Map from Google - Syria

Syria has a Mediterranean climate on the coast, with mild, rainy winters and hot, sunny summers, while it becomes arid subtropical in the vast inland areas, with moderately cold winters and hot and sunny summers. Summer is sunny throughout the country, but the air is humid on the coast, and dry in the rest of the country.
Latakia is located on the coast, and has a Mediterranean climate. The average temperature goes from 53.5 °F in January to 80.5 °F in August, which on the coast (but often even in the interior) is, albeit slightly, the warmest month. Here are the average temperatures.
Average temperatures - Latakia
LatakiaJanFebMarAprMayJunJulAugSepOctNovDec
LatakiaJFMAMJJASOND
Min (°F)464852576370757772645750
Max (°F)616164727581848684817263

Precipitation amounts to 30 inches per year, concentrated from October to April, with a maximum in winter, which is definitely rainy: in December and January, 5.9/6.3 in of rain per month fall. Summer, when it never rains, is hot and sunny, but tempered by the breezes. Here is the average precipitation.
Average precipitation - Latakia
LatakiaJanFebMarAprMayJunJulAugSepOctNovDecYear
LatakiaJFMAMJJASONDY
Prec. (in)5.94.33.71.810.2000.42.83.96.330.3
Days1198631001561060

The Mediterranean Sea is warm enough for swimming from June to October, as we can see in the following table.
Sea temperature - Latakia
LatakiaJanFebMarAprMayJunJulAugSepOctNovDec
LatakiaJFMAMJJASOND
Sea (°F)646363647075818281777266

Behind the coast there is a mountain range, 3,300 feet high, known as An-Nusayriyah, which separates the Mediterranean world from the mainland.

The interior is largely occupied by a plateau, at an altitude between 1,300 and 4,000 feet above sea level, while the Euphrates Valley is lower, about 650/1,000 feet; the temperatures decrease with increasing altitude, especially at night and in winter, while summer is often hot due to the distance from the sea. There are also mountain ranges, notably the Anti-Lebanon range in the south-west, dominated by the 9,200-feet high Mount Hermon.
In inland areas of Syria, the climate is fairly homogeneous, however, it's a bit colder in winter in the north, while the summer gets warmer as we move away from the sea. This means that the more continental areas are those of the north-east, as we can see from the mean temperatures of Qamishli, at 1,300 feet above sea level.
Average temperatures - Qamishli
QamishliJanFebMarAprMayJunJulAugSepOctNovDec
QamishliJFMAMJJASOND
Min (°F)373943505968757368594839
Max (°F)52556372849710410495826655

In Qamishli, 17 inches of rain or snow per year fall, as always concentrated in the cold half year. Here is average precipitation.
Average precipitation - Qamishli
QamishliJanFebMarAprMayJunJulAugSepOctNovDecYear
QamishliJFMAMJJASONDY
Prec. (in)3.12.82.82.41.20.10000.81.62.617
Days1111111061000471071

As for rainfall, the wettest areas of the interior are the north-west and the north-east near the border with Turkey, but also a small portion in the southwest (Daraa Governorate), where precipitation goes from 12 to 20 inches per year. For the rest, the climate is desert. This distinction about aridity, however, only applies to the winter, because the summer as we have said is dry throughout Syria, and is shown in the following map:

Aridity in Syrian inland areas

Aleppo, located in the north, at 1,300 feet above sea level, has a quite cold winter, with a January average of 43 °F, while summer is hot, with an average in July and August of 83.5 °F. Here are the average temperatures.
Average temperatures - Aleppo
AleppoJanFebMarAprMayJunJulAugSepOctNovDec
AleppoJFMAMJJASOND
Min (°F)363641485764707063544337
Max (°F)505563738493979791816354

Precipitation amounts to 13 inches per year, and occurs between October and May, but is moderate only in winter, while it never rains from June to September. During winter, from December to February or early March, cold spells lasting a few days, with snow and frost, may occur. In some years, the rains from December to March can be abundant. Here is the average precipitation.
Average precipitation - Aleppo
AleppoJanFebMarAprMayJunJulAugSepOctNovDecYear
AleppoJFMAMJJASONDY
Prec. (in)2.421.81.40.80.1000.10.81.42.413
Days131410741001471172

The archaeological site of Ebla, is situated in the west at 1,300 feet above sea level and 52 miles from the sea, and has a climate similar to that of Aleppo.
Palmyra, near the modern Tadmur, is located in an oasis in the center of Syria, at 1,300 feet above sea level, and is less rainy, although it has similar temperatures: the daily average is 45 °F in January and 85.5 °F in July and August.
Average temperatures - Palmyra
PalmyraJanFebMarAprMayJunJulAugSepOctNovDec
PalmyraJFMAMJJASOND
Min (°F)373945546168727266574639
Max (°F)54596677889510010093826857

The rainfall amounts to just 5.2 inches per year, concentrated as usual from October to April, with a maximum in winter. Here is the average precipitation.
Average precipitation - Palmyra
PalmyraJanFebMarAprMayJunJulAugSepOctNovDecYear
PalmyraJFMAMJJASONDY
Prec. (in)0.80.80.80.80.300000.40.60.85.3
Days710642000035744

Palmyra, Syria

The capital Damascus is located in the south-west, at 2,300 feet above sea level, about 50 miles from the sea. Even here the climate is not different from that of Aleppo, with a fairly cold winter and a hot summer, ranging from 43.5 °F in January to 80 °F in July and August, but the higher altitude makes the summer nights cooler.
Average temperatures - Damascus
DamascusJanFebMarAprMayJunJulAugSepOctNovDec
DamascusJFMAMJJASOND
Min (°F)323439455257636355483934
Max (°F)555966778693999791826857

The rainfall in Damascus amounts to around 6.3 inches per annum, however, here too from December to February, snow and frost may sometimes occur. Here is the average precipitation.
Average precipitation - Damascus
DamascusJanFebMarAprMayJunJulAugSepOctNovDecYear
DamascusJFMAMJJASONDY
Prec. (in)1.21.20.80.4000000.40.81.66.3
Days97532000025639

During spring, and less often in autumn, Syria may be affected by strong southerly winds, which cause sandstorms and raise considerably the temperature.

When to go

The best times to visit Syria are spring and autumn, and in particular the months of April and October.

What to pack

In winter: in Aleppo, Damascus and the plateau: warm clothes, down jacket, hat, raincoat or umbrella, scarf for the wind-borne sand and dust, hiking shoes; on the coast, spring/autumn clothes, sweater and jacket, raincoat or umbrella.

In summer: in Aleppo, Damascus and the plateau: lightweight clothing, of natural fiber, desert turban, hiking boots, sweatshirt or sweater for evenings outdoors, sleeping bag for overnight stays outdoors. For the coast, light clothing, a light sweatshirt for the evening. For women, it is best to avoid low-cut dresses.