J. Weiskopf Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the J. Weiskopf Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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 8% 5% 4 4 4 5% 7% 37%	101605
	Night	13% 9% 3 8% 5% 4 4 4 5% 6% 39%	98009
Transparency	Day	45% 4 12% 24% 14% 2	105157
	Night	45% 3 11% 24% 15% 2	102598
Seeing	Day	17% 11% 29% 25% 14% 5%	15916
	Night	18% 9% 30% 30% 12% 1	25776

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	11% 14% 10% 14% 5% 5% 4 3 4 4 26%	2527
	11th to 20th	Day	12% 11% 5% 16% 5% 4 4 8% 9% 6% 19%	2530
	21st to end	Day	17% 19% 10% 12% 5% 3 3 3 5% 5% 19%	2957
February	1st to 10th	Day	13% 14% 5% 8% 7% 5% 5% 3 5% 6% 29%	1710
	11th to 20th	Day	13% 11% 3 7% 6% 5% 5% 4 4 6% 36%	1811
	21st to end	Day	13% 14% 5% 11% 5% 4 4 3 5% 6% 31%	1570
March	1st to 10th	Day	13% 11% 4 7% 4 4 3 3 4 6% 39%	3120
	11th to 20th	Day	11% 6% 3 7% 5% 4 4 4 6% 7% 41%	3348
	21st to end	Day	20% 13% 4 12% 6% 5% 5% 4 5% 5% 21%	4123
April	1st to 10th	Day	18% 12% 4 8% 6% 4 5% 3 5% 6% 31%	2280
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 5% 5% 6% 34%	2254
	21st to end	Day	19% 10% 3 8% 4 4 4 3 5% 6% 33%	2326
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 4 4 6% 7% 35%	4141
	11th to 20th	Day	15% 6% 3 6% 4 4 4 3 5% 7% 43%	4399
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 4 6% 9% 40%	4824
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 5% 6% 8% 43%	2660
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 4 7% 9% 46%	2660
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	2660
July	1st to 10th	Day	9% 8% 3 9% 5% 5% 6% 5% 7% 9% 33%	4440
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 10% 50%	4440
	21st to end	Day	8% 7% 2 7% 6% 5% 5% 4 7% 9% 40%	4813
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 7% 10% 42%	4308
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 7% 10% 45%	3764
	21st to end	Day	8% 6% 2 6% 4 4 4 4 5% 8% 48%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 6% 7% 48%	2280
	11th to 20th	Day	13% 4 1 3 3 3 4 3 5% 8% 53%	2210
	21st to end	Day	11% 6% 3 6% 4 4 5% 3 6% 7% 46%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 3 4 3 5% 6% 49%	1957
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 3 4 6% 38%	1900
	21st to end	Day	20% 9% 3 6% 5% 3 3 2 4 6% 38%	1924
November	1st to 10th	Day	12% 9% 3 7% 4 4 4 3 4 7% 45%	1762
	11th to 20th	Day	15% 12% 3 9% 6% 4 4 4 5% 6% 32%	1800
	21st to end	Day	17% 13% 4 8% 4 4 3 3 4 5% 36%	1616
December	1st to 10th	Day	15% 13% 4 9% 5% 5% 4 3 5% 5% 32%	1520
	11th to 20th	Day	13% 13% 5% 10% 6% 5% 6% 4 4 6% 28%	1918
	21st to end	Day	22% 22% 9% 12% 6% 3 4 3 3 4 15%	2783
January	1st to 10th	Night	13% 13% 8% 12% 5% 4 3 3 3 4 31%	4428
	11th to 20th	Night	13% 17% 7% 14% 6% 4 4 6% 7% 4 20%	4440
	21st to end	Night	18% 19% 7% 11% 6% 4 4 3 4 4 21%	4696
February	1st to 10th	Night	14% 14% 4 8% 6% 5% 4 2 4 6% 33%	2470
	11th to 20th	Night	13% 10% 3 8% 4 4 4 3 4 6% 40%	2253
	21st to end	Night	16% 13% 3 8% 4 4 4 2 4 6% 36%	1884
March	1st to 10th	Night	18% 9% 2 6% 4 3 4 3 5% 5% 41%	3733
	11th to 20th	Night	10% 7% 2 6% 5% 4 4 4 5% 7% 47%	3640
	21st to end	Night	23% 11% 5% 10% 5% 4 3 3 4 5% 27%	3506
April	1st to 10th	Night	17% 10% 2 7% 5% 4 4 3 4 5% 39%	1900
	11th to 20th	Night	18% 8% 3 7% 4 4 3 4 4 6% 40%	1875
	21st to end	Night	17% 9% 3 7% 4 4 4 4 5% 7% 37%	1853
May	1st to 10th	Night	16% 7% 3 7% 5% 4 5% 4 5% 7% 36%	2752
	11th to 20th	Night	13% 6% 2 6% 4 5% 5% 4 7% 8% 39%	2506
	21st to end	Night	9% 6% 2 5% 4 4 4 4 7% 8% 45%	2743
June	1st to 10th	Night	8% 5% 2 6% 4 4 5% 5% 6% 9% 47%	1520
	11th to 20th	Night	9% 5% 2 4 4 3 5% 4 7% 8% 48%	1520
	21st to end	Night	8% 6% 2 8% 5% 5% 5% 5% 6% 8% 43%	1520
July	1st to 10th	Night	15% 7% 3 6% 4 5% 4 4 6% 8% 39%	2530
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 6% 7% 56%	2530
	21st to end	Night	6% 5% 2 7% 5% 5% 5% 5% 7% 9% 45%	2750
August	1st to 10th	Night	5% 4 2 5% 4 5% 4 4 6% 8% 52%	2630
	11th to 20th	Night	6% 6% 2 6% 4 4 5% 5% 5% 8% 50%	3066
	21st to end	Night	8% 4 2 5% 4 4 5% 4 6% 9% 51%	3476
September	1st to 10th	Night	9% 6% 2 5% 4 3 3 3 4 6% 55%	1900
	11th to 20th	Night	10% 4 2 5% 3 3 4 3 5% 9% 52%	1950
	21st to end	Night	10% 6% 2 5% 4 4 3 3 6% 7% 49%	2090
October	1st to 10th	Night	11% 6% 2 5% 3 4 3 3 4 5% 52%	2220
	11th to 20th	Night	15% 8% 2 5% 4 3 3 3 4 6% 47%	2280
	21st to end	Night	19% 9% 3 6% 4 4 4 4 5% 6% 36%	2674
November	1st to 10th	Night	12% 7% 3 5% 4 2 3 4 4 6% 49%	2536
	11th to 20th	Night	15% 10% 3 8% 5% 4 3 3 5% 6% 37%	2600
	21st to end	Night	17% 11% 3 7% 4 3 4 3 5% 5% 37%	2715
December	1st to 10th	Night	15% 12% 4 10% 6% 5% 4 3 5% 6% 31%	2660
	11th to 20th	Night	15% 14% 4 8% 5% 4 4 4 5% 6% 32%	3279
	21st to end	Night	22% 16% 6% 10% 6% 5% 5% 4 4 5% 18%	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	63% 2 12% 17% 6%	2528
	11th to 20th	Day	62% 2 13% 14% 9%	2530
	21st to end	Day	68% 2 8% 16% 5%	2957
February	1st to 10th	Day	57% 2 12% 18% 11% 1	1710
	11th to 20th	Day	50% 3 11% 25% 9% 2	1821
	21st to end	Day	55% 4 9% 21% 10% 2	1570
March	1st to 10th	Day	48% 2 11% 25% 11% 3	3120
	11th to 20th	Day	42% 3 11% 33% 11% 1	3348
	21st to end	Day	66% 3 12% 15% 4 1	4123
April	1st to 10th	Day	55% 3 10% 23% 9%	2280
	11th to 20th	Day	50% 4 9% 24% 11% 2	2254
	21st to end	Day	53% 3 11% 18% 13% 1	2326
May	1st to 10th	Day	49% 3 9% 23% 15% 1	4179
	11th to 20th	Day	43% 2 10% 29% 15%	4403
	21st to end	Day	42% 3 10% 24% 17% 5%	4837
June	1st to 10th	Day	38% 4 13% 28% 16% 2	2660
	11th to 20th	Day	35% 4 12% 26% 21% 2	2660
	21st to end	Day	42% 5% 14% 24% 13% 2	2660
July	1st to 10th	Day	44% 6% 13% 25% 10% 2	4371
	11th to 20th	Day	28% 6% 16% 31% 17% 1	4365
	21st to end	Day	38% 6% 16% 18% 19% 3	4662
August	1st to 10th	Day	36% 6% 16% 28% 13% 1	4293
	11th to 20th	Day	33% 5% 12% 34% 15% 1	3800
	21st to end	Day	33% 5% 12% 31% 17% 2	4179
September	1st to 10th	Day	36% 4 10% 27% 20% 3	2280
	11th to 20th	Day	31% 4 11% 31% 21% 3	2210
	21st to end	Day	38% 3 9% 28% 19% 1	2090
October	1st to 10th	Day	24% 3 6% 36% 30% 1	3256
	11th to 20th	Day	33% 3 9% 26% 26% 2	3140
	21st to end	Day	43% 3 9% 29% 13% 3	3149
November	1st to 10th	Day	41% 3 14% 26% 14% 2	1762
	11th to 20th	Day	53% 3 11% 22% 10% 1	1800
	21st to end	Day	52% 3 13% 23% 8% 1	1616
December	1st to 10th	Day	55% 2 14% 19% 8% 1	1520
	11th to 20th	Day	57% 3 14% 17% 8% 2	1915
	21st to end	Day	77% 2 7% 9% 4	2783
January	1st to 10th	Night	59% 2 10% 20% 9% 1	4428
	11th to 20th	Night	66% 2 10% 13% 9%	4440
	21st to end	Night	68% 2 8% 16% 6% 1	4696
February	1st to 10th	Night	55% 2 9% 21% 13% 1	2470
	11th to 20th	Night	47% 3 10% 24% 14% 1	2254
	21st to end	Night	51% 2 10% 22% 11% 4	1884
March	1st to 10th	Night	46% 2 12% 24% 11% 4	3733
	11th to 20th	Night	38% 2 9% 31% 18% 2	3640
	21st to end	Night	62% 2 9% 20% 6% 1	3506
April	1st to 10th	Night	49% 2 10% 23% 14% 2	1900
	11th to 20th	Night	47% 3 6% 22% 18% 4	1875
	21st to end	Night	47% 3 11% 25% 12% 2	1853
May	1st to 10th	Night	49% 3 11% 19% 16% 2	2779
	11th to 20th	Night	43% 2 14% 27% 13% 1	2507
	21st to end	Night	36% 3 11% 25% 22% 3	2751
June	1st to 10th	Night	33% 4 14% 28% 19% 2	1520
	11th to 20th	Night	33% 4 11% 24% 25% 3	1520
	21st to end	Night	38% 5% 14% 22% 18% 3	1520
July	1st to 10th	Night	43% 5% 14% 22% 12% 4	2490
	11th to 20th	Night	26% 6% 13% 34% 20% 2	2482
	21st to end	Night	33% 8% 15% 20% 20% 4	2662
August	1st to 10th	Night	28% 5% 15% 32% 18% 2	2621
	11th to 20th	Night	30% 7% 14% 32% 17% 1	3096
	21st to end	Night	30% 5% 13% 31% 18% 3	3476
September	1st to 10th	Night	31% 3 11% 28% 24% 3	1900
	11th to 20th	Night	31% 3 8% 35% 20% 3	1950
	21st to end	Night	34% 3 10% 31% 20% 2	2090
October	1st to 10th	Night	24% 2 6% 36% 30% 2	3701
	11th to 20th	Night	33% 3 8% 26% 28% 3	3779
	21st to end	Night	42% 3 9% 26% 17% 3	4399
November	1st to 10th	Night	37% 4 9% 29% 18% 3	2536
	11th to 20th	Night	49% 3 12% 23% 13% 1	2600
	21st to end	Night	50% 3 11% 24% 11% 1	2715
December	1st to 10th	Night	56% 3 10% 22% 8% 1	2660
	11th to 20th	Night	53% 3 13% 21% 9% 1	3281
	21st to end	Night	70% 2 8% 15% 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	20% 14% 37% 21% 7% 1	270
	11th to 20th	Day	21% 18% 36% 20% 3 1	270
	21st to end	Day	22% 11% 39% 23% 4 1	297
February	1st to 10th	Day	20% 19% 40% 16% 6%	270
	11th to 20th	Day	21% 22% 33% 20% 3	270
	21st to end	Day	29% 20% 37% 12% 2	225
March	1st to 10th	Day	20% 23% 38% 16% 4	270
	11th to 20th	Day	23% 18% 38% 12% 8% 1	270
	21st to end	Day	28% 14% 31% 18% 7% 1	477
April	1st to 10th	Day	26% 13% 36% 20% 5%	460
	11th to 20th	Day	23% 18% 27% 22% 9% 2	452
	21st to end	Day	25% 14% 28% 22% 10% 2	460
May	1st to 10th	Day	20% 13% 31% 25% 8% 2	496
	11th to 20th	Day	21% 12% 29% 24% 11% 3	675
	21st to end	Day	16% 10% 29% 20% 17% 7%	734
June	1st to 10th	Day	10% 8% 29% 30% 18% 5%	680
	11th to 20th	Day	11% 6% 34% 28% 16% 5%	680
	21st to end	Day	12% 10% 29% 30% 15% 4	680
July	1st to 10th	Day	10% 3 20% 29% 25% 14%	671
	11th to 20th	Day	9% 4 23% 32% 22% 9%	668
	21st to end	Day	8% 4 20% 37% 25% 6%	739
August	1st to 10th	Day	9% 4 19% 33% 25% 11%	676
	11th to 20th	Day	12% 5% 18% 27% 24% 15%	502
	21st to end	Day	9% 4 18% 33% 27% 10%	539
September	1st to 10th	Day	16% 5% 22% 27% 23% 7%	490
	11th to 20th	Day	12% 2 20% 30% 25% 11%	490
	21st to end	Day	14% 11% 26% 27% 17% 6%	490
October	1st to 10th	Day	17% 8% 27% 27% 17% 4	357
	11th to 20th	Day	20% 20% 33% 19% 8%	300
	21st to end	Day	31% 15% 28% 22% 4 1	330
November	1st to 10th	Day	19% 16% 36% 17% 9% 3	300
	11th to 20th	Day	24% 19% 35% 15% 6% 1	300
	21st to end	Day	28% 10% 39% 18% 5%	291
December	1st to 10th	Day	20% 16% 39% 18% 6% 1	270
	11th to 20th	Day	19% 13% 38% 22% 7%	270
	21st to end	Day	25% 21% 38% 14% 2	297
January	1st to 10th	Night	19% 13% 37% 26% 5% 1	950
	11th to 20th	Night	20% 13% 36% 25% 5%	950
	21st to end	Night	22% 12% 33% 28% 6%	1045
February	1st to 10th	Night	22% 12% 36% 24% 5%	950
	11th to 20th	Night	19% 13% 37% 26% 5%	778
	21st to end	Night	22% 14% 38% 22% 4	628
March	1st to 10th	Night	21% 12% 36% 24% 6% 1	760
	11th to 20th	Night	17% 11% 35% 29% 8%	665
	21st to end	Night	24% 10% 34% 26% 5% 1	627
April	1st to 10th	Night	22% 12% 38% 22% 6%	570
	11th to 20th	Night	23% 13% 29% 29% 6%	563
	21st to end	Night	21% 10% 29% 29% 10% 1	570
May	1st to 10th	Night	18% 9% 31% 32% 9%	570
	11th to 20th	Night	18% 8% 31% 32% 11%	561
	21st to end	Night	14% 4 30% 35% 17% 1	617
June	1st to 10th	Night	9% 5% 25% 46% 15% 1	570
	11th to 20th	Night	11% 4 29% 42% 13% 1	570
	21st to end	Night	10% 4 28% 40% 17% 1	570
July	1st to 10th	Night	10% 2 13% 40% 30% 5%	555
	11th to 20th	Night	9% 3 18% 41% 26% 3	552
	21st to end	Night	8% 2 15% 48% 26% 2	624
August	1st to 10th	Night	4 3 13% 47% 31% 3	567
	11th to 20th	Night	7% 2 13% 41% 31% 5%	564
	21st to end	Night	10% 2 15% 42% 28% 3	627
September	1st to 10th	Night	12% 4 18% 35% 27% 5%	570
	11th to 20th	Night	10% 4 20% 35% 29% 3	620
	21st to end	Night	12% 9% 24% 34% 19% 2	760
October	1st to 10th	Night	14% 7% 28% 32% 18% 1	760
	11th to 20th	Night	19% 11% 31% 28% 10% 1	760
	21st to end	Night	25% 11% 32% 25% 8%	836
November	1st to 10th	Night	15% 10% 39% 25% 10% 1	780
	11th to 20th	Night	21% 14% 35% 24% 6%	800
	21st to end	Night	23% 11% 34% 26% 6%	942
December	1st to 10th	Night	23% 12% 36% 23% 5% 1	950
	11th to 20th	Night	23% 11% 33% 27% 5% 1	950
	21st to end	Night	28% 13% 33% 22% 3	1045

Page generated at 2024-11-24 23:22:17. Version ac07

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