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% 8% 3 8% 5% 5% 5% 4 6% 7% 37%	101631
	Night	11% 9% 4 8% 5% 5% 5% 4 6% 7% 37%	97468
Transparency	Day	44% 7% 14% 25% 10% 1	105209
	Night	44% 7% 13% 24% 11% 1	102034
Seeing	Day	15% 9% 32% 27% 14% 3	16069
	Night	14% 9% 33% 32% 11% 1	25941

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% 17% 4 12% 10% 9% 6% 4 5% 5% 18%	2218
	11th to 20th	Day	5% 10% 5% 16% 7% 7% 12% 7% 7% 6% 18%	2399
	21st to end	Day	6% 13% 11% 17% 9% 6% 6% 4 6% 6% 17%	3135
February	1st to 10th	Day	8% 12% 5% 10% 6% 6% 6% 5% 7% 9% 27%	1710
	11th to 20th	Day	6% 10% 4 10% 8% 7% 6% 5% 7% 8% 29%	1664
	21st to end	Day	9% 12% 5% 12% 7% 8% 5% 5% 5% 9% 25%	1465
March	1st to 10th	Day	12% 12% 4 10% 6% 5% 4 4 5% 6% 33%	3372
	11th to 20th	Day	8% 11% 3 8% 5% 4 5% 4 5% 9% 38%	3480
	21st to end	Day	19% 14% 5% 11% 8% 5% 4 3 5% 5% 21%	3814
April	1st to 10th	Day	15% 10% 4 7% 4 4 4 4 5% 7% 36%	2227
	11th to 20th	Day	17% 13% 4 8% 4 4 4 4 5% 6% 32%	2405
	21st to end	Day	23% 10% 4 6% 4 4 4 3 5% 6% 32%	2470
May	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 4 5% 7% 38%	4114
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 5% 8% 43%	4399
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 7% 8% 36%	4824
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 4 4 6% 8% 44%	2825
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 39%	2850
	21st to end	Day	10% 5% 2 6% 4 4 5% 5% 7% 10% 42%	2850
July	1st to 10th	Day	9% 4 2 5% 5% 5% 5% 5% 8% 10% 42%	4750
	11th to 20th	Day	5% 4 1 3 3 3 4 3 6% 8% 59%	4750
	21st to end	Day	6% 4 1 6% 4 4 5% 4 7% 9% 50%	4612
August	1st to 10th	Day	12% 5% 2 5% 3 4 5% 4 6% 8% 45%	4104
	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% 7% 33%	1568
	11th to 20th	Day	9% 14% 6% 13% 7% 6% 5% 4 5% 6% 25%	1520
	21st to end	Day	11% 11% 6% 10% 7% 5% 4 3 6% 7% 30%	1520
December	1st to 10th	Day	8% 12% 5% 15% 8% 7% 6% 4 6% 6% 23%	1520
	11th to 20th	Day	9% 13% 6% 13% 7% 6% 6% 4 5% 7% 25%	1764
	21st to end	Day	20% 14% 5% 14% 8% 3 3 3 4 6% 20%	2442
January	1st to 10th	Night	8% 10% 5% 15% 10% 6% 6% 4 5% 7% 24%	4737
	11th to 20th	Night	5% 11% 5% 13% 9% 10% 12% 7% 7% 6% 16%	4566
	21st to end	Night	6% 15% 11% 14% 5% 6% 5% 4 6% 6% 21%	4532
February	1st to 10th	Night	6% 11% 5% 10% 7% 6% 6% 4 7% 8% 31%	2470
	11th to 20th	Night	6% 10% 4 9% 5% 5% 6% 5% 7% 7% 36%	2403
	21st to end	Night	9% 11% 4 8% 6% 5% 6% 4 6% 7% 33%	2007
March	1st to 10th	Night	8% 8% 3 7% 5% 4 5% 4 6% 7% 43%	3483
	11th to 20th	Night	8% 10% 3 8% 7% 5% 4 4 6% 8% 38%	3480
	21st to end	Night	22% 14% 4 11% 6% 3 3 2 4 5% 27%	3817
April	1st to 10th	Night	15% 10% 4 6% 4 4 3 4 4 6% 40%	1952
	11th to 20th	Night	16% 10% 5% 8% 4 4 3 2 4 6% 36%	1721
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 4 5% 38%	1710
May	1st to 10th	Night	16% 9% 2 6% 4 4 3 2 5% 7% 43%	2775
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 5% 5% 46%	2506
	21st to end	Night	16% 7% 3 6% 5% 3 4 4 5% 7% 40%	2743
June	1st to 10th	Night	11% 4 1 4 4 4 3 3 6% 9% 51%	1351
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 7% 7% 47%	1330
	21st to end	Night	9% 5% 2 4 3 3 4 3 6% 8% 53%	1330
July	1st to 10th	Night	10% 4 2 6% 3 5% 5% 4 7% 10% 44%	2220
	11th to 20th	Night	4 2 1 4 3 3 2 2 5% 7% 67%	2220
	21st to end	Night	5% 3 1 4 4 4 4 4 6% 10% 55%	2948
August	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 6% 7% 52%	2836
	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% 9% 4 7% 5% 4 4 4 6% 6% 39%	2581
	11th to 20th	Night	11% 12% 5% 12% 5% 4 4 4 5% 5% 32%	2660
	21st to end	Night	10% 9% 3 9% 6% 5% 4 4 7% 6% 36%	2660
December	1st to 10th	Night	8% 12% 5% 12% 8% 5% 5% 4 6% 7% 28%	2660
	11th to 20th	Night	10% 12% 5% 13% 8% 6% 6% 4 6% 6% 24%	3428
	21st to end	Night	19% 15% 5% 11% 7% 5% 4 4 5% 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	50% 8% 11% 23% 7%	3372
	11th to 20th	Day	41% 7% 12% 30% 10%	3480
	21st to end	Day	64% 8% 10% 14% 4	3814
April	1st to 10th	Day	46% 7% 11% 21% 13% 1	2227
	11th to 20th	Day	52% 6% 12% 17% 12% 1	2405
	21st to end	Day	52% 7% 13% 19% 8% 1	2470
May	1st to 10th	Day	41% 9% 13% 24% 12% 1	4152
	11th to 20th	Day	37% 8% 12% 29% 11% 3	4403
	21st to end	Day	40% 11% 15% 26% 7% 1	4837
June	1st to 10th	Day	36% 9% 16% 28% 11% 1	2825
	11th to 20th	Day	42% 8% 14% 21% 14% 1	2850
	21st to end	Day	34% 9% 20% 25% 11%	2850
July	1st to 10th	Day	35% 7% 20% 24% 13%	4676
	11th to 20th	Day	23% 4 17% 38% 16% 2	4669
	21st to end	Day	31% 5% 18% 33% 12% 1	4466
August	1st to 10th	Day	36% 5% 13% 32% 12% 2	4090
	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	49% 7% 11% 20% 12% 2	1568
	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	36% 9% 15% 26% 12% 2	3483
	11th to 20th	Night	40% 8% 12% 27% 12%	3480
	21st to end	Night	60% 5% 9% 20% 6% 1	3817
April	1st to 10th	Night	44% 6% 13% 23% 13% 1	1952
	11th to 20th	Night	50% 4 7% 23% 14% 2	1721
	21st to end	Night	47% 7% 11% 22% 12% 2	1710
May	1st to 10th	Night	40% 7% 11% 26% 13% 3	2802
	11th to 20th	Night	36% 9% 10% 24% 18% 3	2507
	21st to end	Night	40% 10% 13% 24% 11% 2	2751
June	1st to 10th	Night	29% 9% 16% 31% 13% 2	1351
	11th to 20th	Night	32% 9% 16% 25% 15% 3	1330
	21st to end	Night	28% 7% 18% 28% 17% 2	1330
July	1st to 10th	Night	35% 7% 21% 21% 15%	2185
	11th to 20th	Night	18% 4 13% 45% 18% 2	2178
	21st to end	Night	25% 6% 18% 34% 15% 2	2855
August	1st to 10th	Night	32% 5% 14% 30% 15% 4	2826
	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% 23% 11% 2	2581
	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% 14% 33% 22% 7%	407
	11th to 20th	Day	26% 14% 26% 23% 9% 1	603
	21st to end	Day	25% 10% 32% 20% 12% 2	650
May	1st to 10th	Day	21% 12% 33% 24% 9% 2	653
	11th to 20th	Day	17% 11% 31% 27% 12% 3	675
	21st to end	Day	17% 7% 31% 31% 13% 2	734
June	1st to 10th	Day	10% 7% 28% 32% 20% 3	680
	11th to 20th	Day	15% 8% 36% 27% 12% 1	680
	21st to end	Day	13% 6% 35% 26% 17% 3	680
July	1st to 10th	Day	7% 3 25% 32% 26% 6%	671
	11th to 20th	Day	7% 5% 31% 33% 18% 6%	668
	21st to end	Day	6% 3 25% 35% 26% 5%	739
August	1st to 10th	Day	10% 3 24% 34% 24% 5%	676
	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	23% 11% 31% 28% 6% 1	279
	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% 10% 36% 28% 6% 1	570
	11th to 20th	Night	21% 11% 33% 27% 9% 1	563
	21st to end	Night	22% 8% 33% 28% 8% 2	570
May	1st to 10th	Night	18% 10% 32% 31% 8% 1	570
	11th to 20th	Night	14% 9% 33% 33% 11% 1	561
	21st to end	Night	13% 8% 28% 39% 12%	617
June	1st to 10th	Night	12% 6% 27% 37% 18% 1	570
	11th to 20th	Night	12% 5% 32% 43% 8%	570
	21st to end	Night	10% 5% 23% 45% 16%	570
July	1st to 10th	Night	6% 2 23% 41% 26% 3	555
	11th to 20th	Night	5% 4 26% 45% 19% 1	552
	21st to end	Night	5% 2 25% 44% 21% 3	624
August	1st to 10th	Night	9% 2 19% 49% 19% 2	567
	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%	943
	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 2024-11-01 21:47:43. Version ac07

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