Jorgenson Obs Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jorgenson Obs Site clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 9% 3 7% 5% 5% 5% 4 5% 7% 38%	127518
	Night	13% 9% 4 8% 5% 4 4 3 5% 7% 39%	124101
Transparency	Day	44% 6% 12% 23% 13% 1	130968
	Night	44% 6% 13% 23% 13% 1	128712
Seeing	Day	17% 15% 33% 22% 10% 3	17590
	Night	17% 11% 34% 28% 9% 1	27525

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	9% 10% 3 9% 6% 7% 6% 4 5% 7% 34%	3625
	11th to 20th	Day	11% 11% 4 8% 6% 5% 6% 5% 6% 7% 31%	3620
	21st to end	Day	11% 15% 5% 12% 5% 4 4 3 4 5% 32%	3982
February	1st to 10th	Day	10% 10% 5% 9% 5% 5% 5% 3 5% 6% 38%	1740
	11th to 20th	Day	10% 11% 5% 8% 5% 4 5% 4 5% 5% 37%	1886
	21st to end	Day	12% 12% 4 8% 5% 4 4 4 6% 6% 34%	1650
March	1st to 10th	Day	12% 9% 2 5% 3 3 4 3 5% 7% 47%	4480
	11th to 20th	Day	10% 8% 3 8% 6% 6% 6% 4 6% 8% 35%	4674
	21st to end	Day	24% 11% 3 9% 5% 5% 4 3 4 5% 28%	5967
April	1st to 10th	Day	15% 13% 3 9% 5% 5% 5% 4 6% 5% 31%	2400
	11th to 20th	Day	20% 10% 3 7% 5% 5% 4 3 6% 5% 33%	2374
	21st to end	Day	19% 9% 3 7% 4 5% 5% 4 5% 7% 33%	2580
May	1st to 10th	Day	14% 6% 2 7% 5% 4 4 3 4 6% 45%	6092
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 4 6% 7% 43%	6319
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	6935
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 4 6% 8% 42%	2800
	11th to 20th	Day	14% 6% 3 7% 6% 5% 5% 4 5% 8% 38%	2800
	21st to end	Day	12% 6% 3 6% 5% 5% 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	6360
	11th to 20th	Day	7% 4 2 5% 4 4 6% 4 6% 9% 49%	6360
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 7% 8% 44%	6924
August	1st to 10th	Day	10% 5% 3 7% 5% 5% 6% 5% 6% 7% 40%	4563
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 5% 8% 46%	4143
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 6% 8% 41%	4312
September	1st to 10th	Day	11% 7% 2 6% 5% 4 4 5% 6% 7% 44%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 5% 6% 9% 44%	2294
	21st to end	Day	10% 7% 3 7% 5% 6% 5% 4 6% 8% 39%	2200
October	1st to 10th	Day	14% 7% 2 6% 5% 4 5% 3 5% 8% 41%	2095
	11th to 20th	Day	18% 10% 2 9% 6% 4 5% 3 5% 7% 31%	1937
	21st to end	Day	19% 9% 3 9% 5% 5% 4 3 4 5% 34%	1980
November	1st to 10th	Day	13% 8% 3 7% 6% 4 5% 4 6% 6% 39%	1852
	11th to 20th	Day	13% 12% 4 9% 7% 6% 5% 4 5% 6% 29%	1846
	21st to end	Day	15% 11% 4 7% 5% 5% 5% 4 6% 7% 31%	1680
December	1st to 10th	Day	9% 13% 4 10% 6% 5% 6% 5% 6% 5% 31%	2690
	11th to 20th	Day	13% 17% 4 8% 5% 5% 5% 3 5% 6% 29%	3088
	21st to end	Day	21% 21% 5% 12% 5% 3 3 2 3 4 21%	4070
January	1st to 10th	Night	10% 11% 6% 10% 5% 4 4 3 5% 6% 37%	6363
	11th to 20th	Night	9% 14% 6% 11% 7% 6% 5% 3 5% 6% 29%	6360
	21st to end	Night	9% 14% 6% 11% 5% 4 4 3 5% 6% 31%	6996
February	1st to 10th	Night	10% 13% 5% 9% 6% 5% 4 3 4 6% 35%	2658
	11th to 20th	Night	9% 11% 4 9% 4 5% 4 4 5% 7% 37%	2399
	21st to end	Night	14% 13% 4 7% 5% 4 4 3 5% 5% 35%	1980
March	1st to 10th	Night	13% 7% 2 6% 3 4 4 3 4 7% 47%	5373
	11th to 20th	Night	10% 9% 4 8% 4 5% 5% 4 6% 9% 35%	5316
	21st to end	Night	21% 9% 3 9% 5% 4 4 3 5% 6% 30%	4962
April	1st to 10th	Night	20% 9% 4 6% 4 5% 4 3 5% 6% 33%	2000
	11th to 20th	Night	21% 8% 2 5% 4 4 3 3 5% 5% 38%	1975
	21st to end	Night	17% 8% 3 7% 5% 4 3 3 6% 6% 38%	1820
May	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 4 6% 44%	3798
	11th to 20th	Night	10% 8% 3 7% 4 4 4 3 4 7% 45%	3596
	21st to end	Night	17% 7% 3 7% 4 4 4 3 5% 7% 39%	3942
June	1st to 10th	Night	10% 5% 2 5% 6% 4 5% 4 6% 8% 46%	1600
	11th to 20th	Night	14% 7% 3 7% 4 4 5% 4 6% 8% 37%	1600
	21st to end	Night	13% 5% 1 5% 5% 4 4 4 8% 7% 45%	1600
July	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 8% 46%	3618
	11th to 20th	Night	8% 4 1 5% 3 3 3 3 6% 8% 57%	3617
	21st to end	Night	8% 5% 2 6% 4 4 3 4 5% 8% 49%	3947
August	1st to 10th	Night	11% 6% 2 6% 5% 5% 4 3 5% 8% 45%	2597
	11th to 20th	Night	11% 4 2 5% 4 4 4 3 6% 8% 48%	2904
	21st to end	Night	10% 6% 2 7% 4 4 4 3 5% 6% 49%	3586
September	1st to 10th	Night	9% 5% 3 5% 3 3 3 3 5% 7% 52%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 51%	2085
	21st to end	Night	13% 7% 2 6% 4 4 3 2 4 7% 47%	2200
October	1st to 10th	Night	15% 6% 2 5% 3 4 4 3 5% 7% 47%	2300
	11th to 20th	Night	16% 9% 3 7% 5% 5% 4 3 5% 6% 37%	2462
	21st to end	Night	18% 10% 3 7% 5% 4 4 3 6% 7% 34%	2860
November	1st to 10th	Night	15% 9% 2 6% 4 4 4 3 5% 6% 41%	2666
	11th to 20th	Night	14% 10% 4 8% 5% 5% 5% 3 4 5% 36%	2772
	21st to end	Night	15% 11% 3 8% 5% 4 5% 4 5% 7% 33%	2940
December	1st to 10th	Night	13% 10% 4 8% 5% 4 4 3 5% 8% 36%	4720
	11th to 20th	Night	13% 15% 6% 12% 7% 5% 4 3 3 5% 28%	5339
	21st to end	Night	23% 16% 5% 8% 4 3 3 3 4 5% 26%	7150

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	49% 6% 11% 22% 11% 1	3626
	11th to 20th	Day	50% 7% 9% 18% 13% 2	3620
	21st to end	Day	54% 6% 11% 16% 12% 1	3982
February	1st to 10th	Day	46% 7% 13% 20% 13% 1	1740
	11th to 20th	Day	46% 7% 13% 22% 11% 2	1896
	21st to end	Day	49% 6% 12% 21% 10% 1	1650
March	1st to 10th	Day	37% 5% 14% 20% 18% 6%	4480
	11th to 20th	Day	44% 6% 17% 21% 11% 2	4674
	21st to end	Day	58% 6% 8% 14% 12%	5969
April	1st to 10th	Day	53% 7% 13% 17% 10% 1	2400
	11th to 20th	Day	51% 7% 9% 20% 11% 1	2374
	21st to end	Day	50% 7% 10% 18% 13% 2	2580
May	1st to 10th	Day	38% 6% 11% 24% 18% 3	6127
	11th to 20th	Day	38% 6% 13% 26% 15% 2	6323
	21st to end	Day	44% 7% 9% 21% 17% 2	6949
June	1st to 10th	Day	37% 8% 13% 25% 15% 2	2800
	11th to 20th	Day	44% 8% 14% 20% 13% 1	2800
	21st to end	Day	40% 8% 15% 24% 12% 1	2800
July	1st to 10th	Day	37% 6% 15% 31% 11% 1	6291
	11th to 20th	Day	30% 6% 14% 36% 13% 1	6285
	21st to end	Day	36% 6% 12% 28% 17% 1	6728
August	1st to 10th	Day	40% 7% 15% 24% 13% 1	4548
	11th to 20th	Day	35% 7% 14% 31% 12%	4184
	21st to end	Day	41% 6% 15% 22% 15% 1	4311
September	1st to 10th	Day	39% 8% 14% 26% 13% 1	2400
	11th to 20th	Day	35% 9% 18% 28% 9%	2294
	21st to end	Day	41% 8% 13% 27% 10%	2200
October	1st to 10th	Day	30% 5% 12% 29% 24%	3407
	11th to 20th	Day	48% 4 10% 23% 13% 1	3127
	21st to end	Day	49% 5% 13% 25% 7%	3180
November	1st to 10th	Day	44% 7% 13% 24% 12% 1	1852
	11th to 20th	Day	55% 7% 12% 17% 9% 1	1846
	21st to end	Day	51% 10% 15% 19% 5% 1	1680
December	1st to 10th	Day	53% 7% 14% 20% 6%	2690
	11th to 20th	Day	56% 6% 11% 15% 13% 1	3085
	21st to end	Day	68% 4 5% 16% 7%	4070
January	1st to 10th	Night	47% 6% 9% 27% 10% 1	6363
	11th to 20th	Night	56% 5% 10% 15% 13% 1	6360
	21st to end	Night	54% 6% 11% 19% 10%	6996
February	1st to 10th	Night	50% 4 10% 21% 12% 2	2658
	11th to 20th	Night	46% 7% 12% 22% 11% 2	2400
	21st to end	Night	50% 5% 12% 22% 9% 1	1980
March	1st to 10th	Night	37% 5% 17% 23% 16% 3	5373
	11th to 20th	Night	42% 6% 18% 21% 11% 1	5316
	21st to end	Night	53% 7% 11% 17% 12% 1	4962
April	1st to 10th	Night	51% 7% 9% 16% 16% 1	2000
	11th to 20th	Night	47% 6% 9% 22% 12% 3	1975
	21st to end	Night	46% 6% 12% 21% 14% 1	1820
May	1st to 10th	Night	39% 6% 12% 24% 15% 3	3825
	11th to 20th	Night	36% 8% 13% 25% 16% 2	3597
	21st to end	Night	44% 6% 11% 23% 13% 3	3950
June	1st to 10th	Night	34% 8% 15% 26% 16% 1	1600
	11th to 20th	Night	43% 8% 12% 21% 15% 1	1600
	21st to end	Night	35% 9% 15% 27% 12% 2	1600
July	1st to 10th	Night	35% 4 18% 30% 9% 2	3580
	11th to 20th	Night	25% 5% 18% 36% 13% 2	3572
	21st to end	Night	32% 5% 15% 26% 21% 1	3832
August	1st to 10th	Night	38% 6% 14% 23% 18% 2	2588
	11th to 20th	Night	34% 6% 15% 29% 14% 2	2929
	21st to end	Night	36% 6% 13% 25% 18% 1	3586
September	1st to 10th	Night	31% 7% 14% 31% 17% 1	2000
	11th to 20th	Night	32% 7% 16% 34% 12%	2085
	21st to end	Night	38% 6% 16% 28% 12% 1	2200
October	1st to 10th	Night	31% 5% 10% 35% 17% 2	3755
	11th to 20th	Night	38% 5% 12% 28% 16%	4011
	21st to end	Night	49% 5% 11% 24% 12%	4610
November	1st to 10th	Night	43% 6% 12% 24% 14% 1	2666
	11th to 20th	Night	49% 7% 14% 18% 11% 1	2772
	21st to end	Night	51% 6% 14% 22% 6% 1	2940
December	1st to 10th	Night	46% 5% 22% 21% 5% 1	4720
	11th to 20th	Night	60% 5% 10% 15% 9%	5341
	21st to end	Night	62% 5% 8% 16% 9%	7150

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	17% 15% 35% 24% 8% 1	303
	11th to 20th	Day	22% 17% 35% 21% 3 1	300
	21st to end	Day	20% 18% 40% 20% 2 1	330
February	1st to 10th	Day	15% 23% 42% 14% 5% 1	300
	11th to 20th	Day	16% 30% 36% 14% 4	300
	21st to end	Day	22% 21% 45% 10% 1	249
March	1st to 10th	Day	17% 26% 42% 14% 2	300
	11th to 20th	Day	21% 22% 36% 15% 6% 1	319
	21st to end	Day	25% 17% 32% 18% 7% 1	550
April	1st to 10th	Day	25% 20% 33% 18% 4 1	500
	11th to 20th	Day	26% 23% 26% 17% 8% 1	492
	21st to end	Day	27% 16% 28% 20% 8% 2	500
May	1st to 10th	Day	19% 18% 36% 19% 6% 2	603
	11th to 20th	Day	18% 18% 33% 22% 7% 2	725
	21st to end	Day	17% 15% 32% 23% 8% 4	789
June	1st to 10th	Day	13% 15% 36% 24% 9% 3	730
	11th to 20th	Day	17% 13% 36% 22% 9% 2	730
	21st to end	Day	15% 14% 32% 25% 10% 3	730
July	1st to 10th	Day	8% 7% 32% 33% 15% 5%	721
	11th to 20th	Day	9% 11% 33% 28% 15% 4	718
	21st to end	Day	7% 7% 34% 30% 16% 7%	794
August	1st to 10th	Day	10% 8% 29% 29% 19% 6%	726
	11th to 20th	Day	14% 6% 24% 29% 20% 7%	577
	21st to end	Day	15% 10% 24% 25% 19% 7%	583
September	1st to 10th	Day	15% 12% 28% 24% 16% 5%	530
	11th to 20th	Day	13% 7% 24% 32% 20% 4	530
	21st to end	Day	15% 15% 25% 26% 15% 3	530
October	1st to 10th	Day	20% 15% 29% 22% 12% 2	425
	11th to 20th	Day	17% 28% 32% 21% 2	330
	21st to end	Day	24% 19% 35% 17% 4 1	363
November	1st to 10th	Day	17% 18% 39% 18% 7% 1	330
	11th to 20th	Day	19% 22% 37% 14% 6% 1	330
	21st to end	Day	23% 19% 40% 14% 4 1	330
December	1st to 10th	Day	16% 18% 42% 19% 4 1	330
	11th to 20th	Day	23% 24% 34% 15% 5%	330
	21st to end	Day	31% 19% 34% 13% 3 1	363
January	1st to 10th	Night	16% 14% 42% 24% 5%	1006
	11th to 20th	Night	19% 15% 39% 20% 6% 1	1000
	21st to end	Night	16% 11% 40% 26% 6% 1	1100
February	1st to 10th	Night	17% 14% 40% 23% 6%	1000
	11th to 20th	Night	16% 17% 38% 24% 6%	844
	21st to end	Night	20% 15% 38% 23% 3	660
March	1st to 10th	Night	17% 13% 36% 27% 7%	800
	11th to 20th	Night	16% 15% 41% 22% 5%	774
	21st to end	Night	21% 11% 33% 28% 7% 1	660
April	1st to 10th	Night	24% 12% 35% 25% 5% 1	600
	11th to 20th	Night	25% 11% 30% 27% 7% 1	593
	21st to end	Night	21% 11% 34% 27% 7%	600
May	1st to 10th	Night	20% 10% 33% 29% 7% 1	600
	11th to 20th	Night	19% 10% 35% 29% 7%	591
	21st to end	Night	14% 7% 32% 35% 12% 1	650
June	1st to 10th	Night	10% 8% 32% 40% 9% 1	600
	11th to 20th	Night	16% 8% 29% 38% 9% 1	600
	21st to end	Night	15% 6% 28% 39% 13% 1	600
July	1st to 10th	Night	7% 5% 24% 45% 18% 1	585
	11th to 20th	Night	10% 4 29% 38% 19% 1	582
	21st to end	Night	11% 3 27% 41% 16% 2	657
August	1st to 10th	Night	9% 2 27% 40% 20% 1	597
	11th to 20th	Night	10% 3 22% 39% 24% 2	594
	21st to end	Night	12% 5% 24% 37% 21% 2	660
September	1st to 10th	Night	12% 5% 29% 31% 20% 4	600
	11th to 20th	Night	12% 6% 27% 33% 20% 2	685
	21st to end	Night	17% 8% 26% 31% 18% 1	800
October	1st to 10th	Night	17% 11% 31% 26% 15% 1	800
	11th to 20th	Night	20% 15% 33% 25% 8%	800
	21st to end	Night	22% 11% 31% 27% 8% 1	880
November	1st to 10th	Night	19% 11% 38% 25% 6% 1	820
	11th to 20th	Night	19% 15% 38% 22% 6%	882
	21st to end	Night	19% 16% 35% 25% 5%	1050
December	1st to 10th	Night	22% 14% 39% 20% 5%	1050
	11th to 20th	Night	20% 14% 36% 23% 5% 1	1050
	21st to end	Night	26% 13% 36% 21% 4	1155

Page generated at 2025-12-30 00:14:10. Version ac07

Copyright © 2025 Attilla Danko.