Alexandria
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alexandria clear sky chart from 2005-11-06 to 2024-04-03.

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	12% 8% 4 7% 6% 4 5% 5% 6% 7% 36%	78613
	Night	11% 9% 4 8% 6% 4 5% 5% 5% 7% 36%	81561
Transparency	Day	46% 7% 14% 23% 10% 1	78308
	Night	46% 7% 13% 22% 10% 1	81386
Seeing	Day	15% 9% 32% 27% 14% 3	15120
	Night	14% 9% 33% 32% 11% 1	25252

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	9% 15% 5% 11% 11% 8% 7% 5% 5% 5% 19%	2218
	11th to 20th	Day	5% 9% 5% 14% 9% 5% 12% 10% 7% 6% 19%	2399
	21st to end	Day	6% 12% 11% 15% 10% 4 7% 5% 6% 6% 17%	3135
February	1st to 10th	Day	8% 11% 6% 9% 7% 5% 7% 6% 6% 9% 27%	1710
	11th to 20th	Day	6% 9% 4 9% 9% 5% 7% 6% 7% 8% 29%	1664
	21st to end	Day	9% 11% 5% 10% 10% 6% 6% 6% 5% 9% 25%	1465
March	1st to 10th	Day	12% 10% 5% 9% 8% 4 5% 5% 5% 7% 29%	2047
	11th to 20th	Day	11% 10% 4 9% 6% 3 5% 5% 5% 7% 34%	2090
	21st to end	Day	17% 11% 5% 9% 6% 4 5% 4 6% 6% 26%	2299
April	1st to 10th	Day	16% 10% 4 6% 5% 3 4 5% 5% 7% 35%	2131
	11th to 20th	Day	17% 12% 4 7% 5% 3 4 5% 5% 6% 32%	2276
	21st to end	Day	23% 10% 4 5% 4 2 4 4 5% 6% 33%	2340
May	1st to 10th	Day	18% 8% 4 6% 6% 3 5% 5% 4 6% 34%	2358
	11th to 20th	Day	16% 7% 3 5% 5% 3 4 4 6% 9% 38%	2479
	21st to end	Day	15% 6% 3 5% 6% 3 5% 5% 6% 8% 38%	2725
June	1st to 10th	Day	11% 5% 2 5% 5% 4 4 5% 6% 8% 43%	2676
	11th to 20th	Day	14% 6% 3 6% 5% 3 4 5% 6% 8% 40%	2700
	21st to end	Day	9% 5% 2 5% 5% 3 5% 6% 7% 11% 42%	2700
July	1st to 10th	Day	6% 3 2 4 4 3 4 6% 6% 10% 51%	2700
	11th to 20th	Day	7% 5% 2 4 4 4 5% 4 6% 8% 52%	2700
	21st to end	Day	6% 3 1 4 4 3 4 6% 6% 9% 53%	2637
August	1st to 10th	Day	8% 6% 2 5% 5% 4 5% 6% 7% 9% 44%	2328
	11th to 20th	Day	12% 5% 2 5% 4 3 4 5% 6% 9% 45%	2301
	21st to end	Day	10% 5% 1 4 4 3 3 3 5% 8% 53%	2561
September	1st to 10th	Day	11% 5% 2 4 4 3 4 4 5% 8% 52%	2160
	11th to 20th	Day	11% 5% 3 5% 5% 3 4 5% 5% 7% 46%	2017
	21st to end	Day	15% 6% 3 4 4 2 4 4 5% 8% 45%	1968
October	1st to 10th	Day	16% 9% 2 5% 5% 3 4 4 5% 7% 39%	1800
	11th to 20th	Day	15% 11% 4 5% 4 2 4 4 5% 7% 39%	1800
	21st to end	Day	18% 10% 4 7% 5% 3 4 4 5% 6% 35%	1922
November	1st to 10th	Day	13% 12% 5% 7% 6% 3 4 4 5% 7% 34%	1541
	11th to 20th	Day	9% 13% 6% 13% 7% 4 6% 5% 5% 6% 26%	1520
	21st to end	Day	12% 10% 6% 9% 8% 4 5% 4 6% 7% 30%	1520
December	1st to 10th	Day	9% 11% 5% 13% 9% 4 8% 5% 6% 6% 24%	1520
	11th to 20th	Day	9% 12% 6% 11% 9% 5% 7% 5% 5% 7% 25%	1764
	21st to end	Day	21% 13% 5% 11% 11% 3 4 3 4 6% 20%	2442
January	1st to 10th	Night	8% 9% 6% 12% 12% 5% 6% 5% 5% 7% 24%	4737
	11th to 20th	Night	6% 10% 5% 11% 11% 7% 12% 10% 7% 6% 16%	4566
	21st to end	Night	7% 14% 12% 13% 6% 4 5% 5% 6% 7% 21%	4532
February	1st to 10th	Night	6% 10% 5% 9% 7% 5% 6% 6% 7% 9% 31%	2470
	11th to 20th	Night	6% 9% 4 8% 6% 4 6% 5% 7% 7% 36%	2403
	21st to end	Night	10% 10% 4 7% 7% 4 6% 5% 6% 7% 33%	2007
March	1st to 10th	Night	11% 9% 4 8% 7% 4 5% 5% 6% 7% 34%	2132
	11th to 20th	Night	11% 10% 4 8% 6% 3 4 4 5% 7% 38%	2090
	21st to end	Night	16% 10% 5% 8% 5% 3 5% 5% 5% 8% 31%	2299
April	1st to 10th	Night	15% 9% 4 6% 4 3 4 4 4 6% 40%	1872
	11th to 20th	Night	17% 9% 5% 7% 5% 3 4 3 4 6% 36%	1630
	21st to end	Night	17% 9% 4 6% 6% 2 3 5% 4 5% 38%	1620
May	1st to 10th	Night	16% 9% 3 5% 5% 4 4 3 5% 8% 39%	1600
	11th to 20th	Night	14% 6% 3 5% 5% 3 4 6% 6% 6% 42%	1416
	21st to end	Night	12% 5% 2 5% 6% 3 4 6% 5% 9% 43%	1551
June	1st to 10th	Night	11% 4 1 3 4 3 4 4 5% 9% 50%	1280
	11th to 20th	Night	10% 4 3 4 4 2 5% 5% 7% 8% 48%	1260
	21st to end	Night	8% 5% 2 4 4 2 4 5% 5% 9% 53%	1260
July	1st to 10th	Night	4 3 2 3 2 3 4 4 6% 10% 59%	1260
	11th to 20th	Night	4 3 2 3 4 2 4 3 5% 8% 63%	1260
	21st to end	Night	4 3 1 3 4 3 4 5% 5% 9% 59%	1678
August	1st to 10th	Night	7% 5% 1 4 4 3 4 4 7% 9% 52%	1611
	11th to 20th	Night	10% 4 2 4 4 2 3 4 5% 8% 55%	1600
	21st to end	Night	8% 3 2 3 3 2 3 3 4 6% 61%	1795
September	1st to 10th	Night	10% 4 2 4 3 2 3 4 5% 8% 55%	1800
	11th to 20th	Night	9% 6% 3 5% 4 2 4 4 5% 6% 53%	1939
	21st to end	Night	11% 6% 3 4 4 2 3 4 5% 6% 51%	1980
October	1st to 10th	Night	13% 8% 2 6% 5% 3 3 4 5% 6% 44%	2153
	11th to 20th	Night	14% 8% 3 5% 5% 3 4 3 5% 7% 42%	2160
	21st to end	Night	16% 10% 4 8% 5% 3 4 4 5% 6% 36%	2432
November	1st to 10th	Night	11% 9% 4 6% 6% 3 4 5% 6% 6% 39%	2535
	11th to 20th	Night	12% 12% 5% 11% 7% 3 5% 5% 5% 5% 32%	2660
	21st to end	Night	10% 9% 4 8% 7% 3 5% 5% 6% 7% 36%	2660
December	1st to 10th	Night	9% 11% 5% 11% 8% 4 6% 5% 6% 8% 28%	2660
	11th to 20th	Night	10% 12% 5% 11% 9% 5% 6% 5% 6% 7% 25%	3428
	21st to end	Night	19% 15% 6% 10% 9% 4 4 5% 4 6% 18%	5225

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	64% 8% 10% 14% 4	2219
	11th to 20th	Day	60% 8% 9% 18% 4	2399
	21st to end	Day	64% 9% 9% 12% 5%	3135
February	1st to 10th	Day	52% 6% 12% 19% 10%	1710
	11th to 20th	Day	50% 7% 14% 21% 6% 1	1673
	21st to end	Day	56% 7% 11% 18% 9%	1465
March	1st to 10th	Day	53% 8% 13% 19% 6% 1	2047
	11th to 20th	Day	48% 7% 13% 24% 9%	2090
	21st to end	Day	57% 7% 14% 17% 6%	2299
April	1st to 10th	Day	47% 7% 11% 21% 12% 1	2131
	11th to 20th	Day	52% 6% 12% 17% 12% 1	2276
	21st to end	Day	52% 7% 13% 19% 9% 1	2340
May	1st to 10th	Day	50% 7% 12% 21% 9%	2358
	11th to 20th	Day	44% 8% 13% 25% 10% 1	2483
	21st to end	Day	42% 8% 14% 24% 10% 2	2725
June	1st to 10th	Day	37% 9% 16% 28% 10% 1	2676
	11th to 20th	Day	41% 9% 14% 21% 14% 1	2700
	21st to end	Day	33% 10% 21% 26% 10%	2700
July	1st to 10th	Day	25% 10% 17% 31% 16% 1	2626
	11th to 20th	Day	29% 8% 16% 29% 15% 3	2619
	21st to end	Day	26% 8% 18% 30% 15% 2	2490
August	1st to 10th	Day	34% 9% 16% 29% 10% 3	2314
	11th to 20th	Day	35% 9% 18% 28% 9% 1	2301
	21st to end	Day	29% 7% 13% 34% 15% 2	2561
September	1st to 10th	Day	32% 6% 16% 33% 12% 1	2160
	11th to 20th	Day	36% 7% 18% 29% 9%	2017
	21st to end	Day	38% 6% 14% 31% 10% 1	1968
October	1st to 10th	Day	44% 7% 13% 25% 12% 1	1800
	11th to 20th	Day	45% 6% 13% 23% 11% 2	1800
	21st to end	Day	50% 6% 13% 24% 7% 1	1922
November	1st to 10th	Day	49% 7% 10% 20% 12% 2	1541
	11th to 20th	Day	59% 6% 13% 15% 8% 1	1520
	21st to end	Day	53% 5% 16% 18% 6% 2	1520
December	1st to 10th	Day	59% 8% 13% 14% 6%	1520
	11th to 20th	Day	57% 7% 15% 16% 4	1761
	21st to end	Day	66% 6% 9% 13% 5%	2442
January	1st to 10th	Night	58% 7% 12% 17% 5%	4737
	11th to 20th	Night	63% 9% 11% 12% 5%	4566
	21st to end	Night	60% 7% 10% 16% 6%	4532
February	1st to 10th	Night	47% 7% 13% 21% 11% 1	2470
	11th to 20th	Night	44% 7% 15% 22% 11% 1	2404
	21st to end	Night	48% 7% 12% 19% 11% 2	2007
March	1st to 10th	Night	48% 7% 14% 21% 8% 1	2132
	11th to 20th	Night	46% 5% 13% 24% 12% 1	2090
	21st to end	Night	51% 6% 12% 22% 7% 1	2299
April	1st to 10th	Night	44% 6% 13% 23% 13% 1	1872
	11th to 20th	Night	51% 4 7% 23% 14% 2	1630
	21st to end	Night	48% 6% 11% 22% 12% 2	1620
May	1st to 10th	Night	44% 7% 12% 23% 13% 1	1600
	11th to 20th	Night	40% 8% 13% 24% 13% 2	1417
	21st to end	Night	37% 8% 15% 24% 14% 3	1552
June	1st to 10th	Night	30% 9% 16% 31% 13% 1	1280
	11th to 20th	Night	30% 9% 17% 25% 15% 3	1260
	21st to end	Night	27% 8% 18% 27% 17% 2	1260
July	1st to 10th	Night	19% 9% 19% 35% 19% 1	1225
	11th to 20th	Night	20% 7% 14% 37% 17% 4	1218
	21st to end	Night	21% 8% 16% 36% 16% 4	1585
August	1st to 10th	Night	27% 9% 16% 31% 12% 4	1601
	11th to 20th	Night	28% 7% 17% 33% 13% 1	1600
	21st to end	Night	24% 6% 13% 33% 21% 3	1795
September	1st to 10th	Night	27% 7% 17% 31% 17% 1	1800
	11th to 20th	Night	32% 7% 16% 31% 15%	1939
	21st to end	Night	33% 6% 14% 32% 15%	1980
October	1st to 10th	Night	40% 5% 12% 27% 14% 2	2153
	11th to 20th	Night	41% 6% 12% 26% 13% 1	2160
	21st to end	Night	48% 6% 12% 22% 11%	2432
November	1st to 10th	Night	42% 7% 13% 24% 12% 2	2535
	11th to 20th	Night	53% 6% 13% 17% 10% 1	2660
	21st to end	Night	46% 8% 17% 22% 6% 2	2660
December	1st to 10th	Night	54% 7% 15% 17% 7%	2660
	11th to 20th	Night	56% 9% 13% 16% 5% 1	3430
	21st to end	Night	66% 7% 10% 12% 5%	5225

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	12% 11% 35% 28% 11% 2	270
	11th to 20th	Day	14% 10% 35% 33% 7%	270
	21st to end	Day	10% 8% 39% 24% 16% 2	297
February	1st to 10th	Day	12% 18% 39% 27% 3 1	270
	11th to 20th	Day	14% 14% 41% 22% 9%	270
	21st to end	Day	14% 12% 40% 29% 4	225
March	1st to 10th	Day	15% 13% 41% 22% 7% 1	270
	11th to 20th	Day	16% 10% 37% 24% 11% 1	270
	21st to end	Day	22% 10% 32% 21% 10% 4	297
April	1st to 10th	Day	24% 13% 33% 22% 7% 1	375
	11th to 20th	Day	27% 13% 26% 24% 10% 1	554
	21st to end	Day	25% 10% 32% 19% 13% 2	600
May	1st to 10th	Day	21% 11% 32% 24% 10% 2	603
	11th to 20th	Day	18% 10% 31% 26% 12% 3	625
	21st to end	Day	17% 7% 31% 30% 13% 2	679
June	1st to 10th	Day	10% 6% 27% 33% 21% 3	630
	11th to 20th	Day	14% 7% 36% 28% 13% 2	630
	21st to end	Day	11% 6% 35% 27% 17% 3	630
July	1st to 10th	Day	6% 3 25% 33% 26% 6%	621
	11th to 20th	Day	7% 6% 32% 33% 17% 6%	618
	21st to end	Day	6% 4 25% 35% 26% 5%	684
August	1st to 10th	Day	9% 2 24% 34% 26% 5%	626
	11th to 20th	Day	13% 5% 21% 31% 23% 7%	615
	21st to end	Day	11% 6% 25% 29% 24% 5%	680
September	1st to 10th	Day	12% 7% 30% 31% 16% 3	450
	11th to 20th	Day	13% 4 31% 32% 16% 4	307
	21st to end	Day	20% 12% 32% 21% 13% 2	270
October	1st to 10th	Day	20% 9% 29% 26% 12% 4	270
	11th to 20th	Day	15% 17% 36% 23% 9% 1	270
	21st to end	Day	24% 14% 36% 17% 7% 1	297
November	1st to 10th	Day	23% 11% 32% 28% 6% 1	270
	11th to 20th	Day	15% 17% 45% 17% 5%	270
	21st to end	Day	18% 17% 37% 23% 4	270
December	1st to 10th	Day	11% 9% 44% 25% 10% 1	270
	11th to 20th	Day	16% 13% 43% 24% 3	270
	21st to end	Day	26% 11% 37% 21% 4 1	297
January	1st to 10th	Night	10% 11% 41% 28% 9% 2	950
	11th to 20th	Night	14% 10% 39% 29% 7% 1	950
	21st to end	Night	15% 11% 39% 28% 7%	1045
February	1st to 10th	Night	10% 10% 38% 33% 9% 1	950
	11th to 20th	Night	11% 10% 44% 25% 8% 1	929
	21st to end	Night	14% 11% 34% 31% 8% 2	751
March	1st to 10th	Night	14% 11% 38% 28% 8% 1	612
	11th to 20th	Night	18% 10% 39% 27% 6% 1	570
	21st to end	Night	19% 9% 29% 29% 12% 1	627
April	1st to 10th	Night	19% 9% 37% 27% 7% 1	546
	11th to 20th	Night	21% 11% 32% 27% 9% 1	533
	21st to end	Night	22% 8% 32% 28% 8% 2	540
May	1st to 10th	Night	18% 9% 33% 31% 8% 1	540
	11th to 20th	Night	14% 9% 33% 32% 11% 1	531
	21st to end	Night	13% 7% 29% 38% 12%	584
June	1st to 10th	Night	12% 5% 26% 38% 18% 1	540
	11th to 20th	Night	11% 5% 32% 44% 8%	540
	21st to end	Night	9% 5% 23% 46% 16%	540
July	1st to 10th	Night	5% 2 24% 42% 26% 2	525
	11th to 20th	Night	6% 4 26% 46% 17% 1	522
	21st to end	Night	5% 2 25% 44% 21% 3	591
August	1st to 10th	Night	8% 2 19% 50% 19% 2	537
	11th to 20th	Night	12% 4 20% 38% 24% 3	534
	21st to end	Night	8% 5% 26% 38% 22% 2	594
September	1st to 10th	Night	11% 5% 26% 40% 15% 2	540
	11th to 20th	Night	11% 7% 27% 36% 17% 1	540
	21st to end	Night	14% 9% 34% 30% 12% 1	540
October	1st to 10th	Night	19% 9% 33% 29% 10%	713
	11th to 20th	Night	20% 10% 35% 27% 8% 1	720
	21st to end	Night	21% 11% 35% 27% 5%	848
November	1st to 10th	Night	16% 11% 33% 32% 8%	925
	11th to 20th	Night	16% 12% 37% 25% 8% 1	950
	21st to end	Night	15% 15% 39% 26% 6%	950
December	1st to 10th	Night	14% 12% 36% 30% 8% 1	950
	11th to 20th	Night	18% 10% 37% 27% 6% 1	950
	21st to end	Night	24% 11% 36% 24% 6% 1	1045

Page generated at 2024-03-31 05:08:54. Version ac07

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