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% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	126066
	Night	11% 9% 4 9% 6% 5% 5% 4 6% 7% 37%	122261
Transparency	Day	44% 6% 13% 25% 11% 1	129605
	Night	45% 5% 13% 24% 12% 1	126802
Seeing	Day	16% 9% 32% 27% 14% 3	17129
	Night	15% 9% 34% 32% 10% 1	26994

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	7% 13% 3 11% 12% 10% 9% 5% 6% 6% 18%	3178
	11th to 20th	Day	6% 9% 5% 14% 6% 7% 11% 8% 10% 8% 17%	3455
	21st to end	Day	4 10% 9% 17% 8% 6% 5% 4 5% 6% 25%	4488
February	1st to 10th	Day	8% 12% 5% 10% 6% 7% 6% 5% 7% 8% 26%	1800
	11th to 20th	Day	5% 10% 4 10% 9% 7% 6% 5% 7% 8% 29%	1754
	21st to end	Day	8% 11% 4 12% 7% 8% 5% 4 6% 9% 25%	1540
March	1st to 10th	Day	11% 9% 3 9% 5% 4 3 3 5% 7% 41%	4846
	11th to 20th	Day	9% 9% 3 7% 5% 4 5% 4 6% 9% 38%	4980
	21st to end	Day	19% 13% 5% 10% 7% 5% 4 3 5% 6% 24%	5462
April	1st to 10th	Day	15% 10% 4 7% 4 4 4 4 5% 7% 36%	2344
	11th to 20th	Day	16% 13% 4 8% 4 4 4 4 5% 6% 32%	2533
	21st to end	Day	23% 9% 3 6% 4 4 4 3 5% 6% 33%	2600
May	1st to 10th	Day	13% 6% 2 5% 4 3 4 3 5% 6% 50%	5908
	11th to 20th	Day	17% 9% 2 5% 4 3 4 3 5% 6% 43%	6319
	21st to end	Day	15% 6% 2 6% 5% 4 5% 4 6% 7% 38%	6935
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 4 4 6% 8% 43%	2974
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 6% 7% 38%	3000
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 7% 10% 42%	3000
July	1st to 10th	Day	8% 3 2 5% 4 4 5% 5% 7% 10% 48%	6800
	11th to 20th	Day	10% 4 2 4 3 3 4 3 6% 8% 53%	6800
	21st to end	Day	8% 4 2 6% 4 4 5% 4 7% 9% 47%	6625
August	1st to 10th	Day	13% 5% 2 5% 3 4 5% 4 6% 8% 45%	4157
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 10% 41%	4041
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 48%	4505
September	1st to 10th	Day	10% 5% 1 5% 3 4 4 3 5% 8% 53%	2280
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 4 5% 8% 47%	2129
	21st to end	Day	15% 6% 3 4 3 3 4 3 5% 7% 47%	2077
October	1st to 10th	Day	16% 9% 2 5% 4 4 4 3 5% 7% 41%	1900
	11th to 20th	Day	15% 11% 4 5% 4 3 4 3 5% 7% 40%	1900
	21st to end	Day	18% 11% 3 8% 4 4 3 3 5% 6% 35%	2028
November	1st to 10th	Day	14% 12% 4 8% 5% 4 4 3 6% 6% 33%	1624
	11th to 20th	Day	10% 14% 6% 13% 7% 6% 5% 4 5% 6% 26%	1600
	21st to end	Day	11% 11% 6% 11% 7% 6% 5% 3 6% 6% 29%	1600
December	1st to 10th	Day	7% 10% 4 14% 12% 11% 7% 4 6% 6% 18%	2610
	11th to 20th	Day	8% 15% 9% 11% 6% 6% 5% 4 6% 6% 25%	2776
	21st to end	Day	16% 17% 5% 19% 8% 3 3 2 3 5% 18%	3498
January	1st to 10th	Night	6% 8% 4 14% 12% 8% 8% 4 6% 7% 23%	6787
	11th to 20th	Night	5% 10% 5% 11% 8% 8% 10% 9% 8% 7% 19%	6520
	21st to end	Night	5% 13% 11% 11% 5% 5% 6% 5% 6% 7% 27%	6490
February	1st to 10th	Night	6% 11% 5% 10% 7% 6% 6% 4 7% 8% 30%	2600
	11th to 20th	Night	5% 10% 4 9% 5% 5% 6% 5% 7% 7% 36%	2533
	21st to end	Night	9% 11% 4 8% 6% 5% 6% 4 6% 7% 34%	2109
March	1st to 10th	Night	10% 6% 2 6% 4 3 4 4 6% 8% 45%	5011
	11th to 20th	Night	9% 8% 3 8% 6% 5% 5% 5% 7% 9% 37%	4980
	21st to end	Night	21% 12% 4 10% 5% 4 4 3 4 5% 28%	5467
April	1st to 10th	Night	15% 10% 4 6% 4 4 3 4 4 6% 40%	2055
	11th to 20th	Night	15% 10% 5% 8% 4 4 4 3 5% 7% 36%	1813
	21st to end	Night	18% 10% 4 7% 5% 3 4 3 4 5% 37%	1800
May	1st to 10th	Night	12% 7% 2 5% 3 3 2 2 4 6% 54%	3991
	11th to 20th	Night	14% 8% 2 6% 4 4 3 3 4 5% 47%	3596
	21st to end	Night	16% 8% 3 7% 4 4 4 4 4 7% 39%	3942
June	1st to 10th	Night	11% 4 1 4 4 4 3 3 6% 8% 50%	1422
	11th to 20th	Night	11% 5% 2 4 4 4 4 5% 7% 7% 47%	1400
	21st to end	Night	10% 5% 2 4 3 3 4 3 6% 8% 52%	1400
July	1st to 10th	Night	9% 4 2 5% 3 4 4 4 6% 9% 50%	3178
	11th to 20th	Night	7% 3 1 4 3 3 3 3 6% 8% 61%	3177
	21st to end	Night	9% 5% 2 5% 4 4 4 3 6% 9% 48%	4243
August	1st to 10th	Night	11% 6% 1 5% 3 3 3 3 6% 7% 52%	2872
	11th to 20th	Night	13% 4 2 5% 4 3 3 3 5% 8% 51%	2798
	21st to end	Night	10% 4 2 5% 3 3 4 3 6% 8% 52%	3161
September	1st to 10th	Night	9% 3 2 4 3 3 3 3 5% 8% 56%	1900
	11th to 20th	Night	9% 6% 2 5% 4 3 3 3 5% 6% 54%	2047
	21st to end	Night	11% 6% 2 4 4 3 3 3 5% 6% 53%	2090
October	1st to 10th	Night	12% 8% 2 6% 4 3 4 3 5% 6% 46%	2273
	11th to 20th	Night	14% 9% 3 6% 4 4 4 2 5% 7% 42%	2280
	21st to end	Night	16% 11% 3 9% 4 4 4 3 5% 7% 35%	2568
November	1st to 10th	Night	11% 10% 4 7% 5% 4 4 4 6% 7% 39%	2672
	11th to 20th	Night	11% 12% 5% 12% 5% 4 4 4 5% 5% 33%	2800
	21st to end	Night	10% 10% 4 10% 7% 5% 4 4 6% 6% 35%	2800
December	1st to 10th	Night	5% 8% 4 11% 12% 8% 6% 5% 7% 8% 26%	4580
	11th to 20th	Night	10% 12% 5% 16% 8% 6% 5% 4 5% 6% 22%	5426
	21st to end	Night	16% 17% 6% 16% 9% 5% 4 3 4 5% 15%	7480

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	63% 5% 8% 15% 8%	3179
	11th to 20th	Day	58% 6% 8% 22% 6%	3455
	21st to end	Day	58% 7% 12% 14% 8%	4488
February	1st to 10th	Day	53% 6% 12% 18% 10% 1	1800
	11th to 20th	Day	50% 7% 13% 22% 6% 1	1763
	21st to end	Day	55% 7% 12% 18% 8%	1540
March	1st to 10th	Day	42% 6% 15% 25% 10% 2	4846
	11th to 20th	Day	42% 5% 14% 30% 10%	4980
	21st to end	Day	61% 6% 11% 17% 5%	5464
April	1st to 10th	Day	46% 7% 11% 21% 13% 1	2344
	11th to 20th	Day	52% 5% 13% 17% 12% 1	2533
	21st to end	Day	51% 6% 13% 20% 9% 1	2600
May	1st to 10th	Day	34% 7% 13% 28% 18%	5946
	11th to 20th	Day	41% 6% 10% 23% 14% 7%	6323
	21st to end	Day	41% 8% 11% 30% 9% 1	6949
June	1st to 10th	Day	36% 8% 15% 28% 11% 1	2974
	11th to 20th	Day	44% 8% 14% 20% 13% 1	3000
	21st to end	Day	35% 9% 19% 25% 12%	3000
July	1st to 10th	Day	31% 5% 20% 32% 12%	6726
	11th to 20th	Day	30% 3 15% 37% 13% 1	6719
	21st to end	Day	34% 4 19% 32% 11% 1	6437
August	1st to 10th	Day	36% 5% 13% 32% 13% 2	4143
	11th to 20th	Day	39% 6% 15% 32% 8% 1	4080
	21st to end	Day	36% 5% 16% 31% 11% 1	4504
September	1st to 10th	Day	31% 6% 16% 35% 12% 1	2280
	11th to 20th	Day	35% 7% 18% 30% 10%	2129
	21st to end	Day	36% 5% 14% 32% 11% 1	2077
October	1st to 10th	Day	34% 4 9% 33% 20% 1	3160
	11th to 20th	Day	40% 4 11% 21% 22% 2	3140
	21st to end	Day	49% 4 14% 26% 8%	3321
November	1st to 10th	Day	50% 6% 10% 21% 11% 2	1624
	11th to 20th	Day	58% 5% 13% 15% 8% 1	1600
	21st to end	Day	55% 5% 15% 17% 6% 2	1600
December	1st to 10th	Day	66% 5% 13% 11% 4	2610
	11th to 20th	Day	58% 4 13% 16% 7%	2773
	21st to end	Day	71% 4 8% 11% 6%	3498
January	1st to 10th	Night	59% 5% 10% 16% 10%	6787
	11th to 20th	Night	59% 7% 10% 13% 10% 1	6520
	21st to end	Night	55% 5% 13% 20% 6%	6490
February	1st to 10th	Night	49% 7% 13% 20% 11% 1	2600
	11th to 20th	Night	43% 7% 14% 22% 13% 1	2534
	21st to end	Night	47% 7% 12% 20% 12% 2	2109
March	1st to 10th	Night	35% 6% 14% 27% 15% 3	5011
	11th to 20th	Night	41% 6% 14% 26% 13%	4980
	21st to end	Night	60% 4 9% 19% 8% 1	5467
April	1st to 10th	Night	44% 6% 12% 24% 13% 1	2055
	11th to 20th	Night	49% 4 8% 23% 14% 2	1813
	21st to end	Night	48% 6% 11% 21% 12% 2	1800
May	1st to 10th	Night	32% 5% 13% 29% 19% 2	4018
	11th to 20th	Night	39% 6% 8% 21% 20% 6%	3597
	21st to end	Night	45% 7% 10% 25% 13% 1	3950
June	1st to 10th	Night	30% 9% 15% 31% 13% 2	1422
	11th to 20th	Night	32% 9% 17% 25% 14% 3	1400
	21st to end	Night	29% 7% 18% 28% 17% 2	1400
July	1st to 10th	Night	32% 5% 20% 28% 15%	3145
	11th to 20th	Night	23% 3 17% 42% 13% 2	3138
	21st to end	Night	34% 4 17% 32% 12% 1	4120
August	1st to 10th	Night	32% 5% 14% 30% 15% 4	2862
	11th to 20th	Night	34% 5% 15% 34% 11% 1	2825
	21st to end	Night	31% 5% 18% 31% 13% 2	3161
September	1st to 10th	Night	26% 7% 17% 33% 16% 1	1900
	11th to 20th	Night	31% 7% 17% 31% 15%	2047
	21st to end	Night	32% 5% 13% 33% 16%	2090
October	1st to 10th	Night	26% 3 7% 36% 23% 4	3793
	11th to 20th	Night	36% 4 11% 28% 20% 2	3779
	21st to end	Night	45% 4 14% 24% 12%	4229
November	1st to 10th	Night	43% 7% 13% 24% 11% 2	2672
	11th to 20th	Night	53% 6% 12% 18% 10% 1	2800
	21st to end	Night	48% 7% 16% 21% 6% 2	2800
December	1st to 10th	Night	55% 4 17% 17% 6%	4580
	11th to 20th	Night	61% 6% 10% 18% 5%	5428
	21st to end	Night	71% 5% 8% 12% 4	7480

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% 12% 35% 28% 11% 3	300
	11th to 20th	Day	15% 11% 34% 33% 7%	300
	21st to end	Day	11% 9% 41% 23% 15% 2	330
February	1st to 10th	Day	15% 17% 39% 26% 3 1	300
	11th to 20th	Day	13% 13% 42% 23% 8%	300
	21st to end	Day	13% 12% 42% 29% 5%	249
March	1st to 10th	Day	15% 13% 41% 23% 7% 1	300
	11th to 20th	Day	16% 11% 37% 24% 11% 1	300
	21st to end	Day	23% 10% 33% 20% 9% 4	330
April	1st to 10th	Day	24% 12% 34% 23% 7%	444
	11th to 20th	Day	26% 14% 27% 23% 8% 1	651
	21st to end	Day	25% 10% 31% 20% 11% 2	700
May	1st to 10th	Day	20% 11% 32% 24% 11% 3	703
	11th to 20th	Day	18% 10% 29% 27% 12% 3	725
	21st to end	Day	18% 7% 30% 31% 13% 2	789
June	1st to 10th	Day	11% 7% 28% 31% 20% 3	730
	11th to 20th	Day	16% 8% 36% 27% 12% 1	730
	21st to end	Day	14% 6% 35% 27% 15% 3	730
July	1st to 10th	Day	7% 3 25% 32% 26% 6%	721
	11th to 20th	Day	8% 5% 31% 33% 18% 6%	718
	21st to end	Day	7% 3 26% 34% 25% 5%	794
August	1st to 10th	Day	10% 3 24% 34% 24% 5%	697
	11th to 20th	Day	13% 4 22% 30% 23% 7%	665
	21st to end	Day	12% 6% 26% 30% 22% 5%	734
September	1st to 10th	Day	12% 7% 30% 32% 16% 3	490
	11th to 20th	Day	12% 5% 32% 32% 16% 4	339
	21st to end	Day	19% 12% 31% 21% 14% 3	300
October	1st to 10th	Day	20% 10% 28% 27% 11% 4	300
	11th to 20th	Day	16% 16% 36% 24% 8% 1	300
	21st to end	Day	24% 15% 36% 17% 8% 1	330
November	1st to 10th	Day	24% 10% 31% 28% 6% 1	300
	11th to 20th	Day	17% 16% 44% 17% 5%	300
	21st to end	Day	19% 17% 38% 22% 4	300
December	1st to 10th	Day	12% 9% 43% 25% 9% 1	300
	11th to 20th	Day	15% 14% 43% 24% 3	300
	21st to end	Day	25% 11% 38% 20% 5% 1	330
January	1st to 10th	Night	10% 11% 41% 28% 9% 2	1000
	11th to 20th	Night	14% 10% 39% 29% 7% 1	1000
	21st to end	Night	14% 11% 40% 28% 6%	1100
February	1st to 10th	Night	11% 10% 38% 32% 9% 1	1000
	11th to 20th	Night	10% 11% 45% 26% 8% 1	979
	21st to end	Night	14% 12% 33% 31% 8% 2	789
March	1st to 10th	Night	14% 11% 37% 29% 8% 1	644
	11th to 20th	Night	17% 10% 40% 27% 6% 1	600
	21st to end	Night	19% 9% 31% 28% 12% 1	660
April	1st to 10th	Night	20% 10% 36% 28% 6% 1	600
	11th to 20th	Night	21% 11% 34% 26% 8% 1	593
	21st to end	Night	23% 8% 32% 28% 8% 2	600
May	1st to 10th	Night	18% 9% 32% 32% 9% 1	600
	11th to 20th	Night	14% 9% 33% 32% 10% 1	591
	21st to end	Night	14% 7% 27% 39% 12%	650
June	1st to 10th	Night	13% 6% 27% 37% 17% 1	600
	11th to 20th	Night	13% 5% 32% 43% 8%	600
	21st to end	Night	11% 5% 23% 45% 15%	600
July	1st to 10th	Night	6% 2 24% 40% 26% 3	585
	11th to 20th	Night	6% 4 25% 46% 18% 1	582
	21st to end	Night	5% 2 25% 45% 20% 3	657
August	1st to 10th	Night	9% 2 19% 49% 18% 2	579
	11th to 20th	Night	12% 4 19% 38% 24% 2	564
	21st to end	Night	8% 5% 26% 38% 22% 2	627
September	1st to 10th	Night	11% 5% 26% 40% 16% 2	570
	11th to 20th	Night	10% 7% 28% 36% 18% 1	570
	21st to end	Night	13% 9% 34% 30% 13% 1	570
October	1st to 10th	Night	18% 9% 33% 29% 10%	753
	11th to 20th	Night	20% 10% 35% 27% 8% 1	760
	21st to end	Night	22% 11% 36% 26% 5%	896
November	1st to 10th	Night	16% 11% 33% 31% 8%	975
	11th to 20th	Night	17% 13% 37% 24% 8% 1	1000
	21st to end	Night	15% 15% 38% 25% 6%	1000
December	1st to 10th	Night	14% 12% 37% 29% 7% 1	1000
	11th to 20th	Night	19% 11% 37% 27% 6% 1	1000
	21st to end	Night	23% 11% 36% 23% 6% 1	1100

Page generated at 2025-08-02 00:32:27. Version ac07

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