Dreyfuss Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dreyfuss Planetarium clear sky chart from 2005-11-06 to 2024-01-12.

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% 6% 2 5% 5% 3 5% 5% 5% 7% 41%	74754
	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 42%	74948
Transparency	Day	27% 38% 8% 15% 10% 2	74459
	Night	27% 38% 8% 15% 11% 1	74777
Seeing	Day	20% 15% 31% 22% 10% 3	13867
	Night	20% 11% 31% 28% 9% 1	23140

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% 7% 2 4 6% 3 5% 4 6% 6% 44%	1517
	11th to 20th	Day	16% 8% 3 5% 6% 3 4 5% 6% 7% 37%	1448
	21st to end	Day	14% 7% 3 5% 5% 3 5% 4 5% 7% 43%	1730
February	1st to 10th	Day	18% 8% 3 5% 4 3 4 4 5% 7% 38%	1624
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 5% 6% 44%	1748
	21st to end	Day	21% 8% 2 5% 4 2 5% 5% 5% 6% 38%	1480
March	1st to 10th	Day	14% 6% 2 5% 5% 2 4 4 5% 7% 44%	1797
	11th to 20th	Day	16% 7% 1 5% 5% 3 6% 5% 6% 7% 39%	1903
	21st to end	Day	20% 7% 3 5% 5% 3 4 4 4 6% 39%	2351
April	1st to 10th	Day	18% 7% 3 6% 4 2 4 5% 5% 7% 40%	2160
	11th to 20th	Day	17% 5% 2 6% 4 3 4 4 5% 8% 42%	2134
	21st to end	Day	20% 5% 2 5% 4 3 4 4 5% 8% 40%	2173
May	1st to 10th	Day	23% 7% 3 4 5% 3 4 4 5% 7% 34%	2340
	11th to 20th	Day	21% 5% 2 5% 4 3 5% 5% 5% 9% 36%	2462
	21st to end	Day	16% 5% 2 5% 5% 3 4 5% 5% 9% 42%	2725
June	1st to 10th	Day	16% 5% 2 5% 5% 3 5% 6% 7% 9% 38%	2520
	11th to 20th	Day	16% 6% 3 6% 6% 3 5% 5% 6% 8% 37%	2520
	21st to end	Day	11% 5% 3 5% 6% 4 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	11% 5% 2 6% 7% 3 6% 6% 7% 10% 37%	2520
	11th to 20th	Day	10% 4 2 4 5% 3 6% 7% 7% 11% 42%	2520
	21st to end	Day	12% 5% 3 6% 6% 4 7% 7% 6% 9% 36%	2744
August	1st to 10th	Day	9% 5% 2 5% 6% 4 6% 7% 7% 9% 38%	2361
	11th to 20th	Day	13% 5% 2 5% 6% 3 6% 5% 6% 9% 40%	2126
	21st to end	Day	13% 6% 2 5% 6% 3 4 5% 6% 7% 42%	2376
September	1st to 10th	Day	14% 6% 2 4 5% 3 5% 5% 5% 8% 43%	2160
	11th to 20th	Day	13% 5% 2 5% 3 2 4 4 5% 7% 50%	2088
	21st to end	Day	17% 5% 3 5% 6% 3 4 5% 5% 6% 42%	1980
October	1st to 10th	Day	21% 6% 2 4 4 3 4 4 4 7% 41%	1872
	11th to 20th	Day	16% 5% 2 5% 4 3 4 4 5% 6% 47%	1800
	21st to end	Day	22% 5% 1 4 4 2 5% 4 4 6% 42%	1846
November	1st to 10th	Day	15% 5% 2 4 4 2 3 4 3 5% 52%	1671
	11th to 20th	Day	18% 5% 2 4 4 2 4 4 4 6% 48%	1710
	21st to end	Day	15% 6% 2 4 4 3 5% 4 5% 6% 45%	1630
December	1st to 10th	Day	19% 8% 2 5% 4 2 5% 3 4 8% 39%	1520
	11th to 20th	Day	23% 5% 2 4 4 2 3 4 4 6% 45%	1906
	21st to end	Day	18% 8% 3 6% 6% 3 5% 6% 5% 7% 32%	2772
January	1st to 10th	Night	16% 6% 3 4 4 2 3 4 5% 5% 46%	2649
	11th to 20th	Night	19% 8% 2 4 5% 3 5% 5% 5% 6% 39%	2541
	21st to end	Night	14% 8% 3 5% 4 2 4 4 5% 7% 45%	2624
February	1st to 10th	Night	18% 9% 3 5% 4 3 4 4 4 6% 41%	2335
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 3 5% 6% 41%	2098
	21st to end	Night	21% 6% 2 5% 4 3 3 4 5% 5% 42%	1776
March	1st to 10th	Night	16% 6% 2 5% 5% 2 4 4 5% 6% 45%	2150
	11th to 20th	Night	17% 7% 3 5% 4 2 4 5% 5% 6% 42%	2052
	21st to end	Night	18% 7% 3 4 4 3 3 4 5% 6% 41%	2003
April	1st to 10th	Night	19% 6% 2 6% 5% 3 4 5% 5% 6% 39%	1800
	11th to 20th	Night	18% 6% 2 5% 4 3 5% 4 4 7% 41%	1775
	21st to end	Night	20% 6% 3 5% 4 3 4 4 5% 6% 42%	1787
May	1st to 10th	Night	22% 7% 3 6% 5% 3 4 4 4 6% 36%	1620
	11th to 20th	Night	19% 7% 3 6% 6% 3 3 4 5% 6% 39%	1446
	21st to end	Night	16% 6% 3 5% 5% 4 4 3 6% 8% 41%	1550
June	1st to 10th	Night	17% 6% 3 4 4 3 5% 4 6% 8% 40%	1440
	11th to 20th	Night	14% 6% 2 5% 5% 3 5% 5% 7% 8% 41%	1440
	21st to end	Night	10% 4 3 5% 5% 3 5% 5% 7% 10% 45%	1440
July	1st to 10th	Night	13% 5% 3 6% 5% 3 5% 5% 6% 9% 39%	1440
	11th to 20th	Night	13% 4 2 4 6% 3 6% 7% 7% 9% 40%	1440
	21st to end	Night	10% 5% 2 5% 5% 3 5% 6% 7% 11% 42%	1574
August	1st to 10th	Night	9% 5% 3 5% 5% 3 5% 6% 6% 9% 43%	1554
	11th to 20th	Night	12% 5% 2 8% 7% 4 6% 6% 6% 8% 37%	1774
	21st to end	Night	14% 5% 2 5% 6% 3 5% 4 5% 9% 42%	1980
September	1st to 10th	Night	15% 5% 2 6% 5% 3 4 5% 5% 6% 43%	1800
	11th to 20th	Night	16% 4 1 4 3 2 3 4 4 7% 52%	1872
	21st to end	Night	17% 6% 2 4 5% 3 3 4 4 6% 45%	1980
October	1st to 10th	Night	20% 6% 3 5% 4 2 3 4 5% 6% 41%	2083
	11th to 20th	Night	15% 4 2 5% 5% 2 3 4 4 6% 48%	2160
	21st to end	Night	21% 7% 2 4 4 2 3 3 5% 5% 43%	2510
November	1st to 10th	Night	13% 5% 1 4 4 2 4 4 4 5% 52%	2407
	11th to 20th	Night	20% 5% 3 4 3 2 3 3 4 6% 48%	2470
	21st to end	Night	14% 5% 2 4 4 3 4 5% 4 7% 47%	2546
December	1st to 10th	Night	20% 6% 3 5% 4 2 4 4 5% 7% 40%	2660
	11th to 20th	Night	20% 7% 2 4 4 3 4 4 5% 6% 42%	3288
	21st to end	Night	20% 9% 3 5% 5% 3 4 5% 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	25% 40% 9% 17% 8%	1518
	11th to 20th	Day	29% 42% 10% 11% 7% 1	1448
	21st to end	Day	26% 38% 8% 18% 9% 2	1730
February	1st to 10th	Day	29% 36% 8% 16% 10% 1	1624
	11th to 20th	Day	28% 38% 9% 16% 9% 1	1757
	21st to end	Day	31% 37% 6% 14% 10% 2	1480
March	1st to 10th	Day	25% 37% 9% 18% 9% 1	1800
	11th to 20th	Day	28% 37% 9% 14% 10% 2	1903
	21st to end	Day	29% 37% 6% 13% 12% 2	2351
April	1st to 10th	Day	28% 37% 7% 14% 11% 3	2160
	11th to 20th	Day	28% 37% 8% 14% 10% 3	2134
	21st to end	Day	27% 38% 7% 13% 12% 3	2173
May	1st to 10th	Day	31% 38% 6% 12% 12% 2	2340
	11th to 20th	Day	28% 39% 8% 12% 10% 2	2466
	21st to end	Day	25% 39% 10% 14% 10% 2	2725
June	1st to 10th	Day	26% 40% 10% 14% 8% 1	2520
	11th to 20th	Day	29% 40% 10% 15% 5% 2	2520
	21st to end	Day	27% 40% 10% 14% 9% 1	2520
July	1st to 10th	Day	26% 41% 9% 13% 9% 1	2450
	11th to 20th	Day	22% 41% 10% 15% 10% 2	2445
	21st to end	Day	25% 42% 9% 12% 10% 1	2591
August	1st to 10th	Day	25% 41% 11% 14% 9% 1	2347
	11th to 20th	Day	27% 38% 9% 15% 10% 1	2126
	21st to end	Day	29% 39% 6% 12% 11% 2	2376
September	1st to 10th	Day	26% 39% 7% 13% 13% 3	2160
	11th to 20th	Day	21% 37% 8% 16% 16% 2	2088
	21st to end	Day	28% 37% 6% 13% 13% 3	1980
October	1st to 10th	Day	27% 37% 8% 16% 12%	1872
	11th to 20th	Day	25% 38% 9% 17% 10% 2	1800
	21st to end	Day	28% 38% 7% 14% 11% 1	1846
November	1st to 10th	Day	20% 41% 7% 16% 14% 1	1671
	11th to 20th	Day	26% 40% 6% 15% 11% 2	1710
	21st to end	Day	26% 40% 7% 19% 7% 1	1630
December	1st to 10th	Day	26% 41% 10% 16% 6% 1	1520
	11th to 20th	Day	29% 33% 11% 17% 9% 1	1906
	21st to end	Day	36% 31% 9% 18% 5%	2772
January	1st to 10th	Night	25% 40% 8% 17% 9%	2649
	11th to 20th	Night	29% 43% 8% 14% 6% 1	2541
	21st to end	Night	25% 38% 7% 18% 12% 1	2624
February	1st to 10th	Night	30% 37% 8% 15% 10% 1	2335
	11th to 20th	Night	30% 38% 8% 12% 10% 3	2100
	21st to end	Night	28% 36% 6% 18% 11% 2	1776
March	1st to 10th	Night	25% 37% 10% 18% 9% 2	2160
	11th to 20th	Night	30% 38% 8% 13% 9% 2	2053
	21st to end	Night	28% 37% 7% 12% 12% 4	2003
April	1st to 10th	Night	29% 38% 7% 12% 11% 3	1800
	11th to 20th	Night	32% 37% 6% 11% 13% 1	1775
	21st to end	Night	27% 38% 7% 12% 13% 3	1787
May	1st to 10th	Night	33% 37% 7% 10% 11% 1	1620
	11th to 20th	Night	29% 39% 6% 10% 13% 3	1447
	21st to end	Night	25% 39% 11% 13% 9% 2	1551
June	1st to 10th	Night	27% 39% 8% 13% 11% 1	1440
	11th to 20th	Night	26% 39% 12% 15% 7% 1	1440
	21st to end	Night	23% 41% 11% 15% 9%	1440
July	1st to 10th	Night	28% 41% 9% 12% 9% 1	1400
	11th to 20th	Night	24% 41% 9% 14% 10% 1	1392
	21st to end	Night	20% 41% 13% 13% 10% 2	1486
August	1st to 10th	Night	24% 40% 12% 14% 9% 1	1544
	11th to 20th	Night	28% 39% 9% 16% 8% 1	1774
	21st to end	Night	29% 40% 8% 11% 10% 1	1980
September	1st to 10th	Night	28% 37% 7% 13% 13% 1	1800
	11th to 20th	Night	20% 38% 8% 17% 16% 2	1872
	21st to end	Night	25% 37% 6% 15% 16% 1	1980
October	1st to 10th	Night	26% 38% 8% 16% 11% 1	2083
	11th to 20th	Night	26% 38% 9% 16% 10% 1	2160
	21st to end	Night	29% 38% 7% 15% 11% 1	2510
November	1st to 10th	Night	19% 41% 7% 15% 15% 2	2407
	11th to 20th	Night	26% 39% 6% 16% 11% 2	2470
	21st to end	Night	25% 40% 8% 17% 10%	2546
December	1st to 10th	Night	26% 40% 9% 15% 8% 1	2660
	11th to 20th	Night	29% 33% 9% 13% 13% 3	3288
	21st to end	Night	36% 29% 10% 17% 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	20% 29% 30% 18% 3 1	180
	11th to 20th	Day	20% 36% 30% 11% 3	162
	21st to end	Day	19% 29% 32% 13% 5% 2	242
February	1st to 10th	Day	28% 26% 31% 13% 2	244
	11th to 20th	Day	24% 20% 37% 16% 2	416
	21st to end	Day	29% 18% 35% 16% 3	346
March	1st to 10th	Day	20% 25% 31% 20% 4	420
	11th to 20th	Day	23% 19% 38% 14% 5% 1	420
	21st to end	Day	27% 21% 33% 13% 5% 1	462
April	1st to 10th	Day	24% 22% 33% 13% 7% 1	420
	11th to 20th	Day	21% 21% 31% 17% 9% 1	412
	21st to end	Day	28% 17% 34% 15% 4 2	420
May	1st to 10th	Day	32% 19% 27% 16% 4 3	423
	11th to 20th	Day	22% 14% 36% 20% 6% 1	447
	21st to end	Day	20% 7% 28% 26% 15% 3	485
June	1st to 10th	Day	16% 13% 30% 26% 12% 3	450
	11th to 20th	Day	17% 13% 32% 26% 9% 2	450
	21st to end	Day	14% 12% 30% 27% 15% 2	450
July	1st to 10th	Day	12% 5% 27% 33% 17% 5%	445
	11th to 20th	Day	10% 4 28% 32% 21% 4	444
	21st to end	Day	14% 7% 29% 31% 15% 3	488
August	1st to 10th	Day	10% 7% 26% 33% 17% 7%	446
	11th to 20th	Day	16% 4 24% 33% 20% 5%	439
	21st to end	Day	12% 8% 28% 30% 16% 6%	495
September	1st to 10th	Day	18% 8% 29% 22% 15% 8%	450
	11th to 20th	Day	15% 7% 29% 26% 14% 10%	450
	21st to end	Day	20% 12% 30% 22% 11% 5%	450
October	1st to 10th	Day	26% 8% 27% 21% 14% 4	450
	11th to 20th	Day	19% 16% 28% 25% 11% 1	450
	21st to end	Day	25% 14% 34% 20% 6% 2	428
November	1st to 10th	Day	16% 14% 32% 23% 12% 3	365
	11th to 20th	Day	23% 18% 31% 21% 6%	370
	21st to end	Day	19% 19% 35% 21% 5% 2	290
December	1st to 10th	Day	23% 26% 32% 12% 8%	180
	11th to 20th	Day	29% 25% 34% 9% 2	180
	21st to end	Day	26% 19% 36% 11% 8%	198
January	1st to 10th	Night	19% 16% 40% 22% 3	760
	11th to 20th	Night	26% 16% 31% 24% 4	727
	21st to end	Night	19% 17% 40% 21% 2	792
February	1st to 10th	Night	22% 15% 35% 25% 3	720
	11th to 20th	Night	23% 18% 39% 19% 1	703
	21st to end	Night	22% 20% 35% 20% 3	592
March	1st to 10th	Night	22% 14% 38% 21% 5%	720
	11th to 20th	Night	23% 14% 38% 21% 4 1	720
	21st to end	Night	21% 15% 37% 21% 6%	792
April	1st to 10th	Night	23% 16% 35% 22% 5%	720
	11th to 20th	Night	22% 14% 34% 24% 6%	711
	21st to end	Night	23% 12% 34% 25% 5% 1	707
May	1st to 10th	Night	24% 12% 34% 23% 7%	540
	11th to 20th	Night	21% 9% 38% 24% 7%	385
	21st to end	Night	20% 5% 23% 38% 14% 1	389
June	1st to 10th	Night	19% 7% 28% 34% 12%	360
	11th to 20th	Night	16% 7% 25% 38% 14%	360
	21st to end	Night	10% 4 23% 52% 10% 1	360
July	1st to 10th	Night	12% 5% 25% 40% 18% 1	350
	11th to 20th	Night	12% 3 22% 43% 19% 1	348
	21st to end	Night	14% 4 21% 45% 15% 1	394
August	1st to 10th	Night	10% 4 20% 44% 21% 1	480
	11th to 20th	Night	15% 3 18% 38% 24% 1	710
	21st to end	Night	15% 4 18% 41% 20% 2	792
September	1st to 10th	Night	16% 5% 24% 35% 18% 2	720
	11th to 20th	Night	18% 3 24% 32% 18% 4	720
	21st to end	Night	21% 5% 26% 32% 15% 2	720
October	1st to 10th	Night	23% 8% 24% 27% 14% 3	720
	11th to 20th	Night	18% 11% 29% 31% 11% 1	720
	21st to end	Night	24% 9% 36% 23% 8%	792
November	1st to 10th	Night	15% 11% 33% 28% 11% 1	740
	11th to 20th	Night	22% 14% 32% 24% 8% 1	760
	21st to end	Night	17% 16% 35% 24% 7% 1	760
December	1st to 10th	Night	24% 14% 32% 24% 5% 1	760
	11th to 20th	Night	23% 13% 37% 25% 3	760
	21st to end	Night	23% 13% 37% 24% 3	836

Page generated at 2024-01-09 07:14:40. Version ac07

Copyright 2024 Attilla Danko