Heartwood Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heartwood Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	15% 9% 3 8% 5% 4 5% 4 5% 7% 35%	126689
	Night	16% 10% 3 8% 5% 4 5% 4 5% 7% 34%	122317
Transparency	Day	47% 6% 13% 22% 12% 1	130243
	Night	48% 6% 12% 20% 12% 1	126849
Seeing	Day	20% 15% 31% 21% 10% 3	15570
	Night	20% 12% 30% 26% 10% 1	24773

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	13% 13% 4 8% 5% 4 4 4 6% 7% 32%	3597
	11th to 20th	Day	10% 19% 7% 14% 7% 6% 4 3 5% 4 20%	3593
	21st to end	Day	15% 18% 7% 13% 6% 5% 5% 3 5% 5% 20%	4466
February	1st to 10th	Day	13% 15% 4 12% 7% 5% 5% 4 6% 5% 24%	1800
	11th to 20th	Day	12% 13% 5% 12% 7% 6% 4 3 5% 5% 27%	1753
	21st to end	Day	19% 13% 4 10% 6% 6% 4 4 4 5% 26%	1585
March	1st to 10th	Day	20% 12% 4 8% 5% 4 3 3 4 6% 32%	4752
	11th to 20th	Day	10% 13% 4 9% 6% 6% 5% 3 6% 7% 32%	4980
	21st to end	Day	19% 10% 3 6% 4 4 3 3 4 5% 40%	5574
April	1st to 10th	Day	19% 12% 3 7% 6% 4 3 4 5% 5% 32%	2400
	11th to 20th	Day	21% 9% 3 6% 4 4 4 2 4 5% 38%	2374
	21st to end	Day	20% 9% 3 8% 6% 4 5% 3 5% 6% 33%	2570
May	1st to 10th	Day	22% 9% 3 8% 4 4 4 4 5% 6% 31%	6160
	11th to 20th	Day	16% 10% 3 8% 4 4 4 4 5% 7% 35%	6318
	21st to end	Day	19% 7% 3 6% 4 4 4 4 5% 8% 35%	6949
June	1st to 10th	Day	16% 8% 2 7% 4 5% 5% 5% 7% 8% 33%	2800
	11th to 20th	Day	12% 6% 2 5% 5% 5% 5% 5% 7% 9% 39%	2800
	21st to end	Day	15% 8% 3 6% 4 4 4 4 7% 8% 37%	2800
July	1st to 10th	Day	10% 5% 3 6% 5% 5% 4 5% 6% 10% 41%	6360
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 6% 9% 46%	6360
	21st to end	Day	7% 4 2 6% 5% 4 5% 5% 7% 9% 46%	6865
August	1st to 10th	Day	15% 7% 3 6% 4 4 5% 4 5% 8% 39%	4234
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 7% 9% 38%	4174
	21st to end	Day	10% 5% 2 6% 4 4 5% 5% 7% 9% 42%	4360
September	1st to 10th	Day	11% 6% 3 5% 4 4 5% 4 6% 8% 45%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 6% 8% 51%	2376
	21st to end	Day	15% 7% 2 7% 4 4 4 4 5% 7% 41%	1987
October	1st to 10th	Day	17% 8% 3 8% 4 4 6% 4 5% 7% 35%	1900
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 6% 32%	1900
	21st to end	Day	23% 10% 3 8% 5% 5% 4 4 5% 6% 27%	2053
November	1st to 10th	Day	14% 9% 3 8% 5% 5% 5% 3 6% 7% 35%	1703
	11th to 20th	Day	15% 11% 3 9% 5% 4 5% 5% 6% 6% 33%	1600
	21st to end	Day	20% 12% 4 8% 6% 4 6% 4 5% 6% 26%	1600
December	1st to 10th	Day	18% 16% 5% 11% 6% 5% 5% 4 4 7% 21%	2600
	11th to 20th	Day	17% 17% 6% 9% 4 5% 4 3 4 4 27%	2986
	21st to end	Day	25% 13% 3 9% 5% 5% 4 3 4 5% 24%	3960
January	1st to 10th	Night	15% 12% 7% 11% 6% 6% 5% 4 5% 5% 25%	6349
	11th to 20th	Night	12% 13% 6% 13% 8% 7% 6% 5% 6% 6% 18%	6288
	21st to end	Night	16% 14% 6% 11% 5% 5% 5% 4 5% 6% 24%	6490
February	1st to 10th	Night	14% 15% 4 10% 6% 5% 5% 4 5% 6% 26%	2600
	11th to 20th	Night	14% 13% 5% 9% 5% 5% 5% 4 6% 6% 28%	2533
	21st to end	Night	15% 10% 4 9% 5% 4 5% 4 6% 7% 31%	2044
March	1st to 10th	Night	18% 11% 3 9% 6% 5% 4 3 5% 5% 30%	5071
	11th to 20th	Night	12% 10% 3 8% 5% 5% 5% 5% 7% 8% 34%	4979
	21st to end	Night	20% 9% 3 6% 3 3 3 3 5% 6% 39%	5343
April	1st to 10th	Night	19% 10% 3 7% 5% 4 4 4 5% 6% 34%	2000
	11th to 20th	Night	18% 8% 2 8% 4 4 4 3 5% 7% 37%	1975
	21st to end	Night	18% 7% 2 8% 5% 4 5% 4 6% 6% 35%	1849
May	1st to 10th	Night	26% 9% 3 6% 4 4 5% 4 5% 5% 29%	3779
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 8% 41%	3575
	21st to end	Night	18% 6% 2 6% 5% 4 5% 4 6% 7% 38%	3926
June	1st to 10th	Night	13% 6% 2 7% 5% 5% 6% 3 7% 8% 38%	1600
	11th to 20th	Night	10% 5% 2 7% 5% 4 4 5% 6% 8% 43%	1600
	21st to end	Night	12% 8% 3 7% 5% 4 4 4 6% 7% 42%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 6% 4 7% 9% 42%	3600
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 5% 7% 49%	3600
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 9% 51%	3971
August	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 6% 8% 47%	2915
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 5% 8% 47%	2875
	21st to end	Night	8% 4 2 5% 4 4 5% 5% 7% 10% 47%	3505
September	1st to 10th	Night	11% 5% 2 6% 5% 4 4 4 6% 8% 47%	2000
	11th to 20th	Night	11% 4 1 5% 3 3 4 3 5% 7% 54%	1981
	21st to end	Night	16% 6% 2 6% 4 4 4 3 4 6% 43%	2190
October	1st to 10th	Night	16% 6% 3 7% 4 4 4 4 6% 7% 38%	2280
	11th to 20th	Night	20% 8% 3 8% 5% 5% 4 3 6% 6% 33%	2280
	21st to end	Night	21% 9% 4 8% 5% 4 5% 3 4 7% 30%	2543
November	1st to 10th	Night	18% 7% 3 5% 5% 4 4 4 5% 7% 39%	2592
	11th to 20th	Night	17% 10% 3 6% 5% 5% 5% 4 6% 7% 32%	2800
	21st to end	Night	21% 10% 3 7% 5% 5% 5% 4 6% 7% 27%	2800
December	1st to 10th	Night	19% 14% 4 8% 5% 4 5% 4 6% 7% 23%	4580
	11th to 20th	Night	18% 14% 4 10% 4 4 4 4 5% 6% 26%	5208
	21st to end	Night	26% 15% 4 9% 4 4 4 3 4 5% 21%	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	50% 6% 13% 21% 10% 1	3598
	11th to 20th	Day	66% 6% 10% 10% 7% 1	3623
	21st to end	Day	66% 5% 9% 10% 10% 1	4466
February	1st to 10th	Day	60% 7% 11% 16% 5% 1	1800
	11th to 20th	Day	58% 5% 10% 15% 10% 1	1762
	21st to end	Day	59% 6% 11% 14% 9% 1	1585
March	1st to 10th	Day	54% 4 12% 14% 15% 1	4755
	11th to 20th	Day	49% 5% 20% 13% 10% 2	4980
	21st to end	Day	47% 4 7% 22% 17% 2	5574
April	1st to 10th	Day	52% 5% 10% 19% 11% 3	2400
	11th to 20th	Day	49% 4 9% 21% 16% 1	2374
	21st to end	Day	52% 6% 9% 18% 13% 3	2570
May	1st to 10th	Day	53% 6% 10% 18% 11% 3	6169
	11th to 20th	Day	48% 7% 11% 17% 16% 1	6322
	21st to end	Day	45% 7% 13% 22% 11% 2	6949
June	1st to 10th	Day	45% 8% 13% 19% 14% 1	2800
	11th to 20th	Day	38% 8% 14% 25% 14% 2	2800
	21st to end	Day	41% 8% 14% 23% 13% 1	2800
July	1st to 10th	Day	37% 6% 14% 29% 14% 1	6290
	11th to 20th	Day	32% 7% 15% 30% 16%	6285
	21st to end	Day	32% 7% 14% 35% 12%	6668
August	1st to 10th	Day	42% 7% 15% 23% 12% 1	4221
	11th to 20th	Day	40% 7% 15% 28% 9%	4213
	21st to end	Day	36% 7% 13% 34% 10% 1	4360
September	1st to 10th	Day	35% 8% 15% 29% 12% 1	2400
	11th to 20th	Day	31% 6% 15% 33% 14% 1	2376
	21st to end	Day	42% 8% 13% 28% 9%	1987
October	1st to 10th	Day	39% 6% 13% 29% 12% 1	3160
	11th to 20th	Day	40% 7% 17% 24% 11% 1	3140
	21st to end	Day	49% 6% 14% 21% 9%	3367
November	1st to 10th	Day	46% 6% 17% 20% 10% 1	1703
	11th to 20th	Day	49% 8% 15% 17% 10% 1	1600
	21st to end	Day	57% 9% 13% 13% 7%	1600
December	1st to 10th	Day	64% 5% 16% 10% 4	2600
	11th to 20th	Day	61% 7% 9% 14% 9%	2986
	21st to end	Day	63% 6% 9% 15% 8%	3960
January	1st to 10th	Night	59% 6% 11% 15% 8%	6349
	11th to 20th	Night	63% 8% 12% 11% 5% 1	6333
	21st to end	Night	60% 6% 11% 11% 11% 1	6490
February	1st to 10th	Night	59% 6% 12% 18% 6% 1	2600
	11th to 20th	Night	55% 6% 11% 19% 9%	2535
	21st to end	Night	50% 7% 12% 21% 9% 1	2044
March	1st to 10th	Night	55% 5% 8% 16% 12% 5%	5081
	11th to 20th	Night	45% 7% 18% 19% 9% 2	4980
	21st to end	Night	45% 4 9% 23% 17% 2	5343
April	1st to 10th	Night	50% 5% 10% 20% 13% 2	2000
	11th to 20th	Night	47% 6% 10% 19% 17% 1	1975
	21st to end	Night	46% 7% 12% 17% 16% 3	1849
May	1st to 10th	Night	55% 7% 9% 14% 11% 4	3786
	11th to 20th	Night	42% 6% 15% 22% 15% 2	3576
	21st to end	Night	44% 7% 13% 19% 15% 2	3927
June	1st to 10th	Night	42% 7% 12% 23% 15% 2	1600
	11th to 20th	Night	36% 7% 14% 22% 19% 3	1600
	21st to end	Night	41% 7% 14% 23% 14% 2	1600
July	1st to 10th	Night	36% 6% 16% 22% 19% 1	3560
	11th to 20th	Night	33% 6% 12% 33% 16% 1	3552
	21st to end	Night	30% 7% 13% 34% 15% 1	3853
August	1st to 10th	Night	35% 6% 13% 30% 15% 2	2904
	11th to 20th	Night	35% 7% 16% 28% 13% 1	2902
	21st to end	Night	30% 8% 13% 37% 12% 1	3505
September	1st to 10th	Night	34% 7% 16% 29% 14% 1	2000
	11th to 20th	Night	29% 8% 13% 33% 17% 1	1981
	21st to end	Night	42% 6% 11% 27% 13% 1	2190
October	1st to 10th	Night	35% 6% 10% 28% 21% 1	3800
	11th to 20th	Night	42% 5% 15% 29% 9%	3779
	21st to end	Night	45% 5% 14% 26% 9%	4179
November	1st to 10th	Night	44% 7% 13% 23% 12% 1	2592
	11th to 20th	Night	49% 7% 13% 19% 10% 2	2800
	21st to end	Night	53% 10% 16% 16% 5% 1	2800
December	1st to 10th	Night	58% 7% 14% 17% 4	4580
	11th to 20th	Night	58% 7% 12% 13% 10% 1	5208
	21st to end	Night	65% 6% 10% 11% 9%	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	24% 24% 33% 13% 7% 1	200
	11th to 20th	Day	27% 22% 32% 15% 3 1	205
	21st to end	Day	27% 24% 30% 15% 4 1	330
February	1st to 10th	Day	29% 28% 30% 11% 2	300
	11th to 20th	Day	26% 27% 37% 8% 1	300
	21st to end	Day	33% 21% 35% 11% 1	349
March	1st to 10th	Day	24% 20% 37% 16% 4	500
	11th to 20th	Day	24% 22% 32% 18% 4	500
	21st to end	Day	31% 18% 31% 13% 6% 1	550
April	1st to 10th	Day	26% 23% 32% 12% 6% 1	500
	11th to 20th	Day	29% 22% 24% 15% 7% 3	492
	21st to end	Day	26% 20% 32% 15% 5% 2	500
May	1st to 10th	Day	24% 14% 33% 18% 7% 4	503
	11th to 20th	Day	18% 12% 35% 23% 10% 3	527
	21st to end	Day	16% 12% 31% 25% 12% 4	573
June	1st to 10th	Day	17% 14% 34% 24% 10% 2	530
	11th to 20th	Day	12% 13% 34% 26% 13% 3	530
	21st to end	Day	18% 11% 32% 25% 11% 2	530
July	1st to 10th	Day	10% 9% 30% 31% 17% 4	525
	11th to 20th	Day	14% 8% 29% 27% 17% 5%	524
	21st to end	Day	9% 8% 34% 32% 12% 4	576
August	1st to 10th	Day	12% 6% 29% 30% 16% 7%	526
	11th to 20th	Day	13% 5% 25% 30% 19% 7%	519
	21st to end	Day	11% 9% 31% 27% 14% 8%	583
September	1st to 10th	Day	13% 9% 28% 28% 13% 8%	530
	11th to 20th	Day	9% 7% 25% 24% 19% 14%	522
	21st to end	Day	15% 13% 28% 24% 15% 5%	490
October	1st to 10th	Day	20% 17% 26% 18% 14% 6%	490
	11th to 20th	Day	22% 18% 32% 18% 8% 2	490
	21st to end	Day	29% 18% 28% 21% 4	519
November	1st to 10th	Day	26% 17% 26% 22% 9% 1	337
	11th to 20th	Day	23% 22% 36% 16% 4 1	200
	21st to end	Day	24% 24% 33% 15% 4 1	200
December	1st to 10th	Day	29% 16% 35% 15% 6%	200
	11th to 20th	Day	31% 25% 31% 12% 2 1	200
	21st to end	Day	35% 17% 31% 13% 3	220
January	1st to 10th	Night	26% 19% 32% 19% 4	800
	11th to 20th	Night	26% 17% 32% 20% 5%	800
	21st to end	Night	23% 15% 35% 23% 5%	880
February	1st to 10th	Night	23% 18% 38% 19% 2	800
	11th to 20th	Night	22% 19% 36% 18% 4	783
	21st to end	Night	24% 18% 34% 21% 3	660
March	1st to 10th	Night	22% 16% 36% 23% 3	800
	11th to 20th	Night	22% 18% 33% 22% 4 1	800
	21st to end	Night	22% 15% 33% 23% 7% 1	880
April	1st to 10th	Night	23% 14% 36% 24% 3	800
	11th to 20th	Night	22% 16% 31% 22% 8% 1	791
	21st to end	Night	22% 12% 31% 25% 10% 1	649
May	1st to 10th	Night	23% 7% 28% 28% 12% 1	482
	11th to 20th	Night	14% 8% 34% 35% 10%	394
	21st to end	Night	12% 6% 33% 34% 14% 1	433
June	1st to 10th	Night	15% 4 35% 33% 13% 1	400
	11th to 20th	Night	12% 7% 29% 36% 16% 1	400
	21st to end	Night	14% 6% 29% 37% 14% 1	400
July	1st to 10th	Night	6% 6% 27% 43% 18% 1	390
	11th to 20th	Night	10% 3 25% 38% 22% 1	388
	21st to end	Night	10% 2 26% 43% 17% 1	458
August	1st to 10th	Night	11% 3 18% 40% 26% 3	597
	11th to 20th	Night	9% 5% 19% 37% 26% 4	594
	21st to end	Night	9% 4 27% 36% 21% 2	825
September	1st to 10th	Night	12% 5% 28% 33% 20% 3	800
	11th to 20th	Night	13% 4 19% 30% 27% 6%	793
	21st to end	Night	19% 8% 21% 30% 19% 4	760
October	1st to 10th	Night	19% 9% 29% 24% 16% 3	760
	11th to 20th	Night	23% 13% 29% 25% 9% 1	760
	21st to end	Night	25% 14% 32% 23% 6%	836
November	1st to 10th	Night	22% 15% 25% 28% 9% 1	780
	11th to 20th	Night	23% 16% 33% 22% 5% 1	800
	21st to end	Night	28% 16% 31% 19% 7%	800
December	1st to 10th	Night	29% 17% 32% 19% 4	800
	11th to 20th	Night	25% 19% 34% 19% 4	800
	21st to end	Night	34% 13% 33% 17% 3	880

Page generated at 2025-09-16 01:49:17. Version ac07

Copyright © 2025 Attilla Danko.