Map from Google - Israel

The climate in Israel is characterized by hot and sunny summers, and by a long period of sunshine, with clear skies from April to October. The rains (where they occur) are concentrated from November to March.
The climate is Mediterranean along the west coast (see Tel Aviv, Haifa, Netanya), with mild and rainy winters, and hot and sunny summers, during which the air is humid but tempered the breeze. In Tel Aviv, the average temperature in January is 14 °C (57 °F), while in July and August it's about 26 °C (79 °F). Heat waves can occur especially in the period from April to June, when the wind from the Egyptian desert can raise the temperature to around 40 °C (104 °F) even on the coast.
Here are the average temperatures in Tel Aviv.
Average temperatures - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)101012141720232322191512
Max (°C)181820232527293029272420
Min (°F)505054576368737372665954
Max (°F)646468737781848684817568

Rainfall, concentrated in the winter months, amounts to around 580 millimetres (23 inches) per annum. Here is the average precipitation.
Average precipitation - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)1451106015400013580125585

On the Mediterranean coast, the sea is warm enough for swimming from June to November.
Sea temperature - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)181717182124262827262320
Sea (°F)646363647075798281797368

In the interior (see Jerusalem, Nazareth), the climate becomes more arid, but also more continental, due both to the greater distance from the sea and the elevation: Jerusalem is located at 700 metres (2,300 feet) above sea level, Bethlehem, in the West Bank, at almost 800 metres (2,600 feet), while Nazareth is located at a lower altitude, around 300/500 metres (1,000/1,600 feet). In these inland areas, winter is still fairly mild, but intrusion of cold air masses from Russia can bring even snow in January and February (and sometimes also in early March), usually above 600 metres (2,000 feet). During summer, the air in this area is dry and the heat is usually tolerable, but sometimes hot air masses from the Egyptian desert can bring some days with scorching heat even here. In Jerusalem, the average temperature in January is 9 °C (48 °F), while in August it's 24.5 °C (76 °F). Here are the average temperatures in Jerusalem.
Average temperatures - Jerusalem
Min (°C)66813161819201917128
Max (°C)121315222528292928251914
Min (°F)434346556164666866635446
Max (°F)545559727782848482776657

Annual precipitation is around 550 mm (21.5 in), concentrated in the winter months, while from May to mid-October it almost never rains. Here is the average precipitation.
Average precipitation - Jerusalem
Prec. (mm)1351209525300001560105555


Further to the east, in the Jordan valley, whose altitude goes even below sea level (not only around the Dead Sea, but also south of the Sea of Galilee), winter is still mild, but summer becomes definitely hot. Rainfall is scarce, and the landscape, excluding the areas where the river's water is used to irrigate fields, is semi-desert. In Tiberias, an average of 415 millimetres (16.5 inches) of rain fall each year, concentrated as usual in the winter months. Here are the average temperatures.
Average temperatures - Tiberias
Min (°C)8810131720222321181410
Max (°C)171922273235363635302419
Min (°F)464650556368727370645750
Max (°F)636672819095979795867566

Further to the south, in the areas bordering the Sinai Peninsula and the Red Sea, we find the Negev Desert, which has mild winters and hot summers. Unlike the Mediterranean coast, summer is not the best time for a beach holiday at the Red Sea: in Eilat, the average daily temperature in July and August is 32 °C (90 °F), with a minimum of 26 °C (79 °F) and maximum of 38 °C (100 °F), but sometimes it can exceed 45 °C (113 °F). In return, in April and October, the maximum temperature is around 30/32 °C (86/90 °F), and sunshine is still guaranteed.
Average temperatures - Eilat
Min (°C)101114182225262625211611
Max (°C)202125303437383836322621
Min (°F)505257647277797977706152
Max (°F)68707786939910010097907970

In Eilat, the rainfall is very poor, since it doesn't even reach 50 millimetres (2 inches) per year.
At Eilat, the Red Sea is a bit cool for swimming in winter, while it is pleasantly warm in the summer months.
Sea temperature - Eilat
Sea (°C)222121232526282828272523
Sea (°F)727070737779828282817773

When to go

The best times to visit Israel are spring and autumn, and in particular the months of April, May and October. Easter is therefore usually a good time to visit the sites of historical and religious interest in the country.
For a beach holiday, on the Mediterranean coast the warm and sunny season is long and runs from mid-May to mid-October, although as we have seen in May the sea is still a bit cool.
The Red Sea is warmer, but as mentioned in summer the heat is too intense for sunbathing, especially from June to September, while in winter, from December to February, pleasant days alternate with others which can be bit cold: so here the best periods for a beach holiday are the month of April, and from mid-October to mid-November.

What to pack

In winter: in Jerusalem and inland hills: warm clothes, jacket, hat, raincoat or umbrella, scarf for the wind, hiking shoes; in the Jordan Valley and Eilat: spring/autumn clothes, shirt for the day, jacket and sweater, umbrella for the area north of the Dead Sea. In Tel Aviv and the Mediterranean coast: spring/autumn clothes, sweater and jacket, raincoat or umbrella.

In summer: in Jerusalem and inland hills: light clothes, sun hat, sweatshirt for the evening, scarf for the wind-borne sand and dust, hiking shoes. In the Jordan valley and Eilat: lightweight clothing of natural fibres, desert turban; in Tel Aviv and the Mediterranean coast, light clothing, a light sweatshirt for the evening.
For the reef, snorkeling equipment, water shoes or rubber soled shoes.