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 2026-10-29.

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	6% 4 2 6% 4 5% 5% 5% 8% 11% 44%	148920
	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	143121
Transparency	Day	31% 11% 21% 20% 13% 4	152693
	Night	34% 10% 18% 20% 13% 5%	147420
Seeing	Day	7% 4 17% 28% 28% 15%	18132
	Night	8% 4 18% 33% 27% 10%	28123

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	7% 5% 2 5% 4 4 4 4 6% 9% 51%	5258
	11th to 20th	Day	18% 9% 3 8% 4 4 5% 3 6% 7% 33%	5280
	21st to end	Day	12% 5% 2 5% 4 3 4 4 6% 8% 47%	6374
February	1st to 10th	Day	11% 5% 2 5% 3 4 4 4 6% 8% 48%	2100
	11th to 20th	Day	7% 3 2 6% 4 3 4 4 5% 7% 55%	2048
	21st to end	Day	7% 5% 2 5% 3 3 3 3 5% 8% 56%	1767
March	1st to 10th	Day	7% 4 2 6% 4 4 4 4 5% 8% 50%	5045
	11th to 20th	Day	5% 3 1 4 3 3 3 3 6% 8% 60%	5090
	21st to end	Day	7% 4 2 4 3 3 4 4 7% 9% 52%	5586
April	1st to 10th	Day	6% 2 2 4 3 4 4 4 7% 8% 55%	2310
	11th to 20th	Day	7% 4 1 5% 4 4 4 4 7% 8% 52%	2286
	21st to end	Day	3 3 1 4 3 4 4 4 7% 9% 59%	2510
May	1st to 10th	Day	3 2 1 4 3 3 4 4 6% 10% 58%	7616
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 9% 51%	7626
	21st to end	Day	3 3 1 4 3 4 4 4 8% 14% 53%	8391
June	1st to 10th	Day	9% 7% 3 7% 5% 6% 5% 5% 8% 10% 34%	2730
	11th to 20th	Day	5% 4 2 6% 5% 6% 7% 7% 11% 15% 33%	2730
	21st to end	Day	6% 5% 2 7% 5% 6% 8% 7% 10% 13% 31%	2730
July	1st to 10th	Day	3 4 2 6% 5% 6% 7% 7% 12% 17% 29%	7680
	11th to 20th	Day	3 3 2 8% 7% 7% 8% 7% 11% 15% 28%	7680
	21st to end	Day	3 3 2 6% 6% 7% 8% 7% 11% 16% 30%	8340
August	1st to 10th	Day	7% 4 2 8% 7% 8% 8% 6% 9% 12% 29%	5860
	11th to 20th	Day	3 3 2 5% 4 5% 6% 5% 9% 14% 45%	5486
	21st to end	Day	4 4 2 8% 5% 7% 8% 8% 11% 15% 28%	5592
September	1st to 10th	Day	6% 5% 2 8% 6% 7% 8% 6% 11% 13% 28%	2310
	11th to 20th	Day	8% 6% 3 7% 6% 6% 7% 6% 8% 13% 31%	2310
	21st to end	Day	6% 5% 3 8% 6% 6% 7% 5% 8% 11% 36%	2310
October	1st to 10th	Day	9% 6% 3 7% 6% 6% 6% 5% 8% 9% 37%	2310
	11th to 20th	Day	5% 3 1 5% 4 3 4 3 6% 9% 56%	2168
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 8% 53%	2280
November	1st to 10th	Day	7% 4 2 5% 4 4 4 4 6% 8% 53%	2057
	11th to 20th	Day	8% 5% 2 5% 4 3 4 4 5% 8% 52%	2100
	21st to end	Day	7% 5% 2 6% 4 3 5% 4 7% 9% 50%	1912
December	1st to 10th	Day	4 3 1 4 4 3 3 4 5% 9% 60%	3020
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 5% 7% 52%	3463
	21st to end	Day	11% 8% 3 8% 5% 5% 5% 4 5% 8% 39%	4565
January	1st to 10th	Night	7% 5% 2 6% 4 4 4 3 5% 8% 52%	7635
	11th to 20th	Night	17% 9% 3 9% 5% 5% 5% 3 5% 7% 32%	7680
	21st to end	Night	12% 6% 2 7% 4 4 4 4 6% 8% 44%	7790
February	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 48%	2520
	11th to 20th	Night	9% 6% 2 5% 3 4 4 3 5% 8% 51%	2458
	21st to end	Night	7% 5% 2 5% 4 3 3 4 6% 8% 54%	2036
March	1st to 10th	Night	8% 6% 2 7% 4 5% 5% 3 6% 7% 46%	5028
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 10% 57%	5090
	21st to end	Night	8% 5% 2 6% 4 4 4 3 6% 7% 50%	5588
April	1st to 10th	Night	6% 4 1 5% 4 4 4 5% 7% 10% 52%	2310
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 3 6% 9% 52%	2283
	21st to end	Night	5% 4 1 4 3 4 4 4 6% 8% 58%	2121
May	1st to 10th	Night	3 3 2 5% 4 4 4 5% 7% 11% 53%	5237
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	5234
	21st to end	Night	4 3 2 5% 5% 4 5% 5% 7% 10% 48%	5761
June	1st to 10th	Night	10% 6% 2 8% 4 5% 5% 4 7% 10% 39%	1890
	11th to 20th	Night	5% 5% 1 7% 5% 4 7% 6% 10% 13% 37%	1890
	21st to end	Night	6% 5% 2 6% 5% 6% 7% 5% 9% 12% 36%	1890
July	1st to 10th	Night	3 5% 2 9% 7% 7% 8% 7% 10% 16% 26%	5278
	11th to 20th	Night	3 3 2 8% 7% 8% 8% 7% 11% 15% 27%	5277
	21st to end	Night	3 6% 3 9% 8% 7% 8% 7% 12% 14% 23%	5736
August	1st to 10th	Night	5% 4 2 8% 6% 6% 7% 7% 11% 14% 29%	4269
	11th to 20th	Night	2 2 1 4 4 4 5% 5% 9% 15% 47%	4560
	21st to end	Night	5% 5% 2 9% 6% 7% 7% 7% 11% 15% 27%	5485
September	1st to 10th	Night	6% 6% 3 8% 7% 7% 7% 7% 9% 11% 28%	2310
	11th to 20th	Night	7% 7% 4 9% 7% 6% 6% 5% 9% 10% 30%	2310
	21st to end	Night	7% 5% 3 8% 7% 5% 7% 5% 8% 10% 34%	2310
October	1st to 10th	Night	9% 6% 3 8% 6% 5% 4 5% 7% 9% 38%	2310
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 3 7% 7% 52%	2449
	21st to end	Night	7% 4 2 5% 4 4 5% 4 6% 8% 50%	2737
November	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 4 6% 8% 51%	2461
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 5% 7% 50%	2520
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 4 6% 8% 48%	2703
December	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 9% 61%	4380
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 51%	4952
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 4 7% 8% 36%	6633

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	28% 6% 11% 23% 25% 7%	5259
	11th to 20th	Day	49% 5% 7% 16% 13% 9%	5280
	21st to end	Day	34% 6% 10% 27% 14% 10%	6374
February	1st to 10th	Day	33% 6% 11% 19% 20% 12%	2100
	11th to 20th	Day	26% 7% 12% 22% 24% 9%	2058
	21st to end	Day	27% 5% 14% 20% 25% 9%	1767
March	1st to 10th	Day	30% 10% 13% 21% 20% 6%	5045
	11th to 20th	Day	20% 6% 13% 25% 20% 16%	5090
	21st to end	Day	26% 6% 10% 30% 21% 6%	5586
April	1st to 10th	Day	24% 8% 19% 24% 19% 6%	2310
	11th to 20th	Day	27% 8% 15% 17% 24% 9%	2286
	21st to end	Day	20% 6% 14% 27% 27% 5%	2510
May	1st to 10th	Day	19% 8% 27% 30% 14% 3	7651
	11th to 20th	Day	27% 9% 21% 26% 15% 1	7630
	21st to end	Day	20% 11% 29% 29% 11%	8405
June	1st to 10th	Day	40% 13% 22% 19% 5%	2730
	11th to 20th	Day	33% 16% 28% 19% 4	2730
	21st to end	Day	37% 16% 29% 14% 3	2730
July	1st to 10th	Day	35% 17% 42% 6%	7615
	11th to 20th	Day	39% 16% 35% 9%	7611
	21st to end	Day	34% 18% 31% 16% 1	8156
August	1st to 10th	Day	43% 15% 30% 11% 2	5847
	11th to 20th	Day	25% 14% 32% 27% 3	5522
	21st to end	Day	38% 17% 34% 11% 1	5592
September	1st to 10th	Day	40% 21% 24% 13% 1	2310
	11th to 20th	Day	40% 17% 24% 15% 4	2310
	21st to end	Day	37% 12% 17% 22% 9% 2	2310
October	1st to 10th	Day	51% 13% 14% 11% 7% 5%	3700
	11th to 20th	Day	23% 7% 9% 21% 34% 6%	3447
	21st to end	Day	25% 5% 9% 24% 28% 8%	3618
November	1st to 10th	Day	26% 8% 12% 24% 21% 10%	2057
	11th to 20th	Day	30% 7% 10% 22% 21% 10%	2100
	21st to end	Day	29% 8% 13% 23% 20% 9%	1912
December	1st to 10th	Day	21% 6% 17% 28% 20% 8%	3020
	11th to 20th	Day	31% 6% 12% 20% 21% 10%	3460
	21st to end	Day	41% 7% 12% 22% 15% 3	4565
January	1st to 10th	Night	28% 6% 10% 25% 26% 5%	7636
	11th to 20th	Night	51% 5% 8% 14% 14% 7%	7680
	21st to end	Night	36% 6% 12% 21% 16% 8%	7790
February	1st to 10th	Night	35% 6% 11% 17% 22% 9%	2520
	11th to 20th	Night	31% 6% 12% 20% 22% 9%	2459
	21st to end	Night	27% 6% 13% 20% 26% 8%	2036
March	1st to 10th	Night	35% 8% 11% 21% 18% 7%	5028
	11th to 20th	Night	20% 7% 17% 22% 19% 16%	5090
	21st to end	Night	31% 6% 9% 27% 21% 5%	5588
April	1st to 10th	Night	25% 8% 18% 25% 16% 6%	2310
	11th to 20th	Night	27% 8% 14% 21% 23% 7%	2283
	21st to end	Night	24% 6% 14% 27% 25% 5%	2121
May	1st to 10th	Night	22% 9% 25% 29% 12% 3	5264
	11th to 20th	Night	29% 8% 20% 26% 15% 1	5235
	21st to end	Night	27% 11% 22% 27% 12%	5771
June	1st to 10th	Night	38% 11% 23% 22% 6%	1890
	11th to 20th	Night	33% 15% 27% 21% 3	1890
	21st to end	Night	38% 17% 25% 15% 6%	1890
July	1st to 10th	Night	42% 18% 35% 5% 1	5235
	11th to 20th	Night	41% 17% 34% 8%	5226
	21st to end	Night	44% 15% 27% 13%	5606
August	1st to 10th	Night	41% 17% 29% 9% 4	4258
	11th to 20th	Night	23% 13% 34% 29% 1	4590
	21st to end	Night	41% 18% 31% 9% 1	5485
September	1st to 10th	Night	44% 19% 24% 12% 1	2310
	11th to 20th	Night	45% 14% 23% 16% 2	2310
	21st to end	Night	41% 13% 18% 19% 7% 2	2310
October	1st to 10th	Night	57% 10% 11% 11% 7% 4	3700
	11th to 20th	Night	25% 6% 9% 28% 25% 6%	3909
	21st to end	Night	28% 5% 9% 26% 26% 6%	4349
November	1st to 10th	Night	30% 6% 14% 20% 20% 11%	2461
	11th to 20th	Night	32% 6% 10% 22% 21% 9%	2520
	21st to end	Night	32% 7% 11% 25% 17% 9%	2703
December	1st to 10th	Night	21% 5% 17% 30% 16% 10%	4380
	11th to 20th	Night	31% 6% 13% 22% 20% 8%	4954
	21st to end	Night	43% 8% 13% 19% 14% 3	6633

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	8% 8% 39% 33% 10% 1	330
	11th to 20th	Day	8% 17% 31% 29% 13% 1	330
	21st to end	Day	13% 12% 38% 30% 7%	363
February	1st to 10th	Day	8% 10% 27% 36% 17% 1	330
	11th to 20th	Day	7% 7% 32% 35% 17% 2	330
	21st to end	Day	8% 7% 30% 36% 16% 3	310
March	1st to 10th	Day	5% 9% 36% 38% 13% 1	540
	11th to 20th	Day	7% 10% 30% 31% 20% 2	540
	21st to end	Day	5% 7% 28% 37% 20% 3	594
April	1st to 10th	Day	7% 6% 30% 37% 19% 2	540
	11th to 20th	Day	7% 7% 26% 38% 18% 4	532
	21st to end	Day	4 3 22% 39% 27% 5%	540
May	1st to 10th	Day	5% 3 20% 39% 26% 6%	543
	11th to 20th	Day	9% 2 16% 35% 29% 9%	567
	21st to end	Day	5% 1 9% 29% 38% 17%	617
June	1st to 10th	Day	12% 2 9% 28% 34% 16%	570
	11th to 20th	Day	6% 2 6% 26% 38% 22%	570
	21st to end	Day	4 4 16% 37% 39%	570
July	1st to 10th	Day	5% 2 2 14% 34% 43%	565
	11th to 20th	Day	6% 2 3 12% 37% 40%	564
	21st to end	Day	6% 1 5% 15% 39% 33%	620
August	1st to 10th	Day	6% 1 5% 13% 37% 37%	566
	11th to 20th	Day	6% 1 4 12% 36% 40%	559
	21st to end	Day	9% 1 6% 18% 37% 30%	627
September	1st to 10th	Day	7% 2 5% 17% 38% 32%	570
	11th to 20th	Day	8% 1 5% 24% 38% 23%	570
	21st to end	Day	6% 1 7% 22% 41% 23%	570
October	1st to 10th	Day	11% 3 16% 32% 31% 7%	570
	11th to 20th	Day	6% 3 16% 31% 35% 8%	570
	21st to end	Day	7% 5% 18% 39% 27% 4	615
November	1st to 10th	Day	8% 4 19% 35% 29% 4	535
	11th to 20th	Day	10% 6% 20% 38% 22% 4	540
	21st to end	Day	5% 5% 28% 35% 23% 4	352
December	1st to 10th	Day	6% 9% 28% 35% 17% 4	330
	11th to 20th	Day	8% 7% 32% 33% 19% 1	330
	21st to end	Day	7% 9% 26% 34% 21% 2	363
January	1st to 10th	Night	9% 9% 30% 35% 16% 1	840
	11th to 20th	Night	13% 8% 26% 37% 15% 1	840
	21st to end	Night	13% 12% 30% 34% 11%	924
February	1st to 10th	Night	13% 6% 28% 35% 17%	840
	11th to 20th	Night	10% 6% 26% 39% 17% 1	823
	21st to end	Night	8% 7% 25% 43% 16%	692
March	1st to 10th	Night	5% 7% 32% 45% 10% 1	840
	11th to 20th	Night	7% 7% 26% 39% 20% 1	840
	21st to end	Night	6% 6% 27% 41% 19% 1	924
April	1st to 10th	Night	7% 7% 24% 43% 19% 1	840
	11th to 20th	Night	7% 6% 28% 41% 17% 1	831
	21st to end	Night	6% 4 25% 44% 21%	793
May	1st to 10th	Night	3 2 15% 43% 33% 4	630
	11th to 20th	Night	8% 2 15% 41% 30% 5%	621
	21st to end	Night	6% 1 9% 35% 42% 6%	683
June	1st to 10th	Night	11% 3 6% 26% 43% 12%	630
	11th to 20th	Night	6% 2 5% 20% 42% 26%	630
	21st to end	Night	7% 1 3 15% 41% 33%	630
July	1st to 10th	Night	7% 1 2 11% 39% 40%	615
	11th to 20th	Night	7% 1 10% 41% 41%	612
	21st to end	Night	5% 1 4 12% 41% 37%	690
August	1st to 10th	Night	5% 1 3 11% 44% 35%	627
	11th to 20th	Night	4 1 3 11% 38% 42%	624
	21st to end	Night	10% 2 4 21% 37% 26%	887
September	1st to 10th	Night	8% 2 6% 21% 41% 20%	840
	11th to 20th	Night	9% 3 9% 26% 39% 15%	840
	21st to end	Night	9% 2 8% 30% 39% 12%	840
October	1st to 10th	Night	11% 3 19% 36% 26% 5%	840
	11th to 20th	Night	6% 4 15% 40% 32% 3	840
	21st to end	Night	8% 4 24% 39% 22% 2	913
November	1st to 10th	Night	8% 4 26% 39% 21% 3	820
	11th to 20th	Night	9% 5% 22% 41% 21% 2	840
	21st to end	Night	8% 5% 25% 40% 20% 1	840
December	1st to 10th	Night	8% 6% 25% 36% 22% 2	840
	11th to 20th	Night	8% 6% 25% 40% 19% 2	840
	21st to end	Night	10% 8% 24% 37% 20% 2	924

Page generated at 2026-10-26 04:37:46. Version ac07

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