Preston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Preston clear sky chart from 2005-11-06 to 2024-01-16.

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%	74826
	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 41%	74960
Transparency	Day	28% 38% 8% 14% 10% 1	74531
	Night	29% 38% 8% 14% 9% 1	74789
Seeing	Day	21% 17% 32% 20% 8% 2	13971
	Night	22% 13% 32% 25% 8% 1	23060

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	15% 7% 3 4 4 3 4 4 5% 6% 45%	1517
	11th to 20th	Day	19% 8% 2 6% 5% 2 5% 4 5% 8% 36%	1480
	21st to end	Day	14% 7% 3 5% 4 2 4 4 5% 7% 44%	1584
February	1st to 10th	Day	20% 7% 3 5% 4 3 5% 5% 5% 7% 36%	1728
	11th to 20th	Day	15% 6% 2 5% 6% 3 5% 5% 4 7% 41%	1748
	21st to end	Day	22% 9% 3 6% 4 3 4 4 5% 5% 37%	1480
March	1st to 10th	Day	17% 7% 3 4 4 3 4 4 5% 6% 44%	1797
	11th to 20th	Day	16% 6% 3 5% 5% 3 5% 5% 6% 7% 39%	1868
	21st to end	Day	20% 8% 3 5% 5% 2 3 4 4 4 40%	2335
April	1st to 10th	Day	20% 6% 3 5% 4 3 4 4 4 7% 39%	2160
	11th to 20th	Day	19% 5% 2 5% 4 3 5% 4 4 7% 43%	2134
	21st to end	Day	23% 5% 2 5% 4 3 4 4 4 7% 38%	2304
May	1st to 10th	Day	26% 7% 2 6% 5% 2 3 4 5% 7% 34%	2340
	11th to 20th	Day	19% 6% 2 5% 4 3 4 4 4 7% 41%	2342
	21st to end	Day	16% 6% 2 4 5% 3 4 5% 5% 8% 43%	2725
June	1st to 10th	Day	19% 5% 2 6% 6% 4 4 5% 6% 8% 35%	2520
	11th to 20th	Day	17% 6% 3 5% 6% 3 5% 5% 7% 9% 35%	2520
	21st to end	Day	14% 7% 3 5% 6% 3 5% 6% 5% 9% 38%	2520
July	1st to 10th	Day	14% 5% 3 5% 6% 3 5% 5% 5% 8% 41%	2520
	11th to 20th	Day	14% 5% 3 5% 5% 3 5% 7% 6% 9% 38%	2520
	21st to end	Day	14% 6% 2 5% 5% 3 6% 5% 6% 9% 38%	2744
August	1st to 10th	Day	12% 6% 3 5% 5% 4 6% 5% 7% 9% 38%	2365
	11th to 20th	Day	14% 6% 2 5% 6% 3 5% 4 6% 9% 40%	2205
	21st to end	Day	11% 6% 3 6% 5% 3 5% 4 6% 8% 41%	2376
September	1st to 10th	Day	15% 6% 2 5% 4 3 4 5% 4 6% 45%	2160
	11th to 20th	Day	14% 5% 2 4 3 3 4 4 4 7% 50%	1998
	21st to end	Day	19% 6% 2 4 4 3 4 4 4 6% 44%	1980
October	1st to 10th	Day	22% 8% 3 5% 4 2 3 3 4 5% 42%	1980
	11th to 20th	Day	15% 6% 2 4 5% 2 4 5% 4 6% 47%	1771
	21st to end	Day	19% 6% 3 6% 5% 3 3 3 5% 6% 42%	1782
November	1st to 10th	Day	15% 7% 2 4 4 3 4 3 4 5% 48%	1671
	11th to 20th	Day	19% 5% 2 4 3 3 4 4 4 6% 46%	1710
	21st to end	Day	17% 5% 2 5% 5% 3 4 4 5% 5% 45%	1710
December	1st to 10th	Day	21% 8% 2 4 5% 3 4 4 6% 6% 37%	1554
	11th to 20th	Day	22% 6% 2 5% 3 2 3 2 3 5% 46%	1906
	21st to end	Day	21% 10% 3 6% 5% 3 4 3 4 5% 36%	2772
January	1st to 10th	Night	16% 7% 3 4 4 2 4 4 4 8% 44%	2649
	11th to 20th	Night	21% 9% 2 5% 5% 3 4 4 4 5% 39%	2591
	21st to end	Night	14% 8% 3 5% 5% 2 4 4 5% 6% 43%	2772
February	1st to 10th	Night	17% 9% 3 6% 5% 3 3 3 4 6% 40%	2227
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 41%	2098
	21st to end	Night	20% 8% 3 5% 5% 3 3 4 4 5% 40%	1776
March	1st to 10th	Night	18% 6% 2 5% 4 3 4 4 5% 7% 43%	2150
	11th to 20th	Night	17% 7% 3 6% 5% 3 5% 4 5% 6% 40%	2089
	21st to end	Night	19% 8% 3 4 4 2 5% 4 4 6% 41%	2017
April	1st to 10th	Night	20% 8% 3 6% 4 3 4 4 4 6% 39%	1800
	11th to 20th	Night	20% 6% 3 5% 5% 3 4 4 5% 6% 40%	1775
	21st to end	Night	23% 7% 3 4 6% 3 4 3 4 5% 40%	1655
May	1st to 10th	Night	22% 9% 2 6% 5% 4 5% 4 5% 6% 33%	1620
	11th to 20th	Night	19% 7% 3 6% 5% 3 4 3 4 8% 37%	1566
	21st to end	Night	17% 7% 3 5% 6% 4 4 5% 5% 8% 37%	1550
June	1st to 10th	Night	18% 6% 3 5% 4 3 5% 4 6% 8% 38%	1440
	11th to 20th	Night	20% 7% 3 4 5% 2 4 5% 5% 7% 39%	1440
	21st to end	Night	14% 8% 2 5% 5% 2 5% 4 6% 9% 41%	1440
July	1st to 10th	Night	13% 6% 2 5% 7% 3 7% 5% 6% 8% 38%	1440
	11th to 20th	Night	16% 7% 3 5% 6% 3 5% 4 8% 10% 35%	1440
	21st to end	Night	13% 6% 3 6% 6% 3 5% 5% 5% 8% 41%	1574
August	1st to 10th	Night	9% 6% 3 6% 5% 3 5% 6% 7% 10% 41%	1553
	11th to 20th	Night	10% 6% 2 7% 6% 4 6% 7% 6% 8% 37%	1693
	21st to end	Night	14% 5% 2 4 4 3 4 4 5% 8% 47%	1980
September	1st to 10th	Night	15% 6% 3 5% 5% 3 5% 4 6% 6% 41%	1800
	11th to 20th	Night	14% 5% 3 5% 4 3 2 4 3 6% 52%	1962
	21st to end	Night	18% 8% 3 5% 4 3 4 4 4 5% 41%	1980
October	1st to 10th	Night	19% 8% 2 5% 5% 3 4 4 4 5% 41%	1980
	11th to 20th	Night	16% 5% 2 6% 4 3 4 4 5% 6% 46%	2186
	21st to end	Night	21% 8% 2 5% 4 2 4 4 4 6% 40%	2574
November	1st to 10th	Night	14% 6% 2 5% 5% 2 3 4 5% 6% 48%	2407
	11th to 20th	Night	19% 7% 2 4 4 2 3 3 4 6% 45%	2470
	21st to end	Night	15% 6% 3 5% 5% 2 4 3 4 7% 46%	2470
December	1st to 10th	Night	21% 7% 3 4 4 3 4 4 4 6% 40%	2624
	11th to 20th	Night	21% 7% 2 4 4 3 4 3 4 6% 42%	3288
	21st to end	Night	22% 10% 3 5% 4 2 3 3 3 5% 38%	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	25% 42% 6% 18% 8%	1518
	11th to 20th	Day	31% 42% 8% 10% 8% 1	1480
	21st to end	Day	24% 38% 8% 18% 10% 2	1584
February	1st to 10th	Day	30% 37% 9% 12% 11% 2	1728
	11th to 20th	Day	27% 39% 8% 16% 8% 2	1757
	21st to end	Day	34% 36% 6% 11% 10% 2	1480
March	1st to 10th	Day	26% 36% 10% 15% 11% 2	1800
	11th to 20th	Day	30% 38% 9% 14% 8% 2	1868
	21st to end	Day	29% 37% 6% 12% 13% 3	2335
April	1st to 10th	Day	28% 37% 8% 14% 11% 2	2160
	11th to 20th	Day	28% 38% 7% 13% 11% 2	2134
	21st to end	Day	28% 38% 7% 13% 10% 3	2304
May	1st to 10th	Day	32% 38% 7% 13% 9% 2	2340
	11th to 20th	Day	28% 39% 8% 13% 10% 2	2346
	21st to end	Day	24% 39% 12% 13% 9% 2	2725
June	1st to 10th	Day	30% 40% 9% 14% 6%	2520
	11th to 20th	Day	29% 41% 10% 12% 8% 1	2520
	21st to end	Day	29% 41% 10% 13% 6% 1	2520
July	1st to 10th	Day	29% 40% 8% 14% 7% 1	2450
	11th to 20th	Day	26% 40% 10% 17% 7% 1	2445
	21st to end	Day	26% 41% 9% 14% 9% 1	2591
August	1st to 10th	Day	28% 40% 11% 13% 8%	2351
	11th to 20th	Day	29% 38% 9% 14% 9% 1	2205
	21st to end	Day	28% 39% 8% 12% 12% 2	2376
September	1st to 10th	Day	27% 38% 7% 13% 13% 2	2160
	11th to 20th	Day	22% 38% 8% 16% 15% 2	1998
	21st to end	Day	29% 38% 5% 13% 14% 1	1980
October	1st to 10th	Day	28% 37% 8% 15% 12% 1	1980
	11th to 20th	Day	25% 38% 9% 18% 8% 1	1771
	21st to end	Day	27% 38% 8% 15% 10% 2	1782
November	1st to 10th	Day	24% 41% 8% 13% 13% 1	1671
	11th to 20th	Day	24% 41% 8% 14% 11% 2	1710
	21st to end	Day	28% 41% 8% 15% 8% 1	1710
December	1st to 10th	Day	29% 41% 9% 13% 8% 1	1554
	11th to 20th	Day	30% 33% 9% 11% 13% 3	1906
	21st to end	Day	38% 28% 9% 17% 7%	2772
January	1st to 10th	Night	26% 40% 9% 16% 9% 1	2649
	11th to 20th	Night	32% 41% 6% 13% 6% 2	2591
	21st to end	Night	25% 38% 8% 17% 11% 1	2772
February	1st to 10th	Night	30% 36% 8% 15% 9% 1	2227
	11th to 20th	Night	30% 38% 7% 15% 8% 2	2100
	21st to end	Night	31% 37% 6% 16% 9% 2	1776
March	1st to 10th	Night	26% 38% 8% 17% 11% 2	2160
	11th to 20th	Night	32% 38% 8% 13% 8% 2	2090
	21st to end	Night	29% 36% 6% 13% 12% 4	2017
April	1st to 10th	Night	29% 38% 7% 14% 10% 2	1800
	11th to 20th	Night	31% 37% 5% 14% 12% 1	1775
	21st to end	Night	31% 36% 7% 13% 9% 3	1655
May	1st to 10th	Night	34% 38% 7% 9% 9% 2	1620
	11th to 20th	Night	30% 38% 8% 12% 10% 3	1567
	21st to end	Night	29% 42% 9% 11% 9%	1551
June	1st to 10th	Night	31% 40% 10% 12% 7%	1440
	11th to 20th	Night	30% 39% 10% 14% 7%	1440
	21st to end	Night	29% 39% 12% 13% 7%	1440
July	1st to 10th	Night	31% 41% 9% 12% 6% 1	1400
	11th to 20th	Night	27% 40% 12% 15% 5% 1	1392
	21st to end	Night	27% 40% 10% 13% 7% 2	1486
August	1st to 10th	Night	28% 41% 11% 14% 6%	1543
	11th to 20th	Night	28% 39% 10% 15% 8%	1693
	21st to end	Night	27% 40% 7% 14% 10% 1	1980
September	1st to 10th	Night	30% 38% 6% 13% 11% 2	1800
	11th to 20th	Night	23% 38% 8% 14% 15% 1	1962
	21st to end	Night	30% 37% 6% 12% 13% 3	1980
October	1st to 10th	Night	29% 36% 8% 14% 11% 2	1980
	11th to 20th	Night	27% 38% 9% 17% 8%	2186
	21st to end	Night	30% 38% 9% 13% 9% 1	2574
November	1st to 10th	Night	21% 41% 9% 14% 12% 2	2407
	11th to 20th	Night	26% 40% 7% 16% 10% 1	2470
	21st to end	Night	26% 39% 9% 16% 8% 1	2470
December	1st to 10th	Night	28% 41% 9% 14% 8% 1	2624
	11th to 20th	Night	29% 34% 11% 12% 13% 1	3288
	21st to end	Night	39% 26% 8% 17% 9%	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	26% 25% 28% 17% 4	180
	11th to 20th	Day	22% 35% 32% 8% 3	170
	21st to end	Day	21% 28% 36% 10% 3 1	176
February	1st to 10th	Day	31% 25% 31% 12% 1	353
	11th to 20th	Day	25% 29% 35% 10% 1	416
	21st to end	Day	27% 27% 35% 9% 3	346
March	1st to 10th	Day	24% 22% 36% 15% 2	420
	11th to 20th	Day	23% 25% 37% 11% 3 1	420
	21st to end	Day	29% 20% 33% 14% 3 1	462
April	1st to 10th	Day	27% 24% 30% 12% 6% 1	420
	11th to 20th	Day	24% 23% 32% 17% 4	412
	21st to end	Day	31% 19% 32% 12% 4 2	420
May	1st to 10th	Day	30% 21% 30% 14% 4 1	423
	11th to 20th	Day	19% 19% 39% 18% 4 1	447
	21st to end	Day	20% 10% 32% 27% 10% 2	485
June	1st to 10th	Day	18% 12% 34% 23% 11% 2	450
	11th to 20th	Day	19% 12% 36% 25% 6% 1	450
	21st to end	Day	16% 13% 32% 27% 10% 2	450
July	1st to 10th	Day	17% 7% 30% 30% 14% 2	445
	11th to 20th	Day	14% 8% 28% 33% 15% 3	444
	21st to end	Day	11% 10% 37% 31% 9% 2	488
August	1st to 10th	Day	12% 9% 32% 32% 11% 4	446
	11th to 20th	Day	16% 5% 31% 30% 13% 5%	439
	21st to end	Day	11% 10% 30% 28% 15% 6%	495
September	1st to 10th	Day	20% 9% 27% 25% 14% 5%	450
	11th to 20th	Day	15% 8% 35% 24% 11% 7%	450
	21st to end	Day	21% 9% 36% 22% 9% 3	450
October	1st to 10th	Day	26% 13% 26% 22% 10% 4	450
	11th to 20th	Day	19% 17% 29% 22% 11% 1	421
	21st to end	Day	24% 17% 32% 18% 7% 2	396
November	1st to 10th	Day	17% 16% 32% 24% 9% 2	365
	11th to 20th	Day	25% 23% 28% 19% 6%	370
	21st to end	Day	21% 21% 30% 20% 8% 1	370
December	1st to 10th	Day	24% 25% 32% 14% 5%	214
	11th to 20th	Day	29% 27% 31% 9% 3	180
	21st to end	Day	24% 26% 32% 12% 4 2	198
January	1st to 10th	Night	21% 21% 36% 19% 3	760
	11th to 20th	Night	29% 20% 32% 17% 3	741
	21st to end	Night	19% 24% 36% 19% 2	792
February	1st to 10th	Night	23% 20% 35% 22% 1	720
	11th to 20th	Night	25% 19% 39% 15% 1	703
	21st to end	Night	26% 21% 35% 15% 2	592
March	1st to 10th	Night	24% 18% 37% 18% 3	720
	11th to 20th	Night	22% 16% 38% 20% 4	720
	21st to end	Night	24% 17% 36% 19% 4 1	792
April	1st to 10th	Night	28% 15% 33% 21% 4	720
	11th to 20th	Night	23% 12% 38% 23% 4	711
	21st to end	Night	25% 11% 37% 21% 6% 1	575
May	1st to 10th	Night	26% 11% 32% 24% 7%	540
	11th to 20th	Night	22% 12% 38% 23% 4	505
	21st to end	Night	22% 5% 26% 34% 11% 1	389
June	1st to 10th	Night	21% 6% 31% 31% 11%	360
	11th to 20th	Night	23% 8% 24% 38% 8%	360
	21st to end	Night	18% 5% 28% 38% 11% 1	360
July	1st to 10th	Night	17% 3 26% 34% 19% 1	350
	11th to 20th	Night	15% 4 20% 45% 15% 1	348
	21st to end	Night	15% 3 25% 44% 13% 1	394
August	1st to 10th	Night	11% 4 25% 45% 14% 1	479
	11th to 20th	Night	14% 3 22% 40% 18% 2	629
	21st to end	Night	15% 4 20% 38% 20% 3	792
September	1st to 10th	Night	19% 4 27% 33% 16% 2	720
	11th to 20th	Night	17% 6% 28% 29% 16% 4	720
	21st to end	Night	21% 7% 29% 28% 14% 2	720
October	1st to 10th	Night	24% 9% 29% 23% 13% 3	720
	11th to 20th	Night	19% 12% 32% 26% 10% 1	720
	21st to end	Night	25% 13% 35% 18% 8%	792
November	1st to 10th	Night	16% 13% 34% 24% 12% 1	740
	11th to 20th	Night	24% 17% 34% 19% 6% 1	760
	21st to end	Night	19% 19% 34% 21% 6%	760
December	1st to 10th	Night	26% 17% 32% 20% 4 1	760
	11th to 20th	Night	25% 19% 33% 19% 4	760
	21st to end	Night	25% 18% 36% 18% 3	836

Page generated at 2024-01-13 01:38:10. Version ac07

Copyright 2024 Attilla Danko