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 2024-11-04.

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% 3 7% 5% 5% 5% 4 6% 7% 40%	100806
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	98243
Transparency	Day	42% 3 10% 21% 18% 6%	104452
	Night	37% 3 11% 20% 20% 9%	102738
Seeing	Day	14% 5% 17% 24% 26% 15%	15004
	Night	15% 4 19% 34% 24% 4	24747

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	19% 8% 3 6% 4 4 4 3 5% 7% 38%	2847
	11th to 20th	Day	18% 7% 2 4 3 2 2 3 4 4 50%	2850
	21st to end	Day	22% 10% 3 7% 4 3 3 3 4 5% 37%	3135
February	1st to 10th	Day	19% 9% 3 7% 5% 3 3 3 3 5% 38%	1754
	11th to 20th	Day	20% 8% 4 7% 4 3 3 3 4 5% 40%	1848
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 4 5% 36%	1689
March	1st to 10th	Day	23% 11% 3 8% 5% 5% 5% 4 4 5% 28%	3436
	11th to 20th	Day	16% 8% 3 8% 5% 5% 4 4 6% 8% 33%	3480
	21st to end	Day	14% 8% 3 9% 5% 4 4 3 4 5% 41%	3814
April	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 3 5% 6% 42%	2253
	11th to 20th	Day	16% 7% 3 7% 4 4 4 4 4 7% 40%	2254
	21st to end	Day	11% 7% 2 6% 5% 5% 6% 4 6% 7% 41%	2280
May	1st to 10th	Day	13% 8% 3 8% 5% 5% 5% 5% 6% 8% 33%	3765
	11th to 20th	Day	13% 6% 3 7% 4 5% 4 4 6% 6% 44%	3764
	21st to end	Day	8% 5% 2 7% 6% 5% 7% 5% 7% 10% 38%	4415
June	1st to 10th	Day	8% 5% 2 7% 8% 6% 6% 5% 7% 9% 35%	2470
	11th to 20th	Day	5% 5% 2 7% 6% 5% 6% 6% 8% 11% 39%	2470
	21st to end	Day	6% 6% 2 7% 6% 7% 7% 5% 10% 9% 35%	2470
July	1st to 10th	Day	7% 6% 2 8% 7% 7% 8% 6% 8% 12% 30%	4120
	11th to 20th	Day	8% 7% 3 8% 7% 8% 8% 7% 10% 11% 26%	4120
	21st to end	Day	16% 10% 3 9% 6% 6% 5% 4 6% 8% 27%	4465
August	1st to 10th	Day	3 3 1 4 3 4 4 4 6% 9% 61%	3918
	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	10% 6% 3 6% 4 4 4 3 4 5% 52%	1888
	11th to 20th	Day	11% 7% 2 6% 4 3 3 3 4 6% 52%	1900
	21st to end	Day	15% 8% 3 6% 4 3 4 4 5% 6% 43%	1900
December	1st to 10th	Day	16% 8% 2 7% 4 3 4 2 5% 5% 43%	1866
	11th to 20th	Day	15% 11% 3 6% 4 4 4 4 5% 6% 38%	2161
	21st to end	Day	18% 8% 3 6% 4 3 3 2 5% 5% 42%	3135
January	1st to 10th	Night	17% 6% 4 5% 4 3 3 2 4 5% 47%	4108
	11th to 20th	Night	15% 7% 3 5% 3 3 3 2 4 4 51%	4120
	21st to end	Night	25% 6% 2 6% 3 3 3 2 4 5% 41%	4532
February	1st to 10th	Night	16% 8% 3 7% 4 3 3 2 5% 6% 42%	2423
	11th to 20th	Night	21% 9% 2 4 3 3 3 2 4 5% 44%	2218
	21st to end	Night	23% 8% 3 7% 3 3 3 3 4 4 39%	1780
March	1st to 10th	Night	24% 9% 4 7% 4 4 4 3 4 5% 32%	3419
	11th to 20th	Night	18% 7% 3 7% 5% 4 4 4 5% 7% 36%	3480
	21st to end	Night	14% 7% 3 6% 4 4 4 4 6% 7% 42%	3817
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 5% 5% 7% 46%	1925
	11th to 20th	Night	14% 6% 2 7% 4 3 4 3 5% 6% 45%	1875
	21st to end	Night	9% 5% 2 5% 4 5% 4 4 6% 7% 51%	1900
May	1st to 10th	Night	16% 7% 3 7% 5% 4 5% 4 5% 8% 37%	3130
	11th to 20th	Night	15% 5% 2 7% 4 4 4 3 5% 6% 44%	3129
	21st to end	Night	9% 5% 3 6% 5% 5% 5% 4 7% 10% 40%	3133
June	1st to 10th	Night	7% 5% 2 5% 4 5% 5% 4 7% 10% 46%	1710
	11th to 20th	Night	4 4 1 4 4 4 4 5% 7% 10% 54%	1710
	21st to end	Night	6% 5% 2 5% 4 4 5% 5% 8% 10% 46%	1710
July	1st to 10th	Night	6% 6% 3 7% 5% 5% 6% 5% 9% 12% 35%	2850
	11th to 20th	Night	6% 5% 2 7% 5% 6% 5% 5% 7% 13% 38%	2850
	21st to end	Night	10% 9% 4 8% 5% 4 6% 4 7% 10% 33%	3098
August	1st to 10th	Night	2 1 2 2 3 3 3 5% 9% 70%	3011
	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	11% 6% 2 4 3 3 3 2 5% 6% 55%	2263
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 5% 5% 53%	2280
	21st to end	Night	14% 7% 3 6% 4 4 3 3 4 5% 46%	2280
December	1st to 10th	Night	17% 8% 3 6% 4 3 3 3 4 5% 45%	2309
	11th to 20th	Night	16% 8% 3 7% 4 3 3 3 4 5% 44%	3036
	21st to end	Night	18% 9% 2 5% 4 3 3 2 4 5% 43%	4532

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	47% 1 3 19% 24% 6%	2848
	11th to 20th	Day	39% 1 6% 19% 27% 7%	2850
	21st to end	Day	53% 1 3 15% 19% 9%	3135
February	1st to 10th	Day	51% 2 6% 15% 17% 10%	1754
	11th to 20th	Day	48% 3 5% 17% 18% 9%	1858
	21st to end	Day	51% 3 6% 13% 17% 10%	1689
March	1st to 10th	Day	60% 2 5% 14% 14% 5%	3436
	11th to 20th	Day	50% 1 7% 22% 16% 4	3480
	21st to end	Day	48% 2 7% 17% 22% 5%	3814
April	1st to 10th	Day	43% 2 8% 21% 16% 9%	2253
	11th to 20th	Day	45% 3 8% 18% 18% 8%	2254
	21st to end	Day	40% 4 7% 23% 21% 5%	2280
May	1st to 10th	Day	50% 2 10% 23% 12% 3	3800
	11th to 20th	Day	41% 2 10% 21% 24% 2	3768
	21st to end	Day	40% 3 15% 30% 11% 1	4427
June	1st to 10th	Day	43% 4 14% 23% 14% 2	2470
	11th to 20th	Day	36% 6% 14% 27% 16% 2	2470
	21st to end	Day	40% 7% 15% 23% 13% 1	2470
July	1st to 10th	Day	45% 6% 22% 21% 5% 1	4056
	11th to 20th	Day	49% 6% 21% 19% 5% 1	4051
	21st to end	Day	55% 5% 14% 21% 5% 1	4325
August	1st to 10th	Day	21% 4 13% 36% 26% 1	3904
	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	35% 3 6% 20% 26% 10%	1888
	11th to 20th	Day	36% 2 6% 16% 28% 12%	1900
	21st to end	Day	43% 3 6% 19% 22% 8%	1900
December	1st to 10th	Day	44% 1 10% 21% 16% 8%	1866
	11th to 20th	Day	48% 2 5% 14% 24% 6%	2159
	21st to end	Day	46% 1 5% 16% 27% 5%	3135
January	1st to 10th	Night	42% 1 4 18% 27% 8%	4108
	11th to 20th	Night	39% 1 5% 20% 25% 10%	4120
	21st to end	Night	48% 1 6% 17% 20% 8%	4532
February	1st to 10th	Night	44% 3 6% 18% 20% 10%	2423
	11th to 20th	Night	46% 2 5% 16% 20% 11%	2219
	21st to end	Night	50% 2 6% 14% 14% 14%	1780
March	1st to 10th	Night	56% 1 7% 15% 15% 6%	3419
	11th to 20th	Night	49% 3 8% 17% 18% 5%	3480
	21st to end	Night	42% 2 10% 16% 24% 5%	3817
April	1st to 10th	Night	37% 3 11% 20% 18% 11%	1925
	11th to 20th	Night	40% 2 10% 18% 18% 12%	1875
	21st to end	Night	33% 3 10% 23% 25% 7%	1900
May	1st to 10th	Night	46% 3 12% 20% 14% 5%	3160
	11th to 20th	Night	42% 2 9% 20% 23% 3	3130
	21st to end	Night	40% 3 18% 25% 13% 1	3142
June	1st to 10th	Night	32% 6% 18% 25% 16% 2	1710
	11th to 20th	Night	24% 6% 17% 33% 16% 3	1710
	21st to end	Night	31% 6% 20% 28% 14% 1	1710
July	1st to 10th	Night	39% 9% 27% 18% 6%	2805
	11th to 20th	Night	37% 8% 27% 23% 5%	2796
	21st to end	Night	46% 8% 21% 19% 6% 1	2999
August	1st to 10th	Night	13% 4 18% 34% 29% 1	3001
	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	31% 2 8% 19% 27% 12%	2263
	11th to 20th	Night	33% 2 7% 18% 25% 14%	2280
	21st to end	Night	41% 3 6% 16% 24% 10%	2280
December	1st to 10th	Night	42% 2 9% 19% 20% 7%	2309
	11th to 20th	Night	44% 2 5% 13% 25% 11%	3037
	21st to end	Night	45% 1 5% 19% 22% 7%	4532

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% 8% 40% 20% 9% 1	270
	11th to 20th	Day	22% 10% 36% 23% 8% 1	270
	21st to end	Day	26% 11% 34% 22% 6%	297
February	1st to 10th	Day	24% 10% 31% 27% 6% 1	270
	11th to 20th	Day	27% 13% 30% 24% 5% 1	270
	21st to end	Day	30% 12% 38% 17% 2	225
March	1st to 10th	Day	24% 11% 27% 30% 8%	270
	11th to 20th	Day	17% 11% 43% 22% 6% 2	270
	21st to end	Day	16% 12% 39% 23% 8% 1	297
April	1st to 10th	Day	20% 8% 32% 27% 13% 1	433
	11th to 20th	Day	21% 11% 25% 26% 15% 3	452
	21st to end	Day	14% 7% 26% 30% 20% 3	460
May	1st to 10th	Day	16% 5% 24% 35% 18% 3	463
	11th to 20th	Day	15% 2 17% 31% 26% 9%	487
	21st to end	Day	9% 3 15% 32% 30% 11%	698
June	1st to 10th	Day	9% 3 8% 29% 37% 15%	680
	11th to 20th	Day	7% 4 8% 30% 35% 16%	680
	21st to end	Day	9% 3 5% 21% 35% 26%	680
July	1st to 10th	Day	7% 3 4 18% 37% 32%	671
	11th to 20th	Day	8% 2 2 14% 40% 35%	668
	21st to end	Day	9% 2 3 18% 40% 29%	739
August	1st to 10th	Day	5% 2 3 13% 41% 36%	568
	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	15% 3 22% 32% 22% 6%	279
	11th to 20th	Day	16% 4 32% 25% 20% 3	270
	21st to end	Day	16% 11% 29% 27% 14% 3	270
December	1st to 10th	Day	21% 5% 30% 26% 14% 4	270
	11th to 20th	Day	23% 8% 32% 27% 9% 1	270
	21st to end	Day	25% 10% 31% 24% 9% 2	297
January	1st to 10th	Night	22% 5% 29% 35% 8%	950
	11th to 20th	Night	21% 5% 30% 34% 10%	950
	21st to end	Night	23% 7% 30% 35% 5%	1045
February	1st to 10th	Night	21% 7% 31% 35% 6%	903
	11th to 20th	Night	26% 8% 29% 32% 5%	743
	21st to end	Night	27% 9% 34% 26% 4	524
March	1st to 10th	Night	20% 5% 29% 38% 7%	570
	11th to 20th	Night	17% 4 34% 37% 8% 1	570
	21st to end	Night	16% 4 32% 36% 11%	627
April	1st to 10th	Night	15% 6% 33% 36% 9% 1	570
	11th to 20th	Night	17% 6% 32% 33% 12%	563
	21st to end	Night	12% 4 29% 39% 16% 1	570
May	1st to 10th	Night	13% 3 24% 43% 15% 1	570
	11th to 20th	Night	11% 2 20% 40% 25% 2	561
	21st to end	Night	6% 3 13% 40% 33% 4	617
June	1st to 10th	Night	8% 1 5% 37% 43% 6%	570
	11th to 20th	Night	5% 2 8% 39% 41% 5%	570
	21st to end	Night	9% 1 5% 29% 44% 12%	570
July	1st to 10th	Night	7% 3 2 24% 51% 14%	555
	11th to 20th	Night	5% 2 3 25% 50% 15%	552
	21st to end	Night	6% 2 2 24% 53% 13%	624
August	1st to 10th	Night	3 1 2 22% 56% 15%	567
	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	13% 2 18% 41% 24% 3	755
	11th to 20th	Night	15% 5% 19% 41% 18% 1	760
	21st to end	Night	18% 6% 24% 37% 13% 2	760
December	1st to 10th	Night	21% 5% 25% 37% 13% 1	789
	11th to 20th	Night	22% 5% 26% 36% 10% 1	950
	21st to end	Night	23% 7% 27% 34% 8%	1045

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