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 2024-11-05.

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% 7% 2 6% 4 4 4 4 5% 7% 39%	101308
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	97733
Transparency	Day	41% 6% 12% 22% 16% 2	104853
	Night	41% 6% 11% 23% 16% 2	102277
Seeing	Day	20% 15% 31% 22% 10% 3	15006
	Night	20% 11% 31% 28% 9% 1	24147

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% 8% 2 5% 4 3 4 3 5% 5% 48%	2517
	11th to 20th	Day	15% 9% 3 6% 5% 5% 4 3 6% 7% 36%	2520
	21st to end	Day	22% 12% 4 7% 4 4 4 3 4 4 33%	3030
February	1st to 10th	Day	18% 8% 3 6% 4 3 3 3 6% 6% 39%	1714
	11th to 20th	Day	15% 7% 3 6% 4 5% 4 3 5% 6% 44%	1848
	21st to end	Day	20% 8% 2 5% 4 3 5% 4 5% 6% 37%	1570
March	1st to 10th	Day	27% 8% 2 6% 4 3 3 3 4 5% 35%	3116
	11th to 20th	Day	10% 6% 1 5% 5% 5% 6% 4 6% 7% 44%	3347
	21st to end	Day	21% 6% 2 6% 3 3 3 3 4 6% 43%	4127
April	1st to 10th	Day	19% 8% 2 6% 4 4 4 4 5% 7% 38%	2280
	11th to 20th	Day	17% 6% 2 6% 4 5% 4 3 5% 7% 42%	2254
	21st to end	Day	19% 6% 2 6% 4 4 4 3 6% 7% 40%	2294
May	1st to 10th	Day	28% 8% 2 5% 4 4 3 3 5% 8% 30%	4106
	11th to 20th	Day	24% 8% 3 7% 5% 4 4 4 5% 7% 30%	4382
	21st to end	Day	14% 5% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	15% 6% 2 5% 5% 4 5% 5% 7% 9% 38%	2660
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 7% 8% 37%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	9% 5% 3 7% 6% 5% 6% 5% 8% 10% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	13% 8% 3 8% 6% 6% 6% 5% 6% 8% 31%	4814
August	1st to 10th	Day	20% 6% 2 6% 5% 5% 5% 4 7% 9% 32%	4139
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 8% 44%	3730
	21st to end	Day	10% 5% 2 6% 5% 4 4 4 6% 8% 46%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 5% 4 4 5% 7% 44%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 50%	2203
	21st to end	Day	19% 6% 3 6% 4 4 4 4 4 6% 40%	2090
October	1st to 10th	Day	20% 6% 2 5% 4 3 4 3 4 7% 41%	1976
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 5% 6% 49%	1900
	21st to end	Day	21% 5% 1 5% 3 4 5% 3 4 6% 44%	1948
November	1st to 10th	Day	15% 5% 2 5% 3 3 3 3 4 5% 52%	1708
	11th to 20th	Day	18% 5% 2 5% 4 3 3 3 4 6% 48%	1710
	21st to end	Day	15% 7% 1 5% 3 4 4 3 5% 6% 45%	1630
December	1st to 10th	Day	18% 8% 2 6% 4 4 4 2 5% 8% 39%	1520
	11th to 20th	Day	23% 5% 2 4 4 2 3 3 4 6% 45%	1906
	21st to end	Day	18% 9% 3 8% 5% 4 5% 5% 6% 7% 32%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 5% 47%	4429
	11th to 20th	Night	17% 9% 3 6% 3 4 4 3 4 4 43%	4440
	21st to end	Night	25% 10% 3 6% 3 3 4 3 4 5% 35%	4624
February	1st to 10th	Night	17% 9% 3 5% 3 4 4 3 5% 6% 42%	2465
	11th to 20th	Night	17% 9% 2 6% 4 4 3 3 5% 6% 41%	2218
	21st to end	Night	20% 7% 2 5% 4 3 3 3 5% 5% 41%	1884
March	1st to 10th	Night	24% 10% 3 6% 4 3 3 2 4 5% 37%	3724
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 6% 47%	3608
	21st to end	Night	20% 9% 3 5% 3 3 3 3 4 5% 42%	3503
April	1st to 10th	Night	19% 7% 2 6% 4 4 4 4 6% 5% 38%	1900
	11th to 20th	Night	18% 7% 2 6% 4 4 5% 3 4 7% 41%	1875
	21st to end	Night	19% 7% 2 5% 3 3 4 3 5% 6% 42%	1886
May	1st to 10th	Night	26% 10% 3 7% 4 4 3 3 4 6% 30%	2834
	11th to 20th	Night	25% 8% 3 7% 5% 4 3 3 4 5% 33%	2540
	21st to end	Night	16% 7% 3 6% 4 4 4 3 6% 8% 41%	2738
June	1st to 10th	Night	16% 7% 2 4 4 4 4 3 6% 8% 40%	1520
	11th to 20th	Night	13% 7% 2 5% 5% 4 4 4 6% 8% 42%	1520
	21st to end	Night	10% 4 2 5% 5% 4 5% 4 7% 10% 44%	1520
July	1st to 10th	Night	10% 6% 3 7% 6% 6% 5% 5% 7% 8% 37%	2520
	11th to 20th	Night	14% 6% 2 6% 5% 5% 6% 5% 7% 9% 36%	2520
	21st to end	Night	17% 8% 2 6% 4 4 5% 4 6% 8% 35%	2739
August	1st to 10th	Night	23% 6% 3 6% 4 4 5% 4 6% 8% 32%	2740
	11th to 20th	Night	14% 6% 3 7% 5% 5% 4 4 6% 7% 39%	3097
	21st to end	Night	10% 5% 2 5% 4 3 4 3 5% 7% 52%	3476
September	1st to 10th	Night	15% 5% 2 7% 4 4 4 3 5% 6% 44%	1900
	11th to 20th	Night	15% 5% 1 4 3 3 3 3 4 7% 51%	1976
	21st to end	Night	18% 7% 2 6% 4 4 3 3 4 6% 44%	2090
October	1st to 10th	Night	19% 6% 3 6% 4 3 4 3 5% 7% 41%	2199
	11th to 20th	Night	15% 4 2 6% 4 3 3 3 4 7% 50%	2280
	21st to end	Night	20% 7% 2 5% 3 3 3 3 5% 5% 45%	2650
November	1st to 10th	Night	13% 5% 1 5% 4 3 4 4 4 5% 52%	2470
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 4 5% 47%	2470
	21st to end	Night	14% 6% 2 5% 4 3 4 4 5% 7% 46%	2546
December	1st to 10th	Night	20% 7% 3 6% 4 3 3 4 5% 6% 40%	2660
	11th to 20th	Night	20% 7% 1 5% 4 4 4 3 5% 6% 42%	3288
	21st to end	Night	20% 9% 3 6% 4 4 4 3 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	38% 3 11% 31% 14% 3	2518
	11th to 20th	Day	45% 6% 13% 23% 12% 2	2520
	21st to end	Day	55% 4 9% 21% 10% 2	3030
February	1st to 10th	Day	45% 5% 13% 23% 12% 2	1714
	11th to 20th	Day	42% 4 10% 25% 17% 3	1857
	21st to end	Day	46% 5% 11% 22% 13% 3	1570
March	1st to 10th	Day	48% 6% 11% 23% 9% 2	3119
	11th to 20th	Day	32% 9% 14% 30% 14% 2	3347
	21st to end	Day	43% 4 9% 15% 24% 5%	4127
April	1st to 10th	Day	44% 5% 11% 20% 16% 4	2280
	11th to 20th	Day	41% 6% 12% 21% 17% 3	2254
	21st to end	Day	43% 6% 11% 20% 17% 4	2294
May	1st to 10th	Day	53% 6% 14% 14% 12% 1	4120
	11th to 20th	Day	50% 8% 13% 18% 9% 1	4386
	21st to end	Day	34% 8% 12% 25% 18% 2	4837
June	1st to 10th	Day	40% 9% 13% 21% 15% 2	2660
	11th to 20th	Day	42% 8% 13% 23% 12% 2	2660
	21st to end	Day	38% 9% 14% 22% 16% 1	2660
July	1st to 10th	Day	41% 10% 13% 21% 15% 1	4370
	11th to 20th	Day	38% 8% 17% 21% 14% 1	4365
	21st to end	Day	47% 8% 12% 16% 15% 2	4661
August	1st to 10th	Day	46% 8% 15% 21% 9%	4125
	11th to 20th	Day	37% 7% 14% 29% 13% 1	3766
	21st to end	Day	35% 6% 11% 27% 20% 2	4180
September	1st to 10th	Day	38% 7% 11% 21% 20% 3	2280
	11th to 20th	Day	33% 5% 11% 24% 24% 3	2203
	21st to end	Day	44% 6% 10% 18% 19% 3	2090
October	1st to 10th	Day	37% 5% 11% 18% 23% 6%	3288
	11th to 20th	Day	25% 4 11% 23% 32% 5%	3140
	21st to end	Day	28% 4 13% 25% 26% 4	3186
November	1st to 10th	Day	35% 4 12% 23% 23% 3	1708
	11th to 20th	Day	39% 5% 9% 23% 21% 4	1710
	21st to end	Day	39% 5% 14% 29% 12% 3	1630
December	1st to 10th	Day	45% 4 15% 21% 14% 1	1520
	11th to 20th	Day	42% 4 14% 24% 14% 2	1906
	21st to end	Day	48% 9% 12% 22% 8%	2772
January	1st to 10th	Night	39% 4 10% 30% 12% 4	4429
	11th to 20th	Night	44% 5% 12% 25% 12% 2	4440
	21st to end	Night	51% 5% 9% 22% 12% 2	4624
February	1st to 10th	Night	44% 5% 12% 24% 13% 2	2465
	11th to 20th	Night	45% 4 9% 19% 18% 5%	2220
	21st to end	Night	44% 5% 8% 25% 16% 4	1884
March	1st to 10th	Night	49% 6% 10% 21% 11% 3	3734
	11th to 20th	Night	35% 8% 13% 25% 17% 3	3609
	21st to end	Night	45% 5% 7% 15% 22% 6%	3503
April	1st to 10th	Night	45% 6% 11% 19% 17% 3	1900
	11th to 20th	Night	44% 5% 9% 20% 19% 2	1875
	21st to end	Night	42% 6% 8% 21% 19% 4	1886
May	1st to 10th	Night	54% 7% 11% 16% 10% 1	2840
	11th to 20th	Night	53% 7% 15% 13% 10% 2	2541
	21st to end	Night	39% 8% 14% 23% 13% 3	2739
June	1st to 10th	Night	41% 7% 10% 21% 20% 2	1520
	11th to 20th	Night	38% 8% 14% 27% 12% 1	1520
	21st to end	Night	35% 9% 15% 24% 16% 1	1520
July	1st to 10th	Night	42% 9% 15% 18% 14% 2	2480
	11th to 20th	Night	41% 9% 14% 23% 12% 1	2472
	21st to end	Night	45% 7% 16% 14% 17% 2	2651
August	1st to 10th	Night	49% 8% 16% 16% 9% 1	2730
	11th to 20th	Night	41% 7% 13% 27% 10% 1	3127
	21st to end	Night	30% 6% 11% 31% 20% 1	3476
September	1st to 10th	Night	40% 7% 10% 23% 19% 1	1900
	11th to 20th	Night	33% 5% 11% 23% 24% 3	1976
	21st to end	Night	42% 5% 8% 21% 21% 2	2090
October	1st to 10th	Night	37% 6% 12% 22% 20% 2	3667
	11th to 20th	Night	24% 4 10% 27% 30% 4	3779
	21st to end	Night	30% 4 11% 29% 23% 2	4362
November	1st to 10th	Night	35% 6% 10% 23% 22% 4	2470
	11th to 20th	Night	40% 4 9% 24% 19% 4	2470
	21st to end	Night	38% 4 13% 28% 15% 2	2546
December	1st to 10th	Night	44% 6% 12% 23% 13% 2	2660
	11th to 20th	Night	43% 5% 12% 20% 17% 4	3288
	21st to end	Night	47% 7% 12% 21% 12%	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	21% 28% 30% 18% 3 1	180
	11th to 20th	Day	23% 34% 29% 11% 3	180
	21st to end	Day	20% 27% 32% 13% 6% 1	272
February	1st to 10th	Day	26% 26% 34% 13% 2	274
	11th to 20th	Day	24% 21% 37% 16% 2	456
	21st to end	Day	27% 19% 35% 15% 3	382
March	1st to 10th	Day	22% 23% 30% 20% 4	460
	11th to 20th	Day	23% 20% 36% 15% 5% 1	460
	21st to end	Day	27% 21% 34% 13% 5% 1	506
April	1st to 10th	Day	25% 22% 32% 13% 7% 1	460
	11th to 20th	Day	22% 22% 31% 16% 8% 1	452
	21st to end	Day	26% 17% 34% 17% 4 2	460
May	1st to 10th	Day	31% 19% 27% 16% 5% 3	463
	11th to 20th	Day	22% 14% 36% 21% 7% 1	487
	21st to end	Day	19% 8% 30% 26% 15% 3	529
June	1st to 10th	Day	16% 12% 30% 27% 12% 3	490
	11th to 20th	Day	16% 13% 32% 27% 11% 2	490
	21st to end	Day	14% 13% 30% 27% 15% 2	490
July	1st to 10th	Day	12% 5% 28% 32% 17% 5%	485
	11th to 20th	Day	10% 5% 29% 32% 21% 4	484
	21st to end	Day	15% 7% 30% 30% 16% 3	532
August	1st to 10th	Day	12% 7% 26% 32% 17% 6%	486
	11th to 20th	Day	15% 5% 24% 32% 19% 5%	479
	21st to end	Day	12% 8% 28% 30% 15% 7%	539
September	1st to 10th	Day	18% 8% 28% 23% 15% 8%	490
	11th to 20th	Day	14% 7% 27% 27% 14% 10%	490
	21st to end	Day	23% 11% 29% 22% 10% 5%	490
October	1st to 10th	Day	24% 8% 28% 22% 13% 4	490
	11th to 20th	Day	18% 16% 29% 24% 10% 2	490
	21st to end	Day	23% 13% 33% 21% 8% 2	464
November	1st to 10th	Day	16% 14% 33% 22% 12% 3	378
	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% 17% 31% 23% 3	760
	21st to end	Night	20% 17% 40% 22% 2	836
February	1st to 10th	Night	22% 15% 36% 25% 3	760
	11th to 20th	Night	23% 18% 40% 18% 1	743
	21st to end	Night	22% 20% 34% 21% 3	628
March	1st to 10th	Night	22% 15% 37% 21% 5%	760
	11th to 20th	Night	22% 16% 37% 20% 4 1	760
	21st to end	Night	22% 15% 37% 20% 5%	836
April	1st to 10th	Night	23% 16% 35% 22% 4	760
	11th to 20th	Night	22% 15% 34% 24% 6%	751
	21st to end	Night	22% 12% 35% 25% 5% 1	746
May	1st to 10th	Night	24% 13% 34% 22% 7%	570
	11th to 20th	Night	22% 10% 37% 24% 7%	406
	21st to end	Night	19% 5% 24% 38% 13% 1	411
June	1st to 10th	Night	18% 7% 28% 35% 11%	380
	11th to 20th	Night	15% 7% 26% 39% 14%	380
	21st to end	Night	11% 4 24% 51% 10% 1	380
July	1st to 10th	Night	12% 5% 24% 41% 18% 1	370
	11th to 20th	Night	13% 3 21% 43% 18% 1	368
	21st to end	Night	15% 4 21% 44% 15% 1	416
August	1st to 10th	Night	12% 3 19% 44% 21% 1	508
	11th to 20th	Night	16% 4 18% 38% 24% 1	750
	21st to end	Night	15% 4 19% 40% 21% 2	836
September	1st to 10th	Night	15% 4 25% 35% 18% 2	760
	11th to 20th	Night	17% 4 23% 33% 19% 4	760
	21st to end	Night	22% 5% 26% 31% 15% 2	760
October	1st to 10th	Night	22% 8% 25% 28% 14% 3	760
	11th to 20th	Night	17% 10% 31% 30% 11% 1	760
	21st to end	Night	23% 9% 35% 25% 9%	836
November	1st to 10th	Night	15% 11% 34% 28% 11% 1	760
	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 2024-11-03 02:11:27. Version ac07

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