Why is there no rain in mediterranean regions during summer compared to tropical regions where there is plenty of rain during summer?

Why is there no rain in mediterranean regions during summer compared to tropical regions where there is plenty of rain during summer?

Here is somewhat of an answer and also an interesting read.
http://www.realclimate.org/index.php/archives/2007/10/sweatin-the-mediterranean-heat/

Around the Mediterranean sea, deserts and desert-like conditions can be found in close proximity to a very warm sea and thus to a marine airmass with a high moisture content, e.g., the coasts of Morocco, Algiers, Tunisia, Libya, and Almeria in Southeastern Spain. These regions were covered with vegetation in historical times, e.g., during the Roman Empire, and in the case of Almeria, just 150 years ago, before the forests were used to fuel lead mines. The question is: did forest removal cause them to run a feedback cycle towards desertification? The reanalysis of results from seventeen EC research projects (Section 5) suggests that this could be the case.
This work shows that the hydrological system in this region is very sensitive to land-use changes and, more recently, to air pollution effects as well. Both of these can combine to exceed critical threshold levels, e.g., the height of the cloud condensation levels with respect to the height of the coastal mountain ranges. This results in the loss of summer storms and tips the regional climate towards desertification and drought. The non-precipitated water vapour returns and accumulates over the Western Mediterranean Basin to heights reaching over 5000 m, for periods lasting from 3 to 10 days in summer.
http://www.springerlink.com/content/t3378w56787ml3hj/

During summer, regions of Mediterranean climate (also known as Dry-Summer Subtropical for the Csa areas) are dominated by subtropical high pressure cells, with dry sinking air capping a surface marine layer of varying humidity but making rainfall impossible or unlikely but for the odd thunderstorm, while during winter the polar jet stream and associated periodic storms reach into the lower latitudes of the Mediterranean zones, bringing rain, with snow at higher elevations. As a result, areas with this climate receive almost all of their yearly rainfall during the winter season, and may go anywhere from 2-5 months during the summer without having any significant precipitation.

Toward the equatorial end, winter precipitation increases. Toward the polar end, total moisture usually increases; in Europe there is more summer rain further north while along the American west coast the winters become more intensely wet and the dry seasons shorter as one moves north.
http://en.wikipedia.org/wiki/Mediterranean_climate

Here is somewhat of an answer and also an interesting read.
http://www.realclimate.org/index.php/archives/2007/10/sweatin-the-mediterranean-heat/

But how does a low pressure system over Iceland and a high pressure system over the Azores cause less rain in mediterranean regions? Does this have to do with barometric pressure?

As far as i know rain in subtropical and tropical regions is convectional in nature. The process of convection in the atmosphere occurs when warm air rises. (Technically speaking, the air is less dense as a result of thermal expansion) When conditions are right, the sun can intensely heat the ground causing warm air to rise. This air cools as it rises in the atmosphere and eventually reaches a dew point temperature causing clouds to form.

Well here's the answer...

Usually we are used to rain falling in Israel only in winter when it's cold. Therefore, in our collective memory rain connects with winter and many of us turn the sensation to the equation: Rain = cold. But, it's appropriate to remember that most rain actually falls in tropical regions where it's very hot, much more so than in cold regions. For proof, in cold Europe amounts of rainfall are usually 500 mm in the south, 700 mm in its center and up to 1000 mm in the north. In the poles annual rainfall amounts are less than 100 mm and they are defined climatically as deserts. In tropical regions amounts of rainfall are usually over 3000 mm and they even reach 10,000 mm and more. The national record is in a region in Burma where annual rainfall there is 14,000 mm. In their contrary, there are tropical regions in which there is no much precipitation like in the Andes of South America.

Rain falls from clouds and these clouds are formed by the evaporation of water from the oceans. The measure of evaporation depends among other things on the temperature of the ocean's water and the hotter the ocean's water the more they evaporate therefore more clouds are formed and there is more rainfall. The average ocean water temperature in the tropics is 30°C as opposed to 12°C in Israel and 1°C in the north Atlantic ocean for example.

The air touching the water surface heats in accordance to the water temperature that's why the hair is warmer in the tropics than at higher latitudes (the terms higher and lower latitudes stem from the fact that the equator is defined as 0° and the poles 90°).

And from here it's important to remember the following facts:

1. When air heats up, like every gas it expands and becomes thinner.
2. Therefore when air heats up it becomes lighter and starts to lift up. We feel that exactly when we sense hot air rising over an oven. Even hot air-balloons are aided by the same principle. You heat the air and inflate the balloon until it becomes lighter than air (aerostat) and starts to lift up. Therefore, above warm oceans and tropical regions in general, hot air rises.
3. The air pressure above sea level is caused by the weight of the air column which starts at sea level and ends somewhere on the edge of the atmosphere. As we go higher in altitude, the air column which puts pressure on us becomes shorter therefore air pressure decreases with altitude. Therefore the hot and light air which climbs upward bumps into the higher layers of lower pressure around it and expands to the sides.
4. All gases (including air) cools down when it expands. The reason for this is that the internal heat of the gas is caused by collisions between its molecules. When a gas is compressed, the collisions intensify (like in a frontal collision between cars) and on the opposite, when it expands, the collisions weaken. Therefore the hot and light air which climbs upward cools down whereas the cold and heavy air which sinks becomes warmer. The rate of heating and cooling is constant and is 1 degree Celsius for every 100 meter height.
5. Like we mentioned, the air above tropical regions is hot and humid therefore, when it climbs upward it cools. Hot and humid air which cools condenses and clouds are formed in it (on condensation in a different article). Therefore above the tropical regions there are a lot of clouds and as aforesaid a lot of rainfall. Not all clouds bring rainfall and there are many conditions required for that but in general low and vertically developed clouds (several km) which have temperatures lower than 0°C may bring rainfall. If only hot air would rise up and cold air would not descend, all the atmosphere would have gone long ago and we would not exist on earth. The fact that the atmosphere is fixed and stable tells us that on every place that hot air rises in, in another place on earth cold air sinks.
6. In places where cold air sinks, it bumps into the higher air pressure on his way down, therefore it is compressed and as mentioned in section 4, it heats up. As air heats up without a change in the amount of water vapor in it, it dries up and when it gets colder it becomes more humid. Therefore in the day it's usually drier while at night there is dew and sometimes even fog. This is also the reason why in a house warmed to a large degree in winter it's very dry and for the parents it's recommended to increase the humidity in it for the babies. In places in which air sinks it's hot and dry, there is no precipitation and they are called - deserts.

And here starts the explanation why in Israel there is no rain in summer. The hot and wet air which rises in equatorial regions cools down there, condenses and gives a lot of rainfall. It sinks back toward earth around latitudes 20°-30° north and south. In places where it sinks, it creates the desert belt. In the northern part of the globe the deserts are: the Sahara, the Arabian Peninsula, Iran, Afghanistan and Pakistan, Gobi desert in China, the Sonoran desert in Mexico and Mojave desert in California. In the winter period in the northern half of the globe, the sinking of the hot air is limited to North Africa and Saudi Arabia but doesn't reach as far as the Mediterranean coasts, which allows cool and and humid air from Europe to infiltrate south and bring precipitation in these regions including Israel. In summer the sinking air spreads north and it covers all Mediterranean regions. Then, in all of these regions there is no rainfall. The descent of the air which causes the heat and dryness in Israel has its origin in the monsoon trough on central Asia. The air which rises there in the summertime sinks in Israel (as after the passage of a low pressure area the weather makes us more stable and bright).

We know it's very hot in summer in Israel. If so, in our country hot air climbs upward and cools down as described in section 5. When dry desert air rises and cools down, it needs to cool down a lot in order for it to be packed together enough and condense. In desert areas this is happening only in altitudes of above 6 km. In these altitudes the temperature is much lower than 0°C. Therefore, when the air condenses and turns into tiny droplets, simultaneously it also freezes and turns into ice, a process called sublimation. If so the clouds above deserts are made of ice, for example Cirrus clouds. This could explain why there is no rainfall in the Negev which is actually a continuation of Sinai desert and the Sahara. About Israel's coastal plane, we remember that in the summer it's hot and humid (such weather is called sultry). The relative humidity in Israel's coastal plane reaches 80%. Hot and humid air should climb upwards, then cool and condense as mentioned in section 5 then why doesn't it happen in Israel?

Very simple, the air indeed starts to climb up and cool (remember, 1°C for each 100 meter altitude?). But alas, when it reaches 1500 m height (by how many degrees did it succeed in cooling?), it bumps into the hot and dry sinking air, that which rose up in the equator and in summer sinks on all Mediterranean regions.

The result:

There usually are no low and developed clouds formed in Israel in summer, even on the Mediterranean coast and if they do form, they are not developed enough and temperature in them in much higher than 0°C therefore they do not bring down any rains.

In a rare manner and for reasons not known, once in two or three years, the sinking of the hot and dry air from the tropics retreats in the middle of the summer back southward. Then the hot and humid air that in the coastal plain doesn't bump into the accepted 1500 m altitude barrier but continues to climb upward, cool down and condense. Developed clouds form and precipitation known as summer rains fall. Rare but it happens and when it comes, the rain refreshes and eases the heat a little bit.

Source: http://www.hagigim.com/modules.php?name=Forums&file=printview&t=3500&start=0

But how does a low pressure system over Iceland and a high pressure system over the Azores cause less rain in mediterranean regions? Does this have to do with barometric pressure?

As far as i know rain in subtropical and tropical regions is convectional in nature. The process of convection in the atmosphere occurs when warm air rises. (Technically speaking, the air is less dense as a result of thermal expansion) When conditions are right, the sun can intensely heat the ground causing warm air to rise. This air cools as it rises in the atmosphere and eventually reaches a dew point temperature causing clouds to form.

Check the equation of state.

However in brief; warm air can contain more water. Around Mediterranean areas the LCL (Lifting Condensation Level) is higher, due to a higher stratosphere. Much due to the water vapor from the ocean being lifted up into the atmosphere as "Particulate matter". With a higher stratosphere, and a higher LCL...you have a higher elevation that is required for air-parcel saturation. It's beyond this saturation that the parcel expends its excess as rain. When elevation of this point is too high, the precipitation gets transformed into water vapor before hitting the ground, further raising the LCL and inherently the Stratosphere. In this climate it can only rain with this process is halted by cooler temperatures via Monsoon or shorter days.

In a colder climate the stratosphere is rather low, and the LCL is even lower. When these are low the water can be precipitated at relatively low temperatures, as with a cold temperature the saturation level is quickly reached. Generally these kind of climates are found continental, as in the summer it's rare for them to get enough surface water or pressure systems bringing in water to allow for precipitation.