(This article is published by me on Wikipedia too)

The climate of Karachi features an arid climate. Karachi is located on the coast and as a result has a relatively mild climate. Karachi has two main seasons; Summer & Winter, while spring and autumn are very short. Summer season persists for longest period during the year. The level of precipitation is low for most of the year. Karachi also receives the monsoon rains from July to September. The city enjoys a tropical climate encompassing mild winters and warm summers. The humidity levels usually remain high from March to November, while very low in Winter as the wind direction in winter is North Easterly. Most visitors, tourists and expatriates come to Karachi during the month of December.

 Factors

The monsoon and the Western Disturbance are the two main factors which alter the weather over Karachi; otherwise, Continental air prevails for rest of the year. Following are the main factors that alter the weather over Karachi city.

• Western Disturbances mostly occur during the winter months and cause drizzle to light showers, temperature also decreases due to it.
• Tropical Storms usually form during the summer months from late April till June and then from late September till November.
• Southwest Monsoon occurs in summer from the month of July till September. Monsoon rains bring much awaited relief from the scorching summer heat. These monsoon rains are quite heavy by nature and can cause significant flooding.
• Continental air prevails during the period when there is no precipitation in the metropolis.

Monthly weather conditions

Like all other cities, Karachi too has four season which is Winter, Summer, Autumn and Spring. The monsoon season also occurs in Summer. The following is a monthly summary of climatic conditions in the city.

January

In the month of January, the cold conditions continue in the city and sometimes get very cold due to the Western Disturbance. Humidity remains low in January. Rains do occur in this month but are moderate in intensity, the sky remains overcast for some part of the month.The highest rainfall during this month was 89.3 millimetres (3.52 in), which occurred in 1995.  On 29 January, 2011, an intense but short spell of rain lashed all parts of the city, at least 9 millimetres (0.35 in) rainfall was recorded. The lowest temperature of 0.0 °C was recorded on 21 January 1934, while the highest temperature of 32.8 °Cwas recorded on 16 January 1965.

February

Until the middle of the month, the cold conditions continue; after that the weather becomes mild throughout the month. The sky remains overcast with drizzle or light rain as a result of the Western Disturbance. Humidity remains between 40-60%.The lowest temperature was 3.3 °C on 11 February 1950 and the highest was 36.1 °C on 25 February 1996. The highest monthly rainfall of 96 millimetres (3.8 in) was recorded in 1979.

March

Till the beginning of March the weather becomes hot. The lowest temperature ever recorded was 7 °C on 9 March 1979 and the highest was 41.5 °C on 2 March 2004.The highest temperature was again recorded in 2010. Being a hot month there is no rain, yet drizzle can occur rarely.March is the annual spring season in Karachi after winter; spring brings beautiful flowers in the city. Many flowers shows are also held in the city in March. A weather system dumped 130 millimetres (5.1 in) rain in 1967, which is March’s highest rainfall for the city.

April

The heatwave of March continues in April as well. Temperatures increase and the weather remains dry and hot. The humidity remains between 65-75%.The highest temperature during April was 44.4 °C, which was recorded on 16 April 1947, while the lowest temperature of 12.2 °C was recorded on 29 April 1967. Rain is rare in this month. The highest monthly rainfall of 52.8 millimetres (2.08 in) was recorded in 1935.

May

May is the hottest month for Karachi, with temperatures touching 40 °C. The hottest May was on 9 May 1938, when temperatures reached 48.8 °C; the lowest temperature ever recorded was on 4 May 1989 when 17.7 °C  was recorded. On May 21, 2011, temperatures touched 46°C, highest in 30 years. The humidity is quite high in this month as well between 70–80%. Rainfall remains rare during this month and the sky remains cloudless. The highest monthly rain that occurred was in May 1933, when 33 millimetres (1.3 in) rain lashed the city. 

In 1999, 2001 and 2010, three major cyclones were formed that came close to Karachi. The 1999 cyclone (the fourth strongest cyclone of the Arabian Sea) hit close to Karachi, while the 2001 India cyclone (the third strongest cyclone of the Arabian Sea) made landfall near the Indian border. Cyclone Phet was formed in May 2010, about 1100 km away from Karachi.

June

The temperatures of May continue through June with a slight decrease. The highest temperature of June was 47 °C on 18 June 1979, and the lowest was 22.1 °C, recorded on 3 June 1997. In the middle of June pre-monsoon rains can start to break the heat spell. Tropical cyclones form close to the sea of Karachi in this month, as in 2007, when Cyclone Yemyin created havoc in the city with strong windstorms of 70 mph and rainstorms killing 200 people. The cyclone produced a heavy rainfall of 110.2 millimetres (4.34 in) for June in 2007, which broke all previous records for June. Cyclone Yemyin was a weak Category 1 hurricane, its strongest winds were observed in Karachi on June 23, 2007 at about 69 mph (111 km/h), winds battered the city in the evening with heavy downpours; the same morning, temperatures reached 44.1 °C.

On June 6, 2010, Cyclone Phet came close to the coast of Karachi as a weak tropical depression, at about 50 km away from the city after a week-long journey. Phet produced a very heavy downpour in the city, which broke the previous record of June 2007. About 152 millimetres (6.0 in) of rain with 35 mph (56 km/h) winds occurred due to Phet. Humidity in June typically remains between 75–85%. In the evening, winds from the southwest blow from the sea, causing light drizzle with cloudy weather.

July

In the month of July, the monsoon reaches its peak and produces heavy to very heavy rainfall. As in June, the sky remains overcast with only 3 to 4 hours of sunlight a day, with occasional drizzle at night or early morning. Two or three rainstorms that continue for few days are common in this month. The highest July temperature of 42.2 °C was recorded on 3 July 1958 and the lowest temperature was 21.2 °C on 22 July 1997. The highest monthly rainfall for July was 429.3 mm (16.90 in) in 1967. In 2003, the monsoon’s low pressure created a flood situation in Sindh, with continuous spells of rain in Karachi; at least 284.5 millimetres (11.20 in) rain was recorded during two days of rain. 

Six years later on July 18, 2009, there was severe flooding due to a tropical depression, in which heavy rainfall of 245 mm occurred in just four hours with 40 mph (64 km/h) winds making it the second highest rainfall in 24 hours in the city, killing 20 and injuring 150 people. The city’s third highest rainfall of 207 mm (8.1 in) in 24 hours was recorded on July 1, 1977. Humidity in July remains high, usually between 80–90%, causing discomfort to the people specially during rush hour. But in the evening, as in June, cool winds blow from the sea in the southwest with light drizzle, making the weather pleasant. In the evenings, cirrus clouds can be seen daily coming from eastern direction, indicating the monsoon’s presence in the city.

August

The weather of August is identical to that of July. One or two weather systems lash the city in this month, causing significant flooding. In 2006, after two years of drought (2004 and 2005), widespread rainfall of 77 millimetres (3.0 in) occurred in the city to break the drought period. On 11 August 2007, a very strong tropical depression produced 191 millimetres (7.5 in) in just two days. Rain continued for three days with intense thunderstorms; another monsoon low produced 80 millimetres (3.1 in) of rain on 22 August 2007 with 98 mph (158 km/h) winds. The monsoon typically starts to get weak from the last week of August, but still has the potential to cause heavy rains in the city, as on 31 August 2009, when 147 millimetres (5.8 in) of rain was recorded with 106 mph (171 km/h) winds. 

But history has shown that the monsoon withdraws from Karachi during the final days of August. The highest rainfall for August is 271 millimetres (10.7 in), which occurred in 2007;  the highest temperature was 41.7 °C on 9 August 1964, while the lowest temperature was 21.0 °C, recorded on 7 August 1984. The city’s highest rainfall in 24 hours also occurred in August, on 7 August 1953, which is about 278.1 millimetres (10.95 in) of rain battered the city.

September

The first two weeks of September can have some rains. After the first two weeks, the monsoon completely withdraws from the city, and the sky remains sunny and dry. There is a slight decrease in humidity, which hovers between 70–80%, and an increase in temperatures. Rain in this month is inconsistent.During the monsoon of 2005 no rain occurred in the city raising the fears of drought but from 12 September till 13 September heavy rainfall lashed the city about 80 millimetres (3.1 in) rainfall was recorded.

On 10 September 2010 a strong low-pressure system approached Karachi and caused a heavy downpour of 78 millimetres (3.1 in), accompanied by strong gusting winds of 35 mph (56 km/h) that destroyed the roofs of some mud houses. The highest rainfall for September was 315.7 millimetres (12.43 in) in 1959. The highest temperature was 42.2 °C on 30 September 1951 and the lowest temperature recorded was 18.2 °C on 23 September 1994. 

October

October is the driest month in the city. The highest temperature of 43.3 °C was recorded on 1 October 1959 and lowest was 10.0 °C recorded on 30 October 1949. Morning is hazy and the winds are calm, the sky remains clear with hot conditions. The highest monthly rainfall for October is 98mm, which occurred in 1959.

During the year of 2004 the city received no significant rains with the exception of a heavy rain which occurred in October due to Cyclone Onil.

November

November has warm days and cool nights. Mid-November conditions are much like those of October. During the last weeks of November there is a gradual decrease in temperature and thus winter begins, but it is not that cold. The first weeks of November are typically dry and the last week of November is cold. Showers occur in this month due to the Western Disturbance and causes the temperatures to decrease.

On 9 November 2010 remnants of Cyclone Jal caused gusty winds in the metropolis. The highest rainfall for November is 83.1 millimetres (3.27 in), which occurred in 1959. The highest and lowest temperatures are 38.5 °C recorded on 2 November 1994 and 6.1 °C recorded on 29 November 1938 respectively.

December

The month of December is the annual winter month, but winter becomes more chilly in January. In December the weather remains cool. The sky remains cloudy for part of the month, causing temperatures to decrease. Rainfalls are common in the city during the month of December due to the western disturbance coming from the Mediterranean sea. The highest monthly rainfall of 63.6 millimetres (2.50 in) was recorded in 1980.

During December 2006 a strong westerly “low” pressure resulted in 60.5 millimetres (2.38 in) of rain in the city. The highest recorded temperature for December was 34.5 °C on 11 December 2001 and the lowest temperature was 1.3 °C which occurred on 14 December 1986.