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 2024-11-12.

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% 37%	101593
	Night	14% 10% 4 7% 5% 4 4 3 5% 6% 38%	97680
Transparency	Day	44% 7% 13% 23% 12% 1	105088
	Night	45% 6% 13% 23% 12% 1	102313
Seeing	Day	17% 15% 33% 22% 10% 3	16042
	Night	17% 11% 34% 28% 9% 1	25912

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	13% 13% 4 9% 6% 6% 5% 4 4 6% 28%	2527
	11th to 20th	Day	12% 12% 5% 11% 6% 6% 5% 5% 6% 6% 25%	2530
	21st to end	Day	14% 18% 6% 12% 5% 4 4 3 4 5% 25%	2783
February	1st to 10th	Day	10% 10% 5% 9% 5% 5% 5% 3 5% 6% 38%	1653
	11th to 20th	Day	10% 11% 5% 8% 4 5% 5% 4 5% 5% 37%	1789
	21st to end	Day	13% 12% 4 7% 5% 4 4 4 6% 6% 35%	1570
March	1st to 10th	Day	11% 10% 3 5% 4 3 3 3 4 6% 46%	3120
	11th to 20th	Day	8% 9% 4 10% 6% 6% 5% 4 6% 7% 36%	3272
	21st to end	Day	20% 12% 4 10% 6% 5% 5% 3 4 5% 27%	4165
April	1st to 10th	Day	15% 12% 4 8% 5% 5% 5% 4 6% 5% 31%	2280
	11th to 20th	Day	21% 10% 3 6% 5% 5% 4 4 5% 5% 33%	2254
	21st to end	Day	18% 9% 2 7% 4 5% 5% 4 5% 7% 33%	2451
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 4 6% 39%	4242
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 6% 7% 44%	4399
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 6% 7% 38%	4824
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 4 6% 8% 42%	2660
	11th to 20th	Day	13% 6% 3 7% 6% 5% 6% 4 5% 8% 39%	2660
	21st to end	Day	12% 6% 3 6% 5% 5% 5% 4 6% 9% 40%	2660
July	1st to 10th	Day	14% 5% 2 6% 5% 5% 5% 4 7% 9% 39%	4440
	11th to 20th	Day	5% 3 2 5% 4 5% 6% 5% 7% 9% 49%	4440
	21st to end	Day	7% 5% 2 7% 5% 5% 4 4 7% 8% 47%	4813
August	1st to 10th	Day	10% 5% 3 7% 5% 5% 6% 5% 6% 7% 40%	4423
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 46%	4014
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 5% 8% 40%	4180
September	1st to 10th	Day	11% 7% 2 6% 5% 4 4 4 6% 6% 44%	2280
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 5% 6% 9% 44%	2180
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	2090
October	1st to 10th	Day	15% 7% 2 5% 5% 4 5% 3 6% 8% 40%	1990
	11th to 20th	Day	19% 10% 2 8% 6% 4 5% 3 5% 7% 31%	1840
	21st to end	Day	20% 10% 3 9% 5% 5% 4 3 4 5% 32%	1881
November	1st to 10th	Day	13% 8% 3 6% 6% 4 5% 4 6% 6% 39%	1762
	11th to 20th	Day	14% 12% 5% 8% 7% 6% 5% 4 6% 6% 28%	1680
	21st to end	Day	15% 11% 3 6% 5% 5% 5% 4 6% 7% 32%	1520
December	1st to 10th	Day	12% 12% 4 8% 5% 4 5% 4 6% 6% 34%	1520
	11th to 20th	Day	15% 14% 4 8% 6% 6% 5% 3 5% 6% 28%	1918
	21st to end	Day	21% 22% 5% 12% 5% 4 3 2 3 4 19%	2783
January	1st to 10th	Night	14% 13% 7% 10% 4 4 4 2 4 5% 33%	4428
	11th to 20th	Night	11% 15% 6% 11% 7% 6% 5% 3 5% 6% 25%	4440
	21st to end	Night	13% 18% 8% 11% 6% 4 4 3 5% 5% 24%	4884
February	1st to 10th	Night	10% 13% 5% 9% 6% 5% 5% 3 4 6% 35%	2526
	11th to 20th	Night	10% 11% 4 9% 4 5% 4 4 5% 7% 37%	2276
	21st to end	Night	14% 14% 4 7% 5% 4 4 3 5% 5% 35%	1884
March	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 4 7% 45%	3733
	11th to 20th	Night	9% 9% 4 8% 4 4 5% 4 5% 7% 38%	3718
	21st to end	Night	17% 10% 4 10% 6% 4 5% 4 5% 6% 29%	3466
April	1st to 10th	Night	19% 9% 4 6% 5% 5% 4 3 5% 6% 33%	1900
	11th to 20th	Night	21% 8% 2 5% 4 4 3 4 5% 5% 38%	1875
	21st to end	Night	17% 8% 4 7% 5% 4 3 4 6% 6% 37%	1729
May	1st to 10th	Night	16% 6% 3 7% 5% 5% 5% 3 5% 7% 40%	2643
	11th to 20th	Night	12% 7% 3 6% 4 4 5% 4 4 7% 43%	2506
	21st to end	Night	16% 6% 3 7% 5% 5% 4 3 5% 7% 39%	2743
June	1st to 10th	Night	10% 5% 2 6% 6% 3 5% 4 5% 8% 46%	1520
	11th to 20th	Night	12% 7% 3 7% 4 4 5% 4 7% 8% 38%	1520
	21st to end	Night	11% 4 1 5% 5% 4 4 4 9% 7% 46%	1520
July	1st to 10th	Night	15% 5% 2 5% 4 4 5% 4 5% 7% 45%	2530
	11th to 20th	Night	6% 3 1 4 3 4 4 3 7% 8% 58%	2530
	21st to end	Night	10% 5% 3 6% 4 4 3 4 4 7% 51%	2750
August	1st to 10th	Night	11% 6% 2 7% 5% 5% 3 3 5% 8% 45%	2517
	11th to 20th	Night	11% 4 2 5% 4 4 4 3 6% 8% 49%	2813
	21st to end	Night	11% 6% 2 7% 4 4 4 3 5% 6% 48%	3476
September	1st to 10th	Night	9% 5% 2 5% 3 4 4 3 6% 7% 53%	1900
	11th to 20th	Night	12% 4 2 4 4 3 4 3 5% 7% 52%	1981
	21st to end	Night	14% 7% 2 6% 4 4 3 3 4 7% 46%	2090
October	1st to 10th	Night	15% 6% 2 5% 3 4 3 3 5% 7% 46%	2185
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 3 5% 6% 37%	2339
	21st to end	Night	18% 10% 3 7% 5% 4 4 3 6% 7% 32%	2717
November	1st to 10th	Night	15% 9% 2 6% 4 4 4 4 5% 6% 41%	2536
	11th to 20th	Night	14% 10% 4 8% 6% 5% 5% 3 5% 5% 35%	2522
	21st to end	Night	15% 11% 3 7% 5% 5% 5% 3 5% 7% 34%	2660
December	1st to 10th	Night	13% 13% 4 9% 5% 4 3 3 5% 6% 34%	2660
	11th to 20th	Night	13% 14% 5% 10% 6% 5% 4 3 4 6% 32%	3279
	21st to end	Night	24% 19% 5% 8% 4 3 3 3 3 4 24%	4884

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	55% 8% 12% 18% 7%	2528
	11th to 20th	Day	56% 9% 13% 16% 6% 1	2530
	21st to end	Day	61% 7% 13% 14% 5% 1	2783
February	1st to 10th	Day	46% 7% 13% 20% 13% 1	1653
	11th to 20th	Day	46% 7% 13% 22% 10% 2	1799
	21st to end	Day	49% 6% 13% 21% 10% 1	1570
March	1st to 10th	Day	38% 6% 14% 20% 19% 2	3120
	11th to 20th	Day	43% 8% 17% 22% 10% 1	3272
	21st to end	Day	58% 8% 10% 16% 9% 1	4165
April	1st to 10th	Day	52% 7% 13% 17% 10% 1	2280
	11th to 20th	Day	51% 7% 9% 20% 11% 1	2254
	21st to end	Day	49% 7% 10% 18% 13% 2	2451
May	1st to 10th	Day	42% 8% 11% 22% 16% 1	4280
	11th to 20th	Day	37% 8% 13% 28% 14% 1	4403
	21st to end	Day	42% 9% 11% 20% 17% 2	4837
June	1st to 10th	Day	37% 8% 13% 25% 15% 2	2660
	11th to 20th	Day	42% 8% 14% 20% 14% 1	2660
	21st to end	Day	39% 8% 15% 24% 12% 1	2660
July	1st to 10th	Day	41% 7% 16% 24% 12% 1	4371
	11th to 20th	Day	27% 8% 13% 34% 16% 2	4365
	21st to end	Day	33% 7% 12% 27% 20% 1	4662
August	1st to 10th	Day	40% 7% 15% 24% 13% 1	4408
	11th to 20th	Day	35% 7% 14% 31% 12%	4055
	21st to end	Day	42% 6% 15% 21% 15% 1	4179
September	1st to 10th	Day	39% 7% 14% 26% 14% 1	2280
	11th to 20th	Day	35% 9% 19% 27% 9%	2180
	21st to end	Day	42% 8% 13% 27% 9%	2090
October	1st to 10th	Day	30% 5% 12% 29% 24%	3302
	11th to 20th	Day	48% 4 10% 24% 13% 1	3030
	21st to end	Day	50% 5% 14% 24% 7%	3081
November	1st to 10th	Day	43% 7% 13% 24% 12% 1	1762
	11th to 20th	Day	55% 7% 12% 16% 9% 1	1680
	21st to end	Day	49% 10% 16% 19% 5% 1	1520
December	1st to 10th	Day	49% 8% 14% 22% 7%	1520
	11th to 20th	Day	55% 8% 13% 15% 8% 1	1915
	21st to end	Day	69% 5% 7% 13% 6%	2783
January	1st to 10th	Night	53% 7% 11% 20% 8% 1	4428
	11th to 20th	Night	59% 7% 12% 18% 5%	4440
	21st to end	Night	61% 7% 11% 16% 5%	4884
February	1st to 10th	Night	50% 4 11% 21% 12% 2	2526
	11th to 20th	Night	45% 7% 13% 22% 10% 3	2277
	21st to end	Night	50% 5% 12% 22% 9% 1	1884
March	1st to 10th	Night	38% 6% 18% 25% 12% 1	3733
	11th to 20th	Night	41% 8% 13% 25% 13% 1	3718
	21st to end	Night	53% 8% 11% 17% 10% 1	3466
April	1st to 10th	Night	51% 7% 9% 16% 15% 1	1900
	11th to 20th	Night	46% 6% 9% 23% 13% 3	1875
	21st to end	Night	46% 7% 12% 20% 13% 2	1729
May	1st to 10th	Night	42% 8% 14% 21% 14% 1	2670
	11th to 20th	Night	37% 8% 14% 26% 13% 2	2507
	21st to end	Night	44% 8% 13% 17% 14% 4	2751
June	1st to 10th	Night	34% 8% 15% 26% 16% 1	1520
	11th to 20th	Night	41% 8% 12% 21% 16% 1	1520
	21st to end	Night	33% 9% 15% 28% 12% 2	1520
July	1st to 10th	Night	38% 5% 17% 23% 13% 3	2490
	11th to 20th	Night	22% 6% 19% 36% 15% 2	2482
	21st to end	Night	33% 5% 12% 27% 22% 1	2662
August	1st to 10th	Night	38% 6% 14% 22% 18% 2	2508
	11th to 20th	Night	33% 6% 15% 29% 14% 2	2838
	21st to end	Night	37% 6% 13% 25% 19% 1	3476
September	1st to 10th	Night	30% 7% 13% 32% 17% 1	1900
	11th to 20th	Night	31% 7% 16% 34% 12%	1981
	21st to end	Night	39% 6% 16% 27% 11% 1	2090
October	1st to 10th	Night	31% 5% 10% 35% 17% 2	3640
	11th to 20th	Night	38% 5% 13% 28% 16%	3888
	21st to end	Night	49% 5% 11% 23% 12%	4467
November	1st to 10th	Night	43% 6% 13% 24% 13% 1	2536
	11th to 20th	Night	50% 7% 14% 17% 11%	2522
	21st to end	Night	50% 7% 14% 22% 6% 1	2660
December	1st to 10th	Night	51% 6% 15% 20% 7%	2660
	11th to 20th	Night	53% 7% 13% 19% 7% 1	3281
	21st to end	Night	65% 6% 8% 16% 5%	4884

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	18% 15% 35% 22% 9% 1	270
	11th to 20th	Day	23% 17% 34% 21% 3 1	270
	21st to end	Day	21% 16% 39% 21% 2 1	297
February	1st to 10th	Day	14% 22% 42% 15% 5% 1	270
	11th to 20th	Day	17% 31% 34% 13% 5%	270
	21st to end	Day	22% 20% 47% 11%	225
March	1st to 10th	Day	16% 26% 43% 14% 2	270
	11th to 20th	Day	21% 20% 36% 16% 7% 1	288
	21st to end	Day	24% 18% 32% 18% 7% 1	506
April	1st to 10th	Day	24% 20% 33% 19% 4 1	460
	11th to 20th	Day	27% 22% 27% 16% 8% 1	452
	21st to end	Day	26% 16% 28% 20% 8% 2	460
May	1st to 10th	Day	20% 19% 36% 17% 6% 2	558
	11th to 20th	Day	18% 18% 33% 21% 7% 2	675
	21st to end	Day	17% 15% 32% 24% 8% 5%	734
June	1st to 10th	Day	13% 16% 36% 23% 9% 3	680
	11th to 20th	Day	16% 14% 36% 22% 10% 2	680
	21st to end	Day	14% 14% 31% 26% 11% 3	680
July	1st to 10th	Day	8% 7% 32% 32% 15% 6%	671
	11th to 20th	Day	9% 11% 32% 27% 15% 5%	668
	21st to end	Day	6% 7% 34% 29% 16% 7%	739
August	1st to 10th	Day	9% 8% 30% 29% 18% 5%	676
	11th to 20th	Day	14% 6% 24% 28% 20% 8%	534
	21st to end	Day	16% 11% 23% 26% 19% 6%	539
September	1st to 10th	Day	15% 11% 26% 26% 17% 5%	490
	11th to 20th	Day	12% 8% 26% 33% 18% 3	490
	21st to end	Day	16% 16% 27% 26% 13% 3	490
October	1st to 10th	Day	21% 15% 29% 21% 11% 2	390
	11th to 20th	Day	18% 30% 30% 19% 2	300
	21st to end	Day	26% 21% 36% 14% 3	330
November	1st to 10th	Day	17% 18% 39% 19% 7% 1	300
	11th to 20th	Day	17% 23% 40% 13% 6% 1	273
	21st to end	Day	21% 20% 39% 14% 4 1	270
December	1st to 10th	Day	17% 18% 40% 20% 4 1	270
	11th to 20th	Day	24% 22% 33% 15% 5%	270
	21st to end	Day	30% 20% 35% 13% 2	297
January	1st to 10th	Night	16% 14% 42% 23% 5%	950
	11th to 20th	Night	19% 15% 39% 21% 6% 1	950
	21st to end	Night	17% 11% 40% 26% 6% 1	1045
February	1st to 10th	Night	16% 14% 40% 23% 6%	950
	11th to 20th	Night	16% 16% 38% 24% 6%	801
	21st to end	Night	21% 15% 38% 23% 3	628
March	1st to 10th	Night	17% 13% 35% 28% 7%	760
	11th to 20th	Night	16% 15% 42% 22% 6%	736
	21st to end	Night	20% 11% 33% 29% 7% 1	627
April	1st to 10th	Night	23% 12% 36% 24% 5% 1	570
	11th to 20th	Night	24% 11% 29% 28% 7%	563
	21st to end	Night	20% 11% 34% 27% 7%	570
May	1st to 10th	Night	21% 10% 33% 29% 6% 1	570
	11th to 20th	Night	19% 10% 34% 29% 7%	561
	21st to end	Night	14% 7% 31% 35% 12% 1	617
June	1st to 10th	Night	10% 9% 32% 39% 9% 1	570
	11th to 20th	Night	15% 8% 30% 38% 9%	570
	21st to end	Night	13% 6% 28% 40% 13% 1	570
July	1st to 10th	Night	7% 5% 24% 45% 19% 1	555
	11th to 20th	Night	10% 3 29% 37% 19% 1	552
	21st to end	Night	10% 3 28% 41% 16% 2	624
August	1st to 10th	Night	10% 2 28% 40% 19% 1	567
	11th to 20th	Night	10% 3 22% 38% 25% 2	564
	21st to end	Night	12% 5% 24% 38% 19% 1	627
September	1st to 10th	Night	11% 5% 29% 32% 19% 4	570
	11th to 20th	Night	12% 6% 28% 34% 18% 2	651
	21st to end	Night	17% 8% 27% 30% 17% 1	760
October	1st to 10th	Night	18% 11% 31% 26% 14% 1	760
	11th to 20th	Night	20% 15% 32% 25% 8%	760
	21st to end	Night	22% 11% 32% 26% 8% 1	836
November	1st to 10th	Night	19% 11% 38% 25% 6% 1	780
	11th to 20th	Night	19% 15% 39% 21% 6%	803
	21st to end	Night	19% 16% 34% 25% 5%	950
December	1st to 10th	Night	22% 14% 39% 19% 5% 1	950
	11th to 20th	Night	20% 13% 37% 23% 6% 1	950
	21st to end	Night	26% 13% 37% 20% 3	1045

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