Stephen F. Wessling Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stephen F. Wessling Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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% 11% 4 9% 5% 5% 4 3 5% 6% 33%	101292
	Night	15% 12% 4 9% 5% 5% 4 3 5% 6% 32%	97772
Transparency	Day	49% 5% 11% 21% 12% 1	104850
	Night	52% 5% 11% 20% 11% 1	102355
Seeing	Day	18% 13% 29% 24% 11% 4	15877
	Night	19% 9% 29% 31% 11% 1	25726

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% 27% 11% 16% 7% 5% 3 3 3 4 8%	2527
	11th to 20th	Day	13% 18% 8% 19% 13% 9% 5% 3 3 3 6%	2530
	21st to end	Day	20% 24% 10% 14% 8% 5% 4 2 2 3 9%	2783
February	1st to 10th	Day	11% 17% 6% 13% 12% 7% 5% 3 3 5% 17%	1699
	11th to 20th	Day	11% 18% 7% 15% 9% 6% 4 3 4 5% 18%	1848
	21st to end	Day	12% 18% 7% 14% 8% 5% 4 3 5% 5% 20%	1570
March	1st to 10th	Day	14% 14% 5% 9% 5% 5% 5% 3 5% 5% 30%	3120
	11th to 20th	Day	11% 8% 7% 12% 6% 5% 4 3 4 6% 34%	3352
	21st to end	Day	20% 11% 6% 10% 5% 4 4 3 4 5% 28%	3997
April	1st to 10th	Day	18% 15% 4 7% 4 4 4 3 5% 6% 30%	2280
	11th to 20th	Day	20% 11% 2 7% 4 4 4 3 5% 6% 33%	2304
	21st to end	Day	17% 10% 3 7% 4 4 4 3 5% 6% 35%	2470
May	1st to 10th	Day	17% 6% 2 7% 4 4 4 3 6% 7% 40%	4082
	11th to 20th	Day	13% 7% 2 6% 4 5% 4 3 6% 7% 42%	4175
	21st to end	Day	11% 7% 2 6% 4 4 5% 5% 6% 9% 42%	4824
June	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 44%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 4 3 4 6% 9% 43%	2660
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 7% 42%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 4 6% 8% 37%	4440
	11th to 20th	Day	7% 3 2 4 4 4 4 4 7% 10% 52%	4440
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 7% 9% 45%	4813
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	4213
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 7% 47%	4044
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	4223
September	1st to 10th	Day	12% 5% 2 7% 5% 5% 5% 3 5% 7% 44%	2266
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 4 5% 7% 47%	2090
	21st to end	Day	13% 7% 3 8% 5% 4 5% 3 5% 7% 40%	2090
October	1st to 10th	Day	17% 8% 3 7% 4 4 3 4 4 5% 42%	2090
	11th to 20th	Day	22% 11% 3 9% 6% 5% 5% 3 5% 6% 24%	1827
	21st to end	Day	24% 13% 4 9% 5% 5% 4 3 4 6% 23%	1881
November	1st to 10th	Day	15% 13% 3 7% 5% 4 5% 3 5% 6% 33%	1699
	11th to 20th	Day	15% 21% 7% 11% 6% 4 4 3 4 5% 20%	1710
	21st to end	Day	22% 18% 7% 11% 5% 3 3 3 3 5% 20%	1704
December	1st to 10th	Day	14% 23% 11% 14% 5% 5% 4 2 4 4 13%	1520
	11th to 20th	Day	14% 21% 9% 15% 8% 5% 3 3 4 4 12%	1918
	21st to end	Day	21% 24% 9% 13% 6% 5% 4 3 4 3 8%	2783
January	1st to 10th	Night	15% 22% 11% 14% 6% 4 3 2 3 4 15%	4428
	11th to 20th	Night	13% 18% 8% 19% 11% 10% 3 2 3 4 9%	4440
	21st to end	Night	19% 23% 9% 15% 7% 4 4 3 3 3 9%	4884
February	1st to 10th	Night	12% 17% 7% 14% 10% 6% 5% 3 4 4 18%	2477
	11th to 20th	Night	13% 14% 6% 13% 9% 5% 5% 4 5% 6% 20%	2218
	21st to end	Night	15% 17% 4 11% 6% 5% 5% 3 5% 6% 23%	1884
March	1st to 10th	Night	17% 9% 4 8% 5% 4 4 3 4 7% 36%	3733
	11th to 20th	Night	9% 10% 3 8% 5% 5% 4 4 5% 8% 41%	3594
	21st to end	Night	16% 13% 5% 9% 5% 5% 5% 4 5% 6% 28%	3663
April	1st to 10th	Night	17% 11% 3 7% 6% 4 5% 3 4 7% 34%	1900
	11th to 20th	Night	20% 10% 4 6% 3 3 4 3 5% 6% 37%	1823
	21st to end	Night	16% 9% 3 7% 4 4 4 4 5% 7% 37%	1710
May	1st to 10th	Night	14% 8% 3 7% 5% 5% 5% 4 5% 8% 37%	2813
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 3 6% 8% 39%	2708
	21st to end	Night	17% 8% 3 6% 4 3 4 3 4 6% 43%	2743
June	1st to 10th	Night	13% 6% 3 5% 3 4 4 4 7% 8% 44%	1520
	11th to 20th	Night	11% 5% 3 5% 4 4 4 4 7% 8% 46%	1520
	21st to end	Night	9% 7% 2 5% 4 4 5% 4 7% 8% 45%	1520
July	1st to 10th	Night	10% 8% 3 6% 4 5% 5% 5% 7% 7% 41%	2530
	11th to 20th	Night	6% 3 2 4 4 4 4 3 5% 8% 56%	2530
	21st to end	Night	8% 5% 2 6% 4 3 4 3 6% 7% 51%	2750
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	2719
	11th to 20th	Night	12% 4 2 5% 3 4 4 3 6% 7% 51%	2785
	21st to end	Night	8% 5% 3 7% 4 4 5% 4 5% 8% 47%	3430
September	1st to 10th	Night	9% 6% 2 6% 5% 3 4 3 5% 6% 52%	1900
	11th to 20th	Night	11% 5% 1 4 3 2 4 2 5% 6% 55%	2083
	21st to end	Night	14% 6% 2 7% 4 5% 3 4 5% 7% 43%	2090
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 2 4 8% 46%	2090
	11th to 20th	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	2351
	21st to end	Night	24% 11% 3 7% 4 4 4 2 4 7% 30%	2717
November	1st to 10th	Night	15% 12% 3 7% 4 5% 4 3 5% 6% 36%	2452
	11th to 20th	Night	15% 18% 4 10% 5% 5% 4 5% 6% 6% 21%	2470
	21st to end	Night	18% 20% 5% 9% 5% 4 4 3 5% 6% 22%	2474
December	1st to 10th	Night	15% 19% 8% 15% 8% 5% 4 3 4 4 15%	2660
	11th to 20th	Night	16% 21% 7% 14% 7% 5% 4 3 4 4 15%	3279
	21st to end	Night	23% 23% 7% 12% 5% 4 3 2 4 4 12%	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	82% 4 8% 6% 1	2528
	11th to 20th	Day	86% 4 5% 4 1	2530
	21st to end	Day	84% 2 8% 5% 1	2783
February	1st to 10th	Day	70% 5% 8% 14% 3	1699
	11th to 20th	Day	69% 4 9% 12% 6%	1858
	21st to end	Day	66% 5% 12% 12% 5%	1570
March	1st to 10th	Day	55% 5% 12% 19% 8%	3120
	11th to 20th	Day	52% 5% 15% 19% 8% 1	3352
	21st to end	Day	59% 4 9% 20% 8% 1	3997
April	1st to 10th	Day	55% 5% 10% 20% 10%	2280
	11th to 20th	Day	52% 5% 9% 19% 13% 2	2304
	21st to end	Day	49% 5% 12% 18% 13% 2	2470
May	1st to 10th	Day	41% 7% 10% 24% 16% 2	4120
	11th to 20th	Day	39% 7% 13% 22% 17% 3	4179
	21st to end	Day	36% 6% 13% 21% 22% 2	4837
June	1st to 10th	Day	38% 5% 11% 27% 18% 2	2660
	11th to 20th	Day	38% 5% 12% 23% 21% 1	2660
	21st to end	Day	40% 6% 13% 23% 17% 2	2660
July	1st to 10th	Day	44% 5% 13% 22% 14% 3	4371
	11th to 20th	Day	26% 6% 15% 31% 20% 2	4365
	21st to end	Day	33% 7% 13% 24% 21% 2	4662
August	1st to 10th	Day	38% 6% 11% 28% 15% 1	4198
	11th to 20th	Day	34% 7% 12% 28% 18% 1	4083
	21st to end	Day	36% 7% 14% 25% 17% 2	4222
September	1st to 10th	Day	39% 6% 13% 26% 14% 2	2266
	11th to 20th	Day	36% 5% 13% 30% 14% 2	2090
	21st to end	Day	43% 4 12% 27% 13%	2090
October	1st to 10th	Day	30% 4 9% 36% 19% 2	3480
	11th to 20th	Day	48% 5% 12% 24% 12%	3004
	21st to end	Day	54% 5% 15% 18% 7%	3081
November	1st to 10th	Day	51% 6% 13% 22% 9%	1699
	11th to 20th	Day	67% 4 12% 12% 4	1710
	21st to end	Day	69% 4 13% 11% 3	1704
December	1st to 10th	Day	77% 4 8% 9% 2	1520
	11th to 20th	Day	76% 3 13% 7% 1	1915
	21st to end	Day	81% 5% 7% 6% 1	2783
January	1st to 10th	Night	74% 4 9% 11% 2	4428
	11th to 20th	Night	82% 4 7% 5% 2	4440
	21st to end	Night	81% 4 8% 5% 1	4884
February	1st to 10th	Night	71% 4 8% 12% 6% 1	2477
	11th to 20th	Night	64% 5% 10% 15% 6%	2219
	21st to end	Night	61% 5% 13% 14% 6%	1884
March	1st to 10th	Night	49% 4 15% 24% 7% 1	3733
	11th to 20th	Night	42% 5% 15% 25% 11% 2	3594
	21st to end	Night	57% 5% 11% 19% 8% 1	3663
April	1st to 10th	Night	51% 4 11% 21% 13% 1	1900
	11th to 20th	Night	49% 4 8% 22% 15% 3	1823
	21st to end	Night	47% 5% 10% 20% 14% 4	1710
May	1st to 10th	Night	44% 7% 12% 19% 16% 3	2840
	11th to 20th	Night	41% 6% 9% 26% 15% 3	2709
	21st to end	Night	44% 5% 9% 20% 18% 4	2751
June	1st to 10th	Night	35% 6% 12% 29% 15% 3	1520
	11th to 20th	Night	34% 7% 13% 24% 20% 2	1520
	21st to end	Night	34% 8% 14% 26% 15% 3	1520
July	1st to 10th	Night	41% 7% 11% 23% 12% 6%	2490
	11th to 20th	Night	25% 5% 13% 34% 20% 2	2482
	21st to end	Night	32% 6% 15% 22% 24% 2	2662
August	1st to 10th	Night	34% 5% 13% 29% 19% 1	2710
	11th to 20th	Night	32% 5% 10% 34% 17% 1	2812
	21st to end	Night	34% 6% 11% 25% 21% 2	3430
September	1st to 10th	Night	32% 8% 11% 29% 18% 2	1900
	11th to 20th	Night	29% 4 12% 37% 17% 2	2083
	21st to end	Night	40% 5% 14% 28% 12% 1	2090
October	1st to 10th	Night	32% 4 9% 37% 17% 1	3480
	11th to 20th	Night	40% 5% 14% 26% 14%	3913
	21st to end	Night	46% 4 12% 28% 10%	4467
November	1st to 10th	Night	48% 6% 13% 22% 11% 1	2452
	11th to 20th	Night	62% 5% 14% 13% 5%	2470
	21st to end	Night	65% 4 16% 13% 2 1	2474
December	1st to 10th	Night	74% 5% 10% 8% 3	2660
	11th to 20th	Night	72% 5% 11% 9% 2	3281
	21st to end	Night	77% 4 10% 6% 2	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% 38% 20% 7% 1	270
	11th to 20th	Day	22% 18% 39% 17% 4	270
	21st to end	Day	29% 13% 33% 21% 4	297
February	1st to 10th	Day	19% 26% 31% 18% 4 2	270
	11th to 20th	Day	18% 26% 37% 15% 3	270
	21st to end	Day	21% 31% 36% 10% 2	225
March	1st to 10th	Day	21% 25% 39% 11% 4	270
	11th to 20th	Day	22% 17% 29% 21% 10% 1	384
	21st to end	Day	27% 13% 30% 23% 5% 2	506
April	1st to 10th	Day	24% 18% 33% 18% 6% 1	460
	11th to 20th	Day	29% 23% 23% 16% 7% 2	452
	21st to end	Day	24% 15% 27% 23% 10% 1	460
May	1st to 10th	Day	22% 17% 29% 21% 7% 3	463
	11th to 20th	Day	19% 13% 29% 25% 11% 4	542
	21st to end	Day	14% 14% 27% 27% 13% 4	734
June	1st to 10th	Day	13% 10% 36% 28% 9% 5%	680
	11th to 20th	Day	16% 10% 32% 28% 11% 3	680
	21st to end	Day	17% 9% 33% 28% 10% 4	680
July	1st to 10th	Day	9% 6% 26% 35% 16% 8%	671
	11th to 20th	Day	9% 7% 29% 34% 15% 6%	668
	21st to end	Day	9% 6% 30% 32% 16% 8%	739
August	1st to 10th	Day	10% 6% 27% 26% 22% 10%	557
	11th to 20th	Day	14% 5% 17% 29% 22% 13%	479
	21st to end	Day	14% 6% 22% 27% 22% 9%	539
September	1st to 10th	Day	16% 7% 23% 28% 19% 7%	490
	11th to 20th	Day	9% 6% 22% 31% 24% 8%	490
	21st to end	Day	16% 12% 24% 26% 16% 6%	490
October	1st to 10th	Day	19% 13% 25% 25% 16% 2	490
	11th to 20th	Day	29% 13% 31% 23% 4	365
	21st to end	Day	34% 14% 33% 12% 4 2	330
November	1st to 10th	Day	24% 20% 31% 17% 7% 2	279
	11th to 20th	Day	22% 23% 39% 12% 4	270
	21st to end	Day	31% 17% 35% 12% 4	270
December	1st to 10th	Day	27% 28% 26% 15% 5%	270
	11th to 20th	Day	23% 26% 35% 13% 3	270
	21st to end	Day	29% 23% 33% 11% 3 1	297
January	1st to 10th	Night	21% 12% 33% 26% 6% 1	950
	11th to 20th	Night	24% 12% 33% 24% 6%	950
	21st to end	Night	22% 9% 33% 30% 7%	1045
February	1st to 10th	Night	18% 13% 35% 30% 4	831
	11th to 20th	Night	21% 11% 34% 28% 5% 1	743
	21st to end	Night	24% 13% 36% 23% 3	628
March	1st to 10th	Night	19% 10% 36% 27% 8% 1	760
	11th to 20th	Night	17% 14% 33% 28% 8% 1	760
	21st to end	Night	20% 11% 29% 31% 8% 1	746
April	1st to 10th	Night	22% 10% 34% 28% 5% 1	570
	11th to 20th	Night	26% 9% 31% 26% 7% 1	563
	21st to end	Night	20% 10% 31% 30% 8% 1	570
May	1st to 10th	Night	18% 10% 31% 34% 7% 1	570
	11th to 20th	Night	17% 7% 31% 34% 12%	561
	21st to end	Night	17% 5% 24% 35% 19% 1	617
June	1st to 10th	Night	12% 5% 28% 41% 13% 1	570
	11th to 20th	Night	14% 5% 28% 36% 16% 1	570
	21st to end	Night	11% 4 25% 43% 17% 1	570
July	1st to 10th	Night	7% 4 18% 42% 26% 3	555
	11th to 20th	Night	7% 4 19% 44% 23% 2	552
	21st to end	Night	9% 3 19% 44% 24% 1	624
August	1st to 10th	Night	9% 2 16% 46% 26% 2	567
	11th to 20th	Night	10% 2 15% 43% 27% 3	564
	21st to end	Night	12% 2 20% 41% 22% 2	627
September	1st to 10th	Night	10% 4 19% 41% 22% 4	570
	11th to 20th	Night	12% 5% 18% 36% 25% 3	753
	21st to end	Night	16% 7% 24% 32% 18% 3	760
October	1st to 10th	Night	21% 6% 23% 34% 15% 2	760
	11th to 20th	Night	26% 9% 27% 29% 8% 1	760
	21st to end	Night	30% 10% 29% 26% 6%	836
November	1st to 10th	Night	22% 10% 30% 29% 9% 1	755
	11th to 20th	Night	23% 15% 31% 25% 6%	760
	21st to end	Night	27% 12% 30% 26% 4	764
December	1st to 10th	Night	24% 14% 33% 24% 4	950
	11th to 20th	Night	25% 14% 35% 20% 4 1	950
	21st to end	Night	30% 12% 34% 22% 1	1045

Page generated at 2024-11-01 22:47:36. Version ac07

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