St. Louis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Louis 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	12% 7% 3 6% 4 4 4 4 6% 8% 42%	101189
	Night	13% 8% 3 6% 4 4 4 3 5% 6% 43%	97898
Transparency	Day	38% 6% 12% 25% 17% 2	104795
	Night	39% 6% 11% 24% 18% 3	102420
Seeing	Day	14% 10% 25% 27% 18% 6%	15621
	Night	16% 8% 29% 32% 14% 1	25420

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% 11% 6% 10% 6% 5% 5% 5% 5% 7% 30%	2527
	11th to 20th	Day	13% 8% 3 8% 7% 9% 6% 4 5% 6% 30%	2699
	21st to end	Day	22% 17% 5% 7% 5% 3 3 2 3 5% 28%	3135
February	1st to 10th	Day	16% 11% 4 9% 4 4 4 4 5% 6% 33%	1710
	11th to 20th	Day	15% 9% 4 7% 6% 4 4 3 4 7% 37%	1846
	21st to end	Day	15% 9% 2 8% 5% 4 4 2 5% 6% 39%	1570
March	1st to 10th	Day	19% 11% 3 7% 3 3 3 2 3 5% 40%	3140
	11th to 20th	Day	12% 8% 2 5% 4 3 4 3 4 7% 48%	3480
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 6% 8% 35%	4036
April	1st to 10th	Day	11% 6% 3 7% 5% 4 5% 4 7% 7% 41%	2280
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 3 5% 5% 42%	2254
	21st to end	Day	16% 8% 2 7% 5% 5% 5% 3 6% 6% 37%	2280
May	1st to 10th	Day	14% 7% 2 7% 5% 5% 6% 5% 7% 10% 33%	3887
	11th to 20th	Day	17% 8% 2 7% 4 4 4 4 5% 7% 38%	4321
	21st to end	Day	9% 6% 2 7% 4 4 5% 4 6% 7% 45%	4824
June	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 7% 8% 47%	2660
	11th to 20th	Day	6% 4 2 5% 4 4 4 5% 7% 9% 50%	2660
	21st to end	Day	4 4 2 6% 5% 4 4 5% 8% 10% 47%	2660
July	1st to 10th	Day	12% 4 2 5% 4 5% 5% 4 7% 10% 42%	4440
	11th to 20th	Day	4 3 2 5% 4 4 5% 5% 8% 11% 48%	4440
	21st to end	Day	6% 5% 2 6% 6% 5% 6% 4 7% 11% 42%	4606
August	1st to 10th	Day	4 4 1 5% 4 5% 5% 4 7% 9% 52%	4054
	11th to 20th	Day	12% 5% 2 5% 4 5% 4 4 7% 8% 43%	3729
	21st to end	Day	5% 5% 1 5% 4 4 5% 4 6% 10% 52%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 5% 6% 51%	2280
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 5% 8% 49%	2243
	21st to end	Day	8% 5% 2 5% 4 4 5% 3 5% 8% 52%	2090
October	1st to 10th	Day	9% 4 2 5% 3 3 4 2 4 6% 57%	1914
	11th to 20th	Day	13% 6% 2 5% 3 3 4 2 4 6% 52%	1900
	21st to end	Day	18% 6% 3 5% 4 3 3 2 4 6% 47%	2056
November	1st to 10th	Day	10% 5% 2 4 3 4 4 3 5% 6% 54%	1699
	11th to 20th	Day	15% 7% 3 6% 4 3 3 3 5% 7% 45%	1710
	21st to end	Day	15% 9% 3 7% 4 4 4 3 4 5% 41%	1658
December	1st to 10th	Day	13% 11% 3 8% 4 4 4 3 5% 7% 37%	1520
	11th to 20th	Day	14% 10% 3 7% 5% 5% 4 4 5% 7% 36%	1918
	21st to end	Day	21% 13% 4 9% 5% 5% 4 3 4 5% 26%	2783
January	1st to 10th	Night	15% 9% 6% 9% 5% 4 4 3 5% 5% 34%	4428
	11th to 20th	Night	14% 11% 4 11% 5% 7% 5% 3 4 5% 30%	4266
	21st to end	Night	24% 15% 5% 8% 4 4 3 3 4 5% 26%	4532
February	1st to 10th	Night	16% 11% 4 8% 6% 4 3 3 4 5% 34%	2470
	11th to 20th	Night	14% 8% 3 8% 5% 4 4 3 5% 5% 42%	2218
	21st to end	Night	15% 9% 4 8% 5% 4 3 2 5% 5% 39%	1884
March	1st to 10th	Night	21% 10% 2 5% 4 3 3 2 4 4 42%	3733
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 4 7% 51%	3488
	21st to end	Night	16% 10% 4 7% 5% 5% 5% 4 5% 5% 35%	3594
April	1st to 10th	Night	12% 6% 3 6% 5% 3 4 3 5% 8% 47%	1900
	11th to 20th	Night	14% 6% 3 5% 4 4 4 4 6% 7% 45%	1875
	21st to end	Night	19% 8% 3 6% 4 3 4 3 4 6% 40%	1900
May	1st to 10th	Night	17% 7% 2 5% 5% 4 4 4 6% 8% 38%	3007
	11th to 20th	Night	15% 7% 2 6% 4 3 3 3 5% 7% 45%	2578
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 7% 9% 41%	2743
June	1st to 10th	Night	8% 4 2 6% 5% 3 5% 4 6% 10% 48%	1520
	11th to 20th	Night	5% 4 1 4 3 4 4 4 5% 10% 55%	1520
	21st to end	Night	4 4 2 4 4 3 4 5% 7% 9% 54%	1520
July	1st to 10th	Night	12% 5% 2 4 3 4 4 4 6% 9% 47%	2530
	11th to 20th	Night	5% 3 2 5% 4 4 4 4 6% 9% 55%	2530
	21st to end	Night	7% 6% 3 7% 6% 5% 6% 5% 8% 9% 39%	2953
August	1st to 10th	Night	5% 3 1 5% 4 3 4 5% 6% 8% 57%	2883
	11th to 20th	Night	10% 5% 2 7% 5% 4 5% 4 6% 7% 47%	3100
	21st to end	Night	5% 3 1 3 4 4 4 4 6% 8% 59%	3476
September	1st to 10th	Night	11% 5% 1 5% 3 2 3 3 5% 7% 53%	1900
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 6% 8% 51%	1916
	21st to end	Night	10% 4 1 4 4 4 4 4 5% 6% 53%	2090
October	1st to 10th	Night	8% 4 2 4 3 3 3 2 5% 6% 58%	2261
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 4 6% 54%	2280
	21st to end	Night	17% 7% 2 4 3 3 4 2 5% 5% 47%	2541
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 4 5% 56%	2452
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 48%	2470
	21st to end	Night	18% 9% 3 5% 3 3 3 3 5% 6% 44%	2517
December	1st to 10th	Night	16% 11% 3 6% 5% 3 4 4 5% 6% 35%	2660
	11th to 20th	Night	16% 9% 3 7% 4 4 4 3 4 5% 41%	3279
	21st to end	Night	19% 17% 5% 10% 5% 4 4 4 5% 4 23%	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	48% 8% 12% 23% 8% 1	2528
	11th to 20th	Day	47% 7% 14% 20% 11%	2699
	21st to end	Day	60% 4 9% 16% 10% 1	3135
February	1st to 10th	Day	50% 6% 11% 22% 10% 1	1710
	11th to 20th	Day	47% 6% 10% 21% 14% 3	1856
	21st to end	Day	45% 6% 10% 20% 15% 4	1570
March	1st to 10th	Day	48% 4 7% 21% 17% 3	3140
	11th to 20th	Day	35% 6% 12% 25% 17% 5%	3480
	21st to end	Day	43% 9% 17% 23% 8% 1	4036
April	1st to 10th	Day	39% 7% 12% 25% 17% 1	2280
	11th to 20th	Day	43% 5% 11% 23% 14% 3	2254
	21st to end	Day	46% 7% 12% 22% 12% 1	2280
May	1st to 10th	Day	42% 10% 16% 23% 9% 1	3922
	11th to 20th	Day	44% 8% 12% 24% 11% 1	4325
	21st to end	Day	35% 8% 11% 23% 21% 1	4837
June	1st to 10th	Day	32% 9% 14% 26% 17% 3	2660
	11th to 20th	Day	28% 8% 13% 24% 24% 3	2660
	21st to end	Day	28% 9% 17% 24% 16% 5%	2660
July	1st to 10th	Day	37% 7% 12% 29% 13% 2	4371
	11th to 20th	Day	27% 7% 15% 31% 17% 2	4365
	21st to end	Day	35% 7% 19% 26% 12% 1	4461
August	1st to 10th	Day	26% 7% 15% 37% 15%	4040
	11th to 20th	Day	37% 7% 14% 25% 17% 1	3765
	21st to end	Day	28% 6% 15% 34% 15% 3	4179
September	1st to 10th	Day	33% 6% 11% 25% 21% 5%	2280
	11th to 20th	Day	33% 5% 10% 25% 25% 3	2243
	21st to end	Day	31% 6% 9% 28% 24% 3	2090
October	1st to 10th	Day	18% 3 6% 20% 45% 8%	3174
	11th to 20th	Day	23% 3 7% 21% 36% 9%	3140
	21st to end	Day	32% 3 7% 28% 27% 3	3370
November	1st to 10th	Day	30% 5% 13% 27% 20% 6%	1699
	11th to 20th	Day	38% 5% 10% 23% 19% 5%	1710
	21st to end	Day	46% 5% 9% 26% 14%	1658
December	1st to 10th	Day	45% 6% 13% 25% 11%	1520
	11th to 20th	Day	47% 6% 11% 22% 13% 2	1915
	21st to end	Day	59% 6% 11% 14% 8% 1	2783
January	1st to 10th	Night	49% 6% 12% 23% 9% 1	4428
	11th to 20th	Night	54% 6% 10% 20% 9% 1	4266
	21st to end	Night	59% 5% 10% 19% 7% 1	4532
February	1st to 10th	Night	51% 6% 10% 21% 12% 1	2470
	11th to 20th	Night	44% 5% 10% 21% 18% 2	2219
	21st to end	Night	46% 5% 11% 21% 13% 4	1884
March	1st to 10th	Night	46% 5% 8% 24% 15% 2	3733
	11th to 20th	Night	35% 5% 12% 23% 21% 4	3488
	21st to end	Night	48% 8% 11% 24% 8%	3594
April	1st to 10th	Night	36% 6% 11% 26% 17% 4	1900
	11th to 20th	Night	37% 6% 11% 23% 19% 4	1875
	21st to end	Night	45% 5% 9% 22% 16% 2	1900
May	1st to 10th	Night	41% 9% 18% 18% 13% 1	3037
	11th to 20th	Night	39% 8% 13% 26% 14% 1	2579
	21st to end	Night	35% 9% 14% 23% 16% 2	2751
June	1st to 10th	Night	30% 8% 14% 25% 19% 4	1520
	11th to 20th	Night	24% 7% 12% 27% 25% 4	1520
	21st to end	Night	24% 9% 16% 27% 18% 7%	1520
July	1st to 10th	Night	33% 6% 13% 29% 16% 3	2490
	11th to 20th	Night	24% 6% 13% 36% 19% 3	2482
	21st to end	Night	38% 7% 17% 24% 12% 2	2859
August	1st to 10th	Night	23% 7% 16% 35% 18% 1	2873
	11th to 20th	Night	35% 7% 13% 23% 20% 2	3130
	21st to end	Night	22% 6% 15% 35% 19% 3	3476
September	1st to 10th	Night	31% 5% 10% 26% 21% 6%	1900
	11th to 20th	Night	31% 6% 9% 25% 24% 5%	1916
	21st to end	Night	29% 7% 9% 22% 27% 5%	2090
October	1st to 10th	Night	17% 3 6% 18% 47% 8%	3768
	11th to 20th	Night	22% 3 7% 22% 39% 8%	3779
	21st to end	Night	31% 4 10% 24% 26% 6%	4177
November	1st to 10th	Night	31% 5% 11% 25% 23% 6%	2452
	11th to 20th	Night	36% 4 9% 25% 22% 4	2470
	21st to end	Night	42% 4 11% 24% 18% 2	2517
December	1st to 10th	Night	47% 5% 11% 24% 12% 1	2660
	11th to 20th	Night	44% 6% 10% 24% 15% 1	3281
	21st to end	Night	61% 7% 10% 14% 7% 1	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	21% 20% 32% 19% 8%	270
	11th to 20th	Day	26% 16% 36% 20% 1	270
	21st to end	Day	19% 20% 41% 15% 5%	297
February	1st to 10th	Day	22% 17% 38% 19% 4	270
	11th to 20th	Day	22% 22% 31% 18% 5% 1	270
	21st to end	Day	23% 20% 36% 21% 1	225
March	1st to 10th	Day	19% 17% 47% 14% 3	270
	11th to 20th	Day	29% 17% 34% 14% 5% 1	270
	21st to end	Day	30% 15% 31% 18% 6%	429
April	1st to 10th	Day	17% 16% 37% 21% 9% 1	460
	11th to 20th	Day	19% 16% 31% 21% 12% 1	452
	21st to end	Day	21% 15% 35% 19% 9% 1	460
May	1st to 10th	Day	16% 14% 32% 27% 9% 2	463
	11th to 20th	Day	17% 10% 27% 26% 16% 4	623
	21st to end	Day	12% 5% 30% 31% 15% 7%	734
June	1st to 10th	Day	10% 6% 21% 29% 24% 9%	680
	11th to 20th	Day	8% 4 19% 38% 24% 8%	680
	21st to end	Day	3 6% 28% 36% 21% 6%	680
July	1st to 10th	Day	11% 2 15% 27% 29% 17%	671
	11th to 20th	Day	5% 3 15% 38% 30% 10%	668
	21st to end	Day	8% 2 17% 34% 29% 10%	739
August	1st to 10th	Day	7% 2 14% 35% 30% 11%	652
	11th to 20th	Day	11% 3 15% 29% 30% 11%	479
	21st to end	Day	9% 1 13% 26% 38% 13%	539
September	1st to 10th	Day	12% 4 14% 27% 32% 11%	490
	11th to 20th	Day	9% 2 11% 31% 31% 16%	490
	21st to end	Day	10% 9% 19% 28% 24% 9%	490
October	1st to 10th	Day	11% 7% 22% 35% 18% 7%	314
	11th to 20th	Day	13% 15% 32% 26% 11% 2	300
	21st to end	Day	26% 20% 25% 22% 8%	330
November	1st to 10th	Day	11% 10% 34% 25% 15% 5%	279
	11th to 20th	Day	19% 17% 31% 20% 11% 1	270
	21st to end	Day	25% 13% 28% 25% 9% 1	270
December	1st to 10th	Day	13% 18% 41% 16% 8% 4	270
	11th to 20th	Day	22% 13% 34% 25% 4 1	270
	21st to end	Day	24% 20% 38% 14% 3 1	297
January	1st to 10th	Night	17% 13% 39% 26% 5%	950
	11th to 20th	Night	21% 14% 38% 22% 5%	950
	21st to end	Night	18% 13% 39% 28% 2	1045
February	1st to 10th	Night	22% 13% 37% 25% 3	950
	11th to 20th	Night	19% 14% 38% 27% 1	743
	21st to end	Night	21% 14% 39% 24% 2	628
March	1st to 10th	Night	20% 13% 38% 25% 4	760
	11th to 20th	Night	22% 10% 35% 30% 4	578
	21st to end	Night	22% 11% 32% 28% 6%	627
April	1st to 10th	Night	16% 11% 39% 27% 7%	570
	11th to 20th	Night	15% 10% 34% 29% 11% 1	563
	21st to end	Night	21% 8% 38% 26% 6% 1	570
May	1st to 10th	Night	17% 6% 32% 35% 9% 1	570
	11th to 20th	Night	17% 6% 26% 34% 17% 1	561
	21st to end	Night	10% 4 22% 40% 20% 4	617
June	1st to 10th	Night	8% 2 20% 42% 27% 1	570
	11th to 20th	Night	6% 2 19% 49% 22% 2	570
	21st to end	Night	5% 2 23% 42% 26% 2	570
July	1st to 10th	Night	9% 1 10% 38% 35% 6%	555
	11th to 20th	Night	4 1 13% 44% 34% 3	552
	21st to end	Night	6% 2 14% 40% 35% 3	624
August	1st to 10th	Night	7% 2 11% 39% 38% 3	567
	11th to 20th	Night	9% 11% 42% 35% 3	564
	21st to end	Night	7% 1 11% 38% 38% 6%	627
September	1st to 10th	Night	12% 2 12% 38% 31% 5%	570
	11th to 20th	Night	11% 2 14% 32% 35% 5%	586
	21st to end	Night	10% 5% 23% 32% 26% 4	760
October	1st to 10th	Night	9% 3 26% 33% 24% 4	760
	11th to 20th	Night	16% 9% 28% 33% 14% 1	760
	21st to end	Night	21% 9% 33% 30% 6%	836
November	1st to 10th	Night	13% 7% 36% 33% 11% 1	755
	11th to 20th	Night	17% 11% 33% 30% 8% 1	760
	21st to end	Night	24% 8% 36% 26% 7%	807
December	1st to 10th	Night	20% 11% 35% 28% 5% 1	950
	11th to 20th	Night	23% 9% 29% 33% 6%	950
	21st to end	Night	24% 14% 35% 24% 4	1045

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