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 2024-02-15.

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	17% 8% 3 6% 5% 3 4 5% 5% 7% 38%	78171
	Night	17% 8% 3 6% 5% 3 4 4 5% 6% 39%	80893
Transparency	Day	46% 6% 11% 22% 14% 2	77876
	Night	45% 6% 12% 22% 14% 2	80722
Seeing	Day	25% 18% 30% 18% 7% 2	13929
	Night	22% 16% 33% 22% 6% 1	23053

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	14% 9% 3 6% 5% 3 4 4 5% 6% 41%	2517
	11th to 20th	Day	17% 9% 4 8% 6% 3 5% 4 4 6% 34%	2520
	21st to end	Day	19% 13% 5% 6% 5% 3 4 4 5% 5% 31%	2772
February	1st to 10th	Day	17% 8% 3 6% 6% 4 6% 5% 5% 7% 33%	1757
	11th to 20th	Day	14% 7% 4 7% 7% 3 5% 4 5% 7% 36%	1788
	21st to end	Day	19% 10% 4 6% 6% 2 4 4 4 4 35%	1480
March	1st to 10th	Day	14% 9% 4 7% 6% 3 3 5% 5% 6% 38%	1797
	11th to 20th	Day	16% 9% 4 5% 5% 3 5% 5% 5% 6% 36%	1858
	21st to end	Day	19% 8% 3 6% 6% 3 3 4 4 5% 39%	2348
April	1st to 10th	Day	17% 10% 4 6% 5% 3 4 4 5% 6% 37%	2160
	11th to 20th	Day	16% 8% 3 5% 5% 3 5% 5% 5% 6% 39%	2134
	21st to end	Day	21% 7% 3 7% 6% 3 4 4 5% 7% 34%	2312
May	1st to 10th	Day	24% 8% 3 6% 5% 3 5% 5% 4 6% 30%	2340
	11th to 20th	Day	19% 8% 2 6% 5% 2 3 4 5% 6% 41%	2400
	21st to end	Day	15% 6% 3 5% 5% 3 5% 5% 5% 9% 40%	2725
June	1st to 10th	Day	18% 6% 3 5% 6% 3 6% 6% 6% 8% 34%	2520
	11th to 20th	Day	17% 6% 3 6% 5% 3 5% 5% 6% 8% 36%	2520
	21st to end	Day	17% 7% 3 7% 6% 3 5% 5% 6% 9% 33%	2520
July	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 5% 7% 7% 39%	2520
	11th to 20th	Day	12% 5% 2 6% 5% 4 6% 6% 6% 10% 39%	2520
	21st to end	Day	13% 5% 3 5% 5% 4 5% 6% 7% 8% 38%	2744
August	1st to 10th	Day	10% 6% 2 6% 5% 4 6% 6% 7% 9% 38%	2409
	11th to 20th	Day	13% 6% 2 5% 6% 4 6% 5% 6% 8% 38%	2222
	21st to end	Day	11% 6% 2 5% 5% 3 4 5% 7% 9% 43%	2376
September	1st to 10th	Day	13% 6% 3 5% 5% 4 5% 5% 6% 7% 43%	2160
	11th to 20th	Day	15% 5% 2 6% 4 2 3 4 4 5% 50%	2012
	21st to end	Day	17% 5% 2 4 4 3 4 4 6% 6% 44%	1980
October	1st to 10th	Day	22% 9% 3 6% 5% 2 4 4 4 4 37%	1980
	11th to 20th	Day	14% 7% 2 6% 5% 3 4 4 5% 6% 44%	1742
	21st to end	Day	22% 9% 3 5% 5% 3 3 3 5% 5% 37%	1782
November	1st to 10th	Day	13% 8% 3 6% 4 2 4 3 4 5% 48%	1671
	11th to 20th	Day	19% 8% 2 4 4 2 4 3 4 6% 43%	1710
	21st to end	Day	17% 7% 3 5% 5% 3 4 5% 5% 6% 40%	1677
December	1st to 10th	Day	20% 7% 4 5% 6% 3 5% 4 5% 6% 34%	1520
	11th to 20th	Day	22% 8% 3 4 5% 3 4 3 4 6% 40%	1906
	21st to end	Day	21% 12% 3 6% 6% 3 4 5% 4 6% 29%	2772
January	1st to 10th	Night	16% 8% 3 5% 4 3 3 4 5% 6% 44%	4429
	11th to 20th	Night	15% 10% 3 8% 7% 4 4 4 5% 6% 34%	4440
	21st to end	Night	21% 13% 5% 7% 5% 3 4 4 4 5% 29%	4884
February	1st to 10th	Night	15% 9% 4 6% 5% 4 5% 6% 5% 5% 37%	2417
	11th to 20th	Night	13% 8% 4 7% 6% 3 5% 5% 6% 7% 37%	2153
	21st to end	Night	17% 10% 3 6% 6% 3 5% 4 6% 5% 36%	1776
March	1st to 10th	Night	15% 9% 3 5% 5% 4 4 4 5% 6% 39%	2150
	11th to 20th	Night	15% 8% 4 6% 6% 4 5% 4 4 6% 38%	2100
	21st to end	Night	17% 9% 3 5% 5% 3 4 4 5% 6% 39%	2003
April	1st to 10th	Night	19% 6% 4 6% 6% 3 5% 5% 5% 6% 36%	1800
	11th to 20th	Night	20% 6% 2 5% 5% 2 5% 4 4 6% 40%	1775
	21st to end	Night	22% 7% 3 5% 5% 3 4 4 4 7% 37%	1646
May	1st to 10th	Night	22% 9% 3 6% 6% 3 5% 5% 5% 6% 30%	1620
	11th to 20th	Night	17% 7% 3 7% 5% 3 4 4 6% 7% 37%	1507
	21st to end	Night	16% 8% 2 5% 6% 3 4 4 5% 8% 39%	1550
June	1st to 10th	Night	19% 6% 3 6% 7% 3 5% 5% 4 4 38%	1440
	11th to 20th	Night	19% 7% 3 5% 6% 3 4 3 5% 8% 37%	1440
	21st to end	Night	16% 5% 2 4 4 3 4 5% 5% 10% 42%	1440
July	1st to 10th	Night	14% 5% 2 5% 6% 3 5% 5% 5% 7% 42%	1440
	11th to 20th	Night	15% 6% 2 5% 5% 4 4 5% 7% 11% 37%	1440
	21st to end	Night	11% 4 2 5% 5% 3 5% 5% 7% 8% 44%	1574
August	1st to 10th	Night	9% 5% 2 6% 6% 3 5% 6% 6% 8% 45%	1509
	11th to 20th	Night	13% 5% 3 7% 7% 4 6% 6% 6% 8% 36%	1676
	21st to end	Night	13% 4 2 5% 5% 3 4 5% 5% 8% 47%	1980
September	1st to 10th	Night	14% 7% 3 4 3 3 4 4 5% 7% 46%	1800
	11th to 20th	Night	13% 6% 2 4 4 2 3 4 3 7% 52%	1948
	21st to end	Night	16% 6% 3 4 5% 2 3 4 5% 6% 46%	1980
October	1st to 10th	Night	21% 6% 2 6% 5% 3 4 4 5% 4 40%	1980
	11th to 20th	Night	16% 6% 2 4 5% 2 4 5% 6% 6% 43%	2215
	21st to end	Night	21% 9% 3 5% 5% 3 3 3 4 6% 37%	2574
November	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 4 6% 46%	2407
	11th to 20th	Night	18% 6% 2 5% 5% 3 5% 3 5% 6% 42%	2470
	21st to end	Night	15% 7% 4 6% 5% 3 4 4 5% 6% 42%	2498
December	1st to 10th	Night	20% 8% 3 6% 5% 3 4 4 5% 6% 36%	2660
	11th to 20th	Night	20% 9% 2 5% 6% 3 4 5% 5% 5% 37%	3288
	21st to end	Night	21% 11% 5% 6% 5% 3 4 4 4 5% 31%	4884

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	43% 4 9% 33% 9% 1	2518
	11th to 20th	Day	51% 5% 11% 23% 9% 1	2520
	21st to end	Day	55% 5% 10% 19% 10% 1	2772
February	1st to 10th	Day	50% 6% 12% 18% 12% 2	1757
	11th to 20th	Day	49% 5% 9% 20% 14% 3	1797
	21st to end	Day	54% 4 8% 19% 13% 2	1480
March	1st to 10th	Day	46% 4 13% 20% 13% 4	1800
	11th to 20th	Day	47% 5% 10% 19% 14% 4	1858
	21st to end	Day	49% 4 8% 16% 18% 5%	2348
April	1st to 10th	Day	49% 5% 9% 21% 14% 3	2160
	11th to 20th	Day	45% 5% 11% 19% 17% 3	2134
	21st to end	Day	51% 5% 8% 18% 15% 3	2312
May	1st to 10th	Day	55% 4 8% 19% 13% 1	2340
	11th to 20th	Day	44% 5% 9% 22% 17% 2	2404
	21st to end	Day	42% 7% 13% 23% 13% 2	2725
June	1st to 10th	Day	47% 8% 12% 22% 10% 1	2520
	11th to 20th	Day	45% 6% 13% 19% 14% 1	2520
	21st to end	Day	46% 8% 12% 21% 13% 1	2520
July	1st to 10th	Day	41% 8% 10% 24% 15% 2	2450
	11th to 20th	Day	41% 7% 14% 26% 12% 1	2445
	21st to end	Day	41% 6% 15% 21% 14% 2	2591
August	1st to 10th	Day	40% 8% 15% 23% 12% 1	2395
	11th to 20th	Day	41% 8% 14% 24% 12% 2	2222
	21st to end	Day	36% 8% 11% 27% 16% 2	2376
September	1st to 10th	Day	40% 6% 12% 24% 16% 1	2160
	11th to 20th	Day	37% 4 8% 25% 23% 3	2012
	21st to end	Day	39% 5% 10% 23% 20% 3	1980
October	1st to 10th	Day	50% 5% 9% 20% 13% 4	1980
	11th to 20th	Day	39% 5% 13% 27% 14% 2	1742
	21st to end	Day	50% 4 12% 19% 12% 3	1782
November	1st to 10th	Day	41% 4 12% 25% 15% 2	1671
	11th to 20th	Day	43% 4 11% 22% 18% 3	1710
	21st to end	Day	44% 5% 14% 24% 11% 2	1677
December	1st to 10th	Day	51% 4 14% 16% 14% 1	1520
	11th to 20th	Day	47% 3 14% 18% 17% 1	1906
	21st to end	Day	55% 6% 10% 18% 10% 1	2772
January	1st to 10th	Night	41% 5% 12% 28% 12% 3	4429
	11th to 20th	Night	50% 4 13% 20% 10% 2	4440
	21st to end	Night	57% 5% 9% 17% 10% 2	4884
February	1st to 10th	Night	47% 7% 13% 20% 12% 2	2417
	11th to 20th	Night	45% 6% 11% 20% 15% 3	2155
	21st to end	Night	49% 4 10% 21% 12% 3	1776
March	1st to 10th	Night	44% 5% 11% 23% 14% 3	2160
	11th to 20th	Night	47% 6% 10% 19% 15% 3	2101
	21st to end	Night	46% 5% 9% 17% 18% 5%	2003
April	1st to 10th	Night	48% 6% 10% 19% 15% 2	1800
	11th to 20th	Night	46% 6% 9% 20% 17% 1	1775
	21st to end	Night	48% 5% 9% 20% 13% 5%	1646
May	1st to 10th	Night	53% 6% 8% 19% 11% 2	1620
	11th to 20th	Night	47% 5% 12% 18% 15% 3	1508
	21st to end	Night	44% 7% 12% 23% 12% 1	1551
June	1st to 10th	Night	48% 6% 10% 23% 12% 1	1440
	11th to 20th	Night	47% 7% 12% 22% 12% 1	1440
	21st to end	Night	38% 9% 15% 23% 15% 1	1440
July	1st to 10th	Night	38% 10% 13% 23% 15% 1	1400
	11th to 20th	Night	41% 8% 15% 26% 9%	1392
	21st to end	Night	36% 8% 15% 26% 14% 2	1486
August	1st to 10th	Night	35% 10% 13% 25% 15% 1	1499
	11th to 20th	Night	42% 8% 14% 23% 12%	1676
	21st to end	Night	35% 7% 11% 27% 18% 1	1980
September	1st to 10th	Night	39% 5% 13% 25% 17% 1	1800
	11th to 20th	Night	35% 5% 10% 24% 25% 2	1948
	21st to end	Night	39% 6% 8% 26% 18% 3	1980
October	1st to 10th	Night	47% 6% 10% 23% 11% 3	1980
	11th to 20th	Night	39% 6% 14% 27% 13% 1	2215
	21st to end	Night	50% 4 12% 21% 12% 2	2574
November	1st to 10th	Night	39% 6% 11% 21% 20% 3	2407
	11th to 20th	Night	44% 4 12% 20% 17% 3	2470
	21st to end	Night	42% 5% 16% 24% 12% 1	2498
December	1st to 10th	Night	49% 5% 13% 20% 12% 1	2660
	11th to 20th	Night	48% 5% 15% 16% 15% 2	3288
	21st to end	Night	53% 5% 9% 22% 10% 1	4884

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	30% 25% 27% 16% 2	180
	11th to 20th	Day	34% 26% 29% 7% 3 1	180
	21st to end	Day	23% 32% 33% 10% 2	198
February	1st to 10th	Day	34% 30% 25% 10% 1	332
	11th to 20th	Day	32% 29% 30% 9% 1	432
	21st to end	Day	33% 25% 30% 10% 1	346
March	1st to 10th	Day	30% 23% 36% 9% 2	420
	11th to 20th	Day	31% 30% 25% 10% 3	420
	21st to end	Day	31% 24% 28% 10% 5% 2	462
April	1st to 10th	Day	32% 23% 28% 10% 6%	420
	11th to 20th	Day	26% 25% 30% 11% 6% 1	412
	21st to end	Day	31% 20% 31% 13% 4 2	420
May	1st to 10th	Day	32% 22% 27% 13% 4 1	423
	11th to 20th	Day	24% 18% 38% 14% 6% 1	447
	21st to end	Day	21% 14% 32% 22% 10% 1	485
June	1st to 10th	Day	20% 16% 32% 21% 9% 2	450
	11th to 20th	Day	20% 15% 34% 22% 7% 2	450
	21st to end	Day	18% 16% 34% 19% 11% 3	450
July	1st to 10th	Day	17% 9% 30% 29% 11% 3	445
	11th to 20th	Day	16% 9% 30% 30% 13% 2	444
	21st to end	Day	16% 9% 35% 27% 10% 2	488
August	1st to 10th	Day	13% 9% 30% 31% 14% 3	446
	11th to 20th	Day	16% 10% 31% 26% 14% 3	439
	21st to end	Day	14% 14% 26% 28% 14% 4	495
September	1st to 10th	Day	20% 10% 29% 22% 14% 5%	450
	11th to 20th	Day	20% 12% 28% 20% 13% 7%	450
	21st to end	Day	19% 15% 31% 19% 12% 4	450
October	1st to 10th	Day	29% 16% 22% 20% 10% 4	450
	11th to 20th	Day	21% 19% 31% 21% 6% 2	419
	21st to end	Day	31% 17% 29% 17% 6% 1	396
November	1st to 10th	Day	24% 16% 31% 18% 9% 1	365
	11th to 20th	Day	31% 23% 26% 16% 4 1	370
	21st to end	Day	29% 20% 32% 15% 4	337
December	1st to 10th	Day	29% 29% 24% 14% 4	180
	11th to 20th	Day	39% 22% 29% 8% 1	180
	21st to end	Day	32% 27% 28% 8% 4 1	198
January	1st to 10th	Night	20% 25% 39% 13% 2	760
	11th to 20th	Night	30% 22% 32% 14% 2	760
	21st to end	Night	21% 25% 34% 17% 2	836
February	1st to 10th	Night	24% 20% 40% 15% 1	760
	11th to 20th	Night	24% 24% 39% 12% 1	722
	21st to end	Night	28% 24% 33% 12% 2	592
March	1st to 10th	Night	25% 22% 36% 14% 2	720
	11th to 20th	Night	23% 26% 34% 15% 2	720
	21st to end	Night	28% 19% 32% 17% 5%	792
April	1st to 10th	Night	27% 20% 34% 17% 2	720
	11th to 20th	Night	25% 19% 32% 20% 4	711
	21st to end	Night	27% 19% 32% 20% 3	566
May	1st to 10th	Night	30% 13% 35% 18% 4 1	540
	11th to 20th	Night	19% 16% 39% 22% 4	446
	21st to end	Night	20% 8% 37% 28% 7% 1	389
June	1st to 10th	Night	22% 10% 31% 29% 9%	360
	11th to 20th	Night	21% 8% 35% 29% 6%	360
	21st to end	Night	17% 5% 33% 38% 8%	360
July	1st to 10th	Night	16% 4 29% 39% 12%	350
	11th to 20th	Night	18% 2 26% 38% 14% 1	348
	21st to end	Night	10% 5% 32% 40% 12% 1	394
August	1st to 10th	Night	11% 5% 29% 41% 13% 1	435
	11th to 20th	Night	14% 6% 26% 38% 14% 2	612
	21st to end	Night	15% 6% 26% 37% 14% 2	792
September	1st to 10th	Night	19% 6% 30% 32% 12% 1	720
	11th to 20th	Night	16% 8% 29% 28% 16% 2	720
	21st to end	Night	19% 9% 30% 29% 11% 3	720
October	1st to 10th	Night	25% 11% 27% 24% 10% 3	720
	11th to 20th	Night	20% 14% 33% 24% 9% 1	720
	21st to end	Night	28% 17% 31% 19% 5%	792
November	1st to 10th	Night	20% 18% 33% 21% 7% 1	740
	11th to 20th	Night	24% 22% 31% 17% 6% 1	760
	21st to end	Night	22% 20% 38% 16% 4	760
December	1st to 10th	Night	29% 21% 33% 14% 4	760
	11th to 20th	Night	26% 23% 34% 16% 3	760
	21st to end	Night	28% 23% 34% 13% 2	836

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