 7% 1	395
	11th to 20th	Day	31% 21% 25% 15% 8% 1	400
	21st to end	Day	37% 16% 29% 17% 1	215
December	1st to 10th	Day	29% 22% 31% 15% 4	200
	11th to 20th	Day	35% 22% 32% 11% 2	200
	21st to end	Day	29% 21% 33% 13% 3	220
January	1st to 10th	Night	26% 22% 29% 20% 3	800
	11th to 20th	Night	28% 19% 28% 20% 4	800
	21st to end	Night	20% 17% 33% 24% 4 1	880
February	1st to 10th	Night	24% 19% 33% 21% 3	781
	11th to 20th	Night	23% 23% 30% 21% 3	743
	21st to end	Night	23% 20% 30% 24% 3	628
March	1st to 10th	Night	29% 14% 29% 24% 4 1	760
	11th to 20th	Night	29% 18% 25% 21% 7%	760
	21st to end	Night	29% 15% 25% 21% 8% 1	836
April	1st to 10th	Night	31% 14% 25% 23% 6%	760
	11th to 20th	Night	30% 15% 24% 22% 8%	751
	21st to end	Night	30% 14% 25% 22% 8%	582
May	1st to 10th	Night	30% 11% 25% 22% 10% 1	527
	11th to 20th	Night	22% 8% 28% 31% 9% 2	374
	21st to end	Night	19% 9% 23% 36% 12%	411
June	1st to 10th	Night	30% 6% 22% 30% 11% 1	380
	11th to 20th	Night	22% 5% 22% 33% 17% 1	380
	21st to end	Night	24% 7% 18% 38% 12% 1	380
July	1st to 10th	Night	16% 4 22% 38% 19% 1	370
	11th to 20th	Night	12% 3 20% 44% 19% 1	368
	21st to end	Night	17% 4 17% 45% 17%	416
August	1st to 10th	Night	19% 4 14% 42% 21% 1	378
	11th to 20th	Night	18% 4 20% 32% 23% 2	600
	21st to end	Night	20% 5% 17% 35% 22% 2	836
September	1st to 10th	Night	19% 6% 19% 35% 18% 2	760
	11th to 20th	Night	20% 5% 17% 31% 23% 4	760
	21st to end	Night	23% 8% 23% 28% 17% 3	760
October	1st to 10th	Night	28% 11% 17% 27% 15% 2	760
	11th to 20th	Night	30% 11% 20% 27% 11% 1	760
	21st to end	Night	33% 13% 26% 22% 5%	836
November	1st to 10th	Night	31% 16% 22% 23% 8% 1	780
	11th to 20th	Night	29% 15% 26% 20% 8% 1	800
	21st to end	Night	31% 18% 26% 18% 6% 1	800
December	1st to 10th	Night	31% 19% 26% 20% 4	800
	11th to 20th	Night	29% 20% 26% 20% 5%	800
	21st to end	Night	30% 21% 25% 21% 3	880

Page generated at 2025-02-03 03:05:49. Version ac07

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