In Slovakia the climate is moderately continental in the plains, while it becomes colder with increasing altitude in the mountainous areas. The west (see Bratislava, Nitra) is flat, in the north there's the vast region occupied by the Tatra Mountains, while the rest of the country is covered with hills and low mountains, belonging (like the same Tatras), to the vast region of the Carpathians.
In the western part of Slovakia, where the capital Bratislava is located, the climate is moderately continental, with cold winters and warm summers. Precipitation is fairly frequent throughout the year, but it's most abundant in late spring and summer, when it can occur in the form of thunderstorm, while in winter it often takes place in the form of light snow.
Here is the average precipitation in Bratislava.
Average precipitation - Bratislava

Bratislava	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Bratislava	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	40	35	35	40	55	60	50	50	50	35	50	50	555
Prec.(in)	1.6	1.4	1.4	1.6	2.2	2.4	2	2	2	1.4	2	2	21.9
Days	14	12	13	11	11	12	11	11	10	10	14	15	144

The daily average temperatures in winter are around freezing (0 °C or 32 °F), while in summer they are around 13/15 °C (55/59 °F) at night, and 25/27 °C (77/81 °F) during the day. During winter, the area of Bratislava can experience dull and wet days, but also intense frosts during outbreaks of cold air from the east, in which the temperature can drop to -20 °C (-4 °F), while in summer sometimes it can experience hot and muggy days, when southern air masses prevail, with peaks of 30/35 °C (86/95 °F).
Here are the average temperatures of Bratislava.
Average temperatures - Bratislava

Bratislava	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Bratislava	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	-4	-2	1	5	10	13	15	15	11	6	1	-2
Max (°C)	2	5	11	16	22	25	27	27	22	15	8	4
Min (°F)	25	28	34	41	50	55	59	59	52	43	34	28
Max (°F)	36	41	52	61	72	77	81	81	72	59	46	39

In Bratislava the sun is rarely seen from November to January, while from May to August it shines for a little more than half the time (compared to the length of the days).
Sunshine - Bratislava

Bratislava	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Bratislava	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	2	3	5	7	8	9	9	9	7	5	2	2

Most places throughout the central and eastern part of the country are located at a few hundred metres of altitude, which is enough to make the climate slightly colder than in Bratislava, so that snow remains on the ground more easily during winter, while summer is generally cool, and the heat, when present, is more bearable.
Here are the average temperatures of Poprad, located at 700 metres (2,300 feet) above sea level.
Average temperatures - Poprad

Poprad	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Poprad	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	-8	-7	-3	1	5	8	10	9	6	2	-3	-7
Max (°C)	0	2	7	12	17	20	22	22	18	13	5	1
Min (°F)	18	19	27	34	41	46	50	48	43	36	27	19
Max (°F)	32	36	45	54	63	68	72	72	64	55	41	34

In the far east, east of Kosice, there is another small flat region, where the climate has the typical continental characteristics of Eastern Europe: colder winters than in the capital, with longer periods below freezing even during the day and weak but frequent snowfalls, and warm summers, with frequent sunshine but also fairly frequent afternoon thunderstorms.

The mountainous areas of Slovakia have a colder climate due to the altitude, especially the northern part, where the Tatra Mountains are located. Here the climate is freezing and snowy in winter, while afternoon thunderstorms are quite frequent in summer. Moreover, the wind is more frequent. At 2,000 metres (6,500 feet) above sea level, the average temperature is around -8 °C (18 °F) in January, and 6/7 °C (43/45 °F) in July.
On top of Peak Lomnický, at 2,634 metres (8,642 feet) of altitude, it's cold and it can snow even in summer. Here are the average temperatures.
Average temperatures - Lomnicky stit

Lomnicky stit	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Lomnicky stit	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	-12	-13	-10	-8	-3	0	1	1	0	-3	-8	-11
Max (°C)	-8	-8	-6	-3	1	3	5	6	3	0	-4	-7
Min (°F)	10	9	14	18	27	32	34	34	32	27	18	12
Max (°F)	18	18	21	27	34	37	41	43	37	32	25	19

The best time to visit Slovakia runs from mid-May to mid-September. In July and August, hot and muggy days may occur, especially in the lowlands (the region of Bratislava and the easternmost plain), while in the rest of the country, nights remain cool even in the hottest periods. In plains and valleys, during the warmest months the rains occur often in the form of short thunderstorms, which do not reduce too much the hours of sunshine, while in the mountains, cloudiness in the afternoon is more frequent.


In winter: warm clothes, sweater, down jacket, hat, gloves, scarf.
In summer: light clothes, T-shirt and shorts, but also long pants, jacket and sweater for the evening or for cooler days; raincoat or umbrella; for the mountains, warmer clothes depending on altitude, hiking shoes and sunglasses.