Hebron Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hebron Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	16% 13% 5% 10% 5% 4 4 4 5% 6% 27%	120343
	Night	16% 14% 6% 11% 6% 4 4 3 4 5% 27%	118740
Transparency	Day	56% 3 10% 19% 10% 1	123978
	Night	59% 3 10% 18% 9% 1	123265
Seeing	Day	28% 18% 28% 16% 7% 3	15530
	Night	26% 18% 28% 20% 8% 1	24924

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% 19% 9% 22% 9% 3 2 2 3 3 15%	3597
	11th to 20th	Day	9% 20% 11% 24% 8% 6% 2 2 3 3 12%	3916
	21st to end	Day	18% 15% 8% 14% 8% 4 4 3 4 5% 17%	4466
February	1st to 10th	Day	14% 19% 7% 14% 8% 5% 4 3 4 4 19%	1800
	11th to 20th	Day	12% 16% 8% 16% 7% 5% 4 4 4 5% 20%	1753
	21st to end	Day	18% 15% 4 12% 7% 4 4 2 5% 5% 25%	1603
March	1st to 10th	Day	16% 14% 8% 10% 5% 3 3 2 3 5% 30%	4888
	11th to 20th	Day	12% 15% 7% 10% 4 3 3 3 4 6% 33%	4980
	21st to end	Day	16% 14% 5% 12% 6% 4 4 3 4 4 28%	5502
April	1st to 10th	Day	16% 18% 4 8% 5% 4 3 4 4 6% 27%	2400
	11th to 20th	Day	17% 15% 3 8% 4 4 5% 4 5% 5% 30%	2374
	21st to end	Day	20% 12% 3 7% 4 4 4 4 4 5% 33%	2525
May	1st to 10th	Day	26% 11% 3 8% 5% 4 4 3 4 5% 25%	5888
	11th to 20th	Day	22% 13% 4 7% 5% 4 5% 4 5% 6% 25%	6313
	21st to end	Day	20% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	6949
June	1st to 10th	Day	17% 8% 3 7% 6% 5% 5% 4 7% 7% 33%	2800
	11th to 20th	Day	18% 7% 2 7% 6% 5% 6% 4 6% 8% 31%	2800
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 5% 6% 7% 35%	2800
July	1st to 10th	Day	10% 7% 2 7% 5% 6% 6% 5% 7% 10% 35%	4580
	11th to 20th	Day	10% 8% 3 7% 5% 6% 6% 5% 8% 9% 34%	4580
	21st to end	Day	10% 7% 3 8% 6% 6% 5% 5% 8% 8% 35%	4505
August	1st to 10th	Day	18% 9% 3 8% 5% 4 5% 5% 6% 9% 28%	4104
	11th to 20th	Day	16% 7% 3 8% 6% 6% 7% 5% 7% 9% 26%	4044
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	4189
September	1st to 10th	Day	18% 8% 2 6% 4 3 4 4 5% 7% 38%	2280
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 5% 6% 42%	2256
	21st to end	Day	22% 9% 4 6% 5% 4 5% 4 5% 6% 31%	1995
October	1st to 10th	Day	18% 10% 3 7% 5% 4 5% 3 6% 6% 33%	1900
	11th to 20th	Day	21% 11% 3 9% 4 4 4 3 5% 6% 31%	1900
	21st to end	Day	23% 16% 3 8% 5% 4 4 3 4 6% 25%	2090
November	1st to 10th	Day	14% 13% 4 8% 4 5% 4 3 5% 6% 35%	1816
	11th to 20th	Day	15% 20% 6% 9% 4 3 4 4 6% 6% 22%	1604
	21st to end	Day	17% 23% 6% 9% 6% 4 4 3 4 6% 19%	1600
December	1st to 10th	Day	17% 23% 13% 20% 5% 3 2 2 3 4 10%	2600
	11th to 20th	Day	18% 19% 8% 13% 7% 3 3 3 3 4 18%	2986
	21st to end	Day	16% 24% 9% 12% 6% 4 3 3 4 4 15%	3960
January	1st to 10th	Night	12% 16% 10% 25% 10% 4 3 2 3 4 11%	6349
	11th to 20th	Night	10% 17% 9% 25% 10% 5% 4 3 4 3 11%	5951
	21st to end	Night	15% 17% 8% 17% 9% 6% 5% 3 3 3 14%	6490
February	1st to 10th	Night	14% 18% 7% 12% 9% 5% 4 3 4 5% 19%	2600
	11th to 20th	Night	12% 16% 7% 15% 8% 6% 5% 4 4 5% 18%	2533
	21st to end	Night	16% 15% 6% 15% 7% 4 4 3 4 4 22%	2026
March	1st to 10th	Night	16% 15% 5% 11% 9% 6% 3 3 4 5% 24%	4938
	11th to 20th	Night	14% 16% 6% 9% 4 3 3 3 4 7% 32%	4979
	21st to end	Night	13% 15% 5% 11% 5% 4 3 3 4 5% 31%	5429
April	1st to 10th	Night	18% 15% 4 9% 5% 4 4 4 4 5% 27%	2000
	11th to 20th	Night	18% 15% 3 7% 4 3 3 3 5% 6% 34%	1975
	21st to end	Night	17% 12% 3 7% 4 4 4 4 5% 7% 34%	1891
May	1st to 10th	Night	28% 13% 3 7% 4 4 3 3 3 5% 26%	4053
	11th to 20th	Night	22% 15% 3 6% 4 4 3 3 4 5% 31%	3580
	21st to end	Night	19% 13% 3 7% 5% 4 4 3 5% 6% 32%	3926
June	1st to 10th	Night	14% 10% 3 7% 5% 4 4 4 5% 9% 36%	1600
	11th to 20th	Night	14% 7% 2 8% 5% 4 4 5% 7% 9% 37%	1600
	21st to end	Night	13% 7% 3 6% 4 4 5% 3 6% 6% 43%	1600
July	1st to 10th	Night	13% 9% 3 7% 5% 4 5% 4 5% 7% 38%	2600
	11th to 20th	Night	11% 7% 3 6% 5% 4 4 3 6% 10% 41%	2600
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 3 5% 7% 41%	3045
August	1st to 10th	Night	18% 10% 2 7% 4 5% 5% 3 5% 6% 35%	2825
	11th to 20th	Night	18% 9% 4 6% 4 4 4 3 6% 6% 37%	2785
	21st to end	Night	11% 7% 2 6% 5% 4 5% 4 6% 8% 42%	3434
September	1st to 10th	Night	15% 8% 2 6% 5% 3 3 2 4 6% 44%	1900
	11th to 20th	Night	13% 8% 2 6% 3 4 4 3 5% 6% 46%	1920
	21st to end	Night	20% 10% 3 7% 4 4 4 3 5% 6% 35%	2184
October	1st to 10th	Night	18% 9% 3 6% 4 4 3 3 4 6% 39%	2280
	11th to 20th	Night	19% 13% 3 7% 4 5% 3 3 5% 7% 32%	2280
	21st to end	Night	24% 16% 3 7% 4 3 4 3 5% 6% 27%	2508
November	1st to 10th	Night	11% 14% 4 8% 4 4 4 3 5% 5% 38%	2480
	11th to 20th	Night	16% 19% 4 8% 5% 5% 4 3 5% 5% 25%	2795
	21st to end	Night	15% 20% 7% 11% 5% 4 4 4 4 6% 20%	2800
December	1st to 10th	Night	15% 19% 12% 21% 6% 4 4 2 3 4 11%	4580
	11th to 20th	Night	18% 18% 9% 15% 5% 4 4 2 3 4 19%	5208
	21st to end	Night	17% 21% 10% 12% 7% 4 3 3 4 4 14%	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	76% 2 5% 11% 5% 1	3598
	11th to 20th	Day	81% 1 5% 9% 4	3950
	21st to end	Day	70% 1 8% 14% 5% 1	4466
February	1st to 10th	Day	70% 2 9% 12% 6% 1	1800
	11th to 20th	Day	67% 3 6% 16% 8%	1762
	21st to end	Day	63% 2 7% 19% 7% 2	1603
March	1st to 10th	Day	58% 2 10% 19% 10% 1	4891
	11th to 20th	Day	54% 3 15% 18% 8% 3	4980
	21st to end	Day	60% 3 4 18% 12% 2	5502
April	1st to 10th	Day	58% 3 9% 18% 12% 1	2400
	11th to 20th	Day	56% 3 9% 17% 13% 1	2374
	21st to end	Day	54% 2 7% 20% 14% 2	2525
May	1st to 10th	Day	61% 3 7% 18% 8% 2	5897
	11th to 20th	Day	60% 3 8% 19% 9%	6317
	21st to end	Day	53% 5% 11% 21% 11% 1	6949
June	1st to 10th	Day	49% 4 11% 21% 13% 1	2800
	11th to 20th	Day	50% 4 13% 20% 11% 1	2800
	21st to end	Day	46% 5% 12% 25% 13%	2800
July	1st to 10th	Day	43% 5% 13% 25% 12% 1	4510
	11th to 20th	Day	43% 6% 13% 24% 14%	4505
	21st to end	Day	43% 6% 13% 26% 11% 1	4361
August	1st to 10th	Day	52% 5% 12% 22% 9%	4091
	11th to 20th	Day	51% 6% 15% 22% 6%	4083
	21st to end	Day	43% 5% 10% 31% 10% 1	4189
September	1st to 10th	Day	47% 3 9% 25% 15% 2	2280
	11th to 20th	Day	43% 3 10% 26% 17% 1	2256
	21st to end	Day	53% 3 9% 21% 13% 1	1995
October	1st to 10th	Day	45% 4 11% 22% 17% 1	3160
	11th to 20th	Day	48% 3 11% 19% 15% 4	3140
	21st to end	Day	45% 4 14% 25% 11% 1	3428
November	1st to 10th	Day	49% 3 10% 21% 13% 3	1816
	11th to 20th	Day	61% 3 8% 18% 7% 3	1604
	21st to end	Day	68% 2 9% 15% 5% 1	1600
December	1st to 10th	Day	81% 2 6% 8% 4	2600
	11th to 20th	Day	70% 2 9% 14% 5%	2986
	21st to end	Day	73% 2 9% 10% 5%	3960
January	1st to 10th	Night	77% 2 5% 10% 5%	6349
	11th to 20th	Night	78% 2 8% 8% 3 1	5992
	21st to end	Night	75% 2 6% 9% 7% 1	6490
February	1st to 10th	Night	69% 2 9% 14% 7%	2600
	11th to 20th	Night	68% 3 7% 15% 6% 1	2535
	21st to end	Night	65% 2 9% 13% 9% 1	2026
March	1st to 10th	Night	64% 3 12% 14% 7% 1	4948
	11th to 20th	Night	53% 3 14% 19% 6% 4	4980
	21st to end	Night	55% 3 6% 17% 16% 2	5429
April	1st to 10th	Night	60% 4 8% 16% 12% 1	2000
	11th to 20th	Night	53% 3 11% 20% 12% 2	1975
	21st to end	Night	51% 3 9% 22% 13% 2	1891
May	1st to 10th	Night	62% 2 7% 17% 9% 2	4060
	11th to 20th	Night	57% 3 12% 18% 9%	3581
	21st to end	Night	55% 3 9% 18% 11% 3	3927
June	1st to 10th	Night	45% 3 11% 25% 14% 1	1600
	11th to 20th	Night	44% 4 15% 25% 11% 2	1600
	21st to end	Night	42% 6% 13% 26% 14% 1	1600
July	1st to 10th	Night	45% 4 12% 21% 16% 1	2560
	11th to 20th	Night	39% 5% 17% 25% 14% 1	2552
	21st to end	Night	42% 4 12% 24% 17% 1	2948
August	1st to 10th	Night	49% 6% 14% 24% 7% 1	2814
	11th to 20th	Night	47% 4 13% 28% 8%	2812
	21st to end	Night	38% 5% 13% 33% 10% 1	3434
September	1st to 10th	Night	42% 2 11% 27% 14% 3	1900
	11th to 20th	Night	39% 3 12% 27% 18% 2	1920
	21st to end	Night	50% 3 12% 21% 13% 1	2184
October	1st to 10th	Night	48% 4 9% 21% 17% 1	3800
	11th to 20th	Night	45% 4 11% 20% 13% 8%	3779
	21st to end	Night	45% 3 15% 24% 12% 1	4120
November	1st to 10th	Night	48% 4 11% 20% 15% 3	2480
	11th to 20th	Night	60% 3 9% 16% 9% 3	2795
	21st to end	Night	65% 3 10% 17% 4 1	2800
December	1st to 10th	Night	78% 4 7% 8% 2	4580
	11th to 20th	Night	70% 2 7% 15% 6%	5208
	21st to end	Night	73% 2 8% 11% 6%	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	38% 25% 26% 9% 3 1	200
	11th to 20th	Day	39% 27% 27% 4 1	277
	21st to end	Day	36% 27% 29% 6% 2	330
February	1st to 10th	Day	42% 26% 27% 5%	300
	11th to 20th	Day	42% 29% 27% 2	300
	21st to end	Day	43% 26% 25% 5%	367
March	1st to 10th	Day	37% 24% 28% 9% 2 1	500
	11th to 20th	Day	34% 27% 26% 10% 2 1	500
	21st to end	Day	36% 22% 27% 11% 3 1	550
April	1st to 10th	Day	35% 26% 25% 10% 3 1	500
	11th to 20th	Day	33% 23% 25% 12% 5% 2	492
	21st to end	Day	30% 21% 32% 13% 3 1	500
May	1st to 10th	Day	32% 21% 27% 11% 7% 2	503
	11th to 20th	Day	31% 16% 28% 16% 6% 2	527
	21st to end	Day	23% 14% 29% 20% 11% 3	573
June	1st to 10th	Day	24% 16% 30% 20% 8% 2	530
	11th to 20th	Day	23% 13% 28% 24% 9% 2	530
	21st to end	Day	20% 15% 34% 23% 7% 2	530
July	1st to 10th	Day	14% 10% 31% 26% 14% 5%	525
	11th to 20th	Day	13% 11% 28% 30% 14% 4	524
	21st to end	Day	14% 11% 34% 27% 10% 4	532
August	1st to 10th	Day	15% 10% 24% 28% 16% 6%	486
	11th to 20th	Day	19% 8% 29% 25% 13% 7%	479
	21st to end	Day	16% 12% 28% 25% 13% 6%	539
September	1st to 10th	Day	22% 12% 23% 23% 14% 7%	490
	11th to 20th	Day	14% 9% 24% 25% 19% 8%	490
	21st to end	Day	28% 15% 24% 18% 11% 3	490
October	1st to 10th	Day	24% 16% 24% 19% 13% 4	490
	11th to 20th	Day	31% 18% 29% 14% 7% 1	490
	21st to end	Day	37% 18% 27% 16% 3	539
November	1st to 10th	Day	28% 19% 28% 16% 7% 2	423
	11th to 20th	Day	39% 21% 29% 9% 1	204
	21st to end	Day	39% 27% 28% 6% 1 1	200
December	1st to 10th	Day	43% 25% 24% 5% 3 1	200
	11th to 20th	Day	43% 26% 26% 5% 2	200
	21st to end	Day	37% 27% 31% 5% 1	220
January	1st to 10th	Night	30% 29% 30% 10% 2	800
	11th to 20th	Night	31% 26% 29% 12% 2	800
	21st to end	Night	31% 28% 28% 12% 1	880
February	1st to 10th	Night	31% 26% 31% 12%	800
	11th to 20th	Night	32% 29% 30% 8% 1	783
	21st to end	Night	35% 26% 28% 10% 2	660
March	1st to 10th	Night	30% 27% 30% 11% 2	800
	11th to 20th	Night	32% 23% 27% 14% 3	800
	21st to end	Night	26% 24% 31% 14% 4 1	880
April	1st to 10th	Night	30% 21% 31% 16% 3	800
	11th to 20th	Night	30% 18% 29% 17% 6% 1	791
	21st to end	Night	25% 14% 33% 23% 4	691
May	1st to 10th	Night	28% 13% 24% 27% 8%	600
	11th to 20th	Night	19% 13% 31% 28% 8%	399
	21st to end	Night	18% 9% 29% 30% 14% 1	433
June	1st to 10th	Night	19% 9% 29% 32% 11% 1	400
	11th to 20th	Night	17% 11% 26% 37% 10%	400
	21st to end	Night	14% 8% 31% 34% 13%	400
July	1st to 10th	Night	13% 4 24% 39% 17% 3	390
	11th to 20th	Night	11% 4 26% 38% 20% 1	388
	21st to end	Night	14% 5% 22% 40% 18% 1	595
August	1st to 10th	Night	14% 4 20% 37% 23% 2	567
	11th to 20th	Night	18% 5% 19% 31% 24% 2	564
	21st to end	Night	15% 8% 20% 32% 23% 2	807
September	1st to 10th	Night	20% 8% 24% 30% 16% 3	760
	11th to 20th	Night	17% 6% 21% 27% 25% 4	760
	21st to end	Night	23% 12% 23% 23% 15% 3	760
October	1st to 10th	Night	23% 12% 27% 19% 16% 2	760
	11th to 20th	Night	28% 15% 30% 19% 6% 1	760
	21st to end	Night	35% 15% 29% 17% 3	836
November	1st to 10th	Night	21% 20% 32% 21% 6%	780
	11th to 20th	Night	31% 20% 29% 14% 5% 1	800
	21st to end	Night	32% 22% 30% 14% 3 1	800
December	1st to 10th	Night	34% 27% 27% 9% 3	800
	11th to 20th	Night	33% 26% 27% 13% 2	800
	21st to end	Night	33% 24% 31% 11% 1	880

Page generated at 2025-07-18 01:09:58. Version ac07

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