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% 6% 3 6% 6% 3 5% 5% 6% 7% 41%	79403
	Night	13% 5% 2 5% 4 3 4 4 5% 7% 47%	80769
Transparency	Day	42% 4 10% 20% 18% 6%	79128
	Night	37% 4 11% 20% 20% 8%	80565
Seeing	Day	14% 5% 17% 24% 26% 15%	14114
	Night	15% 4 19% 34% 24% 4	24060

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% 7% 3 5% 4 3 4 4 5% 7% 38%	2847
	11th to 20th	Day	18% 6% 3 4 3 2 2 3 4 4 50%	2850
	21st to end	Day	22% 9% 3 6% 5% 2 3 4 4 5% 37%	3135
February	1st to 10th	Day	20% 8% 4 7% 5% 2 4 4 3 5% 38%	1754
	11th to 20th	Day	20% 7% 4 6% 5% 2 3 4 4 5% 40%	1848
	21st to end	Day	19% 9% 3 7% 5% 3 4 4 4 5% 36%	1689
March	1st to 10th	Day	20% 8% 3 6% 5% 3 5% 4 4 5% 37%	2090
	11th to 20th	Day	14% 7% 3 8% 7% 4 5% 5% 5% 8% 36%	2090
	21st to end	Day	12% 7% 3 7% 6% 4 5% 4 4 6% 44%	2299
April	1st to 10th	Day	12% 6% 3 6% 6% 4 5% 5% 5% 6% 42%	2157
	11th to 20th	Day	16% 7% 3 6% 5% 3 5% 4 4 7% 40%	2134
	21st to end	Day	11% 6% 3 6% 5% 4 6% 5% 6% 7% 41%	2160
May	1st to 10th	Day	10% 6% 3 6% 5% 3 6% 5% 6% 9% 40%	2160
	11th to 20th	Day	10% 6% 3 6% 5% 4 5% 5% 6% 7% 43%	2124
	21st to end	Day	8% 4 3 5% 8% 4 8% 7% 7% 9% 37%	2497
June	1st to 10th	Day	8% 5% 3 6% 9% 5% 7% 6% 7% 9% 35%	2340
	11th to 20th	Day	6% 4 2 6% 7% 4 7% 7% 8% 11% 39%	2340
	21st to end	Day	7% 5% 3 6% 6% 5% 8% 7% 10% 10% 34%	2340
July	1st to 10th	Day	8% 5% 3 6% 7% 5% 7% 8% 8% 11% 31%	2340
	11th to 20th	Day	8% 5% 3 6% 6% 4 7% 8% 9% 11% 33%	2340
	21st to end	Day	8% 5% 3 6% 7% 5% 7% 8% 8% 10% 33%	2546
August	1st to 10th	Day	5% 4 2 5% 6% 5% 7% 7% 8% 12% 40%	2226
	11th to 20th	Day	7% 6% 3 6% 6% 4 6% 6% 8% 11% 38%	2125
	21st to end	Day	11% 5% 3 6% 6% 4 7% 7% 8% 9% 33%	2376
September	1st to 10th	Day	9% 5% 3 5% 6% 4 5% 6% 7% 9% 40%	2160
	11th to 20th	Day	12% 5% 3 6% 5% 4 5% 5% 6% 8% 41%	2160
	21st to end	Day	11% 5% 2 4 4 3 4 4 5% 7% 52%	1876
October	1st to 10th	Day	6% 4 2 4 5% 3 5% 4 4 6% 58%	1800
	11th to 20th	Day	10% 5% 3 4 4 3 4 4 5% 6% 54%	1800
	21st to end	Day	12% 6% 2 5% 5% 3 2 3 4 5% 53%	1980
November	1st to 10th	Day	10% 5% 3 5% 4 2 4 4 4 5% 53%	1858
	11th to 20th	Day	11% 6% 3 5% 4 2 3 4 3 6% 52%	1900
	21st to end	Day	15% 7% 3 6% 5% 2 4 4 5% 6% 43%	1900
December	1st to 10th	Day	16% 8% 2 6% 5% 2 5% 3 5% 6% 43%	1866
	11th to 20th	Day	16% 10% 3 6% 5% 3 4 4 4 6% 38%	2161
	21st to end	Day	18% 8% 3 5% 5% 2 3 3 5% 5% 42%	3135
January	1st to 10th	Night	17% 5% 4 4 4 2 4 3 4 5% 47%	4108
	11th to 20th	Night	15% 6% 3 4 4 3 3 3 4 4 51%	4120
	21st to end	Night	25% 6% 2 5% 4 2 3 3 4 5% 41%	4532
February	1st to 10th	Night	16% 7% 3 6% 5% 3 3 3 4 7% 42%	2423
	11th to 20th	Night	22% 8% 3 4 3 2 3 3 3 5% 44%	2218
	21st to end	Night	23% 8% 3 6% 4 3 3 3 4 4 39%	1780
March	1st to 10th	Night	18% 7% 3 4 5% 3 4 4 4 6% 43%	2090
	11th to 20th	Night	14% 6% 3 6% 5% 3 5% 5% 5% 7% 41%	2090
	21st to end	Night	13% 6% 3 5% 4 3 4 4 5% 7% 48%	2299
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 5% 6% 7% 46%	1845
	11th to 20th	Night	14% 6% 3 5% 4 3 3 4 5% 7% 45%	1775
	21st to end	Night	8% 4 2 5% 5% 3 4 4 6% 7% 52%	1800
May	1st to 10th	Night	10% 5% 2 5% 5% 3 4 5% 5% 9% 47%	1800
	11th to 20th	Night	9% 5% 2 5% 5% 4 4 5% 5% 7% 50%	1769
	21st to end	Night	6% 3 3 4 6% 4 6% 5% 6% 10% 47%	1775
June	1st to 10th	Night	7% 4 2 4 5% 4 5% 5% 7% 10% 47%	1620
	11th to 20th	Night	3 2 2 3 5% 4 4 6% 7% 11% 54%	1620
	21st to end	Night	7% 4 3 4 4 3 5% 5% 8% 10% 46%	1620
July	1st to 10th	Night	6% 5% 3 4 4 3 6% 7% 10% 12% 40%	1620
	11th to 20th	Night	6% 3 2 5% 4 3 5% 6% 6% 13% 49%	1620
	21st to end	Night	5% 3 2 5% 5% 3 6% 5% 8% 11% 47%	1772
August	1st to 10th	Night	3 2 1 3 4 3 5% 6% 7% 12% 55%	1708
	11th to 20th	Night	4 4 2 5% 5% 3 6% 6% 9% 13% 44%	1776
	21st to end	Night	9% 4 2 6% 6% 4 5% 7% 7% 10% 39%	1980
September	1st to 10th	Night	7% 5% 2 5% 6% 4 6% 6% 6% 8% 46%	1800
	11th to 20th	Night	8% 6% 3 5% 5% 5% 5% 6% 5% 8% 45%	1800
	21st to end	Night	9% 5% 2 3 4 3 4 4 5% 6% 55%	2056
October	1st to 10th	Night	7% 3 1 3 5% 3 5% 5% 4 6% 58%	2160
	11th to 20th	Night	10% 4 2 3 4 2 3 3 3 6% 58%	2160
	21st to end	Night	12% 4 1 4 3 3 3 3 4 4 58%	2376
November	1st to 10th	Night	10% 5% 2 4 3 2 3 3 4 6% 57%	2220
	11th to 20th	Night	12% 5% 2 4 4 2 3 4 5% 6% 54%	2280
	21st to end	Night	15% 7% 3 6% 4 3 3 4 4 5% 47%	2280
December	1st to 10th	Night	17% 7% 3 5% 4 2 4 3 4 6% 45%	2309
	11th to 20th	Night	16% 8% 3 6% 5% 2 4 4 4 5% 44%	3036
	21st to end	Night	18% 9% 2 4 4 2 4 3 4 6% 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	50% 3 6% 16% 16% 8%	2090
	11th to 20th	Day	48% 2 10% 14% 21% 5%	2090
	21st to end	Day	42% 3 8% 19% 23% 4	2299
April	1st to 10th	Day	43% 3 9% 22% 17% 7%	2157
	11th to 20th	Day	45% 3 7% 19% 18% 8%	2134
	21st to end	Day	41% 4 7% 23% 21% 4	2160
May	1st to 10th	Day	40% 4 11% 21% 18% 6%	2160
	11th to 20th	Day	38% 4 12% 27% 16% 3	2128
	21st to end	Day	40% 4 14% 27% 13% 1	2497
June	1st to 10th	Day	44% 4 14% 23% 14% 2	2340
	11th to 20th	Day	36% 6% 14% 28% 15% 1	2340
	21st to end	Day	42% 8% 15% 22% 13% 1	2340
July	1st to 10th	Day	43% 7% 21% 20% 8% 1	2276
	11th to 20th	Day	40% 7% 21% 23% 8% 1	2271
	21st to end	Day	41% 6% 20% 24% 8% 1	2405
August	1st to 10th	Day	34% 5% 22% 27% 11%	2212
	11th to 20th	Day	39% 6% 20% 24% 10% 2	2125
	21st to end	Day	44% 6% 19% 18% 11% 2	2376
September	1st to 10th	Day	40% 5% 16% 19% 16% 4	2160
	11th to 20th	Day	41% 5% 12% 18% 18% 6%	2160
	21st to end	Day	34% 2 9% 23% 22% 9%	1876
October	1st to 10th	Day	28% 3 8% 22% 25% 14%	1800
	11th to 20th	Day	33% 3 3 18% 29% 14%	1800
	21st to end	Day	35% 2 4 17% 28% 14%	1980
November	1st to 10th	Day	35% 3 6% 20% 27% 10%	1858
	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	44% 2 8% 15% 21% 10%	2090
	11th to 20th	Night	44% 4 10% 15% 20% 8%	2090
	21st to end	Night	36% 3 11% 18% 24% 7%	2299
April	1st to 10th	Night	36% 3 11% 20% 18% 10%	1845
	11th to 20th	Night	39% 2 10% 19% 18% 12%	1775
	21st to end	Night	32% 3 10% 23% 25% 6%	1800
May	1st to 10th	Night	35% 4 11% 22% 19% 8%	1800
	11th to 20th	Night	34% 3 13% 24% 22% 4	1770
	21st to end	Night	32% 5% 17% 27% 19% 1	1776
June	1st to 10th	Night	32% 7% 18% 25% 16% 2	1620
	11th to 20th	Night	23% 6% 18% 34% 16% 2	1620
	21st to end	Night	32% 6% 20% 28% 14% 1	1620
July	1st to 10th	Night	33% 9% 26% 21% 10% 1	1575
	11th to 20th	Night	26% 9% 31% 25% 8% 1	1566
	21st to end	Night	27% 10% 28% 24% 10% 1	1673
August	1st to 10th	Night	22% 7% 31% 28% 12% 1	1698
	11th to 20th	Night	29% 9% 24% 26% 10% 2	1776
	21st to end	Night	38% 8% 20% 18% 13% 3	1980
September	1st to 10th	Night	35% 6% 16% 22% 14% 7%	1800
	11th to 20th	Night	36% 5% 14% 19% 17% 8%	1800
	21st to end	Night	31% 3 10% 23% 19% 13%	2056
October	1st to 10th	Night	28% 3 9% 22% 23% 14%	2160
	11th to 20th	Night	29% 2 5% 20% 28% 16%	2160
	21st to end	Night	31% 3 6% 17% 25% 19%	2376
November	1st to 10th	Night	30% 2 8% 19% 28% 13%	2220
	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% 11% 40% 24% 8% 1	297
April	1st to 10th	Day	18% 8% 32% 28% 14% 1	401
	11th to 20th	Day	22% 10% 24% 26% 15% 3	412
	21st to end	Day	15% 7% 25% 31% 20% 3	420
May	1st to 10th	Day	15% 5% 22% 36% 18% 4	423
	11th to 20th	Day	14% 2 16% 32% 27% 10%	447
	21st to end	Day	10% 3 12% 31% 32% 12%	645
June	1st to 10th	Day	8% 3 7% 30% 37% 15%	630
	11th to 20th	Day	6% 4 8% 30% 36% 16%	630
	21st to end	Day	10% 3 5% 21% 36% 26%	630
July	1st to 10th	Day	7% 3 3 18% 37% 32%	621
	11th to 20th	Day	8% 1 2 14% 39% 37%	618
	21st to end	Day	7% 2 2 18% 40% 30%	684
August	1st to 10th	Day	6% 2 2 12% 40% 38%	524
	11th to 20th	Day	7% 1 11% 41% 39%	439
	21st to end	Day	12% 2 5% 15% 30% 36%	495
September	1st to 10th	Day	7% 2 3 15% 42% 31%	450
	11th to 20th	Day	14% 6% 20% 35% 25%	450
	21st to end	Day	14% 2 6% 24% 35% 19%	272
October	1st to 10th	Day	7% 1 17% 28% 33% 13%	270
	11th to 20th	Day	9% 3 19% 29% 27% 13%	270
	21st to end	Day	16% 8% 23% 28% 18% 7%	297
November	1st to 10th	Day	15% 3 21% 33% 23% 6%	270
	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	14% 5% 34% 37% 10% 1	546
	11th to 20th	Night	16% 6% 31% 34% 12%	533
	21st to end	Night	11% 4 29% 40% 16% 1	540
May	1st to 10th	Night	12% 3 24% 43% 16% 1	540
	11th to 20th	Night	10% 2 20% 40% 26% 2	531
	21st to end	Night	6% 3 12% 40% 35% 4	584
June	1st to 10th	Night	8% 1 4 36% 44% 6%	540
	11th to 20th	Night	4 2 7% 40% 42% 5%	540
	21st to end	Night	9% 1 4 28% 44% 13%	540
July	1st to 10th	Night	7% 3 1 24% 50% 14%	525
	11th to 20th	Night	5% 2 2 25% 50% 16%	522
	21st to end	Night	5% 2 2 23% 54% 14%	591
August	1st to 10th	Night	4 1 2 23% 54% 16%	537
	11th to 20th	Night	5% 1 1 22% 56% 15%	534
	21st to end	Night	10% 1 3 20% 47% 19%	594
September	1st to 10th	Night	8% 1 3 23% 53% 12%	540
	11th to 20th	Night	10% 2 4 26% 48% 11%	540
	21st to end	Night	11% 1 5% 33% 44% 7%	625
October	1st to 10th	Night	8% 1 9% 33% 43% 6%	720
	11th to 20th	Night	13% 2 17% 38% 26% 4	720
	21st to end	Night	14% 5% 17% 39% 22% 3	792
November	1st to 10th	Night	12% 2 18% 41% 24% 3	740
	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

Page generated at 2024-03-31 05:08:53. Version ac07

Copyright © 2024 Attilla Danko.