Peterborough
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peterborough clear sky chart from 2005-11-06 to 2025-09-04.

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	15% 10% 3 8% 5% 5% 4 4 5% 7% 34%	126471
	Night	15% 10% 4 9% 5% 4 5% 4 5% 7% 33%	122229
Transparency	Day	48% 7% 12% 21% 11% 1	130039
	Night	49% 7% 12% 20% 11% 1	126747
Seeing	Day	22% 16% 31% 19% 9% 3	15471
	Night	20% 15% 31% 24% 10% 1	24713

By Month

Percentage Cloud Cover NE (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% 14% 5% 9% 5% 5% 5% 4 5% 6% 30%	3597
	11th to 20th	Day	10% 17% 6% 16% 9% 7% 5% 3 4 4 18%	3683
	21st to end	Day	15% 17% 6% 13% 6% 6% 5% 3 5% 5% 19%	4466
February	1st to 10th	Day	13% 17% 6% 13% 6% 5% 5% 3 5% 4 23%	1800
	11th to 20th	Day	12% 15% 5% 13% 7% 6% 4 3 4 5% 26%	1753
	21st to end	Day	17% 14% 5% 9% 7% 6% 4 3 5% 3 25%	1551
March	1st to 10th	Day	18% 13% 5% 9% 5% 4 3 3 4 6% 30%	4843
	11th to 20th	Day	12% 12% 5% 9% 6% 5% 5% 4 6% 8% 29%	4980
	21st to end	Day	18% 11% 3 7% 4 3 4 3 4 5% 38%	5506
April	1st to 10th	Day	20% 12% 4 7% 5% 4 4 3 5% 5% 32%	2400
	11th to 20th	Day	21% 10% 2 6% 4 4 3 3 4 5% 38%	2389
	21st to end	Day	20% 10% 3 7% 5% 5% 4 4 4 5% 34%	2600
May	1st to 10th	Day	21% 9% 3 7% 5% 4 4 4 5% 7% 32%	6072
	11th to 20th	Day	17% 10% 3 8% 4 4 4 4 5% 7% 34%	6318
	21st to end	Day	19% 9% 3 6% 4 4 4 4 5% 8% 35%	6949
June	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 7% 34%	2800
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 10% 39%	2800
	21st to end	Day	15% 8% 3 6% 4 4 4 4 6% 9% 37%	2800
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 9% 39%	6360
	11th to 20th	Day	10% 4 2 5% 4 5% 4 4 6% 9% 45%	6360
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 6% 9% 47%	6755
August	1st to 10th	Day	16% 8% 3 7% 4 4 5% 4 5% 7% 38%	4234
	11th to 20th	Day	13% 7% 3 7% 4 5% 4 4 7% 9% 37%	4174
	21st to end	Day	9% 5% 2 7% 4 5% 5% 4 7% 10% 41%	4421
September	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 5% 8% 42%	2316
	11th to 20th	Day	11% 5% 2 5% 3 3 5% 4 6% 8% 47%	2247
	21st to end	Day	15% 7% 3 8% 5% 4 4 3 5% 6% 40%	2003
October	1st to 10th	Day	17% 10% 3 7% 4 4 5% 3 6% 7% 34%	1900
	11th to 20th	Day	21% 9% 3 9% 5% 5% 4 4 5% 6% 28%	1900
	21st to end	Day	24% 12% 4 8% 5% 5% 3 4 6% 6% 24%	2087
November	1st to 10th	Day	14% 11% 3 7% 4 4 5% 4 5% 7% 35%	1661
	11th to 20th	Day	15% 13% 3 8% 6% 5% 4 5% 6% 7% 28%	1600
	21st to end	Day	20% 14% 4 10% 6% 5% 4 4 5% 6% 23%	1600
December	1st to 10th	Day	16% 20% 6% 11% 6% 6% 4 3 5% 5% 18%	2600
	11th to 20th	Day	18% 17% 5% 9% 5% 4 4 4 4 5% 25%	2986
	21st to end	Day	23% 17% 5% 11% 6% 4 3 3 4 5% 19%	3960
January	1st to 10th	Night	14% 13% 7% 12% 6% 5% 5% 3 5% 6% 24%	6349
	11th to 20th	Night	12% 14% 7% 15% 8% 6% 5% 4 5% 5% 18%	6200
	21st to end	Night	14% 13% 7% 13% 5% 4 4 3 5% 6% 25%	6490
February	1st to 10th	Night	12% 16% 4 10% 8% 6% 6% 4 4 6% 24%	2600
	11th to 20th	Night	13% 13% 5% 11% 7% 5% 5% 4 6% 5% 28%	2533
	21st to end	Night	14% 12% 3 10% 6% 5% 5% 4 6% 7% 29%	2077
March	1st to 10th	Night	16% 11% 4 10% 6% 4 4 3 5% 6% 31%	4980
	11th to 20th	Night	11% 10% 4 8% 5% 5% 5% 5% 6% 8% 33%	4979
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 6% 38%	5425
April	1st to 10th	Night	18% 10% 4 7% 5% 4 5% 4 5% 7% 32%	2000
	11th to 20th	Night	17% 9% 2 6% 4 4 4 3 5% 7% 38%	1975
	21st to end	Night	16% 9% 3 6% 5% 5% 5% 4 6% 6% 34%	1800
May	1st to 10th	Night	24% 8% 3 7% 4 4 5% 3 5% 6% 32%	3869
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 4 5% 7% 41%	3575
	21st to end	Night	17% 7% 3 6% 4 5% 4 3 5% 7% 40%	3926
June	1st to 10th	Night	13% 7% 2 6% 6% 4 5% 4 7% 9% 38%	1600
	11th to 20th	Night	10% 6% 2 5% 5% 5% 5% 5% 7% 8% 44%	1600
	21st to end	Night	12% 6% 4 9% 5% 5% 4 4 6% 7% 40%	1600
July	1st to 10th	Night	8% 5% 3 7% 4 5% 5% 5% 7% 9% 43%	3600
	11th to 20th	Night	10% 6% 2 5% 3 4 4 3 5% 8% 50%	3600
	21st to end	Night	9% 5% 2 4 4 3 4 3 6% 8% 51%	4093
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 48%	2915
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 8% 48%	2875
	21st to end	Night	8% 5% 2 6% 4 3 4 4 6% 8% 48%	3444
September	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 45%	1931
	11th to 20th	Night	11% 4 1 4 3 3 4 4 7% 7% 53%	1928
	21st to end	Night	16% 7% 2 6% 4 4 3 4 5% 6% 44%	2176
October	1st to 10th	Night	16% 7% 4 7% 5% 4 4 4 4 6% 39%	2280
	11th to 20th	Night	20% 9% 3 7% 6% 5% 4 3 5% 7% 32%	2280
	21st to end	Night	21% 11% 4 8% 5% 5% 5% 3 5% 6% 28%	2509
November	1st to 10th	Night	18% 10% 3 6% 4 4 4 3 5% 7% 36%	2636
	11th to 20th	Night	17% 11% 3 8% 5% 5% 5% 4 6% 7% 29%	2800
	21st to end	Night	19% 11% 3 9% 6% 6% 6% 4 6% 7% 24%	2800
December	1st to 10th	Night	18% 15% 5% 9% 6% 5% 6% 5% 6% 7% 19%	4580
	11th to 20th	Night	15% 16% 5% 12% 5% 5% 5% 4 5% 6% 21%	5208
	21st to end	Night	24% 16% 5% 13% 5% 4 4 3 4 5% 17%	6996

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	52% 7% 12% 19% 10% 1	3598
	11th to 20th	Day	70% 6% 9% 9% 7%	3713
	21st to end	Day	67% 5% 9% 10% 9% 1	4466
February	1st to 10th	Day	63% 5% 10% 16% 5% 1	1800
	11th to 20th	Day	61% 5% 8% 14% 11% 1	1762
	21st to end	Day	62% 5% 9% 15% 9% 1	1551
March	1st to 10th	Day	55% 5% 12% 13% 14% 1	4846
	11th to 20th	Day	51% 6% 17% 16% 8% 2	4980
	21st to end	Day	48% 5% 8% 20% 17% 2	5506
April	1st to 10th	Day	54% 5% 9% 19% 12% 2	2400
	11th to 20th	Day	49% 5% 9% 20% 16% 1	2389
	21st to end	Day	52% 6% 9% 18% 13% 3	2600
May	1st to 10th	Day	51% 7% 9% 19% 10% 4	6081
	11th to 20th	Day	49% 8% 10% 16% 18% 1	6322
	21st to end	Day	47% 7% 12% 21% 12% 2	6949
June	1st to 10th	Day	45% 8% 11% 20% 15% 1	2800
	11th to 20th	Day	40% 7% 13% 25% 14% 1	2800
	21st to end	Day	43% 8% 13% 22% 13% 1	2800
July	1st to 10th	Day	38% 7% 15% 26% 14% 1	6290
	11th to 20th	Day	32% 7% 14% 30% 16% 1	6285
	21st to end	Day	31% 8% 14% 34% 13%	6560
August	1st to 10th	Day	44% 7% 14% 23% 12% 1	4221
	11th to 20th	Day	40% 8% 14% 28% 9%	4213
	21st to end	Day	36% 7% 13% 33% 10% 1	4421
September	1st to 10th	Day	39% 8% 13% 29% 11% 1	2316
	11th to 20th	Day	33% 8% 14% 31% 14% 1	2247
	21st to end	Day	44% 7% 14% 25% 10%	2003
October	1st to 10th	Day	40% 7% 13% 28% 12% 1	3160
	11th to 20th	Day	44% 7% 16% 22% 12% 1	3140
	21st to end	Day	50% 7% 15% 19% 8%	3413
November	1st to 10th	Day	47% 8% 15% 20% 9% 1	1661
	11th to 20th	Day	53% 8% 14% 14% 9% 1	1600
	21st to end	Day	61% 8% 12% 14% 5%	1600
December	1st to 10th	Day	68% 6% 13% 9% 4	2600
	11th to 20th	Day	61% 7% 9% 14% 8%	2986
	21st to end	Day	67% 6% 9% 12% 7%	3960
January	1st to 10th	Night	59% 8% 10% 14% 9%	6349
	11th to 20th	Night	64% 8% 11% 12% 5% 1	6245
	21st to end	Night	60% 6% 10% 11% 12% 1	6490
February	1st to 10th	Night	60% 7% 11% 16% 6% 1	2600
	11th to 20th	Night	57% 6% 10% 18% 9%	2535
	21st to end	Night	52% 7% 12% 20% 8% 1	2077
March	1st to 10th	Night	54% 5% 9% 16% 12% 4	4990
	11th to 20th	Night	46% 8% 18% 18% 9% 1	4980
	21st to end	Night	45% 5% 11% 20% 17% 2	5425
April	1st to 10th	Night	52% 5% 9% 20% 12% 2	2000
	11th to 20th	Night	45% 6% 10% 20% 16% 2	1975
	21st to end	Night	47% 8% 11% 17% 15% 2	1800
May	1st to 10th	Night	52% 6% 11% 13% 13% 4	3876
	11th to 20th	Night	41% 8% 13% 21% 16% 2	3576
	21st to end	Night	44% 6% 12% 22% 14% 2	3927
June	1st to 10th	Night	40% 8% 12% 23% 15% 2	1600
	11th to 20th	Night	36% 7% 13% 22% 20% 2	1600
	21st to end	Night	42% 8% 12% 22% 14% 2	1600
July	1st to 10th	Night	35% 8% 16% 23% 17% 1	3560
	11th to 20th	Night	32% 7% 13% 32% 16% 1	3552
	21st to end	Night	30% 7% 13% 35% 15% 1	3973
August	1st to 10th	Night	34% 6% 14% 27% 17% 1	2904
	11th to 20th	Night	34% 6% 18% 28% 12% 1	2902
	21st to end	Night	31% 8% 12% 34% 13% 1	3444
September	1st to 10th	Night	35% 8% 14% 28% 13% 1	1931
	11th to 20th	Night	29% 6% 14% 32% 17% 1	1928
	21st to end	Night	41% 7% 11% 28% 12% 1	2176
October	1st to 10th	Night	34% 6% 11% 28% 21% 1	3800
	11th to 20th	Night	42% 7% 16% 27% 9%	3779
	21st to end	Night	45% 8% 14% 25% 8%	4133
November	1st to 10th	Night	47% 6% 13% 22% 11% 1	2636
	11th to 20th	Night	52% 7% 13% 17% 9% 2	2800
	21st to end	Night	57% 9% 15% 14% 5%	2800
December	1st to 10th	Night	61% 9% 14% 13% 3	4580
	11th to 20th	Night	62% 8% 11% 12% 7%	5208
	21st to end	Night	69% 6% 9% 10% 6%	6996

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	22% 25% 36% 13% 5% 1	200
	11th to 20th	Day	24% 25% 33% 13% 3 1	225
	21st to end	Day	26% 25% 32% 13% 4	330
February	1st to 10th	Day	31% 26% 31% 10% 3	300
	11th to 20th	Day	28% 29% 35% 8% 1	300
	21st to end	Day	36% 22% 34% 9%	315
March	1st to 10th	Day	23% 23% 36% 14% 3	500
	11th to 20th	Day	27% 22% 31% 16% 3 1	500
	21st to end	Day	34% 15% 32% 13% 6% 1	550
April	1st to 10th	Day	31% 22% 30% 12% 5% 1	500
	11th to 20th	Day	32% 21% 23% 13% 7% 4	492
	21st to end	Day	29% 19% 32% 12% 7% 1	500
May	1st to 10th	Day	25% 15% 32% 18% 6% 5%	503
	11th to 20th	Day	19% 14% 35% 19% 11% 3	527
	21st to end	Day	18% 14% 31% 21% 11% 5%	573
June	1st to 10th	Day	18% 13% 37% 22% 8% 1	530
	11th to 20th	Day	16% 11% 33% 23% 13% 3	530
	21st to end	Day	23% 11% 30% 23% 9% 4	530
July	1st to 10th	Day	10% 9% 32% 32% 12% 5%	525
	11th to 20th	Day	15% 10% 28% 25% 14% 8%	524
	21st to end	Day	10% 9% 35% 31% 12% 4	576
August	1st to 10th	Day	11% 8% 28% 30% 16% 7%	526
	11th to 20th	Day	13% 9% 25% 28% 17% 8%	519
	21st to end	Day	12% 9% 32% 26% 14% 6%	583
September	1st to 10th	Day	14% 11% 28% 23% 15% 8%	502
	11th to 20th	Day	11% 9% 26% 21% 20% 13%	490
	21st to end	Day	16% 16% 26% 25% 10% 6%	490
October	1st to 10th	Day	22% 16% 28% 16% 12% 6%	490
	11th to 20th	Day	22% 23% 32% 16% 6% 2	490
	21st to end	Day	29% 22% 28% 16% 5%	536
November	1st to 10th	Day	27% 17% 29% 19% 7%	295
	11th to 20th	Day	23% 28% 34% 12% 3 1	200
	21st to end	Day	30% 18% 36% 15% 2	200
December	1st to 10th	Day	28% 26% 31% 13% 3	200
	11th to 20th	Day	28% 29% 28% 13% 3 1	200
	21st to end	Day	33% 19% 36% 9% 2	220
January	1st to 10th	Night	25% 25% 29% 15% 5%	800
	11th to 20th	Night	26% 19% 30% 19% 6%	800
	21st to end	Night	21% 17% 34% 22% 5% 1	880
February	1st to 10th	Night	22% 22% 37% 17% 2	800
	11th to 20th	Night	21% 22% 36% 18% 3	783
	21st to end	Night	22% 23% 33% 18% 3	660
March	1st to 10th	Night	22% 20% 36% 19% 4	800
	11th to 20th	Night	21% 20% 36% 18% 4 1	800
	21st to end	Night	22% 15% 34% 21% 8%	880
April	1st to 10th	Night	24% 18% 33% 20% 5%	800
	11th to 20th	Night	23% 16% 31% 21% 8% 1	791
	21st to end	Night	22% 12% 31% 27% 9% 1	600
May	1st to 10th	Night	22% 8% 29% 28% 11% 2	520
	11th to 20th	Night	16% 9% 32% 33% 10%	394
	21st to end	Night	14% 8% 31% 34% 13% 1	433
June	1st to 10th	Night	16% 7% 35% 29% 12% 1	400
	11th to 20th	Night	11% 10% 30% 36% 13% 1	400
	21st to end	Night	13% 7% 31% 35% 14%	400
July	1st to 10th	Night	6% 6% 30% 42% 16%	390
	11th to 20th	Night	9% 4 31% 34% 21% 1	388
	21st to end	Night	10% 2 32% 39% 16% 1	504
August	1st to 10th	Night	11% 4 23% 35% 25% 2	597
	11th to 20th	Night	12% 5% 20% 36% 25% 3	594
	21st to end	Night	11% 5% 29% 32% 21% 2	790
September	1st to 10th	Night	12% 8% 28% 30% 19% 3	773
	11th to 20th	Night	12% 5% 25% 29% 24% 5%	760
	21st to end	Night	19% 9% 22% 28% 17% 3	760
October	1st to 10th	Night	19% 11% 30% 23% 15% 2	760
	11th to 20th	Night	24% 16% 29% 21% 9% 1	760
	21st to end	Night	27% 17% 30% 21% 5%	836
November	1st to 10th	Night	24% 16% 28% 24% 7% 1	780
	11th to 20th	Night	22% 21% 32% 19% 6% 1	800
	21st to end	Night	26% 20% 30% 19% 5%	800
December	1st to 10th	Night	28% 23% 30% 15% 4	800
	11th to 20th	Night	24% 22% 33% 17% 4 1	800
	21st to end	Night	30% 19% 31% 16% 3	880

Page generated at 2025-09-01 01:51:02. Version ac07

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