Plum Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plum Island clear sky chart from 2005-11-06 to 2024-01-17.

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% 6% 2 5% 5% 3 4 4 5% 7% 41%	74850
	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	74969
Transparency	Day	29% 36% 8% 15% 10% 2	74555
	Night	30% 36% 8% 15% 10% 1	74798
Seeing	Day	20% 8% 28% 29% 12% 2	13936
	Night	21% 9% 31% 27% 10% 1	22916

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	17% 7% 2 5% 5% 3 3 4 5% 6% 43%	1517
	11th to 20th	Day	20% 8% 3 5% 4 3 5% 4 5% 7% 38%	1488
	21st to end	Day	14% 7% 2 6% 5% 3 5% 5% 5% 6% 43%	1584
February	1st to 10th	Day	19% 9% 3 5% 5% 3 5% 5% 5% 7% 35%	1657
	11th to 20th	Day	14% 7% 3 7% 5% 3 4 4 6% 8% 39%	1748
	21st to end	Day	17% 10% 3 5% 5% 3 6% 5% 5% 5% 37%	1480
March	1st to 10th	Day	18% 7% 3 6% 4 3 4 5% 5% 8% 37%	1797
	11th to 20th	Day	17% 6% 3 6% 5% 2 5% 3 5% 7% 40%	1916
	21st to end	Day	18% 9% 2 6% 5% 3 3 3 4 6% 41%	2265
April	1st to 10th	Day	19% 7% 3 5% 5% 4 4 3 4 6% 39%	2160
	11th to 20th	Day	16% 7% 2 4 5% 3 6% 5% 6% 6% 40%	2177
	21st to end	Day	21% 5% 2 5% 6% 4 5% 5% 5% 6% 37%	2340
May	1st to 10th	Day	23% 7% 3 6% 5% 2 4 4 5% 7% 34%	2340
	11th to 20th	Day	17% 7% 3 4 5% 3 5% 5% 5% 6% 41%	2321
	21st to end	Day	15% 5% 2 5% 4 3 4 4 5% 8% 44%	2725
June	1st to 10th	Day	20% 6% 3 5% 5% 3 5% 5% 5% 8% 35%	2520
	11th to 20th	Day	16% 5% 2 5% 5% 3 5% 5% 5% 9% 41%	2520
	21st to end	Day	17% 5% 2 6% 6% 3 5% 4 6% 8% 37%	2520
July	1st to 10th	Day	14% 5% 2 4 4 3 5% 4 6% 9% 43%	2520
	11th to 20th	Day	11% 6% 3 4 6% 3 5% 5% 6% 10% 42%	2520
	21st to end	Day	11% 4 2 5% 6% 3 5% 5% 7% 9% 42%	2744
August	1st to 10th	Day	9% 4 2 5% 7% 3 6% 7% 6% 8% 42%	2341
	11th to 20th	Day	12% 5% 2 6% 5% 3 6% 6% 6% 9% 41%	2304
	21st to end	Day	13% 3 2 3 4 2 4 6% 6% 8% 48%	2395
September	1st to 10th	Day	13% 5% 2 4 4 3 5% 4 5% 8% 47%	2138
	11th to 20th	Day	15% 5% 2 3 4 3 2 4 3 5% 54%	1980
	21st to end	Day	16% 5% 2 4 4 3 4 4 5% 7% 46%	1980
October	1st to 10th	Day	23% 5% 2 4 4 2 4 3 4 6% 41%	1980
	11th to 20th	Day	16% 4 2 4 4 3 5% 4 6% 6% 46%	1755
	21st to end	Day	21% 6% 2 5% 4 3 4 4 4 6% 40%	1782
November	1st to 10th	Day	16% 7% 2 5% 4 2 4 3 4 4 49%	1671
	11th to 20th	Day	18% 6% 2 5% 4 2 3 4 4 6% 45%	1710
	21st to end	Day	19% 6% 2 4 4 2 4 4 4 6% 44%	1710
December	1st to 10th	Day	20% 7% 4 5% 4 2 4 5% 4 5% 40%	1567
	11th to 20th	Day	22% 7% 2 3 4 2 3 3 3 5% 44%	1906
	21st to end	Day	23% 10% 3 5% 5% 3 4 4 4 6% 34%	2772
January	1st to 10th	Night	17% 8% 2 5% 4 3 4 4 5% 6% 43%	2649
	11th to 20th	Night	19% 10% 3 5% 5% 3 5% 4 5% 7% 34%	2611
	21st to end	Night	15% 7% 4 5% 5% 4 4 5% 5% 7% 40%	2772
February	1st to 10th	Night	17% 8% 3 6% 5% 3 5% 4 5% 6% 37%	2300
	11th to 20th	Night	15% 9% 3 5% 5% 4 4 4 5% 5% 40%	2098
	21st to end	Night	18% 8% 2 5% 4 3 3 4 5% 6% 42%	1776
March	1st to 10th	Night	16% 8% 3 6% 5% 2 4 5% 5% 5% 42%	2150
	11th to 20th	Night	18% 7% 5% 6% 6% 2 5% 4 5% 5% 38%	2043
	21st to end	Night	19% 7% 2 6% 4 3 4 3 4 4 43%	2088
April	1st to 10th	Night	19% 9% 2 5% 5% 2 4 5% 5% 6% 37%	1800
	11th to 20th	Night	19% 7% 2 5% 4 3 4 5% 4 6% 41%	1732
	21st to end	Night	21% 7% 2 6% 6% 3 5% 4 6% 5% 36%	1620
May	1st to 10th	Night	23% 8% 3 6% 5% 3 4 4 5% 6% 33%	1620
	11th to 20th	Night	19% 6% 2 5% 5% 3 5% 6% 6% 7% 35%	1587
	21st to end	Night	19% 7% 3 5% 5% 2 3 4 5% 8% 38%	1550
June	1st to 10th	Night	20% 8% 2 5% 7% 3 4 4 4 6% 37%	1440
	11th to 20th	Night	20% 6% 3 4 5% 3 4 5% 6% 9% 37%	1440
	21st to end	Night	17% 6% 3 5% 5% 2 5% 5% 6% 8% 38%	1440
July	1st to 10th	Night	16% 6% 3 6% 4 3 5% 4 5% 9% 38%	1440
	11th to 20th	Night	12% 7% 3 6% 5% 3 5% 5% 7% 9% 39%	1440
	21st to end	Night	14% 5% 2 5% 4 3 5% 4 6% 8% 43%	1574
August	1st to 10th	Night	10% 5% 2 4 4 3 5% 6% 7% 9% 44%	1580
	11th to 20th	Night	13% 5% 3 6% 7% 4 5% 6% 6% 8% 37%	1596
	21st to end	Night	13% 6% 2 3 4 3 4 5% 5% 8% 46%	1957
September	1st to 10th	Night	13% 4 2 5% 4 3 4 4 6% 7% 47%	1822
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 4 7% 52%	1980
	21st to end	Night	16% 5% 2 5% 5% 3 4 4 4 7% 46%	1980
October	1st to 10th	Night	22% 5% 2 6% 5% 3 3 3 6% 6% 40%	1980
	11th to 20th	Night	16% 4 2 5% 4 3 4 5% 6% 6% 44%	2200
	21st to end	Night	23% 7% 2 5% 4 3 4 4 5% 5% 38%	2574
November	1st to 10th	Night	15% 5% 2 5% 5% 3 4 4 4 6% 47%	2407
	11th to 20th	Night	17% 6% 2 5% 4 3 4 4 4 6% 46%	2470
	21st to end	Night	17% 6% 3 4 4 3 3 4 6% 7% 44%	2470
December	1st to 10th	Night	19% 8% 3 5% 5% 3 5% 5% 4 5% 38%	2611
	11th to 20th	Night	20% 7% 2 5% 4 3 4 4 6% 7% 40%	3288
	21st to end	Night	22% 8% 3 5% 5% 2 4 4 4 6% 36%	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	28% 39% 8% 17% 8%	1518
	11th to 20th	Day	31% 40% 8% 12% 7% 1	1488
	21st to end	Day	26% 37% 8% 19% 9% 2	1584
February	1st to 10th	Day	32% 35% 8% 14% 9% 2	1657
	11th to 20th	Day	27% 37% 9% 17% 8% 2	1757
	21st to end	Day	35% 36% 7% 13% 8% 2	1480
March	1st to 10th	Day	30% 36% 9% 14% 9% 2	1800
	11th to 20th	Day	31% 36% 8% 14% 9% 3	1916
	21st to end	Day	28% 35% 6% 13% 12% 4	2265
April	1st to 10th	Day	31% 35% 7% 12% 12% 3	2160
	11th to 20th	Day	30% 35% 7% 15% 9% 3	2177
	21st to end	Day	31% 36% 6% 14% 11% 3	2340
May	1st to 10th	Day	34% 35% 6% 14% 10% 2	2340
	11th to 20th	Day	30% 37% 6% 12% 12% 3	2325
	21st to end	Day	24% 38% 10% 16% 10% 2	2725
June	1st to 10th	Day	35% 36% 9% 14% 6% 1	2520
	11th to 20th	Day	29% 37% 11% 12% 9% 2	2520
	21st to end	Day	31% 37% 9% 16% 5% 1	2520
July	1st to 10th	Day	28% 38% 10% 15% 8% 1	2450
	11th to 20th	Day	26% 37% 10% 19% 8% 1	2445
	21st to end	Day	25% 38% 10% 17% 8% 2	2591
August	1st to 10th	Day	27% 37% 11% 16% 8%	2327
	11th to 20th	Day	27% 36% 10% 16% 10% 1	2304
	21st to end	Day	23% 37% 10% 16% 12% 2	2395
September	1st to 10th	Day	24% 36% 9% 16% 14% 1	2138
	11th to 20th	Day	22% 35% 7% 21% 14% 2	1980
	21st to end	Day	28% 36% 6% 14% 15% 1	1980
October	1st to 10th	Day	29% 35% 7% 15% 12% 1	1980
	11th to 20th	Day	27% 36% 8% 20% 8% 1	1755
	21st to end	Day	28% 36% 9% 14% 10% 2	1782
November	1st to 10th	Day	25% 39% 6% 17% 11% 1	1671
	11th to 20th	Day	23% 40% 7% 16% 12% 1	1710
	21st to end	Day	28% 39% 11% 15% 7%	1710
December	1st to 10th	Day	30% 39% 10% 11% 9% 1	1567
	11th to 20th	Day	32% 31% 8% 13% 12% 4	1906
	21st to end	Day	42% 25% 9% 17% 7% 1	2772
January	1st to 10th	Night	29% 39% 8% 16% 8%	2649
	11th to 20th	Night	33% 40% 8% 12% 6% 1	2611
	21st to end	Night	28% 37% 6% 16% 12% 2	2772
February	1st to 10th	Night	32% 36% 8% 14% 10% 1	2300
	11th to 20th	Night	31% 37% 7% 15% 9% 1	2100
	21st to end	Night	30% 36% 6% 18% 9% 2	1776
March	1st to 10th	Night	30% 35% 6% 16% 10% 3	2160
	11th to 20th	Night	34% 36% 9% 12% 9% 1	2044
	21st to end	Night	30% 35% 6% 12% 14% 3	2088
April	1st to 10th	Night	31% 36% 8% 13% 10% 2	1800
	11th to 20th	Night	32% 34% 5% 14% 13% 2	1732
	21st to end	Night	33% 36% 6% 12% 10% 2	1620
May	1st to 10th	Night	36% 34% 5% 12% 9% 3	1620
	11th to 20th	Night	32% 37% 7% 11% 10% 4	1588
	21st to end	Night	28% 36% 10% 14% 10% 1	1551
June	1st to 10th	Night	36% 35% 9% 13% 6% 1	1440
	11th to 20th	Night	31% 37% 8% 12% 10% 2	1440
	21st to end	Night	32% 37% 11% 13% 7%	1440
July	1st to 10th	Night	33% 38% 9% 14% 5% 1	1400
	11th to 20th	Night	27% 38% 11% 17% 7%	1392
	21st to end	Night	27% 38% 9% 18% 7% 1	1486
August	1st to 10th	Night	28% 37% 9% 17% 8%	1570
	11th to 20th	Night	30% 36% 9% 15% 9% 1	1596
	21st to end	Night	27% 37% 7% 17% 11% 1	1957
September	1st to 10th	Night	25% 36% 8% 17% 13% 1	1822
	11th to 20th	Night	23% 35% 7% 17% 16% 1	1980
	21st to end	Night	26% 37% 6% 16% 14% 2	1980
October	1st to 10th	Night	31% 36% 7% 15% 10% 1	1980
	11th to 20th	Night	30% 37% 8% 18% 8% 1	2200
	21st to end	Night	31% 36% 8% 14% 10% 1	2574
November	1st to 10th	Night	23% 39% 9% 15% 12% 2	2407
	11th to 20th	Night	26% 39% 7% 16% 11% 1	2470
	21st to end	Night	27% 39% 8% 15% 9% 1	2470
December	1st to 10th	Night	31% 39% 8% 14% 7% 1	2611
	11th to 20th	Night	30% 32% 11% 13% 14%	3288
	21st to end	Night	41% 25% 7% 19% 8%	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	25% 14% 28% 24% 8%	180
	11th to 20th	Day	23% 21% 28% 20% 8%	172
	21st to end	Day	18% 16% 37% 23% 6%	176
February	1st to 10th	Day	27% 15% 37% 19% 2	338
	11th to 20th	Day	24% 16% 42% 16% 3	416
	21st to end	Day	21% 15% 34% 26% 4	346
March	1st to 10th	Day	20% 15% 34% 26% 5%	420
	11th to 20th	Day	19% 17% 36% 24% 4	420
	21st to end	Day	23% 13% 34% 20% 6% 2	462
April	1st to 10th	Day	24% 13% 34% 20% 8% 2	420
	11th to 20th	Day	20% 10% 40% 21% 8% 1	412
	21st to end	Day	26% 6% 33% 29% 5% 1	420
May	1st to 10th	Day	28% 8% 31% 23% 9% 1	423
	11th to 20th	Day	16% 9% 32% 32% 9% 1	447
	21st to end	Day	19% 3 26% 36% 13% 2	485
June	1st to 10th	Day	21% 3 26% 33% 15% 2	450
	11th to 20th	Day	14% 5% 31% 36% 11% 2	450
	21st to end	Day	18% 3 27% 35% 15% 3	450
July	1st to 10th	Day	18% 3 14% 40% 21% 4	445
	11th to 20th	Day	14% 3 17% 36% 24% 5%	444
	21st to end	Day	11% 1 17% 50% 19% 1	488
August	1st to 10th	Day	11% 2 18% 40% 25% 4	446
	11th to 20th	Day	14% 3 19% 36% 23% 5%	439
	21st to end	Day	15% 2 17% 39% 20% 6%	495
September	1st to 10th	Day	18% 2 21% 31% 21% 6%	450
	11th to 20th	Day	19% 7% 22% 27% 19% 6%	450
	21st to end	Day	18% 3 28% 31% 15% 5%	450
October	1st to 10th	Day	26% 7% 23% 24% 17% 4	450
	11th to 20th	Day	18% 8% 26% 32% 13% 2	386
	21st to end	Day	27% 7% 30% 23% 12% 1	396
November	1st to 10th	Day	16% 10% 31% 29% 12% 3	365
	11th to 20th	Day	24% 15% 30% 24% 7% 1	370
	21st to end	Day	23% 15% 28% 24% 9% 1	370
December	1st to 10th	Day	22% 7% 41% 22% 7%	227
	11th to 20th	Day	22% 17% 39% 20% 2	180
	21st to end	Day	22% 16% 31% 23% 6% 2	198
January	1st to 10th	Night	21% 16% 41% 18% 4	760
	11th to 20th	Night	25% 15% 40% 17% 3	747
	21st to end	Night	20% 16% 40% 20% 4	792
February	1st to 10th	Night	24% 15% 38% 21% 2	720
	11th to 20th	Night	20% 16% 46% 15% 3	703
	21st to end	Night	22% 14% 39% 22% 3	592
March	1st to 10th	Night	23% 14% 37% 23% 3	720
	11th to 20th	Night	21% 13% 39% 22% 5%	720
	21st to end	Night	23% 10% 35% 24% 7%	792
April	1st to 10th	Night	24% 11% 32% 26% 6%	720
	11th to 20th	Night	23% 7% 34% 26% 9%	668
	21st to end	Night	24% 7% 31% 30% 6% 1	540
May	1st to 10th	Night	27% 5% 32% 27% 7% 1	540
	11th to 20th	Night	22% 5% 31% 32% 9% 1	526
	21st to end	Night	22% 4 23% 34% 15% 1	389
June	1st to 10th	Night	23% 3 22% 36% 13% 3	360
	11th to 20th	Night	21% 4 22% 40% 12% 1	360
	21st to end	Night	19% 3 20% 41% 15% 1	360
July	1st to 10th	Night	18% 3 14% 45% 17% 3	350
	11th to 20th	Night	15% 1 14% 46% 22% 2	348
	21st to end	Night	16% 3 13% 48% 20% 1	394
August	1st to 10th	Night	11% 2 16% 46% 23% 2	506
	11th to 20th	Night	15% 3 17% 39% 24% 2	532
	21st to end	Night	16% 2 17% 36% 25% 3	769
September	1st to 10th	Night	15% 1 24% 34% 22% 3	720
	11th to 20th	Night	17% 4 23% 32% 19% 5%	720
	21st to end	Night	18% 3 24% 31% 20% 4	720
October	1st to 10th	Night	24% 6% 24% 28% 15% 3	720
	11th to 20th	Night	18% 8% 29% 30% 13% 2	720
	21st to end	Night	26% 9% 31% 26% 8% 1	792
November	1st to 10th	Night	16% 7% 35% 30% 10% 1	740
	11th to 20th	Night	21% 12% 36% 22% 8% 1	760
	21st to end	Night	20% 18% 34% 21% 6% 1	760
December	1st to 10th	Night	23% 13% 41% 19% 5% 1	760
	11th to 20th	Night	21% 16% 36% 22% 4	760
	21st to end	Night	26% 14% 39% 19% 2	836

Page generated at 2024-01-13 23:56:39. Version ac07

Copyright 2024 Attilla Danko