Potomac Astrometric Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Potomac Astrometric Observatory 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	15% 6% 2 6% 4 4 5% 4 6% 7% 41%	101221
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97762
Transparency	Day	40% 6% 11% 25% 16% 3	104859
	Night	40% 5% 11% 25% 17% 3	102264
Seeing	Day	19% 12% 27% 24% 13% 5%	14991
	Night	18% 12% 29% 29% 12% 1	24481

By Month

Percentage Cloud Cover SE (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	18% 5% 1 4 3 4 4 4 5% 6% 47%	2585
	11th to 20th	Day	15% 11% 3 6% 5% 4 4 3 4 5% 40%	2840
	21st to end	Day	19% 9% 3 7% 5% 5% 4 3 4 6% 36%	3124
February	1st to 10th	Day	15% 7% 2 7% 5% 5% 4 4 6% 7% 38%	1710
	11th to 20th	Day	18% 7% 2 5% 5% 3 4 4 6% 6% 40%	1714
	21st to end	Day	18% 8% 3 7% 4 5% 3 3 4 5% 40%	1570
March	1st to 10th	Day	23% 7% 2 5% 4 4 4 3 5% 6% 36%	3374
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 5% 6% 49%	3480
	21st to end	Day	21% 7% 2 5% 4 4 3 3 5% 6% 40%	3875
April	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 3 5% 6% 47%	2254
	21st to end	Day	19% 6% 2 5% 4 4 4 3 6% 7% 41%	2280
May	1st to 10th	Day	24% 7% 2 7% 5% 4 4 4 6% 8% 30%	4015
	11th to 20th	Day	23% 7% 3 6% 4 4 5% 4 6% 9% 29%	4291
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 45%	4823
June	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 7% 9% 43%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 42%	2660
	21st to end	Day	10% 4 2 4 4 4 5% 4 8% 10% 44%	2660
July	1st to 10th	Day	7% 4 1 5% 6% 5% 6% 5% 8% 11% 42%	4440
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 5% 8% 10% 40%	4313
	21st to end	Day	11% 7% 2 7% 6% 6% 5% 5% 7% 8% 36%	4466
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 7% 9% 33%	4104
	11th to 20th	Day	10% 4 2 7% 5% 5% 7% 5% 7% 8% 39%	3867
	21st to end	Day	10% 5% 2 6% 4 4 4 4 5% 8% 47%	4180
September	1st to 10th	Day	14% 5% 2 6% 5% 3 4 4 5% 7% 45%	2280
	11th to 20th	Day	12% 6% 2 4 3 4 4 4 5% 9% 47%	2280
	21st to end	Day	20% 7% 3 6% 4 4 3 4 6% 5% 38%	1998
October	1st to 10th	Day	18% 5% 2 4 4 3 3 3 5% 6% 47%	1900
	11th to 20th	Day	18% 4 2 4 3 3 4 3 4 6% 50%	1900
	21st to end	Day	14% 5% 2 6% 4 4 4 3 5% 5% 49%	2090
November	1st to 10th	Day	13% 5% 1 4 3 4 5% 3 5% 7% 51%	1799
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 6% 6% 49%	1687
	21st to end	Day	15% 6% 1 4 4 4 4 4 5% 5% 47%	1520
December	1st to 10th	Day	16% 6% 3 6% 4 4 4 5% 6% 8% 40%	1520
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 6% 7% 45%	1910
	21st to end	Day	18% 7% 3 6% 5% 4 4 4 5% 8% 36%	2772
January	1st to 10th	Night	15% 7% 2 6% 3 3 4 3 5% 7% 44%	4357
	11th to 20th	Night	19% 7% 2 5% 3 3 4 3 3 5% 46%	4120
	21st to end	Night	21% 9% 3 5% 3 3 3 3 4 6% 41%	4532
February	1st to 10th	Night	15% 6% 3 6% 4 6% 4 4 6% 6% 41%	2470
	11th to 20th	Night	19% 7% 2 6% 4 4 3 3 5% 6% 43%	2351
	21st to end	Night	18% 8% 2 5% 4 4 3 3 4 6% 42%	1884
March	1st to 10th	Night	22% 7% 3 5% 3 4 3 4 4 6% 39%	3464
	11th to 20th	Night	11% 4 2 6% 3 4 4 4 6% 7% 50%	3480
	21st to end	Night	15% 9% 2 6% 4 3 3 3 4 5% 47%	3755
April	1st to 10th	Night	15% 7% 2 6% 4 4 4 5% 5% 6% 41%	1900
	11th to 20th	Night	13% 5% 2 7% 4 3 5% 3 5% 6% 46%	1875
	21st to end	Night	16% 6% 2 4 4 5% 4 3 7% 7% 43%	1900
May	1st to 10th	Night	24% 8% 2 6% 4 4 4 3 5% 7% 32%	2907
	11th to 20th	Night	24% 9% 2 6% 4 4 3 4 5% 7% 32%	2601
	21st to end	Night	11% 4 2 5% 4 5% 5% 4 7% 11% 43%	2732
June	1st to 10th	Night	11% 4 2 5% 4 4 5% 4 7% 9% 45%	1520
	11th to 20th	Night	12% 6% 2 7% 4 4 4 5% 6% 10% 41%	1520
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 11% 43%	1520
July	1st to 10th	Night	7% 5% 2 6% 6% 6% 6% 5% 7% 9% 41%	2520
	11th to 20th	Night	8% 6% 2 7% 5% 6% 6% 6% 8% 8% 38%	2642
	21st to end	Night	12% 7% 2 7% 4 5% 5% 5% 7% 8% 38%	3087
August	1st to 10th	Night	17% 6% 3 7% 5% 4 5% 5% 6% 9% 32%	2825
	11th to 20th	Night	10% 6% 3 9% 6% 5% 6% 4 7% 8% 36%	2927
	21st to end	Night	11% 5% 2 5% 3 3 4 3 6% 8% 50%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	1900
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 6% 50%	1900
	21st to end	Night	20% 6% 3 6% 4 4 4 3 5% 6% 39%	2175
October	1st to 10th	Night	20% 6% 2 4 3 3 4 3 5% 6% 45%	2280
	11th to 20th	Night	15% 4 1 4 3 3 3 2 4 5% 53%	2280
	21st to end	Night	15% 6% 1 5% 3 3 4 3 5% 6% 50%	2508
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	2376
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 6% 50%	2489
	21st to end	Night	14% 4 2 4 4 4 4 4 6% 7% 47%	2660
December	1st to 10th	Night	17% 7% 2 7% 4 4 3 3 5% 7% 41%	2660
	11th to 20th	Night	17% 5% 2 6% 4 4 4 4 5% 6% 43%	3285
	21st to end	Night	17% 6% 2 6% 5% 5% 5% 4 6% 7% 37%	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% 6% 7% 31% 14% 4	2586
	11th to 20th	Day	47% 4 9% 21% 15% 4	2840
	21st to end	Day	50% 4 10% 23% 11% 2	3124
February	1st to 10th	Day	44% 5% 13% 24% 13% 2	1710
	11th to 20th	Day	44% 4 9% 25% 16% 3	1723
	21st to end	Day	47% 4 10% 20% 15% 4	1570
March	1st to 10th	Day	46% 6% 10% 22% 13% 3	3374
	11th to 20th	Day	33% 6% 8% 29% 20% 4	3480
	21st to end	Day	45% 4 10% 21% 17% 3	3875
April	1st to 10th	Day	40% 5% 12% 21% 18% 5%	2280
	11th to 20th	Day	37% 5% 10% 23% 20% 6%	2254
	21st to end	Day	42% 5% 9% 22% 18% 4	2280
May	1st to 10th	Day	52% 5% 11% 20% 11% 1	4052
	11th to 20th	Day	49% 7% 14% 21% 8% 1	4295
	21st to end	Day	35% 6% 15% 25% 16% 3	4837
June	1st to 10th	Day	37% 6% 14% 27% 15% 2	2660
	11th to 20th	Day	38% 8% 13% 25% 14% 2	2660
	21st to end	Day	33% 7% 15% 26% 18% 2	2660
July	1st to 10th	Day	33% 9% 19% 21% 12% 5%	4370
	11th to 20th	Day	36% 7% 16% 29% 11% 2	4238
	21st to end	Day	44% 7% 10% 22% 16% 1	4324
August	1st to 10th	Day	46% 7% 15% 25% 8% 1	4091
	11th to 20th	Day	38% 7% 11% 28% 15% 1	3906
	21st to end	Day	35% 6% 12% 28% 17% 2	4180
September	1st to 10th	Day	38% 6% 12% 20% 19% 4	2280
	11th to 20th	Day	35% 5% 8% 28% 21% 4	2280
	21st to end	Day	46% 5% 8% 20% 19% 2	1998
October	1st to 10th	Day	38% 4 7% 18% 28% 5%	3160
	11th to 20th	Day	25% 4 7% 29% 26% 9%	3140
	21st to end	Day	24% 3 9% 31% 28% 6%	3428
November	1st to 10th	Day	33% 4 11% 26% 21% 4	1799
	11th to 20th	Day	35% 4 10% 24% 22% 4	1687
	21st to end	Day	39% 5% 10% 28% 17% 2	1520
December	1st to 10th	Day	43% 5% 13% 25% 13% 2	1520
	11th to 20th	Day	39% 5% 10% 32% 12% 3	1906
	21st to end	Day	45% 5% 13% 24% 11% 1	2772
January	1st to 10th	Night	40% 5% 9% 27% 16% 3	4357
	11th to 20th	Night	41% 4 10% 25% 17% 3	4120
	21st to end	Night	46% 3 8% 27% 15% 1	4532
February	1st to 10th	Night	43% 5% 12% 25% 14% 1	2470
	11th to 20th	Night	43% 4 9% 22% 18% 4	2353
	21st to end	Night	45% 3 9% 22% 15% 5%	1884
March	1st to 10th	Night	46% 5% 9% 26% 12% 2	3464
	11th to 20th	Night	33% 6% 9% 31% 18% 4	3480
	21st to end	Night	41% 3 8% 23% 19% 5%	3755
April	1st to 10th	Night	42% 5% 9% 20% 19% 4	1900
	11th to 20th	Night	38% 4 12% 22% 18% 6%	1875
	21st to end	Night	39% 6% 10% 25% 15% 5%	1900
May	1st to 10th	Night	52% 4 11% 20% 10% 2	2936
	11th to 20th	Night	51% 6% 11% 20% 10% 1	2602
	21st to end	Night	35% 7% 18% 25% 13% 3	2739
June	1st to 10th	Night	34% 7% 12% 28% 17% 2	1520
	11th to 20th	Night	38% 7% 13% 26% 14% 3	1520
	21st to end	Night	35% 6% 14% 24% 18% 3	1520
July	1st to 10th	Night	38% 8% 20% 18% 13% 4	2480
	11th to 20th	Night	38% 8% 17% 25% 11% 1	2594
	21st to end	Night	42% 7% 13% 21% 16% 1	2988
August	1st to 10th	Night	46% 7% 17% 21% 8% 1	2814
	11th to 20th	Night	43% 7% 10% 27% 12% 1	2954
	21st to end	Night	33% 6% 12% 31% 17% 2	3476
September	1st to 10th	Night	39% 5% 10% 22% 19% 5%	1900
	11th to 20th	Night	34% 4 8% 28% 22% 4	1900
	21st to end	Night	46% 4 9% 20% 17% 3	2175
October	1st to 10th	Night	41% 4 9% 22% 19% 5%	3800
	11th to 20th	Night	25% 3 10% 24% 30% 8%	3779
	21st to end	Night	24% 3 9% 32% 25% 7%	4120
November	1st to 10th	Night	33% 3 9% 27% 22% 5%	2376
	11th to 20th	Night	37% 3 8% 24% 22% 5%	2489
	21st to end	Night	35% 4 11% 26% 20% 3	2660
December	1st to 10th	Night	43% 4 12% 24% 14% 2	2660
	11th to 20th	Night	41% 5% 9% 24% 18% 3	3288
	21st to end	Night	44% 5% 15% 24% 11% 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	32% 17% 35% 13% 3 1	200
	11th to 20th	Day	31% 23% 31% 13% 3	270
	21st to end	Day	25% 22% 35% 12% 4 1	297
February	1st to 10th	Day	24% 20% 40% 11% 4	270
	11th to 20th	Day	32% 20% 39% 7% 2	321
	21st to end	Day	28% 20% 32% 18% 2	382
March	1st to 10th	Day	26% 22% 30% 18% 4	460
	11th to 20th	Day	27% 21% 30% 18% 3 2	460
	21st to end	Day	22% 20% 33% 19% 6% 1	506
April	1st to 10th	Day	22% 20% 33% 17% 6% 1	460
	11th to 20th	Day	18% 17% 34% 17% 11% 3	452
	21st to end	Day	28% 12% 33% 19% 7% 2	460
May	1st to 10th	Day	25% 16% 28% 20% 8% 3	463
	11th to 20th	Day	20% 11% 30% 23% 12% 3	487
	21st to end	Day	16% 6% 25% 29% 20% 5%	529
June	1st to 10th	Day	14% 8% 28% 25% 19% 6%	490
	11th to 20th	Day	16% 6% 24% 31% 18% 5%	490
	21st to end	Day	10% 7% 30% 30% 17% 5%	490
July	1st to 10th	Day	12% 4 17% 34% 22% 11%	485
	11th to 20th	Day	7% 2 21% 34% 28% 8%	484
	21st to end	Day	13% 4 23% 32% 23% 6%	532
August	1st to 10th	Day	13% 4 19% 29% 24% 11%	486
	11th to 20th	Day	11% 4 18% 33% 23% 10%	479
	21st to end	Day	10% 4 18% 34% 21% 12%	539
September	1st to 10th	Day	17% 8% 18% 25% 21% 12%	490
	11th to 20th	Day	13% 5% 19% 32% 19% 12%	490
	21st to end	Day	27% 8% 23% 27% 10% 5%	490
October	1st to 10th	Day	24% 7% 20% 25% 19% 6%	490
	11th to 20th	Day	18% 13% 26% 27% 13% 4	490
	21st to end	Day	17% 12% 33% 25% 11% 2	539
November	1st to 10th	Day	17% 10% 34% 24% 12% 2	425
	11th to 20th	Day	21% 14% 34% 20% 10% 1	347
	21st to end	Day	23% 18% 33% 14% 11% 1	180
December	1st to 10th	Day	23% 18% 34% 19% 3 2	180
	11th to 20th	Day	26% 21% 27% 19% 7%	180
	21st to end	Day	25% 16% 35% 17% 7% 1	198
January	1st to 10th	Night	20% 19% 41% 17% 3	760
	11th to 20th	Night	24% 17% 32% 24% 3	760
	21st to end	Night	21% 19% 36% 22% 2	836
February	1st to 10th	Night	19% 16% 36% 27% 2	760
	11th to 20th	Night	25% 22% 36% 16% 1	743
	21st to end	Night	25% 17% 36% 20% 2	628
March	1st to 10th	Night	21% 17% 38% 21% 3	760
	11th to 20th	Night	23% 19% 33% 19% 4 1	760
	21st to end	Night	19% 18% 38% 20% 4 1	836
April	1st to 10th	Night	21% 21% 32% 22% 4	760
	11th to 20th	Night	16% 14% 36% 26% 7% 1	751
	21st to end	Night	19% 12% 34% 27% 7%	760
May	1st to 10th	Night	22% 8% 30% 28% 12%	627
	11th to 20th	Night	22% 5% 27% 35% 11%	441
	21st to end	Night	15% 5% 22% 39% 18% 1	411
June	1st to 10th	Night	12% 4 27% 35% 22% 1	380
	11th to 20th	Night	13% 3 24% 42% 16% 2	380
	21st to end	Night	9% 3 25% 44% 18% 1	380
July	1st to 10th	Night	10% 3 17% 41% 26% 3	370
	11th to 20th	Night	5% 3 14% 41% 34% 3	438
	21st to end	Night	10% 3 16% 43% 26% 3	624
August	1st to 10th	Night	12% 3 10% 43% 30% 2	567
	11th to 20th	Night	12% 3 13% 44% 24% 3	641
	21st to end	Night	11% 3 15% 39% 27% 4	836
September	1st to 10th	Night	17% 3 21% 29% 26% 4	760
	11th to 20th	Night	11% 5% 15% 36% 29% 4	760
	21st to end	Night	23% 5% 22% 31% 18% 1	760
October	1st to 10th	Night	21% 7% 20% 29% 20% 4	760
	11th to 20th	Night	17% 12% 26% 29% 15% 1	760
	21st to end	Night	19% 11% 31% 26% 13%	836
November	1st to 10th	Night	15% 10% 31% 32% 10% 1	760
	11th to 20th	Night	18% 17% 31% 27% 7% 1	760
	21st to end	Night	16% 16% 33% 28% 7% 1	760
December	1st to 10th	Night	23% 17% 32% 21% 7% 1	760
	11th to 20th	Night	19% 14% 38% 24% 5%	760
	21st to end	Night	19% 16% 34% 28% 3	836

Page generated at 2024-11-03 02:11:21. Version ac07

Copyright © 2024 Attilla Danko.