What is an atmospheric front? Atmospheric fronts, cyclones and anticyclones. Atmospheric fronts - what is it? What are they like

ATMOSPHERIC FRONT (tropospheric front), intermediate, transition zone between air masses in the lower part of the atmosphere - the troposphere. The atmospheric front zone is very narrow compared to the air masses it separates, therefore it is approximately considered as the interface (gap) of two air masses of different density or temperature and is called the frontal surface. For the same reason, on synoptic maps, the atmospheric front is depicted as a line (front line). If the air masses were stationary, the surface of the atmospheric front would be horizontal, with cold air below and warm air above it, but since both masses move, it is inclined to the earth's surface, and the cold air lies in the form of a very gentle wedge under the warm one. The tangent of the slope of the frontal surface (front slope) is about 0.01. atmospheric fronts can sometimes extend to the tropopause itself, but can also be limited to the lower kilometers of the troposphere. In intersection with earth's surface the zone of the atmospheric front has a width of the order of tens of kilometers, while the horizontal dimensions of the air masses themselves are of the order of thousands of kilometers. At the beginning of the formation of atmospheric fronts and during their erosion, the width of the frontal zone will be greater. Vertically, atmospheric fronts are a transitional layer hundreds of meters thick, in which the temperature decreases less than usual with height, or increases, that is, temperature inversion is observed.

At the earth's surface, atmospheric fronts are characterized by increased horizontal air temperature gradients - in a narrow zone of the front, the temperature abruptly changes from values ​​characteristic of one air mass to values ​​characteristic of another, and the change sometimes exceeds 10 ° C. The air humidity and its transparency also change in the frontal zone. In the baric field, atmospheric fronts are associated with troughs reduced pressure(see baric systems). Extensive cloud systems form over the frontal surfaces, giving precipitation. The atmospheric front moves at a speed equal to the normal component to the wind speed front, so the passage of the atmospheric front through the observation site leads to a rapid (within hours) and sometimes abrupt change in important meteorological elements and the entire weather regime.

Atmospheric fronts are characteristic of temperate latitudes, where the main air masses of the troposphere border on each other. In the tropics, atmospheric fronts are rare, and the intratropical convergence zone that is constantly present there differs significantly from them, not being a temperature division. The main reason for the emergence of an atmospheric front (frontogenesis) is the presence of such motion systems in the troposphere, which lead to the convergence (convergence) of air masses that have different temperatures. The initially wide transitional zone between air masses then becomes a sharp front. In special cases, the formation of an atmospheric front is possible when air flows along a sharp temperature boundary on the underlying surface, for example, above the ice edge in the ocean (the so-called topographic frontogenesis). In the process of general circulation of the atmosphere between air masses of different latitudinal zones with sufficiently large temperature contrasts, long (thousands of km), main fronts elongated mainly in latitude - arctic, antarctic, polar - arise, on which cyclones and anticyclones form. In this case, the dynamic stability of the main atmospheric front is violated, it is deformed and moves in some areas to high latitudes, in others - to low latitudes. On both sides of the surface of the atmospheric front, vertical components of the wind speed of the order of cm/s arise. Especially important is the upward movement of air above the surface of the atmospheric front, which leads to the formation of cloud systems and precipitation.

In the front part of the cyclone, the main atmospheric front takes on the character of a warm front (figure, a), when it advances to high latitudes, warm air takes the place of the receding cold air. In the rear part of the cyclone, the atmospheric front takes on the character of a cold front (figure, b) with the advance of the cold wedge forward and with the displacement of warm air in front of it into the high layers. When a cyclone is occluded, a warm and cold atmospheric front combines to form a complex occlusion front with corresponding changes in cloud systems. As a result of the evolution of frontal disturbances, the atmospheric fronts themselves are washed out (the so-called frontolysis). However, field changes atmospheric pressure and winds created by cyclonic activity lead to the emergence of conditions for the formation of new atmospheric fronts and, consequently, to the constant resumption of the process of cyclonic activity on the fronts.

In the upper part of the troposphere, in connection with the atmospheric front, so-called jet streams arise. From the main fronts, secondary atmospheric fronts are distinguished, arising inside the air masses of one or another natural area with some of their heterogeneity; they do not play a significant role in the general circulation of the atmosphere. There are cases when the atmospheric front is well developed in the free atmosphere (upper atmospheric front), but is little pronounced or does not appear at all near the earth's surface.

Lit.: Petersen S. Analysis and weather forecasts. L., 1961; Palmen E., Newton C. Circulation systems atmosphere. L., 1973; Ocean - atmosphere: Encyclopedia. L., 1983.

), are separated from each other by rather narrow transition zones, which are strongly inclined to the earth's surface (less than 1°). a front is a section between having different physical properties. The intersection of the front with the earth's surface is called the front line. At the front, all the properties of the air masses—temperature, wind direction and speed, humidity, precipitation—change dramatically. The passage of the front through the place of observation is accompanied by more or less abrupt changes.

Distinguish fronts associated with cyclones, and climatic fronts.

In cyclones, fronts are formed when warm and cold air meet, while the top of the frontal system, as a rule, is in the center. Cold air meeting warm air always ends up at the bottom. It leaks under the warm, trying to push it up. Warm air, on the contrary, flows onto cold air and if it pushes it, then it itself rises along the interface plane. Depending on which air is more active, in which direction the front is moving, it is called warm or cold.

A warm front moves towards colder air and means an advance warm air. It slowly pushes cold air out. Being lighter, it flows onto the wedge of cold air, gently rising up along the interface. In this case, an extensive zone of clouds forms in front of the front, from which heavy precipitation falls. The precipitation band in front of the warm front reaches 300, and in cold weather even 400 km. Behind the front line, precipitation stops. The gradual replacement of cold air with warm air leads to a decrease in pressure and an increase in wind. After the passage of the front, there is abrupt change weather: rises, changes direction by about 90 ° and weakens, visibility deteriorates, forms, drizzling precipitation may occur.

The cold front moves towards the warm air. In this case, cold air, being denser and heavier, moves along the earth's surface in the form of a wedge, moves faster than warm air, and, as it were, lifts warm air in front of it, vigorously pushing it up. Above the front line and ahead of it, large cumulonimbus clouds form, from which fall torrential rains, occur , are observed strong winds. After the passage of the front, precipitation and cloudiness significantly decrease, the wind changes direction by about 90 ° and weakens somewhat, the temperature drops, air humidity decreases, its transparency and visibility increase; is growing.

The Arctic (Antarctic) front separates the Arctic (Antarctic) air from the air of temperate latitudes, two temperate (polar) fronts separate the air of temperate latitudes and tropical air. A tropical front forms where tropical and air meet, differing in , not in temperature. All fronts, together with the boundaries of the belts, shift towards the poles in summer, and in winter. Often they form separate branches that spread over long distances from. The tropical front is always in the hemisphere where it is summer.

Various air masses are usually in constant motion. At the same time, they can approach and meet, forming the so-called frontal zones - transitional zones between air masses with different physical properties. Their width is a few hundreds of kilometers, their length is thousands of kilometers. They observe rapid changes in all meteorological quantities horizontally - temperature, pressure, humidity, since in fact they represent a "battlefield" between warm and cold air. In the frontal zones, there are interfaces between warm and cold air masses, which are called frontal surfaces (Latin frons (genus item frontis) - forehead, front side). This surface is a narrow strip of several tens of kilometers, but compared to the size of the air masses that it delimits, it appears to be a plane. The angle between the frontal plane and the earth's surface is very small, less than 1°, but it is exaggerated in the figures for clarity. The frontal surface is always inclined towards the cold air, so that the cold dense air is located below, under it, and the warm, less dense and light air- above her. The line of intersection of the frontal plane with the surface of the Earth forms the front line, which is also briefly called the front. All of these listed concepts are often combined with the expression atmospheric front.

Since the pressure step in warm air is greater than in cold air, the distance between the isobaric surfaces on both sides of the frontal surface will be different. A change in the properties of air under conditions of its continuity in the atmosphere is achieved by the formation of a trough of all isobaric surfaces in the front zone. It manifests itself near the earth's surface in the form of a hollow outlined by isobars (Fig. 56). Thus, all atmospheric fronts lie in baric troughs.

Atmospheric fronts are stationary and moving.

If air currents are directed from both sides along the front line and it does not move noticeably either towards warm or towards cold air, then the front is called stationary.

A moving front is formed if one of the air masses has a velocity component perpendicular to the front line. Depending on the direction of movement, moving fronts are divided into warm and cold. A warm front is formed when warm air flows into cold air. The front line moves in the direction of cold air. After the passage of the warm front, warming occurs (Fig. 57). A cold front is formed when cold air flows under warm air.

Rice. 57. Warm front. The names of the clouds are indicated in table 2 (according to I. I. Guralnik)

Rice. 58. Cold front of the first kind (according to I. I. Guralnik)

In this case, the front line moves towards the warm air, which is forced upward. After the passage of the cold front, cooling sets in. There are cold fronts of the first and second kind. A cold front of the first kind is formed in the case of a slow onset of cold air. In this case, warm air quietly rises along the frontal surface and the front line moves slowly (Fig. 58). A cold front of the second kind occurs when cold air moves rapidly and abruptly flows under warm air, which is thrown up. At the same time, the frontal surface rises steeply above the earth's surface due to the fact that the surface layers of air are decelerated by friction. The front line is moving fast (Fig. 59).

In the atmosphere, more complex complex fronts often arise when two main fronts, warm and cold, merge (combine). These are the fronts of occlusion (Latin occlusio - locking). When they form, two cold air masses unite, and warm air is forced into the upper layers of the troposphere and loses contact with the earth's surface. If the incoming cold air is less cold than the previous one, an occlusion front is formed similar to a warm front. If the incoming air is colder than the previous one, an occlusion front occurs according to the type of cold front (Fig. 60).

Frontal activity is most intense in temperate and nearby latitudes. Here, atmospheric fronts systematically arise, move (mainly from west to east) and collapse over several days. They are associated with the formation of atmospheric disturbances of a vortex nature - cyclones (ascending vortices) and anticyclones (descending vortices), which determine various types of weather.

Rice. 59. Cold front of the second kind (according to I. I. Guralnik)

On the climate maps zones are distinguished where, according to long-term average data, air masses are more common various types and subtypes and where atmospheric fronts are most actively formed. Such statistically stable frontal zones are called climatic fronts. In these zones of large horizontal contrasts in temperature, pressure and strong winds large reserves of energy are concentrated, which are spent on the formation of cyclones and anticyclones. Thus, these zones reflect the average long-term most typical position of the series of moving atmospheric fronts.

Climate fronts are divided into primary and secondary fronts.

Main fronts are zones of separation and interaction of the main types of air masses, contrasting primarily in temperature. Between arctic (Antarctic) and polar (temperate) air, they are called respectively Arctic and Antarctic fronts, between polar and tropical air - polar front. The section between warm air masses - relatively dry tropical and humid equatorial - previously considered a tropical front, is a zone of convergence of the trade winds of the northern and southern hemispheres and is currently called intertropical convergence zone(VZK) (Fig. 61, 62).

The features of the main fronts are as follows. First, they are traced up to the stratosphere, often causing the formation of so-called jet streams- very strong winds that reach their greatest value near the tropopause. Secondly, they do not form continuous bands on the Earth, but are torn into separate branches (segments), which have their own names. This is especially noticeable in the example of the polar front, which is divided into a number of branches. Thirdly, these branches move seasonally following the Sun: in summer, fronts, together with series of cyclones arising on them, migrate towards the poles, in winter - towards the equator, and some of them are washed out in certain seasons. Figure 62 shows that in winter the branch of the polar front that separates the sea polar air of the Atlantic from the sea tropical masses of the North Atlantic High is located at the latitude of France. The Mediterranean branch of the polar front, which separates the tropical air from the continental air masses of temperate latitudes, lies over mediterranean sea and further to the east passes into the Iranian branch, but in summer both branches are washed out. Over eastern Transbaikalia and northern Primorye, the Mongolian branch of the polar front forms in summer, separating the continental polar and tropical air masses, and over the Sea of ​​Japan, the Pacific branch between the marine polar and tropical masses.

Rice. 61. Climate fronts in July (according to S. P. Khromov)

Rice. 62. Climate fronts in January (according to S. P. Khromov)

The ends of the polar fronts that penetrate far into the depths of the tropics are called trade-wind fronts. In the tropics, they no longer separate polar and tropical air, but different masses of tropical air, brought from different oceanic subtropical highs by winds called trade winds. Often they arise between two MTS, one of which was formed from EE over warm sea ​​currents the western periphery of the subtropical maxima, and the second one, from the MSP over the cold currents of their eastern periphery (for example, in the summer near the Mexican Highlands, the Kalahari semi-desert, etc.).

Secondary fronts(fronts of the second order) are usually formed between air masses of different subtypes of the same geographical type.

They often occur between maritime and continental polar air, primarily in winter, when the temperature difference between them reaches its greatest values. Such a polar front is outlined above the center of the East European Plain, in connection with which Moscow is figuratively called a “front-line” city. Secondary fronts can be traced to a lower height than the main ones - for several kilometers within the troposphere.

Watching the weather changes is very exciting. The sun gives way to rain, the rain to snow, and gusty winds blow over all this diversity. In childhood, this causes admiration and surprise, in older people - a desire to understand the mechanism of the process. Let's try to understand what shapes the weather and how atmospheric fronts are related to it.

air mass boundary

In the usual perception, "front" is a military term. This is the edge on which the clash of enemy forces takes place. And the concept of atmospheric fronts is the boundaries of contact between two air masses that form over huge areas of the Earth's surface.

By the will of nature, man was given the opportunity to live, evolve and populate ever larger territories. The troposphere - the lower part of the Earth's atmosphere - provides us with oxygen and is in constant motion. All of it consists of separate air masses, united by a common occurrence and similar indicators. Among the main indicators of these masses determine the volume, temperature, pressure and humidity. During the movement, different masses can approach and collide. However, they never lose their boundaries and do not mix with each other. - these are areas where sharp weather jumps come into contact and occur.

A bit of history

The concepts of "atmospheric front" and "frontal surface" did not arise by themselves. They were introduced into meteorology by the Norwegian scientist J. Bjerknes. It happened in 1918. Bjerknes proved that atmospheric fronts are the main links in the high and middle layers. However, before the studies of the Norwegian, back in 1863, Admiral Fitzroy suggested that the furious atmospheric processes begin at the meeting points of air masses coming from different parts of the world. But at that moment, the scientific community did not pay attention to these observations.

The Bergen school, of which Bjerknes was a representative, not only carried out its own observations, but also brought together all the knowledge and assumptions expressed by earlier observers and scientists, and presented them in the form of a consistent scientific system.

By definition, the inclined surface, which is the transition area between different air flows, is called the frontal surface. But atmospheric fronts are a display of frontal surfaces on a meteorological map. Usually, the transition region of the atmospheric front is tied up near the surface of the Earth and rises up to those heights at which the differences between air masses are blurred. Most often, the threshold of this height is from 9 to 12 km.

warm front

Atmospheric fronts are different. They depend on the direction of movement of warm and cold massifs. There are three types of fronts: cold, warm and occlusion, formed at the junction of various fronts. Let us consider in more detail what warm and cold atmospheric fronts are.

A warm front is a movement of air masses in which cold air gives way to warm air. That is, the air of a higher temperature, moving forward, is located in the territory where cold air masses dominated. In addition, it rises up along the transition zone. At the same time, the air temperature gradually decreases, due to which condensation of the water vapor in it occurs. This is how clouds form.

The main signs by which you can identify a warm atmospheric front:

• atmospheric pressure drops sharply;
• increases;
• the air temperature rises;
• cirrus appear, then cirrostratus, and after - high-stratus clouds;
• the wind turns slightly to the left and becomes stronger;
• clouds become nimbostratus;
• precipitation of varying intensity falls.

It usually gets warmer after the precipitation stops, but this does not last long, because cold front moving very fast and catching up with a warm atmospheric front.

cold front

Such a feature is observed: a warm front is always inclined in the direction of movement, and a cold front is always inclined in the opposite direction. When fronts move, cold air wedges into warm air, pushing it up. Cold atmospheric fronts lead to a decrease in temperature and cooling over a large area. As the rising warm air masses cool, the moisture condenses into clouds.

The main signs by which a cold front can be identified are:

• before the front, the pressure drops, behind the line of the atmospheric front it rises sharply;
• cumulus clouds form;
• a gusty wind appears, with a sharp change in direction clockwise;
• heavy rain begins with a thunderstorm or hail, the duration of precipitation is about two hours;
• the temperature drops sharply, sometimes by 10 ° C at once;
• Numerous clearings are observed behind the atmospheric front.

Traveling through a cold front is no easy task for travelers. Sometimes you have to overcome whirlwinds and squalls in conditions of poor visibility.

Front of occlusions

It has already been said that atmospheric fronts are different, if everything is more or less clear with warm and cold fronts, then the front of occlusions raises a lot of questions. The formation of such effects occurs at the junction of cold and warm fronts. The warmer air is forced upward. The main action occurs in cyclones at the moment when a more rapid cold front catches up with a warm one. As a result, there is a movement of atmospheric fronts and a collision of three air masses, two cold and one warm.

The main features by which you can determine the front of occlusions:

• clouds and precipitation of a general type;
• abrupt shifts without a strong change in speed;
• smooth pressure change;
• no sudden temperature changes;
• cyclones.

The occlusion front depends on the temperature of the cold air masses in front of it and behind it. Distinguish between cold and warm occlusion fronts. The most difficult conditions are observed at the moment of direct closure of the fronts. As the warm air is displaced, the front is eroded and improved.

Cyclone and anticyclone

Since the concept of "cyclone" was used in the description of the front of occlusions, it is necessary to tell what kind of phenomenon it is.

Due to the uneven distribution of air in the surface layers, zones of high and low pressure. High pressure zones are characterized by excess air, low - insufficient air. As a result of the air flow between the zones (from excess to insufficient), wind is formed. A cyclone is an area of ​​low pressure that draws in, like a funnel, the missing air and clouds from areas where they are in excess.

Anticyclone - section with high blood pressure, which displaces excess air into low pressure areas. The main characteristic is clear weather, since clouds are also forced out of this zone.

Geographic division of atmospheric fronts

Depending on the climatic zones, over which atmospheric fronts are formed, they are divided geographically into:

1. Arctic, separating cold Arctic air masses from temperate ones.
2. Polar, located between the temperate and tropical masses.
3. Tropical (trade wind), delimiting the tropical and equatorial zones.

Influence of the underlying surface

On the physical properties air masses are affected by radiation and the appearance of the Earth. Since the nature of such a surface can be different, the friction against it occurs unevenly. Difficult geographic topography can deform the atmospheric front line and change its effects. For example, there are known cases of destruction of atmospheric fronts when crossing mountain ranges.

Air masses and atmospheric fronts bring many surprises to forecasters. Comparing and studying the directions of movement of the masses and the vagaries of cyclones (anticyclones), they draw up graphs and forecasts that people use every day, without even thinking about how much work is behind it.

Rain... Snow... Penetrating wind... Burning sun... These manifestations of the weather are familiar to each of us from deep childhood. But even having diligently studied geography at school, we are still sometimes surprised by sudden changes in temperature and unusual natural disasters. Atmospheric fronts are invariably associated with climatic shocks. They shape everyday weather and define the boundaries of the seasons.

atmospheric front

The word "front" (from the Latin "frontis" - forehead, front side) implies a fine line between something. It can pass, for example, between different areas combat operations: areas of concentration of enemy forces and a friendly army. If we use the phrase "atmospheric front", then we mean a boundary in the air, a certain boundary in the atmosphere. What exactly does he share, and how does it affect us?

Mother nature has shaped favorable climate in which a person can exist, multiply and develop. We live in the troposphere, the lower part of the atmosphere, which not only provides us with oxygen, but is in constant motion. Some voluminous in it interact from time to time. In the middle of each of these formations there are small pockets of microclimate, which differ in properties, but are generally homogeneous, maintaining a stable temperature and humidity. Masses move over the surface of the Earth, meet and even collide. But they never mix. The boundary between them is called the atmospheric front.

Main types

The width of the strip between air masses of the same properties reaches tens, sometimes hundreds of kilometers. This is an atmospheric front, where jumps in air pressure, changes in cloudiness and temperature always occur. That is, it is in these areas that one can observe how the hot sun is replaced by cold rain and vice versa. If very close, in fact, homogeneous masses come into contact, an atmospheric front does not arise. As a result, the weather does not change.

There are several fronts. They were formed on the basis of the main indicators of which remain constant.

1. Arctic. Separates cold arctic air from temperate air.
2. Polar. Located between temperate and tropical air masses.
3. Tropical. This is the border between the tropical and equatorial zones.

In the case of complete immobility, the front would take horizontal position. In this case, the layer of cold air would always be at the bottom, and warm - at the top. But as a result of constant cyclicity, it is located at an angle to the surface of the earth.

cold front

Whether the weather in our region will change and what it will be - all this will be demonstrated by a map of atmospheric fronts. It clearly shows that the warm front is always inclined in the direction in which it moves, the cold one - in the opposite direction. When the latter moves into a zone of high temperature, and penetrates into it in a kind of wedge, pushing it up, cooling sets in in this territory. Warm masses gradually cool down, moisture is released from them - this is how clouds and clouds are formed.

The first sign of an approaching cold front is rain cumulus formations that appear on the horizon. At the same time, the wind blows in gusts, sharply changing direction. Wall pouring rain collapses suddenly. The sky is gloomy, it is cut by lightning, thunder rumbles, sometimes there is hail. The storm lasts no more than two hours, after which the precipitation stops. The air temperature drops, sometimes by 5-10 degrees at once, as the space of the atmosphere is completely occupied by a cold front that has displaced the air warmed by the sun.

warm front

It is formed when a zone of high positive temperature "flows" onto a cold mass. She seems to be sliding on it, gradually rising. The weather changes smoothly, without unexpected sudden jumps and drops. Cirrus clouds are the first sign that an atmospheric front is approaching, in the center of which there is quite heat air. There is no wind yet. If it is, then its breaths are always pleasant and light.

Gradually, the clouds melt, and a continuous white veil of small layered formations forms in the sky, which move across the clear blue sky. After some time, they bunch up: a dense layer sinks lower, the wind rises, it drizzles, or light snow falls. Precipitation intensifies, lasts for several hours, sometimes days, after which warming sets in. Good weather does not last long. The atmospheric front, in which the temperature is low, catches up with the heat zone, as it moves faster and more rapidly.

Cyclone

The air on the earth's surface is unevenly distributed. As a result, zones with high and low pressure are formed. In the first region, air is in excess, in the second - in short supply. From the zone of high pressure, it flows out, as if pouring over the edge of the glass, and fills the formed "holes" in the area where we call this phenomenon of nature the wind.

The area of ​​low pressure is the cyclone. It has the shape of a whirlwind. Watch how water flows out of the sink - it forms a funnel. The same principle shows us the weather. Cyclone - the same funnel in the sink, only turned upside down. In its center is a low pressure pole, which draws in air from all sides and rushes up, and it twists clockwise in and against - in the north. It is cloudy inside the cyclone, because together with the wind it "sucks" the clouds into itself. They slide down a hill into it from those areas where the pressure is high.

Anticyclone

It works exactly the opposite. In the center - high pressure, there is a lot of air there, so it spreads in all directions, as if cream squeezed out of a pastry bag. The currents swirl clockwise in the northern hemisphere, counter-clockwise in the southern. To give another example: if you draw a carbonated drink into a straw and then release it, it will invariably drain into a glass. A similar phenomenon occurs in an anticyclone. Only with the help of air and on a global scale.

The weather in the anticyclone is usually clear, as high pressure forces clouds out of this area. At the same time, it is always very hot in summer: there are no barriers in the form of clouds that prevent the sun from warming up the air. In winter, the opposite is true. The sun is low enough, but it cannot warm the air: there are no clouds, and therefore nothing retains heat. As a result, in winter, when the anticyclone comes, the weather is clear but frosty. By the way, by studying atmospheric fronts, cyclones and anticyclones, their movements, modifications and transformation, weather forecasters make a weather forecast for a particular region.

What does the coming day have in store for us?

The most difficult thing, forecasters say, is to predict the weather for the next three days. That is, after collecting all the necessary information, you need to quickly process it, taking into account all the vagaries of atmospheric fronts, changes in cyclones and anticyclones. And only by comparing the data, you can draw a conclusion.

Weather forecasts are as follows:

1. Short-term - a maximum of three days.
2. Medium-term - up to ten days.
3. Long-term weather forecast - for a month or a season.

The first two types are the solution by weather forecasters of the equations of thermodynamics and dynamics, which describe the state of the atmosphere. To do this, experts analyze the possibility of a change in precipitation, expected jumps in pressure and air humidity. A long-range weather forecast is never completely accurate. Even with the latest equipment, weather forecasters cannot foresee all the surprises nature has in store. But in any case, it is necessary to make it, since such a forecast refers to the expected monthly or seasonal weather anomalies.