Jordan Hall Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jordan Hall 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	14% 10% 4 9% 5% 4 4 4 5% 7% 33%	119752
	Night	15% 11% 5% 9% 5% 4 4 3 5% 6% 32%	119549
Transparency	Day	49% 5% 11% 22% 13% 1	123350
	Night	51% 5% 10% 20% 12% 1	124092
Seeing	Day	20% 13% 29% 22% 11% 4	16714
	Night	20% 11% 29% 28% 11% 1	26549

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	12% 16% 11% 21% 7% 4 4 3 4 4 15%	3617
	11th to 20th	Day	12% 15% 9% 16% 10% 7% 6% 4 5% 5% 11%	3620
	21st to end	Day	21% 18% 7% 12% 6% 4 3 2 4 5% 19%	3982
February	1st to 10th	Day	15% 15% 7% 11% 6% 7% 5% 4 5% 5% 20%	1920
	11th to 20th	Day	15% 12% 5% 14% 9% 5% 5% 4 5% 5% 22%	1948
	21st to end	Day	16% 14% 6% 11% 5% 4 4 4 6% 6% 24%	1650
March	1st to 10th	Day	17% 11% 6% 11% 5% 4 3 3 4 5% 31%	4480
	11th to 20th	Day	10% 9% 7% 7% 4 5% 4 4 6% 8% 37%	4790
	21st to end	Day	18% 16% 7% 10% 6% 5% 5% 4 4 5% 21%	5737
April	1st to 10th	Day	18% 14% 3 8% 4 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	16% 11% 3 6% 4 4 4 3 5% 6% 37%	2380
	21st to end	Day	19% 9% 2 7% 4 4 4 4 6% 7% 34%	2600
May	1st to 10th	Day	19% 9% 3 7% 4 4 4 4 5% 8% 33%	5862
	11th to 20th	Day	14% 8% 3 6% 4 4 5% 4 6% 9% 38%	5861
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 43%	6882
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 5% 8% 44%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 6% 5% 5% 6% 8% 42%	2800
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	2800
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 10% 36%	4580
	11th to 20th	Day	7% 4 2 5% 4 5% 4 4 7% 10% 48%	4580
	21st to end	Day	7% 6% 2 7% 6% 5% 6% 5% 6% 8% 42%	4814
August	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 5% 9% 48%	4118
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 45%	3961
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 9% 47%	4180
September	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 6% 8% 46%	2266
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 7% 9% 47%	2090
	21st to end	Day	13% 6% 2 6% 5% 5% 4 3 6% 8% 41%	2090
October	1st to 10th	Day	14% 7% 3 6% 4 4 4 3 5% 7% 43%	2090
	11th to 20th	Day	19% 9% 3 6% 4 4 4 4 6% 7% 35%	1878
	21st to end	Day	22% 10% 4 9% 5% 4 4 4 5% 7% 26%	1881
November	1st to 10th	Day	12% 11% 4 7% 5% 4 4 4 5% 7% 38%	1762
	11th to 20th	Day	16% 18% 3 8% 7% 4 4 3 5% 5% 27%	1800
	21st to end	Day	20% 17% 5% 9% 5% 4 4 3 3 5% 24%	1800
December	1st to 10th	Day	10% 15% 6% 17% 6% 4 3 3 4 5% 26%	2743
	11th to 20th	Day	15% 20% 11% 14% 5% 5% 4 3 4 4 15%	3008
	21st to end	Day	27% 20% 7% 13% 6% 4 4 2 3 4 10%	3982
January	1st to 10th	Night	15% 18% 9% 19% 6% 5% 4 3 4 4 15%	6348
	11th to 20th	Night	13% 20% 8% 16% 8% 7% 5% 4 5% 4 10%	6360
	21st to end	Night	19% 14% 8% 12% 5% 4 3 3 4 5% 21%	6996
February	1st to 10th	Night	17% 16% 5% 10% 7% 4 4 3 5% 4 24%	2472
	11th to 20th	Night	14% 14% 5% 11% 7% 5% 5% 3 5% 6% 25%	2338
	21st to end	Night	20% 13% 5% 11% 5% 5% 5% 3 4 5% 26%	1980
March	1st to 10th	Night	16% 11% 5% 10% 5% 4 4 3 4 5% 33%	5373
	11th to 20th	Night	12% 8% 3 7% 4 5% 5% 3 5% 6% 42%	5169
	21st to end	Night	19% 11% 5% 9% 4 4 4 3 5% 6% 30%	5220
April	1st to 10th	Night	16% 10% 3 6% 5% 4 5% 4 6% 8% 35%	2000
	11th to 20th	Night	18% 9% 3 7% 5% 4 5% 3 4 5% 37%	1967
	21st to end	Night	17% 9% 3 7% 5% 4 5% 3 6% 6% 37%	1800
May	1st to 10th	Night	19% 10% 4 8% 5% 4 4 3 5% 6% 34%	4033
	11th to 20th	Night	16% 7% 2 6% 4 4 3 3 5% 8% 41%	4032
	21st to end	Night	17% 7% 3 7% 4 3 4 3 4 6% 41%	3989
June	1st to 10th	Night	9% 5% 3 5% 3 4 4 4 7% 9% 48%	1600
	11th to 20th	Night	9% 5% 3 6% 5% 4 4 5% 7% 9% 44%	1600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 8% 11% 41%	1600
July	1st to 10th	Night	9% 5% 2 5% 5% 5% 5% 4 7% 10% 43%	2610
	11th to 20th	Night	7% 4 2 4 4 4 3 3 5% 9% 54%	2610
	21st to end	Night	7% 5% 2 6% 4 5% 6% 5% 8% 8% 44%	2750
August	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	2811
	11th to 20th	Night	9% 4 2 6% 3 4 4 3 6% 8% 50%	2866
	21st to end	Night	9% 6% 2 5% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 5% 6% 50%	1900
	11th to 20th	Night	10% 5% 1 4 4 3 3 3 4 8% 55%	2085
	21st to end	Night	15% 6% 3 6% 3 3 3 3 5% 9% 44%	2090
October	1st to 10th	Night	14% 6% 3 5% 4 4 4 3 4 6% 47%	2090
	11th to 20th	Night	18% 11% 3 6% 5% 4 4 3 4 6% 38%	2298
	21st to end	Night	23% 10% 3 8% 4 4 3 3 4 6% 34%	2717
November	1st to 10th	Night	14% 12% 3 7% 5% 4 3 3 4 6% 38%	2536
	11th to 20th	Night	13% 16% 4 9% 5% 5% 5% 4 5% 6% 27%	2600
	21st to end	Night	18% 18% 5% 9% 5% 4 3 3 4 6% 25%	2600
December	1st to 10th	Night	12% 13% 6% 15% 6% 5% 5% 4 5% 7% 23%	4438
	11th to 20th	Night	17% 19% 8% 14% 8% 5% 4 3 4 4 13%	5199
	21st to end	Night	26% 20% 7% 11% 6% 4 4 3 4 4 11%	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	73% 4 10% 11% 3	3618
	11th to 20th	Day	74% 4 8% 7% 6%	3620
	21st to end	Day	69% 3 9% 13% 6%	3982
February	1st to 10th	Day	66% 3 10% 13% 7% 1	1920
	11th to 20th	Day	62% 6% 8% 17% 7%	1958
	21st to end	Day	60% 4 12% 16% 7% 1	1650
March	1st to 10th	Day	57% 3 10% 21% 9%	4480
	11th to 20th	Day	44% 5% 12% 30% 7% 1	4790
	21st to end	Day	66% 4 9% 14% 6% 1	5739
April	1st to 10th	Day	55% 3 10% 20% 12%	2400
	11th to 20th	Day	48% 4 10% 23% 12% 3	2380
	21st to end	Day	48% 4 12% 21% 12% 3	2600
May	1st to 10th	Day	49% 5% 11% 21% 14% 1	5897
	11th to 20th	Day	43% 6% 13% 24% 13% 1	5865
	21st to end	Day	39% 5% 9% 27% 18% 1	6896
June	1st to 10th	Day	38% 5% 11% 26% 18% 2	2800
	11th to 20th	Day	38% 5% 12% 25% 18% 2	2800
	21st to end	Day	38% 6% 15% 22% 17% 3	2800
July	1st to 10th	Day	42% 5% 13% 20% 16% 3	4511
	11th to 20th	Day	29% 6% 14% 32% 17% 2	4505
	21st to end	Day	38% 8% 12% 19% 21% 3	4663
August	1st to 10th	Day	34% 5% 14% 28% 17% 1	4104
	11th to 20th	Day	33% 7% 12% 30% 17% 1	4000
	21st to end	Day	33% 5% 12% 27% 19% 3	4179
September	1st to 10th	Day	36% 5% 15% 22% 18% 4	2266
	11th to 20th	Day	34% 4 11% 29% 20% 2	2090
	21st to end	Day	42% 4 10% 25% 17% 2	2090
October	1st to 10th	Day	30% 3 9% 35% 21% 1	3480
	11th to 20th	Day	39% 5% 9% 21% 26%	3094
	21st to end	Day	49% 4 10% 25% 11% 2	3081
November	1st to 10th	Day	46% 5% 11% 23% 14% 1	1762
	11th to 20th	Day	59% 5% 10% 18% 8% 1	1800
	21st to end	Day	63% 4 11% 17% 5%	1800
December	1st to 10th	Day	61% 3 16% 17% 3	2743
	11th to 20th	Day	72% 4 10% 12% 2	3005
	21st to end	Day	81% 3 6% 8% 2	3982
January	1st to 10th	Night	72% 5% 7% 13% 3	6348
	11th to 20th	Night	75% 4 10% 8% 3	6360
	21st to end	Night	65% 4 12% 12% 7%	6996
February	1st to 10th	Night	63% 4 9% 15% 8% 1	2472
	11th to 20th	Night	60% 4 8% 19% 8% 1	2339
	21st to end	Night	61% 3 10% 16% 8% 2	1980
March	1st to 10th	Night	53% 3 11% 25% 8% 1	5373
	11th to 20th	Night	42% 5% 11% 29% 11% 2	5169
	21st to end	Night	54% 5% 11% 20% 10% 1	5220
April	1st to 10th	Night	48% 4 12% 21% 14% 1	2000
	11th to 20th	Night	49% 4 7% 21% 17% 2	1967
	21st to end	Night	48% 4 10% 22% 14% 2	1800
May	1st to 10th	Night	52% 3 9% 19% 13% 2	4060
	11th to 20th	Night	42% 6% 12% 24% 15% 1	4033
	21st to end	Night	44% 5% 9% 21% 19% 1	3997
June	1st to 10th	Night	32% 6% 11% 30% 18% 3	1600
	11th to 20th	Night	35% 6% 13% 25% 18% 2	1600
	21st to end	Night	35% 6% 17% 24% 16% 2	1600
July	1st to 10th	Night	36% 5% 14% 25% 14% 6%	2570
	11th to 20th	Night	27% 6% 14% 25% 26% 3	2562
	21st to end	Night	33% 8% 14% 17% 24% 3	2662
August	1st to 10th	Night	35% 7% 12% 29% 15% 1	2801
	11th to 20th	Night	31% 5% 12% 31% 19% 1	2893
	21st to end	Night	33% 5% 10% 30% 19% 3	3476
September	1st to 10th	Night	34% 5% 11% 25% 20% 5%	1900
	11th to 20th	Night	29% 3 8% 31% 26% 2	2085
	21st to end	Night	39% 4 10% 28% 18% 2	2090
October	1st to 10th	Night	26% 4 9% 33% 28% 1	3480
	11th to 20th	Night	40% 3 8% 23% 24% 1	3821
	21st to end	Night	40% 4 8% 29% 18% 1	4467
November	1st to 10th	Night	47% 5% 9% 24% 14% 2	2536
	11th to 20th	Night	55% 5% 11% 19% 9% 2	2600
	21st to end	Night	61% 3 11% 18% 6%	2600
December	1st to 10th	Night	59% 5% 15% 18% 4	4438
	11th to 20th	Night	73% 4 10% 10% 3	5201
	21st to end	Night	77% 4 9% 8% 2	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	22% 23% 34% 16% 5% 1	300
	11th to 20th	Day	29% 14% 40% 13% 3 1	300
	21st to end	Day	27% 18% 35% 16% 2 1	330
February	1st to 10th	Day	28% 24% 32% 11% 4	300
	11th to 20th	Day	29% 24% 32% 13% 2	300
	21st to end	Day	32% 26% 33% 8% 1	249
March	1st to 10th	Day	26% 24% 36% 11% 2	300
	11th to 20th	Day	28% 16% 33% 18% 4 1	414
	21st to end	Day	33% 17% 29% 16% 6%	550
April	1st to 10th	Day	28% 16% 31% 19% 5% 1	500
	11th to 20th	Day	27% 22% 26% 15% 9% 2	492
	21st to end	Day	29% 15% 30% 18% 7% 2	500
May	1st to 10th	Day	29% 14% 30% 17% 8% 2	503
	11th to 20th	Day	24% 13% 28% 22% 10% 4	527
	21st to end	Day	15% 12% 33% 22% 13% 5%	762
June	1st to 10th	Day	15% 10% 31% 30% 10% 4	730
	11th to 20th	Day	13% 9% 31% 30% 14% 3	730
	21st to end	Day	13% 9% 32% 28% 13% 5%	730
July	1st to 10th	Day	11% 4 26% 29% 20% 11%	721
	11th to 20th	Day	10% 7% 31% 26% 17% 9%	718
	21st to end	Day	9% 7% 27% 34% 17% 7%	740
August	1st to 10th	Day	11% 3 21% 30% 22% 13%	500
	11th to 20th	Day	11% 6% 25% 30% 21% 7%	479
	21st to end	Day	9% 3 24% 31% 26% 7%	539
September	1st to 10th	Day	14% 8% 24% 30% 17% 7%	490
	11th to 20th	Day	10% 4 20% 28% 27% 10%	490
	21st to end	Day	20% 12% 21% 25% 17% 6%	490
October	1st to 10th	Day	18% 10% 25% 27% 16% 3	490
	11th to 20th	Day	25% 19% 28% 21% 6%	380
	21st to end	Day	32% 15% 37% 13% 2 1	330
November	1st to 10th	Day	18% 19% 37% 18% 6% 1	300
	11th to 20th	Day	32% 22% 26% 16% 4	300
	21st to end	Day	28% 22% 36% 13% 2	300
December	1st to 10th	Day	24% 26% 32% 13% 4 1	300
	11th to 20th	Day	23% 28% 32% 13% 4	300
	21st to end	Day	32% 21% 32% 13% 2	330
January	1st to 10th	Night	25% 16% 34% 20% 5%	1000
	11th to 20th	Night	25% 19% 34% 18% 3 1	1000
	21st to end	Night	24% 16% 34% 23% 3	1100
February	1st to 10th	Night	28% 14% 35% 21% 3	817
	11th to 20th	Night	23% 17% 32% 24% 3	783
	21st to end	Night	28% 16% 35% 16% 4	660
March	1st to 10th	Night	22% 14% 35% 23% 6%	800
	11th to 20th	Night	20% 14% 37% 23% 7%	800
	21st to end	Night	26% 12% 32% 26% 4 1	775
April	1st to 10th	Night	22% 15% 32% 24% 6% 1	600
	11th to 20th	Night	23% 13% 32% 25% 5% 1	593
	21st to end	Night	21% 12% 32% 26% 9% 1	600
May	1st to 10th	Night	21% 9% 31% 31% 8% 1	600
	11th to 20th	Night	21% 6% 28% 32% 12%	591
	21st to end	Night	14% 6% 27% 36% 16% 1	650
June	1st to 10th	Night	10% 6% 29% 41% 14% 1	600
	11th to 20th	Night	13% 4 30% 38% 15% 1	600
	21st to end	Night	11% 3 24% 40% 21% 1	600
July	1st to 10th	Night	8% 2 16% 43% 27% 4	585
	11th to 20th	Night	10% 3 22% 37% 25% 4	582
	21st to end	Night	7% 3 19% 42% 29% 1	624
August	1st to 10th	Night	8% 2 15% 45% 27% 3	567
	11th to 20th	Night	9% 2 16% 43% 26% 5%	564
	21st to end	Night	10% 2 16% 38% 30% 3	627
September	1st to 10th	Night	15% 2 19% 38% 23% 4	570
	11th to 20th	Night	10% 3 19% 37% 26% 4	755
	21st to end	Night	18% 7% 22% 32% 19% 3	760
October	1st to 10th	Night	17% 7% 27% 30% 17% 1	760
	11th to 20th	Night	24% 15% 29% 24% 8% 1	760
	21st to end	Night	28% 13% 31% 22% 6%	836
November	1st to 10th	Night	22% 14% 31% 23% 9% 1	780
	11th to 20th	Night	24% 18% 32% 21% 5% 1	800
	21st to end	Night	27% 14% 33% 22% 5%	800
December	1st to 10th	Night	26% 16% 35% 18% 4	910
	11th to 20th	Night	27% 15% 35% 20% 4	1000
	21st to end	Night	34% 16% 30% 19% 2	1100

Page generated at 2025-07-18 01:10:02. Version ac07

Copyright © 2025 Attilla Danko.