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 2025-08-05.

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	12% 7% 2 7% 5% 5% 5% 4 6% 7% 41%	125260
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	122970
Transparency	Day	41% 3 11% 21% 18% 7%	128867
	Night	38% 3 11% 20% 20% 9%	127440
Seeing	Day	14% 5% 17% 24% 25% 15%	16025
	Night	15% 4 19% 34% 24% 4	25738

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	17% 8% 3 6% 4 3 4 3 4 5% 43%	4077
	11th to 20th	Day	14% 6% 3 5% 4 3 3 3 4 5% 51%	4080
	21st to end	Day	23% 11% 3 6% 4 3 2 3 4 4 37%	4488
February	1st to 10th	Day	19% 9% 3 7% 5% 3 3 3 4 5% 38%	1847
	11th to 20th	Day	20% 8% 4 7% 4 3 3 3 4 5% 39%	1948
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 4 5% 35%	1775
March	1st to 10th	Day	20% 10% 3 8% 4 5% 4 3 4 5% 34%	4936
	11th to 20th	Day	13% 6% 2 7% 4 4 4 3 5% 6% 45%	4980
	21st to end	Day	13% 9% 3 9% 5% 4 4 3 4 6% 40%	5462
April	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 3 5% 6% 42%	2372
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 4 4 7% 41%	2374
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 4 6% 7% 41%	2400
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 32%	5405
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	5404
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	6344
June	1st to 10th	Day	8% 5% 2 7% 8% 6% 6% 5% 7% 9% 36%	2600
	11th to 20th	Day	6% 4 2 7% 6% 5% 6% 6% 8% 11% 39%	2600
	21st to end	Day	6% 5% 2 7% 5% 7% 7% 5% 10% 9% 36%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 8% 6% 9% 12% 31%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 7% 7% 6% 9% 11% 34%	5900
	21st to end	Day	12% 7% 3 7% 5% 6% 5% 5% 7% 10% 33%	6422
August	1st to 10th	Day	3 3 1 4 3 4 4 4 6% 9% 60%	3970
	11th to 20th	Day	4 4 2 4 4 5% 5% 5% 8% 12% 48%	3729
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 4 7% 8% 37%	4180
September	1st to 10th	Day	10% 5% 3 6% 5% 5% 5% 5% 7% 9% 39%	2280
	11th to 20th	Day	12% 6% 3 7% 4 5% 5% 5% 6% 8% 40%	2280
	21st to end	Day	11% 5% 2 4 3 4 4 3 5% 7% 52%	1980
October	1st to 10th	Day	6% 4 2 5% 4 4 5% 3 4 6% 58%	1900
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 4 6% 56%	1900
	21st to end	Day	12% 6% 2 6% 4 3 2 3 4 5% 54%	2090
November	1st to 10th	Day	11% 6% 3 6% 4 4 4 3 4 5% 51%	1958
	11th to 20th	Day	11% 7% 2 6% 4 3 3 3 4 6% 53%	2000
	21st to end	Day	14% 8% 3 6% 4 3 4 4 5% 6% 44%	2000
December	1st to 10th	Day	19% 9% 3 6% 4 3 3 2 4 4 43%	3200
	11th to 20th	Day	14% 10% 3 7% 5% 4 4 3 4 6% 40%	3391
	21st to end	Day	17% 8% 3 5% 3 3 3 2 4 5% 46%	4488
January	1st to 10th	Night	18% 5% 3 4 3 3 3 2 4 5% 51%	5888
	11th to 20th	Night	15% 8% 3 5% 4 4 3 2 4 5% 48%	5900
	21st to end	Night	26% 7% 3 6% 3 4 3 2 4 5% 38%	6490
February	1st to 10th	Night	16% 8% 3 7% 4 3 3 3 5% 6% 43%	2550
	11th to 20th	Night	22% 9% 2 5% 3 3 3 2 4 5% 43%	2338
	21st to end	Night	23% 8% 3 7% 3 3 3 3 4 4 39%	1870
March	1st to 10th	Night	20% 9% 3 6% 4 4 3 3 4 5% 38%	4919
	11th to 20th	Night	17% 6% 2 6% 4 3 4 3 5% 6% 45%	4980
	21st to end	Night	18% 6% 2 6% 4 3 4 3 5% 5% 42%	5467
April	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 7% 45%	2026
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 6% 6% 46%	1975
	21st to end	Night	9% 5% 2 5% 4 5% 4 4 6% 7% 50%	2000
May	1st to 10th	Night	17% 7% 3 8% 5% 4 5% 4 5% 8% 36%	4490
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 5% 8% 41%	4489
	21st to end	Night	10% 6% 3 6% 5% 5% 5% 4 7% 10% 39%	4499
June	1st to 10th	Night	7% 4 2 5% 4 5% 4 4 7% 10% 47%	1800
	11th to 20th	Night	4 4 1 4 5% 4 4 5% 7% 10% 53%	1800
	21st to end	Night	6% 5% 2 5% 4 4 5% 5% 8% 10% 47%	1800
July	1st to 10th	Night	5% 6% 3 7% 5% 6% 6% 5% 10% 13% 34%	4078
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 5% 7% 14% 44%	4077
	21st to end	Night	8% 7% 3 8% 5% 5% 6% 4 7% 10% 37%	4449
August	1st to 10th	Night	2 1 2 2 3 3 3 5% 9% 69%	3047
	11th to 20th	Night	3 3 2 5% 3 4 5% 4 8% 14% 50%	3100
	21st to end	Night	8% 6% 2 8% 5% 5% 5% 5% 6% 8% 43%	3476
September	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 5% 6% 8% 45%	1900
	11th to 20th	Night	8% 6% 2 6% 4 6% 5% 5% 6% 8% 45%	1900
	21st to end	Night	8% 5% 2 4 3 3 4 3 5% 6% 56%	2171
October	1st to 10th	Night	6% 3 1 4 4 4 5% 3 4 6% 59%	2280
	11th to 20th	Night	10% 5% 1 4 3 3 3 2 3 6% 60%	2280
	21st to end	Night	12% 4 1 5% 2 3 3 2 4 4 59%	2508
November	1st to 10th	Night	12% 6% 2 4 3 3 3 2 5% 6% 54%	2340
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 5% 5% 54%	2400
	21st to end	Night	14% 7% 3 6% 4 4 3 3 4 5% 47%	2400
December	1st to 10th	Night	21% 7% 3 6% 4 3 3 2 4 5% 41%	3977
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 4 5% 44%	4816
	21st to end	Night	17% 9% 2 5% 3 3 3 2 4 4 48%	6490

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	45% 1 3 24% 22% 4	4078
	11th to 20th	Day	38% 1 4 19% 28% 9%	4080
	21st to end	Day	53% 1 3 12% 17% 13%	4488
February	1st to 10th	Day	50% 2 6% 15% 18% 10%	1847
	11th to 20th	Day	49% 3 5% 17% 17% 9%	1958
	21st to end	Day	52% 3 5% 13% 17% 10%	1775
March	1st to 10th	Day	55% 1 5% 13% 16% 11%	4936
	11th to 20th	Day	41% 1 5% 17% 20% 15%	4980
	21st to end	Day	47% 2 9% 18% 17% 6%	5464
April	1st to 10th	Day	43% 2 8% 21% 16% 8%	2372
	11th to 20th	Day	44% 3 8% 18% 20% 8%	2374
	21st to end	Day	41% 4 7% 23% 20% 4	2400
May	1st to 10th	Day	51% 2 10% 24% 11% 2	5440
	11th to 20th	Day	42% 3 14% 22% 18% 1	5408
	21st to end	Day	39% 3 13% 29% 14% 2	6357
June	1st to 10th	Day	42% 4 14% 23% 15% 2	2600
	11th to 20th	Day	36% 6% 14% 26% 16% 2	2600
	21st to end	Day	39% 7% 16% 24% 13% 1	2600
July	1st to 10th	Day	43% 6% 24% 21% 5%	5836
	11th to 20th	Day	41% 6% 26% 22% 4	5831
	21st to end	Day	45% 5% 16% 27% 7%	6240
August	1st to 10th	Day	21% 4 13% 36% 26% 1	3956
	11th to 20th	Day	28% 4 21% 38% 8% 1	3765
	21st to end	Day	44% 4 16% 16% 19% 1	4179
September	1st to 10th	Day	41% 5% 16% 18% 15% 5%	2280
	11th to 20th	Day	40% 5% 12% 19% 19% 6%	2280
	21st to end	Day	33% 2 9% 23% 21% 11%	1980
October	1st to 10th	Day	20% 3 5% 15% 21% 35%	3160
	11th to 20th	Day	20% 1 3 17% 31% 28%	3140
	21st to end	Day	26% 2 3 15% 39% 16%	3428
November	1st to 10th	Day	37% 2 6% 19% 25% 10%	1958
	11th to 20th	Day	35% 2 6% 16% 29% 14%	2000
	21st to end	Day	42% 3 6% 19% 22% 8%	2000
December	1st to 10th	Day	47% 1 6% 18% 22% 5%	3200
	11th to 20th	Day	47% 2 6% 13% 22% 10%	3389
	21st to end	Day	43% 1 5% 19% 28% 4	4488
January	1st to 10th	Night	39% 1 5% 24% 26% 6%	5888
	11th to 20th	Night	41% 1 3 18% 27% 10%	5900
	21st to end	Night	52% 1 5% 13% 17% 11%	6490
February	1st to 10th	Night	43% 3 6% 18% 20% 11%	2550
	11th to 20th	Night	47% 2 5% 17% 19% 10%	2339
	21st to end	Night	50% 2 6% 14% 14% 14%	1870
March	1st to 10th	Night	50% 1 5% 12% 21% 10%	4919
	11th to 20th	Night	43% 2 6% 15% 17% 18%	4980
	21st to end	Night	45% 2 9% 16% 21% 7%	5467
April	1st to 10th	Night	38% 3 11% 19% 19% 11%	2026
	11th to 20th	Night	39% 2 10% 18% 19% 12%	1975
	21st to end	Night	34% 3 10% 23% 24% 6%	2000
May	1st to 10th	Night	48% 3 11% 22% 13% 3	4520
	11th to 20th	Night	43% 2 13% 21% 18% 2	4490
	21st to end	Night	41% 3 18% 26% 13% 1	4508
June	1st to 10th	Night	32% 6% 18% 25% 17% 2	1800
	11th to 20th	Night	25% 7% 18% 32% 16% 3	1800
	21st to end	Night	31% 6% 21% 28% 13% 1	1800
July	1st to 10th	Night	39% 8% 29% 18% 5%	4035
	11th to 20th	Night	30% 8% 33% 25% 4	4026
	21st to end	Night	42% 8% 18% 24% 7% 1	4320
August	1st to 10th	Night	14% 4 18% 33% 29% 1	3037
	11th to 20th	Night	23% 6% 27% 35% 8% 1	3130
	21st to end	Night	40% 6% 15% 18% 18% 3	3476
September	1st to 10th	Night	36% 7% 15% 21% 13% 7%	1900
	11th to 20th	Night	36% 5% 14% 19% 18% 8%	1900
	21st to end	Night	30% 3 10% 23% 20% 13%	2171
October	1st to 10th	Night	21% 3 6% 15% 20% 36%	3800
	11th to 20th	Night	17% 1 3 20% 29% 29%	3779
	21st to end	Night	22% 2 4 14% 36% 22%	4120
November	1st to 10th	Night	33% 2 8% 18% 26% 12%	2340
	11th to 20th	Night	33% 2 7% 18% 25% 16%	2400
	21st to end	Night	40% 3 6% 16% 24% 11%	2400
December	1st to 10th	Night	47% 1 6% 15% 26% 4	3977
	11th to 20th	Night	45% 2 6% 14% 24% 9%	4817
	21st to end	Night	42% 1 5% 19% 26% 6%	6490

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	22% 9% 40% 20% 8% 1	300
	11th to 20th	Day	22% 11% 35% 24% 7% 1	300
	21st to end	Day	28% 11% 33% 22% 6%	330
February	1st to 10th	Day	23% 9% 33% 28% 6% 1	300
	11th to 20th	Day	27% 15% 31% 22% 5% 1	300
	21st to end	Day	30% 11% 39% 16% 3	249
March	1st to 10th	Day	24% 12% 28% 29% 7%	300
	11th to 20th	Day	18% 12% 43% 20% 5% 2	300
	21st to end	Day	17% 11% 40% 23% 8% 2	330
April	1st to 10th	Day	21% 9% 31% 26% 12% 1	472
	11th to 20th	Day	20% 12% 26% 25% 14% 3	492
	21st to end	Day	14% 7% 27% 30% 19% 4	500
May	1st to 10th	Day	17% 5% 24% 34% 17% 3	503
	11th to 20th	Day	15% 4 19% 29% 25% 9%	527
	21st to end	Day	10% 3 15% 33% 29% 11%	751
June	1st to 10th	Day	8% 3 9% 30% 36% 14%	730
	11th to 20th	Day	7% 3 8% 31% 35% 15%	730
	21st to end	Day	8% 3 5% 21% 36% 27%	730
July	1st to 10th	Day	7% 3 4 17% 38% 31%	721
	11th to 20th	Day	8% 2 2 14% 40% 34%	718
	21st to end	Day	8% 2 3 18% 40% 29%	794
August	1st to 10th	Day	5% 2 3 12% 43% 35%	588
	11th to 20th	Day	7% 1 1 12% 39% 40%	479
	21st to end	Day	13% 2 4 15% 31% 35%	539
September	1st to 10th	Day	9% 2 4 16% 40% 28%	490
	11th to 20th	Day	14% 1 7% 20% 35% 23%	490
	21st to end	Day	13% 2 9% 25% 33% 19%	302
October	1st to 10th	Day	7% 1 19% 31% 31% 11%	300
	11th to 20th	Day	8% 4 19% 31% 26% 12%	300
	21st to end	Day	16% 7% 22% 28% 19% 9%	330
November	1st to 10th	Day	18% 4 21% 31% 20% 5%	300
	11th to 20th	Day	15% 4 31% 27% 20% 3	300
	21st to end	Day	15% 12% 30% 26% 14% 3	300
December	1st to 10th	Day	22% 5% 30% 25% 13% 4	300
	11th to 20th	Day	22% 8% 33% 27% 8% 1	300
	21st to end	Day	24% 11% 31% 23% 9% 2	330
January	1st to 10th	Night	22% 5% 30% 35% 8%	1000
	11th to 20th	Night	21% 6% 30% 33% 9%	1000
	21st to end	Night	24% 7% 30% 35% 5%	1100
February	1st to 10th	Night	21% 7% 31% 35% 6%	950
	11th to 20th	Night	27% 9% 29% 31% 5%	783
	21st to end	Night	27% 8% 35% 27% 4	550
March	1st to 10th	Night	20% 6% 30% 37% 7%	600
	11th to 20th	Night	17% 6% 33% 36% 8% 1	600
	21st to end	Night	17% 4 32% 36% 11%	660
April	1st to 10th	Night	16% 6% 33% 35% 10% 1	600
	11th to 20th	Night	16% 6% 32% 34% 11%	593
	21st to end	Night	12% 4 29% 39% 15% 1	600
May	1st to 10th	Night	14% 4 24% 43% 15% 1	600
	11th to 20th	Night	11% 3 21% 39% 24% 2	591
	21st to end	Night	7% 3 13% 41% 32% 4	650
June	1st to 10th	Night	8% 1 6% 38% 42% 5%	600
	11th to 20th	Night	5% 3 8% 40% 41% 5%	600
	21st to end	Night	9% 2 5% 27% 46% 12%	600
July	1st to 10th	Night	7% 3 2 24% 51% 13%	585
	11th to 20th	Night	5% 2 2 25% 50% 15%	582
	21st to end	Night	6% 2 2 24% 53% 13%	657
August	1st to 10th	Night	3 1 2 22% 56% 15%	579
	11th to 20th	Night	5% 1 1 22% 56% 15%	564
	21st to end	Night	10% 1 3 22% 46% 18%	627
September	1st to 10th	Night	9% 1 4 23% 51% 12%	570
	11th to 20th	Night	11% 2 4 26% 47% 11%	570
	21st to end	Night	10% 2 5% 33% 43% 6%	660
October	1st to 10th	Night	8% 1 9% 34% 43% 5%	760
	11th to 20th	Night	12% 2 17% 40% 25% 3	760
	21st to end	Night	14% 5% 17% 39% 23% 3	836
November	1st to 10th	Night	14% 2 18% 40% 23% 3	780
	11th to 20th	Night	15% 5% 20% 41% 18% 1	800
	21st to end	Night	17% 6% 25% 38% 13% 2	800
December	1st to 10th	Night	21% 5% 25% 37% 12%	831
	11th to 20th	Night	22% 6% 26% 36% 10% 1	1000
	21st to end	Night	23% 6% 28% 34% 8%	1100

Page generated at 2025-08-02 00:32:29. Version ac07

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