St. Andrews
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Andrews clear sky chart from 2005-11-06 to 2026-07-16.

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	19% 8% 2 6% 4 4 4 4 5% 7% 35%	137801
	Night	20% 9% 3 7% 4 4 4 3 5% 7% 34%	135007
Transparency	Day	47% 6% 13% 21% 12% 2	141332
	Night	49% 5% 12% 21% 12% 2	139566
Seeing	Day	23% 14% 30% 23% 8% 1	18505
	Night	25% 13% 30% 24% 8% 1	26443

By Month

Percentage Cloud Cover NE (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	14% 11% 4 9% 5% 5% 5% 4 6% 7% 31%	4242
	11th to 20th	Day	18% 13% 4 9% 4 3 3 2 3 5% 36%	4650
	21st to end	Day	11% 9% 3 7% 4 5% 6% 4 6% 7% 39%	5573
February	1st to 10th	Day	17% 11% 3 8% 5% 6% 4 4 5% 9% 29%	1890
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 4 6% 7% 35%	1843
	21st to end	Day	12% 10% 3 6% 4 5% 4 4 6% 7% 38%	1730
March	1st to 10th	Day	25% 10% 2 7% 4 5% 5% 3 5% 6% 29%	4796
	11th to 20th	Day	20% 11% 2 6% 5% 5% 4 4 6% 7% 30%	5090
	21st to end	Day	28% 10% 3 7% 4 4 4 4 6% 6% 26%	5587
April	1st to 10th	Day	21% 9% 2 6% 6% 3 5% 3 5% 7% 32%	2448
	11th to 20th	Day	21% 8% 2 6% 4 3 4 3 5% 6% 38%	2702
	21st to end	Day	22% 7% 2 5% 3 4 4 4 5% 6% 38%	2730
May	1st to 10th	Day	28% 10% 2 7% 4 4 4 3 5% 6% 27%	7639
	11th to 20th	Day	19% 7% 2 7% 4 4 5% 4 5% 7% 35%	7819
	21st to end	Day	18% 8% 3 7% 5% 4 5% 4 6% 7% 33%	9298
June	1st to 10th	Day	22% 7% 2 7% 6% 4 5% 3 5% 8% 31%	3150
	11th to 20th	Day	20% 8% 3 6% 3 4 3 4 5% 8% 37%	3150
	21st to end	Day	26% 7% 3 6% 5% 4 5% 4 6% 7% 28%	3150
July	1st to 10th	Day	16% 9% 3 8% 5% 5% 5% 5% 7% 9% 29%	6970
	11th to 20th	Day	17% 9% 2 6% 4 4 4 4 5% 8% 38%	6667
	21st to end	Day	15% 7% 2 6% 4 4 4 4 7% 9% 39%	6589
August	1st to 10th	Day	20% 9% 3 6% 5% 4 4 4 5% 7% 33%	4235
	11th to 20th	Day	16% 8% 3 7% 4 5% 5% 3 6% 9% 35%	4175
	21st to end	Day	11% 5% 2 5% 4 4 4 4 5% 8% 48%	4675
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 8% 50%	2350
	11th to 20th	Day	16% 4 2 5% 3 3 3 4 4 7% 50%	2200
	21st to end	Day	18% 6% 2 5% 4 3 3 3 4 7% 46%	2200
October	1st to 10th	Day	16% 6% 3 6% 3 3 5% 3 6% 6% 43%	2065
	11th to 20th	Day	19% 5% 2 5% 4 4 5% 4 5% 8% 40%	2000
	21st to end	Day	20% 7% 2 7% 5% 4 4 4 5% 6% 36%	2060
November	1st to 10th	Day	21% 7% 2 6% 4 4 4 3 5% 6% 39%	1851
	11th to 20th	Day	20% 6% 1 5% 3 4 4 3 6% 8% 39%	1680
	21st to end	Day	20% 9% 2 7% 5% 4 4 3 6% 6% 34%	1680
December	1st to 10th	Day	22% 9% 2 4 4 3 4 4 7% 8% 35%	2676
	11th to 20th	Day	24% 9% 2 7% 4 4 4 3 4 5% 36%	2688
	21st to end	Day	22% 10% 2 4 3 4 4 4 5% 7% 35%	3553
January	1st to 10th	Night	13% 13% 4 9% 5% 4 4 3 5% 7% 33%	8671
	11th to 20th	Night	19% 12% 4 9% 5% 4 4 3 5% 6% 30%	8235
	21st to end	Night	10% 11% 3 6% 4 4 4 4 6% 6% 41%	8606
February	1st to 10th	Night	17% 11% 3 5% 4 5% 5% 4 6% 8% 31%	2730
	11th to 20th	Night	16% 10% 3 8% 6% 5% 5% 3 6% 5% 34%	2660
	21st to end	Night	17% 11% 3 6% 5% 4 3 3 5% 6% 37%	2076
March	1st to 10th	Night	25% 11% 4 8% 4 4 5% 3 4 4 28%	5263
	11th to 20th	Night	20% 11% 3 7% 5% 4 5% 4 6% 7% 29%	5089
	21st to end	Night	31% 9% 2 5% 4 4 4 3 4 6% 29%	5587
April	1st to 10th	Night	25% 10% 3 7% 5% 4 3 3 3 5% 32%	2168
	11th to 20th	Night	23% 9% 3 6% 4 4 3 4 4 6% 33%	1868
	21st to end	Night	26% 8% 2 6% 4 4 4 3 5% 6% 32%	1890
May	1st to 10th	Night	31% 8% 3 6% 3 3 3 3 4 6% 30%	5259
	11th to 20th	Night	21% 9% 3 7% 4 5% 5% 4 6% 7% 31%	5048
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	4951
June	1st to 10th	Night	24% 8% 2 8% 4 4 4 4 5% 8% 30%	1470
	11th to 20th	Night	20% 8% 3 6% 4 5% 4 3 6% 7% 34%	1470
	21st to end	Night	28% 8% 3 7% 4 3 4 4 5% 6% 28%	1470
July	1st to 10th	Night	16% 10% 3 9% 7% 5% 7% 4 7% 7% 25%	3230
	11th to 20th	Night	19% 10% 3 7% 4 5% 5% 4 6% 8% 30%	3409
	21st to end	Night	16% 7% 3 6% 4 4 5% 4 6% 8% 37%	4216
August	1st to 10th	Night	22% 11% 3 8% 4 4 4 4 5% 6% 29%	2915
	11th to 20th	Night	16% 9% 3 6% 4 5% 5% 4 6% 9% 33%	2872
	21st to end	Night	11% 5% 2 6% 4 3 4 3 5% 9% 47%	3223
September	1st to 10th	Night	16% 7% 2 4 4 4 3 3 5% 7% 45%	2047
	11th to 20th	Night	16% 6% 2 4 4 3 3 3 4 6% 49%	2200
	21st to end	Night	19% 6% 2 4 3 3 4 3 5% 6% 45%	2200
October	1st to 10th	Night	18% 7% 2 4 4 4 4 3 5% 7% 42%	2335
	11th to 20th	Night	19% 7% 2 6% 3 4 4 3 5% 6% 41%	2400
	21st to end	Night	24% 7% 2 6% 4 4 4 3 5% 6% 35%	2777
November	1st to 10th	Night	20% 6% 2 5% 4 4 4 4 5% 7% 38%	2667
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 4 6% 38%	2938
	21st to end	Night	22% 8% 2 6% 4 4 4 3 5% 8% 34%	2940
December	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 5% 7% 36%	4720
	11th to 20th	Night	23% 9% 2 6% 3 3 4 3 5% 5% 36%	5740
	21st to end	Night	22% 11% 3 6% 4 4 4 3 5% 6% 33%	7667

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	52% 4 13% 22% 7% 1	4243
	11th to 20th	Day	53% 4 10% 22% 10% 2	4680
	21st to end	Day	45% 4 10% 19% 19% 2	5573
February	1st to 10th	Day	52% 5% 10% 18% 12% 2	1890
	11th to 20th	Day	48% 6% 12% 20% 12% 2	1851
	21st to end	Day	44% 5% 13% 22% 12% 4	1730
March	1st to 10th	Day	56% 4 10% 13% 13% 4	4800
	11th to 20th	Day	51% 8% 14% 17% 9% 1	5090
	21st to end	Day	58% 4 7% 17% 13% 2	5587
April	1st to 10th	Day	52% 5% 12% 15% 14% 3	2448
	11th to 20th	Day	46% 5% 11% 23% 13% 2	2702
	21st to end	Day	47% 6% 10% 20% 13% 4	2730
May	1st to 10th	Day	57% 6% 12% 17% 8% 1	7651
	11th to 20th	Day	48% 7% 14% 21% 8% 2	7823
	21st to end	Day	48% 8% 13% 16% 14% 1	9299
June	1st to 10th	Day	51% 7% 14% 18% 10% 1	3150
	11th to 20th	Day	46% 6% 12% 21% 14% 1	3150
	21st to end	Day	54% 8% 12% 16% 10% 2	3150
July	1st to 10th	Day	50% 8% 13% 19% 9% 1	6895
	11th to 20th	Day	44% 7% 14% 23% 12% 1	6586
	21st to end	Day	40% 7% 16% 26% 12% 1	6401
August	1st to 10th	Day	49% 7% 13% 22% 8% 1	4222
	11th to 20th	Day	45% 7% 17% 21% 9%	4214
	21st to end	Day	33% 6% 15% 26% 19% 1	4675
September	1st to 10th	Day	32% 6% 14% 29% 17% 1	2350
	11th to 20th	Day	34% 6% 12% 26% 19% 3	2200
	21st to end	Day	39% 5% 10% 28% 17% 1	2200
October	1st to 10th	Day	44% 6% 13% 27% 10% 1	3364
	11th to 20th	Day	36% 4 13% 34% 13%	3240
	21st to end	Day	36% 4 18% 22% 17% 2	3310
November	1st to 10th	Day	47% 5% 12% 22% 13% 1	1851
	11th to 20th	Day	43% 6% 14% 21% 15% 2	1680
	21st to end	Day	51% 5% 15% 20% 8% 1	1680
December	1st to 10th	Day	47% 4 18% 22% 9%	2676
	11th to 20th	Day	52% 3 15% 15% 11% 5%	2688
	21st to end	Day	48% 6% 14% 17% 12% 2	3553
January	1st to 10th	Night	51% 5% 11% 23% 9% 1	8671
	11th to 20th	Night	56% 4 11% 19% 9% 1	8280
	21st to end	Night	42% 4 10% 23% 16% 4	8606
February	1st to 10th	Night	50% 7% 12% 17% 12% 2	2730
	11th to 20th	Night	51% 6% 9% 20% 12% 2	2663
	21st to end	Night	49% 4 10% 21% 14% 3	2076
March	1st to 10th	Night	61% 3 7% 12% 14% 4	5272
	11th to 20th	Night	52% 8% 18% 15% 7% 1	5090
	21st to end	Night	58% 3 7% 18% 13% 1	5587
April	1st to 10th	Night	54% 4 10% 15% 13% 3	2168
	11th to 20th	Night	51% 5% 10% 17% 15% 1	1868
	21st to end	Night	52% 5% 9% 17% 14% 2	1890
May	1st to 10th	Night	57% 4 14% 15% 9% 1	5265
	11th to 20th	Night	50% 8% 12% 20% 9% 2	5049
	21st to end	Night	48% 7% 13% 20% 10% 1	4952
June	1st to 10th	Night	52% 6% 14% 20% 7% 1	1470
	11th to 20th	Night	50% 5% 12% 21% 11% 1	1470
	21st to end	Night	57% 5% 10% 18% 7% 3	1470
July	1st to 10th	Night	56% 7% 12% 17% 8%	3195
	11th to 20th	Night	53% 6% 11% 21% 8%	3367
	21st to end	Night	44% 6% 13% 22% 13% 1	4090
August	1st to 10th	Night	56% 7% 10% 19% 8%	2903
	11th to 20th	Night	47% 6% 16% 22% 8%	2899
	21st to end	Night	34% 6% 14% 26% 18% 2	3223
September	1st to 10th	Night	38% 7% 10% 28% 16% 1	2047
	11th to 20th	Night	37% 5% 12% 25% 20% 1	2200
	21st to end	Night	39% 4 11% 28% 17% 1	2200
October	1st to 10th	Night	43% 5% 12% 27% 13% 1	3816
	11th to 20th	Night	37% 4 12% 28% 19%	3899
	21st to end	Night	44% 3 13% 24% 14% 2	4477
November	1st to 10th	Night	45% 7% 12% 22% 14% 1	2667
	11th to 20th	Night	47% 5% 12% 21% 14% 1	2938
	21st to end	Night	49% 6% 15% 19% 10% 1	2940
December	1st to 10th	Night	49% 5% 10% 28% 9%	4720
	11th to 20th	Night	49% 5% 11% 20% 14% 2	5741
	21st to end	Night	51% 5% 12% 17% 12% 3	7667

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	27% 18% 35% 17% 2	220
	11th to 20th	Day	31% 24% 30% 11% 4 1	220
	21st to end	Day	21% 19% 39% 18% 3	395
February	1st to 10th	Day	25% 19% 33% 19% 4	430
	11th to 20th	Day	22% 25% 36% 14% 3 1	426
	21st to end	Day	23% 22% 35% 18% 3	446
March	1st to 10th	Day	24% 23% 34% 17% 2	540
	11th to 20th	Day	26% 21% 31% 16% 6%	540
	21st to end	Day	29% 21% 30% 15% 4 1	594
April	1st to 10th	Day	30% 21% 31% 15% 3 1	540
	11th to 20th	Day	25% 20% 30% 20% 4 1	532
	21st to end	Day	27% 14% 30% 22% 6% 1	540
May	1st to 10th	Day	29% 17% 31% 19% 5% 1	543
	11th to 20th	Day	22% 15% 33% 23% 6% 1	631
	21st to end	Day	24% 14% 31% 23% 8% 1	845
June	1st to 10th	Day	23% 12% 30% 24% 11% 1	780
	11th to 20th	Day	23% 10% 28% 26% 12% 1	780
	21st to end	Day	30% 6% 25% 27% 11% 1	780
July	1st to 10th	Day	21% 5% 28% 32% 12% 1	770
	11th to 20th	Day	21% 5% 28% 30% 14% 2	645
	21st to end	Day	17% 6% 32% 32% 11% 1	576
August	1st to 10th	Day	15% 6% 30% 33% 13% 2	526
	11th to 20th	Day	19% 6% 24% 32% 17% 3	519
	21st to end	Day	17% 9% 26% 31% 16% 2	583
September	1st to 10th	Day	13% 6% 31% 37% 12% 1	530
	11th to 20th	Day	20% 10% 25% 25% 15% 5%	530
	21st to end	Day	17% 11% 28% 28% 14% 3	530
October	1st to 10th	Day	18% 15% 25% 23% 17% 2	456
	11th to 20th	Day	20% 14% 29% 25% 10% 2	420
	21st to end	Day	25% 15% 31% 22% 6% 1	462
November	1st to 10th	Day	25% 16% 31% 22% 4 1	425
	11th to 20th	Day	26% 20% 31% 17% 6%	430
	21st to end	Day	27% 20% 29% 19% 4	430
December	1st to 10th	Day	28% 18% 34% 16% 4	429
	11th to 20th	Day	34% 25% 26% 12% 3	220
	21st to end	Day	25% 27% 26% 16% 5% 1	242
January	1st to 10th	Night	25% 20% 32% 20% 3	920
	11th to 20th	Night	28% 18% 33% 18% 4	840
	21st to end	Night	17% 23% 39% 17% 3	924
February	1st to 10th	Night	25% 19% 35% 17% 4	840
	11th to 20th	Night	24% 23% 32% 18% 3 1	823
	21st to end	Night	24% 17% 35% 20% 3	692
March	1st to 10th	Night	24% 18% 38% 19% 2	840
	11th to 20th	Night	24% 19% 29% 24% 5%	840
	21st to end	Night	25% 15% 34% 19% 5% 1	924
April	1st to 10th	Night	31% 15% 33% 18% 3	777
	11th to 20th	Night	26% 10% 36% 22% 5%	624
	21st to end	Night	29% 10% 33% 21% 7%	630
May	1st to 10th	Night	28% 8% 32% 25% 7%	630
	11th to 20th	Night	27% 7% 30% 29% 7%	622
	21st to end	Night	23% 7% 35% 27% 6% 1	616
June	1st to 10th	Night	27% 6% 22% 36% 9%	420
	11th to 20th	Night	23% 6% 25% 31% 14% 1	420
	21st to end	Night	32% 2 21% 35% 9% 1	420
July	1st to 10th	Night	23% 4 21% 39% 12%	410
	11th to 20th	Night	24% 5% 19% 38% 14% 1	398
	21st to end	Night	24% 3 21% 39% 14%	561
August	1st to 10th	Night	18% 5% 23% 41% 13%	598
	11th to 20th	Night	23% 4 19% 37% 16%	592
	21st to end	Night	17% 3 22% 38% 18% 2	660
September	1st to 10th	Night	20% 3 29% 32% 15% 2	677
	11th to 20th	Night	19% 6% 25% 27% 21% 2	800
	21st to end	Night	21% 7% 27% 30% 14% 2	800
October	1st to 10th	Night	22% 7% 27% 29% 14% 1	800
	11th to 20th	Night	21% 12% 32% 23% 11% 1	800
	21st to end	Night	26% 16% 29% 22% 7%	880
November	1st to 10th	Night	25% 14% 32% 21% 8% 1	820
	11th to 20th	Night	24% 17% 31% 20% 7% 1	950
	21st to end	Night	28% 19% 30% 18% 5% 1	950
December	1st to 10th	Night	33% 14% 33% 15% 4	950
	11th to 20th	Night	28% 23% 32% 14% 2 1	950
	21st to end	Night	29% 21% 31% 15% 4	1045

Page generated at 2026-07-13 04:14:52. Version ac07

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