Saint Petersburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint Petersburg clear sky chart from 2005-11-06 to 2025-07-06.

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	7% 4 2 6% 4 5% 5% 5% 7% 10% 44%	120070
	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	118696
Transparency	Day	31% 10% 19% 21% 14% 4	123885
	Night	34% 9% 17% 21% 15% 5%	123018
Seeing	Day	7% 4 17% 28% 28% 15%	16309
	Night	8% 4 18% 33% 27% 9%	26366

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	8% 6% 3 6% 4 4 4 4 6% 8% 48%	4056
	11th to 20th	Day	20% 9% 3 7% 4 4 5% 3 6% 7% 32%	4060
	21st to end	Day	13% 5% 2 6% 3 3 4 4 6% 9% 45%	4924
February	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 6% 8% 46%	2000
	11th to 20th	Day	8% 3 2 5% 4 3 4 3 5% 7% 56%	1948
	21st to end	Day	7% 5% 1 5% 4 3 3 3 6% 8% 55%	1685
March	1st to 10th	Day	8% 4 2 6% 4 4 4 4 5% 8% 50%	4935
	11th to 20th	Day	5% 3 1 4 3 3 3 3 6% 8% 60%	4980
	21st to end	Day	8% 4 2 4 4 3 4 4 7% 9% 51%	5465
April	1st to 10th	Day	6% 2 2 4 3 4 4 4 8% 9% 55%	2200
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 7% 9% 51%	2176
	21st to end	Day	4 3 1 4 3 4 4 4 7% 9% 58%	2390
May	1st to 10th	Day	2 2 1 4 3 3 4 4 6% 10% 61%	5862
	11th to 20th	Day	8% 3 2 5% 4 4 4 4 6% 9% 51%	5861
	21st to end	Day	3 3 1 3 3 4 4 4 7% 12% 57%	6433
June	1st to 10th	Day	9% 7% 3 7% 5% 6% 5% 5% 8% 10% 35%	2600
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 7% 10% 14% 33%	2600
	21st to end	Day	6% 6% 2 7% 5% 6% 8% 7% 10% 12% 31%	2600
July	1st to 10th	Day	5% 4 2 7% 5% 6% 7% 8% 11% 15% 30%	4185
	11th to 20th	Day	3 4 2 7% 6% 8% 9% 8% 12% 15% 27%	4120
	21st to end	Day	4 4 2 7% 7% 8% 8% 7% 11% 16% 26%	4466
August	1st to 10th	Day	7% 3 2 7% 7% 7% 8% 6% 10% 13% 30%	3995
	11th to 20th	Day	4 4 2 6% 4 6% 7% 5% 10% 15% 39%	3729
	21st to end	Day	6% 4 3 7% 6% 6% 8% 8% 11% 14% 27%	3841
September	1st to 10th	Day	7% 5% 2 8% 6% 7% 8% 6% 10% 13% 28%	2090
	11th to 20th	Day	9% 6% 2 7% 6% 6% 7% 6% 8% 13% 30%	2090
	21st to end	Day	7% 5% 3 8% 6% 6% 7% 5% 8% 11% 35%	2090
October	1st to 10th	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 8% 39%	2090
	11th to 20th	Day	6% 4 1 5% 4 4 4 4 7% 9% 54%	1962
	21st to end	Day	6% 4 2 5% 3 3 4 4 6% 9% 52%	2090
November	1st to 10th	Day	7% 4 2 5% 4 4 4 4 6% 8% 52%	1957
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 8% 50%	2000
	21st to end	Day	7% 5% 2 6% 4 3 5% 4 7% 9% 49%	1821
December	1st to 10th	Day	3 3 1 4 4 3 3 4 5% 9% 61%	2930
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 5% 7% 52%	3373
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 5% 8% 39%	4466
January	1st to 10th	Night	8% 5% 2 7% 4 4 4 3 6% 9% 48%	5889
	11th to 20th	Night	18% 10% 4 9% 5% 5% 5% 4 5% 7% 28%	5900
	21st to end	Night	13% 6% 3 7% 4 4 4 4 6% 8% 41%	6015
February	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 47%	2400
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 5% 8% 51%	2338
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 52%	1942
March	1st to 10th	Night	8% 6% 2 7% 5% 5% 4 3 6% 7% 47%	4918
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 10% 57%	4980
	21st to end	Night	9% 5% 2 6% 4 4 4 3 6% 7% 50%	5467
April	1st to 10th	Night	6% 4 1 4 4 4 4 5% 7% 10% 53%	2200
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 3 6% 9% 52%	2173
	21st to end	Night	5% 4 1 4 3 4 3 4 6% 8% 58%	2020
May	1st to 10th	Night	2 2 1 5% 3 4 4 5% 7% 10% 57%	4033
	11th to 20th	Night	8% 4 1 5% 3 4 4 4 6% 8% 53%	4032
	21st to end	Night	3 3 1 4 3 3 4 5% 7% 10% 57%	4421
June	1st to 10th	Night	10% 6% 2 7% 4 5% 5% 4 7% 10% 40%	1800
	11th to 20th	Night	5% 5% 2 7% 5% 4 6% 6% 10% 13% 38%	1800
	21st to end	Night	7% 6% 2 6% 6% 6% 7% 5% 9% 13% 35%	1800
July	1st to 10th	Night	5% 5% 2 10% 7% 8% 8% 7% 9% 13% 27%	2885
	11th to 20th	Night	3 4 2 8% 7% 7% 9% 7% 11% 13% 29%	2840
	21st to end	Night	4 7% 4 11% 10% 8% 9% 7% 11% 12% 18%	3087
August	1st to 10th	Night	6% 4 2 7% 5% 6% 5% 6% 9% 14% 35%	2914
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 6% 10% 16% 37%	3100
	21st to end	Night	6% 5% 2 10% 7% 7% 7% 7% 9% 13% 27%	3782
September	1st to 10th	Night	6% 6% 3 8% 7% 7% 7% 7% 10% 12% 29%	2090
	11th to 20th	Night	8% 7% 3 9% 7% 6% 6% 5% 9% 10% 30%	2090
	21st to end	Night	8% 6% 3 8% 7% 5% 7% 6% 8% 10% 33%	2090
October	1st to 10th	Night	9% 5% 3 8% 5% 5% 4 5% 7% 10% 39%	2090
	11th to 20th	Night	6% 4 2 4 5% 4 5% 3 7% 7% 52%	2215
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 4 6% 8% 48%	2508
November	1st to 10th	Night	6% 5% 2 6% 4 5% 4 4 6% 8% 50%	2341
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 4 6% 8% 48%	2400
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 4 6% 8% 48%	2574
December	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 9% 62%	4250
	11th to 20th	Night	9% 6% 2 6% 4 3 3 3 5% 7% 51%	4822
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	6490

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	32% 6% 9% 19% 25% 10%	4057
	11th to 20th	Day	51% 5% 7% 18% 15% 5%	4060
	21st to end	Day	35% 6% 8% 28% 18% 5%	4924
February	1st to 10th	Day	34% 6% 11% 19% 19% 10%	2000
	11th to 20th	Day	26% 7% 12% 21% 25% 10%	1958
	21st to end	Day	27% 5% 14% 20% 24% 9%	1685
March	1st to 10th	Day	30% 10% 13% 21% 20% 6%	4935
	11th to 20th	Day	20% 6% 13% 25% 20% 17%	4980
	21st to end	Day	27% 6% 10% 30% 21% 6%	5465
April	1st to 10th	Day	23% 8% 19% 24% 20% 7%	2200
	11th to 20th	Day	27% 9% 15% 17% 23% 9%	2176
	21st to end	Day	21% 7% 14% 26% 28% 5%	2390
May	1st to 10th	Day	16% 8% 27% 30% 15% 4	5897
	11th to 20th	Day	27% 9% 19% 28% 15% 1	5865
	21st to end	Day	19% 8% 23% 35% 14% 1	6447
June	1st to 10th	Day	39% 14% 23% 20% 5%	2600
	11th to 20th	Day	34% 16% 26% 20% 4	2600
	21st to end	Day	38% 17% 28% 14% 3	2600
July	1st to 10th	Day	37% 16% 38% 8% 1	4120
	11th to 20th	Day	39% 21% 35% 5%	4051
	21st to end	Day	38% 23% 30% 9% 1	4324
August	1st to 10th	Day	39% 15% 31% 11% 3	3982
	11th to 20th	Day	29% 16% 30% 21% 4	3765
	21st to end	Day	40% 19% 29% 12% 1	3841
September	1st to 10th	Day	40% 22% 24% 13% 1	2090
	11th to 20th	Day	41% 17% 24% 14% 4	2090
	21st to end	Day	38% 12% 16% 22% 9% 2	2090
October	1st to 10th	Day	50% 13% 14% 10% 7% 5%	3480
	11th to 20th	Day	23% 7% 9% 22% 34% 4	3241
	21st to end	Day	26% 5% 9% 24% 28% 7%	3428
November	1st to 10th	Day	27% 8% 11% 24% 20% 10%	1957
	11th to 20th	Day	31% 7% 11% 22% 20% 9%	2000
	21st to end	Day	29% 8% 13% 21% 20% 9%	1821
December	1st to 10th	Day	20% 6% 17% 28% 20% 8%	2930
	11th to 20th	Day	31% 6% 12% 20% 21% 10%	3370
	21st to end	Day	42% 7% 12% 21% 14% 4	4466
January	1st to 10th	Night	31% 7% 8% 19% 28% 7%	5889
	11th to 20th	Night	54% 6% 9% 15% 13% 3	5900
	21st to end	Night	39% 7% 10% 21% 17% 6%	6015
February	1st to 10th	Night	36% 6% 11% 18% 20% 8%	2400
	11th to 20th	Night	31% 6% 12% 19% 22% 9%	2339
	21st to end	Night	28% 7% 13% 20% 25% 7%	1942
March	1st to 10th	Night	35% 8% 11% 20% 18% 8%	4918
	11th to 20th	Night	19% 7% 17% 21% 19% 17%	4980
	21st to end	Night	31% 6% 9% 28% 21% 4	5467
April	1st to 10th	Night	24% 8% 19% 26% 17% 7%	2200
	11th to 20th	Night	27% 8% 14% 21% 22% 7%	2173
	21st to end	Night	23% 6% 14% 27% 25% 5%	2020
May	1st to 10th	Night	18% 10% 23% 32% 14% 3	4060
	11th to 20th	Night	26% 9% 18% 30% 16% 1	4033
	21st to end	Night	20% 8% 21% 35% 15% 1	4429
June	1st to 10th	Night	37% 11% 24% 22% 6%	1800
	11th to 20th	Night	33% 16% 26% 22% 4	1800
	21st to end	Night	38% 17% 24% 15% 6%	1800
July	1st to 10th	Night	44% 17% 30% 8% 1	2840
	11th to 20th	Night	40% 19% 35% 6%	2786
	21st to end	Night	53% 17% 22% 6%	2988
August	1st to 10th	Night	36% 17% 30% 13% 5%	2903
	11th to 20th	Night	29% 16% 35% 19% 1	3130
	21st to end	Night	43% 18% 28% 10% 1	3782
September	1st to 10th	Night	41% 20% 25% 13% 1	2090
	11th to 20th	Night	44% 14% 23% 16% 2	2090
	21st to end	Night	41% 13% 18% 18% 7% 2	2090
October	1st to 10th	Night	56% 10% 11% 11% 8% 4	3480
	11th to 20th	Night	25% 6% 9% 29% 25% 5%	3675
	21st to end	Night	28% 5% 10% 26% 26% 5%	4120
November	1st to 10th	Night	30% 6% 14% 20% 20% 10%	2341
	11th to 20th	Night	34% 6% 10% 22% 20% 8%	2400
	21st to end	Night	33% 7% 11% 24% 17% 9%	2574
December	1st to 10th	Night	20% 5% 17% 31% 17% 11%	4250
	11th to 20th	Night	31% 6% 13% 21% 20% 8%	4824
	21st to end	Night	43% 8% 13% 19% 14% 3	6490

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	9% 9% 41% 31% 9%	300
	11th to 20th	Day	8% 18% 29% 30% 13% 1	300
	21st to end	Day	14% 12% 38% 29% 7%	330
February	1st to 10th	Day	9% 9% 26% 37% 18% 1	300
	11th to 20th	Day	7% 6% 33% 32% 18% 2	300
	21st to end	Day	7% 6% 31% 36% 17% 3	284
March	1st to 10th	Day	5% 9% 38% 37% 11% 1	500
	11th to 20th	Day	7% 10% 28% 32% 21% 2	500
	21st to end	Day	6% 8% 29% 37% 19% 2	550
April	1st to 10th	Day	7% 6% 30% 38% 17% 1	500
	11th to 20th	Day	8% 8% 27% 38% 17% 3	492
	21st to end	Day	4 3 21% 40% 27% 5%	500
May	1st to 10th	Day	5% 3 18% 39% 28% 7%	503
	11th to 20th	Day	9% 2 15% 35% 30% 9%	527
	21st to end	Day	5% 1 9% 28% 38% 17%	573
June	1st to 10th	Day	12% 1 8% 28% 34% 17%	530
	11th to 20th	Day	7% 2 6% 24% 38% 23%	530
	21st to end	Day	4 5% 15% 37% 39%	530
July	1st to 10th	Day	6% 2 2 13% 35% 44%	505
	11th to 20th	Day	5% 1 2 10% 38% 44%	484
	21st to end	Day	7% 1 5% 13% 39% 36%	532
August	1st to 10th	Day	6% 1 4 12% 39% 38%	486
	11th to 20th	Day	7% 2 5% 10% 35% 41%	479
	21st to end	Day	9% 1 6% 17% 36% 31%	539
September	1st to 10th	Day	8% 1 5% 16% 37% 34%	490
	11th to 20th	Day	9% 1 6% 22% 39% 24%	490
	21st to end	Day	7% 1 7% 20% 41% 25%	490
October	1st to 10th	Day	11% 2 15% 33% 31% 8%	490
	11th to 20th	Day	7% 3 17% 31% 33% 9%	490
	21st to end	Day	8% 5% 16% 38% 29% 4	539
November	1st to 10th	Day	8% 4 19% 34% 30% 5%	495
	11th to 20th	Day	11% 6% 21% 37% 21% 4	500
	21st to end	Day	6% 5% 28% 35% 22% 3	321
December	1st to 10th	Day	6% 8% 27% 36% 18% 5%	300
	11th to 20th	Day	9% 7% 31% 33% 19% 1	300
	21st to end	Day	7% 9% 27% 35% 20% 2	330
January	1st to 10th	Night	10% 9% 31% 35% 15% 1	800
	11th to 20th	Night	13% 8% 26% 37% 16% 1	800
	21st to end	Night	13% 11% 29% 35% 12%	880
February	1st to 10th	Night	14% 6% 28% 35% 18%	800
	11th to 20th	Night	10% 6% 27% 38% 17% 1	783
	21st to end	Night	8% 6% 26% 44% 16%	660
March	1st to 10th	Night	5% 7% 33% 44% 10% 1	800
	11th to 20th	Night	7% 6% 26% 39% 20% 1	800
	21st to end	Night	6% 6% 27% 42% 18% 1	880
April	1st to 10th	Night	6% 6% 24% 44% 19% 1	800
	11th to 20th	Night	7% 6% 27% 42% 17% 1	791
	21st to end	Night	6% 3 25% 44% 21%	755
May	1st to 10th	Night	3 2 15% 44% 33% 4	600
	11th to 20th	Night	8% 2 14% 40% 31% 5%	591
	21st to end	Night	5% 1 9% 35% 42% 6%	650
June	1st to 10th	Night	11% 2 5% 25% 44% 13%	600
	11th to 20th	Night	6% 2 5% 20% 42% 26%	600
	21st to end	Night	7% 1 4 15% 42% 32%	600
July	1st to 10th	Night	7% 1 2 11% 38% 40%	570
	11th to 20th	Night	6% 8% 42% 43%	552
	21st to end	Night	5% 1 4 10% 42% 39%	624
August	1st to 10th	Night	5% 1 3 11% 45% 36%	567
	11th to 20th	Night	5% 1 3 10% 38% 43%	564
	21st to end	Night	11% 2 4 22% 36% 26%	803
September	1st to 10th	Night	8% 2 6% 21% 43% 21%	760
	11th to 20th	Night	9% 3 7% 26% 40% 15%	760
	21st to end	Night	10% 1 8% 30% 39% 12%	760
October	1st to 10th	Night	11% 3 18% 36% 28% 6%	760
	11th to 20th	Night	6% 4 14% 40% 32% 3	760
	21st to end	Night	9% 4 22% 40% 23% 3	836
November	1st to 10th	Night	8% 4 25% 39% 21% 3	780
	11th to 20th	Night	9% 5% 23% 40% 20% 2	800
	21st to end	Night	8% 5% 26% 42% 19% 1	800
December	1st to 10th	Night	7% 6% 25% 36% 23% 2	800
	11th to 20th	Night	9% 6% 25% 39% 19% 2	800
	21st to end	Night	10% 8% 24% 38% 19% 1	880

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