
Thermal lows, or heat lows, are non-
frontal low-pressure area
In meteorology, a low-pressure area (LPA), low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. It is the opposite of a high-pressure area. Low-pressure areas are commonly associated with incle ...
s that occur over the continents in the
subtropics
The subtropical zones or subtropics are geographical and climate zones immediately to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately ...
during the warm season, as the result of intense heating when compared to their surrounding environments.
[Glossary of Meteorology (2009)]
Thermal Low.
American Meteorological Society
The American Meteorological Society (AMS) is a scientific and professional organization in the United States promoting and disseminating information about the atmospheric, oceanic, and hydrologic sciences. Its mission is to advance the atmosph ...
. Retrieved on 2009-03-02. Thermal lows occur near the
Sonoran Desert
The Sonoran Desert () is a hot desert and ecoregion in North America that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the Southwestern United States (in Arizona and California). It ...
, on the
Mexican Plateau
The Central Mexican Plateau, also known as the Mexican Altiplano (), is a large arid-to-semiarid plateau that occupies much of northern and central Mexico. Averaging above sea level, it extends from the United States border in the north to the T ...
, in California's
Great Central Valley, in the
Sahara
The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...
, in the
Kalahari, over north-west
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
, in
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
, over the
Kimberley
Kimberly or Kimberley may refer to:
Places and historical events
Australia
Queensland
* Kimberley, Queensland, a coastal locality in the Shire of Douglas
South Australia
* County of Kimberley, a cadastral unit in South Australia
Ta ...
region of north-west
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
, over the
Iberian Peninsula
The Iberian Peninsula ( ), also known as Iberia, is a peninsula in south-western Europe. Mostly separated from the rest of the European landmass by the Pyrenees, it includes the territories of peninsular Spain and Continental Portugal, comprisin ...
, and over the
Tibetan Plateau
The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...
.
On land, intense, rapid solar heating of the Earth's surface causes the heating of the lowest layers of the atmosphere, via re-radiated energy in the infrared spectrum. The hotter air is less dense than surrounding cooler air and rises, leading to the formation of a low-pressure area. Elevated areas can enhance the strength of the thermal low because they warm more quickly than the atmosphere which surrounds them at the same altitude. Over water, instability lows form during the winter when the air overlying the land is colder than the warmer water body.
Thermal lows can extend to in height and tend to have weak circulations. Thermal lows over the western and southern portions of North America, northern Africa, and Southeast Asia are strong enough to lead to summer
monsoon
A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...
conditions. Thermal lows inland of the coastline lead to the development of
sea breeze
A sea breeze or onshore breeze is a wind that blows in the afternoon from a large body of water toward or onto a landmass. By contrast, a land breeze or offshore breeze is a wind that blows in the night from a landmass toward or onto a large ...
s which, combined with rugged topography near the coast, can lead to poor air quality. Owing to the very high temperatures in the centre of heat lows, there are relatively few direct observations of thermal lows.
Formation

In deserts, the lack of ground and plant moisture, that would normally provide
evaporative cooling
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
, can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. That, combined with the rise of the hot air, results in a low pressure area called a thermal low.
Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above
sea level
Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
, which creates an associated heat low over the terrain, and enhances any thermal lows which would have otherwise existed. During the cold season, (
winter
Winter is the coldest and darkest season of the year in temperate and polar climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Dif ...
), warm water bodies such as the
Great Lakes
The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes spanning the Canada–United States border. The five lakes are Lake Superior, Superior, Lake Michigan, Michigan, Lake Huron, H ...
can induce an instability low. Thermal lows which develop near sea level can build in height during the warm season, or
summer
Summer or summertime is the hottest and brightest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, daylight hours are the longest and darkness hours are the shortest, with day ...
, to the elevation of the 700 hPa pressure surface, which lies near above sea level. Heat lows normally are stationary and have a weak cyclonic circulation. As they are strongest at the surface and warm near their center, and weaker aloft where the air is more stable, the thermal low is considered warm core. The strongest versions of these features globally are over Arabia, the northern portion of the
Indian subcontinent
The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...
,
Arizona
Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
,
Mexican Plateau
The Central Mexican Plateau, also known as the Mexican Altiplano (), is a large arid-to-semiarid plateau that occupies much of northern and central Mexico. Averaging above sea level, it extends from the United States border in the north to the T ...
, northwest
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
, southwestern
Spain
Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...
, Australia, and northern
Africa
Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...
. The formation of the heat low over northern Africa leads to a low-level westerly
jet stream
Jet streams are fast flowing, narrow thermal wind, air currents in the Earth's Atmosphere of Earth, atmosphere.
The main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east around the gl ...
from June into October.
Role in the monsoon regime

Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. That differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty meters deep, due to the action of wind and buoyancy-generated
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a meter or so. Additionally, the
specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...
of liquid water is significantly higher than that of most materials that make up land. Together, those factors mean that the heat capacity of the layer involved in the seasonal cycle is much larger over the oceans than over land, meaning that the air over the land warms faster and reaches a higher temperature than the air over the ocean. The hot air over the land tends to rise, creating an area of
low pressure. That creates a steady wind blowing toward the land, bringing the moist near-surface ocean air with it.
[Dr. Louisa Watts (2009)]
What causes the west African monsoon?
National Centre for Environmental Science. Retrieved on 2009-04-04. Similar
rainfall
Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. ...
is caused by the moist ocean air being lifted upwards by mountains,
[Dr. Michael Pidwirny (2008)]
CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes.
Physical Geography. Retrieved on 2009-01-01. surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However when the lifting occurs, the air cools due expansion in lower pressure, which in turn produces
condensation
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...
.
In winter, the land cools off quickly, but the ocean retains its heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling.
Monsoons are similar to
sea and land breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale, much stronger, and seasonal.
Role in sea breeze formation

The
sea
A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order section ...
is warmed by the sun to a greater depth than the land due to its greater
specific heat
In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...
. The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the
land
Land, also known as dry land, ground, or earth, is the solid terrestrial surface of Earth not submerged by the ocean or another body of water. It makes up 29.2% of Earth's surface and includes all continents and islands. Earth's land sur ...
rises, the land heats the air above it. The less dense warm air rises, which lowers the
sea level pressure by about 0.2%. The cooler air above the sea, with higher sea level pressure, flows towards the land into the area of lower pressure, creating a cooler breeze near the coast. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea. If the environmental wind field is greater than and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.
Along the California coast, the cooler water creates a surface marine layer that is much cooler than inland areas during the summer. At the same time, the intense heating inland generates a pronounced thermal trough aligned with the Central Valley, and typically linked to the broader thermal low across the North American deserts. As a consequence, a strong pressure gradient is created which draws cool marine air landward. As temperatures plummet, fog and stratus stream in and through the gaps of the Coast Ranges, and especially through the
Golden Gate
The Golden Gate is a strait on the west coast of North America that connects San Francisco Bay to the Pacific Ocean. It is defined by the headlands of the San Francisco Peninsula and the Marin Peninsula, and, since 1937, has been spanned by ...
at San Francisco (''see''
San Francisco fog
Fog is a common weather phenomenon in the San Francisco Bay Area and the entire coastline of California extending south to the northwest coast of the Baja California Peninsula. The frequency of fog and low-lying stratus clouds is due to a co ...
). The same thermal trough is sometimes pushed toward the coast, especially in late fall, when higher pressure develops to the east due to cooling even further east. That situation often brings the warmest temperatures of the year to the normally cool coastline, because the sea breeze stops or is even replaced by a dangerously dry land breeze (see also
Diablo wind and
Santa Ana wind).
Role in air pollution
In hilly or mountainous areas near the coastline, thermally-forced sea breezes, combined with wind circulations up the sides of the mountains, can encourage the production of chemicals which can lead to the development of
smog
Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and ''fog'' to refer to smoky fog due to its opacity, and odour. The word was then inte ...
. Pollution has been tracked into the mid-levels of the
troposphere
The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...
in the form of
ozone
Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
, which is concentrated over the circulation of the thermal low as well as adjacent oceanic areas.
[A.C. Carvalho, A. Carvalho, I. Gelpi, M. Barreiro,
C. Borrego, A.I. Miranda, V. Perez-Munuzuri (2006)]
Influence of topography and land use on pollutants dispersion in the Atlantic coast of Iberian Peninsula.
''Atmospheric Environment'' 40 (2006) 3969–3982. Retrieved on 2009-03-09.
References
{{Cyclones
Types of cyclone
Atmospheric dynamics
Vortices