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 2024-03-23.

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	8% 5% 2 5% 5% 4 6% 6% 7% 10% 42%	79177
	Night	8% 5% 2 6% 6% 3 5% 5% 7% 9% 43%	80710
Transparency	Day	34% 11% 17% 19% 14% 5%	78900
	Night	35% 10% 16% 20% 14% 5%	80506
Seeing	Day	7% 4 17% 28% 28% 16%	14526
	Night	8% 4 18% 33% 27% 10%	24671

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% 8% 4 7% 6% 4 4 5% 6% 9% 37%	2836
	11th to 20th	Day	21% 9% 3 5% 4 2 4 4 5% 8% 35%	2840
	21st to end	Day	12% 6% 3 5% 5% 3 5% 5% 6% 9% 43%	3442
February	1st to 10th	Day	11% 5% 3 4 4 3 4 5% 6% 8% 46%	1900
	11th to 20th	Day	8% 3 2 5% 4 2 4 5% 5% 7% 56%	1848
	21st to end	Day	7% 5% 2 5% 4 2 3 4 5% 9% 56%	1603
March	1st to 10th	Day	5% 2 1 4 3 3 4 5% 4 10% 59%	2090
	11th to 20th	Day	6% 3 1 4 4 2 4 4 5% 9% 57%	2090
	21st to end	Day	7% 4 2 3 4 2 4 4 5% 8% 57%	2200
April	1st to 10th	Day	7% 2 2 4 4 3 4 5% 7% 9% 55%	1980
	11th to 20th	Day	8% 4 2 5% 5% 3 4 5% 7% 9% 47%	1956
	21st to end	Day	4 3 1 4 3 2 5% 5% 6% 9% 58%	2150
May	1st to 10th	Day	4 2 1 2 2 2 3 4 6% 9% 65%	2340
	11th to 20th	Day	7% 4 2 4 5% 3 5% 6% 7% 9% 48%	2301
	21st to end	Day	6% 4 2 3 5% 3 6% 6% 8% 11% 45%	2531
June	1st to 10th	Day	10% 6% 3 6% 6% 5% 6% 7% 8% 10% 33%	2340
	11th to 20th	Day	6% 3 2 5% 6% 4 8% 8% 10% 14% 34%	2340
	21st to end	Day	7% 5% 3 6% 6% 5% 8% 9% 10% 12% 31%	2340
July	1st to 10th	Day	8% 4 3 6% 6% 4 8% 9% 11% 14% 27%	2340
	11th to 20th	Day	5% 4 2 7% 9% 6% 9% 9% 11% 11% 28%	2340
	21st to end	Day	6% 5% 3 7% 8% 6% 9% 9% 10% 13% 25%	2547
August	1st to 10th	Day	6% 3 2 5% 8% 6% 9% 9% 11% 12% 29%	2274
	11th to 20th	Day	6% 6% 3 6% 7% 5% 9% 9% 11% 14% 23%	2125
	21st to end	Day	9% 5% 3 6% 8% 5% 7% 10% 9% 13% 25%	2191
September	1st to 10th	Day	7% 4 2 7% 7% 4 8% 7% 10% 14% 29%	1980
	11th to 20th	Day	9% 6% 3 7% 6% 4 7% 7% 8% 13% 30%	1980
	21st to end	Day	7% 5% 3 5% 8% 5% 6% 7% 8% 11% 35%	1980
October	1st to 10th	Day	7% 6% 2 6% 6% 4 6% 6% 8% 8% 41%	1980
	11th to 20th	Day	6% 3 2 4 4 3 5% 5% 6% 9% 53%	1859
	21st to end	Day	7% 4 2 4 4 3 4 5% 6% 9% 52%	1980
November	1st to 10th	Day	7% 4 2 4 5% 3 5% 5% 5% 8% 53%	1857
	11th to 20th	Day	8% 5% 2 5% 4 3 4 5% 6% 8% 50%	1900
	21st to end	Day	7% 4 2 5% 5% 3 5% 5% 7% 9% 48%	1730
December	1st to 10th	Day	6% 5% 2 5% 6% 3 4 5% 6% 10% 50%	1710
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 5% 5% 7% 48%	2153
	21st to end	Day	13% 8% 3 7% 6% 3 5% 5% 5% 7% 36%	3124
January	1st to 10th	Night	11% 7% 3 8% 6% 4 6% 5% 5% 9% 36%	4109
	11th to 20th	Night	18% 10% 4 7% 6% 4 5% 4 5% 7% 31%	4120
	21st to end	Night	10% 7% 3 7% 6% 4 5% 5% 6% 8% 39%	4211
February	1st to 10th	Night	10% 6% 2 6% 5% 3 4 5% 5% 7% 47%	2280
	11th to 20th	Night	10% 5% 2 4 4 3 4 5% 5% 8% 51%	2218
	21st to end	Night	7% 4 2 4 5% 2 4 5% 6% 8% 53%	1848
March	1st to 10th	Night	4 4 1 5% 5% 3 4 4 6% 7% 56%	2090
	11th to 20th	Night	6% 3 2 4 5% 3 4 5% 7% 9% 52%	2090
	21st to end	Night	5% 4 2 4 4 3 5% 5% 5% 6% 57%	2201
April	1st to 10th	Night	6% 4 1 3 4 3 4 5% 6% 10% 53%	1980
	11th to 20th	Night	6% 4 2 4 6% 4 6% 5% 6% 9% 49%	1953
	21st to end	Night	6% 4 1 4 3 3 4 4 5% 8% 58%	1819
May	1st to 10th	Night	2 2 1 2 3 2 3 4 6% 10% 65%	1620
	11th to 20th	Night	7% 5% 2 5% 5% 3 6% 5% 7% 9% 47%	1592
	21st to end	Night	4 3 2 4 5% 3 4 6% 6% 8% 53%	1744
June	1st to 10th	Night	11% 6% 3 6% 5% 4 5% 6% 6% 10% 39%	1620
	11th to 20th	Night	5% 4 2 5% 6% 3 7% 7% 9% 14% 40%	1620
	21st to end	Night	7% 5% 2 5% 6% 3 7% 6% 9% 13% 36%	1620
July	1st to 10th	Night	6% 5% 2 6% 6% 4 7% 8% 8% 13% 34%	1620
	11th to 20th	Night	5% 5% 2 6% 8% 5% 9% 8% 10% 13% 30%	1620
	21st to end	Night	4 5% 3 6% 9% 5% 7% 7% 11% 14% 30%	1771
August	1st to 10th	Night	5% 3 2 5% 6% 4 6% 8% 10% 15% 36%	1658
	11th to 20th	Night	5% 5% 3 7% 7% 4 7% 8% 10% 13% 31%	1776
	21st to end	Night	9% 6% 2 8% 7% 4 6% 7% 8% 12% 30%	2146
September	1st to 10th	Night	6% 5% 4 7% 7% 4 8% 8% 10% 12% 30%	1980
	11th to 20th	Night	8% 6% 4 8% 8% 4 7% 7% 8% 10% 30%	1980
	21st to end	Night	8% 6% 3 7% 9% 4 6% 6% 8% 11% 33%	1980
October	1st to 10th	Night	7% 5% 3 7% 7% 3 5% 6% 7% 10% 42%	1980
	11th to 20th	Night	7% 4 2 4 5% 3 5% 5% 6% 7% 52%	2098
	21st to end	Night	8% 5% 2 5% 5% 4 4 5% 6% 9% 48%	2376
November	1st to 10th	Night	6% 5% 3 5% 4 3 5% 5% 6% 8% 52%	2221
	11th to 20th	Night	9% 4 3 5% 5% 3 5% 5% 5% 8% 48%	2280
	21st to end	Night	7% 5% 3 5% 6% 4 5% 5% 5% 8% 47%	2445
December	1st to 10th	Night	7% 4 2 5% 6% 3 5% 5% 5% 9% 49%	2470
	11th to 20th	Night	11% 6% 2 6% 5% 3 4 4 5% 8% 47%	3042
	21st to end	Night	14% 8% 3 6% 6% 4 5% 6% 6% 8% 34%	4532

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	41% 7% 10% 20% 15% 6%	2837
	11th to 20th	Day	48% 5% 7% 18% 16% 7%	2840
	21st to end	Day	36% 8% 8% 24% 19% 5%	3442
February	1st to 10th	Day	34% 6% 11% 20% 18% 11%	1900
	11th to 20th	Day	26% 7% 12% 20% 26% 10%	1858
	21st to end	Day	26% 5% 14% 20% 25% 9%	1603
March	1st to 10th	Day	21% 7% 15% 26% 23% 8%	2090
	11th to 20th	Day	23% 6% 14% 30% 18% 8%	2090
	21st to end	Day	24% 6% 12% 28% 24% 6%	2200
April	1st to 10th	Day	25% 8% 20% 23% 19% 6%	1980
	11th to 20th	Day	30% 10% 15% 18% 22% 6%	1956
	21st to end	Day	21% 6% 13% 25% 27% 6%	2150
May	1st to 10th	Day	15% 8% 16% 29% 24% 9%	2340
	11th to 20th	Day	30% 10% 17% 23% 18% 2	2305
	21st to end	Day	29% 12% 17% 25% 17% 1	2531
June	1st to 10th	Day	40% 14% 22% 19% 5%	2340
	11th to 20th	Day	33% 16% 27% 20% 4	2340
	21st to end	Day	38% 17% 27% 14% 3	2340
July	1st to 10th	Day	40% 20% 30% 8% 2	2275
	11th to 20th	Day	42% 22% 27% 8% 1	2271
	21st to end	Day	41% 19% 27% 12% 1	2405
August	1st to 10th	Day	38% 20% 32% 10%	2261
	11th to 20th	Day	42% 22% 26% 10%	2125
	21st to end	Day	44% 19% 25% 11% 1	2191
September	1st to 10th	Day	39% 21% 25% 14% 1	1980
	11th to 20th	Day	41% 17% 24% 14% 4	1980
	21st to end	Day	38% 13% 16% 22% 9% 2	1980
October	1st to 10th	Day	35% 13% 14% 17% 11% 9%	1980
	11th to 20th	Day	26% 9% 12% 23% 24% 5%	1859
	21st to end	Day	27% 7% 13% 20% 21% 11%	1980
November	1st to 10th	Day	27% 8% 12% 24% 19% 10%	1857
	11th to 20th	Day	32% 7% 11% 22% 20% 8%	1900
	21st to end	Day	30% 8% 13% 22% 20% 7%	1730
December	1st to 10th	Day	29% 8% 15% 27% 16% 4	1710
	11th to 20th	Day	34% 6% 13% 19% 18% 11%	2150
	21st to end	Day	45% 8% 10% 20% 14% 3	3124
January	1st to 10th	Night	43% 9% 9% 17% 17% 5%	4109
	11th to 20th	Night	52% 6% 8% 18% 12% 4	4120
	21st to end	Night	39% 8% 12% 16% 19% 6%	4211
February	1st to 10th	Night	36% 6% 11% 18% 20% 9%	2280
	11th to 20th	Night	31% 6% 12% 19% 23% 10%	2219
	21st to end	Night	28% 7% 13% 19% 25% 8%	1848
March	1st to 10th	Night	27% 6% 12% 26% 22% 7%	2090
	11th to 20th	Night	27% 7% 14% 27% 17% 7%	2090
	21st to end	Night	26% 7% 11% 28% 21% 6%	2201
April	1st to 10th	Night	24% 8% 20% 25% 17% 6%	1980
	11th to 20th	Night	30% 8% 15% 22% 20% 5%	1953
	21st to end	Night	25% 5% 13% 26% 25% 5%	1819
May	1st to 10th	Night	16% 6% 13% 33% 23% 9%	1620
	11th to 20th	Night	31% 10% 19% 22% 17% 1	1593
	21st to end	Night	26% 10% 19% 24% 19% 1	1745
June	1st to 10th	Night	39% 11% 23% 21% 7%	1620
	11th to 20th	Night	30% 16% 27% 23% 4	1620
	21st to end	Night	38% 17% 24% 15% 6%	1620
July	1st to 10th	Night	37% 19% 31% 10% 2	1575
	11th to 20th	Night	40% 20% 30% 9%	1566
	21st to end	Night	36% 20% 33% 11% 1	1672
August	1st to 10th	Night	33% 21% 35% 11% 1	1647
	11th to 20th	Night	38% 18% 32% 12%	1776
	21st to end	Night	44% 17% 24% 13% 2	2146
September	1st to 10th	Night	40% 20% 26% 13% 2	1980
	11th to 20th	Night	44% 15% 24% 15% 2	1980
	21st to end	Night	42% 14% 18% 17% 8% 2	1980
October	1st to 10th	Night	34% 12% 15% 19% 13% 7%	1980
	11th to 20th	Night	30% 8% 12% 25% 22% 4	2098
	21st to end	Night	31% 7% 13% 19% 21% 9%	2376
November	1st to 10th	Night	29% 6% 14% 20% 20% 11%	2221
	11th to 20th	Night	34% 7% 10% 22% 20% 8%	2280
	21st to end	Night	34% 7% 11% 25% 16% 8%	2445
December	1st to 10th	Night	32% 6% 13% 30% 16% 3	2470
	11th to 20th	Night	34% 7% 11% 21% 18% 9%	3044
	21st to end	Night	43% 10% 13% 17% 14% 3	4532

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% 37% 33% 10%	270
	11th to 20th	Day	7% 15% 29% 33% 14% 1	270
	21st to end	Day	13% 12% 39% 29% 7%	297
February	1st to 10th	Day	9% 10% 27% 38% 15% 1	270
	11th to 20th	Day	7% 6% 34% 30% 20% 3	270
	21st to end	Day	6% 7% 29% 38% 17% 3	258
March	1st to 10th	Day	4 8% 38% 38% 11% 1	460
	11th to 20th	Day	7% 9% 29% 32% 21% 2	460
	21st to end	Day	6% 6% 29% 37% 20% 3	470
April	1st to 10th	Day	7% 6% 29% 37% 20% 1	420
	11th to 20th	Day	9% 8% 27% 36% 17% 3	412
	21st to end	Day	5% 3 22% 39% 26% 5%	420
May	1st to 10th	Day	5% 3 17% 39% 29% 7%	423
	11th to 20th	Day	9% 1 13% 34% 32% 11%	447
	21st to end	Day	6% 1 8% 26% 39% 20%	485
June	1st to 10th	Day	13% 1 9% 26% 33% 18%	450
	11th to 20th	Day	7% 1 5% 23% 38% 26%	450
	21st to end	Day	4 5% 15% 35% 41%	450
July	1st to 10th	Day	6% 2 2 11% 33% 47%	445
	11th to 20th	Day	5% 1 2 10% 39% 43%	444
	21st to end	Day	7% 1 5% 14% 40% 33%	488
August	1st to 10th	Day	6% 1 3 11% 40% 39%	446
	11th to 20th	Day	8% 2 5% 9% 35% 41%	439
	21st to end	Day	9% 1 6% 16% 36% 32%	495
September	1st to 10th	Day	8% 1 5% 16% 34% 36%	450
	11th to 20th	Day	9% 1 6% 20% 39% 25%	450
	21st to end	Day	6% 1 6% 20% 41% 26%	450
October	1st to 10th	Day	8% 2 15% 35% 32% 8%	450
	11th to 20th	Day	7% 2 14% 31% 35% 10%	450
	21st to end	Day	8% 4 15% 37% 31% 5%	495
November	1st to 10th	Day	9% 4 20% 33% 30% 5%	455
	11th to 20th	Day	11% 7% 22% 37% 20% 3	460
	21st to end	Day	6% 4 28% 37% 22% 3	290
December	1st to 10th	Day	6% 8% 24% 37% 19% 5%	270
	11th to 20th	Day	10% 7% 30% 33% 19% 1	270
	21st to end	Day	6% 9% 26% 36% 21% 2	297
January	1st to 10th	Night	10% 9% 29% 35% 16% 1	760
	11th to 20th	Night	13% 8% 24% 38% 16% 1	760
	21st to end	Night	13% 11% 29% 35% 12%	836
February	1st to 10th	Night	14% 6% 29% 35% 16%	760
	11th to 20th	Night	10% 6% 26% 38% 17% 1	743
	21st to end	Night	8% 6% 25% 45% 16%	628
March	1st to 10th	Night	5% 7% 33% 44% 10% 1	760
	11th to 20th	Night	8% 6% 25% 39% 21% 1	760
	21st to end	Night	6% 5% 26% 43% 19% 1	801
April	1st to 10th	Night	6% 5% 23% 43% 21% 1	720
	11th to 20th	Night	8% 5% 26% 42% 18% 1	711
	21st to end	Night	7% 3 24% 44% 21%	680
May	1st to 10th	Night	3 1 14% 45% 33% 4	540
	11th to 20th	Night	8% 1 14% 40% 32% 6%	531
	21st to end	Night	6% 2 9% 34% 42% 7%	584
June	1st to 10th	Night	11% 2 5% 24% 43% 14%	540
	11th to 20th	Night	5% 1 4 18% 44% 28%	540
	21st to end	Night	8% 1 4 14% 41% 34%	540
July	1st to 10th	Night	8% 1 2 9% 38% 42%	525
	11th to 20th	Night	6% 9% 41% 43%	522
	21st to end	Night	5% 1 4 10% 41% 39%	591
August	1st to 10th	Night	4 3 10% 45% 37%	537
	11th to 20th	Night	5% 1 3 9% 39% 43%	534
	21st to end	Night	11% 2 4 21% 36% 26%	760
September	1st to 10th	Night	8% 1 5% 20% 44% 22%	720
	11th to 20th	Night	9% 2 7% 25% 40% 16%	720
	21st to end	Night	10% 1 8% 29% 40% 12%	720
October	1st to 10th	Night	9% 2 19% 36% 28% 6%	720
	11th to 20th	Night	6% 4 12% 41% 33% 4	720
	21st to end	Night	9% 4 20% 41% 24% 3	792
November	1st to 10th	Night	8% 3 26% 39% 21% 3	740
	11th to 20th	Night	9% 5% 24% 39% 21% 2	760
	21st to end	Night	8% 4 26% 42% 18% 1	760
December	1st to 10th	Night	8% 6% 24% 37% 24% 2	760
	11th to 20th	Night	9% 6% 24% 39% 20% 2	760
	21st to end	Night	10% 7% 23% 39% 20% 1	836

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