Jaffa Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jaffa Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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	6% 10% 3 6% 4 3 4 3 5% 6% 49%	105086
	Night	7% 10% 3 7% 4 3 4 3 5% 6% 48%	103270
Transparency	Day	35% 5% 7% 21% 21% 12%	104851
	Night	37% 4 8% 21% 20% 11%	103094
Seeing	Day	13% 18% 34% 26% 8% 1	15013
	Night	12% 10% 25% 33% 17% 3	25505

By Month

Percentage Cloud Cover SW (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% 18% 6% 13% 6% 4 3 3 4 5% 28%	2858
	11th to 20th	Day	8% 24% 7% 13% 5% 3 3 2 4 6% 25%	2860
	21st to end	Day	8% 17% 6% 10% 4 3 4 3 4 7% 34%	3146
February	1st to 10th	Day	12% 17% 5% 10% 5% 4 5% 3 3 6% 31%	1710
	11th to 20th	Day	9% 18% 6% 11% 4 4 4 3 5% 6% 31%	1734
	21st to end	Day	6% 21% 6% 11% 6% 3 3 3 6% 5% 29%	1570
March	1st to 10th	Day	5% 18% 8% 12% 5% 4 4 3 5% 7% 29%	3355
	11th to 20th	Day	6% 13% 6% 6% 3 4 3 4 5% 6% 44%	3490
	21st to end	Day	9% 24% 8% 12% 5% 4 3 3 5% 5% 22%	3825
April	1st to 10th	Day	10% 15% 4 8% 5% 4 4 4 4 6% 38%	2107
	11th to 20th	Day	9% 16% 4 8% 4 3 4 3 5% 5% 39%	2395
	21st to end	Day	9% 13% 2 6% 4 4 4 3 5% 6% 43%	2470
May	1st to 10th	Day	8% 14% 4 8% 4 4 5% 4 6% 7% 37%	4110
	11th to 20th	Day	6% 5% 2 4 3 3 4 3 7% 8% 55%	4071
	21st to end	Day	8% 9% 2 5% 4 4 5% 4 6% 6% 48%	4477
June	1st to 10th	Day	4 5% 2 5% 4 4 4 4 6% 8% 53%	2503
	11th to 20th	Day	5% 5% 2 4 3 3 3 3 4 6% 61%	2632
	21st to end	Day	2 3 1 3 3 3 3 3 4 5% 70%	2470
July	1st to 10th	Day	1 2 1 2 3 2 3 2 4 5% 77%	4110
	11th to 20th	Day	3 3 1 4 4 4 5% 5% 7% 8% 57%	4110
	21st to end	Day	3 2 1 4 3 4 4 4 5% 6% 64%	4454
August	1st to 10th	Day	3 3 1 4 4 4 4 4 5% 7% 63%	4092
	11th to 20th	Day	2 2 1 4 4 5% 4 4 5% 6% 64%	4028
	21st to end	Day	3 2 1 3 3 3 3 3 5% 6% 69%	4476
September	1st to 10th	Day	3 3 1 3 3 2 4 4 4 5% 69%	2143
	11th to 20th	Day	4 4 1 4 3 3 4 4 5% 5% 63%	2090
	21st to end	Day	7% 3 1 3 2 2 2 2 4 6% 68%	2090
October	1st to 10th	Day	5% 5% 2 3 3 2 2 3 4 3 67%	3333
	11th to 20th	Day	8% 7% 1 4 2 2 3 3 5% 8% 57%	3150
	21st to end	Day	7% 9% 2 5% 3 4 3 2 4 5% 56%	3422
November	1st to 10th	Day	10% 12% 3 5% 4 3 4 3 4 5% 46%	1764
	11th to 20th	Day	13% 10% 3 7% 2 3 3 4 4 6% 45%	1800
	21st to end	Day	8% 15% 4 8% 5% 4 4 2 4 5% 42%	1669
December	1st to 10th	Day	11% 13% 4 7% 3 3 3 3 4 7% 40%	1528
	11th to 20th	Day	11% 14% 5% 10% 5% 5% 4 3 6% 8% 29%	1926
	21st to end	Day	8% 14% 6% 11% 5% 4 4 3 5% 8% 33%	3118
January	1st to 10th	Night	8% 15% 6% 14% 5% 4 3 3 4 6% 33%	4097
	11th to 20th	Night	12% 23% 5% 13% 4 3 3 2 4 5% 28%	4110
	21st to end	Night	6% 16% 5% 9% 5% 4 4 3 5% 7% 36%	4521
February	1st to 10th	Night	10% 14% 5% 9% 5% 4 4 4 4 6% 35%	2470
	11th to 20th	Night	8% 15% 4 9% 6% 3 4 3 4 6% 39%	2332
	21st to end	Night	6% 15% 5% 11% 4 3 4 4 5% 5% 37%	1884
March	1st to 10th	Night	6% 15% 6% 12% 6% 4 5% 4 5% 6% 32%	3519
	11th to 20th	Night	5% 11% 3 6% 3 4 4 4 6% 7% 48%	3490
	21st to end	Night	9% 24% 6% 8% 4 3 4 3 4 4 31%	3828
April	1st to 10th	Night	12% 13% 3 6% 3 3 4 3 4 6% 44%	2071
	11th to 20th	Night	7% 12% 3 6% 4 4 5% 3 6% 7% 43%	1728
	21st to end	Night	10% 11% 2 7% 5% 3 4 3 5% 5% 45%	1710
May	1st to 10th	Night	9% 15% 3 5% 5% 4 4 4 6% 6% 39%	2860
	11th to 20th	Night	6% 4 1 4 3 4 4 3 5% 6% 59%	2834
	21st to end	Night	9% 6% 1 5% 3 4 3 4 6% 7% 52%	3110
June	1st to 10th	Night	7% 5% 2 5% 4 3 4 3 6% 7% 54%	1663
	11th to 20th	Night	6% 4 1 4 2 3 3 3 5% 6% 63%	1531
	21st to end	Night	3 4 2 3 2 2 4 2 6% 7% 66%	1710
July	1st to 10th	Night	3 3 1 3 3 2 3 2 3 4 72%	2860
	11th to 20th	Night	2 4 1 6% 5% 4 5% 5% 7% 8% 53%	2860
	21st to end	Night	3 4 2 5% 4 4 4 4 5% 6% 59%	3109
August	1st to 10th	Night	3 3 1 5% 3 4 5% 4 5% 7% 60%	2847
	11th to 20th	Night	3 3 2 5% 4 5% 5% 3 4 6% 60%	2811
	21st to end	Night	3 3 1 3 2 4 3 3 4 5% 68%	3177
September	1st to 10th	Night	3 4 1 4 3 3 4 3 3 6% 66%	2033
	11th to 20th	Night	5% 4 1 3 2 3 3 3 5% 7% 64%	2090
	21st to end	Night	6% 4 1 3 2 2 3 2 3 3 71%	2090
October	1st to 10th	Night	5% 4 1 3 2 2 3 2 3 4 70%	3640
	11th to 20th	Night	9% 6% 2 4 3 3 2 3 4 5% 59%	3778
	21st to end	Night	8% 8% 2 5% 3 3 4 3 5% 7% 53%	4132
November	1st to 10th	Night	9% 11% 2 5% 2 3 3 4 6% 6% 48%	2535
	11th to 20th	Night	11% 8% 2 5% 3 4 3 3 5% 8% 48%	2600
	21st to end	Night	8% 13% 4 5% 4 3 4 3 4 6% 47%	2706
December	1st to 10th	Night	11% 12% 4 6% 3 3 4 3 5% 7% 44%	2687
	11th to 20th	Night	13% 13% 4 7% 4 4 4 3 5% 7% 35%	3271
	21st to end	Night	9% 13% 4 12% 6% 4 4 3 5% 6% 33%	4576

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	58% 7% 10% 18% 8% 1	2859
	11th to 20th	Day	61% 5% 13% 14% 6% 1	2860
	21st to end	Day	51% 7% 16% 18% 7% 1	3146
February	1st to 10th	Day	56% 5% 12% 17% 9% 2	1710
	11th to 20th	Day	54% 5% 11% 19% 11% 1	1743
	21st to end	Day	54% 6% 10% 19% 10% 1	1570
March	1st to 10th	Day	51% 8% 8% 21% 10% 2	3355
	11th to 20th	Day	39% 6% 10% 34% 10% 1	3490
	21st to end	Day	62% 7% 10% 13% 6% 1	3825
April	1st to 10th	Day	48% 5% 10% 20% 13% 4	2107
	11th to 20th	Day	47% 5% 10% 21% 16% 2	2398
	21st to end	Day	42% 4 8% 27% 18% 1	2470
May	1st to 10th	Day	42% 8% 5% 24% 16% 4	4110
	11th to 20th	Day	25% 5% 4 35% 27% 3	4074
	21st to end	Day	32% 8% 5% 28% 24% 3	4477
June	1st to 10th	Day	28% 5% 6% 24% 28% 9%	2503
	11th to 20th	Day	25% 3 4 20% 28% 18%	2632
	21st to end	Day	18% 3 1 13% 31% 34%	2470
July	1st to 10th	Day	12% 3 2 7% 18% 57%	4046
	11th to 20th	Day	23% 4 2 13% 28% 30%	4041
	21st to end	Day	20% 4 4 11% 30% 31%	4314
August	1st to 10th	Day	23% 3 3 23% 27% 21%	4078
	11th to 20th	Day	22% 3 3 18% 29% 24%	4067
	21st to end	Day	18% 3 3 15% 36% 25%	4475
September	1st to 10th	Day	18% 4 4 18% 30% 27%	2143
	11th to 20th	Day	24% 4 5% 19% 28% 20%	2090
	21st to end	Day	21% 3 4 23% 34% 16%	2090
October	1st to 10th	Day	25% 2 3 19% 37% 15%	3333
	11th to 20th	Day	30% 2 5% 31% 26% 6%	3150
	21st to end	Day	34% 3 7% 30% 23% 3	3422
November	1st to 10th	Day	40% 5% 10% 21% 19% 4	1764
	11th to 20th	Day	41% 4 9% 29% 15% 1	1800
	21st to end	Day	45% 4 9% 26% 14% 3	1669
December	1st to 10th	Day	45% 5% 16% 21% 9% 3	1528
	11th to 20th	Day	53% 6% 10% 22% 9%	1924
	21st to end	Day	48% 8% 11% 17% 15% 1	3118
January	1st to 10th	Night	52% 6% 11% 21% 9% 1	4097
	11th to 20th	Night	59% 6% 11% 15% 7% 2	4110
	21st to end	Night	47% 6% 18% 18% 8% 2	4521
February	1st to 10th	Night	49% 6% 13% 21% 10% 1	2470
	11th to 20th	Night	47% 4 9% 22% 17% 2	2333
	21st to end	Night	47% 6% 9% 23% 13% 2	1884
March	1st to 10th	Night	49% 7% 8% 22% 13% 1	3519
	11th to 20th	Night	35% 5% 10% 33% 15% 2	3490
	21st to end	Night	55% 6% 7% 19% 11% 2	3828
April	1st to 10th	Night	42% 5% 10% 21% 16% 6%	2071
	11th to 20th	Night	39% 5% 8% 24% 21% 3	1728
	21st to end	Night	41% 4 10% 23% 21% 1	1710
May	1st to 10th	Night	39% 9% 5% 23% 21% 2	2860
	11th to 20th	Night	25% 5% 5% 33% 27% 6%	2835
	21st to end	Night	29% 6% 7% 30% 26% 2	3110
June	1st to 10th	Night	29% 4 6% 24% 29% 8%	1663
	11th to 20th	Night	22% 4 5% 18% 31% 20%	1531
	21st to end	Night	19% 3 1 10% 30% 37%	1710
July	1st to 10th	Night	19% 3 2 7% 12% 57%	2815
	11th to 20th	Night	28% 3 2 12% 25% 29%	2806
	21st to end	Night	27% 3 4 12% 24% 30%	3011
August	1st to 10th	Night	25% 3 5% 18% 26% 23%	2838
	11th to 20th	Night	28% 3 4 13% 29% 24%	2838
	21st to end	Night	20% 2 4 10% 35% 28%	3177
September	1st to 10th	Night	21% 3 3 15% 26% 31%	2033
	11th to 20th	Night	22% 2 5% 20% 27% 23%	2090
	21st to end	Night	22% 2 3 18% 35% 21%	2090
October	1st to 10th	Night	21% 2 4 19% 35% 18%	3640
	11th to 20th	Night	31% 2 3 30% 27% 8%	3778
	21st to end	Night	33% 3 5% 29% 25% 5%	4132
November	1st to 10th	Night	35% 4 8% 25% 21% 6%	2535
	11th to 20th	Night	35% 4 11% 28% 18% 3	2600
	21st to end	Night	39% 4 8% 28% 17% 4	2706
December	1st to 10th	Night	41% 4 14% 26% 12% 4	2687
	11th to 20th	Night	48% 4 13% 23% 11% 1	3272
	21st to end	Night	49% 6% 13% 17% 12% 2	4576

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	28% 16% 23% 22% 11%	270
	11th to 20th	Day	18% 23% 30% 19% 9% 2	270
	21st to end	Day	18% 19% 30% 24% 8% 1	297
February	1st to 10th	Day	23% 23% 33% 14% 6%	270
	11th to 20th	Day	19% 23% 35% 17% 5% 1	339
	21st to end	Day	17% 27% 36% 16% 4	382
March	1st to 10th	Day	19% 23% 37% 16% 5% 1	460
	11th to 20th	Day	17% 20% 37% 22% 5%	460
	21st to end	Day	23% 25% 34% 17% 2	506
April	1st to 10th	Day	18% 24% 31% 22% 4 1	460
	11th to 20th	Day	22% 28% 29% 16% 5%	452
	21st to end	Day	19% 25% 27% 22% 7% 1	460
May	1st to 10th	Day	16% 25% 33% 19% 6% 1	463
	11th to 20th	Day	13% 24% 32% 22% 8% 2	488
	21st to end	Day	12% 19% 32% 26% 9% 2	529
June	1st to 10th	Day	6% 16% 40% 30% 6% 2	523
	11th to 20th	Day	9% 18% 32% 27% 13% 1	652
	21st to end	Day	5% 17% 40% 32% 5% 1	490
July	1st to 10th	Day	3 11% 44% 33% 7% 1	486
	11th to 20th	Day	4 12% 40% 32% 9% 2	484
	21st to end	Day	3 5% 43% 36% 11% 2	531
August	1st to 10th	Day	7% 9% 40% 34% 8% 1	487
	11th to 20th	Day	4 11% 35% 38% 9% 2	477
	21st to end	Day	6% 12% 35% 37% 8% 3	539
September	1st to 10th	Day	5% 10% 33% 37% 13% 2	490
	11th to 20th	Day	7% 18% 32% 33% 9% 1	490
	21st to end	Day	9% 14% 30% 35% 10% 2	490
October	1st to 10th	Day	11% 19% 29% 28% 12% 1	398
	11th to 20th	Day	11% 17% 31% 26% 13% 2	300
	21st to end	Day	12% 22% 35% 25% 7%	330
November	1st to 10th	Day	19% 22% 27% 26% 6% 1	300
	11th to 20th	Day	17% 20% 35% 18% 9% 2	300
	21st to end	Day	18% 14% 27% 28% 11% 2	300
December	1st to 10th	Day	21% 13% 31% 25% 8% 2	273
	11th to 20th	Day	20% 13% 36% 23% 7% 1	270
	21st to end	Day	24% 18% 28% 26% 5%	297
January	1st to 10th	Night	19% 9% 25% 27% 17% 4	760
	11th to 20th	Night	17% 9% 26% 29% 14% 5%	760
	21st to end	Night	17% 9% 23% 32% 15% 4	836
February	1st to 10th	Night	16% 11% 29% 30% 11% 3	760
	11th to 20th	Night	15% 12% 27% 30% 14% 2	744
	21st to end	Night	16% 12% 31% 27% 12% 1	628
March	1st to 10th	Night	16% 15% 28% 27% 10% 3	760
	11th to 20th	Night	12% 12% 27% 31% 15% 3	760
	21st to end	Night	15% 17% 25% 30% 10% 2	836
April	1st to 10th	Night	19% 14% 27% 28% 11% 2	741
	11th to 20th	Night	13% 13% 29% 32% 11% 2	563
	21st to end	Night	15% 11% 24% 33% 15% 3	570
May	1st to 10th	Night	12% 10% 31% 33% 13% 2	570
	11th to 20th	Night	9% 10% 24% 33% 20% 4	561
	21st to end	Night	11% 10% 25% 31% 19% 4	617
June	1st to 10th	Night	8% 11% 21% 37% 20% 2	570
	11th to 20th	Night	6% 11% 27% 38% 16% 2	570
	21st to end	Night	5% 5% 26% 39% 21% 5%	570
July	1st to 10th	Night	3 8% 22% 34% 30% 3	555
	11th to 20th	Night	5% 4 22% 43% 22% 4	552
	21st to end	Night	4 4 16% 41% 34% 2	624
August	1st to 10th	Night	5% 5% 23% 40% 26% 2	567
	11th to 20th	Night	5% 5% 20% 40% 27% 3	564
	21st to end	Night	4 6% 24% 40% 23% 3	627
September	1st to 10th	Night	5% 6% 17% 43% 25% 5%	703
	11th to 20th	Night	6% 7% 22% 36% 24% 5%	760
	21st to end	Night	8% 7% 20% 31% 26% 9%	760
October	1st to 10th	Night	9% 7% 27% 33% 20% 5%	760
	11th to 20th	Night	10% 5% 23% 36% 22% 4	760
	21st to end	Night	12% 9% 26% 30% 18% 4	836
November	1st to 10th	Night	15% 11% 23% 30% 17% 4	780
	11th to 20th	Night	14% 10% 29% 27% 15% 5%	800
	21st to end	Night	15% 10% 26% 30% 15% 4	906
December	1st to 10th	Night	17% 11% 25% 31% 15% 1	960
	11th to 20th	Night	21% 10% 27% 30% 11% 2	950
	21st to end	Night	24% 12% 24% 31% 8% 1	865

Page generated at 2024-11-29 08:43:06. Version ac07

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