Map from Google - Egypt

In Egypt there are three types of climate: the Mediterranean climate on the northern coast, the desert climate in inland areas, and finally the climate of the coast of the Red Sea, equally desert but a bit milder.

On the Mediterranean coast (see Alexandria, Marsa Matrouh), winter is very mild, and looks like spring: highs are around 18/19 °C (64/66 °F), and lows around 9/10 °C (48/50 °F), though this is the only period of the year in which weak or moderate rains occur. Summer is long, muggy and sunny, with highs around 30 °C (86 °F), but tempered by sea breezes. The humidity is high especially in the delta of the Nile River.
Here are the average temperatures of Alexandria.
Average temperatures - Alexandria
AlexandriaJanFebMarAprMayJunJulAugSepOctNovDec
AlexandriaJFMAMJJASOND
Min (°C)9911131720232321181411
Max (°C)181921242729303030282420
Min (°F)484852556368737370645752
Max (°F)646670758184868686827568

The rains on the northern coast follow the Mediterranean pattern, but they are scarce and concentrated in winter, since this is the only period when depressions can pass. In Alexandria, only 200 mm (8 inches) of rain fall per year. Here are the average precipitation.
Average precipitation - Alexandria
AlexandriaJanFebMarAprMayJunJulAugSepOctNovDecYear
AlexandriaJFMAMJJASONDY
Prec. (mm)55301541000193055196
Prec.(in)2.21.20.60.2000000.41.22.27.7
Days1196210000351047

The temperature of the Mediterranean Sea is warm from July to October, but all in all is warm enough for swimming even in June, as can be seen in the following table.
Sea temperature - Alexandria
AlexandriaJanFebMarAprMayJunJulAugSepOctNovDec
AlexandriaJFMAMJJASOND
Sea (°C)181717182023252626252220
Sea (°F)646363646873777979777268

In inland areas the climate is desert, with virtually no rain; the temperatures increase gradually as you head south. The diurnal temperature range is remarkable, due to clear skies and low relative humidity. Winters are mild and sunny, but nights are cool or cold, ranging from 5/7 °C (41/45 °F) in the central area (see Luxor), to 10/11 °C in the southern area (see Aswan), though in the coldest nights the temperature can approach freezing (0 °C or 32 °F). The winter days are pleasantly mild or warm, about 20/22 °C (68/72 °F) on average. In inland areas, summers are scorchingly hot, with highs ranging from 36/37 °C (97/99 °F) in the centre-north, to 40/42 °C (104/108 °F) in the south, and during this season the sun hits hard. In the hottest days, in the south the temperature can reach as high as 50/52 °C (122/126 °F).
Here are the average temperatures in Luxor.
Average temperatures - Luxor
LuxorJanFebMarAprMayJunJulAugSepOctNovDec
LuxorJFMAMJJASOND
Min (°C)671116202324242218128
Max (°C)232529353941414139352924
Min (°F)434552616873757572645446
Max (°F)73778495102106106106102958475

The Egyptian south-central desert is actually one of the sunniest places on Earth: clear skies are a permanent feature in all seasons.

Pyramids

Cairo, the capital, is located about 180 kilometres (110 miles) away from the sea, but it's at the edge of the huge Nile Delta, so its climate is halfway between the Mediterranean and the desert climate: the summer temperatures are lower than in the desert area, but higher than in the coast; highs are around 35 °C (95 °F) in July and August, but moisture is quite high, therefore the heat can be stifling. Summer is long: the maximum temperatures exceed 30 °C (86 °F) from May to mid-October. Within the city, the unpleasant sensation of heat is increased by the so-called "urban heat island effect", which occur in the big cities, and also by pollution.
Average temperatures - Cairo
CairoJanFebMarAprMayJunJulAugSepOctNovDec
CairoJFMAMJJASOND
Min (°C)91012151820222221171410
Max (°C)192024283234353433292520
Min (°F)485054596468727270635750
Max (°F)666875829093959391847768

The rainfall in Cairo is poor, since it amounts to only 25 mm (1 in) per year, and it's absent in the long summer: in fact, from the point of view of rainfall, we are already in the desert.
Winter in the capital is similar to that of the north coast: highs are around 18/20 °C (64/68 °F) between December and February, although sometimes cold and cloudy days may occur, when northern currents prevail. The sun, however, frequently shines in all seasons, and it almost always shines in the long period that goes from spring to autumn.

On the coasts of the Red Sea (see Hurghada), the climate is desert, but tempered by the sea: the rainfall is very little or totally absent as in the interior, but the temperature range is lower and the humidity is generally higher. Therefore, in summer the days are very hot and humid, with minimum temperatures around 25 °C (77 °F), and maximum temperatures around 34/35 °C (93/95 °F), except when the wind blows from the desert, raising the temperature while lowering the humidity. On the coast to the east of Sinai (see Sharm el Sheikh), where the prevailing wind in summer comes from the desert, the heat is more intense but moisture is lower. During winter, the temperature on the coast is very mild: in the northern part, highs hover around 21/22 °C (70/72 °F) and lows stay around 10/12 °C (50/54 °F), while the southern part is slightly warmer.
Here is the average temperature in Sharm el Sheikh.
Average temperatures - Sharm el Sheikh
Sharm el SheikhJanFebMarAprMayJunJulAugSepOctNovDec
Sharm el SheikhJFMAMJJASOND
Min (°C)131416202427272827231915
Max (°C)222225303437383835322723
Min (°F)555761687581818281736659
Max (°F)72727786939910010095908173

The temperature of the Red Sea is very high in summer, while from January to March, in the north-central part it is a bit cool for swimming, but all in all it's not impossible.
Sea temperature - Sharm el Sheikh
Sharm el SheikhJanFebMarAprMayJunJulAugSepOctNovDec
Sharm el SheikhJFMAMJJASOND
Sea (°C)222121232426272828272524
Sea (°F)727070737579818282817775

In the south-central part of the coast of the Red Sea (see Marsa Alam), the temperature of the sea is a bit higher in winter, since it doesn't go below 22 °C (72 °F) in February.

In the highlands of Sinai, the climate is cooled by altitude, and temperatures are lower especially at night: during winter there can be some frosts at night, and even some snowfalls. During summer, the heat is intense during the day, while nights remain cool.

The wind in Egypt is often absent or weak, breezy along the coasts, but sometimes, especially in spring (April to June), for a few days the Khamsin can blow, a hot and dry wind which raises dust or sand storms and makes travel and communication difficult.

When to go

To visit the archaeological sites of Egypt, such as the Great Pyramids of El Giza, the best time is winter (November to February), when the weather is normally mild and sunny. The temperature range can be high, especially in the south, so that it can be warm during the day but even cold at night.
The absolute best months are November in the north (Cairo, El Giza) and December in the centre-south (Luxor, Aswan).
In spring the temperatures are higher (in the south, in April the days are already torrid), but nights are cool; sometimes, however, the Khamsin blows.
In summer, on the northern coast and down to El Cairo and Giza, the heat is sweltering, while further south the heat is intense but dry; a relentless sun reigns everywhere. During this period, it can be unpleasant to visit the archaeological sites for those who don't like the heat, though, on the positive side, in this period there's a lower number of tourists.

For swimming and sunbathing it's necessary to distinguish between the coast of the Mediterranean and that of the Red Sea.
On the Mediterranean Coast, the long summer, from mid-May to mid-October, is fine, although as we have seen in May the sea is still cool.
On the coast of the Red Sea, summer is too hot, while winter is a bit cool, so it's preferable to go in spring and autumn.
In Sharm el Sheikh, the best times are April, and from mid-October to mid-November; as we have seen, due to the thermal inertia of the sea, in the latter period the sea is warmer. During winter, the sun often shines but the temperature, though nice, is not always high enough for swimming, while in summer it is often too high.

What to pack

In winter: in Cairo, spring/autumn clothes, a jacket and a sweater, warm clothing for hotels and houses which could not be heated; in Alexandria and the Mediterranean coast, you can add a raincoat or umbrella.
In the desert, light clothing, but also sweater and warm jacket for the evening, scarf for the sand.
On the Red Sea Coast, light clothes for the day, sweater and light jacket for the evening. In Sinai, warm jacket, hiking boots. For boat trips, wind jacket.

In summer: on the Mediterranean Coast and in Cairo, summer clothes; a light sweatshirt for the evening on the coast.
In the desert, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), a wide-brimmed hat, a light scarf, sunglasses (even graduated instead of contact lenses), comfortable and breathable shoes; for women it's best to avoid shorts and miniskirts in rural and non-tourist areas; a sweatshirt for the night, sleeping bag to sleep outdoors; desert boots or sandals.
On the Red Sea Coast, lightweight clothes; for the reef, equipment for snorkeling, water shoes or rubber soled shoes.