Petersham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Petersham clear sky chart from 2005-11-06 to 2025-03-12.

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	16% 8% 3 7% 4 4 4 4 5% 7% 37%	108545
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 38%	110456
Transparency	Day	45% 5% 11% 22% 15% 2	112113
	Night	45% 5% 12% 22% 15% 2	115052
Seeing	Day	25% 18% 30% 18% 7% 2	15315
	Night	23% 16% 33% 22% 6% 1	24419

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	12% 8% 3 12% 6% 5% 5% 4 5% 5% 35%	3597
	11th to 20th	Day	13% 10% 4 7% 4 4 5% 3 5% 6% 40%	3570
	21st to end	Day	15% 11% 4 7% 4 4 4 3 4 5% 39%	3960
February	1st to 10th	Day	16% 9% 3 7% 6% 5% 5% 4 5% 7% 33%	1850
	11th to 20th	Day	13% 8% 3 9% 7% 4 5% 3 6% 6% 35%	1948
	21st to end	Day	18% 11% 3 7% 5% 4 4 4 3 5% 36%	1650
March	1st to 10th	Day	20% 15% 4 9% 4 4 2 3 3 4 30%	3216
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 4 5% 7% 37%	3284
	21st to end	Day	20% 9% 2 7% 5% 4 4 3 5% 6% 34%	4124
April	1st to 10th	Day	17% 12% 3 7% 5% 4 4 3 5% 6% 35%	2280
	11th to 20th	Day	16% 9% 2 7% 4 5% 4 4 5% 6% 38%	2254
	21st to end	Day	20% 8% 3 8% 5% 4 3 4 5% 6% 34%	2441
May	1st to 10th	Day	24% 10% 3 8% 5% 4 4 3 5% 6% 27%	4106
	11th to 20th	Day	22% 9% 3 8% 4 3 3 3 5% 6% 34%	4264
	21st to end	Day	14% 6% 2 5% 3 4 4 4 6% 9% 42%	4837
June	1st to 10th	Day	17% 7% 3 5% 5% 5% 5% 5% 6% 8% 34%	2660
	11th to 20th	Day	16% 6% 3 6% 5% 4 5% 4 7% 8% 36%	2660
	21st to end	Day	17% 7% 3 7% 6% 4 4 4 7% 9% 32%	2660
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	13% 5% 2 5% 5% 5% 6% 5% 7% 10% 37%	4440
	21st to end	Day	19% 8% 3 6% 4 5% 4 4 6% 7% 34%	4814
August	1st to 10th	Day	16% 7% 2 7% 4 5% 6% 4 7% 8% 33%	4243
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	3893
	21st to end	Day	8% 5% 2 5% 4 5% 5% 5% 7% 10% 44%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 6% 7% 44%	2280
	11th to 20th	Day	15% 6% 2 6% 3 3 4 3 3 6% 50%	2123
	21st to end	Day	17% 6% 2 5% 4 4 4 3 6% 6% 42%	2090
October	1st to 10th	Day	21% 9% 2 7% 4 3 3 3 4 5% 38%	2090
	11th to 20th	Day	14% 7% 2 7% 3 3 4 3 5% 6% 45%	1838
	21st to end	Day	21% 9% 2 6% 4 3 4 2 5% 5% 38%	1881
November	1st to 10th	Day	12% 8% 3 7% 4 3 4 2 5% 5% 48%	1761
	11th to 20th	Day	18% 9% 2 5% 4 3 3 3 4 6% 45%	1800
	21st to end	Day	18% 8% 2 5% 5% 3 5% 4 5% 5% 40%	1765
December	1st to 10th	Day	16% 8% 3 6% 5% 3 4 3 6% 6% 41%	2600
	11th to 20th	Day	20% 8% 3 5% 4 3 4 2 4 5% 41%	2986
	21st to end	Day	19% 11% 3 7% 5% 4 3 3 4 6% 35%	3960
January	1st to 10th	Night	13% 7% 3 9% 4 4 4 3 5% 7% 40%	6349
	11th to 20th	Night	13% 10% 3 9% 5% 5% 5% 3 5% 6% 35%	6315
	21st to end	Night	15% 10% 3 7% 5% 4 5% 3 5% 6% 37%	6996
February	1st to 10th	Night	15% 9% 4 7% 4 4 5% 5% 6% 5% 36%	2543
	11th to 20th	Night	13% 8% 3 8% 6% 4 5% 4 6% 6% 36%	2338
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 6% 5% 37%	1980
March	1st to 10th	Night	24% 11% 2 6% 5% 4 4 3 5% 5% 31%	3844
	11th to 20th	Night	12% 8% 3 7% 5% 6% 5% 4 5% 7% 38%	3703
	21st to end	Night	22% 10% 3 7% 4 4 4 3 6% 5% 33%	3503
April	1st to 10th	Night	18% 7% 4 7% 6% 4 4 4 5% 6% 35%	1900
	11th to 20th	Night	20% 8% 2 6% 5% 3 4 4 4 5% 39%	1875
	21st to end	Night	21% 7% 3 6% 4 4 3 3 4 6% 38%	1737
May	1st to 10th	Night	23% 10% 3 7% 4 4 4 4 6% 7% 27%	2834
	11th to 20th	Night	17% 11% 4 9% 5% 6% 5% 4 6% 7% 27%	2657
	21st to end	Night	16% 7% 2 6% 4 4 3 3 6% 9% 40%	2738
June	1st to 10th	Night	18% 8% 2 7% 6% 4 5% 4 5% 4 38%	1520
	11th to 20th	Night	18% 8% 2 6% 6% 4 3 3 6% 8% 37%	1520
	21st to end	Night	16% 6% 1 5% 4 4 5% 4 5% 9% 40%	1520
July	1st to 10th	Night	13% 6% 2 7% 6% 5% 5% 5% 6% 9% 37%	2520
	11th to 20th	Night	16% 7% 2 5% 4 4 3 4 6% 9% 39%	2520
	21st to end	Night	16% 8% 3 7% 4 4 4 3 6% 6% 38%	2739
August	1st to 10th	Night	22% 7% 2 7% 4 4 5% 4 5% 7% 33%	2653
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 3 6% 7% 37%	2932
	21st to end	Night	8% 3 2 4 3 3 4 3 5% 8% 57%	3476
September	1st to 10th	Night	14% 7% 3 4 3 4 4 3 5% 6% 47%	1900
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 3 7% 52%	2057
	21st to end	Night	16% 7% 3 5% 4 3 3 3 6% 6% 44%	2090
October	1st to 10th	Night	20% 6% 2 7% 5% 4 4 3 5% 5% 40%	2090
	11th to 20th	Night	16% 6% 2 5% 4 3 4 4 5% 6% 45%	2339
	21st to end	Night	20% 9% 3 5% 4 4 3 2 4 6% 38%	2717
November	1st to 10th	Night	14% 6% 2 6% 3 4 3 3 5% 6% 47%	2537
	11th to 20th	Night	17% 6% 2 6% 5% 4 4 3 4 6% 43%	2600
	21st to end	Night	15% 8% 3 7% 4 4 3 3 6% 6% 42%	2630
December	1st to 10th	Night	19% 9% 3 6% 4 4 3 3 5% 6% 39%	4580
	11th to 20th	Night	24% 9% 2 5% 4 3 3 3 4 4 38%	5208
	21st to end	Night	19% 11% 4 7% 4 3 3 2 4 5% 37%	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	50% 3 7% 26% 12% 2	3598
	11th to 20th	Day	45% 3 11% 20% 17% 4	3600
	21st to end	Day	48% 3 8% 20% 18% 2	3960
February	1st to 10th	Day	50% 6% 12% 17% 13% 2	1850
	11th to 20th	Day	49% 5% 9% 20% 14% 3	1957
	21st to end	Day	52% 4 9% 20% 14% 2	1650
March	1st to 10th	Day	58% 4 11% 13% 11% 3	3219
	11th to 20th	Day	43% 7% 16% 20% 11% 3	3284
	21st to end	Day	51% 3 7% 16% 18% 4	4124
April	1st to 10th	Day	50% 5% 8% 20% 14% 3	2280
	11th to 20th	Day	46% 5% 11% 18% 17% 2	2254
	21st to end	Day	50% 5% 8% 18% 15% 3	2441
May	1st to 10th	Day	56% 7% 11% 15% 10% 1	4120
	11th to 20th	Day	50% 7% 13% 19% 9% 1	4268
	21st to end	Day	37% 7% 12% 26% 17% 1	4837
June	1st to 10th	Day	46% 8% 12% 23% 11% 1	2660
	11th to 20th	Day	44% 6% 14% 20% 14% 1	2660
	21st to end	Day	46% 8% 12% 20% 13% 1	2660
July	1st to 10th	Day	43% 6% 12% 22% 16% 1	4370
	11th to 20th	Day	41% 5% 16% 25% 12% 1	4365
	21st to end	Day	49% 4 14% 19% 14% 1	4661
August	1st to 10th	Day	48% 5% 16% 21% 9%	4229
	11th to 20th	Day	44% 5% 16% 27% 8% 1	3932
	21st to end	Day	36% 5% 12% 31% 16% 1	4180
September	1st to 10th	Day	39% 6% 11% 25% 18% 1	2280
	11th to 20th	Day	37% 4 8% 26% 23% 3	2123
	21st to end	Day	41% 5% 10% 22% 20% 2	2090
October	1st to 10th	Day	44% 3 8% 23% 19% 2	3480
	11th to 20th	Day	35% 3 8% 26% 25% 3	3028
	21st to end	Day	36% 3 11% 30% 17% 3	3081
November	1st to 10th	Day	40% 4 13% 25% 16% 2	1761
	11th to 20th	Day	42% 4 10% 21% 21% 3	1800
	21st to end	Day	45% 5% 13% 24% 11% 2	1765
December	1st to 10th	Day	45% 3 15% 28% 9% 1	2600
	11th to 20th	Day	47% 2 13% 17% 19% 1	2986
	21st to end	Day	51% 4 7% 18% 15% 5%	3960
January	1st to 10th	Night	44% 3 10% 28% 13% 2	6349
	11th to 20th	Night	49% 3 13% 18% 13% 3	6360
	21st to end	Night	49% 4 10% 16% 17% 4	6996
February	1st to 10th	Night	47% 6% 13% 20% 12% 2	2543
	11th to 20th	Night	46% 6% 10% 20% 15% 3	2340
	21st to end	Night	49% 4 9% 22% 12% 4	1980
March	1st to 10th	Night	53% 6% 10% 19% 10% 2	3854
	11th to 20th	Night	43% 7% 14% 20% 13% 2	3704
	21st to end	Night	50% 4 8% 13% 19% 6%	3503
April	1st to 10th	Night	49% 6% 10% 19% 14% 2	1900
	11th to 20th	Night	47% 6% 9% 20% 18% 1	1875
	21st to end	Night	47% 5% 9% 20% 14% 5%	1737
May	1st to 10th	Night	54% 9% 11% 17% 8% 1	2840
	11th to 20th	Night	55% 8% 13% 14% 8% 2	2658
	21st to end	Night	40% 7% 14% 23% 14% 1	2739
June	1st to 10th	Night	47% 7% 10% 23% 12% 1	1520
	11th to 20th	Night	45% 7% 13% 23% 12% 1	1520
	21st to end	Night	40% 9% 15% 22% 14% 1	1520
July	1st to 10th	Night	43% 7% 13% 22% 15%	2480
	11th to 20th	Night	43% 5% 14% 23% 13%	2472
	21st to end	Night	47% 5% 11% 20% 16% 1	2651
August	1st to 10th	Night	51% 7% 11% 21% 9% 1	2643
	11th to 20th	Night	45% 5% 15% 25% 9%	2959
	21st to end	Night	27% 4 16% 33% 19% 1	3476
September	1st to 10th	Night	38% 5% 12% 25% 18% 1	1900
	11th to 20th	Night	35% 5% 10% 24% 25% 2	2057
	21st to end	Night	41% 5% 8% 25% 18% 3	2090
October	1st to 10th	Night	41% 3 14% 24% 16% 2	3480
	11th to 20th	Night	30% 3 11% 26% 26% 4	3888
	21st to end	Night	36% 2 13% 32% 15% 1	4467
November	1st to 10th	Night	38% 6% 11% 22% 20% 3	2537
	11th to 20th	Night	43% 4 11% 20% 19% 3	2600
	21st to end	Night	43% 5% 16% 24% 12% 1	2630
December	1st to 10th	Night	47% 3 11% 29% 10%	4580
	11th to 20th	Night	50% 3 14% 13% 19% 1	5208
	21st to end	Night	50% 3 8% 20% 15% 4	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	31% 26% 28% 15% 2	200
	11th to 20th	Day	35% 26% 30% 7% 3 1	200
	21st to end	Day	25% 32% 31% 10% 2	220
February	1st to 10th	Day	34% 31% 25% 9% 1	365
	11th to 20th	Day	34% 29% 27% 8% 1	496
	21st to end	Day	31% 26% 31% 10% 2	414
March	1st to 10th	Day	32% 24% 34% 9% 2	500
	11th to 20th	Day	32% 28% 26% 10% 3	464
	21st to end	Day	32% 25% 28% 9% 4 2	506
April	1st to 10th	Day	33% 23% 28% 10% 6%	460
	11th to 20th	Day	28% 25% 29% 11% 6% 1	452
	21st to end	Day	30% 20% 31% 13% 4 2	460
May	1st to 10th	Day	32% 22% 28% 13% 4 1	463
	11th to 20th	Day	24% 17% 37% 15% 6% 1	487
	21st to end	Day	20% 14% 33% 22% 9% 1	529
June	1st to 10th	Day	19% 16% 32% 21% 9% 2	490
	11th to 20th	Day	19% 15% 34% 23% 8% 2	490
	21st to end	Day	18% 17% 35% 18% 10% 2	490
July	1st to 10th	Day	17% 9% 30% 29% 11% 3	485
	11th to 20th	Day	14% 9% 31% 31% 12% 2	484
	21st to end	Day	18% 9% 35% 27% 10% 2	532
August	1st to 10th	Day	14% 8% 31% 30% 13% 3	486
	11th to 20th	Day	17% 10% 30% 25% 14% 3	479
	21st to end	Day	13% 13% 27% 28% 13% 5%	539
September	1st to 10th	Day	19% 10% 29% 22% 13% 6%	490
	11th to 20th	Day	18% 11% 27% 21% 14% 9%	490
	21st to end	Day	20% 14% 31% 19% 11% 5%	490
October	1st to 10th	Day	28% 16% 22% 20% 10% 4	490
	11th to 20th	Day	22% 18% 30% 21% 6% 2	455
	21st to end	Day	30% 17% 28% 17% 6% 1	429
November	1st to 10th	Day	24% 17% 32% 17% 9% 1	395
	11th to 20th	Day	30% 23% 27% 15% 3 1	400
	21st to end	Day	31% 21% 31% 14% 4	365
December	1st to 10th	Day	28% 29% 26% 15% 4	200
	11th to 20th	Day	42% 22% 28% 8% 2	200
	21st to end	Day	31% 25% 30% 10% 4	220
January	1st to 10th	Night	21% 26% 39% 13% 2	800
	11th to 20th	Night	30% 21% 32% 14% 3	800
	21st to end	Night	21% 24% 35% 17% 3	880
February	1st to 10th	Night	24% 21% 40% 15% 1	800
	11th to 20th	Night	26% 23% 38% 12% 1	783
	21st to end	Night	27% 23% 34% 12% 3	660
March	1st to 10th	Night	27% 22% 35% 14% 2	800
	11th to 20th	Night	23% 26% 34% 15% 2	767
	21st to end	Night	28% 19% 32% 16% 5%	836
April	1st to 10th	Night	26% 20% 34% 17% 2	760
	11th to 20th	Night	26% 19% 33% 19% 3	751
	21st to end	Night	26% 20% 31% 20% 3	597
May	1st to 10th	Night	30% 14% 34% 18% 4 1	570
	11th to 20th	Night	19% 15% 39% 22% 4	471
	21st to end	Night	19% 9% 36% 28% 7% 1	411
June	1st to 10th	Night	21% 11% 30% 30% 8%	380
	11th to 20th	Night	20% 8% 36% 29% 6% 1	380
	21st to end	Night	18% 6% 32% 36% 8%	380
July	1st to 10th	Night	16% 4 30% 39% 12%	370
	11th to 20th	Night	17% 2 27% 38% 14% 1	368
	21st to end	Night	12% 5% 31% 39% 12%	416
August	1st to 10th	Night	13% 5% 28% 40% 13% 1	460
	11th to 20th	Night	15% 6% 26% 37% 14% 2	647
	21st to end	Night	15% 5% 26% 38% 14% 2	836
September	1st to 10th	Night	19% 6% 31% 31% 12% 1	760
	11th to 20th	Night	15% 8% 28% 29% 18% 3	760
	21st to end	Night	19% 9% 30% 29% 11% 2	760
October	1st to 10th	Night	24% 11% 26% 26% 10% 3	760
	11th to 20th	Night	20% 14% 33% 23% 9% 1	760
	21st to end	Night	27% 17% 31% 19% 6%	836
November	1st to 10th	Night	20% 19% 33% 20% 7% 1	780
	11th to 20th	Night	23% 23% 31% 17% 6% 1	800
	21st to end	Night	23% 20% 38% 16% 4	800
December	1st to 10th	Night	29% 21% 33% 14% 4	800
	11th to 20th	Night	27% 22% 33% 16% 3	800
	21st to end	Night	27% 22% 34% 14% 2	880

Page generated at 2025-03-09 08:08:44. Version ac07

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