Map from Google - Jordan

In Jordan the climate is subtropical arid, with quite cold winters on the plateau, due to the altitude, and sunny summers, hot but partly tempered by the altitude as well. Most of Jordan is in fact occupied by a plateau, which has an altitude between 700 and 1,200 metres (2,300 and 3,900 feet), broken only to the west by the narrow valley of the Jordan River, which goes even below sea level, and therefore has higher temperatures, with mild winters and very hot summers. In the south, there are also mountains, culminating in Jabal Umm Dami, 1,854 metres high.
In Jordan, the rains are generally scarce, and fall from November to April, with a maximum in winter, between December and February. The total annual rainfall ranges between 250 and 450 millimetres in the north-western area, and it decreases to a desert level, below 100 mm (4 in) per year in the rest of the country, that is, in the south-central Jordan Valley, and in the vast areas of the north-east and the south-east, which are offshoots of the Syrian and Arabian deserts. Therefore, the majority of the country is desert, while the north-western area is semi-desert, and some parts of it, located above 1,000 metres (3,300 feet) are even quite green. During winter, the river beds which remain dry most of the year, called wadis, can suddenly fill up with water, so they can be dangerous to cross.
In Jordan's capital, Amman, the climate is mild and dry; precipitation amounts to 270 mm (10.5 in) per year, of which 60 mm (2.4 in) in January and February, while basically it never rains from May to October. Here is the average precipitation.
Average precipitation - Amman
Prec. (mm)656045153000073050270

The city is located at around 800 metres (2,600 feet) above sea level, and has an average temperature in January of 7 °C (44.5 °F), and in July and August of 25 °C (77 °F). During winter, there are sunny periods, mild during the day and a bit cold at night, alternating with periods of bad weather and rain; from December to February, every now and then cold air masses from the north can arrive here and cause snowfalls, especially in the western suburbs of the city which are located at a slightly higher altitude, around 1,000 metres (3,300 feet), and at night the temperature can drop a few degrees below freezing (0 °C or 32 °F). Summer in Amman is sunny and hot, but the altitude tempers the heat, in fact highs are usually around 30/32 °C (86/90 °F), even though there can be also torrid days, with peaks of 37/40 °C (99/104 °F) from May to September, more likely in July and August.
Here are the average temperatures.
Average temperatures - Amman
Min (°C)4461014171919171495
Max (°C)121417232831323231272014
Min (°F)393943505763666663574841
Max (°F)545763738288909088816857

Irbid, in the north, is rainier than Amman, in fact here precipitation reaches 450 mm (17.5 in) per year, but the pattern is the same, with a maximum in winter and a dry summer. Irbid is located at 600 metres (2,000 feet) above sea level, so it is slightly warmer than Amman.
In the north-east, the climate is desert; winter temperatures are similar to those of Amman, with possible frosts, while summer is warmer, and despite the altitude, daytime temperatures around 35 °C (95 °F) are normal, with peaks of 45 °C (113 °F) and above. Here are the average temperatures of Ruwaished, located in the north-east, at 700 metres (2,300 feet) above sea level.
Average temperatures - Ruwaished
Min (°C)3581317202222201595
Max (°C)121418242933363533272013
Min (°F)374146556368727268594841
Max (°F)545764758491979591816855

Rainfall does not even reach 100 millimetres (4 inches) per year, and is therefore rare and sporadic even in winter. Here is the average precipitation.
Average precipitation - Ruwaished
Prec. (mm)9131011700003101477

The famous archaeological site of Petra is located in the south, 900 metres (3,000 feet) above sea level, and it also has a desert climate. Its temperatures are similar to those of Amman, but it's drier, so that only 40 mm (1.6 in) of rain fall in a year.

Petra, Jordan

The Jordan Valley is very mild in winter and scorchingly hot in summer, but while in the north the rainfall pattern is almost Mediterranean as in Amman, with rains in the period from November to April, which amount to about 300 mm (12 in) per year, going south it becomes progressively more arid, until it becomes desert, already at the latitude of the Dead Sea, the salt lake whose banks are located in a deep depression, 400 metres (1,300 feet) below sea level, where most of the water evaporates due to the heat. In the Dead Sea area, just over 100 mm (4 in) of rain per year fall. In the Jordan Valley, the average temperature in January is around 14/15 °C, while in July and August it ranges from 30 °C (86 °F) in the north to 32 °C (90 °F) in the south, as in Aqabah, the Red Sea port which is also the only small portion of the Jordan territory overlooking the sea.
Here are the average temperatures of Aqabah.
Average temperatures - Aqabah
Min (°C)91013172124252523201510
Max (°C)212226313538393936332722
Min (°F)485055637075777773685950
Max (°F)707279889510010210297918172

In Aqaba the rains are rare and do not reach even 50 mm (2 in) per year. Here is the average rainfall.
Average precipitation - Aqabah
Prec. (mm)55531000023832

At Aqabah, the Red Sea is a bit cool for swimming in winter, while it is very warm in the summer months.
Sea temperature - Aqabah
Sea (°C)222121232526282828272523
Sea (°F)727070737779828282817773

Sometimes Jordan may be affected by strong and hot winds blowing from the Egyptian desert, causing dust and sand storms; this happens more easily in spring and autumn.

When to go

The best times to visit Jordan are spring and autumn, and in particular the months of April, May (which can already be hot) and October.
For a beach holiday in the Red Sea, the best months are April, and from mid-October to mid-November; summer is too hot, in fact it's not easy to sunbathe when the temperature exceeds 40 °C (104 °F) in the shade. Winter in the coast is very mild and sunny, so a stay can be pleasant, but the sea is a bit cool and sometimes the temperature can be cool or even cold at night.

What to pack

In winter: in Amman, Petra and the plateau: warm clothes, jacket, hat, raincoat or umbrella, scarf for the wind-borne sand and dust, hiking shoes; for the valley of Jordan and Aqaba: spring/autumn clothes, a jacket and a sweater, umbrella for the area north of the Dead Sea.

In summer: in Amman, Petra and the plateau: light clothes, sun hat, sweatshirt or sweater for evenings outdoors, desert turban for the wind-borne sand and dust, hiking shoes, sleeping bag for overnight stays outdoors; for the valley of Jordan and Aqaba: lightweight clothes, desert turban. For women, it is better to avoid shorts and miniskirts in rural and non-tourist areas.